Abstract Understanding the volatile cycles at convergent margins is fundamental to unravel the Earth's evolution from primordial time to present. The assessment of fluid-mobile and incompatible element uptake in serpentinites via interaction with seawater and subduction-zone fluids is central to evaluate the global cycling of the above elements in the Earth's mantle. Here, we focus on the carbon (C), nitrogen (N) and C isotope compositions of chlorite harzburgites and garnet peridotites deriving from subduction-zone dehydration of former oceanic dehydration of serpentinite – i.e., metaperidotites (Cima di Gagnone, Swiss Central Alps) with the aim of evaluating the contribution of these rocks to the global C-N cycling. These ultramafic rocks, enclosed as lenses in a metasedimentary melange, represent the destabilization of antigorite and chlorite at high-pressure/temperature (P/T) along a slab-mantle interface. Chlorite- and garnet-bearing rocks have similar ranges in C concentration ([C] = 210 – 2465 ppm and 304 – 659 ppm, respectively), with one magnesite-bearing chlorite harzburgite hosting 11000 ppm C. The average N concentrations ([N]) of the garnet peridotites (54 ± 15 ppm, one standard deviation indicated) are higher than those of the chlorite harzburgites (29 ± 6 ppm). The δ 13 C of total C (TC) and total organic C (TOC) values of the Gagnone metaperidotites range from -12.2 to -17.8‰ and from -27.8 to -26.8‰, respectively, excluding the magnesite-bearing chlorite harzburgites with higher values of -7.2‰ (TC) and -21.2‰ (TOC). The [C] of these rocks are comparable to those of serpentinites form modern and ancient oceanic environments and with [C] of high-P serpentinites. However, the lack of preserved serpentinite precursors makes it difficult to determine whether release of H2O during high-P breakdown of antigorite and chlorite is coupled with significant C release to fluids. The δ 13 C values appear to reflect mixing between seawater-derived carbonate and a reduced C source and a contribution from the host metasedimentary rocks ([C] = 301 ppm; [N] = 33 ppm; TC δ 13 C = -24.4‰; TOC δ 13 C = -27.0‰) cannot be completely excluded. The C-O isotope composition of the carbonate in magnesite-bearing chlorite harzburgites is compatible with progressive devolatilization at oxidized conditions, whereas the signatures of the majority of the other Gagnone samples appear to reflect different degree of interaction with sedimentary fluids. The [N] of the Gagnone metaperidotites are higher than those of oceanic and subducted serpentinites and show a range similar to that of high-P antigorite-serpentinites from mantle wedges. This enrichment is compatible with fluid-mediated chemical exchange with the surrounding metasedimentary rocks leading to strong modification of the Gagnone metaperidotites' geochemistry during prograde subduction along the slab-mantle interface. Comparing the δ 13 C data reported in this study with published δ 13 C values for diamonds, we suggest that the volatile recycling via Gagnone-like metaperidotites in subduction zones could contribute to deep-Earth diamond genesis and in particular to the formation of blue boron (B)-bearing diamonds. Our results highlight that the subduction of secondary peridotites evolved along the slab-mantle interface is a viable mechanism to inject volatiles into the deep mantle, particularly in hotter geothermal regimes such as the ones active during the early Earth's history.

中文翻译：

---

深入和超越：次生橄榄岩中的碳和氮俯冲循环

摘要 了解收敛边缘的波动周期是解开地球从原始时间到现在的演变的基础。通过与海水和俯冲带流体的相互作用，对蛇纹岩中流体流动和不相容元素吸收的评估对于评估地球地幔中上述元素的全球循环至关重要。在这里，我们重点研究绿泥石方辉石和石榴石橄榄岩的碳 (C)、氮 (N) 和 C 同位素组成，这些成分源自蛇纹岩前海洋脱水的俯冲带脱水，即变橄榄岩（Cima di Gagnone，瑞士中阿尔卑斯山）目的是评估这些岩石对全球 CN 循环的贡献。这些超镁铁质岩石，以变质沉积混杂物的形式封闭，代表叶蛇纹石和绿泥石在高压/温度 (P/T) 下沿板片-地幔界面的不稳定。含绿泥石和含石榴石的岩石的 C 浓度范围相似（[C] = 210 – 2465 ppm 和 304 – 659 ppm，分别），一个含菱镁矿的绿泥石方辉石含有 11000 ppm C。 ]) 石榴石橄榄岩（54 ± 15 ppm，标明一个标准偏差）高于绿泥石方辉石（29 ± 6 ppm）。Gagnone变橄榄岩的总C（TC）和总有机C（TOC）的δ 13 C值分别为-12.2～-17.8‰和-27.8～-26.8‰，不包括含菱镁矿的绿泥石菱镁矿。 -7.2‰ (TC) 和 -21.2‰ (TOC) 的值。这些岩石的 [C] 与来自现代和古代海洋环境的蛇纹岩的 [C] 以及高 P 蛇纹岩的 [C] 相当。然而，由于缺乏保存下来的蛇纹石前体，因此很难确定在叶蛇纹石和绿泥石的高压分解过程中释放的 H2O 是否与大量的 C 释放到流体中相结合。δ 13 C 值似乎反映了海水衍生的碳酸盐和还原 C 源之间的混合以及来自宿主变质沉积岩的贡献（[C] = 301 ppm；[N] = 33 ppm；TC δ 13 C = -24.4‰ ; TOC δ 13 C = -27.0‰) 不能完全排除。含菱镁矿绿泥石方辉石中碳酸盐的 CO 同位素组成与氧化条件下的渐进脱挥相容，而大多数其他 Gagnone 样本的特征似乎反映了与沉积流体的不同程度的相互作用。Gagnone变橄榄岩的[N]高于大洋和俯冲蛇纹岩的[N]，其范围与地幔楔中的高P叶蛇纹石-蛇纹岩相似。这种富集与流体介导的与周围变质沉积岩的化学交换相容，导致在沿板片-地幔界面前向俯冲过程中 Gagnone 变橄榄岩的地球化学发生强烈变化。将本研究中报告的 δ 13 C 数据与已公布的钻石 δ 13 C 值进行比较，我们认为，通过俯冲带中类 Gagnone 变橄榄岩的挥发性循环可能有助于深地钻石的成因，尤其是蓝硼 (B) 钻石的形成。我们的研究结果强调，沿板片-地幔界面演化的次生橄榄岩的俯冲是将挥发物注入深部地幔的可行机制，特别是在较热的地热区，例如地球早期历史上活跃的地热区。