Post-garnet transitions in the system Mg4Si4O12–Mg3Al2Si3O12 up to 28 GPa: phase relations of garnet, ilmenite and perovskite

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Abstract

Phase transitions in Mg3Al2Si3O12 garnet (pyrope) were examined at 23–28 GPa and 1600–2000°C using a 6–8-type multianvil apparatus. It was found that pyrope dissociated at 26.5–27 GPa and 1600–2000°C into MgSiO3-rich perovskite solid solution and Al2O3-rich corundum solid solution with a slightly positive dP/dT slope. Perovskite solid solution containing 10 mol% Al2O3 was stabilized at about 26.5 GPa and 1600°C, coexisting with corundum solid solution of 23 mol% MgSiO3.

Powder X-ray diffraction of the single-phase aluminous perovskites in the system MgSiO3–Al2O3 showed that these were orthorhombic and that the increase in the b- and c-axes and a slight decrease in the a-axis accompanied increasing Al2O3 content. Our data and other available X-ray data indicate that the unit cell volume of perovskite in the system MgSiO3–Al2O3 is best expressed as V (Å3)=162.35+6.95x, where x represents the mol fraction of Al2O3 in perovskite (0≤x≤0.25).

Phase relations on the Mg4Si4O12–Mg3Al2Si3O12 join were examined at 20–27 GPa at 1600°C, and also at about 900°C. A wide two-phase field of garnet+perovskite and a relatively narrow field of ilmenite+garnet were delineated at 1600°C. However, at about 900°C at 21–26 GPa, the stability fields of single-phase ilmenite and of ilmenite+garnet were greatly expanded. Thus, in the cool interior of descending slabs near 660 km discontinuity, there may be a significant depth interval over which garnet gives way to ilmenite-bearing assemblages, before ultimate transformation to perovskite takes place.

Introduction

Garnet structured solid solution containing a pyroxene component, called majorite, is generally accepted to be stable in the transition zone of the Earth's mantle along with wadsleyite (β-phase) and ringwoodite (spinel). Valuable insight into the ultimate conversion of majorite to aluminous Mg, Fe-silicate perovskite can be obtained by studying a simplified model system, namely the Mg4Si4O12–Mg3Al2Si3O12 binary join.

In order to clarify mineralogy and chemistry of the transition zone and lower mantle, extensive studies have been carried out on high-pressure phase transitions of Mg3Al2Si3O12 garnet (pyrope) and majorite in the system Mg4Si4O12–Mg3Al2Si3O12. Akaogi and Akimoto (1977) reported the pyroxene–garnet transition under the PT conditions of the transition zone. Under the uppermost lower mantle conditions, Liu (1974) first observed dissociation of pyrope into a mixture of perovskite and corundum. He also reported the garnet→ilmenite→perovskite transitions for the Mg4Si4O12–Mg3Al2Si3O12 join up to 30 GPa (Liu, 1977b). Kanzaki (1987) examined in detail the Mg4Si4O12–Mg3Al2Si3O12 system at 10–24 GPa and 1000°C. Moreover, Liu (1977a) and Kanzaki (1987) synthesized single-phase ilmenite of pyrope composition at 24 and 29 GPa at 1000°C, respectively, while Serghiou et al. (1998) recently synthesized single-phase ilmenite at 21.5 GPa at temperatures exceeding 2000°C. Irifune et al. (1996) and Kubo et al. (1998) showed that pyrope dissociates into aluminous perovskite solid solution and corundum solid solution in the system MgSiO3–Al2O3 at about 27 GPa, and that Al content in perovskite increases as temperature and/or pressure increases. Kondo and Yagi (1998) also showed that pyrope dissociates into perovskite and corundum at above 25 GPa, though they reported that perovskite of pyrope composition was not achieved even at 57 GPa. Ito et al. (1998) synthesized single-phase perovskite of pyrope composition with orthorhombic symmetry at about 37 GPa and 1600°C, while Kesson et al. (1995) synthesized perovskite of essentially pyrope composition at 60–70 GPa, which was recovered with rhombohedral symmetry. Zhang and Weidner (1999) synthesized orthorhombic perovskite of 95 mol% MgSiO3–5 mol% Al2O3 composition at 26 GPa and 1500°C. Akaogi and Ito (1999) calculated a positive dP/dT slope for the majorite–perovskite transition in the system Mg4Si4O12–Mg3Al2Si3O12, based on calorimetry.

High-pressure transformations of natural garnets have also been performed using laser-heated diamond anvil cells (DAC). However, natural garnets contain minor and trace elements other than Mg, Si and Al, and various types of Al-rich phases have been reported as coexisting with perovskite. Ahmed-Zäid and Madon (1995) reported a (Ca, Mg)Al2SiO6 phase with unknown structure; an Al2SiO5 phase with V3O5 structure, and a (Ca,Mg,Fe)Al2Si2O8 phase with hollandite structure. Miyajima et al. (1999) reported a hexagonal phase of M3Al4Si1.5O12 (M=Mg, Fe, Ca, Na, K) composition, which would have the same structure as the hexagonal phase in the system MgAl2O4–CaAl2O4 described by Akaogi et al. (1999). On the other hand, O'Neill and Jeanloz (1990), Irifune (1994), Kesson et al. (1998) and Wood (2000) reported that any Al-rich phase other than aluminous perovskite was absent in pyrolite or peridotite composition at the lower mantle pressure–temperature conditions.

As described above, the previous work on post-garnet transitions is controversial, and further studies are desirable to clarify the nature of the transformation. In particular, the temperature dependence of post-garnet phase relations in the Mg4Si4O12–Mg3Al2Si3O12 system has not yet been elucidated. In the present study, therefore, phase relations in the system Mg4Si4O12–Mg3Al2Si3O12 have been examined at 20–28 GPa and 700–2000°C, so as to clarify temperature dependency of the phase relations. Detailed powder X-ray diffraction data on perovskites in the system are also reported in this study. Host phases of Al in pyrolite mantle and subducted slabs in the uppermost lower mantle conditions are also discussed.

Section snippets

Starting materials

Three kinds of starting materials were prepared: glasses, crystalline pyrope and mixtures of MgSiO3 clinoenstatite, Al2O3 corundum and Al(OH)3 gibbsite.

Glasses in the MgSiO3–Al2O3 system with MgSiO3 component (in mol%) of 100 (=enstatite composition), 97.5, 95, 92.5, 90, 85, 80 and 75 (=pyrope composition) were synthesized from reagent grade chemicals: MgO, SiO2 and Al2O3. These chemicals were thoroughly mixed and melted in a platinum crucible in argon atmosphere at 1600–1760°C. After being

Phase relations for Mg3Al2Si3O12

Table 1 lists the experimental conditions and phases observed in the central part of the run products. The following phases were identified by means of microfocus X-ray diffraction; garnet, perovskite, corundum, ilmenite and stishovite. No other phases were observed. Fig. 1 shows examples of microfocus X-ray diffraction patterns of the central part of the samples recovered from pyrope starting materials. These profiles indicate that pyrope dissociated to an assemblage of perovskite and

Acknowledgements

We thank Prof. E. Ito and Dr. T. Suzuki for their help with the experiments and for many helpful discussions. Discussions with Prof. T. Yagi, Prof. H. Nagasawa, Dr. T. Katsura, Dr. K. Hirose and Dr. H. Kojitani are also acknowledged. We also thank to Dr. T. Ishii for his assistance with EPMA at the Ocean Research Institute, University of Tokyo, and to Dr. T. Fujita for his help with powder X-ray diffractometry at Institute for Study of the Earth's Interior, Okayama University. Constructive

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