The role of partial melts of peridotite in the formation of oceanic island basalts.

Abstract

Despite the classic hypothesis that primary basalts are generated by partial melting of peridotite, several compositional characteristics of oceanic island basalts (OIB) are not easily reconciled with such an origin. Many OIB have higher FeO and lower Al2O3 concentrations than do experimentally-derived partial melts of peridotite; however experiments have not been performed at both the high pressures (3 GPa) and low melt fractions relevant to OIB formation. Trace element concentrations and ratios of many first-row transition elements (FRTE) in OIB and associated phenocrysts may also suggest that non-peridotitic melting lithologies play a role in OIB petrogenesis. This thesis presents the results of high-pressure experiments designed to test the hypothesis that oceanic island basalts (OIB) can be derived solely by partial melting of a peridotite source. Experiments were performed in a piston cylinder apparatus at 3 GPa and temperatures near the solidus of peridotite (1430-1470 °C). Experiments to determine the major-element composition of the incipient melt of garnet lherzolite employed a recently developed technique, modified iterative sandwich experiments (MISE), which simulates near-solidus partial melting while allowing for the preservation of large melt pools to facilitate chemical analysis. The resulting melts are similar to alkalic OIB in many respects, but are still higher in Al2O3 (12.7±0.2 wt.%) and lower in FeO* (9.7±0.1 wt.%) than most OIB. Trace element partitioning experiments were performed to determine mineral/melt partition coefficients for several FRTE (Sc, Ti, V, Cr, Mn, Fe, Co, Zn) and Ga and Ge during partial melting of peridotite. Experiments were cooled slowly from ≥50 °C above the intended dwell temperature to facilitate growth of large crystals for trace element analysis by LA-ICP-MS. Measured partition coefficients form the basis of a partial melting model that predicts FRTE concentrations and ratios in partial melts of peridotite at 3 GPa. Partial melts of peridotite have Fe/Mn < 62, Zn/Fe < 13*10-4, and Co/Fe > 7*10-4. Many OIB have higher Fe/Mn and lower Co/Fe than can be obtained by partial melting of peridotite, at least under relatively reducing conditions. Experiments were performed using the starting compositions from the MISE experiments but with added K2O in the melt to determine the effects of K2O on the composition of near-solidus melts of peridotite at 3 GPa. In resulting melts SiO2 increases and CaO decreases by ~0.5 wt.% for each 1 wt.% increase in the K2O content of the melt. Al2O3 increases and Cr2O3 and Na2O decrease with increasing K2O content of the melt. Increased K2O content moves the compositions of near-solidus melts of peridotite toward the low-CaO, high-SiO2 EM-type OIB, but the effect is not strong enough to match these compositions at comparable K2O concentrations. Spinel lherzolite KLB-1, which is used as a starting material in many high pressure experiments, was analyzed for major and minor elements. To perform the bulk analysis, powdered KLB-1 was glassed by laser melting of aerodynamically levitated spheroids. The glass beads and minerals separates from the powdered sample were analyzed by electron microprobe analysis. This measurement resolves conflicting FeO, CaO, and TiO2 values from two older measurements, and allows for an improved estimate of the modal proportion of mineral phases determined by mass balance.