Synsedimentary versus metamorphic control of S, O and Sr isotopic compositions in gypsum evaporites from the Cameros Basin, Spain

Abstract

Sulfate δ34S and δ18O and Sr isotope compositions are presented for Berriasian (Upper Jurassic) gypsum evaporites from the Cameros Basin, a continental basin in the Iberian ranges of northern Spain. Solute sources to the ephemeral lakes in which the evaporites formed are dominated by weathering of sediments containing older Keuper marine evaporites and granitic/metamorphic Variscan basement. Strontium isotopic ratios of the continental evaporites (87Sr/86Sr = 0.707882 to 0.707933) are elevated compared to the Keuper source (87Sr/86Sr = 0.707605 to 0.707799), most likely reflecting a radiogenic component from basement lithologies. Sulfate δ34S in the continental evaporites (at around 18.2 ‰V-CDT) is higher than any possible mixture of solutes to the basin. The degree of 34S enrichment is small (≈ 3.7‰) and is most likely the result of partial bacterial reduction of lake-water sulfate. The same process would also enrich sulfate in 18O, but by only ≈ 1‰. Evaporite sulfate δ18O (at around 21.7‰V-SMOW) is, in fact, enriched by ≈ 10‰ relative to sulfate sources to the basin. This large effect is most likely the result of re-equilibration of oxygen isotopes during low grade metamorphism of the basinal sequence. Sulfur and strontium isotopes in the evaporites remained internally buffered and thus unchanged during metamorphism while oxygen isotopes were open to exchange with aqueous fluid and/or interbedded carbonates that constituted a large exchangeable oxygen reservoir. The potential of sulfate oxygen to undergo isotopic exchange at elevated temperatures must be taken into account when interpreting the significance of evaporite sulfate oxygen data from units that have undergone deep burial or low grade metamorphism.