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

地球硅酸盐分异过程中稳定 Cr 同位素分馏的研究以及准确的块状硅酸盐地球 delta53Cr 值的重新评估

• 批准号: 383556308
• 负责人: Professor Dr. Ronny Schönberg, Ph.D.
• 金额: --
• 依托单位: Arbeitsgruppe Isotopengeochemie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2017
• 资助国家: 德国
• 起止时间: 2016-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/383556308?language=en
• 关键词: Investigation; stable; Cr; isotopic; fractionation

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.

岩浆岩和储层的Cr同位素组成与硅酸盐分异指标（如Mg#）有一定的相关性。几十年来，人们也知道，尖晶石、橄榄石和辉石等基性矿物除了氧化态Cr3+外，还含有大量的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+比率，这取决于环境氧逸度。因此，在硅酸盐分化过程中，Cr同位素可能会发生分馏，而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（如熔融程度，fO2）的玄武岩储层（MORB， OIB，大陆洪泛玄武岩，太古代科马长岩）的Cr同位素组成；（ii）造山橄榄岩中发现的岩石中Cr同位素变化较大的原因；（iii）橄榄石，辉石，橄榄岩中的尖晶石或石榴石具有可变的Cr同位素组成，如果是这样（iv）这些如何取决于矿物组成以及矿物是否处于同位素平衡。然而，本研究的最终目的是测试稳定的Cr同位素是否可以作为一种工具来追踪部分地幔熔融期间的主要氧逸度。