Compositional control on etching and retention of fission tracks in monazite

Abstract

The application of spontaneous fission tracks in monazite for thermochronology is in its infancy. Monazite may display a wide range of chemical compositions, therefore constraints on the compositional control on etching and fission-track (FT) retention is critical but is currently lacking. To address this need, monazite grains from a heavy mineral sand collected from a beach placer (Pulmoddai, NE Sri Lanka) are used to prepare three separate mounts, each etched for different durations (15, 30 and 60 minutes) in a HCl:H2O solution at 90 °C. For domains with relatively well-etched tracks, FT density, confined track length and the size of FT openings is quantified using both optical and electron microscope images. The full composition and Th–U–total Pb age of the quantified domains is determined by EPMA while LA-ICP-MS analyses are used to evaluate the accuracy of U concentrations and obtain additional U-Pb dates. Principal component analysis is applied to the entire dataset to identify potential correlations between composition and FT properties.

Etching duration introduces a sampling bias related to actinide content. Th–U-rich domains with high FT densities are quickly over-etched and cannot be quantified if a relatively long etching duration (> 15 min) is adopted, whereas Th–U-poorer domains are well-etched after 60 min. The size of FT openings is moderately (15 min etching) to strongly (60 min etching) correlated with Th content, which is thought to reflect the role of radiation damage mainly resulting from the alpha decay of Th. Quantitative results from different analysts show that domains with low FT density, large and variable track openings and significant surface damage can yield up to ±50% difference in FT density values, but that confined track length measurement is not subject to a strong bias (∼5%). Regions of interest defined for FT counting are strongly analyst-dependent and relatively small areas (∼1×104 μm2) may be preferable to avoid abrupt changes in U concentration.

Th–U–total Pb and U-Pb dates reveal that most monazite grains formed between 570 and 480 Ma while individual FT dates range from 7.31 to 0.27 Ma. An accurate calculation of FT dates needs to account for variable monazite densities (5.0–5.3 g.cm−3, calculated using EPMA analyses) and potentially different etchable range values (9.3–13 μm), which appear to depend on Si content. An important finding is the strong, positive correlation between FT date and Si content, interpreted to reflect, in the case of the heavy mineral sand and despite a lack of constraints on its full thermal history, increased track retention with greater Si content. Because monazite is commonly zoned, the main implication is that a single grain might have the potential to record several stages of a thermochronological history in the low (< 50 °C) temperature range.