Instability of sapphirine at high pressures

Abstract

The stability field of Mg-sapphirines is limited at high pressures through the solid-solid breakdown reaction sapphirine⇌pyrope = corundum+spinel, the univariant curve originating from an invariant point located at 22 kb, 880°C to 30 kb, 1350°C. Under water pressures less than 22 kb sapphirines exhibit the same low-temperature breakdown into the assemblage chlorite+corundum+spinel as determined by Seifert (1974) between 1 kb and 7 kb thus resulting in one continuous univariant lower stability limit extending from 1 kb, about 650°C through 10 kb, 770°C to the invariant point at 22 kb, 880°C. If $$P_{{\text{H}}_{\text{2}} {\text{0}}} < P_{{\text{total}}} $$ , the stability field of sapphirine will expand towards lower temperatures. The occurrence of sapphirine in mantle depths requires rather aluminous bulk compositions, high geothermal gradients and/or $$P_{{\text{H}}_{\text{2}} {\text{0}}} < P_{{\text{total}}} $$ , with total pressures not exceeding 30 kb. Thus sapphirine is probably not a stable phase in the lower portions of lithospheric plates and the underlying asthenosphere.