Chondrule porosity in the L4 chondrite Saratov: Dissolution, chemical transport, and fluid flow
Chondrule porosity in the L4 chondrite Saratov: Dissolution, chemical transport, and fluid flow
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DOI：
10.1016/j.gca.2018.08.002
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发表时间：
2018-11-01
影响因子：
5
通讯作者：
Garcea, Serafina C.
中科院分区：
文献类型：
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作者：
Lewis, Jonathan A.;Jones, Rhian H.;Garcea, Serafina C.
孔隙度是陨石和小行星的一个重要物理性质，它影响密度、材料强度和导热性。多孔性还可以通过促进流体和溶解离子的运输来促进化学交换。我们测量了单个球粒的孔隙度从L4普通球粒陨石萨拉托夫，使用X射线显微断层扫描（μ CT）和扫描电子显微镜，检查球粒中的孔隙度的丰度和分布，并了解如何孔隙度与母体过程中的化学交换。我们测量的球粒中孔隙率为1- 2体积%，最大孔径与300 μ m相似。孔隙分布和形貌表明，孔隙是次要特征，大多数>1 μ m的孔隙是由球粒介稳态玻璃溶解形成的。Fe和K优先富集在最多孔区域附近的相中：Fe富集在辉石中，K富集在中稳态中，在那里观察到二氧化硅蚀变相merrihueite或细尺度钾长石出溶于钠长石中。一些孔隙可以被描述为洞穴，因为它们含有自形橄榄石和铬铁矿，具有指示气相沉积的纹理。已知球粒孔隙度，我们估计萨拉托夫的基质孔隙度非常高，为40- 60%。我们认为，在进变质作用期间，来自基质的含水流体溶解了球粒中稳态玻璃，产生了观察到的孔隙度，并将Fe引入到辉石斑晶中。通过高峰期变质作用，流体不太丰富，球粒中稳态玻璃结晶为细粒钠长石。在退变质作用，高温，短时间的突发干燥，含碱流体从小行星内部渗透的孔隙网络，形成了孔洞相，改变硅merrihueite，并介绍了K的次生钠长石。在快速冷却至环境温度的过程中，细鳞钾长石从钠长石中析出。总的来说，L球粒陨石母体在变质过程中孔隙度的发展有助于普通球粒陨石物质的化学演化，并影响了母体小行星的物理性质。(C)2018作者爱思唯尔有限公司出版
Porosity is an important physical property of meteorites and asteroids that affects density, material strength, and thermal conductivity. Porosity can also promote chemical exchange by facilitating the transport of fluids and dissolved ions. We measured the porosity of individual chondrules from the L4 ordinary chondrite Saratov, using X-ray microtomography (mu CT) and scanning electron microscopy, to examine the abundance and distribution of porosity in chondrules, and to understand how porosity relates to chemical exchange during parent body processes. Porosity was 1-2% by volume in the chondrules we measured and maximum pore sizes were similar to 300 mu m. Porosity distribution and morphology indicate that porosity is a secondary feature and most pores >1 mu m were formed from the dissolution of chondrule mesostasis glass. Fe and K are preferentially enriched in phases adjacent to the most porous regions: Fe is enriched in pyroxene, and K is enriched in mesostasis where it is observed as either the silica alteration phase merrihueite, or fine-scale, K-feldspar exsolution in albitic feldspar. Some pores can be described as vugs, as they contain euhedral olivine and chromite, with textures indicating vapor deposition. Knowing the chondrule porosity, we estimate the matrix porosity in Saratov to be very high, 40-60%. We suggest that during prograde metamorphism, an aqueous fluid originating from the matrix dissolved chondrule mesostasis glass, producing the observed porosity, and introducing Fe into the pyroxene phenocrysts. Fluids were less abundant through peak metamorphism, and chondrule mesostasis glass crystallized to fine-grained albite. During retrograde metamorphism, high temperature, short duration bursts of a dry, alkali-bearing fluid from the asteroid interior infiltrated the pore network, formed the vug phases, altered silica to merrihueite, and introduced K to the secondary albite. Fine-scale K-feldspar then exsolved from albite during rapid cooling to the ambient temperature. Overall, development of porosity during metamorphism on the L chondrite parent body contributed to the chemical evolution of ordinary chondrite material, as well as affecting physical properties of the parent asteroid. (C) 2018 The Authors. Published by Elsevier Ltd.