Geological and Geophysical Investigation of the Mid-Cayman Rise Spreading Center: Geochemical Variation and Petrogenesis of Basalt Glasses

Basalt glasses erupted from the transverse spreading center within the Cayman Trough have higher contents of TiO₂, Na₂O, K₂O, P₂O₅, Zr, Y, Sr, and Ba than typical mid-ocean ridge basalts (MORB). These compositional features are unique and the glasses appear to be transitional between fractionated MORB and incompatible-element enriched basalts from certain oceanic regions such as the mid-Atlantic Ridge near 43°N, the triple junction near Bouvet, and Fracture Zone B at the FAMOUS (MAR 37°N) spreading center. The major element compositional variation defines an apparent cotectic, in the normative plagioclase-pyroxene-olivine triangle, which is systematically offset into the plagioclase field compared to most other MORB trends. Model fits based on the assumption of simple, single-stage fractional crystallization are very good for both major and trace elements, with no apparent "decoupling" of the incompatible elements as observed in the FAMOUS (37°N) spreading center. These calculations indicate that, like typical MORB, three-phase crystallization is required to explain the major and trace element variations, although only plagioclase and olivine are observed as liquidus (phenocryst) phases. High-pressure crystallization of clinopyroxene is suggested by mineral assemblages in Cayman Trough gabbros and by analogy with experimental data, and may be favored by the special tectonic setting of the Cayman Trough. Other geochemical peculiarities of the glasses may be related in part to local characteristics of the mantle source beneath the equatorial Atlantic and/or Caribbean plate.