MELTING BEHAVIOUR OF MODEL LHERZOLITE IN THE SYSTEM CAO-MGO-AL2O3-SIO2-FEO AT 0·7-2·8 GPA

Abstract

Fe–Mg exchange is the most important solid solution involved in partial melting of spinel lherzolite, and the system CaO–MgO–Al2O3–SiO2–FeO (CMASF) is ideally suited to explore this type of exchange during mantle melting. Also, if primary mid-ocean ridge basalts are largely generated in the spinel lherzolite stability field by near-fractional fusion, then Na and other highly incompatible elements will early on become depleted in the source, and the melting behaviour of mantle lherzolite should resemble the melting behaviour of simplified lherzolite in the CMASF system. We have determined the isobarically univariant melting relations of the lherzolite phase assemblage in the CMASF system in the 0·7–2·8 GPa pressure range. Isobarically, for every 1 wt % increase in the FeO content of the melt in equilibrium with the lherzolite phase assemblage, the equilibrium temperature is lower by about 3–5°C. Relative to the solidus of model lherzolite in the CaO–MgO–Al2O3–SiO2 system, melt compositions in the CMASF system are displaced slightly towards the alkalic side of the basalt tetrahedron. The transition on the solidus from spinel to plagioclase lherzolite has a positive Clapeyron slope with the spinel lherzolite assemblage on the high-temperature side, and has an almost identical position in <rm>P</rm>–<rm>T</rm> space to the comparable transition in the CaO–MgO–Al2O3–SiO2–Na2O (CMASN) system. When the compositions of all phases are described mathematically and used to model the generation of primary basalts, temperature and melt composition changes are small as percent melting increases. More specifically, 10% melting takes place over 1·5–2°C, melt compositions are relatively insensitive to the degree of melting and bulk composition, and equilibrium and near-fractional melting yield similar melt compositions. FeO and MgO are the oxides that exhibit the greatest change in the melt with degree of melting and bulk composition. The amount of FeO decreases with increasing degree of melting, whereas the amount of MgO increases. The coefficients for Fe–Mg exchange between the coexisting crystalline phases and melt, <rm>K</rm>dFe–Mgxl–liq, show a relatively simple and predictable behaviour with pressure and temperature: the coefficients for olivine and spinel do not show significant dependence on temperature, whereas the coefficients for orthopyroxene and clinopyroxene increase with pressure and temperature. When melting of lherzolite is modeled in the CMASF system, a strong linear correlation is observed between the <rm>mg</rm>-number of the lherzolite and the <rm>mg</rm>-number of the near-solidus melts. Comparison with melting in the CMASN system indicates that Na2O has a strong effect on lherzolite melting behaviour only at small degrees of melting.