CONSTRAINTS ON MELT MIGRATION FROM MANTLE PLUMES - A TRACE-ELEMENT STUDY OF PERIDOTITE XENOLITHS FROM SAVAII, WESTERN-SAMOA

Geochemical signals of high degrees of melting and migration of basalt ic and carbonatitic melts are present in harzburgite xenolithis from t he island of Savai'i, Western Samoa. These xenoliths are extremely dep leted in clinopyroxene and have refractory mineral compositions. Ion m icroprobe trace element data delineate three groups of xenoliths. Peri dotites depleted in the light rare-earth elements (LREE) contain clino pyroxenes which have trace element characteristics of garnet but at lo wer absolute concentrations, and they have the lowest olivine Ca-Al-Na concentrations and the lowest equilibration temperatures (<960 degree s C). This group of peridotites is significantly more depleted in trac e elements than Hawaiian xenoliths and abyssal peridotites. These peri dotites can be modeled as residues of 33-42% fractional melting from a pyrolite composition, with generally more than half of this melting t aking place in the garnet Iherzolite stability field. Compared with ab yssal peridotites, the LREE-depleted Savai'i xenoliths originated in a melting regime characterized by higher rates of upwelling, probably w ithin a mantle plume. Intermediate samples display U-shaped REE patter ns with variable, negative anomalies in Ti and Zr, intermediate olivin e Ca-Al-Na concentrations, and intermediate temperatures (960-1050 deg rees C). These features can be modeled by percolation of basaltic melt s through harzburgite at a transient stage of magma-mantle disequilibr ium. The existence of substantial intragranular trace element heteroge neity, even within single samples, argues against homogeneous porous f low, and clinopyroxene distribution suggests a diffuse channel network with channel spacing on a length scale of 1 cm. LREE-enriched peridot ites, with the highest olivine Ca-Al-Na concentrations and highest equ ilibration temperatures (1020-1170 degrees C), are the end-member of t his melt migration process, in which the mantle has approached equilib rium with through-going melts. The geochemistry of the Savai'i xenolit hs testifies to complex processes of plume-lithosphere interaction, fr actional melting, and heterogeneous buoyancy driven flow of plume-deri ved melt fractions with highly variable compositions.