The May 2003 eruption of Anatahan volcano, Mariana Islands: Geochemical evolution of a silicic island-arc volcano Jennifer A. Wade a, T , Terry Plank a , Robert J. Stern b , Darren L. Tollstrup c , James B. Gill c , Julie C. O’Leary d , John M. Eiler d , Richard B. Moore e , Jon D. Woodhead f , Frank Trusdell g , Tobias P. Fischer h , David R. Hilton i a Department of Earth Sciences, Boston University, Boston, MA 02215, USA b Geosciences Department, University of Texas at Dallas, Richardson, TX 75083, USA c Department of Earth Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA d Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 92215, USA e United States Geological Survey, Tucson, AZ 85719, USA f School of the Earth Sciences, The University of Melbourne, Victoria 3010, Australia g Hawaiian Volcano Observatory, United States Geological Survey, Hawaii National Park, HI 96718, USA h Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA i Fluids and Volatiles Laboratory, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA Received 18 July 2004; accepted 28 November 2004 Abstract The first historical eruption of Anatahan volcano began on May 10, 2003. Samples of tephra from early in the eruption were analyzed for major and trace elements, and Sr, Nd, Pb, Hf, and O isotopic compositions. The compositions of these tephras are compared with those of prehistoric samples of basalt and andesite, also newly reported here. The May 2003 eruptives are medium-K andesites with 59–63 wt.% SiO 2 , and are otherwise homogeneous (varying less than 3% 2r about the mean for 45 elements). Small, but systematic, chemical differences exist between dark (scoria) and light (pumice) fragments, which indicate fewer mafic and oxide phenocrysts in, and less degassing for, the pumice than scoria. The May 2003 magmas are nearly identical to other prehistoric eruptives from Anatahan. Nonetheless, Anatahan has erupted a wide range of compositions in the past, from basalt to dacite (49–66 wt.% SiO 2 ). The large proportion of lavas with silicic compositions at Anatahan (N 59 wt.% SiO 2 ) is unique within the active Mariana Islands, which otherwise erupt a narrow range of basalts and basaltic andesites. The silicic compositions raise the question of whether they formed via crystal fractionation or crustal assimilation. The lack of 87 Sr/ 86 Sr variation with silica content, the MORB-like d 18 O, and the incompatible behavior of Zr rule out assimilation of old crust, altered crust, or zircon-saturated crustal melts, respectively. Instead, the constancy of isotopic and trace element ratios, and the systematic variations in REE patterns are consistent with evolution by crystal fractionation of similar parental magmas. Thus, Anatahan is a type example of an island-arc volcano that erupts comagmatic basalts to dacites, with no evidence for 0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.11.035 T Corresponding author. Tel.: +1 617 353 4085; fax: +1 617 353 3290. E-mail address: jwade@bu.edu (J.A. Wade). Journal of Volcanology and Geothermal Research 146 (2005) 139– 170 www.elsevier.com/locate/jvolgeores