| Titre | High-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems |
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| Auteur | Brown, S T; Turchyn, A V; Bickle, M J; Davis, A C; Alt, J C; Bédard, J H; Skulski, T ; DePaolo, D J |
| Source | Earth and Planetary Science Letters vol. 535, 116101, 2020 p. 1-10, https://doi.org/10.1016/j.epsl.2020.116101 |
| Image |  |
| Année | 2020 |
| Séries alt. | Ressources naturelles Canada, Contribution externe 20190199 |
| Éditeur | Elsevier |
| Document | publication en série |
| Lang. | anglais |
| DOI | https://doi.org/10.1016/j.epsl.2020.116101 |
| Media | papier; en ligne; numérique |
| Formats | pdf (Adobe® Reader®); html |
| Sujets | système hydrothermal; expansion océanique; dorsales médio-océaniques; dynamique des fluides; etudes isotopiques; rapports isotopiques; calcium; rapports strontium-strontium; analyses thermiques;
temperature; cristallisation fractionnée; épidote; péridotites; ophiolites; croûte océanique; modèles; sulfate; géochimie de l'eau de mer; précipitation; silicates; gisements minéraux; genèse des minerais; Méthodologie; pétrologie ignée et
métamorphique; minéralogie; géochimie; géologie économique; Sciences et technologie; Nature et environnement; Phanérozoïque; Cénozoïque; Quaternaire; Mésozoïque; Crétacé; Paléozoïque; Cambrien |
| Illustrations | tableaux; graphiques; modèles |
| Programme | Initiative géoscientifique ciblée (IGC-3), 2005-2010 Appalaches IGC-3 |
| Diffusé | 2020 02 12 |
| Résumé | (disponible en anglais seulement)
Calcium isotope ratios in epidote from epidosites in ophiolites of varying Phanerozoic ages have 44Ca/40Ca ratios that are lower by 0.1 to 0.6 per mille
relative to typical mid-ocean ridge hydrothermal fluids. Epidosites are inferred to form in high-temperature parts of seafloor hydrothermal systems at temperatures above 300°C and where fluid fluxes are high, so the Ca isotopic composition of the
epidote is likely to reflect fractionation during growth of crystals from aqueous solution. Available Ca isotope data from MOR hydrothermal vent fluids and mantle peridotites constrain the delta-44Ca of likely modern hydrothermal fluids to a narrow
range at delta-44Ca= -0.05 ± 0.1. A reactive-transport model is used to evaluate whether the delta-44Ca of hydrothermal fluids might have been higher during the Cretaceous and Late Cambrian, the ages of the Troodos, Oman, and Betts Cove ophiolites
from which we have data. For these calculations we use the epidosite 87Sr/86Sr as a guide to the extent of Ca isotopic exchange that affected the ancient hydrothermal fluids, which were derived from seawater with higher Ca and Sr, and lower sulfate
concentration, than modern seawater. The calculations suggest that the ancient hydrothermal fluid delta-44Ca values were not much different from modern values, with the possible exception of the Late Cambrian example. We infer that the epidote-fluid
Ca isotope fractionation averaging delta-44Ca= -0.2 to -0.6, is most likely due to kinetic isotope fractionation during mineral precipitation. There is evidence from the literature that hydrothermal epidote may commonly form from oversaturated
solutions, which makes the kinetic isotope interpretation plausible. The equilibrium epidote-fluid Ca isotope fractionation is estimated to be delta-44Caeq approximately equal to 0 based on recently reported DFT calculations. Our results suggest that
kinetic calcium isotope fractionation can affect hydrothermal silicate minerals, and may be only slightly smaller in magnitude than the effects observed in Ca-bearing minerals at low temperature. Kinetic isotope effects during mineral growth could
provide new insights into the formation mechanisms of hydrothermal silicate minerals. |
| GEOSCAN ID | 315454 |
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