Garnet–monazite rare earth element relationships in sub-solidus metapelites: a case study from Bhutan
Posted on 27.03.2018 - 10:09
A key aim of modern metamorphic geochronology is to constrain precise and accurate rates and timescales of tectonic processes. One promising approach in amphibolite and granulite-facies rocks links the geochronological information recorded in zoned accessory phases such as monazite to the pressure–temperature information recorded in zoned major rock-forming minerals such as garnet. Both phases incorporate rare earth elements (REE) as they crystallize and their equilibrium partitioning behaviour potentially provides a useful way of linking time to temperature. We report REE data from sub-solidus amphibolite-facies metapelites from Bhutan, where overlapping ages, inclusion relationships and Gd/Lu ratios suggest that garnet and monazite co-crystallized. The garnet–monazite REE relationships in these samples show a steeper pattern across the heavy (H)REE than previously reported. The difference between our dataset and the previously reported data may be due to a temperature-dependence on the partition coefficients, disequilibrium in either dataset, differences in monazite chemistry or the presence or absence of a third phase that competed for the available REE during growth. We urge caution against using empirically-derived partition coefficients from natural samples as evidence for, or against, equilibrium of REE-bearing phases until monazite–garnet partitioning behaviour is better constrained.
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Warren, Clare J.; Greenwood, Lucy V.; Argles, Tom W.; Roberts, Nick M. W.; Parrish, Randall R.; Harris, Nigel B. W. (2018): Garnet–monazite rare earth element relationships in sub-solidus metapelites: a case study from Bhutan. Geological Society of London. Collection. https://doi.org/10.6084/m9.figshare.c.4044323.v1
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Clare J. Warren
Lucy V. Greenwood
Tom W. Argles
Nick M. W. Roberts
Randall R. Parrish
Nigel B. W. Harris
granulite-facies rocks linksinclusion relationshipsrock-forming mineralscase studysub-solidus metapelitesearth element relationshipsequilibrium partitioning behaviourmonazite co-crystallizedsub-solidus amphibolite-facies metapelitesREE-bearing phasessamples showpartition coefficientsaccessory phasesearth elementsgarnetdatasettectonic processesgeochronological informationreport REE datamonazite chemistryBhutanempirically-derived partition coefficients