The subduction of oceanic slabs with diverse lithologies greatly contributes to the geochemical heterogeneity of the mantle. Oceanic metasomatism leads to the formation of rocks with heterogeneous geochemical characteristics, which can be further modified by fluid–rock interactions during subduction zone processing. Investigations of exhumed high-pressure oceanic rocks not only shed light on the compositional variations that occur during oceanic metasomatism and subduction processes but also help in understanding mantle heterogeneity and magma genesis. Here, we present a comprehensive Mg–O–Sr–Nd isotope study on eclogite-facies Mg- and Ca-metasomatized rocks (Ti-clinohumite dikelets and metarodingites, respectively) and their host -serpentinites from the Voltri Massif (Ligurian Western Alps). Compared to their Fe–Ti gabbroic protoliths, the Ti-clinohumite (Ticl) dikelet and metarodingite are characterized by high MgO contents (∼32–38 wt%) and CaO contents (∼17–25 wt%), respectively. These Mg- and Ca-enriched rocks both display low δMg values of −0.54 ‰ to −0.43 ‰ and −1.49 ‰ to −0.82 ‰, respectively, while the serpentinite hosts have mantle-like δMg values of −0.26 ‰ to −0.20 ‰. The Ticl-chlorite rind rimming the metarodingite has a low δMg value of −1.00 ‰. No carbonate minerals were found for metasomatized rocks that have low δO values ranging from +2.0 ‰ to +3.4 ‰, demonstrating that their low δMg signature cannot be attributed to metasomatism by fluids involved with carbonate components. The increase in MgO content and decrease in δMg in the Ticl dikelets are ascribed to fluid-induced mass transfer through low-δMg fluids buffered by serpentinization of the host ultramafic rocks. The metarodingite has comparable MgO but extremely low δMg values compared to those of the gabbroic protolith. By integrating reported Mg isotope data for low-grade rodingites, we propose that the low δMg values of metarodingites are ascribed to isotopic exchange with serpentinizing fluids during progressive rodingitization reactions at the seafloor. However, the extremely low δMg value of the Voltri metarodingite (as low as −1.49 ‰) may be attributed to further metasomatic modification in the subduction zone. High pressure metamorphic recrystallization of these metasomatized rocks play a limited role in their Mg isotope compositions. Therefore, our results show that the oceanic metasomatized metarodingite and Ticl-bearing dikes hosted in serpentinite display systematically low-δMg features. Such isotopically light Mg compositions can be preserved during subduction to eclogite-facies conditions. Hence, subduction of these metasomatized rocks may lead to the transfer of a low-δMg component into the deep mantle, which was overlooked thus far and may further contribute to the origin of low-δMg magmas.

中文翻译：

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俯冲大洋岩石圈中 δ26Mg 值极低的流体交代岩石：对地幔 Mg 同位素异质性和低 δ26Mg 岩浆起源的影响

具有不同岩性的大洋板片的俯冲极大地影响了地幔的地球化学非均质性。海洋交代作用导致形成具有异质地球化学特征的岩石，这些特征可以在俯冲带加工过程中通过流体-岩石相互作用进一步改变。对挖出的高压海洋岩石的研究不仅有助于了解海洋交代作用和俯冲过程中发生的成分变化，而且有助于了解地幔异质性和岩浆成因。在这里，我们对来自沃尔特里地块（利古里亚西部阿尔卑斯山）的榴辉岩相镁和钙交代岩石（分别是钛斜辉石双岩和变硬岩）及其主体蛇纹岩进行了全面的 Mg-O-Sr-Nd 同位素研究。与 Fe-Ti 辉长岩原岩相比，Ti-斜辉长岩（Ticl）双岩和变辉长岩的特点分别是高 MgO 含量（~32-38 wt%）和 CaO 含量（~17-25 wt%）。这些富镁和富钙岩石均表现出较低的 δMg 值，分别为 -0.54 ‰ 至 -0.43 ‰ 和 -1.49 ‰ 至 -0.82 ‰，而蛇纹岩主体的 δMg 值为 -0.26 ‰ 至 -0.20 ‰ 。围绕变辉石的 Ticl-绿泥石外皮具有较低的 δMg 值，为 -1.00 ‰。对于 δ18O 值范围在 +2.0 ‰ 至 +3.4 ‰ 范围内的低 δ18O 值的交代岩石，没有发现碳酸盐矿物，这表明它们的低 δMg 特征不能归因于涉及碳酸盐成分的流体的交代作用。 Ticl 系中 MgO 含量的增加和 δMg 的减少归因于通过由宿主超镁铁质岩石的蛇纹石化缓冲的低 δMg 流体引起的流体诱导传质。与辉长岩原岩相比，变辉岩具有可比的 MgO，但 δMg 值极低。通过整合已报告的低品位罗金石镁同位素数据，我们提出变铜矿的低 δMg 值归因于海底渐进罗金石化反应期间与蛇纹石化流体的同位素交换。然而，Voltri变硬岩的δMg值极低（低至-1.49‰）可能归因于俯冲带的进一步交代改造。这些交代岩石的高压变质重结晶对其镁同位素组成的影响有限。因此，我们的结果表明，蛇纹岩中的大洋交代变硬岩和含 Ticl 岩脉表现出系统的低 δMg 特征。这种同位素轻镁成分可以在俯冲到榴辉岩相条件的过程中得以保存。因此，这些交代化岩石的俯冲可能导致低δMg成分转移到深部地幔中，这一点迄今为止被忽视，并可能进一步促进低δMg岩浆的起源。