(a) Percentage of natural pyroxenite compositions [Lambart et al., 2009a and references therein] that have calculated T5% (Melt-PX) > T5%,peridotite [Katz et al., 2003] as a function of mantle potential temperature (TP) (see text for details). Green and red vertical bands denote the TP ranges for MORB and OIB, respectively, [Courtier et al., 2007]; values at the boundaries of the bands denote pyroxenite percentages with T5% > T5%,peridotite. (b) Na2O + K2O (oxide mol.%) vs. bulk Mg# for the global suite of SD and SE pyroxenites (pyrox). Solid red and green lines separate those pyroxenites with T5% < T5%,peridotite (to the left) from those with T5% > T5%,peridotite (to the right) for TP values of 1500°C (red) and 1350°C (green); equations for the lines are: (Na2O + K2O) = 19.05·Mg# – 15.03 (red) and (Na2O + K2O) = 15·Mg# – 12.16 (green).
This figure illustrates although it is generally assumed that near-solidus temperatures (i.e., T5%) for pyroxenites are below those of fertile peridotite, our modeling results show that along an adiabatic path, between 8 and 20% of pyroxenite compositions start to melt at shallower pressures than fertile peridotite for the range of estimated TP values beneath mid-ocean ridges. This proportion increases to 25% for the range of TP values estimated for oceanic islands.

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