Sulphate incorporation in monazite lattice and dating the cycle of sulphur in metamorphic belts

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• 作者：Antonin T. Laurent…
• 关键词：Monazite ; Sulphate ; U–Pb ; Geochronology ; Metamorphism ; S cycle
• 刊名：Contributions to Mineralogy and Petrology
• 出版年：2016
• 出版时间：November 2016
• 年：2016
• 卷：171
• 期：11
• 全文大小：3,900 KB
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Geology
  Mineral Resources
  Mineralogy
  
• 出版者：Springer Berlin / Heidelberg
• ISSN：1432-0967
• 卷排序：171

文摘

Microgeochemical data and transmission electron microscope (TEM) imaging of S-rich monazite crystals demonstrate that S has been incorporated in the lattice of monazite as a clino-anhydrite component via the following exchange Ca2+ + S6+ = REE3+ + P5+, and that it is now partly exsolved in nanoclusters (5–10 nm) of CaSO₄. The sample, an osumilite-bearing ultra-high-temperature granulite from Rogaland, Norway, is characterized by complexly patchy zoned monazite crystals. Three chemical domains are distinguished as (1) a sulphate-rich core (0.45–0.72 wt% SO₂, Th incorporated as cheralite component), (2) secondary sulphate-bearing domains (SO₂ >0.05 wt%, partly clouded with solid inclusions), and (3) late S-free, Y-rich domains (0.8–2.5 wt% Y₂O₃, Th accommodated as the huttonite component). These three domains yield distinct isotopic U–Pb ages of 1034 ± 6, 1005 ± 7, and 935 ± 7 Ma, respectively. Uranium–Th–Pb EPMA dating independently confirms these ages. This study illustrates that it is possible to discriminate different generations of monazite based on their S contents. From the petrological context, we propose that sulphate-rich monazite reflects high-temperature Fe–sulphide breakdown under oxidizing conditions, coeval with biotite dehydration melting. Monazite may therefore reveal the presence of S in anatectic melts from high-grade terrains at a specific point in time and date S mobilization from a reduced to an oxidized state. This property can be used to investigate the mineralization potential of a given geological event within a larger orogenic framework.