Precipitation and dissolution of chromite by hydrothermal solutions in the Oman ophiolite: New behavior of Cr and chromite
Precipitation and dissolution of chromite by hydrothermal solutions in the Oman ophiolite: New behavior of Cr and chromite
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DOI：
10.2138/am.2014.4473
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发表时间：
2014
影响因子：
3.1
通讯作者：
S. Arai;N. Akizawa
中科院分区：
文献类型：
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作者：
S. Arai;N. Akizawa
摘要铬铁矿是一种典型的耐火火成岩矿物，是在较高温度下从镁铁质岩浆中沉淀出来的。铬铁矿通常存在于沉积岩、变质岩和交代岩中，它们被解释为火成岩相的遗迹，并作为低温含铬矿物的铬源。我们提出的证据铬铁矿成核的热液溶液。我们发现在透辉石岩中有微小的真面状铬铁矿颗粒，被钙铬榴石（钙铬榴石）包裹，交代取代了阿曼蛇绿岩的层状辉长岩。铬钒矿的Cr值[Cr/（Cr+Al）]呈振荡同心环带，铬铁矿仅嵌在铬钒矿的高Cr值区。另一透辉石岩，取代橄榄岩在下伏上地幔部分，包含捕虏晶铬铁矿，这是部分溶解。这表明，热液通过上地幔中铬铁矿的部分至全部溶解收集了Cr，并在下地壳上段中沉淀了铬铁矿，沿着高Cr-no.铬钒矿。所涉及的交代剂是一个CO2，SO2，和Cl-轴承热液含有可观的硅酸盐成分，可以携带铬，可能作为一个复杂的。热液铬铁矿的化学性质与火成岩中常见的相似[例如，Cr ~（2+）= 0.8，Mg/（Mg+ Fe ~（2+））= 0.1-0.2，TiO ~（2+）<0.3wt%，Fe ~（3+）/（Cr+Al+ Fe ~（3+））可达0.3]，但其Cr ~（2+）明显不同于阿曼蛇绿岩中橄榄岩和铬铁矿中地幔铬铁矿的Cr ~（2+）（0.6-0.7）。从这项研究的结果表明，热液起源是可能的铬铁矿在超镁铁岩经历了流体活动，假设有足够的铬铁矿在流体源。
Abstract Chromite is a typical refractory igneous mineral, precipitated from mafic magmas at relatively high temperatures. Chromites commonly occur in sedimentary, metamorphic, and metasomatic rocks, where they are interpreted as relics of an igneous phase and serve as the source of Cr for low-temperature Cr-bearing minerals. We present evidence for the nucleation of chromite within hydrothermal solutions. We have found minute euhedal chromite grains enclosed by uvarovite (Ca-Cr garnet) in a diopsidite, metasomatically replacing the layered gabbro of the Oman ophiolite. The uvarovite shows oscillatory concentric zoning in terms of Cr no. [Cr/(Cr+Al)], and the chromite is embedded only in the high-Cr-no. zones of the uvarovite. Another diopsidite, replacing peridotite in the underlying upper mantle section, contains xenocrystic chromite, which is partly dissolved. This suggests that a hydrothermal solution collected Cr by partial to total dissolution of chromite within the upper mantle and precipitated chromite, along with high-Cr-no. uvarovite, within the lower crust upsection. The metasomatic agent involved was a CO2-, SO2-, and Cl-bearing hydrothermal solution containing appreciable silicate components that could carry Cr, possibly as a complex. The hydrothermal chromite is similar in chemistry to that commonly found in igneous rocks [e.g., Cr no. = 0.8, Mg/(Mg+Fe2+) = 0.1-0.2, TiO2 < 0.3 wt% and Fe3+/(Cr+Al+Fe3+), up to 0.3], but its Cr no. is clearly different from that of mantle chromite (0.6-0.7) in peridotites and chromitites from the Oman ophiolite. The results from this study suggest that a hydrothermal origin is possible for chromites in ultramafic rocks that have experienced fluid activity assuming that there is sufficient chromite at the fluid source.