Fig. 5
From: Platinum-bearing chromite layers are caused by pressure reduction during magma ascent
A physical model for the generation of basaltic melts saturated in chromite alone by a reduction in lithostatic pressure. a Mantle-derived basaltic melts ascending from lower crustal storage regions, or a mantle source, inevitably experience a reduction in lithostatic pressure. This results in shifting of the chromite topological trough and, as a result, basaltic melts located alongside the chromite topological trough, at high pressure regions will become saturated in chromite alone during ascent to shallow-level chambers. Fractional crystallisation of a large volume of these chromite-saturated melts, in an open system where magma can also flow out of the chamber, will produce monomineralic layers of massive chromitites in mafic-ultramafic intrusions. b Phase relations for a primitive basalt (MgO = 15.13 wt.%; Cr2O3 = 0.10 wt.%) in P–T space illustrating the model that basaltic melts located alongside the chromite topological trough first become slightly superheated during their ascent and then saturated in chromite alone after stalling and cooling in shallow-level chambers. Therefore, allowing for the development of massive chromitites in shallow-level chambers. This case is analogous to path C–D in Fig. 3b, c and Fig. 4c, in which multiply-saturated liquids become saturated in chromite alone with pressure reduction. The phase diagram is simplified from Fig. 10 in ref. 28 and is only used to graphically illustrate the principle lying at the heart of our model
