Stable isotope composition of secondary carbonates from basalric lavas of ODP Hole 104-642E

Hole 642E is located near the outer margin of the Voring Plateau in the Norwegian Sea. The thick pile of basaltic lavas penetrated during drilling are variably altered with extensive development of calcite, which fills vesicles and fractures along with saponite and celadonite. delta13C results, determined by mass spectrometry, show that most carbonates above about 1040 m have values between -2.5 and -5.5 per mil (PDB), but a few samples at approximately 1090 m have depleted delta13C values down to -26.3 per mil. Below 1100 m the delta13C values decrease from -6.0 per mil to -12 per mil. The delta 18O values range between -1.9 and -13.7 per mil (PDB), and generally decrease with depth. The results are interpreted as indicating that the calcites were precipitated from cool seawater percolating through the basalt pile at waterrrock of less than 10:1, during seawater incursion at about 54 Ma. The progressive depletion with depth may result from subsequent reequilibration at temperatures below those of formation, and the geothermal gradient on the Wring Plateau appears to have decreased with time. The very depleted values of delta13C for carbonates around the 1090-m level are probably related to organic matter from an underlying volcaniclastic unit.