GEOCHEMICAL FEATURES OF SITE-840 FROM THE TONGA PLATFORM - A COMPARISON WITH THE OTHER SITES FROM LEG-135 ODP

The sedimentary sequence recovered at Site 840 ODP in the Tonga Platfo rm is 597 m thick and ranges in age from Holocene to late Miocene. Her e, from the 49 samples analysed, three classes of sediments can be ide ntified, based on their proportion of biogenic carbonate and volcanoge nic content. These three classes are comparable with those previously differentiated in the Lau Basin (Sites 834-839) and with some sediment s from the Tonga Trench Margin (Site 841). In constrast with neighbour ing Site 834-839 sediments from the Lau Basin, the Site 840 sediment c omposition appears to be exempt of hydrothermal pending material and s eems to be characterized only by biogenic and terrigenous andesiticrhy olitic volcanic inputs. The alteration of volcanic material results pr incipally in the replacement of the volcanic glass by smectite. This a lteration is the major cause of the observed changes in the Ca, Mg, an d K gradients in the interstitial waters. Diagenetic processes seem to be more pronounced in the deeper part of the hole, with the formation of abundant zeolite minerals.