The behavior of Fe3+/ΣFe during partial melting of spinel lherzolite
The behavior of Fe3+/ΣFe during partial melting of spinel lherzolite
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DOI：
10.1016/j.gca.2016.03.019
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发表时间：
2016-07-15
影响因子：
5
通讯作者：
Gaetani, Glenn A.
中科院分区：
文献类型：
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作者：
Gaetani, Glenn A.
本文提出了一个尖晶石二辉橄榄岩部分熔融过程中Fe ~（3+）/σ Fe行为的内洽模型。橄榄石的Fe 3 +/σ Fe比是基于点缺陷热力学计算的，并且其他固相中的铁的氧化态是使用橄榄石与斜方辉石、单斜辉石或尖晶石之间的Fe 3 +/Fe 2+分布来计算的。质量守恒是用来与部分熔体的Fe 3 +/Fe 2+比的初始和残余固体中的Fe 3+和Fe 2+的浓度作为压力，温度和氧逸度的函数。从等压批量熔化计算的结果表明，部分熔体的Fe 3 +/σ Fe比随熔体分数的增加而降低。相反，部分熔体的Fe 3 +/σ Fe比随着减压批料熔融期间熔体分数的增加而增加。上地幔的相对氧逸度取决于铁的氧化态和地幔位温。从增量减压熔融计算，其中1%的熔体产生为每100 MPa的减压，然后从残余固体中除去的结果表明，相对氧逸度计算的铁在玄武岩玻璃中的氧化态并不代表一个独特的值为海洋上地幔，而是反映了熔融制度的下部的条件。地幔潜在温度变化100摄氏度，相对氧逸度的变化量约为0.8个对数单位，与大洋中脊玄武岩玻璃推断的全球范围相似。因此，在得出大洋上地幔铁氧化态不均匀性的结论之前，有必要将由玄武岩玻璃计算的相对氧逸度与潜在温度的代用指标进行比较。模型计算的结果还表明，亚弧地幔本质上比大洋地幔更氧化，因为它更冷。玄武岩玻璃中Fe ~（3+）/Sigma Fe与H_2O之间的全局相关性可能部分反映了后者的固形物降低影响。由于地球在地质时期的冷却，上地幔的相对氧逸度的长期变化似乎没有大到足以影响火山释气的性质，足以促成大氧化事件。(C)2016爱思唯尔有限公司版权所有
This study presents an internally consistent model for the behavior of Fe3+/Sigma Fe during partial melting of spinel lherzolite. The Fe3+/Sigma Fe ratio for olivine is calculated on the basis of point defect thermodynamics, and the oxidation states of iron in the other solid phases are calculated using Fe3+/Fe2+ distribution between olivine and orthopyroxene, clinopyroxene, or spinel. Conservation of mass is used to relate the Fe3+/Fe2+ ratio of partial melt to the concentrations of Fe3+ and Fe2+ in the initial and residual solids as a function of pressure, temperature, and oxygen fugacity. Results from isobaric batch melting calculations demonstrate that the Fe3+/Sigma Fe ratio of the partial melt decreases with increasing melt fraction. Conversely, the Fe3+/Sigma Fe ratio of the partial melt increases with increasing melt fraction during decompression batch melting. The relative oxygen fugacity of the upper mantle depends on both the oxidation state of iron and mantle potential temperature. Results from incremental decompression melting calculations in which 1% melt is produced for each 100 MPa of decompression and then removed from the residual solid indicate that relative oxygen fugacity calculated from the oxidation state of iron in basaltic glass does not represent a unique value for the oceanic upper mantle but, rather, reflects conditions in the lower portion of the melting regime. A 100 degrees C change in mantle potential temperature produces a change in relative oxygen fugacity of similar to 0.8 log units, similar to the global range inferred from mid-ocean ridge basalt glasses. It is necessary, therefore, to compare relative oxygen fugacity calculated from basaltic glass with proxies for potential temperature before drawing conclusions on heterogeneity of the oxidation state of iron in the oceanic upper mantle. Results from model calculations also suggest that the sub-arc mantle is intrinsically more oxidizing than the oceanic mantle because it is cooler. The global correlation between Fe3+/Sigma Fe and H2O in basaltic glasses may, in part, reflect the solidus-lowering influence of the latter. Secular variations in relative oxygen fugacity of the upper mantle due to cooling of the Earth over geologic time do not appear to be large enough to have influenced the nature of volcanic outgassing significantly enough to have contributed to the Great Oxidation Event. (C) 2016 Elsevier Ltd. All rights reserved.