INITIAL DEVELOPMENT OF BADDELEYITE (U-TH)/HE THERMOCHRONOLOGY

Over the past two decades, (U-Th)/He thermochronology has evolved into a reliable technique for constraining the timing and rates of a wide range of geologic processes. Despite its utility, the vast majority of published (U-Th)/He studies have focused on just two minerals, apatite and zircon. Although these minerals are common in a wide variety of lithologies, developing and characterizing new phases for (U-Th)/He dating remains a primary goal of the thermochronologic community. Of specific interest are minerals that either help constrain additional geologic processes or extend the range of lithologies that are amenable to thermochronology.

Baddeleyite (monoclinic ZrO₂) is a common accessory mineral in mafic and ultramafic rocks including basalts, gabbros, and kimberlites, as well as extraterrestrial rocks. Baddeleyite is often one of the primary U-bearing phases in lithologies with a low enough SiO₂activity as to preclude the crystallization of zircon. As such, baddeleyite is a prime target for (U-Th)/He thermochronology.

Shards from the Duluth Gabbro baddeleyite standard yield reproducible (U-Th)/He dates of 922 ± 27 Ma, similar to its crystallization age (U-Pb baddeleyite 1099.6 ± 1.5 Ma). (U-Th)/ He dates on baddeleyite from the Phalaborwa Igneous Complex are ~2100 Ma, also similar to its crystallization age (U-Pb baddeleyite 2059.60 ± 0.35 Ma). Despite the moderate- to high-U concentrations (200-500 ppm) and consequently high radiation dosages experienced by both of these samples (> 1 X 10¹⁷a/g), their laboratory degassing behavior implies relatively high He retentivity. In all baddeleyite degassing experiments, heating temperatures and durations required to completely degas a shard far exceeded conditions required to completely degas He from zircon. This implies that baddeleyite has both a relatively high closure temperature, as well as a relatively high resistance to radiation damage, consistent with previous work indicating that baddeleyite can absorb 2-3 orders of magnitude more radiation damage than zircon without compromising its crystal structure.

Because of its presence in lithologies otherwise devoid of datable minerals, relatively high concentrations of U, and resistance to radiation damage, baddeleyite is a promising new mineral for (U-Th)/He thermochronology.