Boron and lead isotope signatures of subduction-zone mélange formation: Hybridization and fractionation along the slab-mantle interface beneath volcanic arcs
Boron and lead isotope signatures of subduction-zone mélange formation: Hybridization and fractionation along the slab-mantle interface beneath volcanic arcs
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DOI：
10.1016/j.chemgeo.2007.01.009
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发表时间：
2007-04
期刊：
影响因子：
3.9
通讯作者：
R. L. King;G. Bebout;M. Grove;T. Moriguti;E. Nakamura
中科院分区：
文献类型：
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作者：
R. L. King;G. Bebout;M. Grove;T. Moriguti;E. Nakamura
B 和 Pb 同位素系统广泛应用于俯冲过程中发生的再循环过程的示踪剂。将这些互补系统作为一对进行研究取得了相当大的成功，其中 B 主要记录了俯冲板片的热和流体演化，而三方 Pb 系统限制了返回火山弧的俯冲物质的来源。然而，从弧火山记录得出的解释主要取决于有关俯冲带未变质输入成分的假设。很少有研究通过分析高压（HP）和超高压（UHP）变质岩组来直接解决 B 同位素和 U-Th-Pb 的潜在分馏问题，尽管在许多火山弧和洋岛玄武岩研究中已经推断出俯冲带变质作用期间这些系统中的分馏。在这里，我们通过对加利福尼亚州卡特琳娜片岩的混杂岩基质进行 B 和 Pb 同位素测定来研究俯冲物质的变质演化。在卡塔利娜片岩内，通过交代作用和变形的协同作用形成混杂岩基质，影响源自俯冲法拉隆板块的玄武岩和沉积物以及源自上覆地幔楔的橄榄岩。这些端元之间的简单机械混合模型广泛地预测了杂化片状混杂岩基质中的 B 和 Pb 浓度，但要解释这两个系统的同位素比率，需要在变质过程中进行显着的分馏。 B 同位素结果与先前提出的基于 O 和 H 同位素数据的卡塔利娜片岩俯冲带内流体来源和输运模型相一致：角闪岩相混杂岩基质的 δ11B 值与低 T 变沉积岩富集区产生的含 B 流体的渗透一致，而较低品位的硬钠长石-钠长石和硬钠长石-蓝片岩构造变质单元代表了来源的可能类似物这种 B 轴承液体。总体而言，Pb 同位素比作为变质等级的函数无法区分，并且具有高度放射性。我们通过碎屑锆石 U-Pb 年龄谱估计大陆输入成分，限制了放射性大陆碎屑对卡塔琳娜俯冲带的潜在影响。这种基于锆石的沉积代理表明，中生代加利福尼亚安第斯型会聚边缘的潜在影响不能在所有情况下解释卡塔利娜混杂岩基质的放射成因 Pb 特征，似乎需要在硬钠长石 - 钠长石和硬钠长石 - 蓝片岩单元的形成过程中对 U-Th-Pb 系统进行一些分馏。低品位混杂岩基质的 Pb 同位素特征可以用两阶段变质分馏模型来解释，该模型涉及在俯冲过程中脱硫反应导致 Pb 的早期损失，随后硅酸盐 U 的更深损失。角闪岩相混杂岩基质的 Pb 特征表明，要么在变质作用过程中有效保留了原岩 Pb 特征，要么通过变质流体流忠实地转移了分馏的 Pb 特征。卡塔琳娜片岩 B 和 Pb 同位素流体特征对地幔楔的污染可以解释现代弧中观察到的 B-Pb 同位素异常，表明在俯冲带质量传递模型中应考虑混杂岩混合的影响。
The B and Pb isotope systems are widely applied tracers of recycling processes occurring during subduction. Studies examining these complementary systems as a pair enjoy considerable success, where B primarily records the thermal and fluid evolution of the subducting slab, whereas the tripartite Pb system constrains the source of subducted material returned to volcanic arcs. However, interpretations derived from the arc volcanic record critically depend upon assumptions regarding compositions of unmetamorphosed inputs to subduction zones. Few studies have directly addressed potential fractionation of B isotopes and U–Th–Pb by analysis of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic suites, despite that fractionation in these systems during subduction-zone metamorphism has been inferred in many studies of volcanic arcs and ocean–island basalts. Here, we address the metamorphic evolution of subducted material with B and Pb isotope determinations for the mélange matrix of the Catalina Schist, CA. Within the Catalina Schist, mélange matrix formed through the synergistic effects of metasomatism and deformation, affecting basalts and sediments derived from the subducting Farallon plate with peridotites derived from the overlying mantle wedge. Models of simple mechanical mixing among these end-members broadly predict both B and Pb concentrations within hybridized schistose mélange matrix, but an explanation of isotope ratios for both systems requires significant fractionation during metamorphism. The B isotope results are compatible with the previously presented model for sources and transport of fluid within the Catalina Schist subduction zone based on O and H isotope data: δ11B values for the amphibolite facies mélange matrix are consistent with infiltration by B-bearing fluid produced in lower-T metasediment-rich domains, whereas the lower-grade lawsonite–albite and lawsonite–blueschist tectonometamorphic units represent possible analogs for the sources of this B-bearing fluid. Overall, Pb isotope ratios are indistinguishable as a function of metamorphic grade and are highly radiogenic. We constrained the potential influence of radiogenic continental detritus to the Catalina subduction zone by estimation of the continental input component from detrital zircon U–Pb age spectra. This zircon-based sedimentation proxy demonstrates that the potential influence of the Mesozoic California Andean-type convergent margin cannot in all cases explain the radiogenic Pb signature of the Catalina mélange matrix, seemingly requiring some fractionation of the U–Th–Pb system during formation of the lawsonite–albite and lawsonite–blueschist mélange units. Pb isotope signatures of the lower-grade mélange matrix can be explained by a two-stage metamorphic fractionation model involving early loss of Pb by desulfidation reactions, followed by deeper loss of silicate U, during subduction. Pb signatures of the amphibolite facies mélange matrix suggest either efficient retention of protolith Pb signatures during metamorphism or faithful transfer of the fractionated Pb signature by metamorphic fluid flow. Contamination of the mantle wedge by Catalina Schist B and Pb isotope fluid signatures can explain B–Pb isotope anomalies observed for modern arcs, indicating that the effects of mélange mixing should be considered in models of subduction-zone mass transfer.