Trace element fractionation trends of tholeiitic magma at moderate pressure: Evidence from an Al-spinel ultramafic-mafic inclusion suite

Trace element data on an Al-spinel ultramafic-mafic inclusion suite in an analcimite support earlier proposals that the various inclusions are comagmatic and represent fragments of a layered tholeiitic ‘pluton’ which differentiated at pressures close to 8 kb. Ultramafic inclusions are dominantly pyroxenites, often websteritic, whereas the mafic inclusions are largely two pyroxene-plagioclase assemblages. Appropriate experimental data and abundances of Sc, Cr and V indicate that subcalcic clinopyroxene or relatively Ca-rich clinopyroxene was the major ferromagnesian phase fractionated, often accompanied by spinel in the early and middle stages of differentiation and, in the later stages, by titanomagnetite. Comparatively moderate decreases in Ni and Co suggest that olivine was a relatively unimportant fractionating phase. Clinopyroxene fractionation at moderate pressures should be assigned only a comparatively insignificant role in the production of evolved basaltic compositions.