西南天山超高压泥质片岩中石墨的形成:对俯冲带碳迁移、储存的启示

Formation of graphite in ultrahigh-pressure pelitic schists from the southwestern Tianshan:Implications for carbon migration and sequestration in subduction zones

大洋沉积物是俯冲带碳输入的主要贡献者之一,为俯冲带“加工厂”提供了大量的有机碳和无机碳。探索这些碳在俯冲带的命运对于更好地了解地球深部碳循环至关重要。虽然超深金刚石和岛弧碳排放的证据强调了俯冲有机碳在这一循环中的作用,但有机碳俯冲到弧下深度的岩石学证据仍然缺乏。此外,在俯冲带(超)高压((U)HP)条件下形成的非生物成因CH_(4)流体的命运仍未得到充分探索。中国西南天山洋壳型HP-UHP变质带以低地温梯度的冷俯冲为特征,富含含碳(如碳酸盐、石墨、CH_(4)、CO_(2)等)岩石,是研究俯冲带碳循环的理想天然实验室。本文对新疆西南天山造山带超高压泥质片岩中不同类型石墨进行了详细的岩石学观察、拉曼光谱和稳定碳同位素分析。发现两种类型的石墨:Type-1石墨以微小包裹体形式存在于石榴石中,δ^(13) C_(TOC)值较低(-24.3‰~-23.2‰),表明其为生物成因;Type-2石墨呈平行于片理的条带状分布,δ^(13) C_(TOC)值较高,为-14.8‰~-12.5‰,表明其来源于非生物成因的前驱体。扫描电镜图像和拉曼光谱表明,这两种类型的石墨具有相似的形态特征和结晶度。通过热力学模型、金红石Zr温度计和碳质物质的拉曼光谱(RSCM),我们对这两类石墨的形成条件进行了约束。Type-1石墨是由俯冲板块中有机碳在进变质作用中石墨化形成的,温度达到530~555℃。Type-2石墨在峰期榴辉岩相变质条件下(约2.7 GPa、530℃)从COH流体中沉淀出。与Type-2石墨相关的含CH_(4)流体包裹体和热力学模拟表明,CH_(4)流体中的碳可能通过氧化作用沉淀出石墨。石榴石中Type-1石墨与柯石英共存的岩石学特征和相平衡模拟计算表明,有机碳来源的石墨可以俯冲至超过90 km的弧下深度。这为有机碳向地幔深度俯冲提供了岩石学证据,可能为解释δ^(13) C值较轻的金刚石的起源提供线索。Type-2石墨强调了俯冲带碳流动性的局限性,这一过程可能会影响碳在俯冲带的停留时间,并调节弧火山的碳释放。此外,我们的研究结果还强调了环境氧化还原状态在控制俯冲带COH流体的命运和碳迁移方面的重要作用,尤其是在氧逸度变化的岩性界面。进一步研究这些CH_(4)流体在不同氧化还原条件下的迁移,对于更好地理解俯冲带中碳的迁移具有重要意义。

Oceanic sediments,as one of the main contributors of carbon input in subduction zones,provide a large amount of organic and inorganic carbon to the subduction factory.Exploring the fate of these carbon types in subduction zones is critical for a better understanding of Earth’s deep carbon cycle.While the role of subducted organic carbon in this cycle is underscored by evidence from ultra-deep diamonds and arc carbon emissions,petrological evidence for the subduction of organic carbon to subarc depths remains scarce.Additionally,the fate of abiogenic CH_(4) fluids formed under ultrahigh-pressure(UHP)conditions in subduction zones remains underexplored.The Chinese southwestern Tianshan oceanic-type HP-UHP metamorphic belt,characterized by cold subduction with a low geothermal gradient,is rich in carbon-bearing rocks(e.g.,carbonate,graphite,CH_(4),CO_(2))and serves as an ideal natural laboratory for studying the carbon cycle in subduction zones.This study conducts detailed petrological observations,Raman spectroscopic,and stable carbon isotopic analyses of different graphite types in the UHP pelitic schists of the southwestern Tianshan orogenic belt in Xinjiang,China.Two types of graphite were identified:Type-1 graphite,occurring as minute inclusions in garnet with lowδ^(13) C_(TOC) values(-24.3‰to-23.2‰),suggesting a biogenic origin;and Type-2 graphite,occurring as foliation-parallel bands with higherδ^(13) C_(TOC) values(-14.8‰to-12.5‰),indicative of an abiogenic precursor.Both types exhibit similar morphological characteristics and crystallinity,as revealed by BSE images and Raman spectroscopy.Thermodynamic modeling,zirconium-in-rutile thermometry,and Raman Spectroscopy on Carbonaceous Material(RSCM)were used to constrain the formation conditions of these two graphite types.Type-1 graphite formed from the graphitization of organic carbon in subducting slabs during prograde metamorphism,reaching temperatures of 530555℃.Type-2 graphite crystallized from COH fluid at approximately 2.7 GPa and 530℃during eclogite-facies peak metamorphism.CH_(4)-bearing fluid inclusions associated with Type-2 graphite,alongside thermodynamic modeling,suggest the oxidative precipitation of carbon from CH_(4) fluid.Petrological characteristics of Type-1 graphite coexisting with coesite in garnet,and p T calculations,suggest that organic graphite was subducted to subarc depths exceeding 90 km.This discovery provides petrological evidence for the deep subduction of organic carbon to mantle depths and may offer insights into the origin of diamonds with lightδ^(13) C values.Type-2 graphite highlights the limitations of carbon mobility in subduction zones,affecting carbon retention times,and modulating carbon emissions from arc volcanoes.Furthermore,our findings emphasize the significance of environmental redox states,particularly at lithological interfaces with variable oxygen fugacity,in controlling the fate of COH fluids and carbon transport in subduction zones,pointing to the need for further research on CH_(4) fluid migration under varying redox conditions.