Formation temperatures of clays from the volcaniclastic series of Site 841ODP: an oxygen isotopic record of a paleothermal flux into the Tonga forearc

Oxygen isotopic compositions of clay minerals were determined on representa tive samples of the volcano-sedimentary series from Site 841 ODP (Tonga for earc). This isotopic study has demonstrated an abnormally high crystallisat ion temperature of the clay minerals with respect to temperature expected i n burial diagenesis. Formation temperatures obtained using delta(18)O reach up to 200 degrees C in a Fe-chlorite-corrensite paragenesis found in the v icinity of basaltic andesite sills intruded into the Miocene tuffs. The pal eothermal flux resulting from the cooling of the sills has produced very lo w grade contact metamorphism in the Miocene Tonga forearc deposits. The con sequence of this was the formation of a large amount of hydrous silicates c haracterised near the sills by a Fe-clays-analcime mineralogical associatio n.