12:00 PM - 12:15 PM
[SIT07-12] High-pressure phase relations in the system MgCr2O4-Mg2SiO4 with implications to ultra-high pressure chromitites in ophiolites
Keywords:high pressure , chromitite, phase relation
We have recently determined phase relations in MgCr2O4 at pressure up to 28 GPa and temperature to 1600 oC. Magnesium chromite (MgCr2O4) with the spinel structure first transforms to a two-phase assemblage of modified ludwigite (mLd)-type Mg2Cr2O5 + corundum-type Cr2O3 at 13-15 GPa, and the two phases combine into a calcium-titanate (CT) phase at 16-20 GPa (Ishii et al., 2015). Because chromite minerals in natural podiform chromitites are enveloped in olivine crystals, we have extended our experimental study to the system MgCr2O4-Mg2SiO4 to examine phase relations and mutual solubilities in the high-pressure phases.
High-pressure high-temperature experiments were performed in composition of 50mol%MgCr2O4.50mol%Mg2SiO4 up to 26 GPa at 1600 oC using a Kawai-type mulitianvil apparatus. Phase identification of the synthesized samples were made using micro-focus and powder X-ray diffractometers. Composition analysis was performed using a scanning electron microscope with an energy-dispersive X-ray spectrometer. At pressure below about 14 GPa, Mg2SiO4 forsterite coexists with MgCr2O4-rich spinel. At 14-19 GPa, a three-phase assemblages, mLd + garnet (Gt) + wadsleyite (Wd) becomes stable. This assemblage changes into MgCr2O4-rich CT + ringwoodite at about 20 GPa, which further changes into an assemblage of MgSiO3-rich bridgmanite (perovskite) + CT + periclase at about 23 GPa. The mLd composition was close to Mg2Cr2O5, while Gt has a middle composition between Mg4Si4O12 and Mg3Cr2Si3O12. Solubility of Mg2SiO4 component in CT increases with pressure to about 20 mol% at 26 GPa. These results implies that, when podiform chromitites are subducted into the deep mantle, MgCr2O4 spinel and forsterite react to form mLd + Gt at 14-19 GPa which corresponds to pressure range of the upper part of the mantle transition zone. Also, if CT phase containing substantial Mg2SiO4 component goes upward from the deep mantle, it decomposes into the assemblage containing mLd + Gt at 14-19 GPa. To the best of our knowledge, mLd + Gt in association with chromite have not yet been found in the natural chromitites. Therefore, we suggest that possible occurrence of mLd and/or Gt coexisting with chromite is an important indicator which shows that the chromitites were derived from the depth at least upper-part of the transition zone.