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JOURNAL OF PETROLOGY
VOLUME 51
NUMBER 6
PAGES 1355^1379
2010
doi:10.1093/petrology/egq022
Antigorite Peridotite, Metaserpentinite, and other
Inclusions within Diatremes on the Colorado
Plateau, SW USA: Implications for the Mantle
Wedge during Low-angle Subduction
DOUGLAS SMITH
DEPARTMENT OF GEOLOGICAL SCIENCES, JACKSON SCHOOL OF GEOSCIENCES, 1 UNIVERSITY STATION C1100,
UNIVERSITY OF TEXAS AT AUSTIN, AUSTIN, TX 78712, USA
RECEIVED DECEMBER 14, 2009; ACCEPTED APRIL 15, 2010
ADVANCE ACCESS PUBLICATION MAY 16, 2010
Antigorite peridotite, Cr-magnetite dunite, chlorite harzburgite, and
other ultramafic rock fragments included in the Green Knobs diatreme in NW New Mexico are used to investigate the evolution of
the mantle wedge during low-angle subduction. The diatreme, part
of the Navajo Volcanic Field, has a matrix of serpentinized ultramafic microbreccia (SUM). The meta-peridotite inclusions have been
characterized by petrographic, electron microprobe, and bulk-rock
analysis. The assemblage antigorite^diopside^olivine formed and
was stable in the mantle. In a Cr-magnetite dunite, olivine compositions span the range Fa9^3, and chlorite-rich pockets contain extraordinarily pure pyrope (Py94·4Alm5·5Gr0·1). The Cr-magnetite dunite
is a product of prograde metamorphism of a brucite-bearing serpentinite. Second generations of olivine in that dunite and other samples
formed during a sharp temperature increase caused by intrusion of
the magma that triggered the eruption. Histories of lawsonite eclogite, garnetite, and other inclusions in SUM diatremes have been interpreted to provide context: some are from the lithospheric part of
the mantle wedge, hydrated near the trench, tectonically eroded and
transported some 700 km in a me¤lange, and emplaced below the
Colorado Plateau during low-angle subduction of the Farallon
plate. The Cr-magnetite dunite was also derived from that me¤lange,
whereas some other inclusions represent mantle hydrated in place
above the slab.Tectonic erosion of the Colorado Plateau mantle lithosphere, or serpentinite diapirism into that lithosphere, or both, accompanied the low-angle subduction. Serpentinization far from the
trench and low-angle transport of parts of the lithospheric mantle
*Corresponding author. Telephone: 512-452-7768. E-mail: doug@
geo.utexas.edu
wedge are consistent with models of the Laramide orogeny and may
be common during low-angle subduction.
KEY
WORDS:
antigorite; diatreme, mantle; metaserpentinite;
subduction
I N T RO D U C T I O N
Low-angle subduction and serpentinization of the mantle
wedge are poorly understood processes. The Navajo
Volcanic Field (NVF) provides an unusual opportunity to
investigate these processes because of the unique population of mantle inclusions and the setting of their host
rocks. Among the inclusions are peridotites with hydrous
minerals rare in mantle xenoliths (McGetchin & Silver,
1970) and lawsonite eclogites interpreted as fragments of a
subducted slab (Helmstaedt & Doig, 1975). These rocks represent mantle sampled more than 700 km from the plate
boundary following the Laramide orogeny in the western
USA, which has been attributed to low-angle subduction
of the Farallon Plate (Dickinson & Snyder, 1978; Saleeby,
2003; Humphreys, 2009). In this peridotite inclusions with
antigorite and with a variety of unusual textures, and earlier studies have been reinterpreted. Our results provide
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JOURNAL OF PETROLOGY
VOLUME 51
insights into mantle serpentinization, slab^wedge interactions during low-angle subduction and mantle evolution.
The NVF is located in the interior of the Colorado
Plateau, which is a relatively stable block of continental
lithosphere (Fig. 1). Laramide-related deformation began
in the plateau interior between 80 and 75 Ma and ended
at about 36 Ma (Cather, 2004). Regions to the west, south,
and east of the plateau experienced extensive post-tectonic
magmatism that has been partly attributed to mantle hydration during the Laramide orogeny (Humphreys et al.,
2003). Igneous activity on the plateau, including that of
the NVF, is relatively small in volume. Most or all of
the NVF was emplaced within the period 30^21 Ma
(Helmstaedt & Doig, 1975; Laughlin et al., 1986;
McDowell et al., 1986; Nowell, 1993).
Inclusions of mantle and crustal rocks are abundant in
the intrusive rocks of the NVF. Minette, a potassic, mafic
igneous rock, makes up most of the field; however, the peridotite samples studied here are hosted in diatremes of serpentinized ultramafic microbreccia (SUM). Crystal
fragments of pyrope, olivine, and pyroxene in the microbreccia occur in a serpentine-rich matrix. The SUM was
emplaced as crystals and rock fragments entrained in a
water-rich fluid phase and was formed by physical disaggregation of mantle peridotite with subsequent incorporation of fragments from the vent walls during ascent
(McGetchin & Silver, 1970). Although the SUM diatreme
fill was once called kimberlite, there is no evidence that
melt was ever a part of the eruptive mix; additionally, incompatible element concentrations are extremely low relative to those of kimberlites (Roden, 1981). Many of the
inclusions in the SUM are unlike any in the other mantle
wedge suites summarized by Arai & Ishimaru (2008). For
example, peridotites containing chlorite, amphibole, titanian clinohumite, and antigorite are common. Smith
(1979) concluded that at least the first three of these hydrous minerals formed in the mantle before eruption, but
that evidence for a mantle origin of the antigorite was ambiguous. Because there is no evidence that the host SUM
was ever a magma, and because the genetic relationships
between the peridotite inclusions and the source of the
microbreccia are unclear, the rock fragments within the
SUM are here referred to as inclusions rather than
xenoliths.
Relationships to Farallon subduction are controversial
and unresolved. For example, although the lawsonite eclogite inclusions have been interpreted as fragments of the
Farallon slab (e.g. Helmstaedt & Doig, 1975; Usui et al.,
2007), they have also been interpreted as fragments of
Proterozoic continental lithosphere that was recrystallized
during Farallon subduction (Wendlandt et al., 1993, 1996;
Smith et al., 2004). Helmstaedt & Schulze (1991) suggested,
but did not prefer, another possibilityçthat the eclogites
might be fragments of continental lithosphere tectonically
NUMBER 6
JUNE 2010
Fig. 1. Pre-Neogene palinspastic map of the Colorado Plateau and
surrounding region. The arrow near the plate boundary shows the approximate direction of Farallon^North America relative plate motion
at about 60 Ma. The approximate boundaries of the flat slab segment
(bold dashed lines) and exposures of ‘lower plate schist’ (black ovals
NE of subduction zone) are adapted and simplified from Saleeby
(2003). The two stars near the intersection of the four states (Utah,
UT; Arizona, AZ; Colorado, CO; New Mexico, NM) mark locations
of laccolith complexes intruded in the period from about 67 to 74
Ma (Cunningham et al., 1994; Semken & McIntosh, 1997). The
dashed line outlines the Colorado plateau, the continuous line the
Navajo Volcanic Field. i, locations of diatremes of serpentinized
ultramafic microbreccia (SUM); the Green Knobs diatreme, nearly
on the NM^AZ boundary, is marked by a black arrow.
eroded from the SW and dragged down during Farallon
subduction. Garnetites, rocks rich in grossularite-rich
garnet, have been interpreted as the products of metasomatism accompanying serpentinization (Helmstaedt &
Schulze, 1988; Smith & Griffin, 2005), even though the
presence of serpentine in the plateau mantle had not been
established at the time. The possibility of antigorite in the
mantle below the plateau is important not only for understanding the movement of water in the mantle but also for
understanding the associated tectonics, because antigorite
may have a critical influence on subduction dynamics
(e.g. Lee et al., 2008; Hilairet & Reynard, 2009).
Hydrous meta-peridotite inclusions from the Green
Knobs SUM diatreme (Fig. 1) have been characterized by
petrographic, electron microprobe, and bulk-rock chemical analysis. Our goals included answering the following
questions. First, was antigorite present in the mantle
sampled by the NVF eruptions, and what information
does it provide about mantle evolution? Second, do some
inclusions represent material emplaced during Farallon
subduction, or were they all derived from the Proterozoic
1356
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
mantle lithosphere of the plateau? Third, what caused the
ultramafic diatremes to erupt? Fourth, how may the diverse inclusions be related to one another, and how may
their histories constrain hypotheses about low-angle subduction and evolution of the plateau lithosphere?
A N A LY T I C A L M E T H O D S
Table 1: Modes of studied rocks
Rock:*
15
51
Ol
33
62
86
65
85
68
66
70
En
—
3
—
—
—
23
23
—
Di
Electron microprobe analyses were made with a JEOL
JXA-8200 at 15 kV and using a 35 nA beam current.
Typical counting times for minor elements were 40 s on
peak and 20^40 s on background; times were shorter for
some major elements. Qualitative detection limits (Von
Seckendorff, 2000) for most analyses were in the range
0·005^0·02 wt % oxide. Synthetic compounds and minerals were used as standards, and data were processed
using the JEOL ZAF procedure. Secondary standards analyzed in each session were also used by Smith (1979), and
the consistency of new and earlier (1970s) data was
verified.
Three bulk-rock analyses were obtained from the
Washington State University GeoAnalytical Laboratory
in 2008 and 2009: major elements were determined by
X-ray fluorescence (XRF) and trace elements were determined by XRF and inductively coupled plasma mass spectrometry (ICP-MS). Analyses of the same powders of two
of these rocks and of two others discussed here were made
by G. K. Hoops using wet-chemical methods (colorimetric, atomic absorption, and titration) at the University of
Texas at Austin in about 1975. Oxides in the replicate analyses compare extremely well, except for alumina; some
1975 results for alumina are probably too low. The replicate
analyses are presented in Electronic Appendix Table 1
(available at http://www.petrology.oxfordjournals.org/) to
document comparability of the datasets. Where two sets
of data are available for a rock, those from the
GeoAnalytical Laboratory are used.
P E T RO G R A P H Y A N D
M I N E R A LO GY
Seven peridotites were characterized in detail for this
study. Three contain prominent antigorite and, together
with an antigorite-rich rock studied by Smith (1979), provide part of the basis for evaluating the presence of serpentine in the Colorado Plateau mantle. A dunite intrusion
was chosen because it contains large deformed olivine
grains and pockets of an enigmatic sheet silicate. The studied chlorite harzburgite is texturally unlike most other
chlorite-bearing inclusions, because the chlorite is part of
a well-defined metamorphic fabric. A mylonite was analyzed because of its distinctive texture. The most unusual
rock, now known to be a Cr-magnetite dunite, was selected
because Smith & Levy (1976) had analyzed a few olivine
244
147
178
196
1
2
1
tr
3
—
—
—
4
—
—
—
—
Atg
63
25
12
25
3
—
—
—
Chl
tr
5
tr
1
12y
5
7z
20y
Opq
0·1
0·2
tr
—
Mag
3
1
—
0·1
—
4
0·7
—
0·2
0·5
3·1
138
Amph
Chu
0·5
114
—
0·7
—
—
7
2
—
—
—
—
0·3
Prp
—
—
—
—
—
—
—
tr
Phl
—
—
—
—
—
tr
—
—
Modes based on 1000 point counts, except for 51 (300
point), 244 (estimate) and 196 (calculation). Ol, olivine;
En, enstatite; Di, diopside; Amph, amphibole; Atg, antigorite; Chl, chlorite; Opq, opaque minerals; Mag, magnesite;
Chu, titanian clinohumite; Prp, pyrope; Phl, phlogopite;
tr, trace.
*Sample number format Nxxx-GN.
y
Chlorite plus fine-grained inclusions.
z
Intergrowths formed by breakdown of chlorite.
grains and found compositions defining a range from
about Fa5 to Fa7; both the range and the compositions
were subsequently realized to be abnormal for mantle peridotite. Modes of these rocks are given in Table 1.
The contextçtypical peridotite inclusions
Peridotite inclusions from the Green Knobs SUM diatreme
include samples of anhydrous mantle material as well as
samples with abundant hydrous minerals of mantle origin
(Smith & Levy, 1976; Smith, 1979). Even in those samples
that represent anhydrous mantle, very fine-grained sheet
silicates are common along irregular fractures and grain
boundaries. This minor alteration is considered to have
formed either after or during diatreme emplacement, and
has been ignored in subsequent discussions. However, hydrous minerals considered to be of mantle origin are present in most peridotite inclusions. Amphibole and chlorite
(clinochlore) are the most common. Although amphibole
and chlorite make up at most a few per cent of most inclusions, each makes up almost 20% of some samples.
Titanian clinohumite and magnesite are present in minor
abundance in some chlorite-bearing peridotites and have
also been interpreted as mantle minerals by Smith (1979).
Antigorite is present with chlorite in some rocks.
Antigorite peridotite
The four rocks in this group contain abundant antigorite
and olivine together with diopside and minor phases
1357
JOURNAL OF PETROLOGY
VOLUME 51
(Table 1). Antigorite occurs in plates and blades with maximum dimensions up to about 3 mm, both in masses and
as sheets within larger olivine grains (Fig. 2). Some antigorite^olivine intergrowths resemble those described in detail by Boudier et al. (2010) in an ultramafic schist from
the SUM diatreme at Moses Rock in the northern part
of the NVF. In rock N15-GN most of the antigorite appears undeformed, and diopside occurs as prisms within
antigorite masses. In contrast, in rock N114-GN curved
antigorite grains with sweeping extinction occur together with deformed olivine crystals and with trails of diopside grains. No exsolution lamellae were observed
in the diopside. Spinel grains are opaque and occur in irregular clusters surrounded by chlorite. Two inclusions
(N15-GN and N114-GN) lack amphibole and orthopyroxene. Rock N51-GN, characterized by Smith (1979), contains several per cent enstatite, titanian clinohumite
and magnesite. Rock N244-GN contains amphibole and
magnesite and is traversed by prominent antigorite-rich
veins.
The four rocks have similar mineral compositions
(Table 2 and Electronic AppendixTable 2). Antigorite contains Al2O3 in the range 1^2·8 wt %, with much of that
range found in each rock; several higher alumina values
are attributed to the presence of intergrown chlorite. The
analytical totals of most antigorite analyses are in the
range 84^89 wt %, a range greater than expected for analytical error, and too low for stoichiometric antigorite.
Shervais et al. (2005) reported similar analytical problems
and suggested a possible cause might be absorbed water.
NUMBER 6
JUNE 2010
Fig. 2. Images of antigorite-rich peridotite N15-GN. Abbreviations as
in Table 1. (a) Polarized light image of olivine and antigorite. White
scale bar, 1mm. (b) Backscattered electron image of olivine (light
gray), antigorite (dark gray), and diopside (white) (voids are black).
White scale bar, 0·5 mm.
Table 2: Analyses of minerals in two antigorite peridotite inclusions
N15-GN
N244-GN
Ol
Atg
Di
SiO2
40·59
41·84
54·48
0·08
41·69
42·26
55·22
TiO2
0·00
0·00
0·05
0·16
0·00
0·00
0·00
Al2O3
0·01
1·31
1·68
13·06
0·01
1·68
Cr2O3
0·00
0·33
1·07
52·46
0·03
0·83
Fe2O3*
Spl
Ol
Atg
Di
Spl
Chl
Amph
0·09
33·76
58·29
0·11
0·00
0·00
0·45
1·72
12·80
0·67
0·35
47·45
1·90
0·18
1·90
2·86
20·33
FeO*
11·02
4·07
1·48
24·87
9·10
3·42
1·42
24·26
4·40
MnO
0·19
0·06
0·01
0·64
0·15
0·02
0·05
0·69
0·01
0·04
MgO
47·58
36·91
16·10
5·59
50·12
37·61
17·44
4·21
34·09
23·20
CaO
0·00
0·02
22·50
0·01
0·01
0·02
24·74
0·18
0·02
9·34
Na2O
0·00
0·01
1·07
0·04
0·00
0·01
0·53
0·00
0·00
3·37
NiO
0·39
0·20
0·03
0·01
0·40
0·17
0·05
0·14
0·22
0·09
K2O
0·00
0·01
0·00
0·00
0·01
0·01
0·00
0·01
0·01
1·16
Total
99·78
84·76
98·46
99·77
101·51
86·02
100·25
99·16
87·21
98·23
*Total iron as FeO except where ferric iron is calculated from stoichiometry.
Spl, spinel; other abbreviations are as in Table 1.
1358
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
unequilibrated olivine at contacts with antigorite might
have Mn contents different from those in olivine interiors,
as Mn is relatively partitioned into olivine (Table 2): such
differences were sought but not recognized.
Dunite N147-GN
Fig. 3. Partitioning of Ni, Fe, and Mg between intergrown antigorite
and olivine in four inclusions from Green Knobs, compared with
that in eight samples of the Val Malenco antigorite schist
(Trommsdorff & Evans, 1972). Each data point for the Val Malenco
schist represents a separate rock, whereas each Green Knobs data
point represents a pair of analyses of olivine and antigorite in close
proximity.
The low totals also may be due, at least in part, to a failure
of the ZAF procedure to adequately correct analyses of hydrous magnesian silicates (B. W. Evans, personal communication). Spinel, a trace mineral, is Cr-rich. Diopside in
rock N15-GN and has 7^10% Na(Al, Cr) endmembers,
whereas in the other three rocks it has lower values
(1^4%). Analyzed sulfide in N15-GN is heazlewoodite
(Fe0·12Ni2·88S1·99). Amphibole, present only in N244-GN, is
richterite, close to tremolite in composition.
The partitioning of Fe, Mg, and Ni between olivine and
antigorite in the four rocks is consistent with a close approach to equilibrium (Fig. 3). Most olivine^antigorite
pairs have values of the exchange KD [¼(Fe/Mg)atg/
(Fe/Mg)ol)] near 0·45, as do equilibrated antigorite and
olivine in Alpine metaserpentinites such as the Val
Malenco antigorite schist (Trommsdorff & Evans, 1972;
Evans, 2008). The partition of NiO between olivine and antigorite is also like that in the Val Malenco schist. If either
olivine or antigorite formed at the expense of the other,
Dunite N147-GN has a mineral assemblage like that in the
forsterite^antigorite^diopside rocks described above, but
‘chlorite’ is more abundant and antigorite less prominent
(Table 1). No exsolution lamellae were observed in the diopside. Titanian clinohumite occurs in small grains epitaxially intergrown with olivine. The ‘chlorite’ occurs in
pockets up to several millimeters in maximum dimension,
and parts of the pockets are turbid with very fine-grained
inclusions (Fig. 4a). The bulk of the olivine occurs in anhedral grains up to 20 mm in maximum dimension with
well-developed {010} cleavages, some of which are tightly
folded. Much smaller olivine grains occur in apparent fracture zones near and at rims of the larger olivine grains.
Mineral compositions (Table 3) are noteworthy partly
because of the bimodal distribution of olivine. The vast
bulk of the olivine is homogeneous (Fa8·5), but some
small grains are relatively Mg-rich and Ni-poor (Fa5·0)
(Fig. 5). The Mg-rich olivine occurs in independent grains
and overgrowths with maximum dimensions less than a
few tens of micrometers. Contacts are sharp between the
overgrowths and the relatively more Fe-rich olivine. Some
analyses of the late-stage Mg-rich olivine in this rock and
the other three rocks discussed below are unusually rich
in Cr2O3, the highest value being 0·18 wt %. It is possible
that the Cr is in the olivine structure, but it is also possible
that the high Cr is produced by excitation of Cr in spinel
at the margins of the olivine grains and in unobserved inclusions like those discussed by Sobolev et al. (2008).
Diopside has 1^2% of the Na(Al, Cr) endmembers.
Titanian clinohumite (5·25 wt % TiO2) is similar in composition to that common in the SUM diatremes
(McGetchin & Silver, 1970; Smith, 1979).
The material in dunite N147-GN referred to as chlorite is
poor in Al þ Cr compared with clinochlore, although clinochlore is typical of ultramafic rocks in general (Pawley,
2003) and of those from the SUM diatremes (Smith, 1979,
1995). The analyses have Al þ Cr in the gap between compositions for coexisting antigorite and chlorite in the
Green Knobs samples analyzed by Smith (1979). The compositions could represent a mix of intergrown clinochlore
and antigorite, but the five minimum-beam analyses are
similar and so the intergrowth proportions would have to
be uniform. Facer et al. (2009) also have reported compositions intermediate between those expected for clinochlore
and antigorite in dunite xenoliths from Montana.
Although the analyses of ‘chlorite’ in rock N147-GN are
not like those expected, the name will be used without
quotation marks in subsequent discussions.
1359
JOURNAL OF PETROLOGY
VOLUME 51
NUMBER 6
JUNE 2010
Fig. 4. Backscattered electron images of two inclusions with bimodal olivine. Ol2, more magnesian olivine; Chlþ, chlorite with inclusions of
fine-grained reaction products. (a) Dunite N147-GN. Image of olivine and of chlorite turbid with reaction products. Most olivine is uniform in
composition, about Fa8·5, and some grains have folded cleavage, as at the lower right. Thin overgrowths of more magnesian olivine (Fa5·0)
(darker gray) locally are in sharp contact with the dominant olivine; one such contact is indicated by the arrow. White scale bar, 100 mm.
(b) Chlorite harzburgite N178-GN. Chlorite laths terminate in ‘feather-like’ masses of very fine-grained intergrowths dominated by more magnesian olivine. White scale bar, 100 mm.
Table 3: Analyses of minerals in inclusions with bimodal olivine compositions
Dunite N147-GN
Ol
Peridotite mylonite N196-GN
Di
Chu
Chl
Ol1
Ol
En
Di
IG2
Spl
Ol1
SiO2
TiO2
40·87
0·00
54·99
0·00
36·42
5·25
36·69
0·00
41·17
0·03
40·84
0·00
56·59
0·00
54·09
0·00
0·10
0·07
39·28
0·00
41·46
0·02
Al2O3
Cr2O3
0·01
0·00
0·06
0·25
0·01
0·06
7·20
3·53
0·04
0·18
0·04
0·08
1·52
0·43
1·67
1·17
2·11
49·84
11·96
3·29
0·07
0·14
Fe2O3*
FeO*
8·27
1·50
8·27
3·11
4·93
9·59
5·14
1·51
20·00
17·19
5·14
5·37
0·12
35·74
0·05
53·12
MnO
MgO
50·82
17·92
47·88
36·20
53·60
0·18
50·04
16·68
9·55
41·17
CaO
Na2O
0·00
0·01
24·93
0·25
0·00
0·00
0·01
0·02
0·01
0·00
0·04
0·00
0·14
0·02
22·71
0·94
0·01
0·00
0·22
0·07
0·01
NiO
Total
0·36
100·34
0·04
99·94
0·24
98·15
0·22
86·98
0·19
100·15
0·36
101·16
0·08
99·78
0·03
98·80
0·19
99·06
0·22
101·27
0·30
100·61
Chlorite harzburgite N178-GN
Ol
En
Di
Cr-magnetite dunite N138-GN
Spl
Chl
Ol1
Ol
Spl
Chu
Chl
Spl1
Ol1
Prp1
SiO2
TiO2
41·12
0·00
57·97
0·01
54·65
0·00
0·01
0·31
30·80
0·00
41·79
0·01
41·24
0·01
0·02
0·32
37·07
2·96
31·53
0·02
0·06
0·51
42·31
0·03
44·31
0·01
Al2O3
Cr2O3
0·00
0·01
0·32
0·08
0·61
0·47
4·20
48·44
16·90
1·94
0·02
0·39
0·00
0·00
0·99
23·88
0·01
0·11
16·26
0·79
9·31
28·12
0·03
0·16
25·31
0·10
Fe2O3*
FeO*
7·85
5·14
1·55
16·65
23·52
2·47
4·85
8·59
46·31
23·04
5·39
3·96
32·96
22·24
3·29
2·99
MnO
MgO
0·12
51·03
0·16
36·14
0·05
17·62
0·59
5·41
0·01
34·13
53·08
0·12
49·54
0·52
5·35
0·13
52·27
0·00
33·73
0·31
7·19
0·05
55·40
0·03
28·74
CaO
0·00
0·10
24·10
0·00
0·02
0·00
0·03
0·00
0·03
0·08
0·02
0·03
0·04
0·00
0·17
0·03
0·23
0·00
0·32
0·33
0·36
0·13
0·14
0·36
0·11
0·02
99·27
86·52
100·46
99·88
100·80
98·09
86·51
101·06
101·42
101·55
Na2O
NiO
0·40
0·06
0·47
0·02
Total
100·52
99·96
99·53
*Total iron as FeO except where ferric iron calculated from stoichiometry.
1
Reaction product of the late-stage temperature pulse. Abbreviations are as in Tables 1 and 2.
2
The average of broadbeam analyses of intergrowths formed by chlorite breakdown.
1360
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
Chlorite harzburgite N178-GN
Chlorite harzburgite N178-GN is texturally distinct from
typical chlorite-bearing peridotite inclusions in SUM. The
rock has a well-developed foliation defined by tabular
chlorite grains together with the long dimensions of olivine
and orthopyroxene grains and well-developed {010} cleavages in olivine. Olivine grains reach maximum diameters
of about 10 mm. Enstatite grains commonly have kink
bands; their interiors contain fine lamellae of spinel and
clinopyroxene, but some grain margins lack lamellae.
Diopside grains lack exsolution lamellae; some have
well-developed mechanical twinning. About 5% chlorite
is present (Table 1), and it is distributed throughout the
rock, not only in clusters like those common in other inclusions. Some chlorite grains are strained, with undulatory
extinction, together with neighboring orthopyroxene and
olivine grains. Many chlorite laths have feather-like terminations into olivine and orthopyroxene. Within these
terminations are very fine-grained intergrowths that are
mostly olivine together with unidentified minerals, probably including chlorite; although these intergrowths are
most abundant in the feather-like terminations (Fig. 4b),
they also occur elsewhere within chlorite.
The silicate compositions are magnesian (Table 3).
Enstatite (En92) is low in alumina (0·3 wt % Al2O3).
Diopside has about 4% Na(Al, Cr) component. Chlorite
is Cr-bearing clinochlore. Spinel is Cr-rich and contains significant ferric iron, with a calculated Feþ3/
(Cr þAl þ Fe3þ) of about 0·2. Olivine is Fa8, except in
the fine-grained intergrowths, where it is more magnesian
(Fa5) and less Ni-rich (Fig. 5). The composition of the
fine-grained intergrowths was obtained from the average
of three broad-beam analyses, and except for the hydrous
component, oxide proportions are close to those of antigorite. The intergrowth composition is too silica-rich for a
forsterite^chlorite mixture, but consistent with an intergrowth of forsterite plus chlorite plus enstatite.
Peridotite mylonite N196-GN
Fig. 5. Bimodal olivine compositions in four peridotite inclusions
from Green Knobs. (a) Dunite N147-GN. The cross shows the average
composition 1SD of the 35 measurements of secondary standard
P-140 olivine made during the study. (b) Chlorite harzburgite
N178-GN. The two more magnesian analyses are of olivine in the
fine-grained ‘feather-like’ intergrowths at the ends of chlorite laths.
(c) Peridotite mylonite N196-GN. The more magnesian analysis is of
a grain within a fine-grained intergrowth produced by breakdown of
chlorite. (d) Cr-magnetite dunite N138-GN. The two more magnesian
analyses are of thin selvages in contact with volumes of chlorite plus
reaction products.
Rock N196-GN is unusual in both grain size and texture.
The average grain size is less than several tens of micrometers, and so the mode of N196-GN was calculated from
the bulk composition using the procedure of Le Maitre
(1981). The calculated olivine, enstatite, and diopside proportions are 66:23:3, respectively (Table 1). The rock has a
pronounced tectonite fabric defined partly by lenses less
than several hundred micrometers thick, distinguished by
contrasting mineral proportions and grain sizes.
Distinctive clots and lenses consisting of even finer-grained
intergrowths are present (Fig. 6a) in which forsterite and
spinel were identified. Most olivine forms grains with
equant cross-sections less than several tens of micrometers
in maximum diameter, which are interpreted as neoblasts.
Porphyroclasts of olivine are uncommon, and most have
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JOURNAL OF PETROLOGY
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NUMBER 6
JUNE 2010
Fig. 6. Backscattered electron images of peridotite mylonite N196-GN. Voids created during sample preparation appear black. Diopside and
spinel are white, enstatite is dark gray, and olivine is composition-dependent shades of lighter gray. (a) A fine-grained intergrowth mostly of
olivine plus spinel, attributed to chlorite breakdown, is marked (1). White scale bar, 100 mm. (b) The white lines intersect on a relatively iron-rich
olivine porphyroclast embayed by olivine neoblasts (darker gray). A pyroxene-rich lens occupies the left side of the image. White scale bar,
100 mm.
maximum diameters less than 100 mm: some are elongate
parallel to the fabric, and others have irregular, embayed
shapes (Fig. 6b).
Mineral compositions (Table 3) are correlated with texture. Olivine porphyroclasts are relatively iron-richçinterior compositions of four grains fall in the narrow range
9·6^9·7% Fa, although some are zoned to more magnesian
rims (Fig. 7). In contrast, neoblasts have 8·7^6·9% Fa. The
porphyroclasts also contain 50·08 wt % CaO, whereas
typical neoblasts contain 0·08^0·13 wt %. The average of
five broad-beam analyses of the very fine-grained intergrowths has proportions of oxides like those of chlorite
(Table 3), except for about 0·2 wt % CaO, and for the anhydrous nature of the mix. Olivine in such a clot is both
relatively magnesian (5·4% Fa) and Ni-poor (Fig. 5). No
accurate analysis of spinel in the clots was obtained because of the small grain size, but a semi-quantitative analysis established that it is much more aluminous than
spinel not in such intergrowths (435 wt % Al2O3 compared with about 2 wt %).
Fig. 7. Compositions of olivine grains in peridotite mylonite
N196-GN. All grains with small cross-sections were classified as neoblasts. The arrow joins the compositions of the interior and more
magnesian rim of a porphyroclast.
Cr-magnetite dunite N138-GN
Textural associations
Sample N138-GN stands out even in this unusual group of
rocks. Forsterite makes up about 70% of the rock, and volumes of very fine-grained chlorite plus inclusions make up
about 20% (Table 1). An opaque oxide constitutes 7%, far
more than the trace quantities of oxide in the rocks
described above. Magnesite (2%), titanian clinohumite
(0·3%) and trace quantities of pentlandite, ilmenite^geikielite, and pyrope are also present. No pyroxene was identified with certainty, although one minimum-beam
electron microprobe analysis of a fine-grained intergrowth
returned a composition close to that of enstatite. No
Ca-rich mineral was recognized, other than one small
grain of dolomite.
Four mineralogical^textural associations were distinguished in the rock. The associations are intermixed and
locally juxtaposed along small-scale faults that traverse
the thin sections (Fig. 8). The first and most abundant association is dominated by forsterite grains that have maximum diameters up to 7 mm. These olivine grains are
fractured and partly recrystallized to neoblasts, and typically they contain abundant inclusions less than several micrometers in diameter, which are isotropic and have low
optical relief; these inclusions are considered to be of fluid.
The second association (Fig. 8c and d) is particularly
complex. It is dominated by opaque spinel and by extremely fine-grained ‘paste-like’ chlorite packed with
1362
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
Fig. 8. Images of Cr-magnetite dunite N138-GN. (a) Transmitted light image, with labeled examples of the textural associations. Association 1
consists mostly of large zoned olivine grains. Association 2 consists of Cr-magnetite, chlorite, and products of chlorite breakdown. Association
3 contains olivine, spinel, and chlorite in a tectonite fabric. White scale bar, 1000 mm. (b) BSE image overlapping Associations 1 and 3.
Forsterite and spinel (white) dominate the image. Forsterite grain size ranges from about 1mm in maximum diameter on the right
(Association 1) to a few micrometers in the sheared spinel^forsterite volumes in the center^left (Association 3). White scale bar, 300 mm.
(c) BSE image overlapping Associations 1 and 2. Chlorite (Chl) and chlorite reaction products (Chlþ) dominate the left part of the image,
and a single fractured forsterite grain (Ol) occupies much of the right portion. Titanian clinohumite (Chu) is rimmed by intergrown forsterite
(gray) and ilmenite (white). Spinel (Spl) grains are clustered below the clinohumite. White scale bar, 100 mm. (d) BSE image of a cluster of
pyrope grains (Prp) surrounded by chlorite plus chlorite reaction products (Chlþ) within Association 2. Compositional zonation in the
pyrope is indicated by variations in shades of gray, the darker shade being more magnesian. The dark gray inclusions several micrometers in
diameter within pyrope are forsterite. White scale bar, 50 mm.
inclusions. Most of the spinel appears homogeneous in
backscattered electron (BSE) images; however, a few
spinel grains have marginal volumes that embay and
appear to have replaced the dominant composition and
are in sharp contact with it. Compositionally distinct olivine selvages 5^20 mm thick occur as overgrowths at the
borders of some large olivine grains with the chlorite-rich
patches, and some of the contacts between large grains
and selvages are sharp in BSE images. The chlorite is too
fine-grained to identify optically, but it is chlorite in composition and has consistent low relief and birefringence.
Most of the grains included in chlorite also are exceedingly
fine-grained, but some pyrope grains within the chlorite
‘paste’ are recognizable optically. They are anhedral, in
clusters up to several hundred micrometers in diameter,
and have compositional zonation visible in BSE images
(Fig. 8d); inclusions of forsterite up to several micrometers
in diameter are irregularly distributed in the garnet.
Titanian clinohumite occurs as equant grains with rims of
intergrown olivine plus Mg-rich ilmenite; the intergrowths
resemble those of olivine plus vermicular ilmenite
described as breakdown products of titanian clinohumite
by Lopez Sanchez-Vizcaino et al. (2009). Pentlandite is
also present.
The third association is dominated by fine-grained
spinel, olivine, and chlorite in a tectonite fabric (Fig. 8b).
The fourth association is much less abundant than the
other three and consists of clusters of magnesite plus
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JOURNAL OF PETROLOGY
VOLUME 51
chlorite in undeformed grains up to several hundred micrometers in maximum dimension; the single dolomite
grain observed in the rock was in such a cluster.
Mineral compositions
Mineral compositions (Table 3) correlate with texture.
Olivine spans the range Fa9·1^3·2 (Fig. 5d). The interiors of
some large grains, all in the first association, have compositions near Fa9. Typically these grains are zoned to more
magnesian rims, with gradients extending over distances
of several hundred micrometers (Fig. 9); the most magnesian composition found as part of the gradual zonations is
Fa4·6. However, some of these grains have selvages as
Mg-rich as Fa3·2 at contacts with chlorite-rich volumes.
Olivine in the selvages is relatively depleted not only in Ni
(Fig. 5d) but also in Mn; selvage olivine has about
0·06 wt % MnO, whereas all other olivine has MnO contents in the range 0·11^0·15 wt %.
Almost all of the spinel is magnetic Cr-magnetite
(ferrit^chromite), with compositions near Fe3þ:Cr:Al of
63:35:2 (Fig. 10, Table 3). The irregular marginal volumes
that appear to have replaced the dominant spinel are
more aluminous, near Fe3þ:Cr:Al 43:33:24. Neither composition resembles that of spinel in other peridotite
inclusions from Green Knobs or from elsewhere in southwestern North America. The Cr-magnetite, however, is
similar to that stable in crustal antigorite-serpentinite
(Fig. 10b) (Evans & Frost, 1975).
Chlorite is clinochlore; the undeformed chlorite laths of
the fourth association are slightly more iron-rich but with
compositions otherwise like that of the very fine-grained
‘paste’ of the second association. Some minimum-beam
Fig 9. Olivine compositional zonation in Cr-magnetite dunite
N138-GN. Contours for 100 Fe/(Fe þ Mg) and analyzed points (circles) are superimposed on a BSE image of part of an olivine grain.
More magnesian olivine is darker gray in this image. Chlorite (Chl),
an ink dot (Ink), and plucked voids in the thin section are black.
White scale bar, 200 mm.
NUMBER 6
JUNE 2010
analyses of clots of inclusions within the second-association
chlorite are like that of dehydrated chlorite, and one analysis is similar to that of aluminous enstatite. The pyrope
in part is extraordinarily pure in composition. The most
magnesian composition found is Py94·4Alm5·5Gr0·1.
Perhaps the only more pure natural pyrope known is in
the Alpine Dora Maira massif (Chopin, 1984). The compositional zonation (Fig. 8d) at least in part is of the
almandine component; a relatively Fe-rich volume in the
same grain has the formula Py92·8Alm6·8Gr0·2. Garnet
Fig 10. Compositions of spinel. (a) Spinel from Green Knobs peridotite inclusions, together with compositions of spinel in peridotite xenoliths in southwestern North America compiled by Smith (2000).
Twelve analyses of spinel are plotted for Cr-magnetite dunite
N138-GN. Only one representative analysis is plotted for each of the
other rocks. (b) Spinel from Green Knobs peridotite samples with hydrous minerals from (a), together with fields for spinel in prograde
metaserpentinite adapted from Evans & Frost (1975). Compositions
in antigorite^diopside^forsterite rocks extend from the Fe3þ corner to
the arrow, whereas compositions in antigorite^tremolite^forsterite
rocks span the entire range for antigorite-bearing metaserpentinites.
1364
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
stoichiometry requires ferric iron for two analyses, and the
extreme composition is Py69·0Alm8·6Gr11·4And8·0Uvr2·3; it
is possible that these two analyses represent multiphase
aggregates.
Clinohumite has xTi 0·2 (13 cation basis), low in Ti
compared with that common in peridotite in SUM
(Smith, 1979). Fluorine was not observed in wavelength
scans, and in comparison with scans of an apatite of
known fluorine content it is qualitatively constrained to
be close to zero. The olivine intergrown with ilmenite in
rims about clinohumite is less Fe-rich (Fa5·1) and lower in
nickel (0·15 wt % NiO) than most in the rock (Fig. 5d).
No completely satisfactory analysis of the intergrown
ilmenite was obtained because of beam overlap on olivine,
but it is clearly magnesian with a high geikielite component, about Ilm35Geik65. Analyzed sulfide has compositions near Fe9·1Ni8·8S16·1, appropriate for pentlandite.
B U L K- RO C K C O M P O S I T I O N S
Bulk-rock analyses (Table 4) supplement those of Smith
(1979) and Aoki (1981) for Green Knobs samples
(Electronic Appendix Table 3). Four of the rocks (N51-GN,
N147-GN, N178-GN, and N196-GN) have normal compositions for depleted peridotite (Fig. 11). For example, CaO/
Al2O3 values plot near the chondritic ratio, as typical for
off-craton continental mantle xenoliths (Pearson et al.,
2004). CaO, Al2O3, TiO2, and Fetotal/(Fetotal þ Mg) for
Table 4: Bulk-rock chemical analyses of peridotite inclusions
N51-
N138-
N147-
N178-
N196-
N51-
N138-
N147-
N178-
GN
GN
GN
GN
GN
GN
GN
GN
GN
GN
(1)
(2)
(1)
(2)
(2)
(3)
(4)
(3)
(4)
(4)
40·88
43·79
43·78
40·24
0·24
0·20
TiO2
0·04
0·143
0·01
0·008
0·008
Ce
0·40
0·54
0·33
Al2O3
0·96
3·89
0·59
0·95
0·96
Pr
0·044
0·067
0·038
Cr2O3
0·33
1·94
0·35
0·35
0·39
Nd
0·170
0·252
0·122
Fe2O3
2·38
2·39
Sm
0·046
0·041
0·015
0·043
0·027
FeO
6·02
4·61
Eu
0·017
0·0097
0·008
0·0059
0·0096
FeO*
8·16
9·08
0·036
0·033
0·023
MnO
0·13
0·122
0·0062
0·0034
0·0026
MgO
44·24
41·72
45·77
44·81
44·80
Dy
0·038
0·024
0·018
CaO
0·57
0·09
0·33
0·63
0·44
Ho
0·0082
0·0056
0·0052
Na2O
0·00
0·04
0·03
0·04
0·04
Er
0·022
0·019
0·019
K2O
0·00
0·01
0·00
NiO
0·09
0·011
0·31
LOI (%)
0·304
0·29
6·69
6·18
Gd
0·113
0·095
Tb
0·14
0·22
0·009
0·01
0·00
Tm
0·008
0·008
Yb
0·041
0·317
0·296
Lu
0·011
2·88*
Ba
0·05
Th
7·43*
SO3 La
0·07
0·21
0·26
N196-
SiO2
P2O5
34·26
ppm
0·0035
0·022
0·026
0·0041
0·0059
64
0·05
0·0034
0·0041
0·025
0·031
0·0046
20
0·07
0·0057
130
0·05
H2Oþ
4·10
4·14
Nb
H2O
0·16
0·30
Y
0·21
0·16
0·13
CO2
0·48
0·16
Hf
0·04
0·03
0·03
Total
99·96
99·18
99·94
97·79
99·93
Ta
ppm
V
0·06
26·3
U
0·02
0·02
0·02
Pb
0·85
0·27
0·52
Rb
0·31
0·31
0·18
0·05
0·07
102·8
29·3
Ga
6·2
3·2
Cs
Cu
8·4
11·1
Sr
Zn
83·9
46·1
44·8
0·00
19
14
0·04
11
Sc
5·2
6·9
7·2
Zr
1·7
1·4
0·9
(1) Wet chemical analyses by G. K. Hoops (Smith, 1979); (2) XRF by Washington State GeoAnalytical Lab; (3) RNAA by
Roden et al. (1990); (4) ICP-MS by Washington State GeoAnalytical Lab.
*LOI (loss on ignition) from wet chemical analysis in Electronic Appendix Table 3.
1365
JOURNAL OF PETROLOGY
VOLUME 51
Fig 11. Comparisons of bulk compositions of peridotite inclusions
from the Green Knobs diatreme. Analyses are from Table 4, Smith
(1979), and Aoki (1981). Plotted weight per cent oxides are for analyses
normalized to 100 without water, carbon dioxide, and sulfate. The
line in (c) represents chondritic CaO/Al2O3 (McDonough & Sun,
1995).
three of these samples are consistent with extreme magmatic depletion relative to other peridotite inclusions from
Green Knobs. Abundances of the heavy rare earth elements (HREE) in these rocks (Fig. 12) are low, again consistent with magmatic depletion. The REE element profiles
are inflected to higher chondrite-normalized values for
the light REE (LREE) elements. Abundances of some of
the most incompatible elements, particularly those of Cs,
Pb, and Ba, are relatively high compared with those in
the depleted mantle source of mid-ocean ridge basalts;
most other incompatible elements, with the exception of
Sr, are significantly lower (Fig. 12b).
In contrast, the chlorite Cr-magnetite dunite (N138-GN)
has a remarkable bulk composition compared not only
NUMBER 6
JUNE 2010
Fig. 12. Trace element abundances in peridotite inclusions from
Green Knobs. (a) Rare earth element abundances in five rocks
(Table 4) normalized to the chondrite values of McDonough & Sun
(1995), together with the range of four xenoliths that have little or no
antigorite (Roden et al., 1990). (b) Incompatible elements in three
rocks normalized to abundances in the depleted mantle source of
mid-ocean ridge basalts of Salters & Stracke (2004).
with those for other xenoliths from southwestern North
America (Smith, 2000), but also with those from worldwide
occurrences compiled by Pearson et al. (2004). Al2O3 is extremely high for the markedly low CaO (Fig. 11). SiO2 is
lower than that of most other peridotite xenoliths, and
Cr2O3 is very much higher. Although TiO2 and Fetotal/
(Fetotal þ Mg) are like those of fertile mantle, abundances
of incompatible trace elements are similar to those of the
other depleted samples (Fig. 12). Roden et al. (1990) noted
that Yb and CaO are well correlated in xenoliths from
Green Knobs, and the abundances in N138-GN fit well
with that trend (Fig. 13). Although most peridotite xenoliths with Al2O3 above 3 wt % have more than 10 ppm Sc
(Pearson et al., 2004), N138-GN has 5 ppm. Hence, Al is
1366
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
Fig. 13. Correlation of CaO with Yb in Green Knobs inclusions.
, data of Roden et al. (1990).
abnormally high relative not only to Ca, but also relative
to HREE and Sc.
I N T E R P R E TAT I O N
Interpretations of the inclusions in the NVF are hindered
because of the dynamic mantle environment during and
closely following Farallon subduction. Continental lithosphere, subducted slab, and asthenosphere all are conceivable inclusion sources. Temperatures cannot be used to
infer depths of equilibration by reference to a geotherm,
because geotherms below the Colorado Plateau were
inverted and variable during low-angle subduction
(Helmstaedt & Schulze, 1991; Spencer, 1996; Smith &
Griffin, 2005). Interpretations are also hindered because
some of the low temperatures recorded by the inclusions
are blocking temperatures and so may not represent
conditions in the mantle at the time of eruption. To establish a basis for interpretation of the rocks studied here,
first evidence for the genesis of some other types of
inclusions in the SUM diatremes is summarized and interpreted to provide insights into the following questions.
What temperatures and depths are recorded by inclusions
with hydrous minerals of certain mantle origin? Do all
inclusions represent Proterozoic lithosphere, or are some
fragments of the Farallon slab? Do any represent parts of
the mantle wedge displaced during low-angle Farallon
subduction?
Context provided by interpretations of
other inclusions
Peridotite and pyroxenite
Hydrous mineral assemblages in peridotite and pyroxenite
inclusions from the SUM diatremes establish temperature^depth ranges for mantle hydration. Relevant
assemblages include chlorite in garnet and spinel peridotite, diopside^talc and diopside^enstatite^chlorite^titanian
chondrodite in metasomatic zones, and retrograde chlorite^garnet^omphacite in garnet pyroxenite (Mercier, 1976;
Helmstaedt & Schulze, 1979; Smith, 1979, 1995). In a peridotite with partly serpentinized olivine, antigorite has
microstructures typical of high-pressure formation
(Boudier et al., 2010). Temperatures and depths recorded by
the hydrous assemblages are similar for all the diatremes.
Most temperatures are in the range from near 5008C to
about 7508C, and depths range to at least 75 km (e.g.
Smith & Levy, 1976; Smith, 1979, 1995; Helmstaedt &
Schulze, 1988).
Isotopic, trace element, and textural studies provide
additional insights. Roden et al. (1990) reported radiogenic
Nd isotope ratios in clinopyroxene from one Green Knobs
peridotite (eNd ¼ þ42) and in amphibole from another
(eNd ¼þ30); they concluded that the samples represent
Proterozoic continental mantle lithosphere of the
Colorado Plateau, consistent with the texture-based interpretations of Smith (1979) and Helmstaedt & Schulze
(1991). Retrograde hydration of a garnet peridotite
occurred less than 25 Myr before eruption, a time constraint based upon preservation of compositional zonation
in olivine (Smith, 1979). Amphibole is low in LREE and
Ti compared with metasomatic amphibole in most mantle
xenoliths, and so the responsible fluid appears to have
been aqueous and low in incompatible elements (Roden
et al., 1990; Arai & Ishimaru, 2008).
Typical discrete pyrope and olivine grains in the SUM
record protolith histories unlike those of the associated
peridotites. Most olivine grains have compositions near
Fa8·0 and are more magnesian than those in most of the
peridotite inclusions (Smith & Levy, 1976). Growth of
pyrope in a hydrous peridotite is established by discrete
grains of pyrope that have inclusions not only of forsterite
but also of chlorite, amphibole, and unusual hydrous minerals such as carmichaelite (Hunter & Smith, 1981; Wang
et al., 2000). Pairs of pyrope plus included forsterite record
slow cooling to temperatures near or below 5008C
(Hunter & Smith, 1981; Wang et al., 1999). Griffin et al.
(2004) analyzed 294 discrete garnets for major and trace
elements, and their calculated temperatures and minimum
pressures cluster in the ranges 600^8508C and 2^4 GPa.
They also calculated olivine compositions that would be
in equilibrium with the garnets: most values are in the
range Fa7^8, a result consistent with the most common
composition of discrete olivine grains. Because their histories are unlike those of the peridotite inclusions, the
characteristic discrete grains may not represent the
Proterozoic lithosphere of the Colorado Plateau.
Eclogite and garnetite
Lawsonite eclogite fragments are present in SUM diatremes in the northern part of the NVF, but not at Green
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JOURNAL OF PETROLOGY
VOLUME 51
Knobs. Usui et al. (2003) found inclusions of coesite within
garnet, a discovery consistent with the results of garnet^
omphacite^phengite thermobarometry: the eclogites
record conditions within the range 400^7608C and
2·5^5 GPa (Smith et al., 2004; Usui et al., 2006). These
eclogites are similar to those in metamorphosed ophiolite
belts and have been interpreted as fragments of metamorphosed Mesozoic oceanic crust subducted beneath the
Colorado Plateau (Helmstaedt & Doig, 1975; Helmstaedt
& Schulze, 1988; Usui et al., 2007). Zonation within pyroxene and garnet has been interpreted as prograde, recording increasing pressure and temperature during
subduction (e.g. Helmstaedt & Schulze, 1988; Usui et al.,
2007); however, this interpretation has been challenged
(Smith & Zientek, 1979; Harley & Green, 1981). Moreover,
inclusions of blueschist-facies minerals such as glaucophane, albite, and titanite have not been observed in the
garnets of the eclogites (Helmstaedt & Schulze, 1988; Usui
et al., 2007). The P^T path leading to eclogite-facies metamorphism thus remains uncertain.
Geochronology also provides some important constraints. Sm^Nd and U^Pb isochrons for mineral separates
have yielded ages within about 10 Myr of the 30 Ma eruption age of the host Moses Rock and Garnet Ridge diatremes (Wendlandt et al., 1996; Smith et al., 2004; Usui
et al., 2006). Ion-microprobe U^Pb isotopic measurements
of zircons in three eclogites by Usui et al. (2003) yielded
nearly concordant ages spanning a range from 81 to 33
Ma. Similar, nearly concordant, ages in the range 34^70
Ma have been determined by thermal ionization mass
spectrometry U^Pb analyses for zircon separates from
three other eclogites (Smith et al., 2004). However, Smith
et al. (2004) also found a discordant mid-Proterozoic
zircon component in each of the three studied xenoliths.
The discordant separates demand a mid-Proterozoic age
for protolith formation, and they are consistent with the
conclusion of Wendlandt et al. (1993) based on Nd model
ages in the range 1·5^1·8 Ga. Both a Proterozoic genesis
for the protoliths and eclogite-facies recrystallization in
the period from 80 to 30 Ma are well established.
Garnetite inclusions occur in the same SUM diatremes
as the eclogites. They consist of over 95% grossularite-rich
garnet, together with minor rutile, ilmenite, chlorite,
clinopyroxene, and zircon. These garnetites are rodingite
analogues, Ca-rich metasomatic rocks formed as byproducts of serpentinization (Helmstaedt & Schulze, 1988).
U^Pb ages and Hf isotope compositions were determined
by in situ laser ablation (LA)-ICP-MS analysis of zircons
in a garnetite (Smith & Griffin, 2005). Most U^Pb analyses plot on or near concordia from 85 to 60 Ma, but a
few are discordant. The least radiogenic Hf analyses are
of zircon cores, which yield depleted-mantle model ages of
about 1·9 Ga. All values of 176Hf/177Hf in the zircons are
less radiogenic than those expected for the Mesozoic and
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Cenozoic basalts of the Farallon plate. The garnetites
record temperatures in the range 400^6008C (Smith &
Griffin, 2005), and so they are similar to the eclogites, not
only in the timing and temperature conditions of recrystallization but also in their Proterozoic inheritance.
Displacements within the mantle wedge
Tectonic history further constrains the genesis of the eclogites and garnetites. The earliest recognized Laramide deformation of the Colorado Plateau began between about
80 and 75 Ma (Cather, 2004). Spencer (1996) compiled the
ages of the igneous activity and inferred that the Farallon
slab did not contact continental lithosphere below the
NVF until about 70 Ma. High mantle temperatures below
the NVF before 70 Ma are consistent with the 74^71 Ma
ages of laccolith complexes in the adjacent Carrizo and
Ute Mountains (Fig. 1) (Cunningham et al., 1994; Semken
& McIntosh, 1997). The high mantle temperatures inferred
before about 70 Ma are not consistent with the
low-temperature hydrous conditions of recrystallization
and zircon growth documented by both eclogite and garnetite. Because the oldest concordant zircon ages in these
rocks fall in the range 81^85 Ma, they are best explained
as fragments of cooler mantle transported from near the
plate boundary to the SE and emplaced below the plateau.
Helmstaedt & Schulze (1991) noted that if the eclogites
had Proterozoic histories, they might be fragments tectonically eroded from the western margin of the upper plate
and dragged to the NE during Farallon plate subduction.
This hypothesis of Helmstaedt & Schulze (1991) best accounts for all the available data for the eclogites and garnetites and so must be considered in interpretations of the
discrete pyrope and olivine grains and of the metaperidotite samples discussed below.
Interpretations of analyzed peridotites
Mantle antigorite
The former presence of antigorite in the mantle below
Green Knobs is strongly supported by inclusions containing the assemblage antigorite^olivine^diopside. This support is provided by two complementary lines of
evidenceçthe first based on the mineral assemblage and
the second based on comparisons with other inclusions in
the SUM diatremes.
First, the mineral assemblage is consistent with equilibrium. The partitioning of Fe, Mg, and Ni between olivine
and antigorite is similar to that in the Alpine peridotites
described by Trommsdorff & Evans (1972) (Fig. 3). Olivine
in N15-GN is Fa11·6, more Fe-rich than that in any of the
other Green Knobs peridotites characterized by Smith &
Levy (1976), and the sample also has 63 modal per cent antigorite; the Fe-rich composition may be attributed to formation of antigorite and partitioning of Fe into residual
olivine. The diopside grains are clear and lack exsolution
lamellae; calculations indicate all or almost all Al is in
1368
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
six-fold coordination. In contrast, relict clinopyroxene
grains in the less hydrated inclusions described by Smith
(1979) typically have visible lamellae or turbidity, and calculations require significant four-fold Al. Hence, the diopside grains are more probably part of the low-T
assemblage than relict. The sulfide heazlewoodite in rock
N15-GN is characteristic of serpentinized peridotite
(Ramdohr, 1980; Frost & Beard, 2007). Other minor minerals in these samples include chlorite and titanian clinohumite, both present in comparable Alpine rocks (Evans,
1977). Finally, antigorite in some rocks shares sheared deformation textures with intergrown olivine and diopside,
and the deformation must have preceded incorporation in
the gas^solid transporting medium. These characteristics
establish that antigorite and coexisting minerals formed
and were stable in the source of the inclusions; thus, they
were not formed during or after eruption of the host SUM.
The conditions for equilibration of the antigoritebearing assemblages are consistent with those of associated
inclusions of certain mantle origin. Equilibria in Fig. 14
constrain the conditions, assuming water was in excess,
and ignoring minor components. The assemblage
forsterite^antigorite^diopside is stable in the approximate
range 470^6808C at pressures greater than 1·25 GPa, the
pressure at the 45 km depth of the Mohorovicic
Discontinuity below the NVF (Wilson et al., 2005). This
temperature range is closely similar to that recorded by hydrous assemblages of certain mantle origin in the peridotites and pyroxenites as well as to that recorded by the
eclogites. The comparison provides strong support for the
hypothesis that these antigorite^diopside^olivine assemblages also formed in the mantle. The depth otherwise is
only loosely constrained. Amphibole occurs with antigorite^forsterite^diopside in some samples, such as N244-GN
(Table 1), and is restricted to pressures less than 2·5^
3 GPa. Chlorite is present without amphibole in other
rocks, indicating pressures less than 4^5 GPa, because the
10 A8 phase replaces chlorite at higher pressures.
No evidence was observed for prograde metamorphism
in these rocks. For example, the spinel is chromite, not
the magnetite or Cr-magnetite formed during lowtemperature serpentinization and deduced by Evans &
Frost (1975) to be stable in prograde antigorite^diopside
metaserpentinites (Fig. 10). The antigorite peridotites have
Fig. 14. Reactions applicable to the interpretation of the Green Knobs inclusions. Bold dashed curves are for reactions with compositions similar to those of natural peridotites. Other curves are for simple systems. Atg, antigorite: Amph, amphibole; Br, brucite; Chl, chlorite; Chu,
low-Ti OH clinohumite; Di, diopside; Fo, forsterite; Ilm, ilmenite; Prp, pyrope; Spl, spinel; w, water. Sources: 1, Wunder & Schreyer (1997); 2,
Bromiley & Pawley (2003); 3, Thermocalc 2.5: Powell & Holland (1988), Holland & Powell (1990); 4, Ulmer & Trommsdorff (1999); 5,
Fockenberg (1995); 6, Gasparik (2000); 7a^c, Fumagalli & Poli (2005).
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JOURNAL OF PETROLOGY
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histories consistent with those of other peridotite inclusions
interpreted as fragments of the Proterozoic mantle lithosphere of the plateau.
Peridotite samples with bimodal olivine, and effects
of a late-stage thermal pulse
Four of the analyzed rocks contain trace amounts of olivine
in small grains and selvages that are distinctly more magnesian than the dominant forsterite (Fig. 5). The textures
and the relatively Mg-rich, Mn- and Ni-poor compositions
are consistent with formation of this olivine in simplified
reactions such as
Chlorite ¼ Forsterite þ Enstatite þ Spinel þ Water ð1Þ
Chlorite ¼ Forsterite þ Pyrope þ Spinel þ Water: ð2Þ
Histories of the three ‘more normal’ inclusions are interpreted first. The Cr-magnetite dunite (N138-GN) has such
unusual characteristics that it is discussed separately, followed by an assessment of the duration and cause of the
thermal pulse.
Dunite N147-GN has the assemblage forsterite^chlorite^
diopside^antigorite^clinohumite. Chlorite harzburgite
N178-GN also contains enstatite in the otherwise similar
assemblage
forsterite^enstatite^chlorite^diopside^chromite^phlogopite. Because broad-beam analyses of intergrowths within the feather-like extensions of chlorite
grains in rock N178-GN return compositions similar to
that of antigorite, minor antigorite may have been present
until just before eruption. Neither spinel nor pyrope was
identified as a reaction product associated with the
fine-grained relatively Mg-rich olivine in either rock.
Peridotite mylonite N196-GN had the assemblage forsterite^enstatite^chlorite^diopside^chromite until just
before eruption; the chlorite is now represented by very
fine-grained intergrowths composed mostly of forsterite
and aluminous spinel. The olivine porphyroclast compositions are unusually Fe-rich (near Fa10·0) for the depleted
composition of the rock, and their compositions may
record equilibrium with antigorite before neoblast formation. The forsterite neoblasts are more magnesian, more
calcic, and much more abundant than the porphyroclasts
(Fig. 7). The more magnesian compositions of the neoblasts
are attributed to the breakdown of antigorite to forsterite
plus enstatite: the compositional contrast between neoblasts and the relatively small volume of porphyroclasts is
otherwise difficult to explain in a system closed to everything but water. The more calcic compositions of the neoblasts are consistent with growth during heating, because
olivine in equilibrium with diopside is more calcic at
higher temperature (e.g. Kohler & Brey, 1990).
Approximate constraints on pressure^temperature histories are provided by the reactions indicated in Fig. 14,
assuming water in excess. The equilibrium assemblage in
NUMBER 6
JUNE 2010
dunite (N147-GN) records a temperature in the approximate range 470^6808C, just as do the other samples with
antigorite^diopside^forsterite and without enstatite. The
harzburgite and mylonite record temperatures in the field
of forsterite^enstatite^diopside^chlorite^water at pressures
too high to stabilize amphibole, conditions bracketed by
the approximate ranges 620^7808C and 2·5^5 GPa.
Antigorite may have been present in the harzburgite
(N178-GN) and the mylonite (N196-GN) until late in the
pre-eruption history of those rocks: if so, the temperature
was near the high-T limit for antigorite stability in a
narrow field for stability of antigorite^enstatite^forsterite^
chlorite^water like those calculated by Lopez
Sanchez-Vizcaino et al. (2005).
The temperatures recorded by the late-stage reaction of
chlorite to yield very fine-grained magnesian olivine are
not well constrained. Temperatures must have exceeded
those on the breakdown curve for chlorite in the natural
peridotite compositions studied by Fumagalli & Poli
(2005), at least about 7508C (Fig. 14). If heating was rapid
enough to preclude involvement of other minerals, then
temperatures must have exceeded the stability limit of
pure chlorite, greater than about 8308C at pressures less
than 4 GPa in the MASH system.
The bulk-rock compositions of these three rocks are
depleted relative to almost all other analyzed inclusions
from the Green Knobs diatreme (Figs 11 and 12), perhaps
an indication of a difference in their tectonic history. The
relatively high abundances of some of the most incompatible elements could be due, at least in part, to
grain-boundary contamination by the host SUM, a suggestion made by Roden et al. (1990) to explain the high
LREE abundances in the Green Knobs inclusions. The
presence of trace amounts of phlogopite in chlorite harzburgite N178-GN is evidence for pre-eruption metasomatism, however, and the high contents of fluid-mobile
elements such as Cs, Pb, Ba, and Sr are consistent with
trace-element enrichment during hydration.
Cr-magnetite dunite N138-GN: a prograde metaserpentinite
The Cr-magnetite dunite is interpreted as a product of prograde metamorphism of a serpentine-bearing protolith.
Evidence includes the abundant Cr-magnetite (ferrit^chromite), the zonation and highly magnesian compositions of
olivine, and the bulk-rock composition.
Similar magnetite-rich spinel is extremely rare in peridotite xenoliths. In Green Knobs inclusions with retrograde antigorite, aluminous spinel reacted to form chlorite
and chromite poor in ferric iron (Fig. 10). The
magnetite-rich spinel in rock N138-GN demands a different origin, such as serpentinization at significantly lower
temperature. Cr-magnetite (ferrit-chromite) is the dominant spinel in serpentinites. Evans (2008) inferred that magnetite forms directly as olivine reacts to yield magnesian
serpentine. Bach et al. (2006) and Beard et al. (2009)
1370
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
suggested that magnetite forms from brucite and relatively
iron-rich serpentine, not as part of olivine breakdown reactions. Regardless of the reactions involved, the link
between low-temperature serpentinization and magnetite
formation in ultramafic rocks is well established.
Magnetite persists during metamorphism, as it is the most
common spinel in prograde antigorite^diopside^forsterite
assemblages, and it also occurs in higher-grade metaserpentinites (Fig. 10), as discussed by Evans & Frost (1975).
Magnetite formation in a serpentine-bearing protolith is
the most likely cause of the distinctive composition of the
spinel in the Cr-magnetite dunite.
Serpentinization of dunite N138-GN probably took place
within the stability field of brucite. The zonation within
olivine grains from Fa9·1 interiors to Fa4·6 rims relatively
poor in Ni is not consistent with a magmatic process.
However, metaserpentinites contain olivine as Mg-rich as
Fa2, and the NiO contents of such olivine span the range
from almost 0·0 to 0·6 wt % (Vance & Dungan, 1977;
O’Hanley, 1996). Hence, the zonation is consistent with
olivine growth during metamorphism of a serpentinized
protolith. The absence of enstatite is consistent with production of olivine by reaction of antigorite plus brucite, as
breakdown of antigorite alone would produce olivine plus
enstatite, unless the system was open to loss of silica.
The gradual compositional zonation of olivine (Fig. 9)
records the first of at least two stages of prograde metamorphism. If the deduction is correct that the serpentinization occurred at conditions of brucite stability, then
temperatures must have been lower than about 5508C at
pressures below 3 GPa (Fig. 14). The first prograde stage
terminated at temperatures within the stability field of the
low-Ti clinohumite, bounded by reaction (4) in Fig. 14,
below about 6508C if the pressure was less than 3 GPa.
Nickel contents divide the olivine zonation into two
partsçone from about Fa9 to Fa6 at about 0·35 wt %
NiO, and one from about Fa6 to Fa4·6 with NiO contents
as low as 0·15 wt % (Fig. 5). The relatively Ni-poor olivine
may have formed after the observed pentlandite was
stabilized.
The compositional gradients within the dominant olivine grains extend over distances of at least three to four
hundred micrometers (Fig. 9). They are defined only in
two dimensions and probably formed both by grain
growth and by diffusion; nonetheless, a diffusion model
does constrain how long those gradients could have persisted. Diffusion produces major changes in gradients over
distances of several square roots of the product Dt, where
D is diffusivity and t is time. Diffusivities used to calculate
corresponding times are from the ‘global equation’ of
Dohmen & Chakraborty (2007) for [001] interdiffusion of
Fe and Mg in olivine. Regardless of the actual geometry,
these gradients would be substantially homogenized for
values of the square root of Dt exceeding 400 mm. For
temperatures of 6008C, 7008C, 8008C, and 9008C, corresponding times are 974 Myr, 29 Myr, 1·8 Myr, and 162 kyr.
The calculation confirms that the gradients are not inherited from high-temperature mantle, and that they are
consistent with formation during gradual prograde metamorphism of serpentinized peridotite. The gradients could
not have persisted at temperatures above 8008C for longer
than several million years.
The prograde stage that produced the gradual olivine
zonation must have been followed by an abrupt heating
event responsible for selvages of Fa3 olivine in sharp contact with that of Fa5 composition. That abrupt pulse also
caused the formation of the pyrope, the local replacement
of Cr-magnetite by a more aluminous spinel, and the
breakdown of clinohumite. Pressures for this stage must
have exceeded that for the stability of pyrope plus forsteriteçabout 2 GPa (Fig. 14). Temperatures must have exceeded those for the breakdown of chlorite in peridotite
above 2 GPa, at least about 7508C, and perhaps exceeded
those for the breakdown of chlorite alone, in excess of
8008C in the MASH system.
The bulk composition of N138-GN must have evolved by
complex processes. The Cr content is extraordinarily high
and SiO2 unusually low compared with almost all other
peridotites; Ca is also low relative to Al (Fig. 11). Bulk-rock
compositions typically behave as systems open to many
elements during serpentinization^deserpentinization processes, and loss of Ca is common (Coleman, 1967; Frost &
Beard, 2007). However, the abnormally low Ca relative to
Al is unlikely to be due solely to loss of Ca in a fluid
phase, unless CaO and the HREE were equally mobile
(Fig. 13). The domains rich in chlorite plus Cr-magnetite
and those rich in forsterite appear physically mixed together and juxtaposed across faults of at least thin-section
scale (Fig. 8). The domains rich in chlorite plus
Cr-magnetite are analogous to the layers called ‘blackwall’
and ‘chlorite schist’ that form as metasomatic zones
between serpentinite and other rocks (Coleman, 1967).
These chlorite^magnetite volumes are responsible for
much of the Al and Cr and for the low Si. The
forsterite-rich portions represent metaserpentinites.
Mixing of the two components probably caused the unusual bulk composition.
Duration and cause of the late-stage thermal pulse
The abrupt compositional gradients between the dominant
olivine and the selvage overgrowths constrain the duration
of the thermal pulse just before eruption. Because these
gradients are sharp on the scale of BSE images (Figs 4
and 15), they span at most a few micrometers.
Modifications of a sharp gradient for values of the square
root of the product Dt are illustrated for linear diffusion
in Fig. 15. The apparent sharpness could not have been
maintained for values of the square root greater than
about 1·5. Temperature is the most important uncertainty
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JOURNAL OF PETROLOGY
VOLUME 51
NUMBER 6
JUNE 2010
each inclusion with bimodal olivine records deformation
that may have accompanied that intrusion.
Possible source histories
Fig. 15. Diffusion constraints on the duration of the late thermal
pulse. (a) BSE image of an Fe-poor selvage at the contact of olivine
(Ol) with chlorite and its reaction products (Chlþ) in the
Cr-magnetite dunite. The arrow points to the sharp contact, and
values of wt % FeO are indicated at two analyzed spots. White
scale bar, 30 mm. (b) Compositional gradients produced by diffusion between two semi-infinite media in contact at an initially
sharp interface; curves are labeled with values of the square root
of the product Dt, where D is an interdiffusion coefficient and t is
time.
in constraining timing of the late-stage pulse. For values
equal to 0·5, 1·5, and 3·0 mm, the corresponding times
at 7008C are 46, 415, and 1661 years; values at 8008C are
3, 24, and 97 years. Hence, the thermal pulse and the
eruption must have taken place within a few hundred
years.
The only plausible cause for such a sharp temperature
pulse is intrusion of magma. Consequent dehydration triggered eruption of the serpentinized ultramafic microbreccia, in confirmation of a hypothesis proposed by Smith &
Levy (1976). Hence, the xenoliths with bimodal olivine
compositions may be from the root zone of the SUM eruption. The intrusive magma probably was part of the minette magmatism that dominated the NVF. The texture of
The Cr-magnetite dunite probably represents mantle
emplaced during low-angle subduction, because it records
formation of brucite^serpentine^magnetite in a fluid-rich,
low-temperature environment and then deserpentinization
during gradual heating. Magnetite forms during serpentinization both of oceanic lithosphere and of the overlying
mantle wedge (e.g. Blakely et al., 2005; Beard et al., 2009),
and temperatures could be low enough for brucite formation in either environment. In the simpler hypothesis, the
dunite is a sample of the mantle wedge transported from
the SW during low-angle subduction. Serpentinization in
the mantle wedge near the trench is required to explain
the formation of the garnetites, and the dunite documents
a complementary history essential for garnetite formation.
The fluid flow accompanying dunite serpentinization also
may have been responsible for the heavy metasomatic
overprint observed in Colorado Plateau eclogites by Usui
et al. (2006). However, the possibility remains that the
rock is a sample of the Farallon slab. In either case, the
rock probably is a sample of a subduction-related me¤lange,
just as are the eclogites and garnetites. The texture resembles the products of millimeter-scale juxtapositions of
chlorite-rich rock and more rigid lithologies described by
Bebout & Barton (2002) and attributed by them to mixing
in subduction me¤langes at the slab^mantle interface.
The other three inclusions with bimodal olivine compositions also may be from such a me¤lange. All four rock samples have bulk compositions more depleted in Ca than
almost all other peridotite samples from Green Knobs,
and their HREE abundances are also notably low (Figs 11
and 12); thus their distinctive compositions may record an
element of common history. However, other than the
Cr-magnetite dunite, only peridotite mylonite N196-GN
records probable evidence of deserpentinization that produced major amounts of more magnesian olivine.
Together with the Cr-magnetite dunite, it may be from
the root zone from which the Green Knobs SUM erupted.
If so, the root zone may have extended over a substantial
depth range, as chlorite breakdown produced pyrope in
the Cr-magnetite dunite but aluminous spinel in the peridotite mylonite.
DISCUSSION
Possible sources of lithosphere emplaced
below the Colorado Plateau
The suggestion of Helmstaedt & Schulze (1991) that the
NVF eclogites might represent fragments tectonically
eroded from the continental lithosphere is supported by
1372
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
more recent data. The Mojave Province was close to the
plate boundary during the Laramide orogeny. The oldest
known rocks in that province formed at about 1·8 Ga
(Barth et al., 2009), and so it is a plausible source for the inherited mid-Proterozoic component of the eclogite and garnetite zircons. Grove et al. (2003) and Saleeby (2003)
summarized evidence that parts of the mantle wedge of
the Mojave Province were removed during Farallon subduction and replaced by ‘lower plate schists’ now exposed
at locations shown in Fig. 1. The mantle wedge was eroded
during accretion of the schists beginning at about 90 Ma
and continuing until about 50 Ma (Grove et al., 2003).
That time interval includes the 81^85 Ma ages measured
in zircons of the eclogites and the garnetite. The eroded
forearc mantle was presumably partly serpentinized, as it
is now along the plate margin in the northwestern USA
(Wada et al., 2008). Partly serpentinized mantle would be
buoyant and unlikely to sink into the asthenosphere after
erosion. The settings of serpentinization, tectonic erosion,
and transport are illustrated schematically in Fig. 16. The
Navajo eclogites are suggested to be derived from dikes in
a subsequently eroded part of the mantle wedge close to
the plate boundary. The initial eclogite-facies recrystallization and garnetite formation may have been catalyzed
by deformation, pressure increase, and fluid migration
during the tectonic erosion, similar to the processes suggested to account for the eclogite-facies recrystallization
of gabbro in Alpine ophiolites (Bucher & Grapes, 2009).
These processes continued during and after transport about 700 km to the NE above the low-angle Farallon
slab.
Extent and possible importance of mantle
serpentinization
Serpentinization above the subducted Farallon slab may
have caused uplift and subsequent magmatism in much of
the western USA, as suggested by Humphreys et al. (2003),
but only the SUM diatremes provide direct evidence for
serpentinization beneath the NVF. Intrusion of minette
magmas was the most likely cause of the SUM eruptions;
however, although minettes are widely distributed in the
NVF, the SUM diatremes are relatively rare and most
occur near or on monoclines. The monoclines lie above
reactivated shear zones, which were first active during
Proterozoic times (Davis & Bump, 2009). Fluid from the
slab^mantle wedge interface could have been channeled
along the shear zones and weakened them, as in the crustal
examples described by Barnes et al. (2004). Although there
is evidence for mantle hydration in xenoliths from the
Colorado Plateau that are not from SUM diatremes, that
evidence is indirect; it consists of textures plus pyroxene
compositions, oxygen isotope ratios, and trace element
abundances (e.g. Smith, 2000; Lee, 2005; Perkins et al.,
2006; Li et al., 2008).
There is little or no xenolith evidence of serpentine elsewhere in the mantle of the western USA, other than antigorite of possible mantle origin in dunite xenoliths hosted
by minette in Montana (Facer et al., 2009). Possibly the
rarity of antigorite is because almost all mantle xenoliths
are magma-hosted, and antigorite is unlikely to survive
during their exhumation. In addition, the Colorado plateau lithosphere is both thicker and cooler than that in the
bordering regions more affected by Cenozoic magmatism
Fig. 16. Schematic cross-sections illustrating tectonic erosion and displacements of parts of the mantle wedge. Sections are for a SW^NE line
within the area overlying the flat-slab segment illustrated in Fig. 1. Horizontal and vertical scales are identical and are the same in each section.
‘H’ represents hydrated lithospheric mantle wedge and me¤lange. ‘F’ represents accreted ‘lower-plate schists’ analogous to parts of the
Franciscan Complex discussed by Grove et al. (2003). (a) Near-trench section at about 90 Ma, before the start of low-angle subduction, adapted
from Saleeby (2003). (b) Near-trench section at about 80 Ma, shortly after the start of low-angle subduction and the beginning of erosion of
the hydrated lithospheric mantle wedge. (c) Section from the trench to below the Navajo Volcanic Field at about 50 Ma.‘M’ represents faults
associated with monoclines that may have channeled water upwards. Section (c) illustrates only the first proposed alternative, that of tectonic
removal of Colorado Plateau lithosphere below about 90 km depth, not the alternative of serpentinite diapirism from greater depth.
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JOURNAL OF PETROLOGY
VOLUME 51
(Sine et al., 2008). Hence, peridotite fragments included in
the SUM may retain evidence of hydration that was obliterated by heating in the surrounding regions.
Evolution of the lithosphere
The conditions in the mantle lithosphere at the time of diatreme formation can be modeled using data only from inclusions in the SUM. Calculated P^T values for four
eclogite inclusions (Fig. 17) fall within the stability fields of
lawsonite and coesite; the presence of coesite (Usui et al.,
2003) establishes a minimum pressure at 4008C of about
2·5 GPa, corresponding to a depth of about 85 km. The
thickness of the inferred me¤lange that transported the
eclogites is poorly constrained, because the calculated
pressures have large uncertainties. Calculated pressures
for two samples agree at about 2·6 GPa, and the corresponding 90 km depth has been adopted as a guide to the
maximum thickness of the lithosphere above the eclogite
source. Overlying Proterozoic lithosphere may be represented by an undeformed but hydrated garnet peridotite
containing amphibole with an eNd value of þ30 (Roden
et al., 1990); garnet stability requires a pressure near
2 GPa, corresponding to about 70 km depth. Hence, if the
eclogite inclusions record the depths from which they
were erupted, only 70^90 km of lithosphere may have
been preserved below the Colorado Plateau after low-angle
subduction; this possibility is illustrated schematically in
Fig. 16c. If so, then emplacement of the me¤lange must
have been accompanied by tectonic erosion, because the
mantle root of the plateau is expected to have been much
thickerçperhaps 200 km, as discussed by Li et al. (2008).
Such tectonic erosion is consistent with the suggestion by
Spencer (1996) that about 120 km of continental mantle
Fig. 17. Constraints to evaluate the depths of continental lithosphere
and me¤lange during low-angle subduction. Circles represent phengite^garnet^omphacite assemblages in four eclogite samples
(Broadhurst, 1986; Usui et al., 2003; Smith et al., 2004) at positions calculated using the approach of Smith et al. (2004). Eclogites are attributed to me¤lange. Area (1) represents remnant Proterozoic lithosphere
of the Colorado Plateau. Area (2) may represent remnant lithosphere,
or me¤lange, or both.
NUMBER 6
JUNE 2010
was eroded from beneath the Colorado Plateau by
Laramide low-angle subduction, leaving 80 km of lithosphere. The erosion is less extreme than that suggested by
Luffi et al. (2009) for mantle below part of the Mojave
Province between the Colorado Plateau and the plate
margin: those workers suggested that only a few kilometers
of Precambrian mantle remained there.
A contrasting view of the plateau lithosphere is provided
by garnet peridotite xenoliths from a minette intrusion,
The Thumb, in the northeastern NVF (Ehrenberg, 1979,
1982). These xenoliths are very different from the inclusions
in the SUM diatremes. Phlogopite is the only mineral present containing essential water. Temperatures and depths
recorded by most rocks fall in the ranges 1100^12508C and
125^145 km. Sr and Nd isotopic ratios are distinct from
those of the peridotite inclusions in the SUM; some ratios
have characteristics of continental lithosphere, but most
more closely resemble those of ocean island basalts, perhaps as a result of melt-induced metasomatism (Roden
et al., 1990; Alibert, 1994). Os isotopic abundances are consistent with Re depletion about 1·6 Gyr ago and thus with
an old lithospheric source (Lee et al., 2001). Based on these
data, the lithosphere has been inferred to extend to about
145 km depth, not the 90 km inferred from the inclusion
populations in the SUM diatremes.
The minette of The Thumb may have been emplaced
more recently than were the SUM diatremes, and such an
age difference could be relevant for understanding the different histories of the inclusion populations. The age of
The Thumb minette has not been determined, but ages
for nearby minettes are in the range 26^23 Ma (Laughlin
et al., 1986). Two northern SUM diatremes have ages of
about 30 Ma, and that of Green Knobs is presumed to be
25 Ma, the same as SUM at nearby Buell Park (Roden
et al., 1979). These ages fall within or near a period of rapidly changing mantle conditions. Removal of the shallow
slab beneath the NVF between 30 and 25 Ma is consistent
with exhumation and erosion that began in that region at
about 27 Ma (Cather et al., 2008). Humphreys (2009) portrayed the Farallon slab sinking in a complex geometry
after shallow subduction; such a geometry would be
accompanied by irregular flow patterns in the replacing
mantle. The present lithosphere of the Colorado Plateau
may contain components juxtaposed as a consequence of
Farallon slab removal, and one component may be derived
from return flow of mantle that had been displaced
during low-angle subduction. If so, that component might
be represented by the minette-hosted xenoliths at The
Thumb. Li et al. (2008) have pointed out that a hydrous
component, such as that in olivine and pyroxene in the
xenoliths from The Thumb, would reduce mantle viscosity
substantially. The suggested return flow may have been
possible because of lithosphere hydration during initial
interactions with the Farallon slab.
1374
SMITH
COLORADO PLATEAU DIATREME INCLUSIONS
An alternative mantle reconstruction is based on the
possibility of serpentinite diapirs below the Colorado
Plateau. Serpentinite diapirs in the Mariana forearc of
the western Pacific provide possible analogies, as they contain inclusions of antigorite-bearing peridotite interpreted
as hydrated mantle wedge transported deeper during subduction and then carried up into seamounts (Murata
et al., 2009). Ehrenberg & Griffin (1979) suggested that the
SUM diatremes might have erupted from serpentine-rich
diapirs intruded into the lower crust; however, thermobarometry results do not support the hypothesis that such
diapirs reached crustal levels. The formation of nearly
pure pyrope plus forsterite during the late-stage thermal
pulse establishes that the Cr-magnetite dunite was at
greater than 70 km depth just before eruption. The inclusions inferred to be from a me¤lange could be from diapirs
intruded to a depth greater than 70 km. Zircon ages and
Sm^Nd mineral isochrons indicate that recrystallization
of the eclogites continued to within several million years
of the time of diatreme emplacement (Helmstaedt &
Schulze, 1988; Wendlandt et al., 1996; Usui et al., 2003;
Smith et al., 2004), however, and no evidence of a pressure
decrease has been observed in the lawsonite eclogites.
Likewise, no evidence for a pressure decrease has been
recognized in any of the hydrated ultramafic inclusions.
Nonetheless, the hypothesis of serpentinite diapirism
below the Colorado Plateau remains possible. If serpentinite diapirs carried relatively low-temperature hydrous
mantle to depths like those recorded by the eclogite inclusions, then none of the inclusions necessarily documents
tectonic erosion of lithosphere below the Colorado Plateau.
Implications
The extraordinary inclusions in the SUM diatremes document the complex evolution of the mantle beneath the
region. The unique characteristics of the diatremes themselves may be due to an unusual set of circumstances that
caused eruption, not because the necessary mantle processes are rare. Below the NVF, as documented by the
rocks with bimodal olivine compositions, magma intruded
hydrated mantle and triggered the eruptions of the SUM.
The rarity of diatremes with inclusion suites like those of
the NVF may be because heating of hydrated mantle normally occurs more slowly and dehydration is gradual, not
because such mantle wedge^subduction zone interactions
are uncommon.
Tectonic erosion of the continental lithospheric mantle
wedge and dragging of the eroded rock during subduction
are likely to be important elsewhere. Such processes have
been suggested by Johnson et al. (2009) to provide water
for arc magmatism more than 300 km from the trench in
central Mexico. During flat subduction below volcanic
gaps, the process may carry water much further, consistent
with explanations for Laramide magmatism yet further
from the trench (Humphreys et al., 2003). Below western
South America, low-angle subduction both is occurring at
present and has occurred intermittently in the past. Here
delamination accompanying slab steepening has been proposed to account for the removal of continental lithosphere
(e.g. Kay & Coira, 2009); tectonic erosion during low-angle
subduction may also have removed part of the continental
lithosphere. Continental mantle roots change with time,
and one cause, melt-metasomatism, is well documented
(Griffin et al., 2009). Other causes, consistent with the scenario proposed here for the mantle below the Colorado plateau, may include low-angle subduction-induced erosion,
followed by lateral flow of former lithosphere during slab
removal.
S U M M A RY
Inclusions establish the evidence of serpentinization in the
mantle sampled by the SUM diatremes of the Navajo
Volcanic Field. Critical evidence includes the mineral assemblage antigorite^diopside^olivine, the mineral textures
and the similarity of the deduced temperatures to those recorded by hydrous assemblages of certain mantle origin in
associated inclusions. The antigorite-bearing assemblages
formed by retrograde hydration. In contrast, prograde
metamorphism of a brucite-bearing serpentinized protolith is documented by a Cr-magnetite dunite inclusion.
That dunite was most probably transported from near the
trench to about 700 km to the NE during low-angle subduction. It could represent a fragment of either hydrated
mantle wedge or part of the Farallon slab; the former possibility is consistent with current interpretations of the
origin of eclogite and garnetite inclusions in SUM diatremes, the protoliths of which have been established as
having Proterozoic ages. They are interpreted as fragments
of the lithospheric mantle wedge below the Mojave
Province dragged by Farallon subduction, a hypothesis
previously proposed by Helmstaedt & Schulze (1991).
These inclusions are probably samples of a tectonic me¤lange that formed at the interface between the old
Colorado Plateau lithosphere and the underlying subducted Farallon slab. Other possible constituents of the
me¤lange are discrete pyrope and forsterite grains and the
peridotite inclusions with extremely depleted bulk compositions and bimodal olivine compositions.
In peridotite samples with bimodal olivine compositions,
one population dominates the rock and the other is present
only as small relatively Mg-rich grains. These more magnesian grains formed by reactions involving chlorite breakdown caused by a sudden temperature increase; the
increase immediately preceded entrainment of inclusions
in the erupting gas^solid mix that formed the diatremes.
The temperature spike is attributed to intrusion of
magma into hydrated peridotite, the suggested cause of
the SUM eruptions. Among the reaction products in the
Cr-magnetite dunite are nearly pure pyrope and forsterite;
1375
JOURNAL OF PETROLOGY
VOLUME 51
this mineral pair establishes that the SUM source was at
70 km or more in depth.
Two hypotheses are considered for the evolution of the
lithosphere of the Colorado Plateau. In the first, the lithosphere was thinned to 70^90 km thick by tectonic erosion
during low-angle subduction. Lithosphere remaining
above the upper surface of the subducted slab was
hydrated, perhaps mostly along reactivated fault zones.
After subsequent sinking of the slab, the lithosphere was
then re-thickened by return flow of mantle material. An alternative possibility is that the diatreme sources were in
diapirs of serpentinized peridotite intruded to depths of
70^90 km; this possibility does not require tectonic erosion
of the Colorado Plateau lithosphere.
The presence of serpentine in the mantle of the
Colorado Plateau during the Laramide orogeny is consistent with the suggestion of Humphreys et al. (2003) that
mantle hydration played a critical role in subsequent magmatism. Tectonic erosion, the presence of serpentine far
from the trench, and the sub-horizontal transport of volumes of material eroded from the hydrated lithospheric
mantle wedge should all be considered in evaluating the
processes and effects of low-angle subduction.
AC K N O W L E D G E M E N T S
The manuscript benefited from reviews by J. D. Barnes and
M. F. Roden, from journal reviews by B. W. Evans, W. R.
Griffin and S. Swanson, and from comments by B. R.
Frost. S. Levy assisted in point-counting modes of rocks.
Oversight of the electron beam laboratories by K.
Milliken and D. Zhao made analyses possible. The Navajo
Nation is thanked for permission to collect the samples.
The electron beam laboratories used for this investigation
were supported by the Department of Geological Sciences
and the Jackson School of Geosciences, The University of
Texas at Austin.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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