LASER PROBE 40AR/39AR DATING STUDIES ON SUBMILLIGRAM WHOLE-ROCK BASALT SAMPLES - THE AGE OF THE STEENS MOUNTAIN GEOMAGNETIC POLARITY TRANSITION (REVISITED)

The Steens Mountain geomagnetic polarity transition is arguably the most comprehensively studied geomagnetic field reversal trapped in igneous rocks. Over twenty years ago, this reversal was dated at 15.5 +/- 0.3 Ma by the K-Ar method. We have now tested the feasibility of carrying out 40Ar/39Ar dating on < 1 mg samples of fine-grained whole-rock basalt utilizing a laser probe, i.e. on samples approximately 1000 times smaller than have been traditionally analyzed. Twenty samples taken from lava flows below, within and above the reversal (N/R) in Steens Mountain, Oregon, have been studied by this laser incremental heating technique. Eighteen of these specimens yield acceptable plateau ages with a weighted mean age of 16.2 +/- 0.1 Ma (standard error on the mean). This result is in excellent agreement with the results obtained on two bulk samples of whole-rock basalt that average 16.1 +/- 0.1 Ma. An attempt to confirm these ages by fusion of plagioclase laths taken from a few of the individual lava flows, was hampered by their low K content (0.2-0.3%); our best estimate for the age of six plagioclase laths each < 1 mg weight, is 16.5 +/- 0.5 Ma. We suggest the techniques utilized herein could be extended to carry out dating of basaltic material of age < 1 Ma.