Download Terra Nova 2012 Jagoutz

doi: 10.1111/ter.12010
Were ancient granitoid compositions influenced by
contemporaneous atmospheric and hydrosphere oxidation states?
Oliver Jagoutz
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, MA 021394307, USA
ABSTRACT
A feedback loop between subducted oceanic plate composition,
subduction zone processes, arc magma chemistry, and the
newly formed continental crust composition links to atmospheric and hydrosphere oxidation conditions through the low
temperature weathering mechanisms of seafloor basalts. Seafloor weathering and the Na ⁄ K of the altered oceanic crust
strongly depends on f(O2) conditions during alteration, that
changed with earth history. The rise of oxygen at 2600–
Introduction
Geologists are acquainted with the
principle
of
Ôuniformitarianism,Õ
which holds that present day processes
are the key to those that operated in
the past. But the extent this applies to
the processes driving the growth and
differentiation of the continental crust
throughout the Earth history remains
a major controversy in earth sciences.
An important part of the discussion
circles around the predominance of
sodium-enriched
tonalite-trondhjemite-granodiorite (TTG) upper continental crustal rocks in Archean times,
compared to dominant potassiumenriched granite-granodiorite (GG)
associations in the post-Archean
upper continental crust. The transition between TTG and GG dominated
upper crust occurred approximately
around the Archean ⁄ Proterozoic
(A ⁄ P) boundary (i.e. 2400–2500
Ma) and has been suggested to mark
the most pronounced compositional
change of the earthÕs crust in the
geological record (Condie, 1993). In
recent years it has become evident that
the A ⁄ P boundary is also associated
with the rise of oxygen in the hydrosphere and atmosphere. Molybdenum,
sulfur and carbon isotopes indicate
Correspondence: Dr. Oliver Jagoutz,
Department of Earth, Atmospheric, and
Planetary Sciences, Massachusetts Institute
of Technology, 77 Massachusetts Ave.,
Cambridge, Massachusetts, MA 021394307, United States. Tel.: 001 617 324
5514; e-mail: [email protected]
2012 Blackwell Publishing Ltd
2400 Ma triggered associated changes in f(O2) seafloor alteration conditions providing a possible explanation for the
observed change from the Na-rich tonalite-trondhjemite-granodiorite (TTG) rocks in the Archean to the K-rich granodioritegranite (GG) granitoids in post-Archean times.
Terra Nova, 25, 95–101, 2013
the rise of oxygen occurred 2700–
2350 Ma (Farquhar et al., 2000; Holland, 2006; Anbar et al., 2007; Wille
et al., 2007) possibly indicating a yet
poorly understood feedback loop between environmental conditions and
compositional evolution of the upper
continental crust.
General consensus exists that plate
tectonic theory explains the mechanism leading to the present-day GGtype upper crust. For the origin of the
TTGs, however, the importance of
this mechanism, and of plate tectonics
in general, has been questioned and
strongly modified or entirely different
crust forming processes have been
proposed for the Archean (e.g.
Condie, 2005; Martin et al., 2005;
Hamilton, 2007). Accordingly, understanding the dominance of TTG rocks
in the Archean and their reduced
importance after the A ⁄ P boundary
will provide important insights into
the evolution of continental crust
formation mechanism through time.
Previous work on the origin of the
TTG rocks dominantly focused on
their trace element characteristics and
more specifically on their high Sr ⁄ Y
and fractionated MREE ⁄ HREE,
which generally have been interpreted
to indicate the involvement of garnet
in their formation (Hanson et al.,
1971; Arth and Hanson, 1972). As
garnet could be either a residual or
cumulative magmatic phase involved
in three different location ⁄ processes in
a subduction system, three different
explanations have been put forward to
explain TTGs (Martin, 1986; Drummond and Defant, 1990; Smithies,
2000; Müntener et al., 2001; Kleinhanns et al., 2003; Alonso-Perez et al.,
2009). However, none of these models
provides a conclusive explanation for
the major element difference between
Archean TTG and post-Archean GG.
Based on an extensive compilation
of experimental melt compositions,
Moyen and Stevens (2006), have inferred that the Na concentration of
the melt is dependent on the pressure
of melting, with high pressure melts
have high Na concentration. The K
concentration of the melts however
are largely dependent on the source
composition and the degree of melting
(Moyen and Stevens, 2006). In Fig. 1,
the Na ⁄ K systematics of partial melting experiments is compared to the
Na ⁄ K of their respective source. The
Na ⁄ K in the melt is strongly dependent on the Na ⁄ K of the source
whereas the pressure and temperature
effects are less significant. This observation questions the popular interpretation that the switch from TTG to
GG type rocks coincides with transition from subduction of hot shallow
dipping slabs in the Archean to
steeper dipping colder ones in the post
Archean.
Alternatively, the Na ⁄ K of the
source of the granitoids could have
varied through time. One possibility is
that the overall maturity of the sedimentary record changed at the A ⁄ P
boundary (Engel et al., 1974), which
ultimately must reflect changes in the
95
Celadonite • O. Jagoutz
Terra Nova, Vol 25, No. 2, 95–101
.............................................................................................................................................................
Pressure
intervals [kbar]
0 to 10
10 to 20
20 to 30
30 to 40
Temperature intervals
750 to 900 °C
900 to 1000 °C
1000 to 1100 °C
1100 to 1200 °C
1
0.1
1
Na/Kmelt/Na/Ksource
Na/Kmelt
10
Na/Kmelt/Na/Ksource
100
Temperature intervals
750 to 900 °C
900 to 1000 °C
1000 to 1100 °C
1100 to 1200 °C
0.1
10
100
1000
Na/Ksource
0.1
0.01
0.01
1
1
700
900
1100
0
10
20
30
40
P [kbar]
T [°C]
Fig. 1 Na ⁄ K relationship of melts and starting material of partial melting experiments (compiled by Moyen and Stevens, 2006).
The strongest relationship is observed between the source composition and the derived melts, whereas temperature and pressure
have less influence on the Na ⁄ K systematics of the melts.
source region, i.e. the upper crust,
generating to some extent a circular
argument. Additionally it is not obvious how this explanation can explain
the apparent relatively abrupt change
in rock composition.
The aim of this paper is to investigate
how much of the apparent compositional differences can be explained by
assuming that continental crust formed
in the Archean by similar subduction
process as today, but under different
environmental f(O2) conditions.
I present a new hypothesis to
explain the difference in major and
incompatible element chemistry between Archean and post-Archean
rocks and propose a correlation to
the transition with the rise of oxygen.
I propose that the difference between
the two rock suites ultimately relates
to the difference in the atmospheric
f(O2) through a feedback cycle associated with oxic and anoxic (in the
sense of less and more anoxic) alteration mechanisms in the oceanic crust.
As will be discussed in detail later, the
proposed model could explain the
observed change in Na ⁄ K independent of the ultimate formation mechanism of the granitoids (i.e. partial
melting vs. fractional crystallization).
Comparison between present day
arc related and Archean granitoids
The compositional evolution of rocks
through time has been frequently
investigated (for a review see Condie,
96
1997) and the aim is not to reproduce
these results here. Instead I compare
the general characteristics of Archean
TTG and GG rocks with modern arc
GG-type granitoid compositions to
understand the important chemical
differences between the two rock
sequences. Figure 2 illustrates the
average composition of the Archean
TTG and GG sequence compared to
modern average granitoid compositions of island arcs, continental arc
and Archean cratons and the bulk post
Archean granitoid composition from
CondieÕs (Condie, 2008) compilation.
Archean TTG and GG sequences differ significantly in their major and
trace element chemistry. As discussed
before, TTGÕs are generally depleted in
the incompatible element concentrations and in the heavy rare earth
[(H)REE] compared to GG rocks.
The GG are also enriched in high field
strength elements (with the exception
of Ti) and in the incompatible major
elements especially in K2O but are
depleted in Na, Mg, Fe and Ca compared to TTG. The average compositions of plutonic rocks from various
cratons are intermediate between
Archean TTG and GG, but are clearly
dominated by TTG rocks. These
chemical differences are in general
agreement with previous results from
Moyen (2011) and Condie (1993).
An important result of this comparison is that modern arc granitoids and post-Archean granitoids
are indistinguishable within uncer-
tainty from Archean GG-type rocks.
Accordingly, the change in upper
crustal rock composition at the A ⁄ P
boundary is not related to the appearance of a new rock type, or the
chemical evolution of a common rock
association. Rather the A ⁄ P is related
to a switch in the dominant rock series
from TTG-type to GG-type rocks, but
both series are preserved in Archean
rocks (Condie, 2008). In contrast, in
post Archean times GG-type rocks are
strongly dominant and Archean-type
TTG-type rock associations are rare.
Discussion
Based on trace element similarities it is
generally accepted that the GG association, and thereby the bulk postArchean upper continental crust, was
formed in a subduction zone setting,
in which oceanic lithosphere is subducted beneath a younger oceanic or
continental lithosphere. The question
remains whether the observed change
in chemistry between GG and TTG
rocks be reconciled with subduction
zone processes or does it require the
existence of a different crust forming
process in the Archean? The depletion
of the HREE is only indicative of the
involvement of garnet in the formation of the TTG rocks. It neither
supports nor contradicts a subduction
origin for these rocks. It is crucial to
understand the remaining changes in
major element chemistry in order to
understand the possible implications
2012 Blackwell Publishing Ltd
O. Jagoutz • Celadonite
Terra Nova, Vol 25, No. 2, 95–101
.............................................................................................................................................................
2
Normalized to average Archean TTG composition
1
0
SiO2
Al2O3
FeO
MgO
CaO
K2O
Na2O
2
1
post Archean GG
Archean GG
0
Ti
Ta
Nb
Zr
Hf
average continental arc granitoid
average island arc granitoid
average cratonic granitoid
4
3
2
1
0
Ba U Nb Ce Sr Hf Sm Gd Y Lu Ni Cr
Rb Th Ta La Pb Nd Zr Eu Dy Yb Sc Mn V
Fig. 2 Comparison between Archean TTG and GG granitoids and post-Archean
granitoids (all rocks are SiO2 ‡ 65 wt%). Archean GG type rocks are very similar to
modern granitoids from convergent margins (shown is the average continental and
oceanic margin granitoid). Archean GG rocks and post-Archean rocks are significantly enriched in K2O and depleted in Na2O compared to Archean ones. The
average Archean craton composition is dominated by the TTG characteristics
documenting the volumetric dominance of TTGÕs during the Archean. Detailed data
sources and average values plotted are given in the electronic appendix.
of the observed change between TTG
and GG. To do so it is useful to first
briefly review the processes that form
granitoids in arcs and what controls
their incompatible element budget.
The origin of the granitoid crust in
arcs, that ultimately produce the mod 2012 Blackwell Publishing Ltd
ern day upper continental crust, remains poorly understood, and the
dominantly
discussed
formation
mechanisms can be grouped in two
endmember processes that likely operate simultaneously. The evolved compositions of the shallow level plutonic
rocks and their volcanic equivalents
are thought to be either formed by
hydrous medium- to high-pressure
crystal fractionation (e.g. Sisson et al.,
2005; Davidson et al., 2007; Jagoutz,
2010) or by partial melting of a previously hydrated (i.e. amphibolised)
gabbroic (i.e. basaltic) rocks in the
lower arc crust (e.g. Tamura et al.,
2009; Moyen, 2011). In either formation mechanism (fractional crystallization
or
partial
melting)
the
incompatible element budget of the
granitoids is controlled by the incompatible element concentration of the
basaltic protoliths ⁄ melt, derived from
primitive arc melts, and the
restite ⁄ cumulate mineral assemblage.
The incompatible element budget of
arc melts is largely controlled by the
so called slab-derived component
(McCulloch and Gamble, 1991). In
subduction zones, due to increasing
pressure and temperature conditions,
the subducted slab undergoes a sequence of metamorphic dehydration
reactions (Schmidt and Poli, 1998).
These reactions produce a hydrous
slab-derived component (melts, fluids
or supercritical liquids), that is expelled from the subducted material,
carrying large quantities of incompatible elements, which then-migrates
upwards into the overlying mantle
wedge triggering ÔfluxÕ melting to
produce primitive arc melts (Tatsumi
et al., 1986; McCulloch and Gamble,
1991). Accordingly, the element budget of primitive arc melts is controlled
by the relative contribution from the
slab as well as the mantle in its
formation.
Estimates indicate that the mantle
contribution controls the major element chemistry (e.g. Si, Al) whereas
the composition of the fluid released
from the slab exerts a significant
control on the incompatible element
budget of Proterozoic ⁄ Phanerozoic
arc magmas (e.g. Na, K, Rb, Ba, U,
Th etc) and thereby on the incompatible element budget of magmatic arc
granitoids. It is estimated that >90%
of incompatible elements like K and
Na in arc magmas are derived from
the subducted slab (Grove et al., 2002;
Jagoutz et al., 2007). Important carriers of potassium in modern subducted
oceanic lithosphere are subducted
sediments, which have on average 2 wt% K2O and around 2.4 wt%
Na2O but generally only a limited
97
Celadonite • O. Jagoutz
Terra Nova, Vol 25, No. 2, 95–101
.............................................................................................................................................................
Low temperature alteration
mechanism of the oceanic crust:
the role of clay minerals
Qualitative field studies have documented that in the so-called seawater
weathering zone of Gillis and Robinson (1988), the low temperature alteration mineralogy in oceanic basalts
varies significantly with the Fe3+ ⁄
Fe2+and thereby with the oxygen
fugacity (f(O2)) of the system. The
presence of pyrite (FeS) in the alteration assemblage is indicative of
anoxic ⁄ euxinic conditions, whereas
the occurrences of Fe-oxides or Feoxyhydroxides (FeO(OH)) indicate
more oxidizing conditions during
alteration. Clay mineralogy varies
consistently with the presence of pyrite or Fe-oxides: During more oxidizing alteration conditions celadonite (a
K-Fe3+rich mica) forms, in addition
to saponite and is the dominant
K-bearing clay mineral (e.g. Andrews,
1980). As celadonite can have up to
10 wt% K2O, low temperature
alteration under more oxidizing conditions can enrich sea-floor basalts by
up to a factor of 800 in K2O compared
to the values of fresh MORB (Fig. 3).
The excess K2O is generally sequestered from the ocean water such that
seafloor alteration provides a significant sink for the riverine K2O flux into
the ocean (e.g. Andrews, 1980). As
micas can incorporate significant
amounts of Rb, similar relationships
are observed for Rb vs Fe3+ ⁄ FeT. In
contrast, under anoxic conditions,
98
0.008
Rb/Ti
K2O/TiO2
4
2
0
0.004
0
0
0.2
0.4
0.6
0.8
1
Fe3+/FeTot
0
0.2
0.4
0.6
0.8
1
Fe3+/FeTot
4
Na2O/TiO2
thickness (<100 m) (Plank and Langmuir, 1998). In terms of alkali major
elements, the basaltic oceanic crust
and mantle lithosphere are originally
slightly depleted in K2O, with typical
MORB having K2O concentration of
0.1 wt% and Na2O 2.4 wt%.
Alteration, however, affects the uppermost few hundreds of meters of the
basaltic oceanic crust and can result in
significant enrichment in K2O (see
below). Clay minerals play a dominant role in the fixation of alkali
elements in altered basalt. The stability of clay minerals depends among
other parameters on the f(O2) and pH
of the system (e.g. Alt and Honnorez,
1984) and that could provide a viable
explanation of the change in
K2O ⁄ Na2O between TTG and GG
associated with the rise of oxygen.
2
0
0
0.2
0.4
0.6
Fe3+/Fe
0.8
1
Tot
Fig. 3 Interrelation between Fe3+ ⁄ FeT and alkali element concentration in altered
oceanic basalts. Symbols correspond to different DSDP ⁄ ODP drill cores (triangles =
Site 417A; stars = 417D; diamonds = 504B) colors correspond to the presence of
different clay minerals in the alteration assemblage (red: saponite + celadonite, green:
Saponite only, orange: chlorite). Data source is (Alt and Honnorez, 1984; Bach et al.,
2003). The strong correlation between K, Rb and Fe3+ ⁄ FeT (normalized to Ti to
account for fractionation) is dominated by the presence of celadonite. In a saponite
only alteration environment, seafloor alteration does not as much modify the K
concentration of altered oceanic crust. Increasing f(O2) favors the stability of
celadonite + saponite over saponite only. The blue bar represents typical range of
unaltered basalts.
celadonite is consistently absent and
saponite
(a
Na-Mg-Fe2+
rich smectite) is the dominant Kbearing phase. As saponite has significantly lower K2O concentrations
(<<2 wt%) compared to celadonite,
basalt altered under more anoxic conditions is only enriched in K2O by a
factor of 8–10 compared to average
MORB. The strong dependence of
K2O sequestration in altered seafloor
due to the dependence of the stability
of saponite and celadonite on f(O2)
conditions, is documented by the relationship between Fe3+ ⁄ FeT and K2O
in altered basalts (Fig. 3). By contrast,
the Na uptake is essentially independent of the oxidizing conditions and
the Na2O concentration of the altered
oceanic crust and similar to that of a
fresh MORB (Fig. 3), although very
oxidized rocks seem to have a slight
decrease in Na2O resulting in a negative correlation between K2O and
Na2O. The dependence of the stability
of saponite and celadonite on the
f(O2) and their importance for alkali
fixation provide a direct link between
environmental f(O2) and the Na ⁄ K
composition of the altered oceanic
crust. Based on this link differences
in environmental f(O2) could explain
the compositional difference between
TTG and GG that seem to coincide
with the rise of oxygen. The transition
between TTG and GG is likely to
have occurred over a considerable
time interval although available data
document this transition dominantly
to have occurred at the A ⁄ P boundary
2012 Blackwell Publishing Ltd
O. Jagoutz • Celadonite
Terra Nova, Vol 25, No. 2, 95–101
.............................................................................................................................................................
contemporaneously with the (initial?)
rise of oxygen in the hydrosphere and
atmosphere. I speculate that the transition from the dominance of Na-rich
TTG rocks to K-rich GG rocks ultimately relates to changes in the f(O2)
conditions in the seafloor weathering
zone reflecting the rise of oxygen
(Fig. 4). During the Archean, low
temperature seafloor alteration was
generally more anoxic and dominated
by celadonite-absent controlled alkali
fixation. As a result, the upper part of
the subducted oceanic crust was not
enriched in K2O. The lack of K2O
enrichment in the altered basaltic
crust resulted in a low K ⁄ Na slab-
derived component in Archean subduction zones. Based on the generally
low K ⁄ Na in the subducted slab,
granitoids formed in Archean subduction zones, regardless of formation
mechanism, would be dominantly Narich TTG granitoids. With the rise of
oxygen, the oceanic crust became
increasingly more altered under more
oxidizing conditions and celadonite
dominated alteration zones became
more abundant. This resulted in a
change of the alkali-fixation budget of
the altered oceanic crust, which became strongly enriched in K2O. Due
to the subduction zone processes, this
change in K ⁄ Na is translated into the
late Archean
Arc crust
TGG-Type dominated
granitoids
reducing atmosphere
High standing ridge crests?
anoxic ocean
low K2O/Na2O
arc melts
FeS+Saponite dominated
nt
e
O
n
2
Na po
altered oceanic crust
O/ m
with low K2O/Na2O
K 2 d co
w
Lo rive
de
b
early/middle Proterozoic
sla
oxydized atmosphere
GG-Type granitoids
High standing ridge crests?
Flux melting
Oxidized shallow ocean
mildly oxidized oceans ?
high K2O/Na2O
arc melts
Flux melting
O
a2
nt
e
on
N p
O/ m
K 2 co
gh ed
Hi eriv
d
lab
FeO(OH)+Celadonite dominated
alterd oceanic crust
with high K2O/Na2O
present day
s
oxydized atmosphere
Oxidized ocean
low K2O/Na2O
arc melts
Flux melting
t
en
O
a 2 on
/N mp
o
c
gh ed
Hi eriv
d
b
sla
O
K2
alterd oceanic crust
with high K2O/Na2O
FeO(OH)+Celadonite dominated
Fig. 4 Schematics of the model proposed to explain the major element differences
between Archean TTG and Proterozoic ⁄ Phanerozoic GG rocks. Top) During
Archean time saponite dominated alteration of the seafloor prevailed due to the
generally more reducing conditions. The Na ⁄ K of the subducted slab controls the
Na ⁄ K of the arc granitoids. Middle) With increasing availability of free O2 celadonite
alteration become more prominent and saponite only dominant alteration diminished. More oxidized alteration could have been important along high standing mid
ocean ridges in the upper ocean or in a slightly more oxidized deeper ocean. Bottom)
Presently the deep oceans are generally well oxidized producing dominantly arc
granitoids with low Na ⁄ K ratio.
2012 Blackwell Publishing Ltd
post-Archean upper crustal granitoids, which are dominated by K-rich
GG granitoids.
In addition, evidence that different
oxidizing conditions might have dominated the Na and K geochemical
equilibrium between seawater and
ocean crust comes from estimates of
the seawater composition. Inclusion
studies have indicated that the Archean seawater was 90% enriched in K
and 50% depleted in Na (DeRonde
et al., 1997).
Did the rise of oxygen in the
atmosphere change ocean
oxygenation conditions?
The redox evolution of the early
Proterozoic ocean water in response
to the rise of oxygen in the atmosphere is still ill-constrained. Whereas
it is generally accepted that the upper
ocean became oxic during or before
the rise of oxygen in the atmosphere,
the conditions in the deep ocean
remain uncertain and estimates range
from mildly oxic to sulfidic (Canfield,
1998; Holland, 2006). The appearance
of celadonite-dominated alteration is
in line with a mildly oxic deep ocean.
Even under anoxic ferruginous conditions iron enrichment in ancient sediments occurs by the formation of
ferric oxyhydroxides (e.g. Poulton and
Canfield, 2011), which in the basalticocean water system, are generally
associated with celadonite-dominated
alteration. Accordingly, the proposed
change in alteration mechanism does
not conflict with the proposed ferruginous conditions of the deeper
oceans.
Alternatively, it has been proposed
that the average ridge depth of the
mid oceanic ridge system correlates to
the average degree of melting that
produced the basaltic oceanic crust
(higher degrees of melting correspond
to shallower ridge depths, Klein and
Langmuir, 1987). It has been proposed that average mantle potential
temperature, and therefore the average degree of melting, was higher in
the earlier part of earth history (e.g.
Korenaga, 2006). Accordingly, the
global ridge depth structure of the
Archean and Proterozoic oceanic
crust could have been significantly
more shallow than today (e.g. Galer,
1991). In that case it is feasible that a
large part of the ridge system reached
99
Celadonite • O. Jagoutz
Terra Nova, Vol 25, No. 2, 95–101
.............................................................................................................................................................
into oxygenated shallow ocean water
(e.g. Nisbet, 1984) where the transition from saponite to celadonite dominated alteration could have occurred.
Testing the hypothesis
Experimental studies aiming at understanding the f(O2) dependence of the
ocean water MORB system are essential to test the proposed hypothesis as
the f(O2) of the system. The anticipated mechanism, in accordance with
qualitative observations from the oceanic crust, predicts that the alteration
of seafloor basalts is dependent on
the f(O2) of the system. In particular,
the stability field of celadonite in the
ocean water-MORB system should
depend on f(O2) and the pH, which
are coupled by the reaction:
4Fe2þ þ O2 þ 6H2 O< >FeOOH
þ8Hþ
The f(O2) threshold that controls
the transition from saponite dominated to celadonite dominated alteration is unknown but could provide
independent constraint on the f(O2)
evolution of the ocean water. The role
of clay minerals in the fixation of trace
elements is still poorly constrained
and should provide further insight
into the role of variable environmental
f(O2) conditions in seafloor alteration
and ultimately the composition of the
continental crust.
Acknowledgement
Kevin Burke is thanked for stimulating discussion. This work was supported in parts by NSF EAR 0910644.
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Received 4 January 2012; revised version
accepted 20 September 2012
Supporting information
Additional Supporting Information
may be found in the online version
of this article:
Table S1 Average composition of
granitoids with SiO2 ‡ 65 wt%.
101