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首页> 外文期刊>Contributions to Mineralogy and Petrology >Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex, South Africa
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Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex, South Africa

机译：南非布什维尔德（Bushveld）矿区梅伦斯基礁（Merensky Reef）的铬铁矿中金云母-邻苯二酚夹杂物的起源

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About 30 percent of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/(Mg + Fe2 + ) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt percent), Na_2O (2.4 wt percent) and H_2O (2.6 wt percent), and low CaO (1.1 wt percent) and FeO (4.4 wt percent). The 18O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ~7 percent0. This value is consistent with the previous estimates for the Bushveld magma and with the 818O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orth-pyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.
机译：Vlakfontein农场Bushveld Complex的Merensky礁基地处的铬铁矿接缝中，约30％的铬铁矿晶粒具有可变大小，其中包含丰富的复合矿物包裹体。夹杂物为多角形到圆形，并带有径向裂纹，并伸入封闭的铬铁矿中。它们的直径从几微米到几毫米不等，并且集中在亚铬酸盐晶体的核和幔中。电子背散射图谱表明主体亚铬酸盐是单晶，而不是多个晶粒的合并。亚金云母和邻苯二酚含量最高。亚铁角闪石，K-金云母，寡糖和石英较少。富含Cl的磷灰石，金红石，锆石和黄铜矿以痕量水平存在。钠金云母是夹杂物特有的。在布什维尔德综合大楼的其他地方找不到它。铬铁矿层的基质中也存在内含物中的其他矿物质，但它们的成分不同。夹杂物中邻苯二茂的Mg /（Mg + Fe2 +）比略高于基体中邻苯二茂的Mg /（Mg + Fe2 +）比。夹杂物中的K-金云母比基质中含有更多的Na。夹杂物的平​​均组成特征在于高的MgO（26重量％），Na_2O（2.4重量％）和H_2O（2.6重量％），以及低的CaO（1.1重量％）和FeO（4.4重量％）。通过分析富夹杂物和贫夹杂物的铬铁矿估计的截留熔体的18O值为〜7％。该值与先前对Bushveld岩浆的估计以及整个礁石中硅酸盐矿物的818O值一致。质构特征和特殊的化学组成与铬铁矿结晶过程中邻苯二酚被可变数量的富挥发物熔体截留相一致。人们认为，富含挥发性物质的熔体是由室底岩浆与从下面的晶体堆排出的富含Na-K的流体之间的混合程度不同而引起的。据认为，向岩浆中添加流体会导致邻-邻辉石溶解，而使系统仅在亚铬酸盐中饱和。氧同位素值和氢同位素值均与岩浆流体在流体添加和邻二甲苯溶解过程中的参与一致。夹杂物中的大多数Cr和Al是通过主体亚铬酸盐的壁溶解而产生的。与邻二甲苯一起捕获的少量金红石的溶解提供了夹杂物中的大部分Ti。夹杂物中富含Na和K的含水硅酸盐矿物是在冷却过程中通过辉石与捕获的富含挥发物的熔体之间的反应形成的。

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