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

Abstract

The ascent of magmas in the sub-oceanic mantle to crustal-level magma chambers provides an opportunity for mid-ocean ridge basalts (MORBs) to undergo crystallization at moderate (3 to 10 kbar) to high (10 to 25 kbar) pressures. Both MORB liquid lines of descent and oceanic cumulates record effects of crystallization at moderate to high pressures. These effects include the crystallization of magnesian pyroxenes, especially the magnesian clinopyroxene megacrysts common within MORBs. Many ultramafic and gabbroic cumulates include magnesian pyroxenes and would appear to have formed at moderate to high pressures as well. The onset of clinopyroxene crystallization can be detected in 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 observed in 1 atm experiments, it is likely that the chemical variations in the suite of natural basaltic glasses were produced by crystallization at moderate to high pressures. As an example, the CaO contents of basaltic glasses from the Mid-Cayman Rise continuously decrease 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. This inference of early crystallization of clinopyroxene in Mid-Cayman Rise glasses contradicts petrographic data, calculated 1 atm liquid lines of descent, and 1 atm experimental studies, all indicating that clinopyroxene does not crystallize in these samples. It is proposed that the observed liquid line of descent for the Mid-Cayman Rise glasses was produced by the crystallization of basaltic liquids at ∼ 6 kbar, where clinopyroxene would be a near-liquidus phase