MECHANISMS OF RECRYSTALLIZATION IN MONAZITE – INSIGHTS FROM IN SITU MICRO-ANALYSIS OF ALTERED MONAZITE

Monazite is a key mineral used for providing absolute time constraints on event histories and the rates of geological processes. Robust interpretation of age data requires that we understand the mechanisms of monazite growth or recrystallization, and can recognize textural or geochemical indicators for the origin of the monazite we are dating. An integrated micro-analytical study has revealed age and chemical complexity in monazite that was altered during an event subsequent to its formation at high-temperature conditions.

The target rocks for this study are orthopyroxene-sapphirine granulites from the Oygarden Islands, East Antarctica, that were deformed and metamorphosed (T>900°C, P=9-10 kbars) during the Rayner Orogeny at ~930-900 Ma. The textural context of monazite suggests initial growth or recrystallization at, or near peak conditions. Backscattered Electron (BSE) imaging of monazite grains exposed zoning patterns that included BSE-bright rims, or more complex patchy, lobate zones. Some grains were also cut by narrow BSE-bright veins that envelope pores or inclusions. Electron microprobe (EMP) data indicate that bright zones are depleted in Y, HREE, U and Ca, and enriched in Th, Si and LREE. EMP chemical ages reveal partial to complete resetting of ~910 Ma monazite to ages down to ~500 Ma, with resetting correlated with BSE zoning where core-rim geometries are preserved.

Vein-like zoning patterns in monazite indicate that fluid may have been a driver of composition and age alteration. However, the preservation of high-T Rayner mineral textures and compositions in the host rocks and absence of retrogression, suggests that resetting of monazite must have occurred during a relatively low-T event and by low water-activity fluids. In addition, the presence of altered boundaries between monazite and its orthopyroxene host supports the notion of fluid-driven alteration to a radiation-damaged host mineral. Focused-Ion-Beam techniques are being used to extract thin foils for TEM analysis of monazite to target boundaries between core-rim zones and grain-inclusion or grain-grain boundaries, in order to gain a more complete understanding of the crystal integrity of the monazite and therefore elucidate mechanisms driving recrystallization.