Towards resolving disparate datasets for rare earth element diffusion in garnet
Abstract
Garnet is commonly used for both geochronology and geospeedometry in metamorphic rocks. The usefulness of garnet stems from its stability across a wide range of bulk rock compositions and pressure-temperature conditions, and the fact that the thermodynamics of garnet have been quantified extensively. Rare earth element (REE) diffusion in garnet, in addition to carrying implications for geospeedometry [1] and thermobarometry [2], is of paramount importance for interpreting Sm-Nd and Lu-Hf garnet ages [3].
Although Sm-Nd and Lu-Hf garnet geochronology have become commonly used dating techniques, interpretation of these ages suffers from disagreement in the literature over the rate of REE diffusion in garnet. Concisely, [4] reported REE diffusion coefficients that are more than an order of magnitude faster than those reported by [5] and [6]. It has been hypothesized by [6] that the disparity between existing datasets could be due to multiple, concentration-dependent, REE diffusion mechanisms in garnet. The authors further suggested that isotopic re-equilibration of Nd in garnet follows the faster diffusion mechanism, a hypothesis that seems to be supported by natural datasets (e.g. [7]); however, without controlled experimental data, this hypothesis is difficult to assess with any reasonable certainty. In order to attempt to reconcile the existing datasets, we are conducting a large number of experiments on REE diffusion in garnet, in which not only the REE concentrations, but also silica and alumina activities, surrounding stable phase assemblages and fluxes of major cations (Fe, Mg, Ca, Mn), are varied systematically. Although preliminary data does not indicate any dependence of REE diffusivity on REE concentration, we stress that these are preliminary data and that more experiments are underway in order to confirm this observation. Nonetheless, it appears possible that the difference between the existing experimental datasets is due to major element activity and/or multicomponent effects. [1] Jollands et al. (2018) EPSL 481, 143-153. [2] Sun and Liang (2015) Chem. Geol. 393-394, 79-92. [3] Bloch et al. (2015a) CMP 169: 14 [4] Tirone et al. (2005) GCA 69, 2385-2398. [5] Van Orman et al. (2002) CMP 142: 416-424. [6] Bloch et al. (2015b) CMP 169: 12. [7] Smit et al. (2013) EPSL 381, 222-233.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V43H0238B
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRYDE: 1160 Planetary and lunar geochronology;
- GEOCHRONOLOGYDE: 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGYDE: 3640 Igneous petrology;
- MINERALOGY AND PETROLOGYDE: 3640 Igneous petrology;
- MINERALOGY AND PETROLOGYDE: 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGYDE: 8434 Magma migration and fragmentation;
- VOLCANOLOGYDE: 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY