加拿大多伦多大学Corliss K. Sio团队在最新研究中，报道了熔体包裹体形成过程中的化学和同位素分馏。相关论文发表在2025年2月12日出版的《地质学》杂志上。

据悉，熔融夹杂物是研究岩浆起源和演化的主题。然而，它们在多大程度上代表平衡熔体成分，关键取决于熔体中晶体生长速率与扩散速率的比值。如果晶体的生长速度受到营养物质的供应和不需要的成分的耗散的限制，则被捕获的熔体将在相容元素中耗尽而在不相容元素中富集。尽管人们普遍认识到熔体夹带可能捕获不平衡成分，但它们的识别仍然是一个挑战。

研究团队展示了稳定同位素如何为这个问题提供解决方案。夏威夷kilauea火山玄武质软岩中熔体包裹体中，Mg同位素分馏值高达0.49‰（26Mg/24Mg）。这些观察结果表明，化学扩散等动力学过程在起作用，因为平衡矿物-熔体同位素在高温下的分馏要小一个数量级。

我们认为熔体包裹体的重Mg同位素组成是由快速扩散产生的，因此生长的橄榄石优先掺入24Mg，而留下了较慢的扩散剂（26Mg）。当边界层效应显著时，预测不相容元素（如Ca，Ti和K）也会显示出类似的大分数。这些发现表明，稳定同位素可以作为鉴定熔体中不平衡成分的重要工具。

附：英文原文

Title: Chemical and isotopic fractionation during melt inclusion formation

Author: Bruna da Silva Ricardo, Martin Oeser, Kendra J. Lynn, Neil R. Bennett, Xu Chu, Grant Henderson, Yanan Liu, Stefan Weyer, Corliss K. Sio

Issue&Volume: 2025-02-12

Abstract: Melt inclusions are used to study the origin and evolution of magmas. The extent to which they represent equilibrium melt compositions, however, critically hinges on the ratio of crystal growth rate to diffusion rate in melts. If the rate of crystal growth is limited by the supply of nutrients and the dissipation of unwanted components, the trapped melt will be depleted in compatible elements and enriched in incompatible elements. Despite widespread recognition of the potential for melt inclusions to trap disequilibrium compositions, their identification remains a challenge. Here, we demonstrate how stable isotopes provide a solution to this problem. Melt inclusions in basaltic tephra from Kīlauea volcano, Hawai‘i, display Mg isotope fractionation up to 0.49‰ (26Mg/24Mg) relative to bulk rock. These observations indicate that kinetic processes, such as chemical diffusion, are at play, as equilibrium mineral-melt isotope fractionation at high temperatures is an order of magnitude smaller. We propose that the heavy Mg isotopic compositions of the melt inclusions were generated by the faster diffusion and thus preferential incorporation of 24Mg by the growing olivine, leaving the slower diffusant (26Mg) behind. The incompatible elements (e.g., Ca, Ti, and K) are predicted to display similarly large fractionations when boundary layer effects are significant. These findings show that stable isotopes can be a useful tool in identifying disequilibrium compositions in melt inclusions.

DOI: 10.1130/G52248.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G52248.1/652039/Chemical-and-isotopic-fractionation-during-meltredirectedFrom=fulltext