ORIGIN OF CRETACEOUS AND OLIGOCENE KAOLINITES FROM THE IWAIZUMI CLAY DEPOSIT, IWATE, NORTHEASTERN JAPAN

Hydrogen- (delta D = -106 to -97 parts per thousand) and oxygen- (delt a(18)O = +14.0 to +16.6 parts per thousand) isotope compositions of ka olinite from late Cretaceous and Oligocene deposits at Iwaizumi, north eastern Japan, indicate that these clays formed by weathering of volca nic parent rocks, rather than during hydrothermal (>100 degrees C) alt eration. The Iwaizumi kaolinites also are depleted of D and O-18 relat ive to kaolinite formed during modem, tropical weathering, suggesting that the kaolinite developed under cool or cool-temperate conditions. The oxygen-isotope compositions of the kaolinite increase slightly upw ard through the deposits, perhaps implying a modest increase in temper ature from late Cretaceous to Oligocene time. The delta D and delta(18 )O results for kaolinite from the Oligocene deposits closely follow th e kaolinite weathering line. However, a small but systematic deviation from this fine for the Cretaceous kaolinites is most simply explained by post-formational, hydrogen-isotope exchange between these clays an d downward percolating meteoric water.