【摘要】 "白云岩问题"一直是碳酸盐岩研究的热点问题。镁作为白云石的主量元素之一,其同位素组成包含了白云岩化过程和白云岩沉积环境的信息。本文以以色列黎凡特盆地早白垩世末阿尔布期（～100 Ma）Givat Ye’arim组和Soreq组白云岩为研究对象,在岩石学、微量元素和C-O-Sr同位素地球化学研究基础之上,开展了高密度镁同位素分析。结果显示白云岩主要由半自形—自形的泥微晶白云石组成,岩性致密,后期成岩改造作用较弱,因此元素及同位素组成基本可以反映原始白云岩化流体的地球化学特征;C-Sr同位素组成与Albian期海水保持一致,表明同期咸化海水是白云岩化流体并为白云石形成提供镁离子;白云岩δ²⁶Mg值稳定在-2.0‰左右,没有明显的垂向差异,表明白云岩化过程中咸化海水供应充足,流体以渗流方式而不是以扩散方式在松散沉积物中运移,孔隙水化学组成相对均一,白云岩镁同位素组成得以与同期海水相平衡。白云岩中陆源风化指示元素Rb和ΣREE以及海水盐度指标Na元素含量表现出多期旋回性变化,反映半封闭体系对周期性气候和环境变化的响应,然而垂向上稳定的白云岩δ²⁶Mg值表明区域性的气候变化不会显著改变沉积盆地体系中的镁同位素组成,因此对于地质历史时期在半局限海洋环境中沉积的巨厚白云岩而言,其镁同位素组成可以用来示踪全球海水镁同位素波动。更多还原

【Abstract】 The "dolomite problem" is one of the long-lasting problems in sedimentology. Magnesium is a major element in dolomite, and its isotopic compositions may be used to infer the dolomitization processes and sedimentary environments. In this study, we investigated the Mg isotope compositions of dolostones of the Albian Givat Ye’arim Formation and Soreq Formation of Albian from a section near Jerusalem, Israel. The carbonates are mainly composed of subhedreal to euhedral micritic dolomite without notable post-depositional diagenetic alterations. Carbon and strontium isotope compositions of the dolomite are consistent with the Albian seawater, suggesting that the dolomitizing fluid was sourced from the contemporaneous seawater. δ²⁶Mg values of dolomite cluster around-2.0‰ and do not show a clear correlation with depth. Combining Mg isotope and C-Sr isotope compositions of dolomite, it is suggested that the Mg supply from seawater for dolomitization was abundant, leading to Mg isotope equilibrium between seawater and dolomite. The rhythmic fluctuations in Na, Rb and ΣREE contents in the dolomite sequence indicate responses of marine environments in a semi-closed basin to variations in regional paleo-climates and paleo-environments. By contrast, the relatively homogeneous dolomite δ²⁶Mg values across the sedimentary beddings imply that the regional climate and environment changes had limited impacts on Mg isotope compositions of seawater in local basins. Hence, for massive dolomite that was deposited in partly restricted basins, their δ²⁶Mg values likely still trace the Mg isotope composition of seawater in open oceans.更多还原