The magnesium isotope composition of diagenetic dolomites from ODP Hole 112-685A and 201-1230A of the Peru Margin@en

The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250 m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~ 6 m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~ 230 mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid delta 26Mg composition becomes progressively enriched in 26Mg with depth from values similar to seawater (i.e. -0.8 per mil, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8 ± 0.2 per mil within the sediments located below 100 mbsf. Type I dolomites have a delta 26Mg of -3.5 per mil, and exhibit apparent dolomite-pore fluid fractionation factors of about -2.6 per mil consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have delta 26Mg values ranging from -2.5 to -3.0 per mil and are up to -3.6 per mil lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in 26Mg and depletion in total Mg concentration below ~ 200 mbsf is likely the result of Mg isotope fractionation during dolomite formation, The 26Mg enrichment of pore fluids in the upper ~ 200 mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.本研究针对秘鲁边缘海域钻取的250米厚沉积剖面，分析了其中成岩白云岩及其邻近孔隙流体的镁同位素组成。该剖面取自大洋钻探计划（Ocean Drilling Program, ODP）201航次1230站位与112航次685站位。既往研究已识别出两类白云石：I型白云石形成于海底以下约6米（meters below seafloor, mbsf）处，其成因与厌氧甲烷氧化作用引发的碱度升高相关；II型白云石则形成于约230 mbsf以深的富集位点，源于深部流体间歇性注入强甲烷生成带。孔隙流体的δ²⁶Mg组成随深度逐渐富集²⁶Mg：从海底以下顶层数米处接近海水的数值（相对于DSM3镁标准参考物质为-0.8‰），升高至100 mbsf以深沉积物中的0.8 ± 0.2‰。I型白云石的δ²⁶Mg值为-3.5‰，其表观白云石-孔隙流体分馏因子约为-2.6‰，与此前海水沉淀白云石的研究结果一致。与之相对，II型白云石的δ²⁶Mg值介于-2.5‰至-3.0‰之间，较现代孔隙流体的镁同位素组成轻最高达3.6‰。约200 mbsf以深的孔隙流体既发生²⁶Mg富集，又伴随总镁浓度降低，这一现象大概率源于白云石形成过程中的镁同位素分馏作用；而约200 mbsf以浅的沉积序列中孔隙流体的²⁶Mg富集，则可归因于黏土矿物表面镁的解吸作用。本研究结果表明，成岩碳酸盐岩记录的镁同位素信号可有效区分近地表与深部形成的白云石，证明其作为古成岩代用指标的应用价值。