Diagenetic alteration affecting δ18O, δ13C and 87Sr/86Sr signatures of carbonates: A case study on Cretaceous seep deposits from Yarlung-Zangbo Suture Zone, Tibet, China
Diagenetic alteration affecting δ18O, δ13C and 87Sr/86Sr signatures of carbonates: A case study on Cretaceous seep deposits from Yarlung-Zangbo Suture Zone, Tibet, China
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影响碳酸盐岩δ18O、δ13C和87Sr/86Sr特征的成岩蚀变：以中国西藏雅鲁藏布江缝合带白垩纪渗流矿床为例
DOI：
10.1016/j.chemgeo.2016.10.003
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发表时间：
2016
期刊：
影响因子：
3.9
通讯作者：
Duofu Chen
中科院分区：
文献类型：
--
作者：
Hongpeng Tong;Qinxian Wang;Jorn Peckmann;Yuncheng Cao;Linying Chen;Weide Zhou;Duofu Chen
渗碳酸盐提供了过去渗流活动的良好记录，通常被认为保存了原始的、未改变的稳定碳同位素特征。然而，晚期成岩反应可能叠加了原始同位素组成，但这种改变的模式和效果尚不清楚。特别是，关于渗碳酸盐的碳和锶同位素组成如何响应成岩作用，存在很大的不确定性。通过δ13C值和87sr /86Sr比值的一元线性递推分析，评价了次生变质程度，定量约束了原生碳酸盐岩和晚期成岩流体的组成。δ18O值为−11.8‰~−2.2‰，δ13C值为−34.1‰~−12.9‰，87sr /86Sr比值为0.706221 ~ 0.706808。同位素组成的非均质性和δ18O负值在重结晶最广泛的样品中出现的观察结果表明，成岩晚期同位素组成发生了明显的空间非均质性改变。基质泥晶δ13C值与δ18O值之间的线性相关性（R2= 0.54），以及块状碳酸盐87sr /86Sr比值与δ18O值之间的线性相关性（R2= 0.85）最好的解释是埋藏成岩叠印的氧、锶甚至碳同位素组成，而不是大气水的热成作用。外推δ13C值和87sr /86Sr比值与δ18O值（同时期白垩纪海水同位素平衡中沉淀方解石的平均值）的比值为- 2‰，表明原生碳酸盐的端元δ13C值为- 34‰，端元87sr /86Sr比值为0.7072。外推得到的端元同位素值表明，低δ13C值和高87sr /86Sr比值的原生渗碳酸盐是由海底附近甲烷厌氧氧化形成的。实测δ18O值和87sr /86Sr比值反映了以低87sr /86Sr比和高温为特征的埋藏孔隙水为代表的晚期成岩流体。研究结果表明，在成岩晚期，δ13C值只能发生中等程度的变化，87sr /86Sr比值发生显著变化，表明可以定量评价成岩蚀变渗碳酸盐岩的主要成分。
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 δ13C values and87Sr/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 δ18O values range from − 11.8‰ to − 2.2‰, δ13C values from − 34.1‰ to − 12.9‰ and87Sr/86Sr ratios from 0.706221 to 0.706808. The heterogeneity in isotopic compositions and the observation that the most negative δ18O 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 δ13C values and δ18O values for matrix micrites (R2= 0.54), and between bulk carbonate87Sr/86Sr ratios and δ18O 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 δ13C and ratios of87Sr/86Sr against a δ18O value of − 2‰ (average value of calcite precipitated in isotopic equilibrium with coeval Cretaceous seawater) that would characterize the primary carbonate, give an end member δ13C value of − 34‰ and an end member87Sr/86Sr ratio of 0.7072. The end member isotopic values obtained by this extrapolation suggest that the primary seep carbonates with low δ13C values and high87Sr/86Sr ratios were formed by anaerobic oxidation of methane near the seafloor. In contrast, the measured δ18O values and87Sr/86Sr ratios reflect late diagenetic fluids represented by burial pore water characterized by a low87Sr/86Sr ratio and high temperature. Our findings reveal that δ13C values can only be moderately and87Sr/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.