Implications of the serpentine phase transition on the behaviour of beryllium and lithium-boron of subducted ultramafic rocks

• Verfasst von: Vils, Flurin [VerfasserIn]
• Verfasst von: Müntener, Othmar [VerfasserIn]
• Verfasst von: Kalt, Angelika [VerfasserIn]
• Verfasst von: Ludwig, Thomas [VerfasserIn]
• Titel: Implications of the serpentine phase transition on the behaviour of beryllium and lithium-boron of subducted ultramafic rocks
• Verf.angabe: Flurin Vils, Othmar Müntener, Angelika Kalt, Thomas Ludwig
• Jahr: 2011
• Umfang: 23 S.
• Fussnoten: Erstmals am 11.Dezmber 2010 online veröffentlicht ; Gesehen am 08.12.2022
• Titel Quelle: Enthalten in: Geochimica et cosmochimica acta
• Ort Quelle: New York, NY [u.a.] : Elsevier, 1950
• Jahr Quelle: 2011
• Band/Heft Quelle: 75(2011), 5, Seite 1249-1271
• ISSN Quelle: 1872-9533
• DOI: doi:10.1016/j.gca.2010.12.007
• Datenträger: Online-Ressource
• Sprache: eng
• K10plus-PPN: 1826612831

Abstract

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⇒antigorite+brucite;lizardite+talc⇒antigorite;lizardite+tremolite⇒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. <0.001-0.009μg/g in olivine) similar to the metamorphic minerals that replace them (e.g. <0.001-0.016μg/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. 156μg/g Li, max. 318μg/g B) than in antigorite (max. 0.11μg/g Li, max. 12μg/g B). Calculated average B/Li ratios for lizardite (∼1395) and antigorite (∼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.