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首页> 外文期刊>Chemical geology >Precise and accurate Os-186/Os-188 and Os-187/Os-188 measurements by Multi-collector Plasma Ionisation Mass Spectrometry, part II: Laser ablation and its application to single-grain Pt-Os and Re-Os geochronology
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Precise and accurate Os-186/Os-188 and Os-187/Os-188 measurements by Multi-collector Plasma Ionisation Mass Spectrometry, part II: Laser ablation and its application to single-grain Pt-Os and Re-Os geochronology

机译：多收集器等离子体电离质谱法精确，准确地测量Os-186 / Os-188和Os-187 / Os-188，第二部分：激光烧蚀及其在单颗粒Pt-Os和Re-Os地球年代学中的应用

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The abundance of platinum group alloy (PGA) gains available from some alluvial deposits combined with their great chemical resistance to conventional acid attack generates the need for an alternative method of obtaining routine, rapid yet precise Os isotopic data. Laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) is ideally suited for this purpose and has been applied, in a relatively limited extent, to Os-rich samples by previous workers (e.g., Hirata, T., Michinari Hattori, M., and Tsuyoshi Tanaka, T., 1998. In-situ osmium isotope ratio analyses of iridosmines by laser ablation-multiple collector-inductively coupled plasma mass spectrometry. Chem Geol., 144, 269-280.; Malitch, K.N., 2004. Osmium isotope constraints on contrasting sources and prolonged melting in the Proterozoic upper mantle: evidence from ophiolitic Ru-Os sulfides and Ru-Os-Ir alloys. Chem Geol. 208. 157-173.; Walker, R.J., Brandon, A.D., Bird, J.M., Piccolia, P.M., McDonougha, W.F., R.D., Asha, R.D., 2005. Os-187-Os-186 systematics of Os-Ir-Ru alloy grains from southwestern Oregon. Earth Planet. Sci. Lett. 230, 211-226.). The wide variety of PGA minerals yield a broad spectrum of Pt, Re and Os contents which translate into a wide spread in parent-daughter ratios that is an attractive proposition for geochronology using the Re-Os and Pt-Os decay schemes. Here we show that single PGA grains, can contain sufficient internal Re, Pt and Os elemental variability and coupled variations in parent/daughter and daughter isotope compositions that they present an opportunity to obtain 'isochron' ages from two different isotope decay systems. We present a rapid (40 s acquisition time), precise and accurate LA-MC-ICP-MS methodology suitable for applying Pt-Os and Re-Os geochronology approaches to single PGA grains, for use in dating chromitite deposits and identifying and dating multiple sources in alluvial PGA deposits. Because of isobaric interferences from Pt, PGA analyses by LA-MC-ICP-MS are best corrected for instrumental mass fractionation using the Os-189/Os-188 ratio. We demonstrate that, within our analytical uncertainties, mass bias effects for Os isotope ratios during LA-MC-ICP-MS are similar in nature and magnitude to mass bias variations observed during normal solution-mode analysis. While robust elemental interference corrections can be applied for Re and W on the Os mass spectrum during solution analyses (Nowell, G.M., Luguet, A., Pearson, D.A., Horstwood, M.S.A., 2008-this volume. Precise and accurate Os-186/(188)s and Os-187/Os-188 measurements by Multi-Collector Plasma Ionisation Mass Spectrometry (MC-ICP-MS) part 1: solution analyses. Chem Geol.) we show that the accurate correction for the Re interference on Os during laser ablation is problematic at Re-187/Os-188 ratios > - 0.5 which means that Re-corrected Os-187/Os-188 ratios for some PGAs and high Re/Os sulfides must be regarded as problematic. Our interfering element correction approach can also be used for the correction for Os on Pt-190 for obtaining Pt-190/Os-188 ratios, removing the need for an independent Pt monitor isotope. Accurate and precise geochronology by LA-MC-ICP-MS can be further limited by poorly quantified fractionation of the parent/daughter elemental ratio during ablation. Although additional work is required to understand the nature and exact extent of inter-element fractionation of the Pt/Os and Re/ Os ratios during laser ablation anaysis of Pt- and Re-rich PGA grains, our best estimate is that fractionation of the Pt/Os during laser ablation is around 5% or less. We present multi-grain isochrons and single-grain internal 'isochrons' for the Re-Os and Pt-Os isotope systems in PGA grains from Lapland. In all cases, the Pt-Os isotope system gives significantly more precise 'isochrons' than the Re-Os system. We interpret this discordance between the two systems as being most likely due to problems inherent with Re interference corrections at elevated Re/Os ratios. We show tha
机译：从某些冲积矿床中获得的大量铂族合金（PGA）以及它们对常规酸侵蚀的强大耐化学性，导致需要一种获取常规，快速而精确的Os同位素数据的替代方法。激光烧蚀多收集器ICP-MS（LA-MC-ICP-MS）非常适合此目的，并且以前的工作人员（例如，Hirata，T。， Michinari Hattori，M.和Tsuyoshi Tanaka，T.，1998年。通过激光烧蚀-多收集器-电感耦合等离子体质谱法对铱进行定量分析，测定铱的含量。Chem Geol。，144，269-280 .; Malitch， KN，2004. O同位素对对比源的约束和元古代上地幔的长期熔融：胶质Ru-Os硫化物和Ru-Os-Ir合金的证据。Chem Geol。208. 157-173 .; Walker，RJ，Brandon， AD，Bird，JM，Piccolia，PM，McDonougha，WF，RD，Asha，RD，2005年，Os-187-Os-186来自俄勒冈州西南部的Os-Ir-Ru合金晶粒的系统，地球行星，科学编号230 ，211-226。）。各种PGA矿物产生的Pt，Re和Os含量范围很广，这转化为父子比的广泛分布，这对于使用Re-Os和Pt-Os衰减方案的年代学来说是一个有吸引力的主张。在这里，我们表明单个PGA晶粒可以包含足够的内部Re，Pt和Os元素变异性以及父/子和子同位素组成的耦合变化，它们提供了从两个不同的同位素衰变系统获得“等时”年龄的机会。我们提出了一种快速（40 s的采集时间），精确的LA-MC-ICP-MS方法，适用于对单个PGA晶粒应用Pt-Os和Re-Os年代学方法，用于对铬铁矿矿床进行测年以及对多个铬铁矿进行定性和测年冲积PGA矿床的来源。由于Pt的等压干扰，使用Os-189 / Os-188比值对LA-MC-ICP-MS进行的PGA分析进行仪器质量分级的最佳校正。我们证明，在我们的分析不确定性范围内，LA-MC-ICP-MS期间Os同位素比率的质量偏差效应与常规溶液模式分析期间观察到的质量偏差变化在性质和大小上相似。尽管可以在溶液分析期间对Os质谱上的Re和W进行强健的元素干扰校正（Nowell，GM，Luguet，A.，Pearson，DA，Horstwood，MSA，2008年-此体积。精确而准确的Os-186 /多收集器等离子体电离质谱（MC-ICP-MS）第1部分（188）和Os-187 / Os-188测量：溶液分析（Chem Geol。）我们证明了Re对Os干扰的准确校正在Re-187 / Os-188比率>-0.5的情况下，激光烧蚀过程中存在问题，这意味着对于某些PGA和高Re / Os硫化物，必须重新校正Os-187 / Os-188比率。我们的干扰元素校正方法还可用于校正Pt-190上的Os，以获得Pt-190 / Os-188比率，从而无需独立的Pt监测仪同位素。通过消融过程中母体/母体元素比例的定量分馏不佳，进一步限制了LA-MC-ICP-MS的准确和精确的年代学。尽管需要额外的工作来了解在Pt和富Re的PGA晶粒的激光烧蚀分析过程中Pt / Os和Re / Os比的元素间分馏的性质和确切程度，但我们最好的估计是Pt的分馏激光烧蚀期间的/ Os约为5％或更低。我们介绍了拉普兰PGA晶粒中Re-Os和Pt-Os同位素系统的多粒等时线和单粒内部“等时线”。在所有情况下，Pt-Os同位素系统都比Re-Os系统提供更精确的“等时线”。我们认为这两种系统之间的这种不协调很可能是由于在提高的Re / Os比下进行Re干扰校正所固有的问题。我们展示tha

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