Abyssal peridotite samples from slow-spreading oceanic ridges have been interpreted as residues of near-fractional melting processes on the basis of trace-element data, whereas major-element compositions and modes of the same samples require interactions between migrating melts and residual solids, either by equilibrium porous flow, refertilization, or olivine crystallization. Modeling of major- and trace-element data shows that these peridotite samples are consistent with a variety of melting and melt migration histories that include elements or episodes both of near-fractional melting and of equilibrium porous flow. A component of equilibrium porous flow explains peridotite compositions better than olivine deposition or refertilization. Mixing of primary basalt liquids composed of variable proportions of unmodified liquid increments extracted by near-fractional melting, and of liquids transported by equilibrium porous flow generates local trend systematics like those observed in fractionation-corrected basalt compositions at slow-spreading ridges. Both the local trend in basalts and the fractionated trace elements in peridotites are absent at the fast-spreading East Pacific Rise, allowing simpler models of melting and melt migration than those required at Atlantic and Indian ridges and implying a spreading-rate or magma-flux dependence to the mechanism of melt extraction. (C) 1999 Elsevier Science B.V. All rights reserved.
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