Loss of slab-derived volatile and fluid-mobile elements (FME) through the forearc may represent a large, unaccounted for flux out of convergent margins. To address volatile recycling through the forearc and potential changes in fluid source from the progressively dehydrating subducting slab, we collected water samples from 44 different cold and thermal (>40 °C) springs on a transect across the outer forearc (∼20–40 km to the subducting slab), forearc (∼40–80 km to the slab), and arc (∼80–120 km to the slab) in Costa Rica. We focused on the heavy halogens (Cl, Br, I), as well as B and Li, given their high fluid mobility and thus limited potential for modification by subsequent fluid-rock interaction. Chlorine, boron, and lithium stable isotope ratios show dramatic ranges from −1.7 to +1.0 ‰ (n = 43), −12.0 to +30.9 ‰ (n = 35), and −2.4 to + 27.5 ‰ (n = 38), respectively, with distinct changes across the transect consistent with a sedimentary pore fluid component in the outer forearc, fluids sourced from dehydration of the slab (sediment and altered oceanic crust) in the forearc, and minimal slab input near the arc. Variations in elemental ratios (Br/Cl, I/Cl, B/Cl, Li/B) across the transect are also largely consistent with evolving FME sources during subduction. Boron isotope ratios in some higher-temperature fluids are modified due to phase separation, whereas, lithium and chlorine isotope ratios are minimally modified. Springs located in the outer forearc along a major fault (Falla Morote) deviate from the consistent across-arc trend and record almost the full range of observed isotopic values. This extreme range in values along the fault likely represents a combination of adsorption/desorption effects, weathering, and mixing with crustal fluids. Mass balance calculations show that up to ∼ 70 % of Cl, ∼50 % of Br, ∼30 % of B, ∼15 % of I, and minimal Li may be recycled through the outer forearc and forearc springs of the Costa Rican margin, therefore fluid emission via springs in global forearc regions are a potentially significant output for FME at convergent margins.

中文翻译：

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流体流动元素通过前弧的循环：哥斯达黎加泉水的 Cl、B 和 Li 同位素组成的见解


通过前弧的板片衍生的挥发性和流体流动元素（FME）的损失可能代表着大量的、无法解释的从会聚边缘流出的通量。为了解决通过前弧的挥发性循环以及逐渐脱水的俯冲板片流体来源的潜在变化，我们从跨越外弧前（约 20-40 公里）的横断面上的 44 个不同的冷泉和热泉（> 40 °C）收集了水样。哥斯达黎加的俯冲板片）、弧前（距俯冲板片约 40–80 公里）和弧（距俯冲板片约 80–120 公里）。我们重点关注重卤素（Cl、Br、I）以及 B 和 Li，因为它们具有较高的流体流动性，因此通过随后的流体-岩石相互作用进行改性的潜力有限。氯、硼和锂稳定同位素比率显示出从 -1.7 至 +1.0 ‰ (n = 43)、-12.0 至 +30.9 ‰ (n = 35) 和 -2.4 至 + 27.5 ‰ (n = 38) 的巨大范围，分别是，横断面的明显变化与外弧前的沉积孔隙流体成分一致，流体源自弧前板片的脱水（沉积物和蚀变的洋壳），以及弧附近板片的输入量最小。横断面上元素比率（Br/Cl、I/Cl、B/Cl、Li/B）的变化也与俯冲过程中不断演变的 FME 源基本一致。一些高温流体中的硼同位素比率由于相分离而发生改变，而锂和氯同位素比率则发生最小程度的改变。位于外弧沿主要断层（Falla Morote）的泉水偏离了一致的跨弧趋势，并记录了几乎全部观测到的同位素值。沿着断层的这种极端值范围可能代表了吸附/解吸效应、风化以及与地壳流体混合的组合。 质量平衡计算表明，高达~70%的Cl、~50%的Br、~30%的B、~15%的I和最少的Li可以通过哥斯达黎加边缘的外前弧和前弧泉循环，因此，全球弧前区域通过泉水排放的流体是 FME 在收敛边缘的潜在重要输出。