Sx50 electron microprobe

Electron probe micro-analyser (EPMA) analyses were performed using a Cameca SX50 electron microprobe (acceleration voltage of 15 kV, beam current of 12 nA) and a 1–2 µm beam width. Prior to analysis, a 10–20 nm-thick carbon layer was sputter-coated on the samples (Edwards Auto 306). Ca and Si were analysed simultaneously. Ca Kα and Si Kα were analysed using a pentaerythritol crystal and a thallium acid phthalate crystal, respectively. The standards used were albite (NaAlSi₃O₈) for Si and wollastonite (CaSiO₃) for Ca. A ZAF data correction (Merlet, 1994 ▸ ) was applied to the raw data.

The composition of the olivine host crystal (Supplementary Table 1) used in the experiments was determined using a Cameca SX-50 electron microprobe (EMPA). Accelerating potential was 15 kV, 20 nA current and 1 μm beam size, using natural and synthetic mineral standards for calibration.

Chemical analyses were carried out on sample Ac139 using a CAMECA SX50 electron microprobe (WDS mode, 20 kV, 20 nA, 2 μm beam diameter) at CNR-IGG of Padova. Standards used were Kakanui pyrope (New Zealand) from the Smithsonian Museum (TAP analyser crystal, MgKα element emission line); Amelia albite (Virginia) (TAP, NaKα); diopside (TAP, SiKα); Al₂O₃ (TAP, AlKα); MnTiO₃ (PET, TiKα; LIF, MnKα); Cr₂O₃ (LIF, CrKα); Fe₂O₃ (LIF, FeKα); sphalerite (LIF, ZnKα); NiO (LIF, NiKα). Raw data were reduced with the PAP-type correction software provided by CAMECA. The resulting oxides wt% are reported in Supplementary Table 1. Chemical analyses on sample MgCr2 were carried out by standardless EDS analysis on a FEG scanning-electron microscope, using an Oxford Instruments SDD detector and the AZtec TruQ software installed at the Oxford Instruments in Wiessbaden (Germany).

Compensated glasses with Al/Si = {0.20, 0.33, 0.72, 1.00} and 0.0 ≤ X_K = K₂O/(K₂O + Na₂O) ≤ 1.0 were prepared following the protocol described in Le Losq and Neuville^{14 (link)}. Densities of all samples have been measured with the Archimedes method using toluene as the immersion liquid (Table S1). Chemical compositions have been measured using a Cameca SX50 electron microprobe at the CAMPARIS facility of the University Paris VI (France), with a 30 nA current, U = 30 kV, and 5 s of counting. Chemical compositions are the mean of 10–20 individual measurements. All glasses are uncolored and transparent, and no crystallization has been detected by optical microscope, Raman spectroscopy or in the electron diffraction pattern during HRTEM observations.

Textural aspects of the recovered samples were firstly analyzed in thin section at the optical microscope and successively at the Scanning Electron Microscopy (SEM), using a FEI Quanta-400 at the Earth Sciences Department (Sapienza University, Rome, Italy). Phase compositions were analyzed at the Consiglio Nazionale delle Ricerche-Istituto di Geologia Ambientale e Geoingegneria (CNR-IGAG, Rome), by a Cameca SX50 electron microprobe equipped with five wavelength dispersive spectrometers (WDS).

Major and minor element compositions were determined on micro-pumice fragments and/or glass shards (grain size >250 μm) from the tephra layer FIC-12.9 (Fig 3). The analyses were carried out at the Istituto di Geologia Ambientale e Geoingegneria of the Italian National Research Council (IGAG-CNR) (Rome, Italy) using a Cameca SX50 electron microprobe equipped with a five-wavelength dispersive spectrometer. Operating conditions were as follows: accelerating voltage, 15 kV; beam current, 15 nA; beam diameter, 10–15 μm; counting time, 20 s per element. The following standards were used: wollastonite (Si and Ca), corundum (Al), diopside (Mg), andradite (Fe), rutile (Ti), orthoclase (K), jadeite (Na), phlogopite (F), potassium chloride (Cl), baritina (S), and metals (Mn). The Ti content was corrected for the overlap of the Ti-Kα peaks. To evaluate the accuracy of the EMP analyses, three international secondary standards (Kakanui augite, Iceladic Bir-1, and rhyolite RLS132 glasses) were analysed prior to the measurements. The mean analytical precision was <1% for SiO₂ and Al₂O₃, 5% for K₂O, CaO and FeO, and 6–9% for the other elements.