T35 Go Small or Go Home: Microbeam Techniques Applied to Igneous, Metamorphic, and Sedimentary Petrology of Earth and Planetary Materials

Sunday, 25 September 2016
1:30 PM - 5:30 PM
Room 404

SOLUTION-LASER ABLATION MC-ICPMS COMPARISONS OF CONODONT APATITE FOR 87SR/86SR

Paper number: T35-9
Program: Technical Sessions
Time: 3:40 PM – 3:55 PM

Authors

Abstract

The 87Sr/86Sr composition of fossil calcites and apatites enables reconstruction of paleoseawater 87Sr/86Sr, which may constrain chronostratigraphy and paleoenvironmental conditions. Bioapatites, including conodont microfossils, require Sr to be purified via anion exchange columns prior to analysis by MC-ICPMS or TIMS methods. The laborious and expensive nature of conventional solution measuremements, as well as the requirement of homogenzation of large quantities of bioapatite, inspired researchers to analyze conodonts for 87Sr/86Sr by laser ablation (LA) MC-ICPMS. However, this method for determining conodont 87Sr/86Sr has not been calibrated to the established solution MC-ICPMS method. We compared aliquots of conodonts from the same stratigraphic height for 87Sr/86Sr composition by solution and laser ablation analysis using MC-ICPMS. Analysis of samples of basal platforms and albid cones, as well as a range of preservation conditions, enabled evaluation of the potential of chemical heterogeneity on the comparison results. In order to decrease the 40Ca31P16O+ interference on mass 87 during LA analyses, plasma sampling depth was increased and a small amount of N2 was added to the Ar sample gas. 87Sr/86Sr analysis of modern fish bioapatite verified this approach. Medium to high resolution settings may resolve the 40Ca31P16O+ interference in analysis of apatite with more abundant Sr. Preliminary results reveal significantly greater 87Sr/86Sr ratios of laser ablation values compared to solution values. This enrichment generally correlates with Yb/Sr and Er/Sr in the conodonts. As such, rare earth elements (REEs), which accumulate in biological apatite during post-mortem recrystallization, may form doubly-charged spectral interferences on the masses measured for 87Sr/86Sr. The inteferences may accurately be determined by monitoring the half masses of Yb and Er isotopes (83.5, 85.5, 86.5) using an ion counter and assuming natural abundance ratios of the REE isotopes. Conodonts measured for 87Sr/86Sr by this method yield results comparable to conventional solution analyses. This LA MC-ICPMS technique may aid in the rapid construction of seawater 87Sr/86Sr curves to improve chemostratigraphic correlations and paleoceanographic reconstructions.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-284126

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