Effect of pressure on Fe 3+ /ΣFe ratio in a mafic magma and consequences for magma ocean redox gradients
Effect of pressure on Fe 3+ /ΣFe ratio in a mafic magma and consequences for magma ocean redox gradients
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DOI：
10.1016/j.gca.2017.01.023
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发表时间：
2017-05
影响因子：
5
通讯作者：
Hailin Zhang;M. Hirschmann;E. Cottrell;A. Withers
中科院分区：
文献类型：
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作者：
Hailin Zhang;M. Hirschmann;E. Cottrell;A. Withers
在100 kPa ~ 7 GPa、1400 ~ 1750℃条件下，研究了压力对共存Ru和ruo2缓冲的安山岩硅酸盐熔体Fe3+/ΣFe比的影响。Fe3+/ΣFe比值采用室温Mössbauer光谱法测定，但对无后坐力分数的影响进行了修正。在100 kPa至3 GPa之间，淬火玻璃中的Fe3+/ΣFe比率随着压力的增加而降低，这与之前的结果一致（O’neill等人，2006），但在5 GPa以上仅显示出很小的压力影响。比值也随着温度的升高而降低。Mössbauer超细参数表示玻璃杯中Fe3+离子的平均配位，与玻璃杯淬火的压力无关，但显示Fe2+离子的平均配位随压力的增加而增加，从~ 5到~ 6。这些玻璃上的XANES光谱显示了Fe3+/ΣFe系统的前边缘强度和质心位置的变化，但与在100 kPa下淬火的其他相同的安山岩玻璃所建立的变化有所不同（Zhang等人，2016）。这些系统允许构建新的XANES校准曲线，该曲线与适用于高压玻璃的Fe3+/ΣFe的边前亚峰强度相关。与Mössbauer超细参数的解释一致，高压玻璃中的XANES前边峰特征主要是由于压力对Fe2+离子从~ 5.5到~ 6的配位的影响，而对Fe3+离子的影响可以忽略不计。我们利用这些新数据建立了一个热力学模型，该模型与氧逸度和压力对Fe3+/ΣFe的影响有关。我们应用该模型计算了代表岩浆海洋的岩浆柱（恒定Fe3+/ΣFe）中氧逸度的变化，其中fo2通过与熔融铁的平衡固定在底部。这些计算表明，浅层岩浆海表面的氧逸度比深层的要低得多。对于底部压力接近5 GPa的岩浆海，如水星和月球，其表面条件比底部条件降低约1.5 log单位。如果校准到7 GPa压力的结果可以外推到更大的类地行星（如火星或地球）上岩浆海洋的更高压力，那么表面条件分别比底部条件降低了~ 2或2.5个对数单位。因此，覆盖在浅层岩浆海洋上的大气应该被高度还原并富含h2和CO。
Experiments establishing the effect of pressure on the Fe3+/ΣFe ratio of andesitic silicate melts buffered by coexisting Ru and RuO2were performed from 100 kPa to 7 GPa and 1400–1750 °C. Fe3+/ΣFe ratios were determined by room temperature Mössbauer spectroscopy, but corrected for the effects of recoilless fraction. Fe3+/ΣFe ratios in quenched glasses decrease with increasing pressure consistent with previous results between 100 kPa and 3 GPa (O’Neill et al., 2006), but show only small pressure effects above 5 GPa. Ratios also decrease with increasing temperature. Mössbauer hyperfine parameters indicate mean coordination of Fe3+ions of ∼5 in glasses, with no dependence on the pressure from which the glasses were quenched, but show an increase with pressure in mean coordination of Fe2+ions, from ∼5 to ∼6. XANES spectra on these glasses show variations in pre-edge intensities and centroid positions that are systematic with Fe3+/ΣFe, but are displaced from those established from otherwise identical andesitic glasses quenched at 100 kPa (Zhang et al., 2016). These systematics permit construction of a new XANES calibration curve relating pre-edge sub-peak intensities to Fe3+/ΣFe applicable to high pressure glasses. Consistent with interpretations of the Mössbauer hyperfine parameters, XANES pre-edge peak features in high pressure glasses are owing chiefly to the effects of pressure on the coordination of Fe2+ions from ∼5.5 to ∼6, with negligible effects evident for Fe3+ions. We use the new data to construct a thermodynamic model relating the effects of oxygen fugacity and pressure on Fe3+/ΣFe. We apply this model to calculate variations in oxygen fugacity in isochemical (constant Fe3+/ΣFe) columns of magma representative of magma oceans, in whichfO2is fixed at the base by equilibration with molten Fe. These calculations indicate that oxygen fugacities at the surface of shallow magma oceans are more reduced than at depth. For magma oceans in which the pressure at the base is near 5 GPa, as may be appropriate for Mercury and the Moon, conditions at the surface are ∼1.5 log unit more reduced at the surface than at their base. If the results calibrated up to pressures of 7 GPa can be extrapolated to higher pressures appropriate for magma oceans on larger terrestrial planets such as Mars or Earth, then conditions at the surface are ∼2 or 2.5 log units more reduced at the surface than at the base, respectively. Thus, atmospheres overlying shallow magma oceans should be highly reduced and rich in H2and CO.