Re-Os Systematics in Proterozoic Anorthosite Complexes

HannahEAR-0106874元古代地块型斜长岩杂岩的岩浆来源以及在其中一些杂岩中发现的块状硫化物的起源仍然是悬而未决的问题。我们认为，来自两个研究很好的斜长岩套的氧化物、硫化物和硅酸盐的Re-Os(Re-Os)浓度和Os同位素比率将揭示这些问题。大多数基于地球化学数据的模型表明，斜长岩杂岩来自于池塘中的地幔熔体和地壳底部的分馏，然后在上升过程中受到地壳物质的不同程度的污染。然而，从相平衡的考虑表明，母岩浆是由辉长岩来源的部分熔融产生的，推测是在下地壳。OS同位素比率是地壳融化或同化的特别敏感指标，应有助于区分相互竞争的模型。元古界斜长岩杂岩中硫化物的187Os/188Os初始比值较高，表明地壳Os占优势。然而，目前尚不清楚地壳Os是通过选择性挥发和并入地壳硫化物、整体同化地壳，还是通过部分熔融地壳辉长岩源而获得的。我们将比较来自两个系统的Os同位素数据-怀俄明州明显不含硫化物的拉勒米斜长岩杂岩和瑞典西南部含硫化物的哈克夫约登杂岩-以估计硫化物及其所含金属最可能的来源。研究结果为区分贫硫化物和富硫化物体系提供了一种勘探工具。

HannahEAR-0106874The magma source for Proterozoic massif-type anorthosite complexes and the origin of massive sulfide bodies found in some of those complexes remain unresolved problems. We propose that rhenium-osmium (Re-Os) concentrations and Os isotope ratios for oxides, sulfides, and silicates from two well-studied anorthosite suites will shed light on these issues. Most models based on geochemical data suggest that anorthosite complexes are derived from mantle melts that pond and fractionate at the base of the crust, and are then variably contaminated by crustal materials during ascent. Considerations from phase equilibria, however, suggest that parental magmas are derived by partial melting of a gabbroic source, presumably in the lower crust. Os isotope ratios, which are particularly sensitive indicators of crustal melting or assimilation, should help discriminate between competing models. High the initial 187Os/188Os ratios in sulfides from Proterozoic anorthosite complexes suggest dominance of crustal Os. It is unclear, however, whether the crustal Os was acquired by selective volatilization and incorporation of crustal sulfide, wholesale assimilation of bulk crust, or derivation of parental melts by partial melting of a crustal gabbroic source. We will compare Os isotopic data from two systems - the apparently sulfide-free Laramie Anorthosite Complex in Wyoming, and the sulfide-bearing Hakefjorden Complex in SW Sweden - to estimate the most likely source for the sulfides and their contained metals. The results could offer an exploration tool for distinguishing sulfide-poor and sulfide-rich systems.