ABSTRACT

Sulphur is of particular importance in the subduction zone system, as it is thought to be intimately related to the arc redox state and metallogenesis above the slab. Knowing the sulphur isotope compositions of the subducted slab, and identifying the δ³⁴S variations between the pre-subduction oceanic lithosphere and its high-pressure (HP) equivalents is of great importance for understanding the sulphur behaviour in subduction zones and the long-term sulphur recycling of the Earth. In this study, we undertook secondary ion mass spectroscopy (SIMS) in-situ δ³⁴S analyses of sulphides from metasediments, mafic blueschists/eclogites, and ultramafic slab serpentinites from the Southwestern Tianshan HP–UHP metamorphic belt, which represents one of the deepest fossil subduction zones on Earth. By integrating the in-situ δ³⁴S compositions of sulphides studied here and those reported from worldwide HP–UHP metamorphic terranes, we found that prograde to peak metamorphic sulphides of metasediments, metabasites/metagabbros, and slab HP serpentinites have δ³⁴S values ranging from –33 to –6‰, –4 to +4‰, and +2 to +18‰, respectively. However, retrograde metasomatism of HP rocks during exhumation may alter the δ³⁴S compositions of sulphides. The integrated dataset proposes that sulphides in subducting slab sequences retain their protolith’s δ³⁴S signatures during HP metamorphism. The δ³⁴S inheritance indicates that no large isotope fractionation is expected to occur in subduction zones. It further suggests that evaluations of slab subduction effects on the δ³⁴S signature of arc magmatism and the deep mantle should consider pre-subduction processes such as ridge geodynamic setting and hydrothermal alteration.

摘要

硫在俯冲带系统中尤为重要，因为它与板坯上方的电弧氧化还原状态和成矿关系密切。知道了俯冲的硫同位素组成，和识别所述δ ³⁴预俯冲海洋岩石层和其高压（HP）当量S之间的变化是非常重要的用于理解在俯冲带硫行为和长期地球的硫回收。在这项研究中，我们进行原位二次离子质谱法（SIMS）δ ³⁴S分析了来自天山西南部HP–UHP变质带的变质沉积，镁铁蓝晶岩/榴辉岩和超镁铁质板状蛇纹岩中的硫化物，该变质带是地球上最深的化石俯冲带之一。通过整合原位δ ^34种在这里学习硫化物第成分和来自世界各地的高压-超高压变质地体的报道，我们发现，顺行到沉积岩，metabasites / metagabbros和平板HP蛇纹岩的峰期变质硫化物具有δ ^34个价值观范围从–33到–6‰，从–4到+ 4‰，和从+2到+ 18‰。不过，惠普的逆行交代折返过程中岩石可能改变δ ³⁴硫化物的S组成。集成的数据集提出，在俯冲板块序列硫化物保留其原岩的δ ³⁴ HP变质在S签名。的δ ³⁴的继承表示没有大的同位素分馏，预计在俯冲带发生。委员会还建议在δ板坯俯冲效应评估³⁴弧岩浆活动的签名和深部地幔应该考虑预俯冲过程，如脊地球动力学背景和热液蚀变。