Accretionary lapilli from the Sudbury impact event
Accretionary lapilli from the Sudbury impact event
复制标题
萨德伯里撞击事件中的增生火山岩
DOI：
10.1111/maps.12863
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发表时间：
2017
影响因子：
2.2
通讯作者：
C. Koeberl
中科院分区：
文献类型：
--
作者：
M. Huber;C. Koeberl
陨石撞击产生的增生火山砾没有得到很好的研究。最近发现的1850马萨德伯里撞击事件的远端喷出物包含丰富的增生火山砾在影响过程中产生的，并在很大的距离从火山口沉积。我们从岩石学和地球化学角度研究了五个地点（麦克卢尔、康纳斯溪、588号高速公路、松树河和太平洋大干线，距离萨德伯里构造中心约480-750 km）的火山砾。石英、钾长石、方解石、黑云母和方解石矿物的成分在所有样品中彼此相似，尽管矿物的相对比例因地点而异。火山砾存在于粗粒石英、碳酸盐和长石颗粒的基质中。火山砾内的环带似乎是由于粒度分布，而不是成分的变化。内部区域比外部区域的颗粒更粗。方解石、页硅酸盐和方解石的相对丰度在各个火山岩带中相似。陨石成分表示高达1.8 ppb的Ir在一个火山岩从松树河网站，镍和铬的比例是在一个pastritic趋势线的火山岩。以前提出的机制增生火山砾形成似乎不足以解释沉积的远端增生火山砾造成的影响事件。提出了一种新的高层大气吸积机制，即从撞击事件中喷出的灰集中在中性浮力高度，然后在那里吸积，并比弹道就位的颗粒晚沉积。很可能，在混乱的撞击后环境中发生了多个过程。
Meteorite impact‐generated accretionary lapilli are not well studied. The recently discovered distal ejecta from the 1850 Ma Sudbury impact event contain abundant accretionary lapilli generated during the impact and deposited at great distances from the crater. We petrographically and geochemically examined lapilli from five sites (McClure, Connors Creek, Hwy 588, Pine River, and Grand Trunk Pacific, approximately 480–750 km from the center of the Sudbury structure). The compositions of quartz, K‐feldspar, calcite, biotite, and chlorite minerals are similar to each other in all of the samples, although the relative proportions of the minerals vary from site to site. The lapilli occur in a matrix of coarse‐grained quartz, carbonate, and feldspar grains. Zonation within lapilli appears to be due to grain size distribution rather than compositional variation. The inner zones are coarser grained than outer zones. The relative abundances of calcite, phyllosilicates, and feldspars are similar in each zone within individual lapilli. A meteoritic component is indicated by up to 1.8 ppb Ir in one lapillus from the Pine River site, and Ni and Cr ratios are on a chondritic trend line for many of the lapilli. Mechanisms previously proposed for accretionary lapilli formation seem inadequate to explain deposition of distal accretionary lapilli resulting from impact events. A new mechanism for upper atmospheric accretion is proposed, whereby ash ejected from impact events concentrates at altitudes of neutral buoyancy, where it then accretes and is deposited later than ballistically emplaced particles. Likely, multiple processes are taking place in the chaotic postimpact environment.