A new SIMS study of Li, Be, B and δ7Li in mantle xenoliths from Harrat Uwayrid (Saudi Arabia)

Abstract

Concentrations of Li, Be and B and the isotopic composition of lithium have been measured in minerals from a suite of dry and metasomatised spinel peridotite and pyroxenite xenoliths from Harrat Uwayrid, a Pliocene–Quaternary volcanic field in the NW part of the Arabian peninsula, developed during formation of the Red Sea. In most peridotites, Li concentrations in pyroxene and olivine grains show significant systematic zoning, with a core-to-rim decrease in pyroxene but a minor increase in olivine. δ 7Li tends to show a rim-ward increase in clinopyroxene, while it is constant or decreases slightly in olivine. These variations indicate a diffusive transfer of Li from clinopyroxene to olivine during cooling, implying that the partition coefficients K Li Ol/Cpx and K Li Ol/Opx are temperature-dependent and increase with falling temperature. K Li Cpx/Opx, stays nearly constant at ∼ 1. δ 7Li values measured in virtually unmodified cores of olivine, orthopyroxene and clinopyroxene from three peridotites are similar within individual xenoliths but vary among the xenoliths, with individual values ranging from + 1.1 to + 9.7‰. In marked contrast to Li, the concentrations of Be and B are nearly constant in mineral grains, and apparent inter-mineral partition coefficients for these elements are independent of concentrations and xenolith equilibration temperature. Concentrations of Li, Be and B in minerals of un-metasomatized peridotites are relatively low and are similar to values reported previously. Relative abundances of the three light elements in minerals of xenoliths affected by metasomatism have been used to constrain the nature of metasomatic agents. A preferential enrichment of both Li and B relative to Be that is observed in some xenoliths suggests metasomatism by a hydrous fluid, well below the critical point of the solvus between hydrous fluid and silicate melt. A uniform enrichment of all three light elements shown by other xenoliths points to either a silicate melt at subcritical conditions or a supercritical liquid as metasomatic agent.