2002-2003 Science Planning Summary

Geology & Geophysics

Dr. Scott Borg
Program Manager


NSF/OPP 01-25194
Station: McMurdo Station
RPSC POC: Karla College
Research Site(s): Taylor Valley, Bull Pass, Beacon Valley

Calibration of cosmogenic argon production rates in Antarctica
Dr. Paul R. Renne
Berkeley Geochronology Center

Deploying Team Members: Kimberly B Knight . Paul R Renne
Research Objectives: Researchers intend to establish the systematics of cosmogenic argon production required to establish its measurement as a routine surface exposure dating tool analogous to existing methods based on helium-3, beryllium-10, carbon-14, neon-21, and aluminum-26. Cosmogenic argon offers advantages over existing cosmogenic chronometers in that it is stable (hence applicable to long-term or ancient exposure dating) and less prone to diffusive loss than helium or neon.

Argon-38 is produced principally by spallation of calcium and (probably) potassium, and it is most easily measured using neutron-irradiated samples, as has been done routinely on extraterrestrial samples for decades. Initial measurements on antarctic samples demonstrate the viability of this method for terrestrial samples and suggest an average production rate of greater than 100 atoms/gram-calcium/year. Existing data suggest that argon-38/calcium exposure ages younger than 105 years can be accurately determined by this method.

Further work on calcic minerals (apatite, sphene, clinopyroxene, plagioclase, calcite) whose exposure histories are constrained by helium-3 and neon-21 concentration data will be used to determine the calcium-derived production rate. Analogous work on potassium-rich minerals (potassium-feldspars, micas) will be used to constrain the production of argon-38 from potassium, which should theoretically be comparable to that from calcium when the same neutron-activation method is used.

Analytical work will use existing samples plus new samples to be collected from the dry valleys of Antarctica to maximize cosmic radiation dosage for purposes of calibration. Laboratory studies of the retentivity of argon-38 in appropriate minerals will be used to help evaluate results and guide future applications.

Field Season Overview:
Project team members will collect rock samples from stable geomorphic surfaces for separation of minerals to be analyzed for isotopic composition of helium, neon and argon. Helicopter support will be provided for day trips from McMurdo to Hess Mesa, Mount Feather, Kukri Hills, Table Mountain, and Schist Peak.