The lithospheric mantle affected by metasomatism is an important reservoir for halogens and often preserves significant heterogeneity in halogen abundances. However, the halogen compositions of the non-metasomatized mantle rocks with depleted geochemical features have not been well studied, impeding a comprehensive understanding of the halogen distribution in the mantle. The Balmuccia (BM) and Baldissero (BD) peridotite massifs in the Ivrea-Verbano Zone, Italian Alps, represent fragments of the sub-continental lithospheric mantle (SCLM) that are fresh and devoid of subduction-related mantle metasomatism. Here we examine the heavy halogen contents of well-characterized spinel-facies mantle peridotites (n = 20) and pyroxenites (n = 16) from the BM and BD massifs. The results indicate significant variation in whole rock halogen contents of peridotites and pyroxenites, with chlorine (Cl) ranging from <30 to 244 μg/g, bromine (Br) from <0.10 to 0.94 μg/g, and iodine (I) from <0.015 to 0.12 μg/g. Some peridotites and pyroxenites show rather low halogen contents (below the detection limit), consistent with their depleted features. However, a large number of samples display halogen contents far higher than the depleted MORB mantle (i.e., Cl: 3–10 μg/g) and peridotite xenoliths from the strongly metasomatized mantle, although ∼30 % of the halogens were leached out by ultrapure water or dilute nitric acid. The data suggest variable halogen enrichment in many mantle rocks. The halogen contents of different types of mantle rocks do not vary with major elements and incompatible trace elements (e.g., Nb), indicating that halogen compositions are not controlled by magmatic processes such as melting and melt-peridotite reaction. Instead, secondary fluid inclusions are abundant and mainly distributed as trails within olivine and pyroxene in both peridotites and pyroxenites, which were likely trapped during the exhumation of these mantle rocks. Their elevated halogen/Nb ratios compared with MORB mantle and strong correlations with Ba/Nb support the effect of fluids. Micro-Raman analyses identify hydrous serpentines within fluid inclusions. Moreover, mass balance calculation indicates that halogen enrichment is mainly caused by secondary fluids (> 90 %). Together, our findings highlight that the fluid percolation in the mantle rocks during exhumation can intensely modify the halogens, to a similar degree as mantle metasomatism, suggesting the sensitivity of halogens to secondary modification.

中文翻译：

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Ivrea-Verbano 区橄榄岩地块的重卤素成分及其对地幔岩强烈改性的影响


受交代作用影响的岩石圈地幔是卤素的重要储存库，通常在卤素丰度中保持显著的异质性。然而，具有耗尽地球化学特征的非交代化地幔岩的卤素成分尚未得到很好的研究，阻碍了对地幔中卤素分布的全面理解。意大利阿尔卑斯山 Ivrea-Verbano 区的 Balmuccia （BM） 和 Baldissero （BD） 橄榄岩地块代表了次大陆岩石圈地幔 （SCLM） 的碎片，这些碎片是新鲜的，没有与俯冲相关的地幔交代作用。在这里，我们研究了来自 BM 和 BD 地块的特征明确的尖晶石相地幔橄榄岩 （n = 20） 和辉石岩 （n = 16） 的重卤素含量。结果表明，橄榄岩和辉石岩的全岩卤素含量存在显著变化，氯 （Cl） 范围为 <30 至 244 μg/g，溴 （Br） 范围为 <0.10 至 0.94 μg/g，碘 （I） 范围为 <0.015 至 0.12 μg/g。一些橄榄岩和辉石的卤素含量相当低（低于检测限），这与它们的贫竭特征一致。然而，大量样品的卤素含量远高于贫化的 MORB 地幔（即 Cl：3-10 μg/g）和来自强交代地幔的橄榄岩异石，尽管 ∼30% 的卤素被超纯水或稀硝酸浸出。数据表明，许多地幔岩中存在不同的卤素富集。不同类型地幔岩的卤素含量不随主量元素和不相容的微量元素（如 Nb）而变化，表明卤素组成不受熔融和熔融-橄榄岩反应等岩浆过程的控制。 相反，次生流体包裹体很丰富，主要分布在橄榄岩和辉石中的橄榄石和辉石中，这些泥土很可能是在挖掘这些地幔岩时被捕获的。与 MORB 地幔相比，它们的卤素/Nb 比值更高，并且与 Ba/Nb 具有很强的相关性，支持液体的作用。微量拉曼分析可识别液体包裹体中的含水蛇纹石。此外，质量平衡计算表明，卤素富集主要由二次流体 （> 90 %） 引起。总之，我们的研究结果强调，挖掘过程中地幔岩石中的流体渗流可以强烈地改变卤素，其程度与地幔交代作用相似，表明卤素对二次改性敏感。