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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. References Alonso-Perez, R., Müntener, O. and Ulmer, P., 2009. Igneous garnet and amphibole fractionation in the roots of island arcs: experimental constraints on H2O undersaturated andesitic liquids. Contrib. Miner. 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