Noble gases, K, U, Th, and Pb in native gold
Noble gases, K, U, Th, and Pb in native gold
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天然金中的稀有气体、K、U、Th 和 Pb
DOI：
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
L. Bruno
中科院分区：
文献类型：
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作者：
O. Engster;S. Niedermann;Ch. Thalmann;R. Frei;J. Kramers;U. Krähenbühl;Y. Z. Liu;B. Hofmann;R. Boer;W. U. Reimold;L. Bruno
我们提出了稀有气体和Pb同位素丰度以及天然金的K， Th和U浓度的测定。我们的研究结果表明，金是一种很好的地壳挥发物载体，但由于各种原因，使用U、Th-4He、40K-40Ar和U裂变Xe方法对金进行直接测年并不成功。这项工作的主要意义在于金对圈闭气体和原位产生的气体具有很高的敏感性，这为利用金及其流体和固体包裹体研究古气体组成提供了前景。瑞士和意大利的砂金，意大利、南非和委内瑞拉的矿脉金，南非的矿脉金，以及南非的矿脉金的稀有气体库存都有许多核效应。质谱分析得到的脱气模式表明，主要在脉状金中的流体包裹体在500°C左右低温释放挥发物，而固体包裹体和金本身则在高温释放挥发物。低温挥发物代表了金结晶时被捕获的物质。我们研究了以下被捕获的物质：He、Ne、Ar、Kr、Xe和Pb的同位素，以及K、U、Th、H2O和CO2的丰度。被金捕获的地壳气体包括6Li(n,α)3H产生的3He→β−→U、Th、K衰变产生的3He、4He和40Ar，以及238U裂变产生的Xe。第三系金放射性成因圈闭气的比值为4He/40Ar = 3.9，太古宙金的比值为1.4。这些比值与U和K在各自时期的产出比值一致，表明金可以用作古代大气气体的取样器。U和Th的浓度范围从十亿分之几到百万分之几，K和Pb的浓度范围高达百万分之几十。南非百合金矿脉金和石英中铅的Pb-Pb模式年龄约为3000 Ma，表明了铅的赋存年代。金还含有由U、Th和K衰变在原位产生的惰性气体。我们从Witwatersrand West Rand金矿样品中分离出的金获得了高达6.8×10−2 cm3 STP/g （STP：大气压和0°C）的极高浓度的4He，表明该金具有较高的U含量。
We present determinations of the noble gas and Pb isotopic abundances and of K, Th, and U concentrations of native gold. Our results demonstrate that gold is an excellent carrier for crustal volatiles, but direct dating of gold using the U, Th-4He, 40K–40Ar, and U fission Xe methods was not successful for various reasons. The main significance of this work is the great sensitivity of gold for trapped gases as well as for gases that were produced in situ which gives the prospects of using gold and its fluid and solid inclusions for the study of paleogas composition. Numerous nuclear effects characterize the noble gas inventory of placer gold from Switzerland and Italy, vein gold from Italy, South Africa, and Venezuela, and lode gold from South Africa. The degassing patterns obtained by mass spectrometry show a low-temperature release of volatiles around 500°C from fluid inclusions mainly in vein gold and a high-temperature release from solid inclusions and the gold itself. The low-temperature volatiles represent species that were trapped when the gold crystallized. We investigated the following trapped species: the isotopes of He, Ne, Ar, Kr, Xe, and Pb, and the abundances of K, U, Th, H2O, and CO2. The crustal gases trapped by gold comprise 3He from 6Li(n,α)3H → β− → 3He, 4He and 40Ar from the U, Th, and K decay, and Xe from 238U fission. We observe 4He/40Ar = 3.9 for the radiogenic trapped gases of tertiary gold and a ratio of 1.4 for Archean gold. These ratios are consistent with the production ratios from U and K at the respective times and demonstrate that gold can be used as a sampler of ancient atmospheric gases. The concentrations of U and Th range from a few parts per billion to a few parts per million, and those of K and Pb range up to some tens of parts per million. The antiquity of trapped Pb is indicated by the Pb-Pb model age of about 3000 Ma for the lead extracted from vein gold and quartz of the Lily gold mine (South Africa). Gold also contains noble gases produced in situ by U, Th, and K decay. We obtained 4He up to the extremely high concentration of 6.8×10−2 cm3 STP/g (STP: atmospheric pressure and 0°C) for gold separated from a specimen of the Witwatersrand West Rand gold fields, indicating that this gold has a large U content.