Titre
Implications of the serpentine phase transition on the behaviour of beryllium and lithium-boron of subducted ultramafic rocks
Type
article
Institution
UNIL/CHUV/Unisanté + institutions partenaires
Périodique
Auteur(s)
Vils, F.
Auteure/Auteur
Muntener, O.
Auteure/Auteur
Kalt, A.
Auteure/Auteur
Ludwig, T.
Auteure/Auteur
Liens vers les personnes
Liens vers les unités
ISSN
0016-7037
Statut éditorial
Publié
Date de publication
2011
Volume
75
Première page
1249
Dernière page/numéro d’article
1271
Peer-reviewed
Oui
Langue
anglais
Résumé
The Totalp-Platta-Malenco ophiolites in the Eastern Central Alps offer a unique opportunity to study the behaviour of Li, Be and B in ultramafic rocks in response to serpentinization and to progressive Alpine metamorphism. These units represent the remnants of a former ocean-continent transition that was intensely serpentinized during exposure on the Jurassic seafloor of the Ligurian Tethys. From north to the south, three isograd reactions (lizardite double right arrow antigorite + brucite; lizardite + talc double right arrow antigorite; lizardite + tremolite double right arrow antigorite + diopside) have been used to quantify the evolution of the light element content of metamorphic minerals. We determined the Li, Be and B concentrations in major silicate minerals from the ultramafic bodies of Totalp, Platta and Malenco by secondary ion mass spectrometry. Mantle minerals have Be concentrations (e.g. <0.001-0.009 mu g/g in olivine) similar to the metamorphic minerals that replace them (e.g. <0.001-0.016 mu g/g in serpentine). The mantle signature of Be is thus neither erased during seafloor alteration nor by progressive metamorphism from prehnite-pumpellyite to epidote-amphibolite facies. In contrast, the Li and B inventories of metamorphic minerals are related to the lizardite-to-antigorite transition. Both elements display higher concentrations in the low-temperature serpentine polymorph lizardite (max. 156 mu/g Li, max. 318 mu g/g B) than in antigorite (max. 0.11 mu g/g Li, max. 12 mu g/g B). Calculated average B/Li ratios for lizardite (similar to 1395) and antigorite (similar to 115) indicate that Li fractionates from B during the lizardite-to-antigorite transition during prograde metamorphism in ultramafic rocks. In subduction zones, this signature is likely to be recorded in the B-rich nature of forearc fluids. Relative to oceanic mantle the Be content of mantle clinopyroxene is much higher, but similar to Be values from mantle xenoliths and subduction-related peridotite massifs. These data support previous hypothesis that the mantle rocks from the Eastern Central Alps have a subcontinental origin. We conclude that Be behaves conservatively during subduction metamorphism of ultramafic rocks, at least at low-temperature, and thus retains the fingerprint of ancient subduction-related igneous events in mantle peridotites. (C) 2010 Elsevier Ltd. All rights reserved.
PID Serval
serval:BIB_4F854BB5B146
Date de création
2011-10-09T19:09:42.630Z
Date de création dans IRIS
2025-05-20T19:24:47Z