Download Migration of radiogenic strontium during metamorphism

JOURNAL
OF GEOPHYSICAL
RESEARCH
VOL. 69, No. 20
OCTOBER
15, 1964
Migration of RadiogenicStrontium during Metamorphism
G. J. WASSERBURG AND A. L. ALBEE
Division o/Geological Sciences
California Institute o/Technology, Pasadena
•/[. A. LANPHERE
U.S. GeologicalSurvey, Menlo Park, California
Abstract. Study of the concentration and isotopic composition of strontium and rubidium
in hornblende diorite dikes and sills of Precambrian age from the Panamint Mountains of
California showed that these rock systems were enriched in radiogenic Sr during a late
Mesozoic metamorphism. Enrichments in radiogenic Sr were observed for total-rock samples
which yielded apparent ages of up to 34,000 m.y., although there is no obvious petrographic
evidence for such metasomatic changes.These results indicate that some caution is necessary
in interpreting the Sr isotopic compositionof rocks in terms of their original sourceor in
assumingthat 'total rocks'form closedsystems.In general,the initial isotopicSr composition
should be determined, not assumed.
Introduction. This study is an investigation are suggestiveof a lack of gross recrystallizaof the effectsof relatively recentmetamorphism tion or of significant mobilization and metasoon the isotopiccompositionof Sr and the con- matism. The mineral assemblagesand textures
centration of Sr and Rb in some marie igneous in the augengneiss,the granite, and the dioritic
rocks of the Precambrian. The samples were dikes and sills are not significantlyaffectedby
chosenfrom an area in which Lanphere et al. the metamorphismand might be interpreted
[1963, 1964] had demonstratedthat certain reasonablyas the primary mineralogy and texolder Precambrian rocks had lost appreciable ture. Details of the geologyand petrographyare
amounts of radiogenic Sr during a Mesozoic given in the report by Lanphere et al. [1964].
The resultsfrom samplesof the augengneiss
metamorphism.
The samplesstudiedare from the StoneCor- and granite from this area are that the Sr•/SF •
ratios for the various component
ral area in Pleasant Canyon in the southern and Rb87/Sr•ø
part of the Panamint Mountains of California. mineralsof a particular samplelie on a straight
A sequenceof later Precambrian rocks (the line on the Sr evolutiondiagram,with a slope
to an age of 100 to 150 m.y. This
Pahrump series) unconformablyoverliesaugen corresponding
gneissand crosscuttinggranite of the World indicatesthat essentiallycompleteSr isotopic
Beater complex. The crystalline rocks occupy equilibration took place during Mesozoic rethe core of an anticlinal dome and are cut by
gional metamorphism. While such mineral iso-
dioritic dikes,someof which can be traced into
sillsand dikesin the lower part of the overlying
Pahrump sedimentary section. Middle-grade
metamorphic mineral assemblages,characterized by the widespreadoccurrenceof biotite
chronswerefoundfor severaldifferentrocks,it
dena.
fects must be attributed
was found that the total-rock samplesscattered
rather widely and did not lie alongan isochron.
Pb-U ageson zirconindicatedprimary agesof
1800 m.y. for the augen gneissand 1350 m.y.
for the granite, but nearly all the total-rock
and the local occurrenceof andalusite, cord•,er
ire, garnet,and chloritoid,are developedin the sampleslay belowa 1600-m.y.isochron,indicatmanflingsedimentaryrocks.However,the pres- ing youngerapparent ages.These resultsshowed
ervation of fine deposittonalfeatures and the that even total-rock samplesas large as 85 kg
absenceof metamorphic differentiation features were open to Sr or Rb transport during the
period of isotopic redistribution. Apatite and
whichcontainvery little Rb, showed
Contribution
1256,Divisionof Geological
Sci- plagioclase,
large
Sr
•
enrichments;
thus the observed efences, California Institute of Technology, Pasa4395
to Sr redistribution.
4396
WASSERBURG, ALBEE, AND LANPItERE
TABLE
The earlier data showedthat the augen gneiss
had lost radiogenicSr and suggestedthat the
granite had been enriched in radiogenicSr. It
was expected,however,that more spectacular
enrichments
could be observed in the diortrio
dikes and sills which have low Rb/Sr ratios
and which are in contact in many places with
the World Beater complex, which contains
rockshaving high Rb/Sr ratios.
Results. To have a basis for estimating the
original undisturbedSr isotopiccompositionsof
the hornblende diorite dikes and sills, we obtained samplesof 'Pahrump diabase' from localities outside the Stone Corral area. Mafic
dikes and sills which are restricted to the lower
part of the Pahrump series (Crystal Spring
formation) throughout the Death Valley region are considered
to be geneticallyrelatedand
of the sameage [Wright, 1952]. These rocksare
referred to as the 'Pahrump diabase,'although
many of them are not petrographicallyof diabasic compositionand texture. Geologic and
geochronologic
relationsindicatethat the 'Pahrump diabase' is between 600 and 1350 m.y.
old. One sample (CP-440A) was obtainedfrom
a thick sill, intrusive into dolomite, in Warm
Spring Canyon,about 10 miles southeastof the
1.
Ar 4ø -
K 4ø Data
Ar4øra
d
Sample
CP-440A
(hornblende)
CP-440A
K,
%
1.198'
Ar•ør•.a,
10-•ø mole/g
Age,
Ar •ø m.y.
5.24
0.58
231
1.73
0.34
840
1.2121
0.0939*
(pyroxene) 0.09201
* Analysis by flame photometer.
I Analysis by isotope dilution.
Mesozoic intrusive activity or to Tertiary extrusive activity, both of which occurredin this
region. The 840-m.y. pyroxene age lies within
the stratigraphic-geochronologic
restrictionsand
might be considereda reasonable,or at least a
minimum, age for the diabase.However, Hari
and Dodd [1962] have shown that pyroxenes
in metamorphicrocksmay containexcessradiogenic Ar, which results in high ages.Thus, we
must conclude that our K-Ar
results do not es-
tablish the age of the 'Pahrump diabase' in
Warm Spring Canyon.
Analyses of samples of 'total rock' and of
Stone Corral area. This area is immediately ad- mineral separatesare reported in Table 2 and
jacent to the Silver Lady Canyon area studied shown in Figure 1. The 'total rock' sample for
by Wasserburget al. [1959] and Silver ei al. CP-436 was 5 kg, but all others were hand
[1961], who reportedagesin the pre-Pahrump specimensize. In the last column of Table 2
basementcomplex of about 1700 m.y. by the an 'apparent age' is given, based on an asAr•ø-K4ø,Sr•7-Rb%and Pb-U methods.Their sumed initial Sr87/Sr• ratio of 0.703. The raw
data indicate that the rocks in this area are
Sr isotopic data were normalizedto a Sr•/Sr •
relatively undisturbed by any younger meta- ratio of 0.1194. An 83% Sr• tracer was used.
morphic episode.The other sampleis from the The Sr•7/Sr• ratios for the samplesare tabuSaratoga Springs area on the southeastedge lated from both composition runs and spiked
of Death Valley, about 45 miles southeastof runs in several cases. The concentrations of Sr
the Stone Corral area. K-At ageswere measured and Rb are considered accurate to within 0.7 and
on hornblendeand pyroxenefrom the diabasein 2%, respectively.The Rb, Sr, and K analyses
Warm Spring Canyon (CP-440A) in an at- reportedin this work weredoneat the California
tempt to verify that the diabasehad not been Institute of Technology. The Ar analyseswere
disturbed by metamorphismand to date more done by one of us (M.L.) in the laboratory of
preciselythe 'Pahrump aliabase.'The discrep- the United States Geological Survey. The K
ancy betweenthe ages(Table 1) indicatesthat analyseswere done by both flame photomerry
the diabase has not remained undisturbed since
and isotopedilution [Wasserburge• al., 1962].
its primary crystallization,as had beeninferred
The apparent initial Sr isotopiccomposition
from the results in Silver Lady Canyon. The
of the two diabase samples from outside the
235-m.y. hornblendeage is much lessthan the Stone Corral area is certainly different. The
stratigraphicage of the diabaseand indicates initial Srs•/Sr• ratio of TSS-1 is 0.708, corthat the hornblendehas lost radiogenicAr. This
respondingto the value measuredfor the apaAr lossmay be a metamorphiceffect related to rite, which is nearly Rb-free. SampleCP-440A
MIGRATION
OF RADIOGENIC
TABLE
Rock or Mineral
Sample
CP-440A
TSS-1
2.
STRONTIUM
4397
Sr and Rb Data
Normal Sr,
10-6 mole/g
Rb,
10-6 mole/g
SrS7/Srs6
RbS7/SrS6 t X 109,years
Sill, total-rock
plagioclase
4.141
0.170
0. 7047
0.1159
3.126
0.0198
0.7032
0. 7030
0.0179
Dike, total-rock
apatite
2.284
0. 718
0. 7170
0.8876
4.864
0.016
0. 7168
0.7078
0.7078
0.0091
(1.09)
CP-436
Dike, total-rock
plagioclase
0.945
3.24
0.058
0.091
0.8040
0.8046
0.175
0.0799
32.8
59.0
CP-349
Dike, total rock
1.696
0.039
0.7427
0.0655
34.1
CP-409b
Cobble in conglomerate,
total
rock
1.571
3.451
0.7764
6.200
Sill, total rock
4.357
0.0994
0.7415
0.0644
Dike, total rock
2.076
0.792
1.077
Plagioelase
7.823
0.270
0.7273
0.7298
0.7261
0.7231
CP-156
Sill, total rock
5.176
0.295
0.7223
0.7217
0.161
8.15
CP-70C
Flow, total-rock
1.573
2.91
0.479
0.242
0.859
0.234
1.33
Plagioclase
1.630
0.123
0.7191
0.7190
0.7179
0.7176
0.7177
Dolomite
clast
3.69
0. 0054
0.0041
Dolomite
clast
6.78
0. 7080
0. 7098
0.7218
CP-263
CP-155
calcite
CP-426-1
2
CP-151
<0.2
Dolomite, total rock
(Beck Springs)
0.0975
0.85
33.7
1.61
15.3
0.214
0.7101
XRb s* = 1.39 X 10-n year -•
has an initial SF7/Sr86ratio of 0.703, corresponding to the value measured for the plagioclase,
which also is nearly Rb-free. The differencemay
be truly original,or the higher value may reflect
the effect of metamorphismor deuteric alteration. Nevertheless,both values are well within
the range obtained for 'normal' basalts [Faure
and Hurley, 1963], and we infer that they representthe probablerange in original Sr compo-
of the World Beater dome, south of the road
up Pleasant Canyon. Samples CP-436 and CP349 were collected about 0.3 mile apart from
a hornblendediorite dike, 5 meters thick, which
crosscutsthe World Beater complex. Sample
CP-263
is from a sill about 250 feet above the
sition of the dioritic dikes and sills in the Stone
uncomformityat the baseof the Pahrump series
and in the vicinity of the dike. It is presumed
that the dike is a feederto the sill, althoughthis
particular dike has not actually been traced
Corral
into the sill. CP-409b
area.
The samplesfrom the Stone Corral area fall
into three groups and are located with respect
to the geologicmap in Lanphere et al. [1964].
The first group is from the southeast corner
is a marie cobble from a
conglomerate between the sill and the uncon-
formiry; the cobbleconsistsof biotite, garnet,
plagioclase,and ilmenite.
The total-rock sample and the plagioclase
4:398
WASSERBURG, ALBEE, AND LANPHERE
' e,
O•;p
43'6TR,
from this same dike (CP-349) and of the sill
(CP-263), which appearsto be associatedwith
this dike, also showsa distinct enrichmentin
Sr87and yields excessivelyhigh ages,indicating
addition of radiogenic Sr. The enrichment in
Sr• in these rocks is distinctly above the values
expectable for undisturbed Rb-poor systems,
'
GP-436-P
but the enrichment
is below that
obtained
in
the neighboring Rb-rich augen gneiss and
granite, which have Sr•/Sr• values of 0.85 to
1.09 [Lanphere ei al., 1964].
The sampleof the cobblefrom a conglomerate
between the sill and the unconformity has a
0 76
similarSr•/Sr •6ratio, but becauseit hasa much
075--
-
CoP
349-TR
o 74
CP263-TR
greater Rb content it yields a total-rock apparent age of 850 m.y.
The second group of samples is from the
northeastern corner of the World Beater dome,
southwest of Sentinel Peak and about 3 miles
north of the location of the first sample group.
A hornblendediorite dike, lithologicallysimilar
to the first dike, has been traced for over a mile
t
o
•
/
CP-155TR
within the World Beater complex,acrossthe unconformity, and into a hornblende diorite sill.
•
CP-70CP/
%•
o
Sample
CP-156 is a sample from the sill, very
/
o /
/
high in coarse-grainedhornblende,and sample
CP-155 is from the dike, which is nearly a mile
from the sill along the strike of the dike. The
analysesshow a pattern similar to that shown
CP4?O
A•P
07% 01 02 013 014015 06 0•7 08 09
by the first group from the southernpart of the
area. The total-rock sample of the dike is only
Rbs•sr86
3% enriched in Sr• and becauseof a relatively
Fig. 1. Strontium evolution diagram showing high Rb content yields an apparent age of 1.61
the data for total-rock samples and minerals from
b.y. This is, perhaps, consideringthe errors,
dikes and sills.
reasonable in comparison with Pb-U ages of
1800 m.y. for the augen gneissand 1350 m.y.
separatefrom CP-436 have the sameSr isotopic for the granite. However, the plagioclasesepacomposition,the Sr87being 14% enriched.The rate has an almost identical Sr•/Sr • ratio, in
total rock has a higher Rbs•/Sr• ratio because spite of the fact that the RbS•/Sr•6 ratio is a
of the presenceof other more t{b-rich phases. factor of 10 smaller. The slight differencebeThe apparent age of the plagioclaseis twice tween the isotopic compositionsof the plagiothat of the total rock as a direct consequence clase and the total rock is what would be expected from the radioactive decay of Rb• over
of the smaller Rb87/Sr• ratio. This general pattern is seenfor the other samplesas well. Both the past 100 to 150 m.y. Since the plagioclase
the ages are absurdlyhigh and exceedby a fac- has a much higher Srs•/Sr• ratio than that
tor of 8 or more the presently acceptedage of found for the undisturbedrocks,we must conthe earth.
clude that the plagioclasein this rock has been
An Ar4ø-K•ø age of 377 m.y. was determined enriched in Srs• during metamorphism.
from the hornblende of this rock [Lanphere
The related sill (CP-156) also showsan enet al., 1964] indicatingthat there was a partial richment, although it is not as marked as in
lossof radiogenicAr during the metamorphism. some of the other samples. This anomaly is
The total-rock analysis of the other sample small enough so that it might have been er-
-CP440A
TR
MIGRATION
OF RADIOGENIC
STRONTIUM
4399
roneouslyinterpreted to be due to an abnorIt is important to comparethe character of
mally high Sr•'/Sr• ratio in the originalmagma. the 'Pahrump diabase' in the disturbed and
CP-70C is from a maficflow within the Kings- undisturbedareas.The samplesfrom the Warm
ton Peak formation of the Pahrump series, Spring and Saratoga Spring localitiesare both
about I mile east of the World Beater dome
typical 'deuterically altered diabase'; they conand about 3000 feet stratigraphicallyabove the sist of 65 to 75% plagioclasein interlocking
World Beater complex. The flow is about 120 blades5 to 10 mm in length,about 20 to 30% infeet thick and contains pillow-like structures terstitial clinopyroxene,about 5% ilmenite, and
through most of the thickness. CP-70C is a about 1% apatite in elongaterods. The plagioamygduloidal rock consisting of plagioclase clase is zoned, and is multiply and complexly
laths in a fine-grained aggregate of chlorite, twinned; it is extensively saussuritizedwith
calcite, and ilmenite; the amydulesare chlorite abundant clinozoisite in an albite matrix. The
and calcite with minor quartz. A total-rock clinopyroxenehas been almost completely alsample and separatesof plagioclaseand calcite tered to pseudomorphicaggregatesof uralitic
were analyzed. It was not possibleto obtain a amphibole, clinozoisite, calcite, chlorite, and
really pure separateof the calcite, and its ab- green biotite--only rare relics of clinopyroxene
persist.
normally high Rb content is due to fine inclusions of biotite. The total rock yields an
In contrast, the dioritic dikes of the Stone
apparent age,which lieswithin the stratigraphic- Corral area consist of about 60 to 65% horngeochronologic restrictions--although outside blende in interlocking blades about 0.5 mm in
our prejudices.However, the results from the length, about 30 to 35% plagioclase(An 32),
mineral separates again indicate that the sys- 3 to 5% ilmenite, and less than 1% apatite.
tem is badly disturbed.All three sampleshave They may also contain as much as 2 to 3%
essentially the same Sr isotopic composition, biotite, clinozoisite,quartz, calcite,and chlorite.
in spite of a wide range in RbS'/Sr•, indicating The plagioclaseis untwinned,has a rather unthat only a short decay time could have ocur- duloseextinction, and containsvery little trace
red since the disturbance.
of alteration products. The hornblende conSincethe previousdata indicatedthat Sr•7en- tains no pyroxenecoresbut is itself somewhat
richment had occurred at a considerable disaltered on its margins to fine needlesof actinotance from the World Beater complex, it was lite. Hornblende is more abundant in the sills,
decided to look for evidence of really major which may consistalmost entirely of interlockenrichment in the overlying sedimentary rocks, ing hornblendeblades,2 to 3 cm across,periphsuch as would be necessaryto enrich a whole erally altered to fine needles of actinolite,
sedimentarycarbonateunit. Samplesof carbon- biotite, chlorite,clinozoisite,and carbonate,with
ate rock were taken from several stratigraphic about 5% interstitial plagioclaseand 3% ilmenpositions.Sample CP-151 is a sample of dolo- ire. It is of interest to note that the isotopic
mite from the Beck Spring dolomite of the Sr compositionhas not been changed in the
Pahrump series, collected at least 600 feet rocks which show obvious alteration, whereas
stratigraphically above the World Beater com- the isotopic compositionhas been extensively
plex, and samplesCP-426-1 and 2 are dolomite changedin rocks which on the basis of both
clasts from the Kingston Peak formation, col- field relations and petrographic examination
lected about 1000 feet stratigraphically above might have been classedas post-metamorphic
the World Beater complex. CP-151 seems to hornblendediorite dikes. The contactsare sharp
have a normal Sr compositionfor dolomite of and the dikes are undeformed over distances of
that age.CP-426-1 alsohas a normal Sr isotopic up to a mile. The absenceof zoningor of twincomposition,but CP-426-2 showsan enrichment ning in the plagioclaseand the lack of alterawhich is considerablygreater than could be ex- tion might suggesta metamorphicorigin, but
plainedby the in situ decayof Rb. The matrix the 'diabasic' texture and the lack of clinozoiof the conglomeratecontainsabundant biotite site might argue against a metamorphic origin.
formed during the 100- to 150-m.y. metamor- It is only by the correlation and petrographic
phism and the excessradiogenicSr of the clast comparison with the unmetamorphosed but
highly altered 'Pahrump diabase'that the metamay be very locallyderived.
4400
WASSERBURG,
ALBEE, AND LANPI-IERE
morphic origin of the mineral assemblageis
in the different minerals. While it might be pos-
made clear.
sible to account for the Sr •* anomalies in the dike
Conclusions. Excessive ages were obtained rocks by the addition of pure Sr• during metain this study on total-rock samplesof dioritic morphism,followedby subsequentlocal homogcomposition in a Precambrian terrane which enization,it is more reasonablein the light of
has undergoneMesozoicmetamorphism.Many the demonstratedmobility of Sr on the local
samplesyielded ages that are patently absurd. scale to suggestthat the enrichment occurred
Genetically related rocks of similar lithology by a similar metasomaticexchangeprocessbut
from relatively undisturbed areas give initial on a somewhatlarger scale.If the anomaliesin
Srs'/Sr•øratioswhichare normal,indicatingthat Sr*7/Sr•6are due to addition or exchangeof
it is reasonable to assme that the initial Sr
averagetotal-rock Sr of the basementcomplex
isotopic composition of the anomalous rocks with normal Sr of the diorite dikes, an addition
was normal. To obtain reasonable ages from of about 20 ppm would be required. This
these total-rock data it would be necessaryto amount representsa significantcontribution to
assumeinitial Sr87/Sr86
ratios which are far in the presumedoriginal Sr content and is possiexcessof that reported for any igneous rock, bly indicative of grossmetasomaticchangesin
either marie or felsic in composition.The data the major chemical constituents.Despite this
for the total rocks and some of their constituent
isotopicevidenceof exchange,the dike contacts
mineral phasesindicate that only a relatively are sharp and give no suggestionthat such an
short time (about 108 yr) could have elapsed exchangehas occurred.
since the mineral constituents had the same
The isotopically disturbed systems occur in
Sr87/Sr86
composition.For CP-155, in particular, an area of Mesozoic middle-grade metamorpha maximum age of only 200 m.y. can be calcu- ism which involved a relatively old Precambrian
lated for a total-rock-plagioclaseisochron,us- basementcontaininghighly radiogenicSr. These
rocks show no definitive evidence of metasomaing the extreme possibleerrors.
The data for the individual mineral phases tism, as deduced from field and petrographic
demonstratethat the processcertainly involved studies. The significanceof these results is that
the transport of Sr. The relative abundanceof they demonstrate that total-rock systemsmay
Rb and Sr in these rocks is comparable with be open during metamorphism and that these
that found in undisturbed dioritic rocks, and systemsmay take on a Sr isotopic composition
to attribute the observed
which is completely secondarywithout the coneffects to the migration of Sr•. The anomalous' current production of gross petrographic efenrichments of Sr 8' which were observed corfects. It is clear that the Sr isotopic composirespondto the addition of up to i ppm of pure tion of a total-rock samplemay not be assumed
to be indicative of the original sourceof igneous
radiogenicSr8'.
From the study of the World Beater complex and metamorphic rocks unlessit can be demon[Lanphere et al., 1963, 1964], it is clear that a strated that this ratio is unaffectedby secondchemical exchange processhas occurred on a ary processes.Also, any Rb/Sr age based on
local scale (about 10 cm), sincethe Sr has been an assumedinitial Sr value may be suspect,and
isotopically equilibrated between the mineral there is, in general, a need to demonstratethe
phaseswithin the augen gneissand the granite. initial value for such age measurements.
it is not unreasonable
The mineral phaseswere in isotopic equilibrium
with one another and have equilibrated with
the local total-rock
Sr reservoir
at about
100
Acknowledgments. We would like to thank
Mr.
B. Troxel
of the California
Bureau
of Mines
for his very kind efforts in obtaining somesamples
m.y. It therefore follows that relatively unhind- of diabase.
ered Sr mobilization took place locally for all
This work was supported by grants from the
phases in these rocks during metamorphism, National Science Foundation and the Atomic
rather than the simple occlusionof radiogenic Energy Commission.
Sr by particular phases.The data on the mineral
t•EFERENCES
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MIGRATION
OF RADIOGENIC
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the Panamint Range, California, during Cretaceous time (abstract) Geol. Soc. Am. Spec.
Paper 73 (1962 meetings), p. 193, 1963.
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and G. R. Tilton, Redistribution of Sr and Rb
isotopes during metamorphism, World Beater
complex, Panamint Range, California, Isotopic
and Cosmic Chemistry, North-Holland Publishing Company, Amsterdam, 1964.
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4401
Silver, L. T., C. R. McKinney, and L. A. Wright,
Some Precambrian agesin the Panamint Range,
Death Valley, Calif. (abstract), Geol. Soc. Am.
Spec.Paper 68, p. 55, 1961.
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Wright, Ages in the Precambrian terrane of
Death Valley, California, J. Geol., 67, 702, 1959.
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Potassium determinations on amphiboles by
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(Manuscript received April 20, 1964;
revised July 2, 1964.)