Deciphering a mantle degassing transect related with India-Asia continental convergence from the perspective of volatile origin and outgassing
Deciphering a mantle degassing transect related with India-Asia continental convergence from the perspective of volatile origin and outgassing
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DOI：
10.1016/j.gca.2021.07.010
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发表时间：
2021-06
影响因子：
5
通讯作者：
Maoliang Zhang;Sheng Xu;Xiaocheng Zhou;A. Caracausi;Y. Sano;Zhengfu Guo;G. Zheng;Yunchao Lang
中科院分区：
文献类型：
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作者：
Maoliang Zhang;Sheng Xu;Xiaocheng Zhou;A. Caracausi;Y. Sano;Zhengfu Guo;G. Zheng;Yunchao Lang
通过热液气体地球化学（如3He/4He、δ13C和CO2/3He）的系统变化，在板块辐合背景下的不同构造单元上的地幔脱气样带在海洋辐合边缘得到了很好的记录。然而，对于整个大陆辐合边缘的挥发性地球化学的空间变化知之甚少。热液气体δ13C-CO2值（−11.8‰~−3.1‰）和CO2/3He值（1.7 × 108 ~ 7.1 × 1011）变化较大，与多组分混合、热液脱气和方解石沉淀等次生物理化学作用的影响一致。根据热液气样品的3he /4He数据集（0.01 ~ 5.87 RA）及其与火山中心的距离，可以识别出3个He脱气级别。在由幔源岩浆供给的活火山和第四纪火山附近发现了岩浆水平的脱气（35-74%地幔He）。随着离火山中心距离的增加，释出的岩浆挥发物逐渐被地壳成分（如放射性He）稀释，定义了过渡性He脱气水平（13-33%地幔He）。橄榄石斑晶的3he /4He值（8.16 ~ 8.48 RA）表明岩浆和过渡性He脱气水平为MORB（中洋脊玄武岩）型地幔源，并定位于第四纪火山场。背景层He脱气（地幔He <12%）在整个研究区占主导地位，可归因于(i)次大陆岩石圈地幔脱气，和/或（ii）地壳污染程度高于火山中心附近过渡层He脱气。结合本研究报道的橄榄石3he /4He数据和文献中寄主玄武岩的全岩87sr /86Sr数据，认为幔源岩浆的来源成分包括morb型对流地幔、俯冲的印度板块物质和较少脱气的地幔物质，这可能解释了He和Sr同位素系统的解耦合。这些发现描绘了印度-亚洲大陆辐合控制的地幔挥发物的起源和排放，有助于更好地理解这些构造背景下的深层挥发物排放。
Mantle degassing transect across different tectonic units within a plate convergent setting has been well documented for oceanic convergent margins by systematic changes in geochemistry (e.g.,3He/4He, δ13C, and CO2/3He) of hydrothermal gases. However, little is known about spatial variations in volatile geochemistry across a continental convergent margin. In this study, we identify a mantle degassing transect in the southeastern Tibetan Plateau using He-CO2systematics of hydrothermal gases, which extends from India-Asia continental convergent margin to intra-continent extensional region. δ13C-CO2(−11.8‰ to −3.1‰) and CO2/3He (1.7 × 108to 7.1 × 1011) values of hydrothermal gases show large variations that are consistent with modification by secondary physico-chemical processes, such as multi-component mixing, hydrothermal degassing, and calcite precipitation. Three levels of He degassing can be recognized based on3He/4He dataset (0.01–5.87 RA) of the hydrothermal gas samples and their distances to volcanic centers. A magmatic level He degassing (35–74% mantle He) is found near active and/or Quaternary volcanoes fed by mantle-derived magmas. With increasing distance to volcanic centers, the outgassed magmatic volatiles are gradually diluted by crustal components (e.g., radiogenic4He), defining a transitional level He degassing (13–33% mantle He). The3He/4He values (8.16–8.48 RA) of olivine phenocrysts indicate a MORB (mid-ocean ridge basalts)-type mantle source for the magmatic and transitional levels of He degassing that are localized in Quaternary volcanic fields. In contrast, a background level He degassing (<12% mantle He) dominates the entire study area, and can be attributed to (i) degassing of sub-continental lithospheric mantle, and/or (ii) higher degrees of crustal contamination than those of the transitional level He degassing near volcanic centers. Combined with olivine3He/4He data reported in this study and whole-rock87Sr/86Sr data of host basalts from literature, source components of the mantle-derived magmas are suggested to include the MORB-type convective mantle, subducted Indian slab materials, and less degassed mantle materials, which can account for the possible decoupling between He and Sr isotope systematics. These findings delineate the origin and outgassing of mantle volatiles controlled by the India-Asia continental convergence, and would contribute to a better understanding of the deeply-sourced volatile emissions in these tectonic settings.