Trapped melt in IIIAB irons; solid/liquid elemental partitioning during the fractionation of the IIIAB magma
Trapped melt in IIIAB irons; solid/liquid elemental partitioning during the fractionation of the IIIAB magma
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IIIAB 熨斗中残留的熔体；
DOI：
10.1016/s0016-7037(99)00283-5
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发表时间：
1999
影响因子：
5
通讯作者：
J. Wasson
中科院分区：
文献类型：
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作者：
J. Wasson
IIIAB群，最大的铁陨石群，显示了组成趋势（包括三个数量级的Ir浓度范围），表明它是由金属岩浆的分离结晶形成的。由于大约有200个铁可用，并且所有程度的结晶都得到了很好的代表，IIIAB为研究小行星核心所有深度的结晶提供了一套极好的样品。在对数-对数Ir-Au图和Ir-As图上，iiab形成一个宽频带；宽度代表了真正的陨石到陨石的变化，远远超出了实验或抽样的不确定性。一个成功的模型必须解释这一波段的宽度；我认为这主要是由于母岩浆被困在结晶固体中。因为S基本上不溶于金属，所以FeS的丰度是捕获液体的分数的量度。具有较高S含量的铁与推断的岩浆母液组成更接近，这一观察结果支持了圈闭熔体模型。S值最低的是位于IIIAB Ir-Au或Ir-As成分场左包络层的铁，因此这组铁应该被解释为分馏岩浆的固体产物。这简化了结晶过程的建模，并允许对演化的IIIAB系统中其他元素的分布比率进行推断。大型（多吨）约克角铁矿在其截留熔体分数上表现出很大的变化；它们的组成似乎最好地理解为IIIAB岩浆的低初始S含量，约为20毫克/克。推断出Ir的初始IIIAB分布系数为4.6，远高于基于低s系统实验室研究的已发表值；我认为，低s（和低p）分割比测量往往会在统一的方向上出错。在IIIAB中，Au、As和Ni的分布系数在IIIAB铁形成时仍<1，表明初始S含量较低。
Group IIIAB, the largest iron–meteorite group, shows compositional trends (including a three-order-of-magnitude Ir concentration range) indicating that it formed by fractional crystallization of a metallic magma. Because about 200 irons are available, and all degrees of crystallization are well represented, IIIAB offers an excellent set of samples for the study of crystallization at all depths of the asteroidal core. On log–log Ir–Au, and Ir–As diagrams IIIAB forms a broad band; the breadth represents real meteorite-to-meteorite variations, far outside experimental or sampling uncertainties. A successful model must explain the width of this band; I suggest that it mainly resulted from the trapping of parental magma within the crystallizing solid. Because S is essentially insoluble in metal, the abundance of FeS is a measure of the fraction of trapped liquid. The trapped-melt model is supported by the observation that irons having higher S contents plot closer to the inferred composition of the magmatic parental liquid. The lowest S values are found in the irons occupying the left envelope of the IIIAB Ir–Au or Ir–As compositional fields, thus it is this set of irons that should be interpreted as the solid products of a fractionating magma. This simplifies the modeling of the crystallization process and allows inferences regarding the distribution ratios for other elements in the evolved IIIAB system. The large (multiton) Cape York irons show wide variations in their trapped-melt fractions; their compositions seem best understood in terms of a low initial S content of the IIIAB magma, about 20 mg/g. The inferred initial IIIAB distribution coefficient for Ir, 4.6, is much higher than published values based on laboratory studies of low-S systems; I suggest that low-S (and low-P) partition-ratio measurements tend to err in the direction of unity. In IIIAB distribution coefficients for Au, As, and Ni were still <1 when the most evolved IIIAB irons formed, another indication of a low initial S content.