Experimental investigation of the stability of clinopyroxene in mid-ocean ridge basalts: The role of Cr and Ca/Al

The change in the stability field of clinopyroxene in mid-ocean ridge basalt (MORB) as a function of pressure has been used widely as a geobarometer. Based on results from crystallization experiments using MORB-like compositions it has been suggested that MORB differentiation occurs at relatively high pressures at ultraslow- and slow-spreading ridges. However, differentiation requires the loss of substantial heat and it is unclear how this is possible at elevated pressures. To better understand the controls on the stability field of clinopyroxene in MORB-like compositions we report a series of experiments performed at 0.1 MPa in which the temperature of clinopyroxene saturation was determined in melts with variable Cr, Ca/Al and fO2. The results show that increased Cr and Ca/Al lead to an expansion of the clinopyroxene stability field. Incorporating these results into a new model of MORB differentiation shows that realistic parental melt Cr contents can increase the temperature at which clinopyroxene saturation occurs relative to assuming a Cr-free melt (as is commonly the case). Likewise, high Ca/Al melts will saturate clinopyroxene earlier than low Ca/Al melts and their crystallization may provide an explanation for high Mg# clinopyroxene in oceanic gabbros. The newly calibrated geobarometer gives lower crystallization pressures for MORB at the slow-spreading SWIR than previous calibrations, but still suggests relatively higher pressures of crystallization with decreasing spreading rate.