Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones
Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones
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DOI：
10.1016/j.gca.2020.12.001
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发表时间：
2020-12
影响因子：
5
通讯作者：
B. Debret;C. Garrido;M. Pons;P. Bouilhol;E. Inglis;V. L. Sánchez‐Vizcaíno;H. Williams
中科院分区：
文献类型：
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作者：
B. Debret;C. Garrido;M. Pons;P. Bouilhol;E. Inglis;V. L. Sánchez‐Vizcaíno;H. Williams
俯冲蛇纹岩具有控制挥发性和氧化还原敏感元素交换的潜力（例如，Fe，S，C，N）的富集作用。在这里，我们研究铁和锌的蛇纹岩衍生流体的活动性，通过使用它们的稳定同位素（δ 56 Fe和δ 66 Zn）在高压俯冲变质蛇纹岩从Cerro del Almirez Alzagraf（西班牙）。该变质带保存了叶蛇纹石（Atg-蛇纹岩）和叶蛇纹石-橄榄石-斜方辉石（过渡岩性）-含蛇纹岩和含绿泥石的方辉橄榄岩（Chl-方辉橄榄岩）之间的变质前缘，显示出花岗质岩、尖晶石和细粒重结晶结构。这些岩石形成于榴辉岩相条件（1.6-1.9 GPa和680-710 °C）。所有Cerro del Almirez变质蛇纹岩的δ 56 Fe平均值（+0.05 ± 0.01‰）与原始地幔的δ 56 Fe平均值（+0.03 ± 0.03‰）相差不大。δ ~（56）Fe与橄榄岩原岩肥力指标（如：Al 2 O3/SiO2）表明，Cerro del Almirez样品的δ 56 Fe值主要反映原岩成分的变化，可能是由以前的熔体提取事件产生的。相比之下，Cerro del Almirez样品的Zn浓度（[Zn] = 34-67 ppm）和同位素特征（δ 66 Zn = +0.18 - +0.55‰）显示出宽范围的值，不同于原始地幔的值（[Zn] = 54 ppm; δ 66 Zn = +0.16 ± 0.06‰）。Atg型蛇纹岩[Zn] = 34-46 ppm，δ 66 Zn = +0.23 ± 0.06‰，其[Zn]和δ 66 Zn值与其他高压变质蛇绿岩中的板状蛇纹岩相似。过渡性岩性（[Zn] = 45-67 ppm; δ 66 Zn = +0.30 ± 0.06‰）和具有花岗质（[Zn] = 38-59 ppm; δ 66 Zn =+0.33 ± 0.04‰）或刺状（[Zn] = 48-66 ppm; δ 66 Zn =+0.43 ± 0.09‰）结构的Chl方辉橄榄岩中[Zn]和δ 66 Zn均增加。重要的是，Cerro del Almirez过渡岩性和Chl方辉橄榄岩显示相对于深海橄榄岩和蛇纹岩（29-45 ppm）异常高的[Zn]，[Zn]和δ 66 Zn之间存在正相关。这种相关性被解释为反映锌的动员俯冲带流体在高压和高温下，再加上显着的锌稳定同位素分馏。从Atg-蛇纹岩到Chl-方辉橄榄岩，[Zn]和δ 66 Zn的增加与U/Yb、Sr/Y、Ba/Ce和Rb/Ce的增加有关，表明[Zn]和δ 66 Zn都记录了过渡岩性和Chl-方辉橄榄岩与已与变质沉积岩平衡的流体的相互作用。定量模型表明，由于沉积物碳酸盐溶解和随后的Zn与释放流体中的碳酸盐物质络合，变质沉积物衍生的流体可以具有同位素重Zn（例如，[ZnHCO 3（H2O）5+]或[ZnCO 3（H2O）3]）。我们的模型进一步表明，锌络合还原碳物种不能产生重δ 66锌签名的流体，因此解释了在叶绿素方辉橄榄岩中观察到的δ 66锌的变化。因此，对Cerro del Almirez样品中重δ 66 Zn的最直接的解释是蛇纹岩脱水，伴随着在与俯冲有关的变质作用期间释放的富含碳酸盐的氧化沉积物衍生流体的开放系统渗透。
Subducted serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in serpentinite-derived fluids by using their stable isotopes (δ56Fe and δ66Zn) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6–1.9 GPa and 680–710 °C). The mean δ56Fe of all the Cerro del Almirez meta-serpentinites (+0.05 ± 0.01‰) is identical within an error to that of primitive mantle (+0.03 ± 0.03‰). A positive correlation between δ56Fe and indices of peridotite protolith fertility (e.g., Al2O3/SiO2) suggests that the δ56Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations ([Zn] = 34–67 ppm) and isotope signatures (δ66Zn = +0.18 – +0.55‰) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm; δ66Zn = +0.16 ± 0.06‰). The Atg-serpentinites ([Zn] = 34–46 ppm; δ66Zn = +0.23 ± 0.06‰) display similar [Zn] and δ66Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both [Zn] and δ66Zn increase in transitional lithologies ([Zn] = 45–67 ppm; δ66Zn = +0.30 ± 0.06‰) and Chl-harzburgites with granofels ([Zn] = 38–59 ppm; δ66Zn = +0.33 ± 0.04‰) or spinifex ([Zn] = 48–66 ppm; δ66Zn = +0.43 ± 0.09‰) textures. Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high [Zn] relative to abyssal peridotites and serpentinites (29–45 ppm) and a positive correlation exists between [Zn] and δ66Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in [Zn] and δ66Zn from Atg-serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and δ66Zn record the interaction of the transitional lithologies and the Chl-harzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., [ZnHCO3(H2O)5+] or [ZnCO3(H2O)3]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy δ66Zn signature and hence explain the δ66Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy δ66Zn of the Cerro del Almirez samples is thus serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism.