Diffusive fractionation of volatiles and their isotopes during bubble growth in magmas
Diffusive fractionation of volatiles and their isotopes during bubble growth in magmas
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岩浆中气泡生长过程中挥发物及其同位素的扩散分馏
DOI：
10.1007/s00410-017-1384-7
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发表时间：
2017
影响因子：
3.5
通讯作者：
E. Watson
中科院分区：
文献类型：
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作者：
E. Watson
气泡在减压岩浆中通过简单膨胀和向气泡/熔体界面扩散供应挥发物而生长。后一种现象具有重要的地球化学意义，因为扩散可以使溶解组分的元素和同位素（或同位素体）断裂。这就提出了一种可能性，即气泡中挥发性组分的性质可能不反映气泡寿命期间溶解在主熔体中的挥发性组分的性质，即使在没有平衡蒸气/熔体同位素分馏的情况下也是如此。最近的实验已经证实了硅酸盐熔体中挥发性元素Cl扩散的同位素质量效应的存在[Fortin et al.（英安质熔体中化学扩散过程中氯的同位素分馏及其对气泡生长过程中同位素行为的影响（摘要），2016年秋季AGU会议，2016年）]，因此，显然需要理解气泡生长期间扩散分馏的功效。在这项研究中，气泡生长过程中的扩散和质量再分配的数值模型，实现了“被动”挥发物的浓度通常远低于饱和水平和“主动”挥发物，如CO2和H2O，其浓度升高和有限的溶解度是气泡成核和生长的原因。扩散和对流气泡生长的情况进行了探讨。在静态系统中，同位素质量对被分成以恒定速率R生长的气泡的被动挥发物的影响的大小取决于R/DL、Kd和DH/DL（Kd =气泡/熔体分配系数; DH/DL =重同位素和轻同位素的扩散率比）。在对流气泡生长期间，针对生长气泡（宽度xBL）的离散（物理）熔体边界层的存在简化了结果，因为它导致生长期间稳态分馏的快速开始，其大小主要取决于R∙xBL/DL和DH/DL（气泡/熔体分馏在R∙xBL/DL ≈0.1时最大化）。常数R对于大多数真实的系统来说是不现实的，因此通过包括“活性”挥发物的溶解度和EOS来探索其他情况（例如，CO2）的数值模拟。对于合理的减压路径，R随时间呈指数增加，潜在地，分配到不断增长的气泡中的物种的同位素分馏。对于已经测量了同位素质量对扩散影响的挥发性物质（Cl，Li），预测出溶蒸汽中的同位素分馏可以高达Cl的−4‰和Li的−25‰。
Bubbles grow in decompressing magmas by simple expansion and by diffusive supply of volatiles to the bubble/melt interface. The latter phenomenon is of significant geochemical interest because diffusion can fractionate elements and isotopes (or isotopologues) of dissolved components. This raises the possibility that the character of volatile components in bubbles may not reflect that of volatiles dissolved in the host melt over the lifetime of a bubble—even in the absence of equilibrium vapor/melt isotopic fractionation. Recent experiments have confirmed the existence of an isotope mass effect on diffusion of the volatile element Cl in silicate melt [Fortin et al. (Isotopic fractionation of chlorine during chemical diffusion in a dacitic melt and its implications for isotope behavior during bubble growth (abstract), 2016 Fall AGU Meeting, 2016)], so there is a clear need to understand the efficacy of diffusive fractionation during bubble growth. In this study, numerical models of diffusion and mass redistribution during bubble growth were implemented for both “passive” volatiles—those whose concentrations are generally well below saturation levels—and “active” volatiles such as CO2 and H2O, whose elevated concentrations and limited solubilities are the cause of bubble nucleation and growth. Both diffusive and convective bubble-growth scenarios were explored. The magnitude of the isotope mass effect on passive volatiles partitioned into bubbles growing at a constant rate R in a static system depends upon R/DL, Kd and DH/DL (Kd = bubble/melt partition coefficient; DH/DL = diffusivity ratio of the heavy and light isotopes). During convective bubble growth, the presence of a discrete (physical) melt boundary layer against the growing bubble (of width xBL) simplifies outcomes because it leads to the quick onset of steady-state fractionation during growth, the magnitude of which depends mainly upon R∙xBL/DL and DH/DL (bubble/melt fractionation is maximized at R∙xBL/DL ≈0.1). Constant R is unrealistic for most real systems, so other scenarios were explored by including the solubility and EOS of an “active” volatile (e.g., CO2) in the numerical simulations. For plausible decompression paths, R increases exponentially with time—leading, potentially, to larger isotopic fractionation of species partitioned into the growing bubble. For volatile species whose isotope mass effects on diffusion have been measured (Cl, Li), predicted isotope fractionation in the exsolved vapor can be as large as −4‰ for Cl and −25‰ for Li.