Geochemical signals of high degrees of melting and migration of basaltic and carbonatitic melts are present in harzburgite xenoliths from the island of Savai'i, Western Samoa. These xenoliths are extremely depleted in clinopyroxene and have refractory mineral compositions. Ion microprobe trace element data delineate three groups of xenoliths. Peridotites depleted in the light rare-earth elements (LREE) contain clinopyroxenes which have trace element characteristics of garnet but at lower absolute concentrations, and they have the lowest olivine Ca-Al-Na concentrations and the lowest equilibration temperatures (<960 ¿C). This group of peridotites is significantly more depleted in trace elements than Hawaiian xenoliths and abyssal peridotites. These peridotites can be modeled as residues of 33--42% fractional melting from a pyrolite composition, with generally more than half of this melting taking place in the garnet lherzolite stability field. Compared with abyssal peridotites, the LREE-depleted Savai'i xenoliths originated in a melting regime characterized by higher rates of upwelling, probably within a mantle plume.

Intermediate samples display U-shaped REE patterns with variable, negative anomalies in Ti and Zr, intermediate olivine Ca-Al-Na concentrations, and intermediate temperatures (960--1050 ¿C). These features can be modeled by percolation of basaltic melts through harzburgite at a transient stage of magma-mantle disequilibrium. The existence of substantial intragranular trace element heterogeneity, even within single samples, argues against homogeneous porous flow, and clinopyroxene distribution suggests a diffuse channel network with channel spacing on a length scale of 1 cm. LREE-enriched periodotites, with the higest olivine Ca-Al-Na concentrations and higest equilibration temperatures (1020--1170 ¿C), are the end-member of this melt migration process, in which the mantle has approached equilibrium with through-going melts. The geochemistry of the Savai'i xenoliths testifies to complex processes of plume-lithosphere interaction, fractional melting, and heterogeneous buoyancy driven flow of plume-derived melt fractions with highly variable compositions. ¿American Geophysical Union 1994