The very depleted nature of certain primary mid-ocean ridge basalts

Abstract

Many mid-ocean ridge basalts (MORBs) might be ultimately derived from primary magmas that are very depleted in Na 2O and TiO 2. These very depleted primary magmas have 0.60 to 1.50 wt.% Na 2O and 0.10 to 0.50 wt.% TiO 2 compared to MORBs, which typically have > 1.90% Na 2O and >0.60% TiO 2. Evidence for these depleted primary magma compositions is obtained from megacrysts in MORBs, from glass inclusions within these megacrysts, and from the highly calcic plagioclases (An 91–96) and depleted clinopyroxenes (Na 2O mostly between 0.10 and 0.35) in certain abyssal peridotites. Cumulate ultramanfi and gabbroic rocks from the North Arm Mountain Massif of the Bay of Islands ophiolite complex show a progressive increase in the Na 2O and TiO 2 abundances in clinopyroxene crystals with stratigraphic height in the ophiolite. The use of mineral-liquid distribution coefficients and cumulate mineral compositions indicate that the liquids from which these minerals crystallized had 0.10 to 0.20 wt.% TiO 2 and 0.60 to 0.80 wt.% Na 2O for the lowermost cumulate ultramafic rocks, with TiO 2 and Na 2O abundances of liquids increasing progressively to normal MORB abundances during crystallization of higher-level gabbroic cumulates. These data clearly demonstrate that primary basalts that are very depleted in Na 2O and TiO 2 can differentiate to form residual magmas that are indistinguishable from MORBs.