Abstract
Monazite fission-track presents itself as a novel, low-temperature
thermochronometer with annealing studies placing its closure temperature
between ~45 and 25 °C. Previously, monazite has been
unsuitable for fission-track dating due to high abundance of gadolinium
and insufficient investigation of the etching protocol. Gadolinium
causes self-shielding via thermal neutron capture and substantial
associated nuclear heating during irradiation which prevented robust
monazite fission-track dating using the traditional external detector
method. Further, early etching studies were found to be extremely
corrosive to monazite grains. However, developments in LA-ICP-MS
fission-track analysis allow for measurement of 238U and improvements in
monazite fission-track etching protocols mean that dating monazite
through the fission-track method is now viable. In this study, we
present monazite fission-track data from an elevation profile (2260 m,
2000 m, 1600 m, and 1200 m) from the Catalina metamorphic core complex
(Catalina MCC), in southern AZ, USA. We follow the etching protocol
described in Jones et al. (2019), etching the monazites in 6 M HCl for
90 minutes at 90 °C. We measure the 238U concentration via LA-ICP-MS and
compare the dates to other multi-method thermochronology from the same
rocks. Traditional low-temperature thermochronology (apatite and zircon
fission-track, apatite and zircon (U-Th-Sm)/He) from the Catalina MCC
reveals cooling at 25-20 Ma and 18-10 Ma. Preliminary monazite
fission-track analysis yields a date of 6.1 ± 0.4 Ma, far younger than
all the traditional thermochronometric data, in-line its far lower
closure temperature. The 6 Ma monazite fission-track date is consistent
with the youngest phase of hematite (U-Th)/He dates observed in the
nearby Rincon metamorphic core complex and suggest that these dates
correspond to the latest phase of exhumation in response to Basin and
Range extension and/or climate enhanced erosion. These preliminary
results show that monazite fission-track can reveal shallow crustal
processes and contribute to constraining thermal histories below
~60 oC, which are traditionally difficult to resolve.