Condensation in dust-enriched systems
Condensation in dust-enriched systems
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DOI：
10.1016/s0016-7037(99)00284-7
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发表时间：
2000-01-01
影响因子：
5
通讯作者：
Grossman, L
中科院分区：
文献类型：
--
作者：
Ebel, DS;Grossman, L
为了研究由此产生的冷凝物的氧化态，对由尘埃富集系统的总蒸发所产生的宇宙气体中的元素的冷凝序列进行了全面的平衡计算。计算包括23种元素和374种气体，并在10（-3）至10（-6）bar的P-tot范围内进行，并在相对于太阳组成系统的Cl组成的尘埃中富集高达1000倍。由于液体在富尘系统中是稳定的冷凝物，硅酸盐液体的MELTS非理想溶液模型（Ghiorso和Sack，1995年）被纳入计算机代码。在10（-3）bar压力下的冷凝和100 x、500 x和1000 x的尘埃富集分别发生在IW-3.1、IW-1.7和IW-1.2的氧逸度下，在橄榄石从蒸汽中直接冷凝停止的温度下，产生的X-Fa分别为0.019、0.088和0.164。硅酸盐液体在粉尘富集度>类似于10（-3）bar下的12.5x和>类似于10（-6）bar下的425 x时是稳定的冷凝物。在500倍下，液体场>1000 K宽，并且在10（-3）巴下占硅的最大48%，并且在10（-6）巴下为240 K宽，并且占硅的25%。在液相消失温度下，共存橄榄石在100倍、500倍和1000倍时的X-Fa分别为0.025、0.14和0.31，几乎与P-tot无关。在1000倍下，最终液体的Na 2 O和K 2 O含量达到10.1和1.3wt.%，分别在10（-3）bar下，但在10（-6）bar下都可以忽略不计。在10（-3）巴下，硫化铁液体在粉尘富集度至少低至500倍时是稳定的冷凝物，并在1000倍时与硅酸盐液体共存。在10（-6）巴下未发现硫化物液体。在10（-3）bar压力下，在1310 K和560 x的尘埃富集度下，Fe在金属、硅酸盐和硫化物之间的预测分布与H-组辉长岩中发现的分布相匹配，在1330 K和675倍的温度下，与金属损失前的L-族镁合金相匹配。tot和高粉尘富集度的冷凝物的整体化学成分趋势在粉尘和气体可以预期达到平衡的温度下达到典型的IIA型球粒的FeO含量，大于或等于1200 K。然而，即使在这些条件下，预测的冷凝物的组合物的轨迹通过组合物与更多的CaO + Al 2 O3相对于MgO + SiO2比大多数IA型球粒。此外，在wt. Na 2 O与重量% FeO，大多数球粒成分Na 2 O太丰富，以至于不能沿着趋势，该趋势是在P-tot小于或等于10（-3）bar和粉尘富集度小于或等于1000 x时对冷凝物的整体化学成分预测的。总之，这些化学差异表明，单个球粒既不是通过淬火存在于不同温度下的液体+固体冷凝物的样品形成的，也不是通过淬火从这些样品形成的次级液体形成的。然而，除了I型球粒中非常贫FeO、富Na 2 O的玻璃和II型球粒中具有非常高FeO和Na 2 O的玻璃之外，许多球粒玻璃的组成在10（-3）bar下与宇宙气体平衡的液体和在600 x和1000 x之间富集的尘埃中沿着沿着体成分轨迹下降。如果这些陨石球粒是由不同温度下形成的冷凝物混合物的二次熔融形成的，那么具有这些特征的星云区域将是必要的，以防止其玻璃的液体前体通过蒸发而损失Na 2 O和通过还原而损失FeO，假设液体热了足够长的时间，与气体平衡。版权所有（C）2000 Elsevier Science Ltd.
Full equilibrium calculations of the sequence of condensation of the elements from cosmic gases made by total vaporization of dust-enriched systems were performed in order to investigate the oxidation state of the resulting condensates. The computations included 23 elements and 374 gas species, and were done over a range of P-tot from 10(-3) to 10(-6) bar and for enrichments up to 1000x in dust of Cl composition relative to a system of solar composition. Because liquids are stable condensates in dust-enriched systems, the MELTS nonideal solution model for silicate liquids (Ghiorso and Sack, 1995) was incorporated into the computer code. Condensation at 10(-3) bar and dust enrichments of 100x, 500x, and 1000x occur at oxygen fugacities of IW-3.1, IW-1.7, and IW-1.2, respectively, and, at the temperature of cessation of direct condensation of olivine from the vapor, yields X-Fa of 0.019, 0.088, and 0.164, respectively. Silicate liquid is a stable condensate at dust enrichments >similar to 12.5x at 10(-3) bar and >similar to 425x at 10(-6) bar. At 500x, the Liquid field is >1000 K wide and accounts for a maximum of 48% of the silicon at 10(-3) bar, and is 240 K wide and accounts for 25% of the silicon at 10(-6) bar. At the temperature of disappearance of liquid, X-Fa of coexisting olivine is 0.025, 0.14, and 0.31 at 100x, 500x, and 1000x, respectively, almost independent of P-tot. At 1000x, the Na2O and K2O contents of the last liquid reach 10.1 and 1.3 wt.%, respectively, at 10(-3) bar but are both negligible at 10(-6) bar. At 10(-3) bar, iron sulfide liquids are stable condensates at dust enrichments at least as low as 500x and coexist with silicate liquid at 1000x. No sulfide liquid is found at 10(-6) bar. At 10(-3) bar, the predicted distribution of Fe between metal, silicate and sulfide at 1310 K and a dust enrichment of 560x matches that found in H-group chondrites, and at 1330 K and 675x matches that of L-group chondrites prior to metal loss.Only at combinations of high P-tot and high dust enrichment do the bulk chemical composition trends of condensates reach the FeO contents typical of type IIA chondrules at temperatures where dust and gas could be expected to equilibrate, greater than or equal to 1200 K. Even under these conditions, however, the composition trajectories of predicted condensates pass through compositions with much more CaO + Al2O3 relative to MgO + SiO2 than those of most type IA chondrules. Furthermore, on a plot of wt.% Na2O vs. wt.% FeO, most chondrule compositions are too Na2O-rich to lie along trends predicted for the bulk chemical compositions of the condensates at P-tot less than or equal to 10(-3) bar and dust enrichments less than or equal to 1000x. Together, these chemical differences indicate that individual chondrules formed neither by quenching samples of the liquid + solid condensates that existed at various temperatures nor by quenching secondary liquids that formed from such samples. With the exception of very FeO-poor, Na2O-rich glasses in type I chondrules and glasses with very high FeO and Na2O in type II chondrules, however, many chondrule glass compositions fall along bulk composition trajectories for liquids in equilibrium with cosmic gases at 10(-3) bar and dust enrichments between 600x and 1000x. If these chondrules formed by secondary melting of mixtures of condensates that formed at different temperatures, nebular regions with characteristics such as these would have been necessary to prevent loss of Na2O by evaporation and FeO by reduction from the liquid precursors of their glasses, assuming that the liquids were hot for a long enough time to have equilibrated with the gas. Copyright (C) 2000 Elsevier Science Ltd.