Recycling reduced iron at the base of magmatic orogens
Recycling reduced iron at the base of magmatic orogens
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DOI：
10.1016/j.epsl.2019.115827
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发表时间：
2019-12
影响因子：
5.3
通讯作者：
Ming Tang;Cin-Ty A. Lee;G. Costin;H. Höfer
中科院分区：
文献类型：
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作者：
Ming Tang;Cin-Ty A. Lee;G. Costin;H. Höfer
地球的大陆地壳被认为起源于地幔的熔融，但它太长英质，相对于原始地幔熔体来说，它的铁是贫乏的。这种铁的贫化在大陆弧岩浆中也很常见，通常被归因于磁铁矿的结晶作用。然而，弧状岩浆中铁的贫化与铁的相对亚铁的富集度相吻合，这不能用磁铁矿等富铁矿物的去除来解释。深部石榴石辉石岩弧堆积体(榴辉岩)具有富铁的成分，补充了大陆地壳和大陆弧岩浆的贫铁性质，很可能是玄武岩地幔岩浆和长英质大陆地壳之间“缺失的一环”的候选者。为了验证这一观点，我们提供了榴辉岩中石榴石的高精度原位Fe价态数据，并重建了全岩Fe价态。结果表明，由于石榴石中Fe~(3+)/∑~(3+)比值较低，且缺乏磁铁矿，钙铝榴石具有较低的体相Fe~(3+)/∑~(2+)Fe。在高压下，石榴石结晶，而磁铁矿不结晶，前者在岩浆造山带底部优先聚集亚铁。凹凸棒石的分馏作用导致氧化性长英石残液的形成(Fe~(3+)/∑Fe为0.2~0.4)。这种氧化作用可能会深刻地影响岩浆挥发分的形态以及地壳的氧化风化能力。
The Earth's continental crust is thought to originate from melting of the mantle, but it is too felsic and depleted in Fe relative to a primary mantle melt. This depletion in Fe is also commonly found in continental arc magmas and is often attributed to magnetite crystallization. However, Fe depletion in arc magmas coincides with an enrichment in ferric Fe relative to ferrous Fe, which cannot be explained by removal of a ferric Fe-rich mineral like magnetite. Deep-seated garnet pyroxenite arc cumulates (arclogites) have Fe-rich compositions that complement the Fe-depleted nature of the continental crust and continental arc magmas, and are likely candidates for the “missing link” between basaltic mantle magmas and the felsic continental crust. To test this suggestion, we present high precision in-situ Fe valence data for garnets in arclogites and reconstruct whole rock Fe valence states. We show that arclogites have low bulk Fe3+/∑Fe due to the low Fe3+/∑Fe of garnets and the lack of magnetite. At high pressures, garnet crystallizes, but magnetite does not, the former causing preferential accumulation of ferrous Fe at the base of magmatic orogens. Arclogite fractionation thus leads to the formation of oxidized felsic residual liquids (Fe3+/∑Fe of 0.2-0.4). Such oxidation may profoundly influence the speciation of magmatic volatiles as well as the oxidative weathering capacity of the crust.