Melting of a mantle source, consisting of multiple rock types, each with its own melting behavior and chemical and isotopic composition, is believed to be an important factor in producing the range of magma types characteristic of individual igneous provinces. An important example of such a compound source is a dominantly peridotitic mantle with minor eclogitic or pyroxenitic veins—the literature concerning this particular example has grown substantially over last decade. Models of the melting of such mixtures require knowledge of the relationships between melt fraction (F), temperature (T), pressure (P), and bulk composition (X) for both peridotites and pyroxenites. While various parameterizations are available to model the melting behavior of peridotites, none yet exist to model pyroxenite melting.
I present here a parameterization that, while remaining mathematically simple, succeeds in capturing the important features of the behavior of pyroxenites melting.
We have used 193 high-P experiments (20 bulk compositions) in which F has been determined up to the disappearance of clinopyroxene (i.e., cpx-out) for pyroxenites over a substantial range of P and T (0.9–5 GPa; 1150–1675°C) to constrain a model that can be used to estimate T from near the solidus (i.e., T5%, the T at which F = 5%) up to cpx-out (Tcpx) for mantle pyroxenites spanning a wide compositional range.
When applied to the 193 experimental data, the model reproduces experimental F values with an average uncertainty of 15%, absolute; T5% of pyroxenites are reproduced with an average uncertainty of 28°C over a temperature range of ~500°C.
In conjunction with estimates of heats of fusion for pyroxenite and peridotite lithologies, these parameterizations permit calculations of how multi-lithologic mantle sources melt during adiabatic decompression, including the effects of varying the composition and the modal proportion of pyroxenite in such source regions. A potentially important prediction of our modeling is that a significant fraction of mantle-derived pyroxenites have both higher solidus temperatures and higher productivities than fertile peridotite at high pressures and therefore will begin to melt at shallower levels than fertile peridotite.