Mineralogical determination, isotopic compositions and Strontium isotope ratios from an outcrop near Kardio village, Xigaze in Tibet

Seep carbonates provide excellent records of past seepage activities, and have been commonly considered to preserve primary, unaltered stable carbon isotope signatures. However, late diagenetic reactions may overprint original isotopic compositions, but the mode and effect of such alterations are poorly understood. In particular, there are significant uncertainties regarding how carbon and strontium isotopic compositions of seep carbonates respond to diagenesis. This study reports recently discovered Cretaceous hydrocarbon-seep deposits from the Yarlung-Zangbo Suture Zone, Tibet, China that have experienced substantial diagenetic alteration that is shown by recrystallization and secondary veins. Unitary linear recursive analysis was applied to d13C values and 87Sr/86Sr ratios of the seep carbonates to evaluate the degree of secondary modification and to quantitatively constrain the compositions of primary carbonates and late diagenetic fluids. The d18O values range from - 11.8 per mil to - 2.2 per mil, d13C values from - 34.1 per mil to - 12.9 per mil and 87Sr/86Sr ratios from 0.706221 to 0.706808. The heterogeneity in isotopic compositions and the observation that the most negative d18O values occur in samples with the most extensive recrystallization indicate significant and spatially heterogeneous modification of isotope compositions during late diagenesis. The linear correlations between d13C values and d18O values for matrix micrites (R2 = 0.54), and between bulk carbonate 87Sr/86Sr ratios and d18O values (R2 = 0.85) are best explained by burial diagenetic overprinting of oxygen, strontium, and even carbon isotopic compositions rather than by meteoric water hypergenesis. Extrapolated values of d13C and ratios of 87Sr/86Sr against a d18O value of - 2 per mil (average value of calcite precipitated in isotopic equilibrium with coeval Cretaceous seawater) that would characterize the primary carbonate, give an end member d13C value of - 34 per mil and an end member 87Sr/86Sr ratio of 0.7072. The end member isotopic values obtained by this extrapolation suggest that the primary seep carbonates with low d13C values and high 87Sr/86Sr ratios were formed by anaerobic oxidation of methane near the seafloor. In contrast, the measured d18O values and 87Sr/86Sr ratios reflect late diagenetic fluids represented by burial pore water characterized by a low 87Sr/86Sr ratio and high temperature. Our findings reveal that d13C values can only be moderately and 87Sr/86Sr ratios can be significantly altered during late diagenesis, and show that it is possible to quantitatively assess the primary composition of diagenetically altered seep carbonates.

冷泉碳酸盐岩（seep carbonates）是记录古渗漏活动的优质载体，此前普遍认为其能够保存原始、未蚀变的稳定碳同位素组成。然而，晚期成岩作用可能会改造原始同位素组成，但此类蚀变的作用模式与影响机制仍不甚明晰。尤为关键的是，冷泉碳酸盐岩的碳、锶同位素组成如何响应成岩作用，目前仍存在显著不确定性。本研究报道了中国西藏雅鲁藏布江缝合带新近发现的白垩纪烃类冷泉沉积，该套沉积经历了强烈的成岩蚀变，具体表现为重结晶作用与次生脉体发育。研究采用一元线性递归分析方法，对冷泉碳酸盐岩的δ13C值与87Sr/86Sr比值进行分析，以评估次生蚀变程度，并定量约束原始碳酸盐与晚期成岩流体（diagenetic fluids）的组成。该套碳酸盐岩的δ18O值范围为-11.8‰至-2.2‰，δ13C值范围为-34.1‰至-12.9‰，87Sr/86Sr比值范围为0.706221至0.706808。同位素组成的非均质性，以及δ18O值最负的样品对应重结晶作用最强的样品这一观测结果，均表明晚期成岩作用过程中同位素组成发生了显著且空间非均质性的改造。基质微晶碳酸盐（matrix micrites）的δ13C值与δ18O值之间存在线性相关关系（决定系数R²=0.54），且块状碳酸盐的87Sr/86Sr比值与δ18O值之间线性相关程度更高（R²=0.85），上述现象最适宜用埋藏成岩作用（burial diagenesis）对氧、锶乃至碳同位素组成的叠加改造来解释，而非大气水表生蚀变（meteoric water hypergenesis）。以与同期白垩纪海水同位素平衡的方解石的平均δ18O值（-2‰）为基准，对δ13C值与87Sr/86Sr比值进行外推以表征原始碳酸盐组成，得到的端元（end member）δ13C值为-34‰，端元87Sr/86Sr比值为0.7072。通过该外推方法得到的端元同位素数值表明，具有低δ13C值与高87Sr/86Sr比值的原始冷泉碳酸盐岩，形成于海底附近的甲烷厌氧氧化（anaerobic oxidation of methane）过程。与之相反，实测的δ18O值与87Sr/86Sr比值则反映了晚期成岩流体的特征，该类流体以低87Sr/86Sr比值与高温为特征的埋藏孔隙水为代表。本研究结果表明，δ13C值在晚期成岩作用过程中仅发生中等程度的改造，而87Sr/86Sr比值则会发生显著改变；同时也证实了，对经历成岩蚀变的冷泉碳酸盐岩的原始组成进行定量评估是可行的。