In-situ oxygen isotope compositions of Ordovician conodonts using SHRIMP II and laser ablation MC-ICPMSJulie A. Trotter 1 , Ian Williams 1 , Stephen M. Eggins 1 , C.R. Barnes 2 1 Research School of Earth Sciences, Australian National University , Canberra , ACT 0200, Australia Given the potential utility of conodonts as geochemical tracers, new and developing technologies have been investigated to assess the feasibility of extracting isotopic records by in-situ analysis. Reconnaissance work has shown that ion microprobe (SHRIMP II) and laser ablation multi-collector ICPMS offer considerable potential to determine the oxygen and strontium compositions of conodont apatite. The benefits of analysing marine bio-apatites rather than less chemically stable carbonates, together with the capability of utilising high spatial resolution techniques, provide an opportunity to target specific components that are most likely to retain primary compositions. Accordingly, this work has significant implications for better characterising the evolution of seawater chemistry throughout the Palaeozoic. Preliminary Sr isotope datasets of discrete conodont elements determined in-situ by laser ablation MC-ICPMS fall within similar ranges of existing Ordovician records based on labour intensive conventional TIMS analyses of both calcitic brachiopod and conodont samples. Further, LA-MC-ICPMS analyses of conodont albid tissue have yielded less radiogenic 87 Sr/ 86 Sr ratios than hyaline tissue for coeval samples, suggesting that there may be systematic differences between the compositions of the component conodont histologies. Reconnaissance in-situ oxygen isotope analyses using SHRIMP II have yielded considerably higher and more realistic d 18 O values (~16 to 19‰ V-SMOW) than conventional GIRMS data based on calcitic brachiopods (-10 to 25‰ V-PDB) reported in the literature (eg. Shields et al ., 2003), the integrity of which has been the focus of much scepticism. Although the initial SHRIMP results have an analytical error of ±1‰, a temporal shift during the Arenig is apparent. Better constraining this shift as well as the well-known Late Ordovician d 18 O excursion is the focus of further work, which given the recent progress in technique development promises to yield higher external precision for conodont apatite, and thus a significantly improved d 18 O seawater curve for the Ordovician.
References: Shields, G.A., Eggins, S.M., (2006) Chemical Systematics of Conodont Apatite Determined by Laser-Ablation ICPMS. Chemical Geology 233 , 196-216. Shields, G.A, Carden, G.A.F., Veizer, J., Meidla, T., Rong, J., & Rong-Yu, L., (2003) Sr, C, and O isotope geochemistry of Ordovician brachiopods: A major isotopic event around the Middle-Late Ordovician transition. Geochimica et Cosmochimica Acta , 67 , 2005-2025. |