CLINOPYROXENES FROM PLAGIOCLASE PERIDOTITES (ZABARGAD ISLAND, RED-SEA) AND COMPARISON BETWEEN HIGH-PRESSURE AND LOW-PRESSURE MANTLE CLINOPYROXENES

A detailed crystal-chemical study of clinopyroxenes from the peridotit e-pyroxenite association from Zabargad Island (Red Sea) has been carri ed out to define the intercrystalline relationships in mantle-derived clinopyroxenes equilibrated at low pressure conditions (plagioclase fa cies: Pl-Cpx). Pl-Cpx typically show larger cell volume (>437 Angstrom (3)) compared with those from spinel and garnet-spinel peridotite nodu les (Sp-Cpx). The larger cell volume is mainly achieved through higher Mg-M1 and lower Al-VI occupancies, which strongly increase the M1 vol ume. Concurrently, overcharging on the O3 oxygens due to the high Ca-M 2 (greater than or equal to 0.828 atoms per formula unit, a.f.u.) and low Na-M2 (less than or equal to 0.037 a.f.u.) occupancies requires le ngthening of T-O distances and increase of the T volume. Consequently: i) for the same Mt volume, Pl-Cpx have larger cell volume compared wi th Sp-Cpx; ii) for a given trivalent cations (R(3+)) content in M1, Al -IV is higher in Pl-Cpx than in Sp-Cpx. Plots of cell volume vs M1 vol ume and of M1-O2 vs T-O-nbr bond lengths are a simple way to illustrat e the complex intracrystalline relationships which control (Ca --> Na) (M2), (Si --> Al-IV)(T) and (Mg --> R(3+))(M1) substitutions, and thus permit sensitive qualitative discrimination of the pressure regimes o f equilibration of mantle clinopyroxenes.