Trace Element Fractionation in Kamacite and Taenite in IVA Irons
Trace Element Fractionation in Kamacite and Taenite in IVA Irons
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IVA 铁中的铁纹石和镍纹石中的微量元素分馏
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发表时间：
2007
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通讯作者：
T. Mccoy
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作者：
R. Ash;M. Luong;R. Walker;W. McDonough;T. Mccoy
通过对陨石样品[15]的分析和实验室实验，铁陨石中亲铁微量元素在卡玛石和带长石之间的亚固体分异已经成为多项研究的主题[6,7]。然而，在单个铁陨石群中进行的系统的原位工作很少。这可以避免由于分析不相关的群体而产生的不同化学引起的并发症。我们选择分析IVA烙铁，因为许多烙铁表现出良好的威德曼施特模式，其带宽范围反映了各种冷却速率。样品和技术：到目前为止，我们已经分析了五种镍含量和冷却速度范围内的IVA铁，即Alvord， Duchesne，决斗山1854，Gibeon和La Grange。用光学显微镜观察了样品的探针底座，并确定了卡玛长石和带长石的区域。我们使用New Wave UP213，五倍NdYAG紫外激光器（213nm）将适当的材料烧蚀成He流。He流与Ar混合，并引入Thermo Finnigan Element 2单收集器磁扇形ICP-MS。烧蚀点范围为80 ~ 150 m，输出功率约为2 ~ 2.3 jcm。铁陨石Coahuila， Hoba， Filomena和金属SRM 1158和SRM 1263a作为标准。用Co和Ni进行内部归一化，烧蚀后用电子探针（JEOL-8900超级探针）测定单个斑点的值。使用改进版本的Lamtrace进行数据缩减。测量的同位素有：P、V、Cr、Mn、Co、Ni、Cu、Ga、Ga、Ge、As、Mo、Ru、Rh、Pd、W、Re、Os、Pt、Pt和Au。结果：大部分元素在卡玛石和条长石之间几乎没有分异，使得Elementkamacite/Elementtaenite的值非常接近1。图1显示了扁针状岩体中的平均浓度除以带长石中元素的平均组成。磷、V、As倾向于分配到绢云石中，而Cu、铂族元素（PGE）倾向于分配到绢云石中（与Ni一起明显）。总的来说，第5期PGEs （Ru， Rh和Pd）比第6期PGEs （Os, Ir, Pt）更倾向于带状体。在给定的陨石中，大多数元素的卡玛长石和带长石浓度在Ni丰度和微量元素丰度之间显示出正相关关系（例如见图2）。一个明显的例外是钨，它在任何陨石中都没有显示出卡玛石或带长石的系统变化。在带长石中没有获得足够的数据来证明任何明显的相关性，只有决斗山1854提供了足够的数据来暗示Ni和Ge之间的正相关性。
Introduction: The subsolidus fractionation of siderophile trace elements, between kamacite and taenite, in iron meteorites has been the subject of several studies through both analysis of meteorite samples [15] and through laboratory experiments [6,7]. However little systematic in situ work has been done within a single iron meteorite group. This may enable the avoidance of complications associated with disparate chemistry resulting from the analysis of unrelated groups. We have chosen to analyse the IVA irons as many exhibit well developed Widmanstatten patterns with a range of band-widths reflecting a variety of cooling rates. Samples and Technique: To date we have analysed five IVA irons with a range of nickel contents and cooling rates viz Alvord, Duchesne, Duel Hill 1854, Gibeon and La Grange. Probe mounts of the samples were viewed by optical microscope and areas of kamacite and taenite identified. We used a New Wave UP213, quintupled NdYAG ultraviolet laser (213nm) to ablate appropriate material into a stream of He. The He flow was mixed with Ar and introduced into a Thermo Finnigan Element 2 single collector magnetic sector ICP-MS. Ablation spots ranged from 80 to 150 m with an output power of ca. 2-2.3Jcm. Iron meteorites Coahuila, Hoba, Filomena and metals SRM 1158 and SRM 1263a were used as standards. Co and Ni were used for internal normalization with the values for individual spots being determined, post ablation, by electron microprobe (JEOL-8900 superprobe). Data reduction was carried out using a modified version of Lamtrace. Isotopes measured were: P, V, Cr, Mn, Co, Ni, Cu, Ga, Ga, Ge, As, Mo, Ru, Rh, Pd, W, Re, Os, Pt, Pt and Au. Results: Most elements show little fractionation between kamacite and taenite, giving Elementkamacite/Elementtaenite values very close to 1. Figure 1 shows the mean concentration in the kamacite divided by the mean compositon of the element in taenite. Phosphorus, V and As show a preference for partitioning into kamacite whereas Cu, and the Platinum Group Elements (PGE) partition preferentially into taenite (obviously along with Ni). In general the fifth period PGEs (Ru, Rh and Pd) have a stronger preference for taenite than the sixth period PGEs (Os, Ir, Pt). Within a given meteorite both kamacite and taenite concentrations of most elements show a positive correlation between Ni abundance and trace element abundance (for example see Figure 2). The one clear exception is tungsten which shows no systematic variation in any of the meteorites for either kamacite or taenite. Insufficient data were obtained for Ge in taenite for any correlations to be apparent, and only Duel Hill 1854 gave enough data to hint at a positive correlation between Ni and Ge.