Thermal Annealing of Implanted 252Cf Fission-Tracks in Monazite

Jones, Sean1, Gleadow, Professor Andy1, Kohn, Professor Barry1

1University Of Melbourne, , Australia

Monazite ((Ce, La, Nd, Sm)PO4), a rare-earth element (REE) phosphate mineral, is found as an accessory mineral in a variety of rock types. Suitable uranium and thorium content make it a useful mineral for isotopic and chemical dating using the (U-Th)/He and U-Th-Pb methods. However, unlike other uranium-bearing minerals such as apatite, zircon and titanite, apart from a few reconnaissance studies, its potential for fission-track dating has not been systematically investigated. These earlier studies produced very young ages suggesting that fission tracks may be annealed at very low temperatures. This study further assesses its potential for thermochronology studies by determining its thermal annealing properties via a series of isochronal heating experiments.

252Cf fission-tracks were implanted into Harcourt Granodiorite (Victoria, Australia) monazite crystals on polished surfaces oriented parallel and perpendicular to {100} prismatic faces. Tracks were annealed over 1, 10, 100 and 1000 hour schedules at temperatures between 30°C and 400°C. Track length measurements were made on captured digital image stacks, and then converted to calculate mean lengths of equivalent confined fission tracks. In all annealed samples, the mean equivalent confined track length was always less than that in unannealed control samples. As annealing progresses, the mean track length is reduced and monazite fission-track lengths also appear to be anisotropic, as is the case for apatite, with tracks oriented perpendicular to the crystallographic c-axis annealing faster than those oriented parallel. To investigate how the mean track lengths decreased as a function of annealing time and temperature, one parallel and two fanning models were fitted to the empirical dataset. The temperature limits of the monazite partial annealing zone (MPAZ) were defined as length reductions to 0.95 (lowest) and 0.5 (highest) for this study. Extrapolation of the laboratory experiments to geological timescales indicates that for a heating duration of 107 years, estimated temperature ranges of the MPAZ are -44 to 101°C for the parallel model and -71 to 143°C (both ± 6 – 21°C, 2 standard errors) for the best fitting linear fanning model (T0 = ¥). If a monazite fission-track closure temperature is approximated as the mid-point of the MPAZ, then these results, for tracks with similar mass and energy distributions to those involved in spontaneous fission of 238U, are consistent with previously estimated closure temperatures (calculated from substantially higher energy particles) of <50°C and perhaps not much above ambient surface temperatures. Based on our findings it is estimated that the closure temperature (Tc) for fission tracks in monazite ranges between ~45 and 25°C over geological timescales of 106 – 107 years, making this system potentially useful as an ultralow temperature thermochronometer.


Biography

Sean is a Phd Student in the Thermochronology Research Group, University of Melbourne. His research is on developing monazite fission track thermochronology through a series of developmental experiments and application studies.

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