Sx 50 emp

PTS were studied with an Olympus BX60 microscope with transmitted- and reflected-polarized light (PLM). Cross polarized light was used to test birefringence of samples. PTS were then coated with a thin layer of graphite, sputter-coated with an Edwards E306A High Vacuum System, and analyzed with a Cameca SX 50 EMP equipped with four wavelength dispersive spectrometers (WDS) and one energy dispersive spectrometer (EDS), at a beam current of 15.0 nA and an accelerating voltage of 15 kV. The samples were investigated using a defocused beam, as described by Ballirano et al. [11 ]. Standards used for the quantitative chemical analyses were all synthetic, apart from the natural fluorapatite from Durango (Mexico) used for phosphorus (P). The synthetic standards used were barite for sulfur (S), periclase for magnesium (Mg), wollastonite for calcium (Ca), magnetite for iron (Fe), and jadeite for sodium (Na). Electron microprobe (EMP) was used to obtain quantitative chemical analysis of micron-sized crystallites and backscattered electron images, very helpful to find high-resolution spatial variations in the chemical composition of the samples. They were used in conjunction with spot probe analyses, either EDS or WDS methods. At least 16 to 30 points were analyzed for each specimen.

EMP was employed to analyse the chemical composition (major, minor and some trace elements) of the glass in exposed MIs and their host clinopyroxene crystals. The analyses were conducted at the C.N.R., Institute of Geosciences and Georesources in Padova, using a Cameca SX50 EMP (samples AL14, AN18, NEW31, and NS12), and at the Department of Earth Sciences, University of Milano, using a JEOL JXA 8200 Superprobe (samples AN137A, NS9, and NS12). As indicated by replicate analyses of standards and unknown samples at both laboratories, the analyses conducted at Padova and Milano are equivalent and comparable, for the level of accuracy required in the present study (Supplementary Tables 2 and 6). Natural and synthetic standards were used for instrumental calibration. For the measurement of the glass in exposed MIs, a 1 μm beam diameter was used to avoid contamination from the surrounding host minerals. Standard beam current and accelerating voltage conditions were applied (Supplementary Tables 2 and 6).