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 often suggest the presence of ³⁹Ar recoil and excess ⁴⁰Ar, rendering ages invalid or difficult to interpret. This study presents a novel approach to dating basaltic rocks by magnetite (U-Th)/He geochronology. Magnetite is a ubiquitous mineral found in volcanic lithologies, such as basalt, that typically do not contain easily datable mineral phases such as sanidine or zircon. We have successfully developed all analytical procedures ranging from mineral separation, sample air-abrasion for α-ejection correction, He extraction/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, the ages from the two geochronometers 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 example, sample 1003-CF-02 yields a poorly behaved ⁴⁰Ar/³⁹Ar age spectra affected by ³⁹Ar recoil, whereas magnetite (U-Th)/He analyses from the same sample gave a highly reproducible mean age of 3.1±0.3 Ma. In order to assess the He retentivity, we conducted a single 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 He retentivity coupled with excellent (U-Th)/He age reproducibility demonstrates enormous potential for magnetite (U-Th)/He dating as a new reliable basalt geochronometer.