The Roles of Lherzolite and Garnet Pyroxenite

Textural, major element and trace element data for xeno-liths from Salt Lake Crater, Hawaii indicate that typical four phase lherzolite inclusions (olivine, orthopyroxene, chrome diopside and spinel) found worldwide in basalts are not samples of primitive upper mantle. Rather, lherzolite may be the recrystallized residue left after a basaltic melt extracts the easily fused components from pre-existing garnet pyroxenite. Some xenoliths from Salt Lake Crater contain lherzolite in contact with garnet pyroxenite. Previous workers have interpreted these xenoliths to be fragments of deep-seated regions where upper mantle peridotite (lherzolite) was intruded by a melt which crystallized to the pyroxenite assemblage. Textures at the lherzolite-pyroxenite contact indicate that therzolite is younger, having formed from pre-existing garnet pyroxenite. Lherzolite orthopyroxene is derived from both pyroxenite pyroxenes. Lherzolite olivine has formed at the expense of pyroxenite orthopyroxene. Lher-zolite chrome diopside and spinel originate as remnants of pyroxenite clinopyroxene and spinel, most of which are consumed in the transformation event. Rare earth data support the proposed view. Application of crystal/liquid distribution coefficients to pyroxenite "pre-garnet" clinopyroxene REE patterns gives hypothetical liquids unlike any Hawaiian basalt. If the pyroxenites crystallized from such liquids, pyroxenite formation and the current Hawaiian vulcanism are probably genetically unrelated. Garnet pyroxenites have REE patterns capable of giving rise to tholeiitic REE patterns on partial melting. Clino-pyroxenes from the lherzolite and pyroxenite portions of a single xenolith have very similar REE distributions. This is consistent with the textural observation that the chrome diopside forms from pyroxenite clinopyroxene. It is inconsistent with the view that lherzolite has been intruded by a melt which crystallized at depth to the pyroxenite assemblage. Sr isotopic data suggest: (1) some garnet pyroxenites have developed and maintained Sr isotopic disequilibrium for a period of time longer than the age of current Hawai-ian vulcanism. (2) Xenoliths and host basalt have different Sr 8 7 /Sr 8 6 ratios, suggesting the absence of a genetic relationship. 11i The similarity of lherzolite inclusions at Salt Lake Crater to those around the world suggests that garnet pyro-xenite, not lherzolite, may be the dominant rock in the upper mantle. This is in accord with geophysical evidence that the density of the upper mantle may be greater than that allowed by the pyrolite model.