Oxygen Isotope Fractionation During Evaporation of Mg- and Si-rich CMAS-Liquids in Vacuum
Oxygen Isotope Fractionation During Evaporation of Mg- and Si-rich CMAS-Liquids in Vacuum
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真空蒸发富镁和富硅 CMAS 液体过程中的氧同位素分馏
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发表时间：
2010
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通讯作者：
A. Davis
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作者：
R. Mendybaev;F. Richter;M. Spicuzza;J. Valley;A. Davis
简介：FUN CAIs在化学和矿物学上类似于B型CAIs，但其特征在于Mg、Si和O同位素的大质量依赖分馏以及元素数量的未识别核效应（例如，[1，2]）。重同位素富集通常归因于高温熔融阶段CAI前体挥发性组分部分蒸发引起的同位素分馏（例如[2]）。为了验证这一假设，我们进行了一项实验研究，其中富含Mg和Si的CMAS液体在真空中蒸发[3，4]。极富镁橄榄石熔体（FUN1）和组成接近Vig1623 - 5 FUN CAI（FUN2）本体组成的熔体的蒸发导致轨迹相应地接近1623 - 5和C1 FUN CAI的组成。此外，化学成分接近1623 - 5（由FUN 1熔体蒸发产生）和C1（由FUN 2熔体蒸发产生）的蒸发残渣中的Mg和Si同位素组成接近1623 - 5（δ Mg~62 ‰和δ Si~21 ‰）和C1（δ Mg~60 ‰和δ Si~28 ‰）FUN CAIs [2，5]中测量的值。已经报道了几种FUN CAIs的体氧同位素组成（例如，[2，5]）。在Vig1623 - 5的三氧同位素组成图上，尖晶石、镁橄榄石和方钠长石的同位素组成位于与CCAM线相交的δ O =-48 ‰的质量分馏线上。我们用这一点来估计的程度O富集的散装氧的一个给定的FUN CAI。在这里，我们报告的氧同位素组成的蒸发残留物的起始组合物，可能是前体的Vig1623 - 5和C1 FUN夹杂物。实验：蒸发实验在真空炉中在1900 ° C下进行。2.5首先将具有起始材料的直径为10mm的Irwire环置于冷炉中，然后将冷炉泵出，并以自动模式将温度升高至1900 ° C。使样品在该温度下蒸发预定的时间段（0分钟至1小时），然后以每分钟几百度的温度冷却炉（实验细节见[6，7]）。氧同位素组成在威斯康星大学麦迪逊分校测量。将通过用BrF5作为氟化剂的激光蒸发产生的氧（来自约2mg的蒸发残余物碎片）低温纯化，使其通过在线Hg扩散泵并收集在保持在-196 °C的5 μ mol筛上。然后使用液体戊烷淤浆（_131 ° C）加热阱，并将气体转移至第二阱（在_196 ° C）以除去NF3。然后用Finnigan MAT 251质谱仪分析纯化的O2的三种氧同位素。如[8]中所述对数据进行标准化。每天，根据同一天分析的UWG-2石榴石标准[8，9]的3 - 4次分析的平均值调整未知物的δ O和δ O值。结果和讨论：测量的氧同位素数据见下表和图1。
Introduction: FUN CAIs are similar to Type B CAIs chemically and mineralogically, but characterized by large mass-dependent fractionations in Mg, Si and O isotopes as well as by unidentified nuclear effects in number of elements (e.g., [1, 2]). The heavy isotope enrichment is usually attributed to the isotopic fractionation caused by the partial evaporation of volatile components of CAI precursors during high temperature molten stage (e.g. [2]). To test this hypothesis we conducted an experimental study in which Mgand Si-rich CMAS liquids were evaporated in a vacuum [3,4]. Evaporation of extremely forsterite-rich melt (FUN1) and a melt with composition close to the bulk composition of Vig1623-5 FUN CAI (FUN2) resulted in a trajectory passing close to the compositions of 1623-5 and C1 FUN CAI, correspondingly. Moreover, Mgand Si-isotopic compositions in the evaporation residues with chemical compositions close to 1623-5 (produced by evaporation of FUN1 melt), and C1 (produced by evaporation of FUN2 melt) are close to the values measured in 1623-5 (δMg~62‰ and δSi~21‰) and C1 (δMg~60‰ and δSi~28‰) FUN CAIs [2, 5]. The bulk oxygen isotopic composition has been reported for several FUN CAIs (e.g., [2, 5]). On a three oxygen isotope diagram for individual grains of Vig1623-5, the isotopic composition of spinel, forsterite and fassaite lie on a mass fractionation line that intersects the CCAM line at about δO= -48‰. We use this point to estimate the degree of O enrichment of the bulk oxygen of a given FUN CAI. Here we report oxygen isotopic composition of evaporation residues from starting compositions that are possible precursors of Vig1623-5 and C1 FUN inclusions. Experimental: Evaporation experiments were conducted in a vacuum furnace at 1900°C. 2.5 mm dia Irwire loops with the starting materials were first placed into a cold furnace which was then pumped out and temperature increased to 1900°C in an automated mode. The sample was allowed to evaporate at this temperature for a predetermined period of time (0 min to 1 hr) and furnace then cooled at several hundred degrees per minute (see [6,7] for the experimental details). Oxygen isotopic compositions were measured at the University of Wisconsin-Madison. Oxygen (from ~2 mg chips of the evaporation residues) produced by laser fluorination with BrF5 as the fluorinating agent was cryogenically purified, passed through an in-line Hg diffusion pump and collected on 5Å mole sieve held at -196°C. The trap was then warmed using a liquid pentane slush (-131°C), and the gas was transferred to a second trap (at -196°C) to remove NF3. The purified O2 was then analyzed for three oxygen isotopes with a Finnigan MAT 251 mass spectrometer. The data were standardized as described in [8]. Each day, the δO and the δO values of unknowns were adjusted based on the average of 3-4 analyses of UWG-2 garnet standard [8, 9] analyzed on the same day. Results and Discussion: Measured oxygen isotope data are presented in Table below and Fig.1.