Geochemistry of molybdenum in the continental crust
Geochemistry of molybdenum in the continental crust
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DOI：
10.1016/j.gca.2018.06.039
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发表时间：
2018-10-01
影响因子：
5
通讯作者：
Clemens, John D.
中科院分区：
文献类型：
--
作者：
Greaney, Allison T.;Rudnick, Roberta L.;Clemens, John D.
使用钼作为定量的古大气氧化还原传感器是基于钼赋存于上大陆地壳(UCC)的硫化物中的假设。通过确定典型太古宙、元古界和显生界上地壳火成岩中钼的矿物学寄主，从玄武岩到花岗岩的成分范围，验证了这一假设。常见的火成岩硫化物，如黄铁矿和黄铜矿，钼含量很低(通常低于10 ng/g左右的检测下限)，不是重要的地壳钼宿主。相比之下，火山玻璃和含钛相如钛铁矿、钛铁矿、磁铁矿和金红石的钼含量明显较高(例如，钛铁矿中的钼含量高达40微克/克)，并可在大多数岩石中解释整个岩石的钼平衡。然而，在用原位方法分析的10个花岗岩中，有4个没有实现全岩和矿物数据之间的质量平衡，这些花岗岩中的钼可能赋存于未被检测到的微量辉钼矿中。这里分析的几乎所有花岗岩中都存在显著的Mo亏损(即UCC标准化的Mo/Ce和Lt；1)，但在大洋玄武岩或它们的分异中不存在(Greaney等人，2017；Jenner和O‘Neill，2012)。平均而言，花岗岩的钼含量约为其预期含量的60%。这可能有两个原因：(1)Mo分成水岩浆气相/流体相，从冷却的深成岩浆中排出，和/或(2)在演化的岩浆部分熔融和分离结晶过程中，Mo分成含钛相。第一种情况很可能是考虑到氧化钼的高溶解度。然而，几个深成岩套中Mo/Ce和Nb/La的相关关系表明，金红石或Fe-Ti氧化物等分馏相可能将Mo隔离在下地壳岩石或弧形俯冲板块中。(C)2018爱思唯尔有限公司。保留所有权利。
The use of molybdenum as a quantitative paleo-atmosphere redox sensor is predicated on the assumption that Mo is hosted in sulfides in the upper continental crust (UCC). This assumption is tested here by determining the mineralogical hosts of Mo in typical Archean, Proterozoic, and Phanerozoic upper crustal igneous rocks, spanning a compositional range from basalt to granite. Common igneous sulfides such as pyrite and chalcopyrite contain very little Mo (commonly below detection limits of around 10 ng/g) and are not a significant crustal Mo host. By contrast, volcanic glass and Ti-bearing phases such as titanite, ilmenite, magnetite, and rutile contain significantly higher Mo concentrations (e.g., up to 40 mu g/g in titanite), and can account for the whole-rock Mo budget in most rocks. However, mass balance between whole-rock and mineral data is not achieved in 4 out of 10 granites analyzed with in-situ methods, where Mo may be hosted in undetected trace molybdenite. Significant Mo depletion (i.e., UCC-normalized Mo/Ce < 1) occurs in nearly every granitic rock analyzed here, but not in oceanic basalts or their differentiates (Greaney et al., 2017; Jenner and O'Neill, 2012). On average, granites are missing similar to 60% of their expected Mo contents. There are two possible reasons for this: (1) Mo partitions into an aqueous magmatic vapor/fluid phase that is expelled from cooling plutons, and/or (2) Mo is partitioned into titaniferous phases during partial melting and fractional crystallization of an evolving magma. The first scenario is likely given the high solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest fractionating phases such as rutile or Fe-Ti oxides may sequester Mo in lower crustal rocks or in subducting slabs in arc settings. (C) 2018 Elsevier Ltd. All rights reserved.