Clumped-Isotope Constraints On Cement Paragenesis In Septarian Concretions

Abstract

Septarian concretions exhibit multiple generations of cements that include body, fringe, and spar phases. Classic paragenetic interpretations include initial precipitation of the body followed by fringe(s) and then by spar in more or less discrete events. Traditional approaches (e.g., carbon and oxygen isotope analyses) are generally unable to distinguish paragenetic trends as they relate to specific formation environments (e.g., precipitation during burial or with meteoric influx). Here we present carbonate clumped-isotope, δ13C (δ13Ccarb), and δ18O (δ18Ocarb) values for septarian concretions taken from four host units in order to assess cement paragenesis and overcome traditional shortcomings. Clumped-isotope and δ18Ofluid data exhibit a wide range of values, with carbonate precipitation temperatures between ∼ 20 and 50°C and δ18Ofluid compositions of ∼ −14 to +4‰ (VSMOW). In stable-isotope cross-plots, specific cement phases group together and confirm the paragenesis indicated by superposition. In some cases, samples analyzed from concretion bodies yield temperature and δ18Ofluid values that indicate formation at shallow depths, consistent with independent data (e.g., high minus-cement porosity, external laminae deflection). In contrast, other concretion-body analyses indicate relatively high body temperatures that conflict with shallow-formation indices. Petrographic and backscatter scanning electron microscopy (SEM) reveal that concretion bodies partially consist of a secondary, replacement phase, which could explain the higher temperatures expressed in bulk body samples. Based on data for different phases in these septarian concretions, we suggest that initial body-cement precipitation occurred at relatively shallow depths from unmodified seawater, followed by fringe formation at elevated temperatures that likely coincided with the emplacement of the secondary body phase. When considered together, late-stage spar phases exhibit temperatures and δ18Ofluid values supportive of spar precipitation from fluids with a significant meteoric component, possibly during uplift.