Trace element and Pb–B–Li isotope systematics of olivine-hosted melt inclusions: insights into source metasomatism beneath Stromboli (southern Italy)

Abstract

We studied the elemental and isotopic (Pb, B and Li isotopes) composition of melt inclusions hosted in highly forsteritic ) olivines that were collected from San Bartolo lava and pumice (ST79p, ST82p and ST531p) samples erupted by Stromboli in historical times. The studied melt inclusions have primitive calcalkaline to shoshonitic basaltic compositions. They cover a compositional range far wider than that exhibited by the wholerocks and differ in key trace element ratios. San Bartolo melt inclusions are characterized by lower incompatible trace element abundances, higher ratios between fluidmobile (B, Pb, U and LILE) and less fluid-mobile (REE, Th, HFSE) elements and lower La/Yb ratios relative to the pumice-hosted melt inclusions and pumiceous melts erupted during paroxysmal events. Trace elements, along with different Pb, B and Li isotopic signatures, attest to source heterogeneity on the small scale and provide new insights into subducted components beneath Stromboli. Results of a mixing model suggest that metasomatism of the mantle source of pumice-hosted melt inclusions was driven by solute-rich high-pressure fluids (\20%) expelled from the deep portion of the slab. Heterogeneous Pb isotopic composition together with light d 11 B (-8.6 to -13.7%) and d 7 Li (?2.3 to -1.7%) indicates that high-pressure liquids were released in variable proportions from highly dehydrated metabasalts and metasediments. On the other hand, the elemental and isotopic (d 11 B * -1.9 to -5.9%) composition of San Bartolo melt inclusions is better explained by the addition of a prevalent aqueous component (*2 to 4%) escaped at shallower depths from sediments and altered basaltic crust in almost equivalent proportions, with a smaller contribution by high-pressure fluids. Owing to the high-angle dip of the subducted cold Ionian slab, aqueous fluids and high-pressure fluids would rise through the mantle wedge and locally superimpose on each other, thus giving origin to variously metasomatized mantle domains.

Boron isotope fractionation in magma via crustal carbonate dissolution

“…, −13.7‰ at Stromboli; ref. 30 ). The boron concentration of the Vesuvius starting glass ranges from 12 to 14 μg/g, and is hence close to the reported range of 14 to 36 μg/g for Vesuvius erupted products 29 .…”

Geochemical heterogeneities in magma beneath Mount Etna recorded by 2001–2006 melt inclusions

“… (a) La/Sm versus Ba/Nb and (b) Ba/La versus K/La diagrams. The composition of Type‐1 olivine and clinopyroxene‐hosted MIs are compared with those of pre‐1971 low‐K and post‐1971 whole rocks [ Viccaro and Cristofolini , ; Correale et al ., ], fall stratified (FS) Etnean rocks [ Kamenetsky et al ., ], MIs from Stromboli [ Schiano et al ., ; Schiavi et al ., ], Vulcano Island, and Hyblean Mts. [ Schiano et al ., ].…”

“…On the less radiogenic end, neither an EM2‐like [ Zindler and Hart , ] nor a DMM‐type [ Rehkamper and Hofmann , ] component alone can account for the observed isotopic variations, which are better reconciled with a mixture of EM2 and DMM (Figure ). A mantle wedge with similar characteristics has been hypothesized as a plausible (premetasomatic) source for magmas erupted from the near Aeolian Islands, in particular from Stromboli volcano [ Schiavi et al ., ]. In the absence of a complete isotopic characterization, our data suggest that the fingerprint of the Etna magma source is likely to result from a mixing between three end‐members with geochemical and Pb isotopic characteristics of EM2, DMM, and HIMU components (Figures and ).…”

“…According to Hoefs [2010], diagrams of critical trace element ratios that are not affected by processes of global differentiation can be more effective in focusing on specific source differences. So we used Ba/Nb versus La/Sm and Ba/La versus K/La diagrams (Figure 7) to compare mantle source fingerprinting of the studied melts with that of typical Etnean and arc-related [Viccaro and Cristofolini, 2008;Correale et al, 2014], fall stratified (FS) Etnean rocks [Kamenetsky et al, 2007], MIs from Stromboli [Schiano et al, 2004;Schiavi et al, 2012], Vulcano Island, and Hyblean Mts. [Schiano et al, 2001].…”

Analytical Method for Lithium Isotopes Determination by Thermal Ionization Mass Spectrometry: A Useful Tool for Hydrogeochemical Applications

“…In the last few decades, lithium isotopes have been investigated to reveal important information concerning a wide range of processes, such as hydrothermal activity, alteration of the oceanic crust, and riverine variability [6,[12][13][14][15]. Coupled with boron isotope systematics [16], lithium isotopes can provide information on the role and nature of the recycled crustal materials in a mantle source and on the metasomatism of a mantle wedge [17][18][19][20][21][22]. Lithium isotopes can also be useful as tracer of degassing processes [23,24] and, coupled with Fe isotope systematics [25], have become used in high temperature geochemistry to study planet formation and magmatic differentiation.…”