ZEOLITE DISTRIBUTION IN VOLCANICLASTIC DEEP-SEA SEDIMENTS FROM THE TONGA TRENCH MARGIN (SW PACIFIC)

605 m of sediments were cored in Hole 841 of the Ocean Drilling Progra m (ODP) at the Tonga Trench margin. The sedimentary sequence consists mainly of Miocene vitric siltstones, vitric sandstones, and volcanic c onglomerates. A major consideration for selecting this site was the pr esence of abundant authigenic minerals (40% to 70% of the whole rock), which consist of K-feldspars, clays, thaumasite (Ca3Si(OH)(6)CO3SO4, 12H(2)O), and zeolites. The zeolite minerals include phillipsite, clin optilolite, analcime, mordenite, chabazite, heulandite, wairakite, and erionite. The increasing amount of analcime from 257 mbsf to 470 mbsf , and the joint occurrence of mordenite and wairakite in this zone of Miocene tuffs, seems to be induced by the heat flow from a major intru sive sequence of basaltic andesite sills and dikes. This abundance of analcime in response to the thermal pulse could explain the unusual Na -depleted pore-water compositions observed in ODP Hole 841.