Laser argon dating of melt breccias from the Siljan impact structure, Sweden: Implications for a possible relationship to Late Devonian extinction events
Laser argon dating of melt breccias from the Siljan impact structure, Sweden: Implications for a possible relationship to Late Devonian extinction events
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瑞典 Siljan 撞击结构熔体角砾岩的激光氩测年：与晚泥盆世灭绝事件可能存在关系的影响
DOI：
10.1111/j.1945-5100.2005.tb00965.x
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发表时间：
2004
影响因子：
2.2
通讯作者：
C. Koeberl
中科院分区：
文献类型：
--
作者：
W. Reimold;S. Kelley;S. Sherlock;H. Henkel;C. Koeberl
摘要-在早期的研究中，瑞典直径65-75公里的锡尔扬撞击结构被认为与晚泥盆世大灭绝事件有关。震旦纪撞击事件已被K-Ar和Ar-Ar年代学确定为342-368 Ma，通常引用的年龄为362.7±2.2 Ma(2 S̀，用目前公认的衰变常数重新计算)。直到最近，公认的弗拉斯尼亚/法门期界线和相关的灭绝事件的年龄为364 Ma，这在较早的Siljan年龄的误差范围内。本文报道了用激光光斑和激光阶梯加热技术提取的几个来自Siljan的熔融角砾岩(解释为冲击熔融角砾岩)的新的Ar-Ar年龄。分析结果显示出一些分散性，这种分散性在蚀变范围较大的样品中更大；这些样品通常产生较年轻的年龄。一个样品的激光斑点年龄为377.2±2.5 Ma(95%可信区间)，另一个样品的激光斑点年龄为376.1±2.8 Ma(95%可信区间)，激光阶跃加热平台年龄超过70.6%~(39)Ar释放量为377.5±2.4 Ma(2 S̀)。我们对Siljan年龄的保守估计为377±2 Ma(95%置信限)，这与以前接受的F/F边界年龄和先前引用的Siljan年龄都有很大不同。然而，国际地层学委员会最近将F/F界线的年龄修正为374.5±2.6 Ma，这与我们的新年龄大致相同。然而，目前可用的年龄数据并不能证明席尔詹撞击事件与F/F边界灭绝之间存在联系。这一新结果突显了陨石撞击的年龄测定的双重问题，在陨石撞击中，细粒熔岩往往是唯一可以进行同位素测年的，以及限制生物地层边界的绝对年龄，这只能受到年龄外推的限制。还需要进一步开展工作，以发展和改进陆地撞击年龄记录，并测试陆地撞击通量是否在某些时间显著增加，可能导致地球生物地层记录中的重大灭绝事件。
Abstract— In earlier studies, the 65‐75 km diameter Siljan impact structure in Sweden has been linked to the Late Devonian mass extinction event. The Siljan impact event has previously been dated by K‐Ar and Ar‐Ar chronology at 342‐368 Ma, with the commonly quoted age being 362.7 ± 2.2 Ma (2 s̀, recalculated using currently accepted decay constants). Until recently, the accepted age for the Frasnian/Famennian boundary and associated extinction event was 364 Ma, which is within error limits of this earlier Siljan age. Here we report new Ar‐Ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from Siljan (interpreted to be impact melt breccia). The analytical results show some scatter, which is greater in samples with more extensive alteration; these samples generally yield younger ages. The two samples with the least alteration yield the most reproducible weighted mean ages: one yielded a laser spot age of 377.2 ± 2.5 Ma (95% confidence limits) and the other yielded both a laser spot age of 376.1 ± 2.8 Ma (95% confidence limits) and a laser stepped heating plateau age over 70.6% 39Ar release of 377.5 ± 2.4 Ma (2 s̀). Our conservative estimate for the age of Siljan is 377 ± 2 Ma (95% confidence limits), which is significantly different from both the previously accepted age for the Frasnian/Famennian (F/F) boundary and the previously quoted age of Siljan. However, the age of the F/F boundary has recently been revised to 374.5 ± 2.6 Ma by the International Commission for Stratigraphy, which is, within error, the same as our new age. However, the currently available age data are not proof that there was a connection between the Siljan impact event and the F/F boundary extinction. This new result highlights the dual problems of dating meteorite impacts where fine‐grained melt rocks are often all that can be isotopically dated, and constraining the absolute age of biostratigraphic boundaries, which can only be constrained by age extrapolation. Further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times, perhaps resulting in major extinction events in Earth's biostratigraphic record.