Spatially heterogeneous argon-isotope systematics and apparent 40Ar/39Ar ages in perlitised obsidian
Spatially heterogeneous argon-isotope systematics and apparent 40Ar/39Ar ages in perlitised obsidian
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DOI：
10.1016/j.chemgeo.2017.05.018
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发表时间：
2018-03-05
期刊：
影响因子：
3.9
通讯作者：
Sherlock, Sarah C.
中科院分区：
文献类型：
--
作者：
Flude, Stephanie;Tuffen, Hugh;Sherlock, Sarah C.
对美国圣胡安27 Ma Cochetopa穹丘附近的水化和脱玻黑曜石进行原位激光烧蚀Ar同位素分析，揭示了初始Ar的脱气和大气Ar的吸收之间复杂的相互作用。这些过程局部修改了黑曜岩的Ar同位素组成，并导致虚假的，空间异质性的Ar同位素和40 Ar/39 Ar年龄数据。小珠在边缘处比核心处表现出更老的明显Ar-ages。这被解释为在边缘处明显过量的40 Ar，其产生是由于a）在用过量的含40 Ar的火山气体冲洗熔岩期间过量的40 Ar扩散到珠中，或者由B）在初始Ar脱气期间的同位素分馏，导致珠边缘处Ar-36比40 Ar优先损失。第二种解释是有利的相对富集的Ar-36在核心的一个无微晶（不良脱气）流带，沿着，并通过缺乏年龄的变化，在一个较大的，新鲜的，良好的脱气微珠的微珠。这些同位素梯度后来在玻璃水合过程中通过吸收具有接近大气成分的Ar而被叠加，导致在玻璃珠的边缘处的Ar-36升高和放射性40 Ar * 产率降低。这些复杂的相互作用基本上代表了三种不同的Ar储层的混合：初始捕获的Ar可能会或可能不会被分馏，在水合过程中引入的同位素大气Ar组分，和放射性40 Ar *。这种储层混合是同位素相关图相关性差和难以验证非放射成因Ar组分组成的根本原因。因此，我们建议，高Ar-36产量的组合不完全脱气的初始（可能是岩浆）Ar和Ar的收益之间的相互作用黑曜石和流星/大气流体。我们的分析强调了具有挑战性的性质的40 Ar/39 Ar定年的黑曜石样品，但也指出可能的解决方案，通过仔细的样品表征和选择高度脱气samples.In原位激光烧蚀Ar同位素分析的水化和脱玻的黑曜石从类似的27马Cochetopa圆顶，圣胡安，美国，揭示了复杂的相互作用之间的脱气初始Ar和大气Ar的吸收。这些过程局部修改了黑曜岩的Ar同位素组成，并导致虚假的，空间异质性的Ar同位素和40 Ar/39 Ar年龄数据。小珠在边缘处比核心处表现出更老的明显Ar-ages。这被解释为在边缘处明显过量的40 Ar，其产生是由于a）在用过量的含40 Ar的火山气体冲洗熔岩期间过量的40 Ar扩散到珠中，或者由B）在初始Ar脱气期间的同位素分馏，导致珠边缘处Ar-36比40 Ar优先损失。第二种解释是有利的相对富集的Ar-36在核心的一个无微晶（不良脱气）流带，沿着，并通过缺乏年龄的变化，在一个较大的，新鲜的，良好的脱气微珠的微珠。这些同位素梯度后来在玻璃水合过程中通过吸收具有接近大气成分的Ar而被叠加，导致在玻璃珠的边缘处的Ar-36升高和放射性40 Ar * 产率降低。这些复杂的相互作用基本上代表了三种不同的Ar储层的混合：初始捕获的Ar可能会或可能不会被分馏，在水合过程中引入的同位素大气Ar组分，和放射性40 Ar *。这种储层混合是同位素相关图相关性差和难以验证非放射成因Ar组分组成的根本原因。因此，我们建议，高Ar-36产量的组合不完全脱气的初始（可能是岩浆）Ar和Ar的收益之间的相互作用黑曜石和流星/大气流体。我们的分析强调了40 Ar/39 Ar定年黑曜石样品的挑战性，但也指出了通过仔细的样品表征和选择高度脱气的样品可能的解决方案。
In situ laser ablation Ar-isotope analyses of variably hydrated and devitrified obsidian from the similar to 27 Ma Cochetopa Dome, San Juan, USA, reveal complex interplay between degassing of initial Ar and absorption of atmospheric Ar. These processes have locally modified the Ar-isotope composition of the obsidian and led to spurious, spatially-heterogeneous Ar-isotope and 40Ar/39Ar age data. Small perlite beads exhibit older apparent Ar-ages at the rims than the cores. This is interpreted as an apparent excess of 40Ar at the rims, produced either by a) diffusion of excess 40Ar into the bead during flushing of the lava with excess 40Ar-bearing volcanic gas, or by b) isotopic fractionation during degassing of initial Ar, causing preferential loss of Ar-36 over 40Ar at the bead rims. The second interpretation is favoured by a relative enrichment of Ar-36 in the core of a perlite bead along a microlite-free (poorly degassed) flow band, and by a lack of age variation in a larger, fresh, well-degassed perlite bead. These isotopic gradients were later overprinted during glass hydration by absorption of Ar with near atmospheric composition, resulting in elevated Ar-36 and reduced radiogenic 40Ar* yields at the rims of perlite beads. These complex interactions essentially represent the mixing of three distinct Ar reservoirs: initial trapped Ar that may or may not be fractionated, an isotopically atmospheric Ar component introduced during hydration, and radiogenic 40Ar*. Such reservoir mixing is the underlying reason for poor correlations on isotope correlation diagrams and the difficulties in validating the composition of the non-radiogenic Ar component. We thus suggest that high Ar-36 yields are a combination of the incomplete degassing of initial (possibly magmatic) Ar and the gain of Ar during interaction between the obsidian and meteoric/atmospheric fluids. Our analyses emphasise the challenging nature of 40Ar/39Ar dating obsidian samples, but also point to possible solutions by careful sample characterisation and selection of highly degassed samples.In situ laser ablation Ar-isotope analyses of variably hydrated and devitrified obsidian from the similar to 27 Ma Cochetopa Dome, San Juan, USA, reveal complex interplay between degassing of initial Ar and absorption of atmospheric Ar. These processes have locally modified the Ar-isotope composition of the obsidian and led to spurious, spatially-heterogeneous Ar-isotope and 40Ar/39Ar age data. Small perlite beads exhibit older apparent Ar-ages at the rims than the cores. This is interpreted as an apparent excess of 40Ar at the rims, produced either by a) diffusion of excess 40Ar into the bead during flushing of the lava with excess 40Ar-bearing volcanic gas, or by b) isotopic fractionation during degassing of initial Ar, causing preferential loss of Ar-36 over 40Ar at the bead rims. The second interpretation is favoured by a relative enrichment of Ar-36 in the core of a perlite bead along a microlite-free (poorly degassed) flow band, and by a lack of age variation in a larger, fresh, well-degassed perlite bead. These isotopic gradients were later overprinted during glass hydration by absorption of Ar with near atmospheric composition, resulting in elevated Ar-36 and reduced radiogenic 40Ar* yields at the rims of perlite beads. These complex interactions essentially represent the mixing of three distinct Ar reservoirs: initial trapped Ar that may or may not be fractionated, an isotopically atmospheric Ar component introduced during hydration, and radiogenic 40Ar*. Such reservoir mixing is the underlying reason for poor correlations on isotope correlation diagrams and the difficulties in validating the composition of the non-radiogenic Ar component. We thus suggest that high Ar-36 yields are a combination of the incomplete degassing of initial (possibly magmatic) Ar and the gain of Ar during interaction between the obsidian and meteoric/atmospheric fluids. Our analyses emphasise the challenging nature of 40Ar/39Ar dating obsidian samples, but also point to possible solutions by careful sample characterisation and selection of highly degassed samples.