TESTING THE ACCURACY OF THE GEOMAGNETIC POLARITY TIME-SCALE (GPTS) AT 2-5 MA, UTILIZING 40AR/39AR INCREMENTAL HEATING DATA ON WHOLE-ROCK BASALTS

Recent 40Ar/39Ar dating and astrochronologic calculations indicate that the K-Ar derived GPTS underestimates the ages of geomagnetic field reversals in the interval approximately 2-0 Ma. We report tests to check the accuracy of K-Ar data utilized to date the Reunion Event (approximately 2.1 Ma) and part of the early Pliocene portion (approximately 4.6 Ma) of the GPTS. 40Ar/39Ar incremental heating studies on whole-rock basalts from the island of Reunion yield an age of 2.14 +/- 0.03 Ma for the Reunion Event, somewhat older than the earlier K-Ar value of 2.07 +/- 0.02 Ma on the same set of lava flows. Study of a single specimen from the Newer Volcanics, Australia, indicates that the earlier K-Ar date was accurate; but the new 40Ar/39Ar results yield a plateau age (4.56 Ma) with a standard error of approximately 0.6%, as compared to the K-Ar value approximately 3%. The improved precision and accuracy resulting from such 40Ar/39Ar incremental heating studies will play a key role in calibrating more precisely the GPTS for the interval 10-0 Ma.