Md. Feigenson et al., CONSTRAINTS ON PARTIAL MELTING IMPOSED BY RARE-EARTH ELEMENT VARIATIONS IN MAUNA-KEA BASALTS, J GEO R-SOL, 101(B5), 1996, pp. 11815-11829
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.