北京大学Ping-Ping Liu和中国科学院海洋研究所Vadim Kamenetsky的合作团队，在研究硼和氧稳定同位素对Toba火山(印度尼西亚)系统脱水流体、沉积物衍生熔体和地壳同化作用的限制中取得新成果。2023年12月15日出版的《地质学》杂志发表了这项成果。

本文研究了印度尼西亚Toba火山体系中，后—破火山口玄武岩中分区橄榄石颗粒的氧同位素组成，以及这些玄武岩和玻璃熔体包裹体中硼的含量和同位素，这些包裹体位于硅质凝灰岩的石英和斜辉石中。玄武岩中橄榄石的高镁质(≥87 mol% Fo [镁橄榄石])核的δ¹⁸O值在5.12‰到6.14‰之间，表明Toba之下的地幔源区不同程度的富集¹⁸O。Fo＜87 mol%的橄榄石的δ¹⁸O值变化较大(4.8-7.2‰)，但δ¹⁸O值总体增加，反映了高δ¹⁸O的地壳物质在分数结晶过程中的同化作用。质量平衡计算将玄武岩的地壳同化总量限制在≤13%。

硼数据进一步限制了富¹⁸O玄武岩熔体的形成过程，这些数据表明，地幔中添加了＜0.1 wt%的流体，40%的流体来自蛇纹岩，其他来自蚀变洋壳和沉积物。这部分流体只能使岩浆的δ¹⁸O增加约0.02‰。为了产生δ¹⁸O值与高Fo橄榄石核的δ¹⁸O值平衡的玄武质熔体，还需要地幔楔中约6-9%的沉积物衍生熔体混合。另一方面，同生的硅质凝灰岩除了地壳同化作用外，似乎还记录了更高比例的由沉积物衍生的流体向地幔源区的流体添加的过程。因此，研究勘测证明了联合应用硼和氧同位素来区分源自蛇纹岩和俯冲板块沉积物的熔体和流体的可行性，这种实践可以应用于全球范围内的弧岩浆。

据介绍，岛弧岩浆产生于地幔楔，可能还有的来自于蛇纹岩和俯冲板块沉积物的流体和熔体。识别各种岩浆来源及这些岩浆的相关贡献非常具有挑战性；特别是在大陆弧上，在那里，岩浆同化作用可能会叠加初始的地球化学特征。

附：英文原文

Title: Constraints of boron and oxygen stable isotopes on dehydration fluids, sediment-derived melts, and crustal assimilation of the Toba volcanic system (Indonesia)

Author: Ping-Ping Liu, Dian-Bing Wang, Mei-Fu Zhou, Xian-Hua Li, Qiu-Li Li, Glenn A. Gaetani, Brian Monteleone, Vadim Kamenetsky

Issue&Volume: 2023-12-15

Abstract: Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ¹⁸O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in ¹⁸O. Olivine with <87 mol% Fo has highly variable (4.87.2‰), but overall increased, δ¹⁸O values, interpreted to reflect assimilation of high δ¹⁸O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the ¹⁸O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ¹⁸O of the magma by only ～0.02‰. Approximately 69% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ¹⁸O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab—an application that can be applied to arc magmas worldwide.

DOI: 10.1130/G51690.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G51690.1/630706/Constraints-of-boron-and-oxygen-stable-isotopes-onredirectedFrom=fulltext