Monazite Microstructures and Their Interpretation in Petrochronology

Abstract

The phosphate mineral monazite (LREE,Y,Th,Ca,Si)PO4 occurs as an accessory phase in peraluminous granites and Ca-poor meta-psammopelites. Due to negligible common Pb and very low Pb diffusion rates at high temperatures, monazite has received increasing attention in geochronology. As the monazite grain sizes are mostly below 100 μm in upper greenschist to amphibolite facies meta-psammopelites, and rarely exceed 250 μm in granulite facies gneisses and in migmatites, microstructural observation and mineral chemical analysis need the investigation by scanning electron microscope and electron probe microanalyzer, with related routines of automated mineralogy. Not only the microstructural positions, sizes and contours of the grains, but also their internal structures in backscattered electron imaging gray tones, mainly controlled by the Th contents, can be assessed by this approach. Monazite crystallizes mostly euhedral to anhedral with more or less rounded crystal corners. There are transitions from elliptical over amoeboid to strongly emarginated grain shapes. The internal structures of the grains range from single to complex concentric over systematic oszillatory zonations to turbulent and cloudy, all with low to high contrast in backscattered electron imaging gray tones. Fluid-mediated partial alteration and coupled dissolution-reprecipitation can lead to Th-poor and Th-rich rim zones with sharp concave boundaries extending to the interior. Of particular interest is the corona structure with monazite surrounded by apatite and allanite, which is interpreted to result from a replacement during retrogression. The satellite structure with an atoll-like arrangement of small monazites may indicate re-heating after retrogression. Cluster structures with numerous small monazite grains, various aggregation structures and coating suggest nucleation and growth along heating or/and enhanced fluid activity. Microstructures of monazite fluid-mediated alteration, decomposition and replacement are strongly sutured grain boundaries and sponge-like porosity and intergrowth with apatite. Garnet-bearing assemblages allow an independent reconstruction of the pressure-temperature evolution in monazite-bearing meta-psammopelites. This provides additional potential for evaluation of the monazite microstructures, mineral chemistry and Th-U-Pb ages in terms of clockwise and counterclockwise pressure-temperature-time-deformation paths of anatectic melting, metamorphism and polymetamorphism. That way, monazite microstructures serve as unique indicators of tectonic and geodynamic scenarios.

Petrochronology of Monazite-Bearing Garnet Micaschists as a Tool to Decipher the Metamorphic Evolution of the Alpine Basement

“…The principal question is, how are these P-T path segments related to monazite crystallisation and ages, or which monazite generation may be linked to specific P-T path segments? Potential methods for performing such correlations to garnet are the monazite microstructures [48], monazite inclusions in garnet, monazite mineral chemistry [76], the stability fields of monazite and allanite [77][78][79] in reference to P-T path segments, and a combination of these.…”

“…In contrast, monazite may crystallise after garnet formation along cracks or pits of the host mineral. Monazite could then be younger or of the same age as the hosting garnet [48]. In many samples, Permian monazites were enclosed in spessartine-poor garnet two porphyroblasts, signalising their post-Permian crystallisation [27,29].…”

“…Of special interest were the various modifications of the monazite corona structures (Figure 2b-e). Such coronas of apatite end epidote around Permian monazite document a post-Permian retrograde metamorphic period [26,48]. Monazite corona structures were enclosed in second generation garnet with low Mn contents, indicating a prograde metamorphism subsequent to the retrogression period (Figure 2d,e).…”

“…metamorphism were observed from monazites [48]. These were the late partial decomposition of Carboniferous monazite to apatite (Figure 2a), and the satellite structure (Figure 2e,f).…”

“…The other samples were garnet micaschists from a western series and from an eastern part of the sequence [47]. Typical microstructures of multistage metamorphism were observed from monazites [48]. These were the late partial decomposition of Carboniferous monazite to apatite (Figure 2a), and the satellite structure (Figure 2e,f).…”