The effects of K_2O on the compositions of near-solidus melts of garnet peridotite at 3 GPa and the origin of basalts from enriched mantle

摘要

Enrichment in K_2O in oceanic island basalts (OIB) is correlated with high SiO_2, low CaO/Al_2O_3, and radiogenic isotopic signatures indicative of enriched mantle sources (EMI and EM2). These are also chemical characteristics of the petit-spot lavas, which are highly enriched in K_2O (3-4 wt%) compared to other primitive oceanic basalts. We present experimentally derived liquids with varying concentrations of K_2O in equilibrium with a garnet lherzolite residue at 3 GPa to test the hypothesis that the major element characteristics of EM-type basalts are related to their enrichment in K_2O. SiO_2 is known to increase with K_2O at pressures less than 3 GPa, but it was previously unknown if this effect was significant at the high pressures associated with partial melting at the base of the lithosphere. We find that at 3 GPa for each 1 wt% increase in the K_2O content of a garnet lherzolite saturated melt, SiO2 increases by ~0.5 wt% and CaO decreases by -0.5 wt%. MgO and K_D~(Fe-Mg) each decrease slightly with K_2O concentration, as do Na_2O and Cr_2O_3. The effect of K_2O alone is not strong enough to account for the SiO_2 and CaO signatures associated with high-K_2O OIB. The SiO_2, CaO, and K_2O concentrations of experimentally derived partial melts presented here resemble those of petit-spot lavas, but the Al_2O_3 concentrations from the experimental melts are greater. Partitioning of K_2O between peridotite and melt suggests that petit spots, previously considered to sample ambient asthenosphere, require a source more enriched in K_2O than the MORB source.