SIGNIFICANCE OF RARE HYDROUS ALKALINE MELTS IN HAWAIIAN XENOLITHS

Two types of melt pockets occur in Hawaiian mantle xenoliths: amphibol e-bearing (AMP) and spinel-bearing (SMP). AMPs contain amphibole (kaer sutite), olivine (Fo(92)), clinopyroxene (with 7-11% Al(2)O3), vesicle s and glass. SMPs contain olivine, clinopyroxene, spinel, glass, and v esicles. The glasses in SMPs (SiO2=44-45%, 11-12% alkalis, La=90-110 p pm) and AMPs (SiO2=49-54%, 6-8.5% alkalis, La=8-14 ppm) are distinct i n color and composition. Both glasses are generally characterized by L REE-enriched (chondrite-normalized) patterns. Amphibole and clinopyrox ene have gently convex upward-to-moderately LREE-enriched patterns. Mi neral/glass trace element abundance ratio plots show a strong negative Ti anomaly and a gentle negative Zr anomaly for clinopyroxene/glass; whereas amphibole/glass patterns show a distinctive positive Ti spike. The amphibole/glass trace element ratios are similar to published meg acryst/lava values. An earlier study showed that the Hawaiian spinel l herzolites (lithosphere) have largely been metasomatized during post-e rosional Honolulu magmatic activity. REE abundances of SMP glasses (me lts) overlap the REE abundances calculated for such metasomes. The occ urrence of hydrous, alkaline, mafic melt pockets in Hawaiian upper man tle xenoliths implies that (1) such hydrous liquids are generated in t he upper mantle, and (2) water plays a role in magmatic activity assoc iated with the Hawaiian plume. Although we are uncertain about the sou rce (plume, lithosphere, or asthenosphere) of this water, we speculate that such melts and other alkalic lavas erupted on Oahu and on the se a-floor over the Hawaiian arch were generated from a broad ''wet'' rim of a radially layered Hawaiian plume, whose hot and ''dry'' core supp lied the shield-forming magmas.