Formation of Chromitite Seams and Associated Anorthosites in Layered Intrusion by Reactive Volatile-rich Fluid Infiltration
Formation of Chromitite Seams and Associated Anorthosites in Layered Intrusion by Reactive Volatile-rich Fluid Infiltration
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富含反应性挥发性流体渗透的层状岩体中铬铁矿矿层和伴生斜长岩的形成
DOI：
10.1093/petrology/egaa109
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发表时间：
2021
影响因子：
3.9
通讯作者：
Boudreau, Alan E
中科院分区：
文献类型：
--
作者：
Marsh, Jennifer S;Pasecznyk, Michael J;Boudreau, Alan E
与静水复合体J-M礁铂族元素存款开发有关的钻探，在不协调斜长岩体中的斜长岩和苏长岩之间返回了一个稀有铬铁矿煤层的样品。与斜长岩侧（平均Ancore 82·5;平均Ancore- Anrim= +1 stats 0）相比，斜长石芯部的An浓度略高，在苏长岩侧（平均Ancore= 83·8;平均Ancore-Anrim= -1 stats 1）呈中度带状分布。斜长岩的平均斜长石粒度也比苏长岩中的斜长石稍小（分别为1.41 mm和1.54 mm）。铬铁矿中可含有单相或多相的斜方辉石、斜长石、角闪石、黑云母和富氯磷灰石包裹体。这些和其他成分和纹理的功能，以及推断从其他不协调的斜长岩体中的带状系列，都是一致的铬铁矿沉淀在反应前的色谱模型作为苏长岩原岩反应与富氯的含水流体饱和斜长石单独。铬铁矿接缝形成模拟使用渗透交代模型，其中流体变得逐渐不饱和辉石，因为它上升到较热的部分的晶体堆。当这种辉石不饱和流体通过正长质原岩时，它溶解含铬斜方辉石，产生斜长岩。由于镁和铬从辉石中的释放，铬铁矿沉淀在斜长岩和苏长岩之间的反应前沿。当辉石溶解前沿向流体流动方向移动时，铬铁矿发生连续的再溶解和再沉淀，收集从斜长岩中损失的Cr。由于Cr主要作为中性二价阳离子络合物CrCl（OH）0溶解在溶液中，但作为三价阳离子并入铬铁矿中，所需的氧化还原反应可涉及硫化物与铬铁矿的同时沉淀。这种机制与最近的一些模型不同，因为斜长岩本身是置换体，不是岩浆的原始沉淀物，也不是由部分熔融造成的镁铁质物质损失形成的。结果表明，需要在T和P条件下的实验矿物溶解度数据适合于上地壳镁铁质-超镁铁质侵入体。
Drilling related to development of the platinum-group element deposit of the J-M Reef of the Stillwater Complex returned samples of a rare chromitite seam between anorthosite and norite in a discordant anorthositic body. Plagioclase core An concentrations are marginally higher and modestly reversely zoned on the norite side (average Ancore= 83·8; average Ancore– Anrim= –1·1) as compared with the anorthosite side (average Ancore82·5; average Ancore– Anrim= +1·0). The anorthosites are also characterized by a slightly smaller average plagioclase grain size than plagioclase in the norite (1·41 mm and 1·54 mm, respectively). The chromite can contain single and polyphase inclusions of orthopyroxene, plagioclase, amphibole, biotite and Cl-rich apatite. These and other compositional and textural features, as well as inference from other discordant anorthositic bodies in the Banded series, are all consistent with a chromatographic model of chromite precipitation at a reaction front as a norite protolith reacts with a Cl-rich aqueous fluid saturated in plagioclase alone. Chromitite seam formation is modeled using an infiltration metasomatic model, in which a fluid becomes progressively undersaturated in pyroxene as it rises into the hotter part of the crystal pile. As this pyroxene-undersaturated fluid moves through a noritic protolith, it dissolves the Cr-bearing orthopyroxene to produce an anorthosite. Chromite precipitates at the reaction front between the anorthosite and the norite owing to liberation of Mg and Cr from pyroxene. Continuous redissolution and reprecipitation of chromite occurs as the pyroxene dissolution front moves in the direction of fluid flow, collecting the Cr lost from the anorthosite. Owing to Cr dissolved mainly as a neutral divalent cation complex, CrCl(OH)0, in the solution, but incorporated as a trivalent cation in chromite, the required redox reaction can involve concurrent precipitation of sulfide with chromite. This mechanism differs from some recent models in that the anorthosites are themselves replacement bodies and are not original precipitates from a magma nor formed by loss of mafic material by partial melting. The results show the need for experimental mineral solubility data atTandPconditions appropriate to upper crustal mafic–ultramafic intrusions.
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DOI：
--
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DOI：
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发表时间：
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DOI：
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发表时间：
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