PRESERVATION OF ND ISOTOPE HETEROGENEITY DURING METAMORPHISM, AND SOME IMPLICATIONS FOR TRACKING MASS TRANSFER IN THE CRUST AND MANTLE (Invited Presentation)

Metamorphic processes are intimately associated with material transfer within - and between - the crust and mantle, as exemplified by subduction zone mass transfer between subducting slabs and the overlying mantle wedge, and by the generation of granitic magmas via anatexis of continental crust. Sm-Nd isotopes have been extensively used to track and quantify these mass transfer processes, but these applications includes an assumption that Nd isotope equilibration is reached at the time of metamorphism. Presently, there is little empirical evidence to support this assumption.

Recent analytical developments allow microscale in situ Sm-Nd isotope measurements of REE-rich minerals using laser ablation multi-collector ICP-MS. We apply this technique to various metamorphic rock suites to investigate the degree of Nd isotope re-equilibration between REE minerals during prograde metamorphism. We focus on sedimentary protoliths that formed from diverse continental sources, and so were isotopically heterogeneous at the grain scale prior to metamorphism.

The Mt Lofty Ranges (South Australia) preserves metasedimentary rocks metamorphosed at low P (~0.4 GPa) up to migmatite grade. Accessory REE minerals (apatite, allanite, titanite, xenotime, monazite) all have homogenous Nd isotope compositions similar to the bulk rock value at metamorphic temperatures above 350-400 ºC; the exception is apatite, which preserves diverse detrital isotopic compositions up to 550 ºC. These results indicate that sedimentary rocks undergoing low-P, high-T metamorphism are likely to reach Nd isotopic equilibrium prior to anatexis and granitic magma generation. Hence, apparent isotopic disequilibria situations between leucosomes and melanosomes in migmatites are not caused by the melting of an isotopically heterogeneous source, but rather may be due to influx of externally-derived melt.

By contrast, REE minerals in previously subducted metasediments from New Caledonia (blueschist facies) and the Italian Western Alps (UHP eclogite facies) preserve a range of detrital/inherited Nd isotope compositions, even at conditions up to ~4.0 GPa, ~700 ºC. Therefore, caution is needed when using Nd isotopes to calculate subducted sediment contributions to arc magmas or for determining rates of crustal recycling through subduction zones.