Absence of density crossover between basalt and peridotite in the cold slabs passing through 660 km discontinuity

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L24607, doi: /2004gl021306, 2004 Absence of density crossover between basalt and peridotite in the cold slabs passing through 660 km discontinuity Konstantin Litasov, Eiji Ohtani, Akio Suzuki, and Takaaki Kawazoe Institute of Mineralogy, Petrology and Economic Geology, Faculty of Science, Tohoku University, Sendai, Japan Kenichi Funakoshi Spring-8, Japan Synchrotron Radiation Research Institute, Kouto, Hyogo, Japan Received 20 August 2004; revised 28 October 2004; accepted 10 November 2004; published 23 December [1] Interaction between subducting slabs and surrounding mantle near 660 km discontinuity is one of the key issues in the dynamic of deep Earth. Distinguishing phase transformations occur in the both major parts of the slab: basaltic crust and underlying peridotite. Post-spinel transformation in peridotite takes place at 660 km, whereas post-garnet transformation in basalt is considered to occur at km creating density crossover. In this paper we determined the post-garnet phase boundary in Mid-Ocean Ridge basalt (representative for oceanic crust) by in situ x-ray diffraction studies. The phase boundary can be expressed as P(GPa) = T(K) The present results imply that the basaltic component of the cold slab transformed to perovskite assembly at lower pressure than the surrounding mantle and provide direct evidence for penetration of the oceanic crust into the lower mantle without gravitational separation from the peridotite body of the slab. INDEX TERMS: 1025 Geochemistry: Composition of the mantle; 3630 Mineralogy and Petrology: Experimental mineralogy and petrology; 3924 Mineral Physics: High-pressure behavior; 3954 Mineral Physics: X ray, neutron, and electron spectroscopy and diffraction; 8124 Tectonophysics: Earth s interior composition and state (1212). Citation: Litasov, K., E. Ohtani, A. Suzuki, T. Kawazoe, and K. Funakoshi (2004), Absence of density crossover between basalt and peridotite in the cold slabs passing through 660 km discontinuity, Geophys. Res. Lett., 31, L24607, doi: /2004gl Introduction [2] The problem, whether the subducting slabs or their components deflect and flatten at 660 km or penetrate into the lower mantle, is associated with continuous seismological and geochemical debates on whole mantle versus layered convection through Earth s history and the existence of fertile lower mantle which is only occasionally involved in the production of surface rocks [e.g., Hofmann, 1997; Van der Hilst et al., 1998]. As yet, there is no consensus on this matter although many models have already been proposed [e.g., Kellogg et al., 1999; Helffrich and Wood, 2001; Turcotte et al., 2001; Albarede and Van der Hilst, 2002]. The most likely scenario of subduction was suggested by Ringwood [1994] and it asserts that mature thick and cold slabs with high thermal inertia may penetrate into the lower mantle, whereas young, thin and relatively hot Copyright 2004 by the American Geophysical Union /04/2004GL slabs are supposed to be deflected above 660 km and retained in the transition zone. [3] Subduction slabs are mainly composed of basaltic crust and underlying peridotite. Post-spinel transformation in peridotite takes place at 660 km [Ito and Takahashi, 1989; Shim et al., 2001; Katsura et al., 2003; Fei et al., 2004], whereas post-garnet transformation in basalt was considered to occur at km creating density crossover which may cause a separation of basaltic crust from slab above 660 km [Irifune and Ringwood, 1993] and lead to formation of so-called garnetite layer [Anderson and Bass, 1986; Ringwood, 1994]. Recently, Hirose and Fei [2002] reported results of quench multianvil experiments at GPa and suggested that the transformation of Mid- Ocean Ridge basalt (MORB, representative for oceanic crust) to perovskite-bearing lithology occurs near 720 km. They predicted that the density crossover between basalt and peridotite should be too narrow for delamination of the basaltic crust near 660 km. In this work we determined the post-garnet phase boundary in MORB by in situ x-ray diffraction studies and showed absence of the density crossover between peridotite and basalt near the 660 km discontinuity in the cold subducting slabs. 2. Experiments [4] In situ X-ray diffraction experiments were conducted at the synchrotron radiation facility Spring-8 in Hyogo prefecture, Japan. We used a Kawai-type multi-anvil apparatus, SPEED-1500, installed at a bending magnet beam line BL04B1. Starting material was synthetic glass representing average MORB [Litasov and Ohtani, 2004] mixed with the Au pressure marker as 15:1 by weight. The truncated edge length of the WC anvil was 2.0 mm. The sample assembly was composed of a sintered Co-doped MgO pressure medium, a cylindrical LaCrO 3 heater, molybdenum electrodes, and a graphite sample capsule isolated from the electrodes by MgO insulators. Temperature was monitored by a W3%Re W25%Re thermocouple with a junction located at the same position as where the x-rays pass through the sample relative to the center of the furnace assembly. The generated pressure was calculated from the unit cell of gold using equations of state (EOS) proposed by Anderson et al. [1989]. Uncertainty of the unit cell volume of gold calculated by the least squares method typically gives the pressure uncertainty less than GPa. [5] In order to constrain the phase boundaries at high pressures we must establish first a pressure marker with a L of5

2 Table 1. Experimental Conditions and Results a Run No Load (tons) T (K) V Au (Å 3 ) P And (GPa) P Sh (GPa) Result Sample mbk-1 No (2) Hp No (4) Hp + Gt No (4) Hp! Gt No (5) Hp! Gt No (1) Hp + Gt No (2) Gt! Hp Sample mbk-2 No (3) Hp No b (2) Hp No31 >2473 b Part. melting No b (6) Hp Sample mbk-3 No (3) Hp No (2) Hp + Gt No (1) Hp + Gt No (2) Hp! Gt No (4) Hp! Gt Sample mbk-4 No (5) CaMgPv No (3) Hp No (4) Hp + Gt No (7) Hp + Gt No (5) Hp + Gt No (5) Hp! Gt a Hp, high-pressure perovskite-bearing assembly; Gt, garnet-bearing assembly; CaMgPv, metastable Ca-Mg-perovskite-bearing assembly. Arrows indicate fast reaction from high-pressure to garnet-bearing assembly and backward. Pressure calculated from Au equation of state by Anderson et al. [1989]P And and Shim et al. [2002]P Sh. b Temperature estimated from temperature/power ratio. reliable EOS. Au, Pt, and MgO are typically used for in situ high-pressure estimations in multianvil apparatus [e.g., Ono et al., 2001; Hirose et al., 2001]. The calculated pressure differences between different EOS of Au [Jamieson et al., 1982; Anderson et al., 1989; Shim et al., 2002] may exceed 2.5 GPa at high temperatures. It was shown that the Anderson s scale [Anderson et al., 1989] underestimates pressure at least for about 1.5 GPa at 20 GPa and high temperatures [Matsui and Nishiyama, 2002; Okube et al., 2002; Matsui and Shima, 2003]. In this paper we Figure 1. Post-garnet transformation boundary in anhydrous MORB determined by in situ x-ray diffraction experiments using pressures calculated from equation of state of gold by Anderson et al. [1989]. Open symbols, garnet in; filled symbols, garnet out. Crossed symbols indicate fast disappearance of Mg-perovskite. used gold scale by Anderson et al. [1989] and Shim et al. [2002]. Additionally, we placed unit cell volumes of Au at high-pt conditions in Table 1 for possible pressure re-estimations if a newly corrected gold pressure scale becomes available. 3. Results [6] The pressure and temperature conditions of the present experiments are summarized in Table 1. The compositions of the phases after experiments are presented in Table 2 and are consistent with those previously reported for observed minerals [Hirose and Fei, 2002; Litasov and Ohtani, 2004]. The phase relations were determined at GPa and temperature up to 2473 K (Figure 1). Table 2. Representative Compositions of Experimental Products a Sample mbk-1, 1973 K, 26.1 GPa Sample mbk-2, 1943 K, 29.2 GPa Mineral SM MgPv CaPv St CF NAL Gt MgPv CaPv St CF SiO (0.9) (1.0) (0.1) 47.9 (0.3) (0.7) TiO (0.51) (0.07) (0.06) 2.22 (0.02) 0.70 (0.03) Al 2 O (0.7) (0.7) (0.1) 4.06 (0.19) (0.6) FeO (1.1) (1.0) (0.1) 5.13 (0.18) (0.4) MgO (0.4) (0.5) (0.1) 3.10 (0.11) 12.2 (0.6) CaO (0.10) (0.09) (0.01) 36.4 (0.4) 1.50 (0.09) Na 2 O (0.07) (0.3) (0.01) 0.75 (0.05) (0.1) K 2 O (0.01) (0.02) (0.01) 0.17 (0.06) 0.03 (0.02) Total Density b a SM, starting material [Litasov and Ohtani, 2004]. b Zero-pressure density in g/cm 3. MgPv, Mg-perovskite; CaPv, Ca-perovskite; St, stishovite; CF, Al-rich phase with Ca-ferrite structure [Akaogi et al., 2002]; NAL, Na-Al hexagonal phase [Gasparik et al., 2000]; Gt, garnet. Mineral compositions of recovered samples were measured by the electron microprobe (JEOL Superprobe, JXA-8800) under the operating condition of 15 kv and 10 na specimen current. 2of5

3 Phase formation began from appearance of metastable Ca-Mg-perovskite at about 1273 K followed by transformation to Mg- and Ca-perovskite-bearing assemblies at about 1573 K. Appearance of garnet was observed with increasing temperature in all runs except one (mbk-2) at highest pressures of GPa. In the run mbk-1 we observed a reversible phase transformation from a perovskite-bearing high-pressure assembly (Mg- Perovskite, Ca-perovskite, Al-rich CF phase, and stishovite) to a garnet-bearing assemblage (garnet, stishovite, Ca-perovskite, Al-rich phases CF or NAL) and backward (Figure 2). Appearance of garnet was observed at 2073 K and 26.4 GPa. At 2173 K we monitored fast growth of garnet and at 2273 K Mg-perovskite peaks had almost disappeared in the x-ray diffraction pattern (Figure 2). Then, with decreasing temperature, we observed fast transformation of a garnet- to perovskite-bearing assemblage at 1973 K and 26.1 GPa. Disappearance of garnet was confirmed by x-ray diffraction and electron microprobe study of the recovered sample. We could find only one small grain of garnet for the microprobe analyses, whereas Mg-perovskite was abundant. The results of other experiments are consistent with those of mbk-1 and post-garnet phase boundary can be expressed as P(GPa) = T(K) Results for mbk-1 show that the pressure interval of coexisting garnet and Mg-perovskite should be very narrow (less than 0.5 GPa). In the other runs (mbk-3 and mbk-4) this interval is much wider; however it can be explained by sluggish transformation of Mg-perovskite to garnet at temperatures below 2000 K. 4. Implication to 660 km Discontinuity [7] Based on the quench experiments it was demonstrated [Hirose and Fei, 2002] that the post-garnet phase transformation in MORB occurs at 27.0 GPa and 1900 K with a slightly positive dp/dt (+0.8 MPa/K). In the in situ study, Oguri et al. [2000] determined the post-garnet transformation boundary in natural pyrope at 25.0 GPa and 1900 K with a significant positive dp/dt (+6.4 MPa). Our data are inconsistent with the previous quench experiments by Hirose and Fei [2002] possibly due to corrections for thermal pressure resulting in a large positive dp/dt of the post-garnet transformation in MORB (+4.1 MPa/K). Recent studies [Katsura et al., 2003; Fei et al., 2004] also indicate Figure 2. Representative x-ray diffraction patterns of the sample mbk-1 at the indicated P-T conditions. Peak identifications are: Pv, Mg-perovskite; Gt, garnet; St, stishovite. The peaks of other minerals are not marked. Figure 3. (a) Comparison of the post-garnet transformation boundary in MORB with post-spinel transformation boundaries in pyrolite (average mantle peridotite) and Mg 2 SiO 4 systems. The mantle geotherm is after Akaogi et al. [1989]. The cold subduction geotherm is after Thompson [1992]. Post-spinel boundaries are after Katsura et al. [2003] (K), Fei et al. [2004] (F), Shim et al. [2001] (SH) and Ito and Takahashi [1989] (I&T). Solid lines are based on the Au EOS after Anderson et al. [1989] (And). Dotted lines Speziale EOS of MgO [Speziale et al., 2001] (MgO). Dashed lines Au EOS after [Jamieson et al., 1982] (Jam) and Ruby scale (SH). Symbols: Rw, ringwoodite; Pv, Mg-perovskite; Pc, periclase; Gt, garnet. (b) Comparison of density profiles of anhydrous MORB and pyrolite at different geotherms corresponding to normal mantle and cold subduction slabs. Density calculations were carried out along normal mantle (solid lines) and cold subduction (dashed lines) geotherms using a third order Birch-Murnaghan equation of state and the set of thermoelastic parameters after Litasov and Ohtani [2004]. 3of5

4 that the negative Clapeyron slope of the post-spinel transformation boundary of Mg 2 SiO 4 should be smaller than previously estimated by the quench multianvil and diamond-anvil cell experiments [Ito and Takahashi, 1989; Shim et al., 2001]. [8] It has been suggested that density crossover between peridotite and basalt can lead to a separation of the basaltic crust from peridotite near 660 km and formation of a garnetite-bearing (former basalt) layer at the base of the transition zone [Anderson and Bass, 1986; Ringwood, 1994; Karato, 1997]. The present experimental study demonstrates that at the low temperatures of the slabs the density crossover between peridotite and basalt might be entirely absent (Figure 3). The pressure and temperature condition of the crossover of the two phase boundaries lies close to the temperature path of a cold subducting slab and is almost independent on the choice of pressure scales (Figure 3a). [9] Kubo et al. [2002] showed recently that the postgarnet transformation in natural pyrope is very sluggish, and cannot be completed in geological time scales of yrs at 1600 K under the significant overpressure conditions which suggest that the garnet-bearing basaltic crust is likely to be separated from the slab at the depths around 660 km. However, the present results suggest that kinetics of pyrope is not applicable for the transformation of the basaltic crust component, since a dense metastable assemblage containing CaMg-perovskite can be formed in the system under dry conditions (around GPa and 1300 K) [Asahara et al., 2004]. Formation of metastable CaMg-perovskite phase enhances the kinetics of the post-garnet transformation. Further, recent experimental results show that water enhances the phase transformation kinetics significantly in the deep mantle [Ohtani et al., 2004]. Since there is evidence for possible existence of water in the subducting slabs in the transition zone [Bercovici and Karato, 2003; Ohtani et al., 2004], the effect of the transformation kinetics could be negligible for water-bearing slabs. [10] Thus, the present results provide direct evidence for penetration of the oceanic crust component into the lower mantle without gravitational separation from the peridotite body of the slab. The seismic reflectors with a positive density jump [e.g., Niu et al., 2003] may suggest that the basaltic crust layers with the post-garnet lithology penetrated into the lower mantle. [11] Acknowledgments. We thank C. Prewitt, T. Kondo, and two anonymous reviewers for discussion and comments and T. Kubo, S. Kato, and R. Ando for technical help during experiments in Spring-8. This work was supported by the Grant in Aid for Scientific Researches from Ministry of Education, Culture, Sports, Science and Technology, Japan (no ) to E. Ohtani. 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