Chemical equilibrium and kinetic constraints for chondrule and CAI formation conditions
Chemical equilibrium and kinetic constraints for chondrule and CAI formation conditions
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球粒和 CAI 形成条件的化学平衡和动力学约束
DOI：
10.1016/j.gca.2004.03.030
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发表时间：
2004
影响因子：
5
通讯作者：
C. Alexander
中科院分区：
文献类型：
--
作者：
C. Alexander
尽管存在许多不确定因素，但一个动力学蒸发-冷凝模型表明，a型球粒和紧凑的a型和B型富钙铝包裹体（CAIs）可能是在与典型太阳星云中2-3 AU的预测一致的条件下由类似ci的前体形成的。B型和富铝球粒，以及C型cai，另一方面，可能是由分异前体形成的。以前的研究主要基于球粒和CAI同位素组成，由于没有考虑到气体-熔体交换的影响，得出了不同的结论。假设类似ci的前体成分，平衡硅酸盐熔体的元素组成与A型球粒的元素组成相似，可以在广泛的条件下形成（T, ppt，固体/气体/太阳能）。当温度≥1600℃时，金属不稳定。当T < 1600°C时，球粒中金属的丰度和组成似乎不如硅酸盐成功地再现，例如，在给定温度下，预测II型球粒中的金属比通常观察到的要多，并且在某些条件下，预测II型球粒比i型更富金属。如果i型是由比CI更还原的前体形成的，如果ii型是在金属硅酸盐分馏后形成的，则这些差异可以克服。熔融cai的形成条件比球粒更受限制，这可能部分解释了它们丰度较低的原因。在非fun cai中，如果它们在~ 1400 ~ 1500°C之间形成，在类cai平衡熔体稳定的区域，Mg、Si和O同位素质量分馏可以重现，但它们没有完全达到与气体的平衡。ppt = 10 - 4-10 - 3bar时的CAI形成时间与B型CAI冷却时间的估计一致，但远低于此压力需要的形成时间太长。FUN cai的同位素质量分馏可以解释，如果它们形成于或低于固体/气体/太阳比的范围，在此范围内，类cai平衡成分是稳定的。在这些条件下，FUN包裹体的气熔交换比非FUN包裹体少。FUN CAI地层温度与1400 ~ 1500℃的地层温度一致，但可能更高。对球粒/CAI中O质量无关分馏分（MIF）分布的两种一般解释进行了探讨：MIF在球粒/CAI形成之前产生，以及MIF在球粒/CAI形成期间产生。如果MIF是在球粒/CAI形成之前建立的，那么最有希望的解释是H2O（可能是冰）和具有相反MIF符号的硅酸盐尘埃是从剩余气体中分离出来的。加热时，气体中的水和熔体交换。如果MIF是在球粒/CAI形成过程中产生的，那么它一定是在H2O中产生的，因为它与熔体的交换最迅速，而质量平衡需要在CO中产生相反符号的MIF。自屏蔽紫外线辐射是一种可能性，但这种效果可能在高温下被淬灭。非rrkm分子内动力学同位素效应是另一种可能性，但可能需要连续的辐射源来防止气相反应接近平衡。
While many uncertainties remain, a kinetic evaporation-condensation model is used to show that type A chondrules, and compact Type A and B calcium-aluminum-rich inclusions (CAIs) could have formed from CI-like precursors under conditions that are consistent with predictions for 2–3 AU in a canonical solar nebula. Type B and Al-rich chondrules, and Type C CAIs, on the other hand, may have formed from fractionated precursors. Based primarily on chondrule and CAI isotopic compositions, previous studies have reached different conclusions because they did not take into account the effects of gas-melt exchange. Assuming CI-like precursor compositions, equilibrium silicate melts with elemental compositions like those of type A chondrules could have formed over a wide range of conditions (T, Ptot, solid/gas/solar). Metal is not predicted to be stable when T ≥ 1600°C. When T < 1600°C, the abundances and compositions of metal in chondrules appear to be less successfully reproduced than the silicates, e.g., at a given temperature more metal is predicted in type II chondrules than is generally observed, and under some conditions type IIs are predicted to be more metal-rich than type Is. These differences could be overcome if type Is formed from precursors that were more reduced than CI, and if type IIs formed after significant metal-silicate fractionation. The formation conditions of molten CAIs are much more restricted than for chondrules, perhaps in part explaining their lower abundances. The Mg, Si and O isotopic mass fractionations in non-FUN CAIs can be reproduced if they formed between ∼1400 to 1500°C in regions where CAI-like equilibrium melts were stable, but they did not quite reach equilibrium with the gas. CAI formation times at Ptot= 10−4-10−3bars are consistent with estimates of Type B CAI cooling times, but pressures much below this require formation times that are too long. The isotopic mass fractionations in FUN CAIs can be explained if they formed at or below the ranges of solid/gas/solar ratios where CAI-like equilibrium compositions are stable. Under these conditions, FUN inclusions undergo less gas-melt exchange than non-FUN CAIs. The FUN CAI formation temperatures are consistent with formation at 1400 to 1500°C, but may have been higher. Two general explanations for the distribution of O mass independent fractionations (MIF) in chondrules/CAIs have been explored: creation of the MIF before chondrule/CAI formation, and creation of the MIF during chondrule/CAI formation. If the MIF was established before chondrule/CAI formation, the most promising explanation is that H2O (presumably as ice) and silicate dust with MIFs of opposite sign are fractionated together from the remaining gas. On heating, the H2O now in the gas exchanges with the melt. If the MIF was generated during chondrule/CAI formation, it must be generated in the H2O, because it exchanges most rapidly with the melt, and mass balance requires creation of MIF of opposite sign in CO. Self-shielding from UV radiation is one possibility, but the effect may be quenched at high temperatures. Non-RRKM intramolecular kinetic isotope effects are another possibility, but a continuous source of radiation may be needed to prevent gas phase reactions from approaching equilibrium.