Chemistry and He isotope systematics of olivine as tracers of source in mantle xenoliths from Harrat Hutaymah and Harrat Kishb Cenozoic lava fields, Western Saudi Arabia

Abstract

Mantle xenoliths from the Cenozoic volcanic fields of Harrat Hutaymah and Harrat Kishb, located in the western part of the Arabian Shield, consist of proto-granular lherzolite, harzburgite, and to a lesser extent dunite and wehrlite. The olivine displays variations in Mn, Ni, and Ca contents relative to the forsterite (Fo) content that is similar to that of residual mantle olivine, which crystallized after being exposed to low to intermediate degrees of melt derived from mantle peridotite. The elemental ratios of Ca-Fe-Ni-Mg-Mn in the olivine further indicate a contribution from peridotite. Pressure-temperature calculations and previous geochemical studies indicate that the peridotite xenoliths were entrained from below the crust-mantle boundary (~35 km in the mantle lithosphere) under the Arabian Shield. Olivine ³He/⁴He ratios range from 6.1 to 8.5 R_a, which overlap the range of <7 to 9 R_a recently reported for off-craton-derived subcontinental lithospheric mantle (SCLM) xenoliths and is consistent with mantle beneath the region. This tends to rule out the overprinting of the SCLM by fluids/melts derived from the deep upwelling mantle sampled by the Afar plume. However, the influence of deep mantle fluid may be cryptic and warrants further exploration because the He content of the Afar mantle plume is unknown.