Lead isotopic disequilibrium between sulfide and plagioclase in the bushveld complex and the chemical evolution of large layered intrusions
Lead isotopic disequilibrium between sulfide and plagioclase in the bushveld complex and the chemical evolution of large layered intrusions
复制标题
DOI：
10.1016/s0016-7037(02)01294-2
复制
发表时间：
2003-05
影响因子：
5
通讯作者：
E. Mathez;T. Waight
中科院分区：
文献类型：
--
作者：
E. Mathez;T. Waight
采用激光烧蚀多接收极等离子体质谱法（LAMC-ICPMS）测定了Bushveld杂岩中斜长石和硫化物的铅同位素组成。这些样品位于综合体东北角的上临界区，是从岩芯和地下矿井暴露物中采集的。所有的岩石都是新鲜的，没有表现出任何证据的蚀变，风化，或中断的铅同位素系统的入侵后的初始冷却。此外，个别斜长石和硫化物晶体不包含足够的U，以保证校正放射性生长。由于这些原因，测得的铅同位素比值接近初始值。斜长石的~（207）Pb/~（206）Pb比值为0.98 ~ 1.02，~（208）Pb/~（206）Pb比值为2.26 ~ 2.35。低的207 Pb/206 Pb和208 Pb/206 Pb比值表征了晶界和部分退火的微裂纹，其中一些包含硫化物和其他相的微小碎片，这解释了大部分（如果不是全部的话）单个样品所表现出的不均匀性。然而，来自不同岩性层的斜长石存在真实的成分差异。例如，斜长石207 Pb/206 Pb值从梅伦斯基辉石岩下的苏长岩中的1.004到矿化辉石岩中的1.009和上覆苏长岩中的0.997不等。在大多数硫化物和斜长石共存的样品中，硫化物的207 Pb/206 Pb比值比斜长石中相应的比值低，208 Pb/206 Pb比值比斜长石中相应的比值高。例如，在矿化Merensky礁样品中，硫化物207 Pb/206 Pb和208 Pb/206 Pb的平均比值分别为0.993和2.313，而斜长石中的比值分别为1.000和2.292。在一个样品中，硫化物非常不均匀，207 Pb/206 Pb和208 Pb/206 Pb的比值低至0.84和2.12。在这个特定的样品中，组合物必须代表一个孤立的出现，另外一个年轻的铅成分。硫化物和斜长石组合物的阵列需要多个来源的铅在结晶时或不久之后。斜长石和硫化物之间的不平衡意味着一些铅来自同位素不同的围岩，并在温度下引入的硫化物，但不是斜长石的组合物可以修改。因此，Bushveld硫化物，在一定程度上斜长石，不可靠地记录的初始铅同位素组成的母岩浆（S）。
The Pb isotopic compositions of coexisting plagioclase and sulfide from the Bushveld Complex were determined by laser ablation multi-collector ICPMS (LA MC-ICPMS). The samples are of the upper Critical Zone in the northeast corner of the Complex and were collected from drill core and underground mine exposures. All the rocks are fresh and exhibit no evidence for alteration, weathering, or disruption of the Pb isotope systematics subsequent to the initial cooling of the intrusion. Furthermore, individual plagioclase and sulfide crystals do not contain enough U to warrant correction for radiogenic in-growth. For these reasons, the measured Pb isotope ratios approximate the initial ones. For plagioclase,207Pb/206Pb ranges from 0.98 to 1.02 and208Pb/206Pb from 2.26 to 2.35. Low207Pb/206Pb and208Pb/206Pb ratios characterize grain boundaries and partially annealed microcracks, some of which contain minute fragments of sulfide and other phases, and this accounts for most, if not all, the heterogeneity exhibited by individual samples. Real compositional differences exist, however, in plagioclase from different lithologic layers. For example, plagioclase207Pb/206Pb values vary from 1.004 in norite beneath the Merensky pyroxenite to 1.009 in the mineralized pyroxenite, and 0.997 in overlying norite. In most samples in which sulfide and plagioclase coexist, the sulfide207Pb/206Pb ratio is lower and208Pb/206Pb ratio higher than the corresponding ones in plagioclase. For example, in a mineralized Merensky reef sample, average sulfide207Pb/206Pb and208Pb/206Pb ratios are 0.993 and 2.313, respectively, while those in plagioclase are 1.000 and 2.292. In one sample, the sulfide is extremely heterogeneous, with207Pb/206Pb and208Pb/206Pb ratios as low as 0.84 and 2.12. In this particular sample, the compositions must represent an isolated occurrence of addition of a young Pb component. The array of sulfide and plagioclase compositions requires multiple sources of Pb at the time of crystallization or soon thereafter. The disequilibrium between plagioclase and sulfide implies that some of the Pb originated from the isotopically distinct country rocks and was introduced at temperatures at which the composition of sulfide but not plagioclase could be modified. Thus, Bushveld sulfide, and to some extent plagioclase, do not reliably record the initial Pb isotopic composition(s) of the parent magma(s).