New constraints on carbonate diagenesis from integrated Sr and S isotopic and rare earth element data, Jurassic Smackover Formation, U.S. Gulf Coast

Abstract

Isotopic (Sr, S, O, C) and trace element, including rare earth element (REE), compositions of calcite cements, metal sulfides and sulfate minerals from oolitic carbonate grainstones of the Smackover Formation (Jurassic, southwest Arkansas shelf) constrain the mechanism of sulfate reduction and establish temporal relations between sulfur and carbonate diagenesis. Strontium isotopic values of calcite cements range from near Jurassic-age sea water values (0.7068) up to values of 0.7096. This evidence, coupled with variation in O and C isotopic compositions and in cement zoning under cathodoluminescence, suggests that calcite cement precipitated over the course of burial diagenesis. Strontium isotopic compositions of pyrite, marcasite and native S range up to maximum values observed for calcite cements, but galena and sphalerite are less radiogenic. The Sr contents of sulfides and native S are <30ppm, but the Rb content is sufficiently low that in-situ growth of 87Sr would not dramatically alter the initial Sr isotope ratio. Diagenetic sulfate minerals also exhibit a range of Sr isotopic compositions: anhydrites range from 0.7071 to 0.7081, while celestites are more radiogenic (0.7096). The REE patterns of early and late calcite cements and framework grains differ. Using Sm/Yb ratios as an indicator of heavy vs light REE enrichment, early calcites have Sm/Yb ratios <1 whereas late calcite cements have ratios of 1.8–3.2. Importantly, both cement signatures differ from framework grain ratios (1.3–1.7) indicating lack of rock-buffering in fluids from which the cements precipitated. Furthermore, the sympathetic increase in Sm/Yb and Sr isotopic ratios from early to late calcites is consistent with input of light REE from siliciclastic sources. Sulfur isotopic compositions of metal sulfide minerals range from −32 to +8% CDT. Native S and anhydrite values (+18%) are near Jurassic-age sulfate values while celestites are heavier (+28%). Although thermochemical sulfate reduction has been proposed to explain H 2S in deeply buried carbonate units associated with evaporite S, lack of S isotopic homogenization between sulfate and sulfide minerals, lack of S isotopic equilibration of oil with reservoir SO 4, and the limited distribution of H 2S-rich brines in southwest Arkansas demand a more complex origin.