210Pb and 210Po Abundances in Dacites Erupted May, 2003 From Anatahan Volcano: Implications for the Time-scales of Magma Generation and Degassing

Six samples of dacite pumice and scoria erupted in May, 2003 and collected from a number of sites around Anathan by Tobias Fischer and David Hilton were analyzed for ²¹⁰Po activities on three occasions between early June and the time of this writing. With two exceptions, all scoria and pumice samples have ²¹⁰Po activities that plot on a single growth curve indicating initial (²¹⁰Po)=0.15 +/-0.07 (1σ ) dpm/g and (²¹⁰Pb)=1.08+/-0.20 dpm/g. More precise values for these initial activities will be presented at the meeting after further analyses are performed in November. Preliminary alpha spectrometry analyses for U and Th, and ICPMS analyses distributed by Terry Plank suggest that the average (²³⁸U) is about 0.53 dpm/g. Equilibrium (²²⁸Th)/( ²³²Th) ratios indicate that these samples do not have excess ²²⁸Ra. Assuming that (²¹⁰Pb )< (²²⁶Ra) because of minor degassing of ²¹⁰Pb (see Gauthier and Condomines, 1999, EPSL, v. 172), the degassing efficiency factor for ²¹⁰Po is greater than or equal to about 0.85, which is identical to the value calculated for a basaltic andesitic lava from Arenal volcano in Costa Rica (Gill et al., 1985, GRL, v. 12). This is surprising, as the May 10 plinian eruption of Anatahan should have resulted in more closed-system degassing than a lava eruption. This and the similar ²¹⁰Po values for the scoria and pumice samples suggest that the shallow-level degassing history has little impact on the efficiency of polonium degassing. The scoria and pumice samples from sample 8-1e both have significant excesses of ²¹⁰Po over the calculated initial (²¹⁰Pb) value for the other samples. These excesses were partially leachable, indicating that ²¹⁰Po was sublimated onto these samples, and that these ejecta resided in the vent before being ejected and redeposited The high inferred (²²⁶Ra)/( ²³⁰Th) for the Anatahan dacites despite the nearly equilibrium (²³⁸U)/( ²³⁰Th) value measured for one sample contrasts with the values for these ratios in more mafic Mariana samples, which are characterized by a direct correlation between radium and uranium excesses (Turner et al., 2001, Science, v. 292). Assuming that these trends hold up after additional analysis, the most straightforward interpretation of our alpha spectrometry data is that the dacite was generated by distillation of more mafic intrusives over a period of time that is significant compared to the half-life of ²³⁰Th, and that radium was added to the dacite recently either by inmixing a Ra-enriched basalt or a crust-derived fluid.