40Ar39Ar geochronology of rhyolites erupted following collapse of the Yellowstone caldera, Yellowstone Plateau volcanic field: implications for crustal contamination
40Ar39Ar geochronology of rhyolites erupted following collapse of the Yellowstone caldera, Yellowstone Plateau volcanic field: implications for crustal contamination
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黄石高原火山场黄石破火山口塌陷后喷发的 40Ar39Ar 流纹岩地质年代学：对地壳污染的影响
DOI：
10.1016/0012-821x(96)00088-x
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发表时间：
1996
影响因子：
5.3
通讯作者：
M. Mcwilliams
中科院分区：
文献类型：
--
作者：
C. Gansecki;G. Mahood;M. Mcwilliams
单晶激光探针40 Ar 39 Ar测年的133粒透长石和斜长石，使我们能够解决喷发年龄的上盆地成员rhythmia-熔岩流和相关的凝灰岩内爆发的黄石火山口后不久，其崩溃630 ky前的熔岩溪凝灰岩喷发。从东部环形断裂带喷发的两个熔岩和一个凝灰岩的喷发年龄为481 ± 8 ka，而从西部环形断裂带喷发的两个熔岩流的喷发年龄为516 ± 7和198 ± 8 ka。大多数单位包含古老的捕虏晶，解释了为什么以前尝试用传统的K-Ar法测定这些最早的破火山口后单位的年代，结果分辨率很差，在某些情况下，异常的年龄。凝灰岩显示出最严重的污染。颗粒从一个单一的浮石火山砾在凝灰岩显示大的年龄范围从散装玻璃斑岩，表明捕虏晶被纳入岩浆前，其近地表爆炸性破碎。扩散计算表明，捕虏晶不可能保持在岩浆中超过几年，没有脱气，并给出年龄与斑晶无法区分。因此，由捕虏晶代表的污染可能发生在断裂和管道传播过程中，而不是在破火山口坍塌过程中，后者发生在100 ky之前。捕虏晶的表观年龄及其成分的电子探针测定表明，始新世Absaroka火山岩是主要的污染物，与单一捕虏晶可能来自前寒武纪基底岩石。大多数捕虏晶很难从光学或化学上与长石斑晶区分开来，这说明了单晶体对许多年轻火山岩精确定年的必要性。
Single-crystal laser-probe40Ar39Ar dating of 133 grains of sanidine and plagioclase has enabled us to resolve the eruption ages of the Upper Basin Member rhyolites — the lava flows and related tuffs that erupted within the Yellowstone Caldera shortly after its collapse 630 ky ago on eruption of the Lava Creek Tuff. Two lavas and a tuff that erupted from the eastern ring-fracture zone yield an eruptive age of 481 ± 8 ka, whereas two flows from the western ring-fracture zone yield eruptive ages of 516 ± 7 and 198 ± 8 ka. Most of the units contain old xenocrysts, explaining why previous attempts at dating these earliest post-caldera units by the conventional KAr method yielded poorly resolved and, in some cases, anomalous ages. The tuff shows the most severe contamination. Grains from a single pumice lapilli in the tuff show as large an age range as those from bulk vitrophyre, indicating that the xenocrysts were incorporated in the magma prior to its near-surface explosive fragmentation. Diffusion calculations indicate that the xenocrysts could not have remained in the magma for more than a few years without degassing and giving ages indistinguishable from the phenocrysts. Thus, the contamination represented by the xenocrysts probably occurred during fracturing and conduit propagation, rather than during caldera collapse, which took place more than 100 ky earlier. The apparent ages of xenocrysts and their compositions as determined by electron microprobe suggest that the Eocene Absaroka volcanics are the main contaminant, with a single xenocryst probably coming from Precambrian basement rocks. Most of the xenocrysts are difficult to distinguish optically or chemically from feldspar phenocrysts, illustrating the necessity of single-crystal to date many young volcanic rocks accurately.