Resolving the Paradox of fO2 in Arcs Using V/Sc Systematics of Primitive Lavas as a Proxy for Mantle fO2: Towards an Internally Consistent Dataset

这项提议的目标是研究上地幔的氧化潜力(氧逸度)，特别是在俯冲带。氧逸度很重要，因为它决定了元素的氧化状态，从而决定了化学行为。特别令人感兴趣的是，氧逸度在俯冲环境中是如何变化的，从地幔楔体内的熔融开始，到地壳水平的岩浆侵位和喷发结束。一个需要检验的具体假设是，人们普遍认为，弧状熔岩的地幔源区的氧逸度比大洋中脊下的氧逸度高得多。这一观点在很大程度上源于这样一种观察，即弧状熔岩本身似乎被高度氧化，而板块衍生的流体交代地幔楔体很可能是氧化的。虽然估算岩浆的氧逸度相当简单，但估算地幔熔体源区的氧逸度就不那么简单了，因为需要一种方法来“看穿”岩浆的分异过程。这一建议试图利用原始弧玄武岩的V/Sc比值来观察地幔。V的分配行为是氧化还原敏感的，而SC的分配行为不是。对于原始岩浆，这一比例不太可能因橄榄石的早期结晶而显著改变。因此，V/Sc比值可能能够“看穿”早期的岩浆分异过程，并进入地幔。初步数据表明，V/Sc比值与流体贡献指数(如Ba/Yb)略有关联，但总体上，弧岩浆的V/Sc比值与大洋中脊的V/Sc比值没有显著差异。将努力在这些调查的基础上，对选定的几个弧形系列进行更详细的调查。这项拟议的研究将在两年的时间里支持一篇博士论文和一到两名本科生。

The goal of this proposal is to investigate the oxidizing potential (oxygen fugacity) of the upper mantle, with special emphasis in subduction zones. Oxygen fugacity is important because it dictates an element's oxidation state, and hence chemical behavior. Of particular interest is how oxygen fugacity varies in subduction environments, beginning with melting within the mantle wedge and ending with emplacement and eruption of the magma at crustal levels. A specific hypothesis to be tested is the widespread notion that the mantle source regions of arc lavas are characterized by oxygen fugacities much higher than that beneath mid-ocean ridges. This view derives largely from the observation that arc lavas themselves appear to be highly oxidized and that slab-derived fluids metasomatizing the mantle wedge are likely to be oxidizing. While estimating the oxygen fugacity of magmas is fairly straightforward, estimating that of the melt source region in the mantle is not so straightforward because a method is required that can "see through" magmatic differentiation processes. This proposal seeks to use V/Sc ratios of primitive arc basalts to see into the mantle. The partitioning behavior of V is redox-sensitive, whereas that of Sc is not. For primitive magmas, this ratio is unlikely to be modified significantly by early crystallization of olivine. Accordingly, the V/Sc ratio may be able to "see through" early magmatic differentiation processes and into the mantle. Preliminary data suggest that V/Sc ratios are slightly correlated with indices of fluid contributions (e.g., Ba/Yb), but that overall, the V/Sc ratios of arc magmas do not significantly differ from that of mid-ocean ridges. Efforts will be made to build on these investigations in more detail on several selected arc series. The proposed study will support 1 PhD thesis and one or two undergraduates over the course of two years.