The occurrence of LREE enrichment and HREE fractionation (LaN/YbN = 18) led many authors to hypothesize the presence of residual phases in the source of Etnean alkaline magmas that preferentially retain HREE, such as garnet. The occurrence of garnet was commonly bounded to lherzolites, thus implying the onset of partial melting at depth greater than 80 km. This would indicate the occurrence of a thermal regime related to an astenospheric plume-like structure, whose evidence is still matter of debate. On the other hand, if such a structure does not occur, the melting process should be mainly under the control of slight pressure changes induced by regional lithospheric stretching at more shallow depth. Recent work has shown that partial melting of a source composed of both spinel lherzolite and garnet pyroxenite can produce sub-alkaline magmas, which can preserve a garnet signature at more shallow depth. Here, we investigate the role that partial melting of a heterogeneous metasomatized source, constituted of spinel lherzolite and garnet pyroxenite, can have in producing a trace element signature consistent with that observed for Etnean alkaline magmas. Xenoliths of deep provenance found in the Hyblean area indicate that silicate melts, and related metasomatic fluids, may have intruded a spinel lherzolite mantle to give origin to a garnet-bearing pyroxenite. As a result, the spinel lherzolite mantle is veined by garnet pyroxenite where phases related to metasomatizing agents, such as amphibole and phlogopite, have been stabilized. Mass balance calculations have been performed for simulating partial melting of such a source. Results have been compared with three representative mantle-equilibrated magmas, calculated by adding an ultramafic assemblage to compositions of the prehistoric lavas of Mt. Maletto, and of the 1763 and 2005 eruptions. Different proportions of spinel lherzolite and garnet pyroxenite, along with variable modal contents of metasomatic phases, can account for the trace element signature of the large spectrum of Etnean alkaline magmas and for their geochemical variability through time.
A new perspective on the geochemical signature of Mt. Etna alkaline magmas
GIACOMONI, Pier Paolo;
2008
Abstract
The occurrence of LREE enrichment and HREE fractionation (LaN/YbN = 18) led many authors to hypothesize the presence of residual phases in the source of Etnean alkaline magmas that preferentially retain HREE, such as garnet. The occurrence of garnet was commonly bounded to lherzolites, thus implying the onset of partial melting at depth greater than 80 km. This would indicate the occurrence of a thermal regime related to an astenospheric plume-like structure, whose evidence is still matter of debate. On the other hand, if such a structure does not occur, the melting process should be mainly under the control of slight pressure changes induced by regional lithospheric stretching at more shallow depth. Recent work has shown that partial melting of a source composed of both spinel lherzolite and garnet pyroxenite can produce sub-alkaline magmas, which can preserve a garnet signature at more shallow depth. Here, we investigate the role that partial melting of a heterogeneous metasomatized source, constituted of spinel lherzolite and garnet pyroxenite, can have in producing a trace element signature consistent with that observed for Etnean alkaline magmas. Xenoliths of deep provenance found in the Hyblean area indicate that silicate melts, and related metasomatic fluids, may have intruded a spinel lherzolite mantle to give origin to a garnet-bearing pyroxenite. As a result, the spinel lherzolite mantle is veined by garnet pyroxenite where phases related to metasomatizing agents, such as amphibole and phlogopite, have been stabilized. Mass balance calculations have been performed for simulating partial melting of such a source. Results have been compared with three representative mantle-equilibrated magmas, calculated by adding an ultramafic assemblage to compositions of the prehistoric lavas of Mt. Maletto, and of the 1763 and 2005 eruptions. Different proportions of spinel lherzolite and garnet pyroxenite, along with variable modal contents of metasomatic phases, can account for the trace element signature of the large spectrum of Etnean alkaline magmas and for their geochemical variability through time.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
