PRIMARY DIFFERENCES IN OXYGEN FUGACITY AND DEPTH OF MELTING IN THE MANTLE SOURCE REGIONS FOR OCEANIC BASALTS

Abstract

Most abyssal basalt can be divided into two groups based on major and minor element compositions of the glassy rinds of pillow lavas. Crystal differentiation models cannot relate one group to the other. Instead, depth of partial melting may exert the primary control. The first group (MORB-1) is characterized by low TiO2 and Na2O and higher MgO, and is derived from a primary melt originating from the spinel peridotite facies. The higher TiO2-NaO2 group (MORB-2) appears to have been derived from the lower-pressure (shallower depth) plagioclase peridotite facies. Mössbauer spectra show that MORB-1 is from a mantle source region with lower oxidation states (between the QFM and NNO buffers) than for MORB-2 (around the NNO buffer).