Marine dolomitization processes are characterized by complex variations in hydrological conditions and pore-fluid chemistry. Deposition of massive dolomitized carbonates on the Comanche Platform of south Texas, USA, during the Albian coincided with multiple fluctuations in sea level, thereby providing an ideal setting to study the response of magnesium isotopes to dolomitization during eustatic sea-level change. In this study, we conducted Mg, C, and O isotope analyses and complimentary mineralogical and petrographic investigations of dolomitized massive carbonates of the Albian Edwards Group in the Comanche Platform. Petrographic observations indicate that the carbonate rocks in the studied units were not altered by deep burial diagenesis or hydrothermal fluids. Based on our petrographic observations and the trace element and C–O isotope data, we infer the dolomites were formed via syn-depositional dolomitization during a period of low sea level. The δ²⁶Mg values of the dolomites increase rapidly from −2.5‰ to −1.8‰ in the basal part of the unit, reflecting a change in fluid chemistry caused by dolomitization in a restricted marine environment. Subsequently, δ²⁶Mg_dolomite values gradually decrease back to their initial value of approximately −2.5‰ due to seawater replenishment during transgression. The observed variability in dolomite δ²⁶Mg values reflects changes in the connectivity of the platform with the open ocean during marine transgressions. However, the δ²⁶Mg_dolomite values do not vary with the high-frequency eustatic sea-level change recorded in the lithological variations, indicating uniform hydrologic conditions of the massive dolomitization system despite the hydrodynamic variations in the sedimentary environment. Therefore, we propose that massive dolomitization systems are mostly fluid-buffered and, as a result, Mg isotopes of dolomites can be used to trace changes in the paleo-marine environment, such as basin connectivity.

海洋白云石化过程的特征是水文条件和孔隙流体化学的复杂变化。在Albian期间，美国得克萨斯州南部的Comanche平台上沉积大量的白云石化碳酸盐，这与海平面的多次波动相吻合，从而为研究镁同位素对欣喜的海平面变化过程中白云石化的响应提供了理想的环境。在这项研究中，我们对Comanche平台中Albian Edwards集团的白云石化大块碳酸盐进行了Mg，C和O同位素分析，以及免费的矿物学和岩石学研究。岩相学观察表明，深埋成岩作用或热液作用并没有改变研究单元中的碳酸盐岩。根据我们的岩石学观测以及痕量元素和C–O同位素数据，我们推断白云岩是在低海平面时期通过同沉积白云石化作用形成的。δ在单元底部，白云石的²⁶ Mg值从-2.5‰迅速增加到-1.8‰，反映出在受限的海洋环境中白云石化引起的流体化学变化。随后，δ ^26个镁_白云石值侵期间逐渐减小回到它们的大约-2.5‰初始值由于海水补水。在白云石δ所观察到的变化^26个镁值反映在海侵与开放的海洋平台的连接性的变化。然而，δ ²⁶镁_白云石数值不随岩性变化中记录的高频自然海平面变化而变化，这表明尽管沉积环境中存在水动力变化，但大块白云石化系统的水文条件是一致的。因此，我们提出大规模白云石化系统主要是流体缓冲的，因此，白云岩的Mg同位素可用于追踪古海洋环境的变化，例如盆地连通性。