Iron partitioning between basaltic melts and clinopyroxene as a function of oxygen fugacity
Iron partitioning between basaltic melts and clinopyroxene as a function of oxygen fugacity
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DOI：
10.2138/am-2004-11-1214
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发表时间：
2004-11
影响因子：
3.1
通讯作者：
M. McCanta;M. Dyar;M. Rutherford;J. Delaney
中科院分区：
文献类型：
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作者：
M. McCanta;M. Dyar;M. Rutherford;J. Delaney
摘要 氧逸度 (fO2) 是一个重要参数，因为它影响岩浆的结晶顺序以及所得矿物的成分。传统上，fO2 是使用硅酸盐熔体中共存的 Fe-Ti 氧化物成分或 Fe3+/Fe2+ 比率来估算的，为此已经校准了 fO2 和 Fe3+ 之间的关系。当系统中不存在氧化物和/或玻璃（熔体）时，或者当氧化物被怀疑已重新平衡时，就会出现问题。因此，开发观察火山岩中 fO2 的替代方法非常重要。在这项工作中，我们描述了一组实验来建立单斜辉石的 Fe3+ 含量与玄武岩熔体之间的关系，作为氧逸度的函数。选择代表火星母体熔体的起始整体成分，其液面上含有辉石（辉石或辉石），以便结果适用于火星陨石岩浆源区。实验条件范围为：对于变辉石/熔体对，在 1165 °C 时，在 1235 °C 时，实验条件为 fO2 = IW 到 ~IW+8.6；对于辉石/熔体对，实验条件范围为，在 ~1190 °C 时，fO2 = IW-1 到 IW+4。结果表明，在此实验条件范围内，Fe3+ 占总铁的 0-65% 不等，而退变辉石和辉石的质量 D（wt%Fe3+Pyx/wt%Fe3+ 熔体）分别在 0.75-1.44 和 0.00-0.77 范围内。 XANES 和穆斯堡尔分析方法在两种技术的已知误差范围内给出了相似的结果，其中 Fe3+ > 10% 的成分最为一致。我们的数据表明，只有在 fO2 高值（>IW + 3.5，或 ≈QFM）下，辉石中的 Fe3+ 分布才能用于估计结晶氧逸度。
Abstract Oxygen fugacity (fO₂) is an important parameter because it influences the crystallization sequences of magmas, as well as the composition of the resultant minerals. Traditionally fO₂ has been estimated using coexisting Fe-Ti oxide compositions or Fe3+/Fe2+ ratios in silicate melts, for which the relationship between fO₂ and Fe3+ has been calibrated. Problems arise in systems where oxides and/or glasses (melts) are absent, or when oxides are suspected to have been re-equilibrated. Therefore, it is important to develop alternate ways to look at fO₂ in volcanic rocks. In this work, we describe a set of experiments to establish the relationship between the Fe3+ content of clinopyroxene and a basaltic melt as a function of oxygen fugacity. The starting bulk compositions were chosen to be representative of martian parental melts, with pyroxene (either augite or pigeonite) on their liquidi, in order for the results to be applicable to the martian meteorite magma source regions. Experimental conditions ranged from fO₂ = IW to ~IW+8.6 at 1165 °C for pigeonite/melt pairs and at 1235 °C for melts alone, and fO₂ = IW-1 to IW+4 at ~1190 °C for augite/melt pairs. Results show that Fe3+ varies from 0-65% of the total Fe over this range of experimental conditions, and mass D (wt%Fe3+Pyx/wt%Fe3+ melt) ranged from 0.75-1.44 and from 0.00-0.77, for pigeonite and augite respectively. XANES and Mössbauer analytical methods give similar results within known error bars of the two techniques, with the best agreement in compositions with >10% Fe3+. Our data show that only at high values of fO₂ (>IW + 3.5, or ≈QFM) can Fe3+ distribution in pyroxene be used to estimate crystallization oxygen fugacity.