T130 Monazite: The Ultimate Geologic Record

Monday, 10 October 2011
1:30 PM - 5:30 PM
Room 102BC

LINKING ACCESSORY MINERAL GROWTH AND BREAKDOWN TO MAJOR MINERAL EVOLUTION IN METAMORPHIC ROCKS – WHERE ARE WE AT FOR MONAZITE?

Paper number: T130-4
Program: Technical Sessions
Time: 2:25 PM – 2:40 PM

Authors

Abstract

Monazite is a complex phosphate mineral that occurs in accessory abundance in rocks such as “S-type” granites and metapelites. Linking monazite U-Pb geochronology and trace element geochemistry to the pressure-temperature (P-T) evolution of metamorphic rocks has become a major focus of petrological studies as monazite appears to grow rather readily at medium to high grade crustal conditions and quite commonly defines a tighter age distribution than zircon in the same rock. Moreover, monazite is commonly microstructurally located within coronas (etc), suggesting a strong link between the growth of monazite and the host (e.g. coronal) mineral. Therefore, constraining the role of P-T and rock composition (X) on the stability and growth/breakdown of monazite with respect to the major metamorphic minerals in a rock is of fundamental importance. Since monazite is a phosphate, control on its growth and dissolution exerted by major metamorphic minerals (e.g. silicates) is probably limited. Monazite stability is probably most closely linked with the stability of other phosphate and rare-earth-element phases, as has been suggested many times in the past.

A first-pass assessment of the stability of monazite as a function of P-T-X space was done by considering melt-bearing systems only, i.e. the monazite dissolves into melt and grows out of melt. That study proved very instructive, particularly when coupled with zircon, in addressing a commonly observed trend in U-Pb geochronological data, namely that monazite ages are typically younger than zircon ages. More recent work on the P-T-X stability of monazite by Frank Spear & co has included other phosphate phases in calculations so that monazite stability can now be investigated to for sub- and supra-solidus conditions. The connection to the major metamorphic minerals of a rock is via an yttrium exchange vector between garnet and monazite.

These studies, along with analogous studies involving zircon, have been instrumental in expanding our capability with respect to the important question of “what does geochronological data mean with respect to the evolution of the rock that hosts the accessory minerals?” The current state of play and avenues/limitations to further progress will be covered here.