Compositional variations of basaltic glasses from the Mid‐Cayman Rise spreading center

Abstract

Basaltic glasses from the Mid‐Cayman Rise (MCR) spreading center have a significant range of compositions, from 7.7% to 5.4% MgO. Well‐defined trends in the major element and trace element contents of these glasses are consistent with their development by up to 50% crystallization of parental magmas with only a small range of compositional variability. Calculated liquid lines of descent and 1 atm phase equilibria studies of primitive MCR glasses, as well as petrographic relations, indicate that olivine and plagioclase (±spinel) crystallize from MCR liquids at low pressures until the residual liquid has <6.3% MgO at which point clinopyroxene crystallizes along with olivine and plagioclase. The CaO versus MgO variations in MCR glasses, however, clearly indicate that clinopyroxene + olivine + plagioclase crystallization has occurred throughout this suite, not just at the most evolved end. The onset of clino‐pyroxene saturation at ∼7.7% MgO in the MCR glasses, compared to at 6.3% in 1 atm experimental studies, is attributed to crystallization at ∼6 kbar of pressure where the most primitive MCR glasses are expected to be cosaturated with olivine, plagioclase, and clino‐pyroxene. The major element and trace element variations in the MCR glasses are consistent with the extraction of olivine gabbro cumulates similar in composition to the relatively simple MCR gabbroic rocks. Although there is a significant range in the compositions of potential primary magmas for the MCR glasses, the suite could have been produced by the accumulation of all individual melt fractions produced by 10–12% fractional melting of undepleted mantle or 3–6% melting of previously depleted mantle.