C and H geochemistry of altered oceanic crus of ODP/IODP Hole 1256D

Carbon and hydrogen concentrations and isotopic compositions were measured in 19 samples from altered oceanic crust cored in ODP/IODP Hole 1256D through lavas, dikes down to the gabbroic rocks. Bulk water content varies from 0.32 to 2.14 wt% with dD values from -64per mil to -25per mil. All samples are enriched in water relative to fresh basalts. The dD values are interpreted in terms of mixing between magmatic water and another source that can be either secondary hydrous minerals and/or H contained in organic compounds such as hydrocarbons. Total CO2, extracted by step-heating technique, ranges between 564 and 2823 ppm with d13C values from -14.9per mil to -26.6per mil. As for water, these altered samples are enriched in carbon relative to fresh basalts. The carbon isotope compositions are interpreted in terms of a mixing between two components: (1) a carbonate with d13C = -4.5per mil and (2) an organic compound with d13C = -26.6per mil. A mixing model calculation indicates that, for most samples (17 of 19), more than 75% of the total C occurs as organic compounds while carbonates represent less than 25%. This result is also supported by independent estimates of carbonate content from CO2 yield after H3PO4 attack. A comparison between the carbon concentration in our samples, seawater DIC (Dissolved Inorganic Carbon) and DOC (Dissolved Organic Carbon), and hydrothermal fluids suggests that CO2 degassed from magmatic reservoirs is the main source of organic C addition to the crust during the alteration process. A reduction step of dissolved CO2 is thus required, and can be either biologically mediated or not. Abiotic processes are necessary for the deeper part of the crust (>1000 mbsf) because alteration temperatures are greater than any hyperthermophilic living organism (i.e. T > 110 °C). Even if not required, we cannot rule out the contribution of microbial activity in the low-temperature alteration zones. We propose a two-step model for carbon cycling during crustal alteration: (1) when "fresh" oceanic crust forms at or close to ridge axis, alteration starts with hot hydrothermal fluids enriched in magmatic CO2, leading to the formation of organic compounds during Fischer-Tropsch-type reactions; (2) when the crust moves away from the ridge axis, these interactions with hot hydrothermal fluids decrease and are replaced by seawater interactions with carbonate precipitation in fractures. Taking into account this organic carbon, we estimate C isotope composition of mean altered oceanic crust at ? -4.7per mil, similar to the d13C of the C degassed from the mantle at ridge axis, and discuss the global carbon budget. The total flux of C stored in the altered oceanic crust, as carbonate and organic compound, is 2.9 ± 0.4 * 10**12 molC/yr.

本次研究对大洋钻探计划（Ocean Drilling Program, ODP）/综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）1256D钻孔取获的19件蚀变洋壳样品开展了碳、氢浓度与同位素组成分析，样品涵盖熔岩、岩墙至辉长岩类岩石。样品总含水量介于0.32至2.14 wt%之间，氘同位素（δD）值范围为-64‰至-25‰。相较于新鲜玄武岩，所有蚀变样品均富集水分。针对氘同位素组成的解译表明，其源于岩浆水与另一端元的混合，该端元可为次生含水矿物，或是烃类等有机化合物中赋存的氢。通过阶梯加热技术提取的总二氧化碳（CO₂）含量介于564至2823 ppm之间，碳同位素（δ¹³C）值范围为-14.9‰至-26.6‰。与水的特征一致，蚀变样品的碳含量同样高于新鲜玄武岩。碳同位素组成可通过两端元混合模型进行解译：(1) δ¹³C为-4.5‰的碳酸盐端元，以及(2) δ¹³C为-26.6‰的有机化合物端元。混合模型计算结果显示，19件样品中的17件，其总碳中超过75%以有机化合物形式赋存，而碳酸盐占比不足25%。该结论得到了磷酸浸取后CO₂产率独立估算的碳酸盐含量数据的佐证。将本次研究样品的碳浓度与海水溶解无机碳（Dissolved Inorganic Carbon, DIC）、溶解有机碳（Dissolved Organic Carbon, DOC）及热液流体的碳浓度进行对比后可知，从岩浆储库脱气的CO₂是蚀变过程中洋壳新增有机碳的主要来源。因此，溶解态CO₂需要经历还原步骤，该过程可由生物介导，亦可不依赖生物作用。对于洋壳深部（>1000 海底以下米，meters below seafloor, mbsf），非生物过程是必要的，因为蚀变温度高于所有超嗜热生物的生存温度（即温度＞110 ℃）。即便并非必需，我们也无法排除微生物活动在低温蚀变带中的贡献。我们提出了洋壳蚀变过程中碳循环的两步模型：(1) 当“新鲜”洋壳在洋中脊轴部或其附近形成时，富含岩浆CO₂的高温热液流体开始发生蚀变作用，通过费托型反应生成有机化合物；(2) 当洋壳远离洋中脊轴部时，与高温热液流体的相互作用减弱，转而被海水与裂隙中碳酸盐沉淀的相互作用取代。考虑到该部分有机碳，我们估算得到平均蚀变洋壳的碳同位素组成为约-4.7‰，与洋中脊轴部地幔脱气产生的碳的δ¹³C值相近，并对全球碳预算展开了讨论。以碳酸盐和有机化合物形式赋存于蚀变洋壳中的碳总通量为2.9 ± 0.4 × 10¹² molC/yr。