In situ geochemistry of Lower Paleozoic dolomites in the northwestern Tarim basin: Implications for the nature, origin, and evolution of diagenetic fluids

Abstract

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 RREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high RREE, 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 RREE, 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.

Fluid mixing induced by hydrothermal activity in the ordovician carbonates in Tarim Basin, China

“…; Zhang et al . ). Hence, it is possible that the late‐stage diagenetic minerals in this study are hydrothermal in origin or are closely related to the hydrothermal activity.…”

“…; Zhang et al . ). In contrast, carbonate reservoirs away from granite intrusions and diabase dikes have relatively high porosity (locally >10%).…”

“…; Zhang et al . ), as supported by their high homogenization temperatures in fluid inclusions, positive Eu anomalies as well as characteristic δ 18 O, δ 13 C, and 87 Sr/ 86 Sr isotopic values of the deep burial diagenetic minerals. Previous studies have argued that hydrothermal fluids during this period may have been related to Permian magmatic activity based on the detailed petrological and geochemical evidence (Cai et al .…”

In situ geochemistry of middle Ordovician dolomites of the upper Mississippi valley

“…(d) Binary diagram of UCC normalized values showing Ce/Ce* and Pr/Pr* relationship after Bau and Dulski () and Zhang et al. (). Field a: neither Ce nor La anomaly; Field b: positive La anomaly, no Ce anomaly; Field c: negative La anomaly, no Ce anomaly; Field d: true positive Ce anomaly; Field e: true negative Ce anomaly.…”

“…() demonstrated that even 2% contamination by shale can obliterate diagenetic elemental anomalies and result in relatively flat REE patterns. Due to the high REE concentration of shales and clays, a positive correlation between these incompatible elements and ∑REE should be present if contamination has occurred (Bolhar, Kamber, Moorbath, Fedo, & Whitehouse, ; Frimmel, ; Zhang et al., ). Thorium values for dolomite range between 0.02 and 1.14 p.p.m., and Al values range from 87 to 6220 p.p.m.…”