Experimental constraints on the deep magma feeding system at Stromboli volcano, Italy

Abstract

New experiments have been performed on a high-K basalt (PST-9) from Stromboli volcano, Italy, to constrain the physical conditions of golden pumice magmas at their storage level and discuss their petrogenesis. Fluid-present, H2O- and CO2-bearing, near-liquidus experiments were performed at 1150°C between 100 and 400 MPa and under oxidizing conditions. Glasses were analyzed by Fourier transform IR spectroscopy and their H2O and CO2 concentrations compared with those in glass inclusions. Most glass inclusions cluster near the 200 MPa isobar, suggesting entrapment at a depth of ~8 km. Golden pumice magmas have viscosities of 7–9 Pa s and densities of 2·48–2·57 g/cm3. Compositions of experimental fluids coexisting with melts along the liquidus have been estimated by mass balance. They range from CO2-rich (XH2O ~ 0·2) at 400 MPa to H2O-rich (XH2O ~ 0·8) at 100 MPa. The free fluid phase present at the reservoir level has an XH2O of ~0·6, consistent with equilibration with a near-liquidus golden pumice magma below 300 MPa. Clinopyroxene is the liquidus phase in all high-pressure experiments, either fluid-absent (H2O) or fluid-present (H2O–CO2). In contrast, at 0·1 MPa, cpx and ol appear together on the liquidus and olivines are more Fo-rich (up to 89·1) than those crystallizing at high pressures (up to 87·3). The composition of cotectic liquids multiply saturated in cpx and ol has been experimentally determined. Most pumices and a majority of melt inclusions have compositions of cotectic melts. In contrast, PST-9 plots in the cpx field and is representative of less evolved ankaramitic magmas parental to golden pumices. Melt inclusions trapped in Fo >87 olivines form a group of ultra-calcic compositions plotting in the cpx field, interpreted as boundary layer melts locally generated by cpx dissolution in the deep reservoir. Ankaramitic melts in the Stromboli feeding system are proposed to derive from primitive mantle melts by combined crystallization, mixing, wall-rock interaction and assimilation.