CORRECTIONS AND FURTHER DISCUSSION OF THE PRIMARY MAGMAS OF MIDOCEAN RIDGE BASALTS, 1 AND 2

An error has been detected in the methodology for estimating batch mel t compositions from plagioclase and spinel lherzolites at mantle condi tions presented by Kinzler and Grove (1992a,b) (hereafter referred to as K&G). In a mass balance calculation employed by K&G the melt fracti on was not properly transformed from weight units to molar units, resu lting in an overestimate of the effective melt fraction employed in th e calculation of the Mg # (Mg/[Mg+Fe2+]) of the melt, at melt fraction s greater than 0.01. In the batch melts estimated by K&G, the abundanc e of FeO increases then decreases in the melt with increasing melt fra ction from 0.01 to 0.20. Using the corrected methodology described her e, the abundance of FeO in melts produced via batch melting increases as melt fraction increases from 0.01 to 0.20 (up to the point of exhau stion of clinopyroxene). The correction has essentially no effect on t he compositions of melts produced via the near-fractional melting proc esses described in K&G because melt fractions employed in the near-fra ctional melting calculations never exceeded 0.01. In addition we apply the methodology of K&G to estimate the mantle temperature, average pr essure of melting and average extent of melting beneath a mid-ocean ri dge segment from the FeO and Na2O content of a mid-ocean ridge basalt from that segment.