Investigation of stable Cr isotopic fractionation during Earths silicate differentiation and reassessment of the accurate Bulk Silicate Earth delta53Cr value

已观察到岩浆岩和储集层的铬同位素组成与硅酸盐分异指标(如镁#)相关。几十年来，人们还知道，镁铁质矿物，如尖晶石、橄榄石和辉石，除了Cr3+的氧化态外，还可以含有大量的Cr2+。此外，最近的研究表明，在部分地幔熔融过程中形成的熔体具有不同的Cr2+/Cr3+比值，这取决于环境的氧逸度。因此，在硅酸盐分异过程中，铬同位素可能发生分馏，这种分馏的大小取决于当时的氧逸度。在这项研究中，我们打算研究(I)具有不同熔融条件(如熔融度、fO2)的玄武岩储集层(MORB、OIB、大陆泛滥玄武岩、太古代科马提岩)的铬同位素组成，(Ii)造山橄榄岩岩体岩石中发现的大的铬同位素变化的原因，(Iii)橄榄岩中的橄榄石、辉石、尖晶石或石榴石是否具有不同的铬同位素组成，如果是的话，(Iv)这些变化如何与矿物组成有关，以及矿物是否处于同位素平衡状态。然而，这项研究的最终目的是测试稳定的铬同位素是否可以作为一种工具来追踪部分地幔熔融过程中的主要氧逸度。

It has been observed that Cr isotope compositions of magmatic rocks and reservoirs correlate with indicators for silicate differentiation (e.g. Mg#). It is also known for decades that mafic minerals such as spinel, olivine and the pyroxenes can contain significant amounts of Cr2+ besides the Cr3+ oxidation state. Moreover, recent studies show that melts formed during partial mantle melting have different Cr2+/Cr3+ ratios depending on the ambient oxygen fugacity. It is thus possible that Cr isotopes fractionate during silicate differentiation and that the size of this fractionation is depending on the prevailing oxygen fugacity. In this study we intend to investigate (i) the Cr isotopic compositions of basaltic reservoirs (MORB, OIB, continental flood basalts, archean komatiites) that had variable melting conditions (e.g. degree of melting, fO2), (ii) the cause for the large Cr isotopic variations found in rocks of orogenic peridotite bodies, (iii) whether olivine, pyroxene, spinel or garnet in peridotites have variable Cr isotopic compositions and if so (iv) how these may be dependent on the minerals composition and whether the minerals are in isotopic equilibrium. The ultimate goal of this study however is to test whether stable Cr isotopes can be used as a tool to trace prevailing oxygen fugacity during partial mantle melting.