Subduction of hydrated oceanic lithosphere represents a profound coupling of the Earth’s surface and its interior, with significant implications for both the internal dynamics and chemistry, as well as the surface habitability of our planet. Serpentinised lithospheric mantle is increasingly recognised as an important component in the geochemical cycling of many volatile species. However, our understanding of the extent of hydration and heterogeneity in volatile geochemistry at depth is limited by a lack of samples from in situ lithospheric mantle. In this study, we apply coupled analyses of halogen abundances and noble gas isotopes to a suite of mineral separates from ultramafic lithologies from the Leka Ophiolite Complex, Norway, in order to trace the sources of fluids and critically assess the relative timing of high- and low-temperature fluid infiltration episodes. Results indicate extreme halogen abundance heterogeneity in the mantle and lower crustal transition zone of the Leka ophiolite, covering much of the known global range of serpentinite compositions. The data extend from near seawater- like compositions (as wt/wt: Br/Cl = 3.47 x 10^-3, I/Cl = 3.04 x 10^-6) to very high values of Br/Cl (≤ 9.9 x 10^-3) and I/Cl (≤ 6.7 x 10^-3), and are characterised by a marked decoupling of halogens, both from one another and from noble gases. Relative enrichments in I, ⁸⁴Kr, ¹³²Xe and radiogenic ⁴⁰Ar in serpentinites and fresh minerals are attributed to supra-subduction zone fluid infiltration at high temperature, followed by serpentinisation involving variable mixtures of seawater, and fluid-derived from terrigenous sediment and organic-rich sedimentary pore fluid. In addition, alteration by brine or assimilation of brine during hydrothermal circulation has resulted in enrichment of Br in some samples. These enrichments are absent, or much less pronounced in harzburgite samples suggesting these rocks preserve an earlier episode of seawater dominated serpentinisation within an SSZ setting. Together, these observations allow deconvolution of a series of metasomatic events and indicate that hydration was asynchronous, resulting in low and high temperature events mutually overprinting one another. Although this complicates our understanding of Leka and other ophiolites as straightforward natural laboratories for oceanic serpentinisation, it demonstrates that complex multi-phase volatile histories can be preserved in ophiolitic materials over timescales of at least ∼0.5 Gyr. Ophiolites therefore have the potential to be employed as archives of a diverse range of fluid processes operating over multiple temporal and spatial scales during subduction.

水合海洋岩石圈的俯冲作用代表着地球表面及其内部的深刻耦合，对地球的内部动力学和化学性质以及地球的表面可居住性都具有重要意义。蛇形岩石圈地幔被越来越多地认为是许多挥发性物种地球化学循环中的重要组成部分。但是，由于缺乏原位样品，我们对深层挥发性地球化学中水化程度和非均质性的了解受到限制岩石圈地幔。在这项研究中，我们将卤素丰度和稀有气体同位素的耦合分析应用于挪威Leka Ophiolite Complex的超镁铁岩性岩层中的一组矿物分离物中，以便追踪流体的来源并严格评估高，低同位素的相对时间。低温流体浸润发作。结果表明，在Leka蛇绿岩的地幔和下地壳过渡带中，卤素的丰度极度不均匀，涵盖了许多已知的全球蛇纹石成分范围。数据从类似海水的成分（以wt / wt：Br / Cl = 3.47 x 10 ^-3，I / Cl = 3.04 x 10 ^-6）扩展到非常高的Br / Cl值（≤9.9 x 10 ^-3））和I / Cl（≤6.7 x 10 ^-3），其特点是卤素彼此之间以及与稀有气体之间都具有显着的卤素去偶联作用。I，⁸⁴ Kr，¹³² Xe和放射源^40的相对富集蛇纹岩和新鲜矿物中的Ar归因于高温下的超俯冲带流体渗透，然后发生蛇纹石化，涉及海水的各种混合物，以及源自陆源沉积物和富含有机物的沉积孔隙流体的流体。另外，在热液循环过程中，盐水引起的改变或盐水的同化作用导致某些样品中Br的富集。这些富集是不存在的，或者在哈兹伯格岩样品中不那么明显，这表明这些岩石在SSZ环境中保留了海水主导的蛇纹岩化的早期事件。总之，这些观察结果使一系列变元事件发生反卷积，并表明水合作用是异步的，从而导致低温事件和高温事件相互叠加。尽管这使我们对Leka和其他蛇绿岩作为海洋蛇纹石化的直接自然实验室的理解更加复杂，但它表明，在至少约0.5 Gyr的时间尺度内，复杂的多相挥发性历史可以保存在脂滑石材料中。因此，蛇绿岩有可能被用作俯冲过程中在多个时空尺度上运行的各种流体过程的档案。