187Os–186Os systematics of Os–Ir–Ru alloy grains from southwestern Oregon
187Os–186Os systematics of Os–Ir–Ru alloy grains from southwestern Oregon
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DOI：
10.1016/j.epsl.2004.11.009
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发表时间：
2005-01
影响因子：
5.3
通讯作者：
R. Walker;A. Brandon;J. Bird;P. Piccoli;W. McDonough;R. Ash
中科院分区：
文献类型：
--
作者：
R. Walker;A. Brandon;J. Bird;P. Piccoli;W. McDonough;R. Ash
对产于俄勒冈州西南部中生代约瑟芬蛇绿岩中的100多个Os-Ir-Ru合金颗粒进行了主、次元素和187 Os/188 Os同位素组成的研究。16个晶粒跨越187 Os/188 Os比率的范围内存在的较大的套件进行了分析，以获得高精度的186 Os/188 Os比率。个别谷物的主要和次要元素丰度的范围是，在一般情况下，类似的组合物以前报道的这种谷物在世界各地。套房内的成分变化相当大。大多数颗粒可能从较大的共生簇中分解出来，并根据质地细分为基质和层状。个别基质颗粒内的组合物的变化是有限的，而个别lamiculum颗粒内的变化是相当大的。在±0.1%的分辨率水平上，单个颗粒的187 Os/188 Os比值是均匀的。整个系列的187 Os/188 Os比值为0.1173 ~ 0.1468。所有颗粒的平均比值为0.1241，类似于约瑟芬溪采样的三种铬铁矿的组成，并与中生代上地幔的形成相一致。16个晶粒的平均~（186）Os/~（188）Os比值为0.119838。除一粒外，所有谷物的同位素比率均在该比率的±20 ppm范围内。结果表明，187 Os/188 Os在mm 3尺度上的变化可能是相当大的上地幔内，模仿上地幔物质的手和露头尺度样品中观察到的非均匀性。相对于具有南极187 Os/188 Os的环境上地幔的高度亏损成分要求某些非均质性必须> 1Ga。186 Os/188 Os比率的同质性并不支持上地幔内的同位素不均匀性可以解释在推定的羽流熔岩中观察到的186 Os-187 Os耦合富集的论点。
More than 100 Os–Ir–Ru alloy grains from southwestern Oregon, presumably originating in the Mesozoic Josephine ophiolite, were examined for major and minor element compositions and187Os/188Os isotopic compositions. Sixteen grains spanning the range of187Os/188Os ratios present in the larger suite were analyzed for high-precision186Os/188Os ratios. The ranges of major and minor element abundances of individual grains were, in general, similar to compositions previously reported for such grains worldwide. Within-suite compositional variations were quite large. Most grains were likely disaggregated from larger intergrown clusters, and were subdivided based on texture into matrix and lamellae. Composition variations within individual matrix grains were limited, whereas variations within individual lamellae grains were considerably larger. The187Os/188Os ratios of individual grains were isotopically homogeneous at the ±0.1% level of resolution. The187Os/188Os ratios of the entire suite of grains ranged from 0.1173 to 0.1468. The average ratio for all grains combined was 0.1241, similar to the composition of three chromitites sampled from the Josephine Creek, and consistent with formation in the Mesozoic upper mantle. The average186Os/188Os ratio of the subset of 16 grains was 0.119838. All grains but one had isotopic ratios within ±20 ppm of that ratio. Results indicate that187Os/188Os variations on a scale of mm3may be quite large within the upper mantle, mimicking heterogeneities observed in both hand-and outcrop-scale samples of upper mantle materials. Highly depleted compositions relative to ambient upper mantle with chondritic187Os/188Os require that some of the heterogeneities must be >1 Ga. The homogeneity of the186Os/188Os ratios lend no support to the contention that isotopic heterogeneities within the upper mantle may explain coupled186Os–187Os enrichments that have been observed in putative plume derived lavas.