Boron isotope fractionation in subducted serpentinites: A modelling attempt
Boron isotope fractionation in subducted serpentinites: A modelling attempt
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DOI：
10.1016/j.lithos.2020.105768
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发表时间：
2020-12
期刊：
影响因子：
3.5
通讯作者：
E. Cannaò
中科院分区：
文献类型：
--
作者：
E. Cannaò
硼(B)及其同位素是俯冲过程的强有力的地球化学示踪剂，使人们能够了解流体在收敛边缘的大规模质量传递中所起的作用。在这种背景下，大洋和弧前蛇纹岩是最重要的B和重B同位素储集层，有助于弧状熔岩的形成。文献资料支持在温石棉/荔枝石向反榴石(ATG)相变和部分至完全反榴石脱水形成次生橄榄岩的过程中释放出大量的B。然而，关于与这些反应相关的B同位素分馏的信息有限。在这封信中，我试图通过从头算B同位素分馏计算来模拟影响俯冲蛇纹岩的B同位素分馏，该计算提供了矿物和流体中三配位和四面体配位的B。所获得的结果主要证实了蛇纹岩是目前所分析的天然高压蛇纹岩所显示的δ11B正向深度的主要载体。然而，缺乏有关pH条件的信息，阻碍了对蛇纹岩中B在俯冲带进动演化过程中的分馏作用的正确估计。这可能代表了这里提出的方法的局限性，因为弱碱性(接近中性)的pH条件可能会导致低(潜在的负)δ11Batg签名。最后，这封信试图解决新的研究趋势和值得研究的主要课题，以更好地揭示蛇纹岩及其脱水产物中的B循环。
Boron (B) and its isotopes are powerful geochemical tracers of subduction processes, enabling to understand the role played by fluids in large-scale mass-transfer at convergent margins. In such settings, oceanic and forearc serpentinites are the most important reservoirs for B and heavy B isotopes contributing to the genesis of arc lavas. Literature data support the release of significant amount of B during the chrysotile/lizardite to antigorite (atg) phase transition and during the partial to complete antigorite dehydration to form secondary peridotites. However, the information concerning the B-isotope fractionation associated to these reactions is limited. In this Letter, I attempt to model the B-isotope fractionation affecting subducted serpentinites through theab initioB-isotope fractionation calculations provided for trigonal- and tetrahedral-coordinated B in both minerals and fluids. The obtained results mainly confirm serpentinites as the main vector of positive δ11B to depth as shown by natural high-pressure serpentinites analysed so far. However, a lack of information on pH conditions hampers the proper estimate of the fractionation of B in serpentinites during their prograde evolution in subduction zones. This may represent a limit in the approach proposed here, since poor-alkaline (close to neutral) pH conditions can result in low (potentially negative) δ11Batgsignatures. Finally, the Letter try to address new research trends and major topics worth investigating to better unravel the B cycle in serpentinites and their dehydration products.