Abstract
Abstract 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.
Highlights
Inclusion-free outer rims of different major and trace element composition that align with the foliation have overgrown inclusion-rich cores and inner rims in samples LG-09-61 and LG-10-83 (Fig. 2d)
Due to the presence of monazite inclusions in the garnet inner rims in samples LG-09-6g, LG-09-61 and LG-10-83, we suggest that the chemical composition of the matrix monazite cores reflects the composition of monazite that grew in equilibrium with the garnet rims
Recent analytical developments that allow increasing spatial precision in geochemical analyses have allowed geochronological information recorded in zoned accessory phases to be linked to the PT information recorded in zoned major rock-forming minerals
Summary
Operating conditions for major-element analyses involved a beam current of 20 nA, an accelerating voltage of 20 kV and data collection times of 10– 30 s depending on the element. Sample analyses were bracketed by secondary standard analyses to check for major-element reproducibility of at least 1%. Mg, Ca, Mn and Y concentrations in garnet were mapped at 1–10 μm resolution depending on grain size, with a 1 μm beam and 35 ms per pixel. U, Y, Th and Ce concentrations were mapped in monazite at 1 μm resolution with a 1 μm beam and collection times of 35 ms per pixel. The same operating conditions were used to generate all monazite maps and relative concentrations of Ce, Th and Y are assessable between grains in the same sample and between samples. Concentrations of Ce, Th and Y in monazite are described below as ‘low’, ‘average’ and/or ‘high’
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