Constraints on partial melting imposed by rare earth element variations in Mauna Kea basalts

Abstract

Thirty‐one basalts from Mauna Kea collected through the Hawaii Scientific Drilling Project [1994] have been analyzed by inductively coupled plasma (ICP) mass spectrometry for rare earth element (REE) concentrations. The systematic variations in REE abundance provide constraints for theoretical models of mantle partial melting. Lavas are given a first‐order correction for the effects of clinopyroxene and olivine fractionation or accumulation so that all lavas can be considered direct mantle melts. Failure to account for fractionation generates errors in calculated source mineralogies and REE patterns. Incremental models of melt generation (including fractional, aggregated fractional, and continuous melting) are found to reproduce the observed lava compositions only under special conditions: a garnet‐free source, a fractionated heavy 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 provides a significantly better match to the observed lava chemistry and is less sensitive to changes in initial conditions. The equations for batch melting can be inverted to compute initial source REE patterns and partitioning behavior and are internally consistent with the calculated forward models. Successful batch melting models include several with an approximately flat chondrite‐normalized source REE pattern, degrees of melting between about 1 and 18%, and a mantle residue containing garnet and clinopyroxene.