New Mexico Geological Society Annual Spring Meeting — Abstracts

Comparison of ⁴⁰Ar/³⁹Ar age determinations and geomagnetic polarity with published geomagnetic polarity time scales is used as an independent check of geochronologic accuracy, specifically, in refining analytical error. Basaltic plugs from 13 volcanic necks located in the Rio Puerco Valley (RPV) east ofMt. Taylor have been dated by the ⁴⁰Ar/³⁹Ar method and analyzed by alternating-field (AF) demagnetization. Results are given in the table below.

Eleven of thirteen samples meet the requirements ofa plateau age. Uncertainty ofeach plateau age is calculated by quadratically combining the standard deviation ofplateau gas fractions ages with the measured uncertainty in irradiation parameter J and uncertainties in isotopes produced by interfering reactions during irradiation. Samples range in age from 4.36-2.40 Ma. Errors (1α) detennined by the above method range from 0.02-0.05 Ma. A total of 109 oriented samples were collected from 13 sites in the RPV. Principal component analysis (PCA) was performed on all samples to detennine polarity and the direction of characteristic remanent magnetization. Site-mean directions and cones of 95% confidence (i.e. α₉₅) were determined using Fisher statistics.

The analytical accuracy of ⁴⁰Ar/³⁹Ar dates is determined by comparing each site's magnetic polarity and age to the geomagnetic polarity time scale. Inconsistencies arise when RPV samples don't correlate with polarity direction ofthe time scale. For example, there are 2 sites which have reverse polarity but fall within a normal chron on the time scale. These inconsistencies either indicate the geomagnetic polarity time scale is wrong or, more likely, the la error resolved by ⁴⁰Ar/³⁹Ar error analysis methods is wrong. When 2σ errors are applied to these problematic samples polarity reversals agree, within error, to the geomagnetic polarity time scale. The true error may lie somewhere between 1σ and 2σ.

Paleomagnetic analysis is a well-established, inexpensive method for correlation of radiometric dates. As technology advances, isotope detection limits increase and analytical errors become smaller, geomagnetic polarity correlation will continue to be a reliable, independent check on accuracy of radiometric dates.