New Mexico Geological Society Annual Spring Meeting
April 7, 2017

Abstract
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Detrital Sanidine 40Ar/39Ar Dating: Transforming Sedimentary Rock Geochronology

Matthew T. Heizler1, Karl Karlstrom2, Matthew Zimmerer1, Jake Ross1, Laura Crossey2 and William McIntosh1

1New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM, 87801, matt.heizler@nmt.edu
2University of New Mexico, Albuquerque, NM, 87131

Several pilot studies and recently published papers are demonstrating the potential to transform sedimentary rock geochronology via detrital sanidine (DS) 40Ar/39Ar dating. Discrete volcanic ash layers are relatively lacking in the sedimentary record and thus only provide a limited opportunity for chronostratigraphic studies. In contrast, detrital sanidines appear to be ubiquitous is western USA sedimentary rocks younger than about 300 Ma. These represent cryptic tephra layers and also grains transported by wind and rivers thus potentially providing the age of the rock unit and important provenance information. Current studies have primarily focused on late Cretaceous/Paleocene rocks from the San Juan Basin and post 10 Ma river terrace deposits from NM, AZ, CO, UT and WY. Combined, these studies reveal the tremendous potential of the method as youngest age populations often equal or approach actual depositional ages and are up to 100 times more precise than detrital zircon U/Pb analyses. Thus DS grains can be linked to individual ignimbrites sources. Also, comparison of DS maximum deposition ages to river terraces dated by low precision and costly cosmogenic nuclide burial isochrons demonstrates unprecedented accuracy of DS to determine the terrace age with remarkable improvement in precision. Thus far our studies of river incision rate, paleodrainage patterns, biostratigraphy and neotectonics are indicating that we are on the verge of a geochronology revolution similar to the impact that has been made by detrital zircon geochronology during the past couple of decades. In order to fully utilize the potential of DS geochronology, several steps are required. Ultra-high precision dating of ignimbrites is needed so that individual DS grains can be better linked to source rocks. A complete and comprehensive interactive database of sanidine location, age, and composition is required to provide simple means of linking DS grains to potential source rocks. Improvements in workflow efficiency, especially the picking of sanidine grains from bulk K-feldspar separates is needed and we propose this can be accomplished robotically. Lastly, a more comprehensive chemical fingerprint of DS grains that expands the commonly obtained K/Ca ratio to elements such as Na, Sr, and Ba for each DS grain will be more diagnostic of source. To accomplish this we intend to measure Ne, Kr and Xe noble gases that are produce during irradiation of Na, Sr, and Ba, respectively. Over the next decade, research utilizing DS geochronology, coupled with technique advances, will become the next revolutionary breakthrough in geochronology.

Keywords:

40Ar/39Ar, sanidine

pp. 35

2017 New Mexico Geological Society Annual Spring Meeting
April 7, 2017, Macey Center, New Mexico Tech campus, Socorro, NM