Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique

Abstract

Abrasion combined with an improved paramagnetic separation technique eliminates 90 to 100 percent of discordance so that ages of unprecedented accuracy (±1 to 3 m.y.) can be achieved for virtually all 2700 m.y. old zircon populations from plutonic or volcanic rocks. The procedures work even better for younger zircons. Besides removing outer layers that may have been leached, high-U parts are preferentially removed by abrasion because they are softened by radiation damage. Altered and cracked zircons also tend to be eliminated. In most cases, the new concordant data move up the line established by previously analysed paramagnetic fractions but a number of anomalies have been found where old data give upper intersections that are in error by as much as 25 m.y. Reducing or eliminating paramagnetically correlated Pb loss greatly enhances our ability to define mixing lines for igneous or metamorphic rocks when two ages of zircon are present. The abrasion technique allows detection of an inherited component if it exists by enhancing the sample in core material. Abrasion in many cases removes about 80 percent of the common lead, thus allowing a direct evaluation of this component. When the outer parts of grains are removed, the correlation between magnetic susceptibility and uranium content is maintained but the usual correlation of uranium level with lead loss is reduced or eliminated. Therefore, only near surface uranium is involved in the classic discordance versus uranium level correlation of Silver (1963).