A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system

Abstract

This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50-25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo 88-91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180B20 7C) and similar volatile abundances. The H 2 O, S and Cl contents are relatively high, whereas magmatic CO 2 concentrations are very low, probably due to CO 2 loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K 2 O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs.

High-K ankaramitic melt inclusions and lavas in the Upper Cretaceous Eastern Srednogorie continental arc, Bulgaria: Implications for the genesis of arc shoshonites

“…However, the total of most electron microprobe analyses is between 95 and 98 wt.%, which indicates that the glasses contaiñ 3 wt.% H 2 O. This value is close to the results obtained by Gioncada et al (1998) for the melt inclusions in the most mafic magmas from Volcano Island (Aeolian Islands), which are compositionally the closest match to our ankaramitic melt inclusion compositions. Using SIMS and FTIR analyses, these authors obtained H 2 O contents between 1.8 and 2.9, and 2.1 and 3.8 wt.%, respectively.…”

“…Similar rock associations are also typical for several other continental and oceanic shoshonitic series, e.g. the Aeolian Islands Volcanic Arc (Gioncada et al, 1998), the West Sulawesi Volcanic Province (Elburg et al, 2006) and Lihir Island, Papua New Guinea (Kennedy et al, 1990). This may signify that ankaramites, instead of absarokites, are the parental magma of the shoshonitic series.…”

“…Hesse and Grove, 2003). The occurrence of more mafic high-K ankaramitic melt inclusions and lavas within high-Fo olivines (up to Fo 91 ) in the Srednogorie zone, Aeolian Islands (Gioncada et al, 1998) and South Sulawesi (Elburg et al, 2006) suggest, however, that nephelinenormative potassic ankaramites are a more likely parental magma type for the more evolved members of the shoshonitic series.…”

“…Volcano and Stromboli (Metrich and Clocchiati, 1996;Gioncada et al, 1998) and the West Sulawesi Volcanic Province, Indonesia (Elburg et al, 2006). The two groups of unusually Ca-enriched (ankaramitic) magmas likely originate from different sources.…”

“…In the past, their high-Ca nature has been interpreted as the result of accumulation of clinopyroxene in normal Lithos 113 (2009) Contents lists available at ScienceDirect Lithos j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s picrites or olivine basalts (Gunn et al, 1970;Hughes, 1982). Following the discovery of olivine-hosted melt inclusions with nephelinenormative ankaramitic compositions (Della-Pasqua and Varne, 1997;Gioncada et al, 1998), island-arc ankaramites have been designated as a distinctive magma type (Schiano et al, 2000). Recently, however, doubt has been also cast on the ankaramitic characteristics of the melt inclusions in the high-Mg olivines.…”