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

Many mid-ocean ridge basalts (MORBs) might be ultimately derived from primary magmas that are very depleted in Na₂O and TiO₂. These very depleted primary magmas have 0.60 to 1.50 wt.% Na₂O and 0.10 to 0.50 wt.% TiO₂ compared to MORBs, which typically have > 1.90% Na₂O and >0.60% TiO₂. 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 (Na₂O 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 Na₂O and TiO₂ 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.% TiO₂ and 0.60 to 0.80 wt.% Na₂O for the lowermost cumulate ultramafic rocks, with TiO₂ and Na₂O 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 Na₂O and TiO₂ can differentiate to form residual magmas that are indistinguishable from MORBs.