The Line Islands revisited: New 40 Ar/ 39 Ar geochronologic evidence for episodes of volcanism due to lithospheric extension A. S. Davis Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-0628, USA (davisa@mbari.org) L. B. Gray Drake Marine Consulting, 1711 Quail Hollow Road, Ben Lomond, California 95005, USA (LBG2DD@earthlink.net) D. A. Clague Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-0628, USA (clague@mbari.org) J. R. Hein U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025-3591, USA ( jhein@usgs.gov) [1] New 40 Ar/ 39 Ar ages of mineral separates and whole rock samples from nine volcanic edifices in the northern Line Islands region, between latitudes 20°N and 6°N, are incompatible with single or multiple hot spot models. Instead, two major episodes of volcanism, each lasting 5 Ma and separated by 8 Ma, occurred synchronously over long distances, not just along the main chain but also at nonaligned edifices. Volcanism during the older episode (81–86 Ma) extended over a distance of at least 1200 km along the eastern part of the complex seamount chain. Volcanism during the younger episode (68–73 Ma) was concentrated in the western part of the chain and may have extended over a distance of >4000 km. Chemical analyses of 68 samples represent a compositionally diverse suite, including tholeiitic, transitional, and alkalic basalt, strongly alkalic basanite and nephelinite, and alkalic differentiates ranging from hawaiite to trachyte. The most diverse assemblage of rocks was recovered from a cross- trending seamount chain south of Johnston Atoll. Although compositions of rocks from the two volcanic episodes overlap, compositions from the younger episode generally are more alkalic and include a larger proportion of highly differentiated compositions. None of the samples from the older episode, but many from the younger one, contain hydrous mineral phases such as amphibole and biotite. Extensive coeval volcanism along major segments of the chain is compatible with decompressional melting of heterogeneous mantle due to diffuse lithospheric extension along pre-existing zones of weakness. Episodes of volcanism are probably related to broad upwarping of the Superswell region in the eastern South Pacific, where these lavas originated. Components: 10,825 words, 10 figures, 3 tables. Keywords: Central Pacific; Line Islands; Mesozoic volcanism; geochronology; igneous petrology; trace elements. Index Terms: 1035 Geochemistry: Geochronology; 3640 Mineralogy and Petrology: Igneous petrology; 3670 Mineralogy and Petrology: Minor and trace element composition; 9609 Information Related to Geologic Time: Mesozoic. Received 6 June 2001; Revised 2 January 2002; Accepted 3 January 2002; Published 14 March 2002. G 3 G 3 Geochemistry Geophysics Geosystems Published by AGU and the Geochemical Society AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Geochemistry Geophysics Geosystems Article Volume 3, Number 3 14 March 2002 10.1029/2001GC000190 ISSN: 1525-2027 Copyright 2002 by the American Geophysical Union 1 of 28