Petrogenesis of Basalt Glasses from the Tamayo Region, East Pacific Rise

Samples collected by the Tamayo scientific team both by dredging and by submersible along 75 km of the East Pacific Rise (EPR) up to its intersection with the Tamayo fracture zone allow exploration of the systematics of ocean ridge basalt petrogenesis in the vicinity of a major transform fault. To investigate these systematics twenty-nine samples of hand-picked glasses have been analyzed for major elements, REE and other trace elements. There are distinct chemical differences between samples collected far from the Tamayo transform and close to the transform. Samples furthest from the transform, where the EPR is characterized by a broad swell, are, for the same MgO content, lower in abundances of all incompatible elements than samples near the transform where the ridge morphology is characterized by a rift. We call such chemical systematics the transform fault effect (TFE). Possible models for the transform fault effect include: (1) fractional crystallization at low or high pressure; (2) partial melting; and (3) open system fractionation and mixing. These models have been evaluated using accurately calculated liquid lines of descent for the major elements, and an inversion technique for the trace elements. Although low and possibly high pressure fractionation are important, partial melting accounts best for the variations among the Tamayo parental magmas. Basalts erupted in close proximity (< 16 km) to the transform (‘rift’ samples) are derived from melts generated by smaller extents of melting than those erupted farther (45–75 km) from the transform (‘swell’ samples). Inversion of the Tamayo trace element data shows that batch melting can account quantitatively for the trace element variations, provided that the extents of melting are very small (1–5 per cent). Continuous melting seems more physically realistic and allows slightly higher extents of melting although the total amount of melt removed from the mantle would be of the order of 5 per cent. Both melting models account for the data better if the swell samples are derived from magmas formed at greater depths where garnet is a stable phase while the rift samples are derived at shallower levels where garnet is not stable. This suggests that melting may occur near the garnet/spinel phase transition in the mantle. The lesser extents of melting near the transform could be caused directly by the cooling effects of ridge truncation, or by perturbation of mantle flow and magma dynamics at ridge/transform in