鲕粒原生矿物识别及对海水化学成分变化的指示意义

鲕粒是碳酸盐沉积过程中一类非常特殊的颗粒类型, 为研究当时的沉积背景、水动力条件、气候环境, 甚至储层特征提供了重要线索。然而, 鲕粒的矿物组成及控制因素问题, 长期受到忽视。组成鲕粒的原生矿物类型在地质历史时期呈周期性变化, 在显生宙表现为三个以文石和高镁方解石占主导的时期以及两个以低镁方解石占主导的时期, 这也被称作“文石海”和“方解石海”时期。原生矿物的组成, 制约着鲕粒的纹层结构、保存程度以及成岩特征, 还蕴含着海水化学成分变化的线索。鲕粒原生矿物识别主要依据:①原生纹层结构;②保存程度;③微量元素浓度, 尤其是Sr-Mg的浓度。文石质鲕粒受文石不稳定性的影响, 原生结构保存程度较差;一般保存有典型的文石残余纹层结构(例如砖砌结构、溶解变形结构以及偏心结构等);在封闭成岩环境下原生矿物为文石质的鲕粒Sr浓度往往大于2 000 ppm;纹层结构主要为切线状(占主导)和放射状。方解石质鲕粒包括低镁方解石和高镁方解石两种类型:低镁方解石为稳定矿物, 原生结构一般保存良好。尽管高镁方解石也为亚稳定矿物, 但成岩转换后的保存程度好于文石。两者Sr含量一般均低于1 000 ppm, Mg含量一般在0~20 mol % MgCO₃(两者以4 mol % MgCO₃为界)。高镁方解石受成岩作用影响, 在纹层中往往保留有微粒白云石包裹体;海相地层中保存的方解石质鲕粒为放射状或同心-放射状结构。另外还存在一类由两种矿物共同构成的双矿物鲕粒, 可以通过分析两类纹层在结构和保存特征上的差异进行区分。鲕粒原生矿物成分随时间的波动变化受到海水化学条件, 尤其是Mg/Ca比值, 大气二氧化碳分压以及碳酸盐饱和度的控制。Mg/Ca比值的波动决定着鲕粒原生矿物类型的长期变化规律。一些突发性事件可能会扰动(区域)短时间尺度下鲕粒原生矿物的组成, 造成鲕粒原生矿物的转换。通过研究碳酸盐鲕粒原生矿物特征以及控制因素进而了解海水的化学特征, 是独立于古生物学和地球化学分析之外的一种较为可靠的沉积学方法。

Identification of Ooid Primary Mineralogy: A clue for understanding the variation in paleo-oceanic chemistry

As one type of coated grains in the process of carbonate deposition, ooids and oolite are usually used as proxies for sedimentary background, hydrodynamic condition, paleoclimate, hydrocarbon reservoir, etc. In last two centuries, numerous studies were carried out promoting the understanding of such “charming” carbonate grains. However, the mineral composition and controlling factors of ooid have been neglected in Chinese literatures for a long time. The CaCO₃ polymorph mineral of ooid had been undergone secular periodic variations in geological history. In Phanerozoic time, three “aragonite and high-Mg calcite” dominated intervals (also called “aragonite sea” periods) and two “low-Mg calcite” dominated intervals (also called “calcite sea” periods) were identified firstly in the light of the mineralogical differences between original aragonitic (with high-Mg calcitic) and low-Mg calcitic ooids. The primary mineralogy of ooids implies the great influence on cortical fabric, degree of conservation, as well as diagenetic history, and may further provide a clue for understanding the variation in paleo-oceanic chemistry. Interpretation of the original mineralogy of ooid cortices requires assessment of (1) morphology and orientation of crystallites in cortices, (2) manner of preservation of cortical layers, and (3) elemental concentration data, especially of Sr and Mg. Primary aragonite ooids exhibit poorly preserved morphologies and recrystallization of cortical fabrics, remaining relics with selective diagenetic dissolution fabrics with respect to aragonite, and high concentrations of Sr (generally >2 000 ppm); Primary high-Mg calcite ooids generally exhibit low-level deformation replacement of radial (or radial-concentric) fabrics, low concentrations of Sr (generally <1 000 ppm) and high values of Mg, sometimes containing microdolomite inclusions in cortical layers; Primary low-Mg calcite ooids exhibit well-preserved radial (or radial-concentric) fabrics, low concentrations of Sr and Mg. In addition, the composition of bimineralic ooids presumably has two patterns, one is consisted of calcitic inner cortex and aragonitic outer cortex, the other is alternated between calcitic and aragonitic layers. Manner of differences in fabrics and preservation styles could help us infer the bimineralic type. It is noted that the original mineralogy of ooid is thought to be strongly influenced by seawater chemistry, especially by Mg/Ca ratio, atmospheric CO₂ partial pressure and carbonate saturation state. The Mg/Ca ratio plays a critical role in determining the original mineralogy of ooids in a long timescale. In some temporal evens, however, suggested other factors may induce the fluctuation of primary ooid mineralogy with the stabilized ratio of Mg/Ca. For instance, glacio-eustatic cycles caused temperature fluctuations (associated with CO₂ fluctuations) and lately supposed to disturb the carbonate saturation state, extremely arid climate might increase local carbonate saturation state, and the volcanism released a large quantity of CO₂ that elevated temperature and atmospheric CO₂ partial pressure and finally causing the decreased in carbonate saturation state. Therefore, the study on features and controlling factors of original ooid mineralogy may provide a new perspective to understand the paleo-oceanic chemistry.