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

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

Jan 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49..289e&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 49, Issue 1, pp.289-298

Computer Science

5

Scientific paper

Many mid-ocean ridge basalts (MORBs) might be ultimately derived from primary magmas that are very depleted in Na 2 O and TiO 2 . These very depleted primary magmas have 0.60 to 1.50 wt.% Na 2 O and 0.10 to 0.50 wt.% TiO 2 compared to MORBs, which typically have > 1.90% Na 2 O 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 2 O 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 2 O 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 2 O for the lowermost cumulate ultramafic rocks, with TiO 2 and Na 2 O 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 2 O and TiO 2 can differentiate to form residual magmas that are indistinguishable from MORBs.

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