Osmium isotopes and crustal sources for platinum-group mineralization in the Bushveld Complex, South Africa
Osmium isotopes and crustal sources for platinum-group mineralization in the Bushveld Complex, South Africa
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南非布什维尔德杂岩中铂族矿化的锇同位素和地壳来源
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发表时间：
1991
期刊：
影响因子：
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通讯作者：
J. Ruíz
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作者：
T. Mccandless;J. Ruíz
南非的Bushveld杂岩是世界上最大的层状基性侵入岩，也是铂族元素（PGEs）的主要产地。在梅伦斯基礁和UG2和UG1铬铁矿中发现了经济矿化，包括含锇褐铁矿（Ru, Ir, Os） s2。含褐铁矿样品的187os / 186os比值为1.28 ~ 1.60，比幔源2.0 Ga Os（~ 0.9）的预测更具放射性成因，表明地壳Os的全系统同化作用。UG1铬铁矿在地层上最低，放射性成因最少（1.28），而Merensky Reef最高，放射性成因最多（1.41-1.60;Hart and Kinloch, 1989）。幔源岩浆或热液中地壳同化的放射性成因氧可以解释这些高同位素比率。同化模型要求Os以及由此推断的其他pge是在富al的拉斑岩(A)岩浆中携带的，而不是在高mg的超镁质(U)岩浆中携带的。与侵入体接触的黄铁矿黑色页岩可能具有较高的放射性成因氧，在A岩浆中小于1%的页岩同化可以解释上述比值。在侵入热的驱动下，晚期热液流体也可能将邻近岩石中的o和其他pge带到布什维尔德的特定层。os同位素数据进一步证明，层状基性侵入体中的pge可能来自地壳，而不再仅仅被认为是幔源岩浆过程的产物。
The Bushveld Complex in South Africa is the largest layered mafic intrusion in the world and a major producer of platinum-group elements (PGEs). Economic mineralization is found in the Merensky Reef and the UG2 and UG1 chromitites and includes osmium-bearing laurite (Ru, Ir, Os)S 2 . The laurite-bearing samples have 187 Os/ 186 Os ratios of 1.28-1.60, which are more radiogenic than predicted for mantle-derived, 2.0 Ga Os (∼0.9) and indicate a system-wide assimilation of crustal Os. The UG1 chromitite is stratigraphically the lowest and the least radiogenic (1.28), whereas the Merensky Reef is the highest and most radiogenic (1.41-1.60; Hart and Kinloch, 1989). Radiogenic Os from crustal assimilation in mantle-derived magmas or from hydrothermal fluids can account for these high isotopic ratios. Assimilation models require that Os, and by inference other PGEs, are carried in Al-rich tholeiitic (A) magmas rather than high-Mg ultramafic (U) magmas. Pyritic black shales in contact with the intrusion may be high in radiogenic Os, and less than 1% shale assimilation in an A magma can explain the observed ratios. Late-stage hydrothermal fluids driven by the heat of the intrusion could also have carried Os and other PGEs from adjacent rocks into specific layers of the Bushveld. The Os-isotope data add to mounting evidence that PGEs in layered mafic intrusions may be derived from crustal sources and can no longer be considered as solely a product of mantle-derived magmatic processes.