Major element chemical and isotopic compositions of refractory inclusions in C3 chondrites: the separate roles of condensation and evaporation☆
Major element chemical and isotopic compositions of refractory inclusions in C3 chondrites: the separate roles of condensation and evaporation☆
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C3球粒陨石中难熔包裹体的主量元素化学和同位素组成：凝结和蒸发的单独作用☆
DOI：
10.1016/s0016-7037(00)00396-3
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发表时间：
2000
影响因子：
5
通讯作者：
N. Parsad
中科院分区：
文献类型：
--
作者：
L. Grossman;D. Ebel;S. Simon;A. Davis;F. Richter;N. Parsad
CV3 球粒陨石中大多数粗粒 A 型和 B 型夹杂物的主要元素氧化物组成的文献数据可能存在错误，因为由于样本量较小，尖晶石相对于其他相的取样不具代表性。当通过添加或减去尖晶石将报告的成分校正为太阳 CaO/Al2O3 比率时，氧化物-氧化物图上会产生明显的趋势。这些趋势接近于在各种条件下针对太阳能或富含灰尘的气体计算的平衡冷凝物的整体成分的轨迹，但在 MgO 与 SiO2 含量图上除外，其中数据点相当分散到冷凝轨迹的贫 MgO 一侧。从液滴到不饱和氢气的蒸发质量损失的不可逆过程被建模为一系列小的平衡步骤。该模型用于表明，CMAS 液体成分的演化路径对于从 1 × 10−15 到 1 bar 的所有 PH2 下的蒸发都是相同的，蒸发的 SiO2 分数与 MgO 分数的比率随着温度从 1700 K 升高到 2000 K 以及随着起始成分 SiO2 含量的增加而增加。这些计算表明，大多数 B 型夹杂物的成分可以通过 10 至 30% 的 MgO 和 0 至 15% 的 SiO2 在 1700 K 下从液滴非平衡蒸发成 H2 气体来解释，该液滴的成分源自许多可能的平衡冷凝轨迹中的任何一种。某些 As 型可能会遭受类似的 MgO 和 SiO2 蒸发损失，但温度更高。该蒸发程度与 A 型和 B 型夹杂物中观察到的 Mg 和 Si 同位素质量分馏量一致。蒸发可能发生在大多数镁和硅从星云气体中去除成较低温度的冷凝物之后。
Literature data for major element oxide compositions of most coarse-grained Types A and B inclusions in CV3 chondrites may be in error due to non-representative sampling of spinel relative to other phases because of small sample sizes. When reported compositions are corrected to the solar CaO/Al2O3ratio by addition or subtraction of spinel, distinct trends result on oxide–oxide plots. These trends lie close to trajectories of bulk compositions of equilibrium condensates calculated for solar or dust-enriched gases under various conditions, except on a plot of MgO vs. SiO2contents, where there is considerable scatter of the data points to the MgO-poor side of the condensation trajectory. The irreversible process of evaporative mass loss from a liquid droplet into an unsaturated H2gas is modeled as a series of small equilibrium steps. This model is used to show that evolutionary paths of CMAS liquid compositions are identical for evaporation at all PH2from 1 × 10−15to 1 bar, with the ratio of the fraction of the SiO2evaporated to that for MgO increasing both with increasing temperature from 1700 to 2000 K and with increasing SiO2content of the starting composition. Such calculations show that compositions of most Type B inclusions can be explained by non-equilibrium evaporation of 10 to 30% of the MgO and 0 to 15% of the SiO2into an H2gas at 1700 K from liquid droplets whose compositions originated on any one of many possible equilibrium condensation trajectories. Some Type As may have suffered similar evaporative losses of MgO and SiO2but at higher temperature. This degree of evaporation is consistent with the amount of Mg and Si isotopic mass fractionation observed in Types A and B inclusions. Evaporation probably happened after most Mg and Si were removed from the nebular gas into lower-temperature condensates.