COMPOSITIONAL VARIATIONS OF BASALTIC GLASSES FROM THE MID-CAYMAN RISESPREADING CENTER

Basaltic glasses from the Mid-Cayman Rise (MCR) spreading center have a significant range of compositions, from 7.7% to 5.4% MgO. Well-defin ed trends in the major element and trace element contents of these gla sses are consistent with their development by up to 50% crystallizatio n of parental magmas with only a small range of compositional variabil ity. Calculated liquid lines of descent and 1 atm phase equilibria stu dies of primitive MCR glasses, as well as petrographic relations, indi cate that olivine and plagioclase (+/- spinel) crystallize from MCR li quids at low pressures until the residual liquid has < 6.3% MgO at whi ch point clinopyroxene crystallizes along with olivine and plagioclase . The CaO versus MgO variations in MCR glasses, however, clearly indic ate that clinopyroxene + olivine + plagioclase crystallization has occ urred throughout this suite, not just at the most evolved end. The ons et of clinopyroxene saturation at similar to 7.7% MgO in the MCR glass es, compared to at 6.3% in I atm experimental studies, is attributed t o crystallization at similar to 6 kbar of pressure where the most prim itive MCR glasses are expected to be cosaturated with olivine, plagioc lase, and clinopyroxene. The major element and trace element variation s in the MCR glasses are consistent with the extraction of olivine gab bro cumulates similar in composition to the relatively simple MCR gabb roic 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 pro duced by 10-12% fractional melting of undepleted mantle or 3-6% meltin g of previously depleted mantle.