EVOLUTION OF THE LITHOSPHERE BENEATH OAHU, HAWAII - RARE-EARTH ELEMENT ABUNDANCES IN MANTLE XENOLITHS

Rare earth element contents of clinopyroxenes in Hawaiian mantle xenol iths from Oahu were determined with an ion microprobe. The analyzed xe noliths are from four vents of the alkali Honolulu Volcanics (HV). Thr ee (Kaau, Pali and Kalihi -KPK) are located close to the caldera of th e extinct Koolau shield volcano, and the fourth, Salt Lake Crater (SLC ), is on the periphery of the shield volcano. Systematic differences e xist in REE contents between clinopyroxenes of the KPK and SLC xenolit hs: (1) KPK pyroxenes are typically zoned in REE contents whereas SLC pyroxenes are homogeneous, (2) the LREE-depleted (chondrite-normalized ) patterns that characterize many of the KPK xenoliths are not found i n SLC xenoliths, and (3) the convex-upward REE patterns that are chara cteristic of SLC xenoliths are not found in KPK xenoliths. Relative to abyssal peridotites, the LREE-depleted Hawaiian lherzolite pyroxenes (interpreted to be residual oceanic lithosphere) have higher contents of REE, Na2O, TiO2 and FeO, and more modal clinopyroxene. These LREE-d epleted Hawaiian xenoliths represent deeper, less-depleted parts of th e melting column, whereas the abyssal peridotites represent the upperm ost, more strongly depleted part of the mantle. The spoon-shaped, LREE -enriched and convex-upward REE patterns in the xenoliths have resulte d from metasomatic enrichment of the lithosphere caused by reaction wi th magmas that formed the Honolulu Volcanics. A model for the evolutio n of the oceanic lithosphere is presented in which fractures were the main mode of transport of the Honolulu Volcanics. Metasomatic enrichme nt resulted from interaction between percolating Honolulu Volcanics ma gmas and wallrock. The differences between SLC and KPK xenoliths are a ttributed to chromatographic fractionation effects: SLC xenoliths are postulated to have come from a greater depth where they equilibrated t o a larger extent with the percolating magmas than the KPK rocks.