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The activity of chromite in multicomponent spinels: An experimental study with implications for the metamorphic history of equilibrated ordinary chondrites.

机译:多组分尖晶石中亚铬酸盐的活性:对平衡普通球粒陨石变质史的实验研究。

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摘要

An experimental technique for evaluating the activity of chromite in multicomponent spinels was developed by equilibrating the spinel of interest with a Pt-alloy under a controlled temperature and oxygen fugacity. The thermodynamic properties of the ternary Pt-Fe-Cr system was evaluated, such that activities of Cr and Fe in an equilibrated Pt-alloy can be used to calculate the activity of chromite in the spinel of interest.; The ternary activity model formulation used is based on the characterization of each bounding binary system, Pt-Fe, Pt-Cr, Fe-Cr, with the addition of ternary interaction terms. The Pt-Fe and Pt-Cr systems are described as asymmetric regular solutions with interaction parameters of WPtFe = −138.0 ± 3.3, WFePt = −90.8 ± 24.0, and WPtCr = −129.1 ± 1.2, WCrPt = −80.9 ± 4.4, and DPtCr = +94.4 ± 2.5 kJ/mol (1σ), respectively. Combined with literature thermodynamic properties for the Fe-Cr system, the ternary interaction parameters in the Pt-Fe-Cr system were found to be C Cr = 0, CPt = +115.7, and C Fe = −68.6 kJ/mol.; Using this technique, the metamorphic history of equilibrated ordinary chondrites was evaluated by examining the compositions and textures of olivines, pyroxenes, spinels, and alloys. Equilibrium temperatures based on Fe-Mg exchange between olivine and spinel exhibit a range of 680–796°C in H, L, and LL ordinary chondrites spanning petrographic type 4 to 6. Type 4 chondrites in all groups record variable temperatures that are lower than or equal to those of types 5 and 6 chondrites, implying decoupling of metamorphic temperature from petrographic type. Cooling rates near 800°C were found to be 1–3 K/Ma, slow enough to allow continuous re-equilibration of spinel grains from peak metamorphic temperatures to the olivine-spinel equilibration temperature.; The temperature-oxygen fugacity relationships in equilibrated H chondrites were constrained from spinel-alloy and olivine-pyroxene-alloy phase assemblages based on the mineral compositions and activity-composition models. Log10fO2 values based on the assemblage olivine-pyroxene-alloy are −1.75 ± 0.02 log units below Iron-Wüstite (IW) buffer, regardless of petrographic type. The log10fO 2 values calculated based on the spinel-alloy coexistence are at least ∼1.5 log units more oxidizing than those based on the olivine-pyroxene-alloy if olivine-spinel equilibration temperatures (728–820°C) are assumed. This probably indicates that closure for spinel-alloy equilibria occurred under retrograde conditions at temperatures below 700°C.
机译:通过在控制温度和氧气逸度下用Pt合金平衡目标尖晶石,开发了一种评估多组分尖晶石中亚铬酸盐活性的实验技术。评估了三元Pt-Fe-Cr系统的热力学性质,从而可以使用平衡Pt合金中Cr和Fe的活性来计算目标尖晶石中铬铁矿的活性。所用的三元活性模型公式是基于每个有边界的二元体系Pt-Fe,Pt-Cr,Fe-Cr的特征,并添加了三元相互作用项。 Pt-Fe和Pt-Cr系统被描述为不对称正则解,其相互作用参数为 W PtFe = -138.0±3.3, W FePt < / sub> = −90.8±24.0, W PtCr = −129.1±1.2, W CrPt = −80.9±4.4,和<斜体D PtCr = +94.4±2.5 kJ / mol(1σ)。结合Fe-Cr体系的热力学性质,发现Pt-Fe-Cr体系的三元相互作用参数为C Cr = 0,C Pt = +115.7,而C Fe = -68.6 kJ / mol。使用该技术,通过检查橄榄石,辉石,尖晶石和合金的组成和织构来评估平衡的普通球粒陨石的变质历史。基于橄榄石和尖晶石之间的Fe-Mg交换的平衡温度在H,L和LL普通球粒岩(岩石学类型4至6)中表现出680-796°C的范围。所有组中的4型球粒球体记录的可变温度低于等于或等于5型和6型球粒陨石,这意味着变质温度与岩石学类型的去耦。冷却速度接近800°C时为1-3 K / Ma,其速度足以使尖晶石晶粒从峰值变质温度连续重新平衡到橄榄石-尖晶石平衡温度。基于矿物组成和活度-组成模型,从尖晶石合金和橄榄石-辉石-合金相组合约束了平衡H球粒陨石中的温度-氧气逸度关系。 Log 10 f O 2 <不论岩石类型如何,基于橄榄石-辉石合金组合的/ rm> 值均低于铁-威斯特(IW)缓冲液以下-1.75±0.02 log单位。 log 10 f O 如果基于橄榄石-尖晶石平衡,则基于尖晶石合金共存计算的2 值的氧化度至少比基于橄榄石-py烯合金的氧化度高至少约1.5 log个单位。假定温度为(728–820°C)。这可能表明尖晶石合金平衡的关闭发生在低于700°C的逆行条件下。

著录项

  • 作者

    Kessel, Ronit.;

  • 作者单位

    California Institute of Technology.;

  • 授予单位 California Institute of Technology.;
  • 学科 Geochemistry.
  • 学位 Ph.D.
  • 年度 2002
  • 页码 205 p.
  • 总页数 205
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 地质学;
  • 关键词

  • 入库时间 2022-08-17 11:46:13

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