CONSTRAINTS ON PARTIAL MELTING IMPOSED BY RARE-EARTH ELEMENT VARIATIONS IN MAUNA-KEA BASALTS

Thirty-one basalts from Mauna Kea collected through the Hawaii Scienti fic Drilling Project [1994] have been analyzed by inductively coupled plasma (ICP) mass spectrometry for rare earth element (REE) concentrat ions. The systematic variations in REE abundance provide constraints f or theoretical models of mantle partial melting, Lavas are given a fir st-order correction for the effects of clinopyroxene and olivine fract ionation or accumulation so that all lavas can be considered direct ma ntle melts. Failure to account for fractionation generates errors in c alculated source mineralogies and REE patterns, Incremental models of melt generation (including fractional, aggregated fractional, and cont inuous melting) are found to reproduce the observed lava compositions only under special conditions: a garnet-free source, a fractionated he avy rare earth element (HREE) source pattern, and aggregation of melt fractions. These conditions are not consistent with the high-pressure phase equilibria for Mauna Kea lavas. Equilibrium batch melting provid es a significantly better match to the observed lava chemistry and is less sensitive to changes in initial conditions. The equations for bat ch melting can be inverted to compute initial source REE patterns and partitioning behavior and are internally consistent with the calculate d forward models. Successful batch melting models include several with an approximately flat chondrite-normalized source REE pattern, degree s of melting between about 1 and 18%, and a mantle residue containing garnet and clinopyroxene.