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Alteration History of Mount Epomeo Green Tuff and a Related Polymictic Breccia,
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Ischia Island, Italy: Evidence for Debris Avalanche
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Bulletin of Volcanology
S. Altaner1, C. Demosthenous2, A. Pozzuoli3, G. Rolandi3
(1) Department of Geology, University of Illinois, 1301 West Green St, Urbana, IL 61801, USA
(2) Department of Geology, University of Wisconsin Oshkosh, 800 Algoma Blvd, Oshkosh, WI 54901, USA
(3) Dipartimento di Scienze della Terra, Università degli Studi di Napoli ‘Federico II’, Largo San Marcellino
10, 80138 Napoli, Italy
S. Altaner
Email: altaner@illinois.edu
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Electronic Supplementary Material
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Methods
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X-ray Analyses
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Bulk sample mineralogy and clay mineralogy were determined by X-ray diffraction (XRD) using a Siemens
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D-500 X-ray diffractometer equipped with a graphite monochrometer and a Cu tube, operating at 40 kV and
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30 mA. Samples were scanned with a 0.040 °2 step size and a 1 s collection time. Samples for bulk
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mineralogy were crushed in a mortar and pestle and then ground in isopropyl alcohol for 6 minutes in a
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micronizing mill. Samples were then side loaded into an aluminum sample holder and X-rayed. Samples for
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clay mineralogy were gently crushed in a mortar and pestle, sonified in deionized water for 2-3 minutes, and
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centrifuged to the desired size fraction (Moore and Reynolds 1997). If flocculation occurred, ~25 mg of Na-
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hexametaphosphate were added, and the sample was centrifuged again. Supernatant was pulled through a
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<0.45 µm Millipore© filter by a vacuum pump, depositing clay onto the filter. The clay was transferred to a
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glass slide, allowed to air dry, and X-rayed. To analyze for the presence of smectite, clay slides were
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exposed to an ethylene glycol atmosphere for 2-3 days at room temperature and then X-rayed.
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Quantitative sample mineralogy and clay mineralogy were calculated from XRD peak intensities
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following a procedure described by Bayliss (1986). Mineral intensity factors (MIFs) were experimentally
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derived and calculated from Bayliss (1986). MIF describes the relationship between the intensity ratio of
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specific XRD peaks and the weight fraction ratio of two minerals in a mixture. Experimental MIFs were
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generated by mixing the desired mineral with quartz (or calcite, both of which have known MIF values from
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Bayliss, 1986) in a 1:1 weight ratio, collecting an XRD pattern of the mineral mixture, and solving for the
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MIF of the accompanying mineral using the equation: IA/IQtz = MIFA/MIFQtz where IA and IQtz = intensities
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of a selected XRD peak of mineral A and quartz, respectively and MIFA and MIFQtz = mineral intensity
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factor for mineral A and quartz, respectively (Reynolds 1989). Similarly MIFs for clay minerals were
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determined from XRD patterns calculated using the program NEWMOD© (Reynolds 1985). Determination
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of % illite layers in interstratified illite/smectite (I/S) was complicated by high iron contents in I/S, which
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significantly reduced intensity of the peak near 5 Å. The % illite in I/S was estimated using the intensity ratio
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of the low angle saddle (near 3 °2) to the 17 Å peak on glycol-treated samples.
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Bulk sample X-ray fluorescence analyses were conducted at XRAL Laboratories in Don Mills, Ontario,
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Canada.
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Electron Microprobe
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Preparation of the <0.5 µm size fraction for microprobe analysis of clay minerals followed the procedure
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described above for XRD analysis of clay mineralogy, except that following centrifugation the supernatant
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was poured into a beaker and dried in an oven at 60 °C. 10 mg of separated clay were pressed into 3 mm
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diameter pellets, glued to a glass slide with silver paint, and carbon coated. Pressed pellets were analyzed
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with a Cameca SX-50 microprobe at the University of Chicago, using a beam width of 16 µm, a current of 25
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nA, and a voltage of 15 kV. Standard analyses were collected from similarly prepared hydrothermal illite
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and smectite samples, which had also been analyzed by X-ray fluorescence. Zeolites, feldspars, and clay
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minerals were also analyzed from polished thin sections using a 5 µm beam spot.
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Microscopy
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Mineral textures were studied with an optical microscope and scanning electron microscope. Thin sections
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were made by Spectrum Petrographic in Winston, Oregon and examined using an optical microscope. Rock
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chips were mounted on an aluminum stub, Au/Pd coated with an SPI module sputter coater at 30 mA for 60
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s, and examined using a JEOL JSM-840A scanning electron microscope with a Kevex 7500 energy
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dispersive X-ray spectrometer.
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Table 1 Average structural formulae of phillipsite (from this study and previous studies), analcime,
pyrogenic K-feldspar, authigenic K-feldspar, plagioclase feldspar, and clay minerals (Fe-illite plus
interstratified illite/smectite, I/S); n = number of analyses
Phillipsite in altered trachyte tuff
Green Tuff (n = 11)
Scarrupata (n = 2)
de’ Gennaro et al. (1990)
de’ Gennaro et al. (2000)
Phillipsite in altered silicic tuff from saline alkaline
lakes
Hay (1964)
Sheppard and Gude (1968)
Sheppard and Fitzpatrick (1989)
Analcime
Green Tuff (n = 3)
polymictic breccia (n = 6)
Pyrogenic K-feldspar
Green Tuff (n = 13)
Scarrupata (n = 19)
polymictic breccia (n = 14)
Authigenic K-feldspar
polymictic breccia (n = 3)
Plagioclase Feldspar
Scarrupata (n = 2)
polymictic breccia (n = 1)
Illite plus I/S clay minerals
K1.62Na1.41Ca0.23Mg0.02Fe0.04Al4.46Si11.74O32 • nH2O
K1.80Na1.48Ca0.04Fe0.01Al4.46Si11.80O32 • nH2O
K2.40Na0.60Ca0.78Mg0.14Fe0.06Al4.71Si11.21O32 • nH2O
K2.28Na0.98Ca0.59Mg0.16Fe0.03Al5.01Si11.26O32 • nH2O
K1.23Na1.79Ca0.30Mg0.12Fe0.15Al3.68Si12.13O32 • nH2O
K1.69Na2.06Mg0.05Fe0.11Al3.67Si12.21O32 • nH2O
K1.46Na2.17Ca0.03Mg0.24Fe0.14Al3.62Si12.13O32 • nH2O
Na0.72Al0.82Si2.20O6 • nH2O
Na0.68Al0.84Si2.20O6 • nH2O
K0.70Na0.27Ca0.03Al1.04Si2.96O8
K0.68Na0.29Ca0.03Al1.04Si2.96O8
K0.63Na0.34Ca0.03Al1.05Si2.95O8
K0.95Na0.02Ca0.03Al1.00Si3.01O8
Na0.55Ca0.38K0.10Fe0.02Al1.27Si2.68O8
Na0.55Ca0.38K0.10Fe0.02Al1.27Si2.68O8
Green Tuff (n = 4)
(K0.26Na0.07Ca0.07)(Mg0.42Fe3+0.25Ti0.03Al1.33)(Al0.16Si3.84)O10(OH)
polymictic breccia (n = 4)
(K0.64Na0.04Ca0.07)(Mg0.36Fe3+0.58Ti0.05Al0.96)(Al0.35Si3.65)O10(OH)
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