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

doi:10.13745/j.esf.sf.2024.6.80

摘要/Abstract

摘要：

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CH₄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、CH₄、CO₂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,δ¹³C_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,δ¹³C_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CH₄流体包裹体和热力学模拟表明,CH₄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δ¹³C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CH₄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。

关键词: 石墨化, 流体沉淀石墨, 有机碳, COH流体, 西南天山, 俯冲带深部碳循环

Abstract:

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 CH₄ 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, CH₄, CO₂) 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 low δ¹³C_TOC values (-24.3‰ to -23.2‰), suggesting a biogenic origin; and Type-2 graphite, occurring as foliation-parallel bands with higher δ¹³C_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 530—555 ℃. Type-2 graphite crystallized from COH fluid at approximately 2.7 GPa and 530 ℃ during eclogite-facies peak metamorphism. CH₄-bearing fluid inclusions associated with Type-2 graphite, alongside thermodynamic modeling, suggest the oxidative precipitation of carbon from CH₄ 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 light δ¹³C 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 CH₄ fluid migration under varying redox conditions.

Key words: graphitization, fluid-deposited graphite, organic carbon, COH fluid, southwestern Tianshan, deep subduction zone carbon cycle

中图分类号:

胡晗, 张立飞, 彭卫刚, 兰春元, 刘志成. 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[J]. 地学前缘, 2024, 31(6): 282-303.

HU Han, ZHANG Lifei, PENG Weigang, LAN Chunyuan, LIU Zhicheng. Formation of graphite in ultrahigh-pressure pelitic schists from the southwestern Tianshan: Implications for carbon migration and sequestration in subduction zones[J]. Earth Science Frontiers, 2024, 31(6): 282-303.

图/表 11

图1 中国西南天山高压—超高压变质带地质图 (据文献[20]修改) a—中国天山西部的简化构造格架;b—中国西南天山剖面示意图,显示塔里木板块向北俯冲到中天山板块之下,以及超高压和高压亚带的相对位置;c—中国西南天山高压—超高压变质带简化地质图及采样点分布,采样点1有H1811D、H1811H、H1814B和H1817样品,采样点2有H1813B样品,采样点3有H1713-6样品,采样点4有H1840、H1842和H1845样品。

Fig.1 Geological map of the high-pressure-ultrahigh-pressure metamorphic belt in the southwestern Tianshan, China. Modified after [20].

图2 西南天山含Type-1石墨的泥质片岩的代表性手标本照片、显微照片及石榴石中的柯石英、石墨包裹体 (据文献[69]) a, b—含Type-1石墨(Gph)的泥质片岩的代表性手标本照片和薄片扫描图,Type-1石墨呈浸染状分布于岩石中;c—含Type-1石墨的泥质片岩的代表性显微照片,主要由多硅白云母(Phn)、石英(Qz)和石榴石(Grt)组成,多硅白云母、石英和石榴石中含有微量石墨;d—Type-1石墨包裹体沿石榴石的生长环带发育;e—具有放射状裂纹特征的石榴石及其中包裹的石墨和柯石英(Coe);f—石榴石中的柯石英和石墨包裹体;g—石榴石中柯石英的拉曼光谱。Ttn—榍石;Rt—金红石。

Fig.2 Representative hand specimen photographs and photomicrographs of the Type-1 graphite-bearing pelitic schists from the southwestern Tianshan, and the coesite and graphite inclusions in garnet. Adapted from [69].

图3 西南天山含Type-2石墨的泥质片岩的代表性手标本照片和显微照片 (据文献[70]) a, b—含Type-2石墨(Gph)的泥质片岩的代表性手标本照片和薄片扫描照片;c—条带状定向的Type-2石墨和多硅白云母(Phn)确定了岩石的片理方向;d—f—共生的Type-2石墨和金红石(Rt);e—石榴石(Grt)覆盖了先前存在的Type-1石墨+金红石组合;b—f—平面单偏振光照片。Ap—磷灰石;Qz—石英;Py—黄铁矿;Pg—钠云母;Czo—斜黝帘石。

Fig.3 Representative hand specimen photographs and photomicrographs of the Type-2 graphite-bearing pelitic schists from the southwestern Tianshan. Adapted from [70].

图4 西南天山含Type-2石墨的泥质片岩中流体包裹体的岩相学特征及拉曼光谱特征 (据文献[70]) a—c—含石墨的石榴石中纯气相 (CH4+ N2+ CO2) 流体包裹体;d, e—含Type-2石墨的泥质片岩中与石榴石斑晶相邻的含石墨石英中纯气相 (CH4+ N2+ CO2) 流体包裹体;f, g—气液两相(富液相少气相,均为H2O + N2)流体包裹体。需要注意,图d和图f的视域相同,但聚焦深度不同。矿物代号含义同前图。

Fig.4 Petrographic characteristics and Raman spectra of fluid inclusions in Type-2 graphite-bearing pelitic schists from the southwestern Tianshan. Adapted from [70].

图5 西南天山含石墨泥质片岩中Type-1石墨和Type-2石墨的晶体及集合体的形貌特征(d—f据文献[70]) Dol—白云石;其他矿物代号含义同前图。

Fig.5 Morphologies of Type-1 and Type-2 graphite crystals and aggregates in graphite-bearing pelitic schists from the southwestern Tianshan (d—f adapted from [70])

图6 含石墨泥质片岩中两类石墨的代表性拉曼光谱及每类石墨对应的R2值直方图 (据文献[70])

Fig.6 Representative Raman spectra of the two types of graphite in the graphite-bearing pelitic schists and histograms of the corresponding R2 values for each graphite type. Adapted from [70].

表1 中国西南天山泥质片岩中碳酸盐和石墨的稳定同位素分析及有机碳含量 (据文献[69-70])

Table 1 Stable isotope analysis of carbonates and graphite, and organic carbon content in pelitic schists from the southwestern Tianshan, China. Adapted from [69-70].

类型	样品编号	δ¹³C_TOC/‰	TOC质量分数/%	δ¹³C_TIC/‰	δ¹⁸O/‰
含Type-1石墨泥质片岩	H1811D	-24.26	0.75	<b.d.l.	<b.d.l.
	H1813B	-23.15	0.23	-6.52	14.17
	H1811H	-23.58	1.07	<b.d.l.	<b.d.l.
	H1817	-24.14	1.24	-10.67	11.48
含Type-2石墨泥质片岩	H1842	-12.48	0.42	-6.76	15.80
	H1840-2	-14.33	0.11	-6.71	10.78
	H1840-1	-14.78	0.06	-6.63	10.65
	H1821-2	-14.48	1.64	-7.03	12.52
	H1845	-13.48	0.65	-5.00	18.78
	H1845(重复样)	-13.37	0.63	<b.d.l.	<b.d.l.
	H1713-6	-14.55	0.49	-7.03	12.45
	H1713-6(重复样)	-14.51	0.51	<b.d.l.	<b.d.l.
含石榴石云母片岩	H1814B	-23.26	0.12	-11.31	14.19

图7 两类石墨的δ13CTOC值和总有机碳(TOC)质量分数,以及含石墨泥质片岩中碳酸盐的δ13CTIC和δ18O值 (据文献[70]) a—含石墨泥质片岩中两种类型石墨的δ13CTOC值和总有机碳(TOC)质量分数。为了比较,我们还加上了中国西南天山贫石墨的石榴石云母片岩(样品H1814B)和含脉状石墨基性榴辉岩[38]中石墨的δ13CTOC值和TOC质量分数;b—与地幔碳[31]、碳酸盐[9,11,38]和有机碳[74]等不同碳源的石墨碳同位素组成对比;c—含石墨泥质片岩中石墨的δ13CTOC值和碳酸盐的δ13CTIC值;d—含石墨泥质片岩中碳酸盐的δ13CTIC和δ18O值。

Fig.7 δ13CTOC values and total organic carbon (TOC) mass fractions for the two types of graphite, and δ13CTIC and δ18O values for carbonates in graphite-bearing pelitic schists. Adapted from [70].

图8 含Type-1和Type-2石墨的泥质片岩的p—T视剖面图 (据文献[69-70]) a, b—含Type-1石墨的泥质片岩H1813B的p—T相图与石榴石、多硅白云母的等值线[69];c, d—含Type-2石墨的泥质片岩H1842的p—T相图与石榴石、多硅白云母、金红石的等值线。体系为NCKFMASHTi+CO2(+quartz/coesite + rutile + F);石榴石中的XPrp、XGrs和XAlm等值线分别用绿色、粉色和紫色虚线绘制;灰色曲线为H1813B和H1842折返的p—T轨迹;金刚石—石墨转变线引自文献[76]。

Fig.8 p—T pseudosections for the investigated Type-1 and Type-2 graphite-bearing UHP pelitic schists. Adapted from [69-70].

图9 C—O—H三元图展示了石墨饱和曲线;p—XO图展示了碳饱和COH流体的原子碳含量和氧逸度(据文献[70]) a—温度为500 ℃时的C—O—H三元图,显示了0.1、0.5、1.0、2.0和2.7 GPa时石墨的饱和曲线。请注意,2.7 GPa和2.0 GPa的石墨饱和曲线几乎重叠。带黑边的虚线圈表示碳饱和表面对应的氧逸度。b, c—p—XO图显示碳饱和COH流体的碳原子摩尔分数和氧逸度关系,XO=n(O)/[n(O)+n(H)],在27 GPa和500 ℃条件下,富CH4还原性流体的氧逸度增加会导致石墨沉淀,同时流体碳含量降低。

Fig.9 A C—O—H ternary diagram showing the graphite saturation curve, and p—XO diagrams showing the atomic carbon content and oxygen fugacity of carbon-saturated COH fluids. Adapted from [70].

图10 俯冲带中有机碳的命运示意图(据文献[69]) 俯冲带海相沉积物中的无序有机物(organic matter,OM,δ13COM<-20‰)会随着温度的升高而部分或完全石墨化。本研究的结果表明,这些来自洋壳沉积物的无序有机碳俯冲到弧下深度(约90 km,柯石英稳定域),形成了δ13COM为-24‰的石墨。地表有机碳的这种俯冲作用可能有助于形成δ13C值较轻的蛇绿岩型金刚石(δ13C峰值为-25‰)。

Fig.10 Schematic diagram of the fate of organic carbon in a subduction zone. Adapted from [69].

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