Latypov Rais, Costin Gelu, Chistyakova Sofya, Hunt Emma J, Mukherjee Ria, Naldrett Tony
School of Geosciences, University of the Witwatersrand, Johannesburg, 2050, South Africa.
Department of Earth Science, Rice University, Houston, 6100, TX, USA.
Nat Commun. 2018 Jan 31;9(1):462. doi: 10.1038/s41467-017-02773-w.
Platinum-bearing chromitites in mafic-ultramafic intrusions such as the Bushveld Complex are key repositories of strategically important metals for human society. Basaltic melts saturated in chromite alone are crucial to their generation, but the origin of such melts is controversial. One concept holds that they are produced by processes operating within the magma chamber, whereas another argues that melts entering the chamber were already saturated in chromite. Here we address the problem by examining the pressure-related changes in the topology of a MgSiO-CaAlSiO-SiO-MgCrO quaternary system and by thermodynamic modelling of crystallisation sequences of basaltic melts at 1-10 kbar pressures. We show that basaltic melts located adjacent to a so-called chromite topological trough in deep-seated reservoirs become saturated in chromite alone upon their ascent towards the Earth's surface and subsequent cooling in shallow-level chambers. Large volumes of these chromite-only-saturated melts replenishing these chambers are responsible for monomineralic layers of massive chromitites with associated platinum-group elements.
诸如布什维尔德杂岩体这类镁铁质 - 超镁铁质侵入体中的含铂铬铁矿，是人类社会具有战略重要性金属的关键储存库。仅在铬铁矿中饱和的玄武质熔体对其形成至关重要，但其熔体的起源存在争议。一种观点认为它们是由岩浆房内发生的过程产生的，而另一种观点则认为进入岩浆房的熔体在铬铁矿中已经饱和。在此，我们通过研究MgSiO - CaAlSiO - SiO - MgCrO四元体系拓扑结构中与压力相关的变化，以及通过对玄武质熔体在1 - 10千巴压力下结晶序列的热力学建模来解决这个问题。我们表明，位于深部储层中所谓铬铁矿拓扑槽附近的玄武质熔体，在向地球表面上升并随后在浅部岩浆房中冷却时，仅在铬铁矿中变得饱和。大量这些仅在铬铁矿中饱和的熔体补充这些岩浆房，形成了块状铬铁矿与相关铂族元素的单矿物层。
