The subduction factory: Its role in the evolution of the Earth s mantle

Transcription

1 The subduction factory: Its role in the evolution of the Earth s mantle Yoshiyuki Tatsumi and Tetsu Kogiso Research Program for Geochemical Evolution, Institute for Frontier Research on Earth Evolution (IFREE) Introduction Subduction zones, where the oceanic lithosphere is foundering into the Earth s interior, have been working as factories and have contributed significantly to the evolution of the solid Earth. Raw materials, such as pelagic/terrigenous sediments, oceanic crust, and mantle lithosphere, are supplied into the factory (Fig. 1). In the process of transportation and processing of these raw materials, the factory causes vibrations as earthquakes. The major products of the factory are arc magmas and their solidified materials, continental crust. Subduction zones are creating >20% of the current, terrestrial magmatic products and have formed 7.35 ~10 9 km 3 of andesitic crust throughout the Earth s history (Taylor and McLennan, 1995). Although the continental crust occupies less than 1% of the total mass of the solid Earth, the origin of such a differentiated component should provide a clue to understanding of the evolution of the Earth. The waste materials processed in the subduction factory, such as chemically modified lithospheric materials and delaminated lower continental crust, sink into the deep mantle (Fig. 1). Basaltic oceanic crust with an average thickness of 7km has constantly been accumulating somewhere in the deep mantle. Assuming that such basaltic materials could be stored at the base of the mantle for the last 3 billion years, the volume of such a layer, with a composition different from both the overlying peridotitic mantle and the underlying metallic core, would be ~ km 3, occupying ~4% of the total mass of the solid Earth. Such voluminous basaltic materials may thus be regarded as the anti-crust at the base of the mantle. Water should have been transported from the surface to the interior of the Earth with the sinking oceanic lithosphere. Although H 2 O bound in altered oceanic crusts and subducting sediments is largely recycled to the surface through fluid migration in accretionary prisms and arc magmatism, the sinking oceanic crust could carry ~1wt% of H 2 O to depths of >200km (e.g., Poli and Schmidt, 1995), indicating that ~ kg of H 2 O, about a quarter of the present sea water, has been injected into the mantle. The presence of such H 2 O should have profound effects on the rheological property of the mantle and have governed the dynamic processes in the Earth s interior. This report will discuss the role of subduction factories in the formation of geochemical reservoirs in the Earth mantle. Evolution of geochemical reservoirs in the deep mantle The presence of variable components within the mantle is well established from geochemical studies on MORBs and ocean island basalts (OIBs). The isotopic compositions of such lavas are suggestive of the presence of at least four end-member components or geochemical reservoirs (DMM, EMI, EMII, and HIMU; Zindler and Hart, 1986), in addition to the primitive mantle (Fig. 2). Among these, DMM is distinct in its isotopically depleted signature, and the other three s enriched signatures. The origin of enriched mantle components is essential for understanding the dynamic processes and evolution of the deep mantle, as such components typify magmas rising from deep-seated hotspots. It has been repeatedly suggested that subducted crustal materials significantly contribute to these enriched end-member components (e.g., Chase, 1981; Hofmann and White, 1982; Chauvel et al., 1992; Kogiso et al., 1997), although some authors argued against such scenarios (Kamber and Collerson, 2000; Rudnick et al., 2000). Here we review the chemical differentiation processes in subduction zones relevant to the isotopic evolution of sinking materials and discuss the role of the subduction factory in the evolution of the deep mantle. Oceanic crusts and sediments: origin of HIMU and EMII reservoirs The upper-crustal portion of subducting oceanic lithosphere mainly consists of MORB-like basalts. The upper portion of the basaltic oceanic crust is hydrated and altered by hydrothermal processes at mid-oceanic ridges and through mechanical fracturing before subduction. However, the rest of the crust remains anhydrous and may keep its original compositions until it reaches the subduction zone. Since the solidus temperature of anhydrous basalt is higher than estimated temperature distributions along the foundering lithosphere (Yasuda et al., 1994; Pertermann and Hirschmann, 1999), the fresh (anhydrous) part of basaltic oceanic crust does not melt upon subduction and thus sinks into the deep mantle without changing its composition. In contrast, the hydrated part of basaltic crust dehydrates to release fluids with increasing pressures and temperatures until all hydrous minerals in the crust break down (e.g., Schmidt and Poli, 1998). Also, hydrated basaltic crust may partially melt if the slab temperature is high enough to cross the solidus of hydrous basalts (Peacock, 1993; Rapp and Watson, 1995). Therefore at least three compositionally different materials should be regarded as basaltic components that are transported, by plate subduction, into the deep mantle: fresh MORB, melting and dehydration residues of hydrous MORB. Since basaltic oceanic crust is generated by partial melting of a depleted MORB source mantle (DMM) at mid-oceanic ridges, the trace element signatures of the subducting fresh MORB crust can be estimated from those of DMM. MORB crust may possess higher Rb/Sr and lower Sm/Nd ratios than its source DMM peridotites, because the order of compatibility 59

2 to mantle peridotites (spinel or garnet lherzolies) is Rb<Sr<Nd<Sm (Green, 1994; Hauri et al., 1994; Blundy et al., 1998). Although the partition coefficient of U between melt and mantle minerals, especially garnet and clinopyroxene, is highly variable (Watson et al., 1987; Hauri et al., 1994), it is smaller than 0.1 (Pertermann and Hirschmann, 1999), and largely identical to that of Pb (Hauri et al., 1994). Consequently, U/Pb ratio of MORB crust is likely to be close to that of the DMM source. It may be thus concluded that long-term residence of fresh MORB crust in the mantle results in higher 87 Sr/ 86 Sr, lower Nd/ 144 Nd, and similar 206 Pb/ 204 Pb values compared with DMM (Fig. 2), suggesting that accumulated MORB crusts may contribute to the isotopic diversity of the mantle. However, characteristic high 206 Pb/ 204 Pb ratios for the HIMU reservoir cannot be explained solely by the involvement of the MORB crust. Several lines of evidence compel a significant majority of researchers to believe that the subducting oceanic crust does not melt in most normal subduction zones in the modern Earth (e.g., Tatsumi and Eggins, 1995). However, in subduction zones with unusually high-temperature conditions, partial melting of the sinking hydrous basaltic crust may take place. This might develop where a young and hot lithosphere subducts (Drummond and Defant, 1990; Peacock, 1990; Furukawa and Tatsumi, 1999) and thus may have existed more widely during the Earth s early history (Martin, 1986). Furthermore, slab melting and subsequent melt-mantle interactions may produce andesitic magmas having major and trace element compositions largely similar to those of the bulk continental crust (Kelemen, 1995; Tatsumi, 2000b). It is therefore interesting to assess the complementary formation of the continental crust and the mantle geochemical reservoirs. Tatsumi (2000b) examined this process, based on geochemical formulation of partial melting and melt-solid reactions and demonstrated that the Sr-Nd-Pb isotopic compositions of melting residues in the subducting slab, which may have foundered and been stored in the deep mantle, do not match those of any proposed geochemical reservoir. Dehydration reactions within subducting hydrated basaltic crust occur continuously from very shallow levels to over 300km depth (e.g., Schmidt and Poli, 1998), but experimental studies on trace element behavior during dehydration are almost limited to those related to the amphibolite-eclogite transformation (Kogiso et al., 1997) and element partitioning between aqueous fluids and garnet/clinopyroxene (Brenan et al., 1995a, 1995b; Keppler, 1996; Ayers et al., 1997; Stalder et al., 1998). A notable feature demonstrated by these experiments is that Pb is more preferentially partitioned into H 2 O fluids than U and Th, leaving the residue, after dehydration, to have higher U/Pb and Th/Pb ratios than its original compositions. It is also demonstrated that Rb and Nd are released from the subducting crust more readily than Sr and Sm (Brenan et al., 1995b; Keppler, 1996; Ayers et al., 1997; Kogiso et al., 1997). Thus, residual basaltic crust post the amphiboliteeclogite transformation will have lower 87 Sr/ 86 Sr, higher Nd/1 44 Nd, and higher 206 Pb/ 204 Pb ratios than hydrated basaltic crust (Fig. 2). 206 Pb/ 204 Pb ratios of the dehydrated residue are likely to be significatly greater than that of the HIMU component, implying that subducted dehydrated basaltic crust may contribute to the genesis of this mantle component (Brenan et al., 1995b; Kogiso et al., 1997). The Sr and Nd isotopic evolution of the dehydrated crust is dependent on Rb/Sr and Sm/Nd ratio changes during partial melting at mid-oceanic ridges and dehydration reactions in subduction zones. Although it is difficult to estimate quantitatively, suitable parent/daughter ratios to produce HIMU-like Sr and Nd isotopic ratios could be explained through the above two processes including accumulation of both fresh and dehydrated MORB crusts (Fig. 2). The role of subducting sediments in the formation of EMII, one of the enriched geochemical reservoirs in the mantle, has been emphasized by several authors because oceanic sediments generally have high 87 Sr/ 86 Sr and relatively low Nd/ 144 Nd ratios (e.g., Devey et al., 1990; Weaver, 1991). However, oceanic sediments that are subducted into the mantle contain significant amounts of hydrous phases, all of which will decompose to release fluids, ultimately causing significant fractionation of trace elements through such fluid migration (Aizawa et al., 1999; Johnson and Plank, 1999). Experiments on sediment dehydration by Aizawa et al. (1999) has demonstrated that ancient subducted oceanic sediments, while experiencing compositional modification in the subduction factory, may evolve to an enriched component having high 87 Sr/ 86 Sr and 206 Pb/ 204 Pb ratios. They further indicated that the isotopic signature of the EMII component can be achieved by the addition of small amounts (~1wt.%) of dehydrated sediments to DMM-like mantle or primitive mantle (Fig. 2). Delaminated pyroxenite: origin of EMI Trace element modeling (Tatsuni, this volume) suggests that the geochemical characteristics of bulk continental crust can be reasonably explained by mixing of mantle-derived basaltic and crust-derived felsic magmas. In order to make an andesitic continental crust, the melting residue after extraction of felsic melts should be removed and delaminated from the initial crust. It is thus interesting to examine the isotopic evolution of a delaminated anti-crust component, based on inferred parent-daughter element concentrations, and to compare such signatures with those of the mantle reservoirs. Seawater is characterized by an extremely high 87 Sr/ 86 Sr ratio of and a rather high abundance of Sr (8ppm). Its incorporation into the amphibolitic oceanic crust through alteration processes at oceanic ridges must strongly affect the Sr isotope composition of the subducting oceanic crust. Furthermore, it is well known that seawater Sr isotope ratios have changed through time, with values decreasing with increase in age (e.g., Veizer and Compston, 1976). We have evaluated the effect of amphibolisation at ridges on the Sr isotope compositions of an oceanic crust by the method proposed by Tatsumi (2000a). The temporal variation in seawater compositions is formulated for the last 2.5 b.y. using the following equation which is based on the data by Veizer and Compston (1976): ( 87 Sr/ 86 Sr) t = t (Ga). For t >2.5Ga, the ratio is assumed to be constant at The Nd and Pb isotope ratios of oceanic crust were assumed to be unchanged during the alteration processes at ridges and those ratios of an ancient subducting amphibolite were calculated based on both the estimates of primitive mantle compositions (Sun and McDounough, 1989) and the isotopic ratios in the present bulk 60

3 mantle. The isotopic compositions of primary arc basalt magma can be obtained from those of the subducting amphibolite, of the original mantle (primitive mantle), and the aforementioned formulation. An important factor for the evaluation of the isotopic evolution of melting residues in the initial basaltic crust is the degree of segregation of felsic melts from partially molten crust. The viscosity of a partial melt is one of the most critical parameters governing the velocity of melt migration. A felsic melt may have a viscosity of about 2 orders of magnitude higher than a basaltic melt under the same hydrous conditions. It is thus suggested that perfect separation of such viscous melts from the solid restite is unlikely. Although quantitative estimation of the degree of melt separation is difficult, the present modeling assumes that the slab restites contain 0-15% of the trapped melt component, that possess a composition identical to an extracted felsic melt. The results of the calculation are shown in Fig. 3, together with the isotope compositions of the geochemical reservoirs in the mantle. Quite distinct evolutionary curves with large variations in isotope ratio are obtained, due to differences in the degrees of involvement of the felsic partial melt with the residuum, i.e., the delaminated component. However, a pyroxenite with a ~15% felsic melt component can reasonably explain the EMI isotopic signature. Simple mixing of the bulk silicate Earth component, which is likely to occupy the deep mantle, and a 3-4 billion-years-old delaminated component could form the EMI component. Conclusion The subduction factory is the major site of injection of surface materials into the Earth s interior. The raw materials are processed into magmas, which cause characteristic arc volcanism and makes continental crusts through complex processes including remelting of an initial basaltic crust and magma mixing. The waste materials, such as chemically modified subducting sediments/crusts and melting residues delaminated from the initial crust, have been accumulated and are likely to have evolved into enriched geochemical reservoirs in the deep mantle. Magmas, which tap such mantle components, are erupting at hotspots where mantle plumes are rising from the deep mantle. Recycling of surface materials through subduction factories and mantle plumes may have been playing the central role in the evolution of the solid Earth. References Aizawa, Y., Y. Tatsumi, and H. Yamada, Element transport during dehydration of subducting sediments: implication for arc and ocean island magmatism, The Island Arc, 8, 38-46, Ayers, J. C., S. K. Dittmer, and G. D. Layne, Partitioning of elements between peridotite and H2O at GPa and degrees C, and application to models of subduction zone processes, Earth Planet. Sci. Lett., 150, , Blundy, J. D., J. A. C. Robinson, and B. J. 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5 Figure 1. The processes occurring in the subduction factory. Raw materials, such as oceanic sediments, oceanic crust, and mantle lithosphere, are fed into the factory and are manufactured into arc magmas and continental crust. The waste materials processed in this factory, such as chemically modified oceanic crust/sediments and delaminated lower continental crust, sink into the deep mantle and are likely to have greatly contributed to the mantle evolution. Figure 2. Variation of isotopic compositions of subducted fresh MORB, dehydrated residue of hydrous MORB and sediments. Ages of subduction are shown with symbols and lines. Compositions of the fresh MORB are calculated with Rb/Sr and Nd/Sm ratios 1% (upper curve) to 10% (lower curve) higher than the MORB source. U/Pb ratios of the fresh MORB are assumed to be same as the source. The MORB source is assumed to be derived from the primitive mantle at 4.0Ga with parent/daughter ratios which changed continuously from 4Ga to present. Present isotopic composition of the MORB source is: 87 Sr/ 86 Sr=0.7026, Nd/ 144 Nd=0.5131, 206 Pb/ 204 Pb=17.5, 207 Pb/ 204 Pb=15.4. Compositions of dehydrated MORB residue and sediments are taken from Kogiso et al. (1997) and Aizawa et al. (1999), respectively. Figure 3. Isotopic evolution of an inferred pyroxenitic restite with 0-15% contribution of felsic magmas. The ages of formation of such delaminated, anti-continental components are shown in Ga. The pyroxenitic restite was produced by partial melting of an initial basaltic crust, delaminated from the crust, and stored in the deep mantle. The isotopic signature of the EMI reservoir may be explained by mixing of the primitive mantle, which represents normal mantle compositions, and delaminated/accumulated pyroxenite with a 10-15% felsic magma component (stars). Isotopic compositions of other mantle components such as the depleted MORB source mantle (DMM), high- µ (HIMU), and enriched mantle II (EMII) are also shown. 63