DISTRIBUTION OF ND AND SR IN METASEDIMENTARY ROCKS AND IN-SITU ND AND SR ISOTOPE MEASUREMENTS ON ACCESSORY MINERALS FROM A PROGRADE LP/HT SEQUENCE: IMPLICATIONS FOR GEOCHRONOLOGY AND ISOTOPE EQUILIBRATION

In this study we investigate how Sr and Nd are distributed in typical metasedimentary rocks and at what temperatures Sr and Nd isotope systems equilibrate between different accessory mineral phases. Several cases of Nd and Sr isotope disequilibrium during crustal anatexis have been claimed that can affect the interpretation of the magma source and temporal constraints of crustal evolution processes. Furthermore, isotope disequilibrium in accessory phases commonly used for geochronology can lead to misinterpretations of obtained ages. To explore Nd isotope equilibrium/disequilibrium and fractionation on a sub-mineral scale, and to better understand reactions amongst Nd-rich accessory phases, we have conducted in-situ Nd isotope analyses by LA-MC-ICP-MS on apatite, allanite, titanite, xenotime and monazite in psammo-pelitic samples. We chose the Mount Lofty Ranges, South Australia, as a study area that represents a well-characterized Cambro-Ordovician sequence of prograde LP/HT metamorphism ranging from ~350˚C to partial melting (~700˚C) at ~4.5 kbar. Our results show that apatite retains the original, probably detrital, highly variable Nd isotopic signature until at least 475˚C before being isotopically homogenized at ~600˚C, irrespective of textural context within the rock. Once equilibrated, apatite retains its isotopic Nd signatures throughout anatexis. Alternatively, allanite and titanite are equilibrated at temperatures as low as 350˚C and do not change their Nd isotope signatures with increasing grade. Accessory phases in high-grade rocks (~600˚C) show very similar eNd values at the time of metamorphism. Our results suggest that under these metamorphic conditions Nd isotope disequilibrium in crustal melts is not due to isotopic variability of accessory phases and hence the Sm/Nd system is a reliable system for crustal provenance studies and age determination. We are currently measuring the Sr isotope signatures on the same minerals on which we measured Nd isotopes.