A PROPOSED METHODOLOGY FOR LA-ICP-MS DATA REDUCTION FOR U-Pb CALCITE GEOCHRONOLOGY

Calcite is a ubiquitous mineral in subsurface and surficial geologic systems whose precipitation may be directly related to significant geologic events. For this reason, calcite is a valuable candidate for geochronology. Recent research has focused on utilizing laser ablation mass spectrometry (LA-ICP-MS) to date calcite with the U-Pb radiogenic system. This methodology is plagued by difficulties inherent to calcite such as sub-millimeter scale U and Pb heterogeneity, high abundance of Pb in comparison to U resulting in lack of spread on isochron plots, and the susceptibility of calcite to dissolution, reprecipitation, and alteration that make relating ages to events difficult. High failure rates and imprecise ages are common in attempting calcite U-Pb geochronology. Furthermore, detailed descriptions of data reduction methodology are often lacking in manuscripts with calcite ages, thus leaving out information potentially vital to the reproducibility of results. The purpose of this presentation is to offer a potential method of data reduction for time-resolved U-Pb signals from LA-ICP-MS using the popular Iolite software platform to (1) increase the chances of successful, accurate age production and (2) decrease researcher bias and develop a consistent methodology to maximize the accuracy and reproducibility of results. This is accomplished by discussing results from the U-Pb analysis of calcite veins from six sedimentary basins of the United States to (1) establish consistently observed signal characteristics; (2) demonstrate the susceptibility of the data reduction process to researcher bias; (3) establish a descriptive process of data reduction involving discrete signal selections, isochron plotting, and the trimming/cleaning of erroneous or redundant data points; and (4) discuss the validity of the resulting ages using other constraints, including temperature of formation and δ¹³C and δ¹⁸O signals.