Major element geochemical composition was established for 59 tephra horizons from Ocean Drilling Program Sites 1150 and 1151, located in the Japan forearc. These data, encompassing typically between 15 and 30 individual shard analyses per tephra horizon, were used to investigate the degree to which sediment reworking, postdepositional geochemical alteration, and geochemical uniqueness of individual eruptives facilitate or impede the potential for establishing a tephrostratigraphical framework for the Japan Trench, as well as usage of the tephra record to document arc evolution. Evidence was found that hydration (termed phase 1 alteration) of glass shards increases with age in the Pliocene-Pleistocene, but there is no indication that element leaching (phase 2 alteration) has occurred. Post- or syn-depositional differences in preservational style are shown to have no significant bearing on tephrogeochemical homogeneity and suitability for tephrostratigraphical analysis. Overall, therefore, the volcaniclastic record is suitable for investigating medium- to long-term changes in arc geochemistry and, provided consideration is given to the potential for nonunique geochemical signatures, is suitable for erecting tephrochronological frameworks. A limited number of Pleistocene tephra correlations are suggested in furtherance of this framework goal.
1Hunt, J.B., and Najman, Y.M.R., 2003. Tephrochronological and tephrostratigraphical potential of Pliocene-Pleistocene volcaniclastic deposits in the Japan forearc, ODP Leg 186. In Suyehiro, K., Sacks, I.S., Acton, G.D., and Oda, M. (Eds.), Proc. ODP, Sci. Results, 186 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/186_SR/107/107.htm>. [Cited YYYY-MM-DD]
2Centre for Environmental Change and Quaternary Research, GEMRU, University of Gloucestershire, Swindon Road, Cheltenham GL50 4AZ, UK.
3Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK. y.najman@glg.ed.ac.uk
Initial receipt: 2 January 2002
Acceptance: 30 January 2003
Web publication: 27 June 2003
Ms 186SR-107