Pyroxenite layers in orogenic mantle massifs have been interpreted as melt-dominated products of melt-rock interaction in the lithosphere and have been suggested to play a key role in the genesis of basaltic melts. The Ligurian ophiolites in Northern Italy contain abundant pyroxenite layers, allowing for detailed study of their formation mechanisms and interaction with wall rock peridotites. They are considered to represent a portion of the Jurassic Tethyan ocean floor obducted during the Alpine orogeny. Highly siderophile elements (HSE) and 187Os/188Os signatures have been shown to be a useful tool to study melt-rock interaction in the mantle, because of their wide range of compatibility during partial melting and their affinity to sulfides, offering complementary insights compared to lithophile trace elements and their isotope systems. For this study, HSE concentrations and 187Os/188Os compositions were analysed in pyroxenites and associated wall rock peridotites from the Mt. Aiona, Mt. Prinzera, Mt. Gavi, and Rio Strega sites of the External Ligurian ophiolites. The Ligurian pyroxenites show patterns enriched in incompatible HSE such as Pd and Re, supporting a melt-dominated origin from material having undergone previous episodes of HSE fractionation, such as recycled oceanic crust. Radiogenic 187Os/188Os values support the notion of long-term Re/Os-enrichment in the source of the parental melts. The associated lherzolites show flat chondritenormalized HSE patterns, most consistent with a multi-stage history of depletion of incompatible HSE followed by replenishment of these elements to the peridotite budget through interaction with the pyroxenite-forming melts. The HSE patterns and concentrations are consistent with a proposed origin of the pyroxenite layers through partial melting of subducted eclogites, as suggested previously for the Ligurian pyroxenites (e.g. Montanini and Tribuzio, 2015) as well as for other suites (e.g. van Acken et al., 2010). These results reassert the presence of recycled material enriched in incompatible HSE in the form of pyroxenites in the oceanic mantle, and hence potentially in the source for oceanic basalts, with implications for the nature of the source of basaltic melts in the oceanic lithosphere.
Pyroxenites as tracers for melt-rock interaction and recycled material in the oceanic mantle / Van Acken, D.; Luguet, A.; Montanini, A.; Tribuzio, R.; Daly, J. S.. - ELETTRONICO. - (2018). (Intervento presentato al convegno GeoBONN 2018).
Pyroxenites as tracers for melt-rock interaction and recycled material in the oceanic mantle.
Montanini A.;
2018-01-01
Abstract
Pyroxenite layers in orogenic mantle massifs have been interpreted as melt-dominated products of melt-rock interaction in the lithosphere and have been suggested to play a key role in the genesis of basaltic melts. The Ligurian ophiolites in Northern Italy contain abundant pyroxenite layers, allowing for detailed study of their formation mechanisms and interaction with wall rock peridotites. They are considered to represent a portion of the Jurassic Tethyan ocean floor obducted during the Alpine orogeny. Highly siderophile elements (HSE) and 187Os/188Os signatures have been shown to be a useful tool to study melt-rock interaction in the mantle, because of their wide range of compatibility during partial melting and their affinity to sulfides, offering complementary insights compared to lithophile trace elements and their isotope systems. For this study, HSE concentrations and 187Os/188Os compositions were analysed in pyroxenites and associated wall rock peridotites from the Mt. Aiona, Mt. Prinzera, Mt. Gavi, and Rio Strega sites of the External Ligurian ophiolites. The Ligurian pyroxenites show patterns enriched in incompatible HSE such as Pd and Re, supporting a melt-dominated origin from material having undergone previous episodes of HSE fractionation, such as recycled oceanic crust. Radiogenic 187Os/188Os values support the notion of long-term Re/Os-enrichment in the source of the parental melts. The associated lherzolites show flat chondritenormalized HSE patterns, most consistent with a multi-stage history of depletion of incompatible HSE followed by replenishment of these elements to the peridotite budget through interaction with the pyroxenite-forming melts. The HSE patterns and concentrations are consistent with a proposed origin of the pyroxenite layers through partial melting of subducted eclogites, as suggested previously for the Ligurian pyroxenites (e.g. Montanini and Tribuzio, 2015) as well as for other suites (e.g. van Acken et al., 2010). These results reassert the presence of recycled material enriched in incompatible HSE in the form of pyroxenites in the oceanic mantle, and hence potentially in the source for oceanic basalts, with implications for the nature of the source of basaltic melts in the oceanic lithosphere.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
