Clinopyroxenite generation via high-pressure crystallization of a moderately evolved MORB-type basalt: Experiments from 1.0 to 2.5 GPa

Abstract

Pyroxenites represent widespread lithological heterogeneities found in the upper mantle in various geodynamical settings. Their origin can be attributed to different magmatic or metamorphic processes or a combination of them. Several mantle pyroxenites are believed to result from melt segregation at high-pressure conditions (P ≥ 1 GPa) of mantle-derived evolved melts. In this study, we performed experiments at 1–2.5 GPa and 1100–1300 °C to investigate the mineralogy and mineral chemistry in pyroxenites formed through high-pressure crystallization of a MORB-like evolved (X_Mg = 0.6) basalt. High temperature (T > 1200 °C) isothermal experiments revealed that clinopyroxene is the liquidus phase. In order to simulate closed-system crystallization after deep melt segregation in the mantle, some experiments experienced isobaric cooling down to 1100 and 1150 °C. Garnet precipitates after clinopyroxene from the basaltic melt at P ≥ 1.5 GPa. At 1 GPa, low-forsterite olivine was observed also in association with plagioclase at 1100 °C. Upon cooling, X_Mg in clinopyroxene decreases (down to 0.66) accompanied by a slight decrease in Ca and an increase in Ti content. At increasing pressure, clinopyroxenes exhibit progressively higher Na and Al contents with decreasing Ca and Cr concentrations. Grossular and Almandine contents in garnet are inversely correlated with temperature. Pyroxenites generated through infiltration and crystallization of MORB-type basalt are garnet clinopyroxenites at P from 1.5 to 2.5 GPa and plagioclase+olivine clinopyroxenites at 1 GPa. Mineral compositions and modal abundances derived from experiments have been employed to estimate the bulk compositions of pyroxenites formed through pure high-pressure crystallization. Computed bulk-rock resemble some garnet clinopyroxenites documented in orogenic and ophiolitic ultramafic massifs.