俯冲带硫的地球化学行为及硫循环SCIEI北大核心CSCD

俯冲带是全球最大的物质循环系统,控制着硫(S)在地球内部圈层及表层的循环,影响着大气圈、水圈、生物圈、岩石圈的稳定性以及地球的宜居性。厘清S在俯冲带中的地球化学行为和循环特征对理解地球各储库的氧化还原状态、岩浆作用与演化、成矿物质聚集、以及地球大气成分等具有重要意义。本文首先总结了进入俯冲带之前的大洋岩石圈的S结构模型,对S在大洋板片中的分布状态和地球化学特征进行了系统归纳。随后,系统阐述了俯冲带高压-超高压变质岩记录的板片变质及脱水过程中硫的地球化学行为。岩石学研究表明俯冲板片中的S多以硫化物相存在,硫酸盐矿物在弧前深度就已被释放或分解。相较于熔体,俯冲带流体中S的溶解度更高,是运移硫的更有效方式。DEW模型计算结果显示,流体中S含量总体较低,但在俯冲板片~90km处其含量有一个峰值(浓度0.5%~1.0%)。岩相学证据、地球化学测试结果、磷灰石S近边吸收结构(S-XANES)特征以及模拟结果都显示俯冲深部流体中S多以HS^(-)及H_(2)S形式存在,不含大量的SO_(4)^(2-)及硫酸盐;中f_(S_(2))流体有利于S迁移出俯冲板片,从而促进俯冲带大规模S循环,而高f_(S_(2))流体在流-岩交换过程沿流体通道发生S的锁固作用而不利于俯冲带S循环。质量平衡计算显示全球俯冲带S输入通量为4.65×10^(13)g/yr,弧下深度板片S输出通量为2.91×10^(12)g/yr,板片-岛弧S循环效率仅6.3%。俯冲板片在弧下深度可能存在一个短暂高效的S释放窗口,释放流体的δ^(34)S值为-2.1±3.0‰。基于高压-超高压变质岩中硫化物的研究,初步厘清了俯冲板片中S的地球化学行为,首次从板片角度全面、定量地限定了俯冲带的脱硫通量、效率、种型和同位素特征,提出俯冲带循环的S不是岛弧岩浆的氧化剂,与岛弧环境的正δ^(34)S值也无直接因果联系,对解析俯冲带S循环和理解地球长期的S循环具有重要意义。最后,本文还展望了俯冲带S循环的未来发展方向,应在俯冲带流体氧化还原性质(硫酸盐的命运)、俯冲沉积物对S循环的制约、俯冲带环境下多硫同位素的分馏效应、S循环与其它挥发分(如C等)循环之间的耦合关系、地球历史上深部S循环等方向做出探索,更深入地理解俯冲带及全球S循环过程。

Geochemical behavior and recycling of sulfur in subduction zones

Subduction zone is the largest material recycling system on the Earth, mediating the sulfur exchange between Earth's interior and surface. Sulfur is one of the key elements that connects the atmo-, hydro-, bio-, and lithosphere and plays a critical role in Earth habitability. Knowing the behavior and recycling of S in the subduction zone is critical to understand the redox state of geological reservoirs, arc magmatism and ore metallogenesis, as well as change and evolution of Earth's atmosphere. This paper first presents the sulfur structure in the pre-subduction oceanic lithosphere, reviewing the occurrences of S-bearing phases and their geochemical compositions. Then, this study systematically reviews the S behavior during the metamorphic and dehydration processes in the subducted slab recorded by high-pressure and ultrahigh-pressure ( HP-UHP) metamorphic rocks. Petrologic investigation shows that S in the subducting slab occurs mostly in the form of sulfide, and that sulfate is mostly dissolved or broken down at the fore-arc depths. Sulfur solubility in slab fluids is much higher in comparison to melts, suggesting that fluid is the more effective agent for transferring S from the slab to the mantle wedge. DEW ( Deep Earth Water) modeling indicates that S concentration in slab fluids is generally low; however, it reveals a distinct S] peak ( 0. 5% similar to 1. 0%) at similar to 90km depth. All evidence from petrologic observations, chemical analyses, apatite S-XANES ( X-ray Absorption Near Edge Structure) characteristics, and DEW modeling show that S species in slab fluids are dominated by reduced HS- and H2S at given subduction zone P-T-f(O2) conditions, without significant SO42- or sulfate. Medium-f(S2) fluids can transport S and thus are effective in transferring S out of the slab, whereas high-f(S2) fluids cause S sequestration along fluid pathways and considerably reduce long-distance S transfer. Mass-balance calculations demonstrate that the global S influx is estimated at 4. 65 x 10(13) s g/yr, slab outflux at 2. 91 x 10(12) g/yr at sub-arc depth, resulting in a slab-arc S recycling efficiency of similar to 6. 3% . A major S release window is constrained at depths of 70 similar to 100km, during which the released slab fluid has a negative delta S-34 value of -2. 1 +/- 3. 0 parts per thousand. Based on researches of HP-UHP metamorphic rocks, the recent studies shed light on the S behavior in the subduction zone, and for the first time from the slab perspective, reveal the slab desulfuration flux and efficiency, sulfur species and isotopic composition during the slab-arc S recycling. It is suggested that slab S is not an oxidant accounting for the high f(O2) of arc magmas and also not related to the heavy delta S-34 signature observed in arc volcanic rocks. The study sheds light on S recycling in subduction zone and long-term global S cycle. At the end, this paper proposes some research directions in the future that will likely be fruitful in understanding subduction zone S recycling, including redox state of slab fluids ( fate of sulfate) , controls of metasediment on sulfur cycle, multiple sulfur isotope fractionation in subduction environment, the coupling/decoupling recycling between S and other volatiles ( C and etc. ) , and deep sulfur cycle through geological time.