HR: 0800h
AN: GP41E-04    [Abstracts]
TI: Magnetic properties of volcanic ash at Yasur, Vanuatu: Variations with time and implications for devitrification processes
AU: * Kelley, J L
EM: jlkelley@siu.edu
AF: Southern Illinois University at Carbondale, 1259 Lincoln Drive, Carbondale, IL 62901, United States
AU: Ferré, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University at Carbondale, 1259 Lincoln Drive, Carbondale, IL 62901, United States
AU: Bani, P
EM: philipson.bani@univ-nc.nc
AF: University of New Caledonia, LGPMC-EA 3325, BP R4, Noumea Cedex, 98851, New Caledonia
AB: Yasur is a small stratovolcano located on Tanna Island in the New Hebrides island arc of the SW Pacific. This volcano has experienced nearly continuous strombolian and vulcanian eruptions for at least the past 1,000 years. The cone height is 365 m with three vents that erupt identical magma. The ash deposits form regular subhorizontal strata in a nearby plain where they have been deeply incised by erosional gulleys. Thirty-three specimens have been collected across a 17 m-thick profile, sampling volcanic activity over a period of about 1,000 years. The regional climate has been hot and humid for thousands of years and hence favorable to rapid devitrification. As the age of ash deposits increases with depth, the degree of devitrification is also expected to increase. During devitrification, new crystallites are formed gradually from the volcanic glass. The present study aims to determine the origin of magnetic properties of the deposits and whether the new crystallites include magnetic minerals such as pyroxene or titanomagnetite, or consist just of feldspars. The magnetic susceptibilities of the samples, measured in low field and in increasing low field, range from 3.6 x 10^-3 to 1.3 x 10^-2 [SI]. Thermomagnetic experiments reveal the presence of a titanomagnetite component with low Curie temperature. All specimens exhibit a strong ferromagnetic behavior characterized by hysteresis properties typical of pseudo-single domain magnetic grain sizes. First Order Reversal Curve (FORC) analysis provides additional information on magnetic grain size distribution. Temperature- and frequency- dependent, and high field measurements will supplement the magnetic dataset and aid interpretation. Degree of devitrification will be assessed by performing XRD and IR spectroscopic analysis. Since magnetic measurements are strongly grain-size dependent and increasing devitrification leads to an increase in grain size, a magnetic granulometry analysis compared with degree of devitrification should yield insight to the relationship between these two parameters. Ultimately, specific magnetic parameters could be used as a proxy for the degree of devitrification. The results of this research potentially impact nearly all studies of volcanic materials, since the rate of devitrification with time is yet poorly understood.
DE: 1519 Magnetic mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly