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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Modelling
  5. Slab dehydration
  6. Discussion
  7. Implications
  8. Acknowledgements
  9. References

Terra Nova, 25, 130–136, 2013

Abstract

High-Al2O3 tonalite, trondhjemite and granodiorite (TTG) magmas characterise felsic Archaean crust, yet are uncommon in the post-Archaean rock record. Consequently, understanding the petrogenesis of these rocks provides valuable insights into early Earth processes. Fluid-absent slab melting represents the dominant hypothesis for the origin of these rocks; however, the absence of voluminous magmas of intermediate composition formed concurrently with these TTGs is incompatible with expectations of slab water loss prior to slab melting. This study demonstrates that for reasonable Archaean mantle temperatures, slab-derived water is captured by an anatectic zone near the slab surface, which melts via reactions that consume quartz, clinopyroxene and water to produce high-Al2O3 Archaean trondhjemite. Late in the Archaean, the mantle cooled sufficiently to prevent wet melting of the slab, allowing slab water to migrate into the wedge and produce intermediate composition magmatism, which has since been associated with subduction zones.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Modelling
  5. Slab dehydration
  6. Discussion
  7. Implications
  8. Acknowledgements
  9. References

The Palaeo- to Meso-Archaean felsic crust is dominated by tonalite, trondhjemite and granodiorite (TTG) series rocks (Jahn et al., 1981; Martin et al., 1983), and in particular, the ‘high-Al2O3’ type of TTG (i.e. Al2O3 > 15 wt% at SiO2 = 70 wt%) (Barker and Arth, 1976). The bulk of Earth’s old cratonic crust formed around nuclei of TTG composition, thus these rocks probably formed Earth’s oldest continental crust. Consequently, studies focused on the petrogenesis of these granitoids form the basis for our understanding of the creation and early evolution of Earth’s continental crust. TTG granitoids are uncommon in the post-Archaean rock record. This suggests that a fundamental change occurred on Earth towards the end of the Archaean Eon, which changed the composition of new magma additions to the continental crust from sodic, leucocratic TTG magma to magmas of intermediate composition (andesites and diorites) (e.g.Shirey and Hanson, 1984).

Archaean TTG granitoids are characterised by high Na : Ca and Na : K ratios (e.g. Moyen, 2011) and are therefore distinct from the calcalkaline magmas, which typify post-Archaean arcs (Martin, 1994). This study focuses on the high-pressure (HP) variety (Moyen, 2011) of the high-Al2O3-type TTGs. These rocks are characterised by high Sr content, as well as high La/Yb and low Nb/Ta ratios, which are, respectively, interpreted to indicate the absence of plagioclase, a substantial fraction of garnet (>15%), and the presence of rutile, in the residuum from which magmas separated (Martin and Moyen, 2002; Rapp et al., 2003; Schmidt et al., 2004; Moyen and Stevens, 2006). Consequently, these magmas are interpreted to arise by partial melting of an eclogite facies metabasalt (Rapp et al., 2003; Moyen, 2011).

Various different geodynamic scenarios have been proposed for the formation of Archaean TTG magmas (e.g. Zegers and van Keken, 2001; Foley et al., 2003; Bédard, 2006); however, the required high pressure of partial melting has been interpreted to indicate their formation via anatexis of the upper portions of slabs within Archaean subduction zones (Condie, 1981; Martin, 1994, 1999; Smithies and Champion, 2000; Rapp et al., 2003; Smithies et al., 2003) as a consequence of higher Archaean mantle temperatures (Rudnick, 1995; Albarède, 1998; Prelevic and Foley, 2007).

Melting has generally been considered to occur through fluid-absent processes (e.g. Foley et al., 2002; Rapp et al., 2003; Moyen and Stevens, 2006). However, in contrast with the Phanerozoic rock record, there is little evidence to suggest that slab fluids metasomatise the overlying mantle wedge during the Archaean, as no significant volume of andesites or diorites are commonly produced concurrently with Palaeo- to Meso-Archaean TTGs. Thus, fluid-absent melting fails to account for the substantial amount of water likely to be lost from the subducting slab prior to the conditions for fluid-absent anatexis being attained (Schmidt and Poli, 1998). A potential solution to this problem is to consider the possibility of TTG production by fluid-present melting, as proposed by Drummond and Defant (1990). This notion is supported by recent experimental findings: Laurie and Stevens (2012) produced trondhjemitic melts via water-present melting of an eclogite facies metabasalt, which are very similar in trace and major element composition to the HP-type TTG (Fig. 1). Melting in these experiments occurred via the reaction: Qtz + Cpx1 + Grt1 + H2O = Melt + Cpx+ Grt2, with the involvement of omphacitic clinopyroxene as a reactant being important in shaping both the characteristic major and trace element signature.

image

Figure 1.  The major and trace element chemistry of experimental glasses (red circles) produced via water-present melting of eclogite facies metabasalt (red star) from Laurie and Stevens (2012), compared with the compositions of high-pressure (HP)-type Archaean tonalite, trondhjemite and granodiorite (TTG) defined by Moyen (2011) (grey diamonds). (a) The Ab(albite)-An(anorthite)-Or(orthoclase) ternary feldspar diagram (O’Connor, 1965) with the granitoid fields defined by Barker (1979). Gr-granite; Tdj-trondhjemite; Ton-tonalite; Grd-granodiorite; (b) Ni (p.p.m.) vs. SiO2 (wt%); (c) Sr/Y vs. Y (p.p.m.); (d) La/Yb vs. Yb. La and Yb are normalised to chondrite (Thompson, 1982). The experimental melt compositions are trondhjemitic and are characterised by light rare earth element (LREE) enrichment; heavy rare earth element (HREE) and Y depletion; and have high Ni contents for a leucocratic melt; they are very similar in composition to the natural TTGs.

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This study evaluates the viability of water-fluxed eclogite melting to produce HP-type trondhjemites within likely Archaean subduction scenarios using numeric and metamorphic models to predict the pressure–temperature evolution of different layers within the subducting slab as well as water release from within the slab.

Modelling

  1. Top of page
  2. Abstract
  3. Introduction
  4. Modelling
  5. Slab dehydration
  6. Discussion
  7. Implications
  8. Acknowledgements
  9. References

Given the significant uncertainty in many of the parameters that govern the dynamics and thermal structure of assumed Archaean subduction systems (including slab age, subduction velocity, mantle wedge temperature, thermal structure of the overriding plate, latent heat from thermodynamic reactions, and the composition and volume of the magmas produced), a simple modelling approach using first order parameters for a hypothetical Archaean subduction scenario was applied to test the viability and implications of water-fluxed slab melting. The P–T profiles of the upper portions of subducting slabs were modelled as a function of mantle temperature [from assumed present-day mantle temperature of 1350 °C (e.g. van Hunen and Moyen, 2012) to 1650 °C], age of the subducting crust and subduction rate. These parameters are considered the primary factors, which control the P–T paths followed by subducted crust (Peacock et al., 1994).

Modelling setup

Citcom (Moresi and Gurnis, 1996; Zhong et al., 2000) was used to produce a model of the thermal structure within the subduction zone, using a setup similar to that of model 1a in van Keken et al. (2008) and the results are benchmarked against those produced by the van Keken et al.’s (2008) study. Thus, the study uses an analytical corner flow mantle and slab flow model, which is coupled to a numerical temperature advection–diffusion solver; however, the thermal structure of the inflowing slab on the left-hand size boundary was rotated to the slab dip angle. The model resolution used was 2.5-km finite element size. Modelled slab temperatures using this simple scenario are lower than those produced using similar flow models with kinematically prescribed slab motion, but with more complex mantle wedge rheology (e.g. van Keken et al., 2002; Syracuse et al., 2010). Fully dynamic slab models, such as Arcay et al. (2007), on the other hand, show similarly cooler slab temperatures to those produced in this study. We adopt this simple mantle rheology model and note that slab temperatures should be considered as lower estimates of possible values. Furthermore, the profiles of present-day subduction zones, which have been modelled using more complex mantle wedge rheology by Syracuse et al. (2010) and van Keken et al. (2011), are illustrated in Fig. 2bi for comparison.

image

Figure 2.  Modelled PT paths followed by the upper portion of subducting slabs. Bold lines represent the upper slab interface surface and dashed lines represent the slab at a depth of 2.5–2.8 km below the slab surface. (a) Upper slab PT paths within hypothetical Archaean subduction zones modelled by applying a simple mantle rheology model and an overriding lithospheric thickness of 50 km (this study). (b) Upper slab PT paths modelled by applying a complex mantle rheology model and a slab mantle decoupling until the depth of 80 km (Syracuse et al., 2010; van Keken et al., 2011). (i) Mantle temperature is maintained at modern approximate temperatures (1350 °C) (Moyen and van Hunen, 2012) and slab age and subduction speed (v) are varied while all other parameters are kept constant. Disregarding different model parameters, the models in (a) and (b) are similar and indicate that the profiles of young slabs are hotter than those of older slabs (e.g. brown vs. green profiles); slow subduction rates produce hot upper slab profiles with weak internal temperature gradients (red profiles); and fast subduction rates produce upper slabs profiles with prominent temperature inflections and strong internal thermal gradients in the upper portion of the slab (green and brown profiles). (ii) The PT paths of upper slabs from reasonably young and fast subduction zones modelled for increased mantle temperatures. This is proposed to be the configuration which would satisfy water-fluxed slab melting during the Archaean Eon to produce high-pressure-type trondhjemite. WBS-Wet basalt solidus within plagioclase stability limits represents the haplotonalitic solidus (an = ∼50) (Johannes and Holtz, 1996) and at pressures above plagioclase stability, the wet (fluid-present) K-free metamafic solidus is extrapolated from Kessel et al. (2005).

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Results

The results of the modelling (Fig. 2ai) indicate that the P–T profiles followed by the upper portions of subducting slabs are dependent on both the age of the slab and the speed of subduction. Younger crust is always warmer and the speed of subduction strongly controls the shape of the P–T paths. Fast subduction produces a strong temperature inflection of the slab-surface paths towards higher temperatures, whereas the paths of slab interiors show only moderate temperature increase. The temperatures of slow subducting slabs increase approximately consistently over the entire pressure range, with only a very small temperature inflection of the slab surface. Disregarding differences in model parameters, the modelling results are essentially similar to those by Peacock et al. (1994) and cooler yet similar in shape to those of Syracuse et al. (2010) and van Keken et al. (2011) (Fig. 2bi).

Within reasonably young and fast subduction zones, the upper slab surface undergoes near-isobaric heating on progression past the wedge tip and remains the hottest portion of the slab, whilst the underlying slab interiors remain cool. This may provide the most appropriate subduction profile for water-fluxed melting of the upper slab, as the cool hydrated slab interiors may release fluid to flux melting within the hot upper slab due to a warmer Archaean mantle wedge. Although young and slow subduction produces the hottest slabs, this configuration will not satisfy water-fluxed slab melting as slab interiors will dehydrate shortly after the upper slab and when the upper slab reaches temperatures sufficient for water-present melting, there will be no water to flux slab melting. Consequently, the P–T paths for young and fast subduction were modelled for increased mantle temperatures and the resultant slab P–T path shift to higher temperatures (Fig. 2ii).

Slab dehydration

  1. Top of page
  2. Abstract
  3. Introduction
  4. Modelling
  5. Slab dehydration
  6. Discussion
  7. Implications
  8. Acknowledgements
  9. References

Water-present melting processes have been argued against on the grounds of the very low porosity of high-grade metamorphic rocks (Peacock et al., 2005). In essence, this follows conventional thinking on the fluid-state during anatexis of the continental crust; however, during the Archaean, fast subduction would create pronounced temperature inversion within the upper portions of slabs on progression past the wedge tip (Fig. 2ii). Oceanic crust is known to be hydrated to considerable depths (2–3 km with 3–5 wt% H2O) (Bickle et al., 1994; Alt and Teagle, 2000; Dick, 2000) and the likely degree of hydration for an Archaean subducting slab is proposed to be similar to that of modern crust (e.g. Kusky et al., 2001; Furnes et al., 2007). Thus, anatexis of the hottest portions of fast Archaean slabs may indeed be driven by ascending water that is derived from dehydration reactions in deeper slab rocks.

Information on water release by the slab, extracted from a pseudosection (modelled metamorphic phase diagram), predicts that all mineral-bound water (16 mol.% or 5 wt%) within the upper 2.8 km of the slab is released as the slab is heated between 450 and 700 °C (Fig. 3). Details of the pseudosection modelling are provided in the caption to Fig. 3. Dehydration reactions produced within the slab are temperature-dependent, due to near-isobaric upper slab heating, and include the breakdown of chlorite, lawsonite, zoisite and amphibole, which release 5 mol.%, 5 mol.%, 2 mol.% and 4 mol.% water, respectively, as they leave the assemblage.

image

Figure 3.  A comparison between Archaean and present-day fast subduction of young slabs. (a) Modelled young (5 Ma) and fast (10 cm a−1) subducting Archaean slab assuming a 1650 °C mantle with a simple mantle wedge rheology. (b) Modelled young (10 Ma) and fast (7.5 cm a−1) Archaean subducting slab assuming a 1650 °C mantle with a complex mantle rheology. (c) Modelled young (5 Ma) and fast (10 cm a−1) subducting slab with proposed present-day mantle temperature (1350 °C). (i) The PT paths of the slab surface and the slab at 2.8 km below the slab for each of the three examples (a, b and c). (ii) The thermal structure model of the subduction zone for each of the three examples (grey, dashed lines). Strong temperature gradients exist in the upper portions of the slab, near the slab interface (black line) and produces hot anhydrous upper slab portions that overlie cooler hydrated slab portions beyond the base of the lithosphere of the overriding plate (horizontal black line). In both the Archaean examples (hotter mantle), the upper portions of the slab are above the relevant water-present solidus whilst the underlying slab portions dehydrate. This metamorphic water rises and fluxes melting of the upper slab. In the present-day mantle scenario, the slab is cooler and does not reach temperatures sufficient for water-present or water-absent melting to occur within reasonable depths. Slab dehydration occurs at deeper levels and is more sparsely distributed within the subduction zone compared with within Archaean slabs. Consequently, slab fluids interact with mantle rocks deeper within the wedge that are above their water-present solidus and induce water-fluxed mantle melting. The metamorphic facies boundaries (green lines) and water isopleths (blue dashed lines) are extracted from a Na-Ca-Fe-Mg-Al-Si-H-Ti-O pseudosection (metamorphic equilibrium phase diagram), which was constructed for a basaltic composition (Laurie and Stevens, 2012). The pseudosection was calculated using the software program PERPLEX (Connolly, 1990, 2005; Connolly and Petrini, 2002), which used the Holland and Powell (1998) thermodynamic dataset. Solid solution models: Gt(HP), Cpx(HP), Opx(HP), TiBio(HP), Am(DHP), feldspar, Ep(HP), Chl(HP) and IlGkPy. 5 wt% water was assumed in the bulk composition and 5% of iron as Fe2O3, to allow for epidote stability at lower PT conditions. g, garnet. Grt-in (Green, 1982). Rt-in (Zamora, 2000). Plag-out (Holland, 1980). DBS-Green (1982). WPS-Wyllie (1979).

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Modelling
  5. Slab dehydration
  6. Discussion
  7. Implications
  8. Acknowledgements
  9. References

The results of the thermal and metamorphic modelling suggest that during the Archaean, when Earth’s mantle was significantly hotter (e.g. Tm = 1650 °C), fast subducting slabs were characterised by a hot and anhydrous upper portion, which was above the temperature of the relevant water-present eclogite solidus (Kessel et al., 2005) and that this zone was underlain by cooler hydrous metamorphic rocks capable of undergoing substantial dehydration (blue-shaded area Fig. 3a). Consequently, interaction of the metamorphic fluid with the overlying eclogitised slab triggered water-present melting via reactions of the type investigated by Laurie and Stevens (2012) (red-shaded area Fig. 3a) to produce HP-type Archaean TTG. The same conclusions can be drawn from slab models created using more complex mantle wedge rheology (Fig. 3b).

This mechanism provides a regenerating configuration for water-present slab melting that may result in large integrated melt volumes. This is due to the continual supply of new hydrated crust overlain by a new fertile anatectic zone, by the subduction process and allows zones of slab anatexis to function as capture sites for water, which is released from the slab. This accounts for the lack of evidence for Archaean water-induced mantle wedge magmatic products (e.g. andesites or diorites) forming concurrently with Palaeo- to Meso-Archaean HP-type TTG granitoids. The slab dehydrates (blue-shaded area in Fig. 3) at shallow levels within Archaean subduction zones as a consequence of hotter mantle temperatures and thus only metasomatises a limited portion of the mantle wedge at shallow levels in the subduction zone (green-shaded area Fig. 3). This portion of the mantle is not hot enough for the water to induce mantle wedge melting. Dehydration curves (blue-dashed curves in Fig. 3) are at shallow angles to the slab surface, which promotes water-fluxed melting as ascending slab fluid must traverse the hotter portions of the slab that are above their water-saturated solidus.

The model predicts that fluid-absent melting of amphibole, as proposed by many previous studies, is impossible in this scenario as amphibole reacts out of the assemblage prior to anatectic temperatures being reached. This is consistent with experimental findings on amphibole melting behaviour in metabasalts (e.g. Wyllie and Wolf, 1993). Consequently, if the slab is to melt at attainable temperatures, it must melt by water-present reactions. In addition, the upper portions of the slab are likely to become quartz-depleted, as a consequence of fluid-present melting, making the slab unable to produce further felsic melts via dry-melting at greater depths.

Within the scenario modelled in this study, melting is limited by quartz and mineral modes derived from the pseudosection modelling suggest that the source is capable of producing ±20 wt% (±35 vol.%) trondhjemitic melt. This is consistent with melt production estimated from quartz-free mantle eclogite xenoliths considered to be the counterparts to melting in late Archaean subduction zones (Jacob and Foley, 1999; Barth et al., 2001) and with the results of experiments investigating the water-present melting of eclogite at relevant pressures (Laurie and Stevens, 2012). If the anatectic zone is assumed to extend 0.5 km below the slab surface and the subduction rate is 10 cm a−1 (100 km Ma−1), then the volume of trondhjemitic melt which can be produced per km of arc length is 17.5 km3 Ma−1. These production rates appear consistent with the volume of the arc-derived Meso-Archaean trondhjemites, which were emplace over 60 Ma within the Barberton granitoid-greenstone terrain in South Africa at c. 3.23 Ga (Moyen et al., 2006; Kisters et al., 2010). This suggests that the melting mechanism proposed by this study provides a viable explanation for volumous Archaean HP-type TTG production.

Implications

  1. Top of page
  2. Abstract
  3. Introduction
  4. Modelling
  5. Slab dehydration
  6. Discussion
  7. Implications
  8. Acknowledgements
  9. References

The geometries of P–T paths of the upper portions of fast subducting slabs and the water-present metabasaltic solidus are similar as they both inflect towards higher temperatures at pressures coincident with the wedge tip (Fig. 3). This suggests that a reasonably small decrease in the temperature of the mantle wedge towards the end of the Archaean, which causes the isobaric heating of the slab surface, will induce the abrupt shift away from Archaean TTG magmatism. On cooling of the mantle, slab melting, which formerly acted as a capture mechanism for metamorphic slab fluid, no longer occurred as slabs seldom reached temperatures sufficient for water-present melting. For the first time, slab dehydration fluids, which were now produced deeper within the subduction zone (Fig. 3cii), migrated into the mantle wedge. This mantle material had previously been metasomatised by slab melts, which had not managed to traverse the wedge (orange-shaded area Figs. 3aii and bii). Interaction of this metasomatised mantle with slab water acted to spawn an initial pulse of water-induced melting, which produced enriched mantle melts (sanukitoids) (e.g. Martin et al., 2009; Rapp et al., 2010) and from this time frame onwards, arc-related intermediate calcalkaline magmatism characterised arc settings and arc-related continental growth products (Fig. 3c).

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Modelling
  5. Slab dehydration
  6. Discussion
  7. Implications
  8. Acknowledgements
  9. References

This research was supported by NRF funding to Gary Stevens as part of the SARChI program. JvH was supported by the European Research Council (ERC StG 279828). We thank P.E. van Keken for providing access to models for various model subduction scenarios. S.F. Foley and P.E. van Keken are thanked for providing constructive reviews that were helpful in improving the manuscript.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Modelling
  5. Slab dehydration
  6. Discussion
  7. Implications
  8. Acknowledgements
  9. References
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