T174 Unravelling Timescales of Magmatism, Metamorphism, and Crustal Evolution (Posters)

Tuesday, 17 October 2023
8:00 AM - 5:30 PM
Hall B

40Ar/39Ar Petrochronology of Sanidine: Case Study from Peach Springs and Apache Leap Tuffs (SW, USA)

Paper number: T174-21
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

Over the last 20 years, radioisotopic dating by the 40Ar/39Ar technique has experienced substantial community-wide advancements in metrologic traceability, interlaboratory reproducibility, precision, accuracy, and intercalibration with other dating methods (e.g., U-Pb zircon ages, astronomical time scale). In addition, in the last 10 years, advances in analytical uncertainty (∼0.1%) on dates from a new generation of noble gas multicollector mass spectrometers have revealed significant dispersion in dates from individual minerals or hand samples, including fluence monitors. Identification of this complexity demands a deeper consideration of the processes (e.g., geologic, analytical, reactor-induced) responsible. One approach to better interpret the origins and significance of such complex datasets is to examine relationships between high-precision 40Ar/39Ar dates and isotopic and geochemical compositions of individual crystals. Here we introduce a new 40Ar/39Ar petrochronologic dating method on whole sanidine grains by adding geochemical tracer information (major & trace elements, Sr & Pb isotopes) to 40Ar/39Ar dates from the same crystals. Our test samples are the Miocene (~18.8 Ma) Peach Springs and Apache Leap tuffs (SW, USA), which have been considered coeval from previous dating. We introduce a workflow of LA-ICP-MS on irradiated crystals prior to subsequent noble gas analysis which does not alter bulk mineral composition or degas Ar, establishing a robust and routine 40Ar/39Ar petrochronologic method. We observe large differences between Apache Leap (Ba=1309-17853 ppm, Sr=112-770 ppm, Li=6-25 ppm) and Peach Spring (Ba=23-1767 ppm, Sr=16-233 ppm, Li=0.3-19 ppm) sanidine compositions. Ti-in-Kspar crystallization temperatures are on average 50-100°C hotter for the Peach Springs sanidine. Also present is a radiogenic subpopulation within the Peach Springs sanidine denoted by 87Sr/86Sri >0.713 and 208Pb/204Pb >40. We postulate on the petrogenetic significance of these early petrochronoloic signatures and comment on future plans and other possible applications for this technique.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-396786

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