Phosphate minerals in LL chondrites: A record of the action of fluids during metamorphism on ordinary chondrite parent bodies
Phosphate minerals in LL chondrites: A record of the action of fluids during metamorphism on ordinary chondrite parent bodies
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DOI：
10.1016/j.gca.2014.01.027
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发表时间：
2014-05-01
影响因子：
5
通讯作者：
Shearer, Charles K.
中科院分区：
文献类型：
--
作者：
Jones, Rhian H.;McCubbin, Francis M.;Shearer, Charles K.
普通球粒陨石含有两种磷酸盐矿物，即镁线石和氯磷灰石，这两种矿物都是球粒陨石母体上变质作用产生的次生矿物。我们研究了岩石学类型为3.9-6的四种LL钙钛矿的磷酸盐矿物学，以确定这两种矿物的岩石成因并解释它们形成的条件。镁橄榄石和磷灰石的表征包括结构观察、电子探针微区分析测定的矿物组成、微量元素和挥发性阴离子元素丰度的离子探针分析。磷酸盐矿物在轻度变质作用期间的初始形成，到岩石学类型4条件，导致最初并入金属中的P的氧化，以及作为金属颗粒内的夹杂物的镁橄榄石的生长。这两种磷酸盐矿物的后续发展是由于扩散平衡、可能从流体中沉淀以及与流体相互作用引起的置换反应。多孔性和脉填充的纹理在这两个merrilite和氯磷灰石，以及纹理表明更换merrilite氯磷灰石，支持一个模型，其中流体发挥了重要作用，并建议在交代作用的界面耦合溶解-再沉淀机制。磷酸盐矿物与铬铁矿斜长石组合的一些协会表明，磷酸盐矿物也可能与影响过程有关，无论是作为沉淀的影响熔体或作为一个结果，与流体或蒸气来自一个影响熔体的相互作用。在低温变质作用下，流体成分最初可能是含水的，但后来演变成富含卤素且非常干燥。后期富卤素流体的LL球粒陨石材料的冷却过程中占主导地位，可能已经来自于部分熔体在母体内部的蒸发。总体而言，LL球粒陨石母体经历了一个复杂的化学演化过程，其中交代作用起着重要作用。(C)2014爱思唯尔有限公司版权所有。
Ordinary chondrites contain two phosphate minerals, merrillite and chlorapatite, both of which are secondary minerals that developed in response to metamorphism on the chondrite parent bodies. We have studied the phosphate mineralogy of four LL chondrites, of petrologic types 3.9-6, in order to determine the petrogenesis of the two minerals and interpret the conditions under which they formed. Characterization of merrillite and apatite includes textural observations, mineral compositions determined by electron probe microanalysis, and ion microprobe analyses of trace element and volatile anion elemental abundances. Initial formation of phosphate minerals during mild metamorphism, to petrologic type 4 conditions, resulted in oxidation of P that was originally incorporated in metal, and growth of merrillite as inclusions within metal grains. Subsequent development of both phosphate minerals occurred in response to diffusional equilibration, possible precipitation from fluids as well as replacement reactions resulting from interactions with fluids. Porosity and vein-filling textures in both merrillite and chlorapatite, as well as textures indicating replacement of merrillite by chlorapatite, support a model in which fluid played a significant role and suggest an interface-coupled dissolution-reprecipitation mechanism during metasomatism. Some associations of phosphate minerals with chromite-plagioclase assemblages suggest that phosphate minerals could also be related to impact processes, either as precipitation from an impact melt or as a result of interactions with a fluid or vapor derived from an impact melt. Fluid compositions may have been water-bearing initially, at low temperatures of metamorphism, but later evolved to become halogen-rich and very dry. Late-stage halogen-rich fluids that dominated during cooling of LL chondrite material may have been derived from vaporization of partial melts in the interior of the parent body. Overall, the LL chondrite parent body underwent a complex chemical evolution, in which metasomatism played a significant role. (C) 2014 Elsevier Ltd. All rights reserved.