The pyroxene paradox in MORB glasses—a signature of picritic parental magmas?

Abstract

A persistent conclusion reached in studies of glass in the chilled margins of mid-ocean ridge basalts (MORB) is that occult clinopyroxene fractionation is required in order to explain the observed variation in glass compositions¹. Successful crystal-fractipnation models commonly require crystallizing assemblages which are dominated by plagioclase and clinopyroxene rather than the plagioclase and olivine which dominate the phenocryst assemblages. The ‘pyroxene paradox’ arises from the observation that clinopyroxene phenocrysts are uncommon in lavas containing more than 8.5 wt% MgO (refs 2,3) and the fact that melting experiments on liquids with such compositions indicate that clinopyroxene appears only at 20–40°C below liquidus temperatures^4–7. Based on a study⁸ of compositional relationships between glasses and chilled margins of pillow lavas from the Baffin Bay picrite suite, I suggest here that the ‘pyroxene paradox’ is an illusion which arises in part because of the compositional dispersion of olivine fractionation trends from picritic parental magmas between the limiting cases of equilibrium and fractional crystallization.