Clinopyroxenes from plagioclase peridotites (Zabargad Island, Red Sea) and comparison between high- and low-pressure mantle clinopyroxenes

Abstract

A detailed crystal-chemical study of clinopyroxenes from the peridotite-pyroxenite association from Zabargad Island (Red Sea) has been carried out to decline the intercrystalline relationships in mantle-derived clinopyroxenes equilibrated at low pressure conditions (plagioclase facies:Pl-Cpx).Pl-Cpx typically show larger cell volume (>437 °A3) compared with those from spinel and garnet-spinel peridotite nodules (Sp-Cpx). The larger cell volume is mainly achieved through higher Mgm, and louver AlVI occupancies, which strongly increase the M l volume. Concurrently, overcharging on the O3 oxygens due te, the high CaM2 (≥ 0.828 atoms per formula unit, a.f.u.) and low NaM2 (< 0.037 a.f.u.) occupancies requires lengthening of T-0 distances and increase of the T volume. Consequently: i) for the saine M 1 volume,Pl-Cpx have larger cell volume compared withSp-Cpx; ii) for a given trivalent cations (R3+) content in M1, AlIV is higher inPl-Cpx than inSp-Cpx. Plots of cell volume vs Ml volume and of M1-O2 vs T-Onbr bond lengths are a simple way to illustrate the complex intracrystalline relationships which control (Ca → Na)M2, (Si → AlIV)T and (Mg → R3+)M1 substitutions, and thus permit sensitive qualitative discrimination of the pressure regimes of equilibration of mantle clinopyroxenes.