HR: 1330h
AN: V32B-1012    [PDF]
TI: Geochemical Composition of Volcanic Rocks from the May 2003 Eruption of Anatahan Volcano, Mariana Islands
AU: * Wade, J A
EM: jwade@bu.edu
AF: Department of Earth Sciences, Boston University 685 Commonwealth Avenue, Boston, MA 02215 United States
AU: Plank, T
EM: tplank@bu.edu
AF: Department of Earth Sciences, Boston University 685 Commonwealth Avenue, Boston, MA 02215 United States
AU: Stern, R
EM: rjstern@utdallas.edu
AF: Geosciences Department, University of Texas at Dallas, Richardson, TX 75083 United States
AU: Hilton, D
EM: drhilton@ucsd.edu
AF: Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093 United States
AU: Fischer, T P
EM: fischer@unm.edu
AF: Department of Earth and Planetary Sciences, Northrup Hall University of New Mexico, Albuquerque, NM 87131 United States
AU: Moore, R
EM: rbmoore@usgs.gov
AF: United States Geological Survey, Volcano Hazards Program, Denver, CO 80225 United States
AU: Trusdell, F
EM: trusdell@usgs.gov
AF: United States Geological Survey, Hawaii Volcano Observatory, Volcano, HI 96702 United States
AU: Sako, M
EM: maurice@usgs.gov
AF: United States Geological Survey, Hawaii Volcano Observatory, Volcano, HI 96702 United States
AB: The first historical eruption of Anatahan volcano began on May 10, 2003, from the easternmost of the island's two craters. Samples of tephra, scoria, and bombs, collected in May by a MARGINS-supported rapid-response team, were analyzed for 34 trace elements by solution ICP-MS at Boston University and Sr-Nd-Pb isotopic composition at the University of Texas-Dallas. The new eruptive materials can be compared with an extensive suite of pre-existing volcanics (basalts through dacites) from Anatahan sampled by the USGS in 1990 and 1992, and analyzed by XRF and INAA. While most Mariana volcanoes erupt basalts and basaltic andesites, Anatahan is unusual for erupting a wide range of compositions, from basalt to dacite, and thus provides the best opportunity for addressing questions of magma evolution in this classic island arc. The newly erupted scoria and pumice are andesites and dacites that are among the most silicic materials erupted in the northern Mariana islands. The recent eruptives are highly homogeneous; 13 samples vary by only 3-5% relative standard deviation for incompatible trace elements. Isotopic compositions (0.703450 +/- 2 87Sr/86Sr and 18.806 +/- 5 206Pb/204Pb) are within the range of previously measured samples from Anatahan and other volcanic centers in the Marianas. The combined dataset for Anatahan defines virtually a single liquid line of descent. This is consistent with nearly-parallel REE patterns, and small variations in the ratios of the most incompatible trace elements (e.g., Th/Rb varies by $<$10% over the entire fractionation trend). Low values of Th/La and Th/Zr in Anatahan volcanics provide evidence against partial melting of crustal material as a source of the silicic magmas, as these ratios are highly senstive to apatite- and zircon- saturated crustal melts. Instead, the basalts, andesites and dacites of Anatahan appear to be related predominantly by crystal fractionation with little evidence for assimilation of crustal melts. The new data can also be used to make new inferences as to the source characteristics of Anatahan magma. Trace element ratios Th/La and Sm/La distinguish island-to-island differences in the subducted sediment components incorporated into the Mariana arc magmas. Most Mariana volcanics plot on a mixing line between depleted mantle and the bulk subducting sediment Th/La (0.14). Anatahan, however, mixes to slightly higher Th/La (0.16), which could be caused by the shallow loss of the top 50 m of the sedimentary column (pelagic clay) during subduction.
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting