Magnetite (U-Th)/He dating and its application to basalt geochronology

K-Ar and 40Ar/39Ar dating have been the traditional geochronometric tools in obtaining high-precision age constraints on felsic to mafic volcanic rocks, employing step-heating or total fusion approaches. However, 40Ar/39Ar age spectra and inverse isochron age information from whole-rock analyses of basalt and basaltic andesite samples are often characterized by the presence of 39Ar recoil and excess 40Ar, rendering ages invalid or difficult to interpret. This study presents a novel technique to dating basaltic rocks by magnetite (U-Th)/He geochronology. Magnetite is a ubiquitous mineral found in many volcanic lithologies, such as basalt, that typically do not contain easily datable mineral phases such as sanidine, zircon, or titanite. We have successfully developed all analytical procedures ranging from mineral separation, sample air-abrasion for á-ejection correction, He extraction and measurement, sample dissolution, and anion-exchange column chemistry procedures for magnetite (U-Th)/He dating. To test the reliability of this new geochronometer, four basalt to basaltic-andesite samples have been dated by both magnetite (U-Th)/He and 40Ar/39Ar methods. With the exception of one sample, 40Ar/39Ar and magnetite (U-Th)/He ages are in excellent agreement (<1%). These multi-aliquot magnetite (U-Th)/He ages (n>7) exhibit 3-12% (2- σ) variation about the mean age, indicating that reproducibility in these new basaltic magnetite (U- Th)/He ages is comparable to apatite and zircon (U-Th)/He analyses. These exciting results suggest that magnetite (U-Th)/He dating holds great promise as an alternative basalt geochronometer, particularly in cases where samples yield inconclusive or uninterpretable 40Ar/39Ar ages. For this study, a basalt sample with a poorly behaved 40Ar/39Ar age spectra affected by 39Ar recoil, yielded a magnetite (U- Th)/He ages characterized by a highly reproducible mean age of 3.1±0.3 Ma; indistinguishable from a 40Ar/39Ar age of 3.1±0.4 Ma of correlated basalt. Assessing the He retentivity, we conducted a magnetite helium diffusion experiment, yielding a well-behaved Arrhenius relationship and a closure temperature of ~250 °C (dT/dt = 10°C/m.y.). Magnetite's high retentivity coupled with excellent age reproducibility demonstrates potential for magnetite (U-Th)/He dating as a new reliable basalt geochronometer.