Evidence for variations in magma production along oceanic spreading centers: a critical appraisal.

Recent geologic, geophysical, and geochemical studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the “magma budget,” defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading, decreases as fracture zones are approached. Seismic reflection and refraction studies indicate that the crust becomes anomalously thin near fracture zones, but reference must be made to the boundary between residual upper mantle peridotites and overlying cumulate rocks in order to assess potential variations in the magma budget. The position of this interface, however, generally is not constrained by geophysical studies. Geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached, but they could also be produced by variations in open-system mag-matic processes with no change in the extent of partial melting. Although a decrease in the magma budget as fracture zones are approached is consistent with these data, so are alternative models that incorporate a constant magma budget.