Anorthitic plagioclase in plagioclase ultraphyric MORB: new considerations

Abstract:

Nekvasil & Schaub (2019) proposed that the anorthite-rich plagioclase in plagioclase ultraphyric mid-ocean ridge basalts may arise from the behavior of plagioclase and olivine produced by the peritectic melting reaction sp+cpx+opx->ol+pl+liquid at the spinel/plagioclase lherzolite boundary. There is potential that this plagioclase and olivine, if separated from the residuum and carried along with the rising asthenosphere, could melt in the incongruent reaction ol+pl->sp+liquid arising from the pressure-induced solidus depression of anorthite. Our experimental investigations indicate that for plagioclase compositions between An₈₅ and An₉₃, this melting reaction will occur at temperatures much lower than would be anticipated by projection of the typical solidus from more albitic compositions. The slight positive P-T slope of this reaction would allow for retention of liquid+sp during adiabatic ascent until the thermal gradient steepens at shallow depths. At this stage, liquid+sp->ol+pl, but the plagioclase forming would be more anorthitic than even the plagioclase produced at the spinel/plagioclase lherzolite boundary because the standard topology and temperature of the plagioclase melting loop is reestablished below 0.7 GPa. Importantly, the ratio of olivine to plagioclase may not reflect the ratio indicated by the original melting reaction at the spinel/plagioclase lherzolite boundary. If plagioclase can be more readily incorporated into the rising asthenosphere, perhaps due to its lower density, then the final product of this process could contain a much larger fraction of plagioclase than the initial material. Once crystallized at shallow depths, this plagioclase and olivine would be available for incorporation into MORB, and the plagioclase would be too anorthitic to have ever been in equilibrium with MORB.