Mineralogy and diagenesis of 3.24Ga meteorite impact spherules
Mineralogy and diagenesis of 3.24Ga meteorite impact spherules
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3.24Ga陨石撞击球体的矿物学和成岩作用
DOI：
10.1016/j.precamres.2011.12.001
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发表时间：
2012
影响因子：
3.8
通讯作者：
G. Byerly
中科院分区：
文献类型：
--
作者：
A. E. Krull;D. Lowe;G. Byerly
南非巴伯顿绿岩带S3层中的球状体是3.24 Ga大撞击事件后在撞击羽流中形成的远端沉降物。从那时起，成岩作用和低绿片岩级变质的球粒改变了矿物学，但形状和纹理在很大程度上被保存下来。S3层的蚀变产生了由石英、层状硅酸盐、钛和铁氧化物以及一些富镍铬铁矿组成的球粒。最初，玻璃球palagonitized和二氧化硅胶结的小球开始在低温与海水的相互作用。进一步蚀变的硅和钾丰富的流体导致的石英，长石和粘土的矿物组合。结晶矿物被溶解-沉淀过程所取代，保留了残余结构。进一步的硅质胶结作用导致岩层完全石化。这种蚀变大部分发生在海底和浅埋过程中。随着埋藏的继续，无定形二氧化硅重结晶为微晶石英。后来，在区域变质作用期间，在300-320°C的峰值温度下，粘土重结晶为云母。晚期剪切作用和成矿作用优先影响带的北方地区。不同地区的样本记录了当地非常不同的情况。水深，沉积前运输量，带内的位置，以及接近火成岩岩脉都影响S3球粒的成岩作用。在深水沉积环境中，二氧化硅和重晶石的浓度较低，碳酸盐的浓度较高。根据对整个BGB多个剖面的研究，可以推断成岩和较低绿片岩级变质作用期间的元素活动性。最不活泼的元素是Al、Zr、Ti、Sc，高场强元素和目前的元素比值可以用来推断球粒的原始成分。大离子亲石元素是高度移动的，轻稀土元素（REE），这是特别容易在碳酸盐成岩和磷酸盐自生活动。稀土元素中，Ce和Eu的变异性最大，表明在成岩作用和低级变质作用过程中有明显的活化作用。一致的Cr/Ir比，特别是在高浓度下，表明有限的流动性，和富镍铬铁矿轴承小球铂族元素的偏析。硫化物矿化并没有影响在S3层中的Ir浓度。
Spherules in the S3 bed of the Barberton greenstone belt, South Africa are distal fallout formed within an impact plume after a large impact event at 3.24Ga. Since that time, diagenesis and lower greenschist grade metamorphism of the spherules has changed the mineralogy, though shape and texture are largely preserved. Alteration of the S3 bed has resulted in spherules composed of quartz, phyllosilicates, Ti- and Fe-oxides, and some Ni-rich chromites. Initially, glassy spherules were palagonitized and silica cementation of the spherules began during low-temperature interaction with seawater. Further alteration by Si- and K-rich fluids resulted in a mineralogical assemblage of quartz, feldspar, and clays. Crystalline minerals were replaced by dissolution-precipitation processes, preserving relict textures. Further silica cementation resulted in complete lithification of the bed. Most of this alteration occurred at the seafloor and during shallow burial. With continued burial, amorphous silica recrystallized to microcrystalline quartz. Later recrystallization of clays to micas occurred during regional metamorphism at peak temperatures of 300–320°C. Late-stage shearing and mineralization preferentially affected the northern region of the belt. Samples from different sections record highly variable local conditions. Water depth, the amount of pre-depositional transport, location within the belt, and proximity to igneous dikes all affect the diagenesis of the S3 spherules. Silica and barite concentrations are lower, and carbonate concentrations are higher in the deep-water depositional environments. Element mobility during diagenetic and lower greenschist grade metamorphism can be inferred based on studies of multiple sections throughout the BGB. The most immobile elements are Al, Zr, Ti, Sc and the high field strength elements and present element ratios can be used to infer original composition of the spherules. The large ion lithophile elements are highly mobile, as are the light rare earth elements (REEs), which are particularly susceptible to mobilization during carbonate diagenesis and phosphate authigenesis. Of the REEs, Ce and Eu show the largest variability, suggesting significant mobilization during diagenesis and low-grade metamorphism. Consistent Cr/Ir ratios, particularly in the high concentrations suggest limited mobility, and segregation of platinum group elements into Ni-rich chromite-bearing spherules. Sulfide mineralization has not affected the Ir concentration in the S3 layer.