Primordial compositions of refractory inclusions
Primordial compositions of refractory inclusions
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DOI：
10.1016/j.gca.2008.04.002
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发表时间：
2008-06-15
影响因子：
5
通讯作者：
Mayeda, T. K.
中科院分区：
文献类型：
--
作者：
Grossman, L.;Simon, S. B.;Mayeda, T. K.
对来自 CV3 球粒陨石的 17 种 A 型和 B 型耐火夹杂物中的每一种进行了整体化学成分和 O、Mg 和 Si 同位素组成的测量。在实验室中针对非代表性取样对大量化学成分进行校正后，使用每个夹杂物的镁和硅同位素成分来计算其原始化学成分，假设这些元素的重同位素富集是由于伴随着它们从 CMAS 液体中蒸发的瑞利分馏所致。由此产生的预蒸发化学成分与高温星云凝聚物平衡热力学计算所预测的一致，但前提是不同的夹杂物是从总压在 10(-6) 至 10(-1) bar 范围内的星云区域凝结而成的，无论它们是在太阳成分系统中形成，还是在富含普通球粒陨石成分的尘埃中形成，而普通球粒陨石成分相对于气体是太阳成分的 10 倍。这类似于太阳星云动态模型预测的温度处于硅酸盐凝结温度范围内的区域的总压力范围。或者，如果发生偏离平衡凝结和/或星云中凝结物的非代表性采样，则推断的总压力范围可能更小。简单的蒸发动力学模型成功地根据推断的蒸发前成分再现了大多数包裹体的观察到的化学成分，这表明封闭系统同位素交换过程对其同位素成分没有显着影响。预蒸发成分与观察到的成分的比较表明，相对于更易挥发的 MgO + SiO2，难熔 CaO + Al2O3 的富集 80% 是由于初始冷凝，20% 是由于 A 型和 B 型夹杂物的后续蒸发。 (c) 2008 Elsevier Ltd. 保留所有权利。
Bulk chemical and O-, Mg- and Si-isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg- and Si-isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates, but only if different inclusions condensed from nebular regions that ranged in total pressure from 10(-6) to 10(-1) bar, regardless of whether they formed in a system of solar composition or in one enriched in dust of ordinary chondrite composition relative to gas by a factor of 10 compared to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al2O3 relative to more volatile MgO + SiO2 is due to initial condensation and 20% due to subsequent evaporation for both Types A and B inclusions. (c) 2008 Elsevier Ltd. All rights reserved.