The crystallization of mid-ocean ridge basalts at moderate and high pressures

MORB liquid lines of descent and oceanic cumulate both record effects of crystallization at moderate to high P, particularly in the magnesian clinopyroxene megacrysts common within MORBs. The onset of clinopyroxene crystallization can be detected in the MORB liquid lines of descent by a decrease in the CaO contents; if the MgO content at which this decrease in CaO begins is significantly higher than is found in 1 atm experiments, it is likely that the chemical variations in a suite of natural basaltic glasses were produced by crystallization at moderate to high P. As an example, the CaO contents of basaltic glasses from the Mid-Cayman Rise decrease continuously from 11.1 to 8.7% as MgO decreases from 7.7 to 5.4%, indicating that the apparent liquid line of descent for these glasses was produced by the co-crystallization of olivine, plagioclase and clinopyroxene (in contradiction of 1 atm studies indicating that clinopyroxene does not crystallize in these samples). It is suggested that the liquid line of descent for the Mid-Cayman Rise glasses was produced by the crystallization of basaltic liquids at approx 6 kbar, where clinopyroxene would be a near-liquidus phase.