HR: 16:30h
AN: V12H-03    [PDF]
TI: Extreme $^{210}$Pb-$^{226}$Ra Disequilibria Observed in arc Lavas: Implications for the Time Scales of Magma Degassing
AU: * Turner, S
EM: sturner@els.mq.edu.au
AF: GEMOC, Department of Earth and Planetary Sciences, Macquarie University, North Ryde, Sydney, NSW 2109 Australia
AU: Black, S
EM:
AF: Postgraduate Research Institute for Sedimentology (PRIS), University of Reading, Whiteknights, PO Box 227, Reading, RG6 6AB United Kingdom
AB: We have undertaken $\alpha$-counting measurements of $^{210}$Pb activity in 39 arc lavas previously analysed by TIMS for U-Th-Ra and, more recently, U-Pa disequilibria from the Lesser Antilles, Tonga, Vanuatu, Philippines, Marianas, Sunda, Kamchatka and the Aleutians. The lavas were erupted between 1953 and 1999 and show extreme variation in $^{210}$Pb -$^{226}$Ra disequilibria with age corrected ($^{210}$Pb /$^{226}$Ra) activity ratios ranging from 0.36 to 3.14. In detail, the majority (25) of the lavas analysed preserve $^{210}$Pb deficits with 17 having ($^{210}$Pb /$^{226}$Ra) $<$ 0.9 and 7 ($^{210}$Pb /$^{226}$Ra) $<$ 0.8 whilst 5 are below ($^{210}$Pb /$^{226}$Ra) = 0.6. Of the 14 lavas that have $^{210}$Pb excess, 6 have ($^{210}$Pb /$^{226}$Ra) $>$ 1.2. Whereas $^{210}$Pb deficits are found across the compositional spectrum of lavas analysed (silica = 47-65 percent), ($^{210}$Pb /$^{226}$Ra) appears to increase with increasing silica in those lavas that have $^{210}$Pb excesses. The $^{210}$Pb deficits are most readily interpreted in terms of protracted magma degassing and the numerical model of Gauthier and Condomines 1999 (EPSL 172: 111-126) suggests that the typical duration of degassing is on the order of 10's of years but may reach 45 years in the case of the largest $^{210}$Pb deficits at Yasur in Vanuatu, Mt Mayon in the Philippines, Avachinsky in Kamchatka and Spurr, Redoubt and Shishaldin in the Aleutians. These estimates for the duration of degassing represent minimum time scales since they assume 100 percent efficient degassing of $^{222}$Rn and no magma replenishment during that period. Therefore, it appears that the majority of arc magmas undergo efficient and protracted degassing for decades prior to eruption. By contrast, there is no simple model for explaining the $^{210}$Pb excesses. Mass balance calculations indicate that plagioclase accumulation cannot account for the observed excesses. Instead, we suggest that inefficient gas release and/or sublimation of $^{210}$Pb produced by decay from $^{222}$Rn during gaseous transport through the magma may be responsible for the observed $^{210}$Pb excesses. The increasing viscosity of higher silica magmas may act to slow bubble transport and increase the likelihood of sublimation and the development of $^{210}$Pb excess leading to the observed correlations.
DE: 1035 Geochronology
DE: 8409 Atmospheric effects (0370)
DE: 8414 Eruption mechanisms
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting