T352 Improvements and Challenges in Geochronology: Organizing the Past while Planning for the Future (Posters)

Wednesday, 25 October 2017
9:00 AM - 6:30 PM
Halls 4EF

ADVANTAGES OF CONTINUOUS RAMP HEATING AND MACHINE-LEARNING IN 40AR/39AR THERMOCHRONOLOGY

Paper number: T352-2
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Traditional 40Ar/39Ar thermochronology is performed by incrementally heating minerals following pre-defined heating schedules. This rigid approach repeatedly subjects the sample to unnaturally high rates of heating and cooling that promote grain fracturing and other problematic behavior. In addition, the step-heating approach is inefficient since gas is not continuously extracted from the sample. Fixed heating schedules may under or over resolve 39Ar release. Because analytical solutions also exist for heating schedules linear in 1/T, more flexible (and natural) approaches to release Ar from geologic samples are possible. We utilize the National Instruments Compact Reconfigurable Input Output (cRIO) platform to control all aspects of vacuum generation, gas extraction, handling, and mass spectrometry on a Linux real-time operating system. Real-time control permits stable temperature regulation (±1°C) over 300-1500°C range. The ability to dynamically update heating schedules based on signal size allows for on-the-fly optimization of data acquisition and results in better resolved 40Ar/39Ar age and 39Ar diffusion data sets in runs as short as 4 hours. The 980 nm diode laser system efficiently heats 3 mm diameter x 0.5 mm thick Ta packets (10 mg) that contain either a single crystal or grain aggregates in the 0.2-2.0 mg range. Automated positioning of the optical pyrometer ensures reproducible temperature monitoring and can be used to assess thermal gradients. Temperature calibration of optical pyrometer is accomplished by: (1) determining the melting point of Ta-encapsulated Al disks; and (2) measuring the Arrhenius properties of 39Ar release from single spherical beads of Ta-encapsulated K-feldspar glass over a 300-1500°C range. Ar sensitivity and detector mass fractionation is constrained via measurement of a 40Ar-39Ar-38Ar-36Ar reference gas developed in collaboration with the U.S.G.S (Menlo Park) using the same analysis routines employed for unknowns. We present applications based upon analysis of single K-feldspar crystals from plutonic and detrital settings to demonstrate the enhanced efficiency and quality of continuous heating relative to the incremental heating approach performed with resistance furnace apparatus.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-304575

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