Comment on “MORB—A Mohole misbegotten?” by M.J. O'Hara

Abstract

One can only applaud O'Hara's call for further research into magmatic processes that take place at and near oceanic spreading centers (EOS, June 15, 1982). In particular, the desirability of having deep drill holes close to active spreading centers is indisputable and needs no elaboration. One of the major reasons for encouraging further research on midocean ridge basalt (MORB) petrogenesis is the need to resolve a controversy now being debated in the literature over the composition and depth of origin of primary MORB magmas [Rhodes, 1982]. O'Hara [1968a] and, recently, several others [Green et al., 1979; Stolper, 1980; Elthon and Scarfe, 1980] have argued that primary MORB magmas are picritic in composition and generated at various pressures from 15 to 30 kbar, depending on the author. Others contend that primary MORB magmas are close in composition to the least‐fractionated MORB glasses (which are not picritic but have mg numbers as high as 73) and are generated at more moderate pressures of about 8–10 kbar [Green and Ringwood, 1967; Kushiro, 1973; Presnall et al., 1979]. O'Hara did not mention any of the recent papers that disagreed with his concept of picritic primary magmas at spreading centers. In order to restore some balance, the following brief discussion is offered.