High-pressure phase transitions in MgCr (sub 2) O (sub 4) .Mg (sub 2) SiO (sub 4) composition; reactions between olivine and chromite with implications for ultrahigh-pressure chromitites

Phase relations in the Mg (sub 2) SiO (sub 4) -MgCr (sub 2) O (sub 4) system were investigated in the pressure range of 9.5-27 GPa at 1600 degrees C to examine the possible deep mantle origin of ultrahigh-pressure (UHP) chromitites in ophiolites. The experimental results indicate that MgCr (sub 2) O (sub 4) -rich chromite (Ch) coexists with Mg (sub 2) SiO (sub 4) -rich olivine (Ol) below approximately 13.5 GPa in the equimolar Mg (sub 2) SiO (sub 4) .MgCr (sub 2) O (sub 4) composition. Above approximately 13.5 GPa, they react to form a three-phase assemblage: garnet (Gt) solid solution in the Mg (sub 4) Si (sub 4) O (sub 12) -Mg (sub 3) Cr (sub 2) Si (sub 3) O (sub 12) system, modified ludwigite (mLd)-type Mg (sub 2) Cr (sub 2) O (sub 5) phase and Mg (sub 14) Si (sub 5) O (sub 24) -rich anhydrous phase B (Anh-B). At approximately 19.5 GPa, Anh-B is replaced by Mg (sub 2) SiO (sub 4) -rich wadsleyite (Wd). At 22 GPa, MgCr (sub 2) O (sub 4) -rich calcium titanate (CT) phase coexists with Mg (sub 2) SiO (sub 4) -rich ringwoodite (Rw). The assemblage of CT+Rw changes to CT + MgSiO (sub 3) -rich bridgmanite (Brg) + MgO periclase at 23 GPa. These sequential phase changes indicate that Ch+Ol do not directly transform to CT+Rw but to the three-phase assemblage, Gt+mLd+Anh-B (or Wd), that becomes stable at pressures corresponding to the upper and middle parts of the mantle transition zone. Our results suggest that the UHP chromitites that have been studied so far did not reach transition zone depths during mantle recycling processes of the chromitites, because there is no evidence of the presence of the reaction products of Ol and Ch. If the reaction products, in particular mLd and Anh-B, are found in the UHP chromitites, they are good indicators to estimate the subduction depth of the chromitites.