Origin of compositional trends in clinopyroxene of oceanic gabbros and gabbroic rocks: A case study using data from ODP Hole 735B

The Ocean Drilling Program Hole 735B near the Southwest Indian Ridge represents the most complete in situ section of oceanic lower crust. The drill core samples are gabbros with several sub-divisions defined by varying amounts of minor phases. These samples have been subject to many studies. Among several yet to be understood phenomena is an apparently unexpected observation in the compositions of the constituent clinopyroxene (cpx). TiO2 and Na2O in cpx do not show expected inverse correlations with MgCpx#. Instead, both TiO2 and Na2O increase with decreasing MgCpx#, reaching a maximum at MgCpx# = ~ 0.76–0.73. With continued MgCpx# decrease, TiO2 decreases sharply whereas Na2O declines gently and flattens out. This observation has been previously interpreted as resulting from boundary layer crystallization within a steady-state magma chamber. In this study, we show that the Na2O–MgCpx#and TiO2–MgCpx#co-variations in cpx can be explained as a straightforward consequence of basaltic magma evolution and related cpx crystal stoichiometry control without invoking complex magma chamber processes. Our interpretation is superior to the boundary layer crystallization model (1) because the latter requires a steady-state magma chamber that is inconsistent with the observation of melt emplacement as thin sills, making convective and steady-state magma chambers unlikely at the slow-spreading Southwest Indian Ridge; and (2) because the TiO2 and Na2O maxima in cpx correspond to the onset and continued crystallization of Fe–Ti oxides or the basalt–andesite transition stage of tholeiitic basaltic melt evolution.