Geochemical Constraints on the origin of Mantle Pyroxenites

Pyroxenites form 1-5 percent of the shallow lithospheric upper mantle. Lithologies range from orthopyroxenite through websterites to clinopyroxenite, with spinel, garnet and olivine as varietal minerals. The origin of mantle pyroxenites is controversial. Previous workers have suggested that they may be metamorphic segregations of the host peridotite; remnants of subducted oceanic crust; frozen mafic silicate magmas; high-pressure crystal segregates; or formed by replacement reactions between peridotite and percolating melts. A supra-subduction zone setting has also been suggested for some pyroxenites. This paper will use whole-rock geochemical data, mineral trace element results and isotope data to test these various hypotheses.

Whole-rock major and trace element compositions of mantle pyroxenites show variations between at least three end-members: two of these resemble clino- and orthopyroxene, while the third component is similar to basalt. Only a few pyroxenites have bulk compositions that resemble oceanic crust. Most are LREE-depleted, with lower absolute concentrations than MORB, and most lack Eu anomalies. Trace element patterns in clinopyroxenes from pyroxenites from massifs are usually LREE-depleted, whereas those in xenoliths are frequently LREE-enriched. Most show depletions in Zr, Hf and Sr, although some display significant Sr peaks, attributed to the influence of subduction-related fluids or melts. Oxygen isotope analyses of clinopyroxenes from most mantle pyroxenites yield typical mantle values, but rare garnet pyroxenite layers from ultramafic massifs show higher values that reflect a recycled crustal component. Sr, Nd, Pb and Hf isotopic ratios of most pyroxenites fall in the depleted mantle field, although with a much wider range of values than MORB. Some show higher 87Sr/86Sr values and lower δHf than mantle peridotites, indicating the presence of an enriched component. Most mantle pyroxenites are best explained as products of crystal accumulation of mantle-derived magmas, together with variable amounts of trapped melt. Only samples from a few ultramafic massifs show strong evidence of being subducted recycled oceanic crust. Others may be the product of interaction between magma and peridotite. Few mantle pyroxenites show good evidence for derivation by either incipient melting of the host peridotite or by metamorphic segregation.