Lower Paleozoic sedimentary rocks in the northwestern Tarim basin were strongly altered by complicated geofluids, which resulted in the occurrence of various diagenetic minerals (e.g., dolomite). Here, in situ major, trace, and rare earth element geochemistry of Lower Ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features, the nature, and origin of the diagenetic fluids. The results indicate that different geochemical information can be detected within a single sample, even within a single dolomite grain. Five generations of diagenetic dolomite have been identified based on geochemical signatures, resulting from four distinct types of diagenetic fluids: (1) HREE enrichment (PAAS‐normalized), low ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high ΣREE, high Mn, Fe, and low Sr content are likely to be associated with Devonian deep‐circulating crustal hydrothermal fluids; (3) flat or LREE enrichment pattern with obviously positive Eu anomaly is inferred to be linked to Permian magmatic hydrothermal fluids; and (4) flat REE pattern, moderate ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to late burial dolomitizing fluids. The significances of in situ method demonstrated in this study, compared with the whole rock analysis, include not only contamination‐free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain. Thus, for the geochemical study of complicated diagenetic process, in situ method should be preferentially considered.

The study on Lower Cambrian dolostones in Tarim Basin can improve our understanding of ancient and deeply buried carbonate reservoirs. In this research, diagenetic fluid characteristics and their control on porosity evolution have been revealed by studying the petrography and in situ geochemistry of different dolomites. Three types of diagenetic fluids were identified: (1) Replacive dolomites were deviated from shallow burial dolomitizing fluids, which might probably be concentrated ancient seawater at early stage. (2) Fine-to-medium crystalline, planar-e diamond pore-filling dolomites (Fd1) were likely slowly and sufficiently crystallized from deep-circulating crustal hydrothermal fluids during Devonian. (3) Coarse crystalline, non-planar-a saddle pore-filling dolomites (Fd2) might rapidly and insufficiently crystallize from magmatic hydrothermal fluids during Permian. Early dolomitizing fluids did not increase the porosity, but transformed the primary pores to dissolution pores through dolomitization. Deep-circulating crustal hydrothermal fluids significantly increased porosity in the early stages by dissolving and then slightly decreased the porosity in the late stage due to Fd1 precipitation. Magmatic hydrothermal fluids only precipitated the Fd2 dolomites and slightly decreased the porosity. In summary, Devonian deep-circulating crustal hydrothermal fluids dominated the porosity evolution of the Lower Cambrian dolostone reservoir in the Tarim Basin.

The Snowball Earth hypothesis postulates that the whole earth was covered by ice sheets for millions of years in the Neoproterozoic. In Tarim Basin, there are four intervals of glacial deposits recorded in the Quruqtagh area during the Neoproterozoic. In chronological order, they are Bayisi, Altungol, Tereeken, and Hankalchough glaciations. In this study, we analyzed carbon, oxygen, and strontium isotopes; whole-rock geochemistry; and acid-insoluble residua of the ∼5-m-thick Altungol cap dolostone (ten samples) in Quruqtagh unit of the Tarim basin. The geochemical data of whole rock are used to explain the ocean environmental changes, while the geochemical data of acid-insoluble residua reflect the changes of the continental environment. The δ¹³C_PDB of cap dolostone varies from −2.5 to −1 ‰, which is likely due to the influence of organics. From the bottom to the top of the Altungol cap dolostones, the chemical index of alteration of acid-insoluble residua rises from 72 to 77, but the index of chemical variation drops from ∼3 to ∼1. Ni, Zn, Cu, and Y contents of acid-insoluble residua are enriched in 2.5 m. The values of iron maintain at a high level in all of the whole-rock samples (>5.15 %), but in the samples of acid-insoluble residua, the values of iron are higher than in the whole rock (19∼60 %) below 1.5 m. The variations in chemical composition were probably associated with the changes of continental weathering. The geochemistry suggests that ocean and continent environmental variations had taken place after the Altungol glaciation, including the destruction of ocean stratification and the changes of chemical weathering. During this period, the source of this area had changed. At first, the weathering residues of the parent rocks were exhausted by meltwater. Then, the vigorous weathering of freshly exposed continental crust provided a new source.