Constraining dolomitization by Mg isotopes: A case study from partially dolomitized limestones of the middle Cambrian Xuzhuang Formation, North China

Abstract

The “dolomite problem” refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record, as well as failure of laboratory inorganic dolomite precipitation at near Earth‐surface temperature. Novel Mg isotope systematics provides a promising tool in resolving the “dolomite problem”. Here, we develop a protocol to place constraints on the dolomitization process by using Mg isotopes. In this study, we measured Mg isotopic compositions ( δ26Mg) of two batches of partially dolomitized limestone samples from the middle Cambrian Xuzhuang Formation in North China. δ26Mg varies between −0.55‰ and −3.18‰, and shows a negative linear correlation with 1true[Mgtrue], suggesting that δ26Mg can be described by a binary mixing between the calcite and dolomite components. Mg isotopic composition of the dolomite component ( δ26Mgdol) for the lower sample set that is collected from a 4 m stratigraphic interval containing three high‐frequency ribbon rock‐packstone cycles is −1.6‰, while δ26Mgdol for the upper sample set (from a thick sequence of ribbon rock) is significantly higher (−0.3‰). However, neither mineralogical and elemental compositions, carbon and oxygen isotopes, nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples. δ26Mgdol of the Xuzhuang limestone is simulated by the Advective Flow (AF) and the Diffusion‐Advection‐Reaction (DAR) models. The AF model assumes that Mg is transported by advective fluid flows, while the DAR model simulates a contemporaneous seawater dolomitization process, in which Mg is delivered by diffusion. The AF modeling result indicates that δ26Mg of the dolomitization fluid is +0.4‰ and +1.7‰ for the lower and upper sample sets, respectively. These values are significantly higher than modern and Cenozoic seawater Mg isotopic composition, suggesting that the dolomitization fluid is not contemporaneous seawater. The AF model also predicts spatially heterogeneous δ26Mgdol with progressive enrichment in 26Mg along the fluid flow pathway. In the DAR model, both dolomite content and δ26Mgdol of the lower sample set can be simulated by using seawater Mg isotopic composition of −0.75‰, thus contemporaneous seawater dolomitization may explain δ26Mgdol of the Xuzhuang limestone. Furthermore, the DAR model demonstrates spatially homogeneous δ26Mgdol. To differentiate the AF and DAR models, samples from multiple sections are required. Nevertheless, this study implies that Mg isotope might be a useful tool in the study of dolomitization.

Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

“…26 Mg values ranging from -2.07% to -2.04% (Table 1), which is within error of the accepted value (-2.05% ± 0.05% (2r) (Huang et al 2015;Peng et al 2016). d…”

Constraining ribbon rock dolomitization by Mg isotopes: Implications for the ‘dolomite problem’

“…In order to guarantee complete dissolution, centrifuge tubes were placed in an ultra-sonic bath for 30 min. Such procedure guarantees complete dissolution of calcareous component (calcite and dolomite), and avoids contamination from clay minerals (Peng et al, 2016). After centrifuge, supernatants were collected for Mg isotope and elemental composition [键入文字] analyses.…”

“…Magnesium was purified by cation exchange chromatography at Peking University. The detailed procedure of column chemistry has been reported in previous studies (Huang et al, 2015;Peng et al, 2016), thus only a brief description will be given. Two column procedures were applied to separate Mg from other metals.…”

“…Numerical models for the Mg isotope systematics of complete dolomitization and partial dolomitization have been established in recent studies. These studies indicate that Mg isotopic compositions of dolostone/dolomite are controlled by the processes of dolomite formation, and are related to temperature, sedimentation rate, reaction rate of dolomite formation, isotopic fractionation, and seawater Mg isotopic compositions (Huang et al, 2015;Peng et al, 2016), explaining the absence of correlation between the types of dolomite and Mg isotopes (Geske et al, 2015a). Third, the isotopic fractionation during dolomite formation at ambient temperature has been extensively studied in recent years.…”

Oligocene incursion of the Paratethys seawater to the Junggar Basin, NW China: insight from multiple isotopic analysis of carbonate

“…For magnesium isotope analysis, magnesium was extracted and purified at Peking University following [50][51][52][53] . Powder samples were loaded with 5 mL of 1 N acetic acid in a tube, which was then placed in an ultrasonic bath for two days to ensure complete dissolution of carbonate.…”