Dating silica sinter (geyserite): A cautionary tale
Dating silica sinter (geyserite): A cautionary tale
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DOI：
10.1016/j.jvolgeores.2020.106991
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发表时间：
2020-09-15
影响因子：
2.9
通讯作者：
Paces, James B.
中科院分区：
文献类型：
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作者：
Churchill, Dakota M.;Manga, Michael;Paces, James B.
我们描述了一项新的努力，利用共沉积有机质的C-14， u系列和宇宙成因Be-10方法来确定黄石国家公园上间歇泉盆地的热液硅烧结矿床（硅辉石）的年代。大多数样品是从地层剖面收集的，主要是在河滨、巨人和城堡间歇泉。从41个C-14分析得到的年龄范围从现代到12.1 cal ka BP。几乎所有的C-14年龄都与地层位置不一致，对同一样品进行的多次重复C-14分析结果显示年龄差异很大。烧结矿中有机材料的δ C-13值从-26.6‰到-12.7‰不等。较高的富集值归因于微生物对溶解无机碳（DIC）的固定作用，DIC具有较重的δ C-13值，并且相对于大气CO2耗尽了c -14，导致年龄明显变老。u系列分析4个样品的年龄在2.2至7.4 ka之间。由于低铀浓度以及高Th-232和相关的初始Th-230，烧结矿中Th-230/U年龄的不确定性很大，使得这些年龄在全新世矿床中使用不精确。单一宇宙成因的Be-10暴露年龄为596 +/- 18 ka，比下伏流纹岩的年龄要老得多，因此是不可靠的。这种明显的衰老是由于被困在欧泊石中的陨星Be-10污染造成的，而欧泊石覆盖了非常少量的宇宙源Be-10。通过介绍我们遇到的问题和讨论其可能原因，本文强调了获得可靠、高精度的地质年代学数据的困难，这些数据是将烧结矿床作为古环境和古热液档案所必需的。Elsevier B.V.出版
We describe a new effort to date hydrothermal silica sinter deposits (geyserite) from the Upper Geyser Basin of Yellowstone National Park using C-14 of co-deposited organic matter, U-series and cosmogenic Be-10 methods. A majority of the samples were collected from stratigraphic sections, mainly at Riverside, Giant, and Castle Geysers. Ages obtained from 41 C-14 analyses range from modern to 12.1 cal ka BP. Nearly all the C-14 ages show inconsistencies with their stratigraphic positions, and several replicate C-14 analyses from the same sample result in significantly different ages. The delta C-13 values of the organic material in the sinter range from -26.6 parts per thousand to -12.7 parts per thousand. The more enriched values are attributed to microbial fixation of dissolved inorganic carbon (DIC), which has heavier delta C-13 values and is C-14-depleted relative to atmospheric CO2, leading to an apparent older age. U-series analyses on 4 samples yielded ages between 2.2 and 7.4 ka. Large Th-230/U age uncertainties in the sinter, due to low uranium concentrations along with elevated Th-232 and associated initial Th-230, make these ages imprecise for use on Holocene deposits. A single cosmogenic Be-10 exposure age of 596 +/- 18 ka is considerably older than the age of underlying rhyolite and is thus unreliable. This apparent old age results from contamination by meteoric Be-10 trapped in the opal that overprints the very small amount of cosmogenic Be-10. By presenting the problems we encountered and discussing their probable cause, this paper highlights the difficulty in obtaining reliable, high-precision geochronological data necessary to use sinter deposits as paleoenvironmental and paleo-hydrothermal archives. Published by Elsevier B.V.