Sr and Pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the Bushveld Complex, South Africa: microdrilling and progressive leaching evidence for sub-liquidus contamination within a crystal mush
Sr and Pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the Bushveld Complex, South Africa: microdrilling and progressive leaching evidence for sub-liquidus contamination within a crystal mush
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南非 Bushveld 杂岩中共存的斜长石和斜方辉石之间的 Sr 和 Pb 同位素不平衡：微钻和渐进浸出证据表明晶体糊状物中存在亚液相线污染
DOI：
10.1007/s00410-011-0691-7
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发表时间：
2012
影响因子：
3.5
通讯作者：
A. Boudreau
中科院分区：
文献类型：
--
作者：
N. I. Chutas;E. Bates;S. Prevec;Drew S. Coleman;A. Boudreau
从临界带和低带的斜长石和斜方辉石颗粒中进行的斜长石Sr同位素的渐进浸出和Sr和Pb同位素的微钻表明，这些矿物不处于同位素平衡。浸出表明临界区斜长石要么失去了Rb或有更多的放射性斯里边缘相对于核心，而斜长石从上层区样品是同位素均匀的斯里。与共存的斜方辉石相比，下部和临界带斜长石的微钻分析始终具有较高的初始87 Sr/86 Sr（Sri）和较低的放射成因模拟238 U/204 Pb成分（μ2）。斜长石和斜方辉石的Sri范围分别为0.70506-0.70662（34）和0.70290-0.70654（36）。矿物质对之间的Sri平均差异为0.00095。斜长石和斜方辉石的μ2值分别为9.42-10.30（平均9.7）和9.83-15.75（平均10.1）。斜长石和斜方辉石的~（208）Pb/~（206）Pb比值范围分别为34.757-36.439（33）和36.669-41.845（85）。一个正辉石岩没有证据表明有一个以上的晶体粒度分布群体，但其Sri = 0.70654（36），较大颗粒的计算μ2为10.32，而较小颗粒的Sri = 0.70290（32），计算μ2为9.97。本研究的同位素结果表明，全岩同位素数据可能无法提供适当的详细程度，以解决布什维尔德复杂的一些过程。然而，系统性变化有可能阐明缓慢冷却的晶体-液体-蒸气糊状系统内发生的其他过程（晶体老化、压实驱动的重结晶和矿物出溶）的污染时间。
Progressive leaching of plagioclase for Sr isotopes and microdrilling for Sr and Pb isotopes from grains of plagioclase and orthopyroxene from the Critical Zone and the Lower Zone indicates that these minerals are not in isotopic equilibrium. Leaching suggests Critical Zone plagioclase either lost Rb or had a more radiogenic Sri rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sri. Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial 87Sr/86Sr (Sri) and a less radiogenic modeled 238U/204Pb composition (μ2) than coexisting orthopyroxene. The range of calculated Sri for plagioclase and orthopyroxene is 0.70506–0.70662(34) and 0.70290–0.70654(36), respectively. The average difference in Sri between mineral pairs was 0.00095. The range of calculated μ2 for plagioclase and orthopyroxene is 9.42–10.30 (average 9.7) and 9.83–15.75 (average 10.1), respectively. The range of measured 208Pb/206Pb for plagioclase and orthopyroxene is 34.757–36.439(33) and 36.669–41.845(85), respectively. One orthopyroxenite without evidence for more than one population of crystal size distribution, nonetheless had Sri = 0.70654 (36) with calculated μ2 of 10.32 for larger grains as compared with Sri = 0.70290 (32) and calculated μ2 of 9.97 for smaller grain-size fractions. Isotopic results from this study demonstrate that whole-rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the Bushveld Complex. However, systematic changes have the potential to elucidate the timing of contamination with regard to other processes (crystal aging, compaction-driven recrystallization, and mineral exsolution) occurring within a slowly cooled crystal–liquid–vapor mush system.