俯冲带碳酸盐化对深部碳循环的启示:以中国西南天山碳酸盐化云母片岩为例

Implications for the deep carbon cycle from the carbonation in subduction zones:A case study of carbonated micaschists from Chinese southwestern Tianshan

俯冲带可将地球表层碳输送至深部地幔,同时也记录着俯冲板片来源碳质流体的迁移沉淀机制,对地球深部碳循环具有重大影响。近年来,俯冲带脱碳机制的研究表明流体溶解脱碳作用是冷的大洋俯冲板片释放COH流体的重要方式,而上覆板块(尤其地幔楔)则被认为是缓冲这些COH流体的重要场所,甚至是俯冲带CO_2的唯一"归宿"。事实上,俯冲带岩石本身的固碳能力却受到了忽视,而对俯冲带岩石捕获和固存CO_2(carbon capture and storage,CCS)能力的评估对全球碳通量的估算尤为重要。本文以中国西南天山高压-超高压变质带中碳酸盐化云母片岩为例,探讨俯冲带岩石的碳酸盐化对深部碳循环的影响。西南天山长阿吾子一带的碳酸盐化云母片岩记录了俯冲板片起源的碳质流体对俯冲带云母片岩的交代作用,地球化学特征表明蛇纹岩释放的富水流体溶解俯冲洋壳中的碳酸盐可能是产生COH流体的重要机制。基于碳质流体对多硅白云母(Si(a.p.f.u.)=3.58～3.73)的交代及相对高压的碳酸盐矿物(主要为白云石和菱镁矿)与金红石的共生,结合区域上碳酸盐化云母片岩与高压碳酸盐化蛇纹岩(HP-ophidolomite)的伴生,我们认为云母片岩的碳酸盐化作用可能发生在俯冲板片峰期稍后的高压折返阶段。俯冲带云母片岩的固碳作用表明除了上覆板块,俯冲带岩石本身对于碳质流体也具有很好的吸收能力。初步估算表明俯冲带云母片岩的碳酸盐化每年可固存至少2.46～6.68Mt/yr,约占俯冲板片每年进碳量的4%～17%。

Subduction zones play a pivotal role in the deep carbon cycle-they carry carbon from the surface into the deep mantle and document mechanisms of mobility and precipitation of carbonic fluids from subducted slabs. Recent researches on the decarbonation in subduction zones indicated that fluid-induced dissolution of carbonates in cold oceanic slabs might be an essential mechanism for the release of COH fluids. Nevertheless,the overlying plate,in particular the mantle wedge,has been considered as an important,or even the only reservoir to buffer the liberated C-bearing fluids. As a matter of fact,the potential of carbon capture and storage（ CCS） in lithologies from subduction zones,which has long been overlooked,could actually influence the estimates of global carbon fluxes.Here,we report carbonated micaschists from Changawuzi in the Chinese southwestern Tianshan HP-UHP metamorphic belt to investigate the significance of the carbonation of lithologies in subduction zones. The carbonated micaschists have been formed by the interaction between micaschists and COH fluids derived from the subducted slab. Geochemical signatures suggest the dissolution of carbonates in the subducted oceanic crust,a process induced by aqueous fluids from the dehydration of serpentinites,may be responsible for the formation of COH fluids. We speculate that the carbonation occurs at a HP stage during exhumation,after peak metamorphism,based on the carbonation of phengite（ Si（ a. p. f. u.） = 3. 58 - 3. 73） and the association of HP-carbonates（ mainly dolomite and magnesite） with rutile, in accordance with the PT conditions of associated HP carbonated serpentinite（ HPophidolomite）. The sequestration of CO2 in carbonated micaschists implies that the lithologies in subduction zones,other than the overlying plate,could also be exerted as potential CO2 sequestrators,with an evaluated C storage of at least 2. 46 - 6. 68Mt/yr accounting for nearly 4% - 17% of the total carbon influx in the downgoing slab.