Evidence for pervasive melt-rock reaction within the uppermost mantle at Hess Deep

Abstract:

A suite of spinel harzburgites from ODP Site 895 at Hess Deep have been analysed for the major and trace element compositions of the major mineral phases and of the bulk rock to investigate the effect of melt rock reaction on mineral and bulk rock geochemistry. The harzburgites are at the depleted end of the global array of abyssal peridotite compositions in terms of moderately incompatible elements such as Al₂O₃, CaO, V and Sc. The whole-rock HREE abundances can be modeled as the residues of 15-25% near fractional melting of DMM however the LREE have much higher concentrations than predicted by this model and the samples show a significant positive Eu anomaly. The data can be fit well by a model of near-fractional melting followed by 0.5 to 2% precipitation of plagioclase that has a trace element composition in equilibrium with MORB. Plagioclase impregnation is common in the mantle section drilled at Site 895 although plagioclase is not observed petrographically in the samples studied here. The rocks are 20-70% altered and we hypothesize that plagioclase was entirely replaced during this alteration. The LREE-enrichment, relative to a melting residue, observed in the bulk-rock is not observed in clinopyroxene compositions. One explanation for this could be that the rocks were relatively cool when plagioclase impregnation occurred meaning diffusion was inefficient at modifying the clinopyroxene compositions [e.g. 1]. Whether melt-rock reaction occurs on- or off-axis is currently being investigated.

Refs: [1] Niu, 2004. Journal of Petrology. Volume 45 (12), 2423-2458.