Diagenetic controls on the isotopic composition of carbonate‐associated sulphate in the Permian Capitan Reef Complex, West Texas

Abstract

Late Palaeozoic-age strata from the Capitan Reef in west Texas show faciesdependent heterogeneity in the sulphur isotopic composition of carbonateassociated sulphate, which is trace sulphate incorporated into carbonate minerals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate-associated sulphate. These processes variously modify the sulphur isotopic composition of incorporated sulphate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector inductively-coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic textures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawater sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marinephreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic compositions of Permian seawater. Some early and all late meteoric calcite cements have carbonate-associated sulphate with a sulphur isotopic composition distinct from that of Permian seawater. Detailed petrographic and sedimentary context for carbonate-associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high-energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater's sulphur isotopic composition.

Variability in sulfur isotope records of Phanerozoic seawater sulfate

“…Diagenetic processes may exchange sulfate with the primary carbonate and alter its isotopic composition (Fichtner et al, 2017;Murray et al, 2020;Present et al, 2015Present et al, , 2019. Kampschulte & Strauss (2004) suggested that the variability of multiple δ 34 S analyses from contemporaneous stratigraphic successions could be used to quantify the effect of diagenesis on the CAS record.…”

“…Further, CAS analyses may be contaminated by sulfur from co-occurring phases, including sulfide and disulfide minerals, sulfur-bearing organic material, and sulfate salts (Edwards et al, 2019;Marenco, Corsetti, Hammond, et al, 2008;Present et al, 2015;Theiling & Coleman, 2015;Wotte et al, 2012). Recent application of plasma-source mass spectrometry for sulfur isotope analysis has permitted δ 34 S analyses on less than one-thousandth as much sulfate, corresponding to 5 mg to 50 mg of carbonate Paris et al, 2013;Present et al, 2015Present et al, , 2019Rennie et al, 2018). Well-preserved biogenic grains, recrystallized grains, matrix, and cements contain CAS with δ 34 S compositions varying by as much as 25‰, spanning most of the range of CAS analyses from the entire Phanerozoic (Present et al, 2015(Present et al, , 2019.…”

“…Recent application of plasma-source mass spectrometry for sulfur isotope analysis has permitted δ 34 S analyses on less than one-thousandth as much sulfate, corresponding to 5 mg to 50 mg of carbonate Paris et al, 2013;Present et al, 2015Present et al, , 2019Rennie et al, 2018). Well-preserved biogenic grains, recrystallized grains, matrix, and cements contain CAS with δ 34 S compositions varying by as much as 25‰, spanning most of the range of CAS analyses from the entire Phanerozoic (Present et al, 2015(Present et al, , 2019. Therefore, much of the variability of CAS δ 34 S data may not reflect the δ 34 S composition of ancient seawater sulfate.…”

“…CAS can reflect the δ 34 S of syndepositional seawater sulfate if the carbonate component did not recrystallize after precipitation, if recrystallization and cementation occurred in contact with a low-sulfate fluid, or if the δ 34 S of pore fluid sulfate was not fractionated from seawater (Gill et al, 2008;Lyons et al, 2004;Rennie & Turchyn, 2014). Alteration occurs if the sediments recrystallize above the depth at which sulfate is completely consumed by MSR but deep enough that some distillation of δ 34 S within sediment pore fluid has occurred (Edwards et al, 2019;Fike et al, 2015;Present et al, 2019;Rennie & Turchyn, 2014;Witts et al, 2018). Additionally, some ancient carbonates contain CAS with anomalously low δ 34 S interpreted to result from the incorporation of sulfate from sulfide that was reoxidized during diagenesis or weathering (Baldermann et al, 2015;Edwards et al, 2019;Fichtner et al, 2017;Fike et al, 2015;Present et al, 2015Present et al, , 2019Rennie & Turchyn, 2014;Riccardi et al, 2006;Yan et al, 2013).…”

“…Alteration occurs if the sediments recrystallize above the depth at which sulfate is completely consumed by MSR but deep enough that some distillation of δ 34 S within sediment pore fluid has occurred (Edwards et al, 2019;Fike et al, 2015;Present et al, 2019;Rennie & Turchyn, 2014;Witts et al, 2018). Additionally, some ancient carbonates contain CAS with anomalously low δ 34 S interpreted to result from the incorporation of sulfate from sulfide that was reoxidized during diagenesis or weathering (Baldermann et al, 2015;Edwards et al, 2019;Fichtner et al, 2017;Fike et al, 2015;Present et al, 2015Present et al, , 2019Rennie & Turchyn, 2014;Riccardi et al, 2006;Yan et al, 2013). Carbonates recrystallizing during burial may also be prone to diagenetic modification of the δ 34 S of CAS if the burial fluids were sulfate rich (Fichtner et al, 2017(Fichtner et al, , 2018Present et al, 2015).…”