Abstract
Picritic liquids, formed by 20% to 30% partial melting of upper-mantle 'pyrolite' at 60- to 70-km depth and a temperature of 1400 to 1450 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 lherzolite diapirs at shallow depths ( 25 km). These liquids are termed 'second-stage' melts because they are extracted from lherzolite 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.