A Simple Thermodynamic Model for Peridotite Melting in the System NCFMASOCr

Abstract:

Experimental studies have allowed great progress in our understanding of the mineralogy of the upper mantle and its melting behaviour. Whilst experiments provide excellent constraints on mineral and melt compositions at fixed P and T, they are inadequate for examining the continuous range of conditions and small melt fractions relevant to mantle melting. A thermodynamic approach may be used to interpolate between, and extrapolate, experimental conditions, particularly to inaccessible low melt fractions and subsolidus conditions.

We present a new thermodynamic model for the system NCFMASOCr, which allows phase relations in peridotite to be calculated up to 60 kbar. This model builds on the Holland and Powell (2011) dataset to include Fe³⁺ and Cr end-members in pyroxenes, garnet, spinel and melt. These additional components result in an increased stability of spinel and a broadening of the PT region where spinel and garnet coexist. All phase compositions are pressure-insensitive in subsolidus spinel-lherzolites, indicating that a suitable barometer for this lithology may not be found. A simple model has been developed for basaltic melts, presented in terms of 8 endmembers and calibrated from experimental data. Despite the simplifications of the melt model and system (K₂O, TiO₂ and volatiles are neglected), it reproduces other (non-calibration) experimental solidi, liquidi, melt productivity, mineral and melt compositions very well. The model is therefore a useful tool for exploring parameter space not yet examined or accessible by melting experiments. In particular, it is successful in predicting realistic nominal 0% melt fraction compositions, allowing the exploration of the fractional melting origin of basalt.

The inclusion of Fe³⁺ allows us to investigate the relationship between bulk Fe₂O₃, P, T and fO₂, finding that fO₂ and solid-melt Fe³⁺ partitioning is highly sensitive to mineral assemblage.

(1) Holland TJB & Powell R (2011) J Metamorph Geol 29, 333–383.