Inhibited eclogite formation: The key to the rapid growth of strong and buoyant Archean continental crust

M.G. Bjùrnerud Geology Department, Lawrence University, Appleton, Wisconsin 54912, USA H. Austrheim Physics of Geological Processes Project and Department of Earth Sciences, Postbox 1047, University of Oslo, Oslo N-0316, Norway

ABSTRACT Subduction is the principal mechanism by which the hydrosphere and interior of Earth interact. Today, subduction involves the dehydration of ocean crust at depths of 60±120 km depending on the age of the slab. Release of the water leads to generation of arc magmas (future continental crust), and the slab is then transformed into denser eclo- gite that helps to pull more of the slab into the trench. However, it is unlikely that the ®rst continental crust formed this way. Growing geochemical evidence indicates that large volumes of continental crust were produced over a short period of time in the Archean, when the planet was probably too hot for modern plate tectonics to operate. A signi®cant increase in the kinetics of eclogite-forming reactions may have been the key to the tran- sition from Archean to modern tectonics. Under the higher geothermal gradients of the Archean, tectonically buried ocean crust would have been severely dehydrated before reaching eclogite facies pressures. Because rapid eclogitization is dependent on water as a medium for advective ion transport, the very shallow dehydration in the Archean may have inhibited the formation of eclogite facies minerals. The importance of water in eclo- gite metamorphism is illustrated by a complex of partly eclogitized ma®c granulites in Holsnùy, western Norway, in which reaction progress was limited by the availability of water. When water is scarce or absent, metastable granulite facies mineral assemblages can persist at eclogite facies depths owing to the extremely slow reaction kinetics when diffusion is the only chemical transport mechanism. Such dehydrated but uneclogitized ma®c crust would have been very strong and too buoyant to sink into the mantle, and it may have formed the substrate for the ®rst continental lithosphere.

Keywords: Archean tectonics, continental crust formation, eclogite, slab dehydration, kinetics of metamorphic reactions.

ARCHEAN TECTONICS change to subduction-dominated tectonics may have been an acceler- Two of the most fundamental unresolved questions about the solid ation in the rates of eclogite-forming reactions, related to the cooling phase Earth are (1) when subduction-dominated plate tectonics began of Earth and associated changes in the extent and depth of slab de- and (2) howÐand how early in the planet's historyÐthe ®rst signi®- hydration near the Archean-Proterozoic boundary. cant volumes of continental crust formed. These questions are closely In modern subduction zones, the dehydration of downgoing ocean related. New continental crust forms principally in arc settings where, crust is dependent on the manner in which a subducting slab ``drags'' at depths of 60±120 km, water is released from hydrous minerals in its surface isotherms with it as it descends. The depths at which the subducting ocean slabs and acts as a ¯ux for the overlying mantle slab begins to dehydrate and later recrystallize to eclogite are artifacts wedge, producing low-temperature melts. The slab is eventually con- of Earth's modern thermal regime. In Archean time, when Earth's geo- verted to dense eclogite, which then helps to pull more ocean crust thermal gradient was higher, the physics of the process would have down into the mantle for dehydration and transformation. Eclogite for- been quite different. First, mantle convection would have been more mation is arguably the essential component in Earth's self-sustaining vigorous, and slabs would have been younger, hotter, and thicker than tectonic system, setting the pace for subduction and preventing the they are today upon arrival at a subduction zone (or the Archean equiv- crust from becoming too thick (Anderson, 1989). However, this mech- alent) (Davies, 1992). Second, a subducted or tectonically buried slab anism for incremental production of continental crust does not explain would have encountered signi®cantly higher temperatures at any given the growing evidence for a burst of crustal formation in Middle Ar- depth than is true today. Geophysical models and the geochemistry of chean time, or possibly earlier (Bowring and Housh, 1995; Wilde et Archean peridotites and komatiites suggest that the upper mantle was al., 2001; Hamilton, 2003), nor is it consistent with the higher radio- 200±400 ЊC hotter than present-day asthenosphere (Hamilton, 2003; genic heat production and higher geothermal gradients in Archean Breuer and Spohn, 1995; Herzberg, 1993). Under these higher Archean Earth, which would probably have precluded modern-style subduction temperatures, ocean crust being buried via protosubduction or crustal (Davies, 1992; Vlaar et al., 1994; Hamilton, 2003; Martin and Moyen, thickening (Fig. 1) would have encountered granulite facies conditions 2002). That is, most of Earth's original continental crust appears to (700±900 ЊC at 0.5 to ϳ1.3 GPa) before being subject to eclogite facies have formed long before the onset of true plate tectonics (Rudnick, pressures (Ͼ1.3 GPa). Even under the comparatively cold conditions 1995). The key to both the early formation of continental crust and the in modern subduction zones, the slab-dehydration process is highly

᭧ 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; September 2004; v. 32; no. 9; p. 765±768; doi: 10.1130/G20590.1; 2 ®gures. 765 may have been common in Archean time. Although these rocks resided for perhaps several millions of years at eclogite facies conditions, eclo- gite mineral assemblages occur only in patches and zones, mainly along fractures and in reaction fronts emanating from them (Fig. 2A). This observation suggests that the dry granulite persisted metastably under eclogite pressures until fracturing allowed the in®ltration of wa- ter that then facilitated the transformation to eclogite through advective ion transport (Austrheim and Grif®n, 1985; Boundy et al., 1997; Jamt- veit et al., 2000; Bjùrnerud et al., 2002). The Holsnùy eclogites are within the Bergen arcs, a stack of ar- cuate thrust sheets of Proterozoic gneisses and lower Paleozoic ophiol- ite and island-arc complexes that were emplaced onto the margin of the Baltic craton during the formation of the Scandinavian Caledoni- des. The eclogites occur near the western edge of the largest exposed allochthonous sheet, the LindaÊs nappe. The island of Holsnùy is un- derlain by Middle(?) Proterozoic anorthositic, gabbroic, and locally pe- ridotitic rocksÐperhaps originally a layered ma®c intrusive complexÐ that were subject to temperatures of 800±900 ЊC and pressures of ϳ1.0 GPa ca. 945 Ma (Austrheim and Grif®n, 1985; Boundy et al., 1997). This granulite facies event left the rocks almost completely dehydrated, as indicated by nearly anhydrous mineral assemblages: garnet, Al-rich diopside, and plagioclase (An45±60), together with minor amounts of orthopyroxene, hornblende, and scapolite (Austrheim and Grif®n, 1985). Loss on ignition (LOI) values for the granulite rocks are Ͻ0.2 wt%, and the rocks may have been even drier immediately following the metamorphic peak. The rocks on Holsnùy cooled by an unknown amount between 945 Ma and ca. 420 Ma, when tectonic burial subjected them to pres- sures of Ͼ1.5 GPa (Ͼ50 km) and temperatures of 650±700 ЊC, as Figure 1. A: Schematic drawing showing how shallow dehydration of recorded by the eclogite facies assemblages omphacite (Jd50), garnet, thickened ma®c crust under higher geothermal gradients of the Ar- kyanite, and clinozoisite (Austrheim, 1987). The conversion of the chean may have built strong, buoyant lower crust and also led to formation of tonalite-trondhjemite-granodiorite (TTG) magmas that are rocks from granulite to eclogite was virtually isochemical except for typical of Archean terranes. P, TÐpressure, temperature. B: Contrast- the addition of an aqueous ¯uid in a concentration of one volumetric ing P-T-time paths (wide arrows) for modern subducted slab vs. Ar- part in 107±108 (Jamtveit et al., 1990). However, the granulite mineral chean tectonically buried crust. Ellipses show approximate ranges of assemblage persisted in ϳ50% of the rock volume, and the transfor- metamorphic facies ®elds (P-PÐprehnite-pumpellyite facies). Thin black arrow shows partial P-T history of Holsnùy rocks, which un- mation of the rock to eclogite was localized and heterogeneous, limited derwent granulite facies conditions ca. 945 Ma and eclogite conditions almost entirely by the availability of water (Engvik et al., 2000; ca. 420 Ma. Bjùrnerud et al., 2002). The anhydrous Holsnùy granulites were apparently very strong prior to their conversion to eclogite. They contain pseudotachylyte ef®cient. Studies using cerium as an analogue for hydrogen (and thus water content) in ocean basalts have shown that 97% of the water is sheets, some traceable for hundreds of meters, in which eclogite facies extracted from a subducting slab before it reaches the middle of the minerals occur as microlites (Austrheim and Boundy, 1994). This ev- upper mantle (Dixon et al., 2002). In the hotter world of the Archean, idence for frictional melting indicates that the granulites, while residing the dehydration process would presumably have been even more ef®- metastably at eclogite conditions in the lower crust, stored large cient, and would have occurred at a shallower depth. amounts of strain energy until a series of massive earthquakes fractured the complex. These seismic events appear to have set in motion the ECLOGITIZATION AS A HYDRATION PHENOMENON long-delayed eclogitization process by allowing water to enter the Today, eclogite formation is related to progressive dehydration of rocks via macroscopic fractures. Reaction rates that had previously a subducting slab, although in detail the reaction mechanisms are more been limited by diffusion were suddenly increased by orders of mag- complex. First, the reaction rates are slowed by the coldness of the nitude in the presence of a medium for advective ion transport (Rubie, slab, and large pressure oversteps are required before signi®cant con- 1986; Baxter and DePaolo, 2000). Once the conversion to eclogite version occurs (Kirby et al., 1996). Second, although the mineral phas- began, it was an initially self-perpetuating process, because the asso- es that are diagnostic of the eclogite facies are not hydrous, water is ciated volume reduction (ϳ10%) gave rise to a network of microfrac- critical in facilitating the essential eclogite reaction albite → jadeite ϩ tures that allowed water to make further inroads into the rock mass quartz (Hacker, 1996). Consequently, eclogite reactions tend to occur (Jamtveit et al., 2000). Ultimately, however, the process was self- at depths where still-hydrous gabbros release water and rehydrate al- limiting, because the hydrated eclogite was mechanically much weaker ready dry gabbro on the outer part of the slab (Kirby et al., 1996). In than the dry, rigid granulite (Fig. 2B). Once the volume of granulite the Archean, slabs would have been hotter and diffusion rates faster, fell below a critical value (ϳ70%), ductile eclogite limited the bulk but thorough dehydration would have happened before a slab ever rock strength, so that fracturing was no longer possible and ¯uids could reached eclogite conditions, and this circumstance would likely have not readily reach unconverted rock (Bjùrnerud et al., 2002). An inter- inhibited the formation of eclogite phases. esting implication of this change in rheology is that once a lower- A remarkable complex of partly eclogitized granulites in Holsnùy, crustal rock mass is thoroughly dehydrated, it may be impossible to western Norway, provides a possible analogue for a phenomenon that rehydrate it completely. What remained when no more water could

766 GEOLOGY, September 2004 Figure 2. A: Band of eclogitized rock (dark, upper left to lower right) on either side of meter-scale fracture in ma®c granulite, Holsnùy, western Norway. Compositional foliationÐlayers of garnet and pyroxene alternating with plagioclase-rich domainsÐdeveloped during granulite metamorphism and can be traced across eclogitized zone. Shorter transecting fractures may be caused by volume reduction associated with formation of dense eclogite phases. Pencil is 15 cm long. B: Eclogite shear zone showing strong rheological contrast between ductile eclogite (right) and dry, brittle foliated granulite (left). Reduction in rock strength upon conversion to eclogite may limit extent of further eclogitization, because once strong granulite bodies are separated from each other, they no longer form stress- supporting framework capable of large-scale fracturing, and no new conduits can carry water necessary for eclogite reactions. Lens cap is 5 cm in diameter. enter the rocks was a partly eclogitized granulite mass too buoyant to mantle material overlies the downgoing plate. Collisions between oce- be subducted. anic plateaus could have produced areas of particularly thick crust that may have formed the substrates for the ®rst continents (Stein and Gold- WATER-STARVED ECLOGITE REACTIONS LED TO stein, 1996). ARCHEAN CONTINENTS One implication of this style of convergence is that the pressure- The Holsnùy rocks may have relevance for Archean Earth. Even temperature-time path of a tectonically buried rock on its way to the if subduction in the modern sense was not possible in Archean time, eclogite facies ®eld would have been very different in Archean time there had to be some process complementary to the production of new than it is today (Fig. 1B). While modern oceanic slabs typically pass ocean crust at upwelling convective sites (primitive spreading centers). through the low-temperature, high-pressure blueschist ®eld before un- Whether by shallow subduction or collision and thickening, some dergoing eclogite facies conditions, Archean ocean crust probably tra- ocean crust would have become tectonically buried and, under the high versed the amphibolite and granulite ®elds en route to the eclogite geothermal gradients of the Archean, would have been dehydrated at facies. The rarity of blueschists and eclogites older than 2 Ga is con- depths shallower than those at which slab dehydration occurs today. sistent with this hypothesis, although it could also be an artifact of Geochemical studies of the tonalite-trondhjemite-granodiorite (TTG) preservation (MoÈller et al., 1995). The Holsnùy rocks demonstrate that suites that are typical of Archean terranes (Condie, 1981; Wyllie et al., the history of a rock mass can have an important effect on the way 1997) show that the source melts for TTG rocks older than 3.0 Ga did the rock responds to subsequent metamorphic conditions. If ma®c not interact with the mantle (Smithies, 2000; Martin and Moyen, 2002). rocks arrive in the eclogite ®eld with very little water, they may be These analyses suggest that prior to Late Archean time, convergence unable to equilibrate to the new pressure conditions and will retain between slabs of ma®c crust occurred by stacking or low-angle under- lower-density, metastable mineral assemblages. The Holsnùy complex thrusting (Fig. 1A), not subduction sensu stricto, in which a wedge of suggests that such thoroughly dehydrated rocks may never transform

GEOLOGY, September 2004 767 entirely to eclogite even if external water later becomes available, be- Austrheim, H., and Boundy, T., 1994, Pseudotachylytes generated during seis- cause the conversion to eclogite progressively reduces rock strength, mic faulting and eclogitization of the deep crust: Science, v. 265, p. 82±83. which eventually suppresses the formation of large-scale fractures that Austrheim, H., and Grif®n, W., 1985, Shear deformation and eclogite formation act as pathways for water to reach the interior of the rock mass. This within granulite facies anorthosites of the Bergen arcs, western Norway: point should be taken into consideration in models that invoke density- Chemical Geology, v. 50, p. 267±281. driven delamination as a mechanism for crustal recycling in Archean Baxter, E., and DePaolo, D., 2000, Field measurements of slow metamorphic reaction rates at temperatures of 500 to 600ЊC: Science, v. 288, time (Rudnick, 1995; Zegers and van Keken, 2001). p. 1411±1414. 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