Thermal effects on rare earth element and strontium isotope chemistry in single conodont elements

Abstract

Abstract-A low-blank, high sensitivity isotope dilution, ICP-MS analytical technique has been used to obtain REE abundance data from single conodont elements weighing as little as 5 g. Sr isotopes can also be measured from the column eluants enabling Sr isotope ratios and REE abundance to be determined from the same dissolution. Results are comparable to published analyses comprising tens to hundreds of elements.To study the effects of thermal metamorphism on REE and strontium mobility in conodonts, samples were selected from a single bed adjacent to a basaltic dyke and from the internationally used colour alteration index (CAI) "standard set." Our analyses span the range of CAI 1 to 8. Homogeneous REE patterns, "bell-shaped" shale-normalised REE patterns are observed across the range of CAI 1 to 6 in both sample sets. This pattern is interpreted as the result of adsorption during early diagenesis and could reflect original seawater chemistry. Above CAI 6 REE patterns become less predictable and perturbations from the typical REE pattern are likely to be due to the onset of apatite recrystallisation. Samples outside the contact aureole of the dyke have a mean 87 Sr/ 86 Sr ratio of 0.708165, within the broad range of published mid-Carboniferous seawater values. Our analysis indicates conodonts up to CAI 6 record primary geochemical signals that may be a proxy for ancient seawater.

The environmental index of the rare earth elements in conodonts: Evidence from the Ordovician conodonts of the Huanghuachang Section, Yichang area

“…Based on the above discussion, it can be concluded that the bell-shaped REE patterns of the Ordovician conodonts from the Huanghuachang section are controlled by palaeotectonogeographic and palaeobiogeographic variations in frequency and time. Thus the bell-shaped REE patterns may be the typical REE distribution patterns of the conodonts in a foreland basin, resulting from the combined effects of water depth and the introduction of terrigenous materials on the seawater REE rather than from diagenesis, which is in agreement with the CAI range (2-3) for conodonts [18,23,34]. …”

Protracted diagenetic alteration of REE contents in fossil bioapatites: Direct evidence from Lu–Hf isotope systematics

“…A meta-analysis of REE compositions of fossil bones and teeth from terrestrial, marine and coastal environments demonstrates broadly environment-specific REE patterns (Trueman et al, 2004), suggesting that the total REE composition in fossil bone retains a component incorporated during early diagenesis reflecting local differences in early depositional environments rather than late diagenetic overprinting (e.g., Trueman and Benton, 1997;Laenen et al, 1997;Patrick et al, 2004;Trueman et al, 2004). Moreover, thermal metamorphism does not appear to alter REE compositions of conodonts until temperatures exceed of 550°C (Armstrong et al, 2001). These datasets support retention of an early diagenetic component within fossil bones and teeth, but again do not prove closed system behaviour.…”

Nd isotopic composition of Late Devonian seawater in western Thailand: Geotectonic implications for the origin of the Sibumasu terrane

“…There is, however, a difference that conodont crowns have relatively low Nd contents (from 25 to 280 ppm) whereas basal bodies have very high concentrations, above 300 ppm. Systematic investigation of hundreds of conodont samples has revealed that their Nd isotopic signatures and Nd contents are extremely stable even when subjected to late diagenetic or low metamorphic alterations (Armstrong et al, 2001;Dopieralska, 2003). Thus, the REE spectra and ε Nd values of conodonts are considered to represent a signatures of a single reservoir, namely that of seawater (Dopieralska, 2003;Lecuyer et al, 2004).…”