Rare Earth distributions in lherzolite and garnet pyroxenite xenoliths and the constitution of the upper mantle

Abstract

Spinel lherzolite occurs at Salt Lake Crater, Hawaii, intimately associated in single xenoliths with garnet pyroxenite. Investigations of the rare earth element (REE) distributions in these two-assemblage rocks show that (1) pyroxenite mineral REE patterns sum very nearly to their whole-rock pattern; (2) chondrite-normalized REE patterns for pyroxenites are gently curved and convex upward (they have maximums in the range Pr to Gd, with total REE contents 4 to 10 times the REE content of chondrites); (3) lherzolites are depleted in REE relative to associated pyroxenites, but show a greater light REE enrichment; and (4) lherzolite chrome diopsides and associated pyroxenite clinopyroxenes have very similar REE distributions. The data suggest the following interpretations: (1) Xenoliths are not contaminated by the host basalt. (2) Solid-liquid distribution coefficients applied to pyroxenite clinopyroxenes give liquids with REE patterns unlike those of any Hawaiian basalt. If the pyroxenites crystallized from some liquid, this liquid was chemically dissimilar to Hawaiian lavas, and pyroxenite formation was probably unrelated to the Hawaiian volcanism. Pyroxenites have REE patterns whose shapes are very similar to the shape of Hawaiian tholeiites and have REE contents about half the REE contents of the tholeiites. The pyroxenites, therefore, may be parental to tholeiites. (3) The similarity in REE patterns of the lherzolite and the pyroxenite clinopyroxenes from a single xenolith is inconsistent with the usual genetic interpretation of these bimodal xenoliths, i.e., that the pyroxenite was a liquid that intruded upper mantle lherzolite (peridotite) and crystallized at depth. The REE data are consistent with textural observations that suggest that the lherzolite has formed as residue when basaltic components were removed from the original pyroxenite by a permeating melt. Relative to Iherzolite, garnet pyroxenite is the more primitive upper-mantle material.