U-loss associated with laser-heating of hematite and goethite in vacuum during (U-Th)/He dating and prevention using high O₂ partial pressure

Abstract

Single-aliquot (U-Th)/He dating of hematite has been used to study iron-oxide precipitation in various environments, but we show there is an important challenge to the method: highly retentive hematite samples require temperatures of > 1000° C to be completely degassed, whereas the temperature for major U-loss is ∼980° C. This leads to erroneously high (U-Th)/He ages. Through the analysis of U, Th, and Sm of hematite and goethite samples, we show the degree of U-loss at this temperature and demonstrate that prolonged heating at temperatures of 950° C can lead to U-loss. We show that loss of U in goethite and hematite samples is associated with phase change from hematite to magnetite as Fe is reduced. The onset temperature of vacuum reduction of hematite can be increased from about 800-900° C in vacuum to approximately 1250° C in an oxygen partial pressure of 100 mbar. We show that samples can be outgassed to extract helium at 1150° C without U-loss in an O₂-rich atmosphere during heating, which does not increase the analytical blanks. We describe our implementation and automation of the procedure. An average age calculated on a reference hematite sample from replicate aliquots (n=12), which were analyzed using this procedure, has a relative uncertainty of 2% (1σ), and is within uncertainty of the previously measured two-aliquot age. We suggest this O₂ degassing procedure as a way to precisely and reproducibly determine single-aliquot hematite and goethite (U-Th)/He ages.