Magnetite (U–Th)/He dating and its application to the geochronology of intermediate to mafic volcanic rocks

Abstract

We present a new approach to dating intermediate to mafic volcanic rocks using magnetite (U–Th)/He geochronology. Magnetite is common in volcanic rocks that typically do not contain easily datable minerals such as sanidine or zircon. Analytical procedures for producing magnetite (U–Th)/He ages have been developed, including mineral separation, sample air-abrasion to correct for α-ejection effects, He extraction/measurement, sample dissolution, and anion-exchange column chemistry procedures. Dated magnetite crystals were non-skeletal, euhedral to subhedral, and 100–300 μm in size. To test the reliability of this new geochronometer, four basaltic to andesitic samples lacking sanidine or zircon were dated by both magnetite (U–Th)/He and whole-rock 40Ar/ 39Ar methods. For two samples, the ages from the different geochronometers are in excellent agreement (< 1%). A third sample with a poorly behaved 40Ar/ 39Ar age spectrum affected by 39Ar recoil yielded a well-defined magnetite (U–Th)/He age that is consistent with 40Ar/ 39Ar age data from similar nearby volcanic rocks. The final sample, however, exhibited a near 40% discrepancy between the two methods, despite yielding reproducible magnetite (U–Th)/He ages. In all cases, the multi-aliquot magnetite (U–Th)/He ages ( n > 7) exhibit 3–11% (2σ) variation about the mean age, indicating that reproducibility for magnetite (U–Th)/He ages is comparable to that of apatite and zircon (U–Th)/He analyses. 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 (d T/d t = 10 °C/myr). Magnetite’s high He retentivity coupled with (U–Th)/He age reproducibility demonstrates good potential for magnetite (U–Th)/He dating as an alternative volcanic geochronometer, particularly in cases where samples yield inconclusive or uninterpretable 40Ar/ 39Ar ages.