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

本建议的目的是研究上地幔的氧化电位（氧逸度），特别强调在俯冲带。氧逸度很重要，因为它决定了元素的氧化态，从而决定了元素的化学行为。特别令人感兴趣的是，氧逸度在俯冲环境中是如何变化的，从地幔楔内部的融化开始，到地壳水平岩浆的就位和喷发结束。一个需要检验的具体假设是一个广为流传的观念，即弧熔岩的地幔源区域的特征是氧逸度远远高于洋中脊之下的氧逸度。这种观点主要来自于这样的观察，即弧熔岩本身似乎是高度氧化的，而交代地幔楔的板岩衍生流体很可能是氧化的。虽然估计岩浆的氧逸度相当简单，但估计地幔中的熔融源区域就不那么简单了，因为需要一种能够“看透”岩浆分化过程的方法。这个建议试图使用原始弧玄武岩的V/Sc比率来观察地幔。V的分配行为对氧化还原敏感，而Sc的分配行为则不敏感。对于原始岩浆，这一比值不太可能因橄榄石的早期结晶而发生显著改变。因此，V/Sc比值可以“看穿”早期岩浆分异过程，进入地幔。初步资料表明，V/Sc比值与流体贡献指标（如Ba/Yb）有轻微的相关性，但总体而言，弧岩浆的V/Sc比值与洋中脊没有显著差异。将努力在这些调查的基础上，对几个选定的弧线系列进行更详细的研究。建议研究将在两年的时间内支持1篇博士论文和1至2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.