Noble gas composition and ⁴⁰Ar/³⁹Ar age in eclogites from the main hole of the Chinese Continental Scientific Drilling project

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• 作者：Jens Hopp ; Winfried H. Schwarz ; Mario Trieloff…
• 关键词：CCSD project ; Eclogites ; Ar–Ar ages ; Noble gases
• 刊名：Contributions to Mineralogy and Petrology
• 出版年：2016
• 出版时间：October 2016
• 年：2016
• 卷：171
• 期：10
• 全文大小：2,227 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

We present the first comprehensive noble gas study on eclogites. The four eclogite samples were recovered during the Chinese Continental Scientific Drilling and are from two distinct profile depth sections differing in their degree of interaction with meteoric water, based on their δ18O-values (surface related and of mantle-type). Hence, noble gas analyses offer the potential to further discriminate between shallow (meteoric) and deep (mantle) fluid sources. Noble gas compositions reveal typical crustal fluid compositions, characterized by a variable mixture of atmospheric gases with significant contributions of nucleogenic neon, radiogenic 4He*, radiogenic 40Ar*, fissiogenic 131–136Xe, and presumably bariogenic 131Xe, but no significant addition of mantle gases. This signature can be also considered to represent one endmember component of eclogitic diamonds. Concentrations of non-radiogenic noble gases are rather low, with depletion of light relative to the heavier noble gases. Eclogites from lower depth which experienced a higher degree of interaction with meteoric water also showed higher contributions of atmospheric gas compared with eclogites recovered from greater depth. This is interpreted to result from interaction with high-salinity fluids during ultrahigh pressure (UHP). It demonstrates that the atmospheric noble gas abundance is a proxy for interaction with surface related fluids. 40Ar/39Ar (inverse) isochron ages of two phengite separates (241.2 ± 0.4 Ma and 275.0 ± 1.8 Ma, 1σ-errors) predate the main phase of UHP metamorphism (ca. 220 Ma). Biotite yields an integrated age of about 1100 Ma. These age values are interpreted to reflect the likely addition of excess 40Ar without any chronological meaning.