Mg and Sr isotopic evidence for basin wide alteration of early diagenetic dolomite in the Williston Basin by ascending crustal fluids

Kimmig, Sara R.; Nadeau, Matthew D.; Swart, Peter K.; Holmden, Chris

doi:10.1016/j.gca.2021.06.006

Cited by 12 publications

(1 citation statement)

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“…Recent studies have used variations in δ 44 Ca and δ 24 Mg values to constrain the extent of rock‐buffered and fluid‐buffered systems in early marine diagenesis, and then applied this information to the interpretation of the stratigraphic variability of other geochemical signatures (for example, δ 13 C carb and δ 7 Li; Higgins et al ., 2018; Crockford et al ., 2020; Fantle et al ., 2020; Ahm et al ., 2021; Murphy et al ., 2022). However, in ancient successions, late diagenetic processes, such as hydrothermal fluids migrating along fractures, can also significantly alter the δ 44 Ca and δ 24 Mg values of carbonates (Walter et al ., 2015; Riechelmann et al ., 2020; Kimmig et al ., 2021), overprinting the signal of early marine diagenesis. A recent investigation, using a combination of multiple geochemical techniques (Δ 47 , δ 44 Ca, δ 24 Mg and δ 34 S values), has shown the different responses of diverse geochemical proxies in the Bahamas (Murray et al ., 2021).…”

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confidence: 99%

A clumped isotope diagenetic framework for the Ediacaran dolomites: Insights to fabric‐specific geochemical variabilities

Lu,

Koeshidayatullah,

et al. 2023

Sedimentology

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While marine dolomites formed under near surface conditions have been considered to be potentially reliable archives of past oceanic conditions, this interpretation comes with significant challenges because diagenetic alteration frequently produces diverse fabrics with large geochemical variability. It has been suggested that the Ediacaran dolomites in South China (Hamajing Member, Dengying Formation) recorded the oceanic conditions present at the time they formed, yet these dolomites are composed of five different fabrics (stromatolitic, micritic, oolitic, saddle dolomites and fibrous–radial dolomite cements) and show large variations in multiple geochemical isotope proxies (carbon, oxygen, clumped, magnesium and the sulphur of carbonate‐associated sulphate). This study establishes a paragenetic sequence for these dolomites by combining the clumped and the oxygen isotopic compositions, thereby assessing whether they are geochemically representative of the original seawater. Using this diagenetic framework, the micritic and stromatolitic dolomites show a closed‐system behaviour (low water–rock ratios; <0.3) and are largely resistant to the hydrothermal alteration during late diagenesis. In contrast, the ooid and cement fabrics have been affected by the hydrothermal fluid precipitating saddle dolomite in the open‐system condition with the high stimulated water–rock ratios (>1). Furthermore, in a closed‐system environment, the elevated δ24Mg and δ34S values in the stromatolitic dolomite reflect the isotopic Rayleigh fractionation that enriches the 26Mg and 34S through rock‐buffered recrystallization, coupled with microbial sulphate reduction. These results demonstrate that the complex signals in early marine dolomite should be carefully evaluated when used as a palaeoproxy.

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