Abstract

Picritic liquids, formed by 20% to 30% partial melting of upper-mantle “pyrolite” at 60- to 70-km depth and a temperature of 1,400 to 1,450 °C, are parental to the majority of ocean-floor basalts. Other more-refractory liquids must be generated beneath mid-ocean spreading ridges to provide a source for the very refractory megacrysts entrained by ocean-floor tholeiites and for some cumulate sequences in ophiolites (thought to be segments of oceanic crust). Such liquids have the composition of extremely LREE-depleted magnesian quartz tholeiite (LREE = light rare-earth element) or olivine-poor tholeiite and are derived by small degrees (5% to 10%) of anhydrous melting of refractory Iherzolite diapirs at shallow depths (≤25 km). These liquids are termed “second-stage” melts because they are extracted from Iherzolite diapirs, which are the residue of a previous partial melting of upper-mantle “pyrolite” that yielded first-stage picritic liquids. Rocks representative of these second-stage melts can be recognized among the uppermost lavas in several ophiolites.

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