The remelting of hydrothermally altered peridotite at mid-ocean ridges by intruding mantle diapirs

Most gabbroic cumulates found at ocean spreading centres are thought to have been generated by the fractional crystallization of melts with the composition of mid-ocean ridge basalt (MORB)1. There are exceptions, however, including some cumulates which appear to have come from melts that contain more silica than MORB and are much more depleted in the incompatible elements (those elements that do not readily substitute into the main mineral phases)2. These unusual rocks bear witness to relatively deep petrological processes that are not accessible through the study of melts erupted on the sea floor, and their origin is still debated. Fortunately, the same lithologies can be studied in detail in ophiolites (sections of oceanic crust accreted to a continent). In a fossil mantle diapir of the Oman ophiolite3,4, we have observed the same dichotomy between a suite of ‘normal’, MORB-type, cumulates (‘N-cumulates’) and a suite of cumulates issued from silica-enriched but incompatible-element-depleted melts (‘D-cumulates’). While the N-cumulates crystallized inside the diapir, the D-cumulates occur essentially as intrusions surrounding the diapir. The combination of silica enrichment, extreme depletion in incompatible elements, and seawater isotopic signature indicates that the D-cumulates were formed by the remelting at low pressure of hydrated residual peridotites left after MORB extraction at the ridge axis. The distribution of the D-cumulates relative to the N-cumulates suggests that such depleted melts are produced episodically at ridge axes when the lithospheric mantle is reheated by a new diapiric pulse.