Characteristics and origin of crystalline dolomite: A case from Paleogene lacustrine fine-grained rocks in Jiyang depression, Bohai Bay Basin, China

Abstract

Crystalline dolomite is a common carbonate mineral in lacustrine fine-grained rocks, but it has a confusing origin that has been a focus of much debate in recent years. In our research, the fine-grained rocks rich in crystalline dolomite were sampled from the upper fourth member (UMbr 4) and lower third member (LMbr 3) of the Shahejie Formation in the Jiyang Depression, and analyzed by a variety of analytical techniques, including field emission scanning electron microscopy (FESEM), cathodoluminescence (CL), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), micro-area carbon, oxygen, strontium isotopes and electron probe analyses. The results show: ①The crystalline dolomites in the lacustrine fine-grained rocks are primarily composed of ferroan-poor dolomite (FeO < 1%), type I ferroan dolomite (FeO ≈ 5%) and type II ferroan dolomite (FeO ≈ 16%); ②The δ18O of ferroan-poor dolomites in mudstone is between −0.89 and 0.32‰, the precipitation temperature is less than 60 °C, and their formation is related to the microbial action during the early diagenetic stage A, while the ferroan-poor dolomites in dolostone are primarily formed by the recrystallization of micritic dolomite; ③The δ18O of type I ferroan dolomites ranges from −3.74 to −3.89‰, the precipitation temperature ranges from 68.4 to 69.4 °C, and they are primarily formed by the microbial action and clay mineral transformation during the early diagenetic stage B; ④The δ18O of type II ferroan dolomites is between −7.78 and −8.33‰, the precipitation temperature is more than 90 °C, and their formation is related to the dissolution of early carbonate and clay mineral transformation during the middle diagenetic stage; ⑤The origin mechanism of crystalline dolomite indicates the diagenetic evolution of fine-grained rocks and the migration path and storage of shale oil.