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

doi:10.13745/j.esf.sf.2024.6.80

Abstract

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

CLC Number:

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.

Figures/Tables 11

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

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].

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

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

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])

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].

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

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].

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

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].

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

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