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 (4.8–7.2‰), 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 6–9% 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.

中文翻译：

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硼和氧稳定同位素对多巴火山系统脱水流体、沉积物衍生熔体和地壳同化的限制（印度尼西亚）

弧岩浆由地幔楔产生，可能添加来自俯冲板片中的蛇纹岩和沉积物的流体和熔体。确定各种来源及其对此类岩浆的相关贡献具有挑战性；特别是在大陆弧处，地壳同化可能会掩盖最初的地球化学特征。这项研究介绍了来自破火山口后玄武岩的分区橄榄石颗粒的氧同位素组成，以及这些玄武岩和印度尼西亚多巴火山系统硅质凝灰岩的石英和单斜辉石中的玻璃状熔体包裹体的硼含量和同位素。玄武岩中高镁质（≥87 mol% Fo [镁橄榄石]）橄榄石核的 δ18O 值范围为 5.12‰ 至 6.14‰，表明多巴下面的地幔源区 18O 富集程度不同。Fo 含量 <87 mol% 的橄榄石具有高度可变性 (4.8–7.2‰)，但总体上增加了 δ18O 值，解释为反映了分级结晶过程中高 δ18O 地壳材料的同化。质量平衡计算将玄武岩的地壳同化总体积限制为≤13%。导致富含 18O 的玄武岩熔体的过程进一步受到硼数据的限制，硼数据表明向地幔添加了 <0.1 wt% 的流体，其中 >40% 的流体来自蛇纹岩，其他流体来自蚀变的洋壳和沉积物。这个量的流体只能使岩浆的δ18O增加约0.02‰。进一步需要在地幔楔中大约 6-9% 沉积物衍生的熔体杂交才能产生 δ18O 值与高 Fo 橄榄石核心平衡的玄武岩熔体。另一方面，除了地壳同化作用之外，同生硅质凝灰岩似乎还记录了较高比例的流体添加，其中以沉积物衍生的流体为主。因此，我们的勘察研究表明，可以应用 B 和 O 组合同位素来区分源自俯冲板片中蛇纹岩和沉积物的熔体和流体，这一应用可应用于全世界的弧岩浆。