Fission Track Chronology and Uranium Content of Basalts from DSDP Leg 34

Abstract

Natural basaltic glasses from Leg 34 Holes 319A and 320B on the Nazca plate have been dated using fission track techniques. Fission track ages of 17.8 ±3.0 m.y. for Hole 319A and 25.2 ±3.4 m.y. for Hole 320B concur with the ages of sediments directly overlying the basalts at these sites. 40Ar/39Ar dating techniques have also been applied to material from Hole 319A (Reynolds, this volume). Although the average age (~19 m.y.) obtained using the 40Ar/39Ar method is slightly greater than the fission track age, the agreement in ages obtained from two totally different techniques is well within the limits of error imposed on either method. Uranium content of the glasses used for dating purposes is very low and may reflect original, unaltered uranium values. Fission track techniques were also utilized to determine amounts of uranium present in various basalt samples fromHoles319A, 320B, and Site 321. Uranium concentrations, which are very low in fresh basalts, increase significantly in basalts which show even the slightest signs of alteration. EXPERIMENTAL PROCEDURE Glasses from Leg 34, Holes 319A and 320B were dated in essentially the same way as described by Fleischer and Price (1964). A piece of glass from each site was sliced along its longest dimension to expose two fresh glassy surfaces. One part of the sample was retained for counting spontaneous tracks resulting from the decay of 238U, while the other part was irradiated in a highly thermalized neutron flux to induce fission of 235U in the sample. The neutron dose for each sample was monitored separately by standard glasses of known uranium content. The standard glass, the irradiated, and nonirradiated portions of the same natural glass sample were all placed together in the same mount. This ensured that all samples were polished identically, and even more important, were etched under identical conditions, a critical factor in fission track counting on an abosolute basis for the purpose of dating (Reimer, 1974). Only the fresh, well-polished surfaces of the glasses were used for counting purposes. Densities of spontaneous tracks in the natural unirradiated glass from both sites were so low that the mounts had to be reground, polished, and etched several times to increase the area for counting spontaneous tracks. Error limits imposed on the ages are the cumulative total of errors resulting from the counting statistics in the natural, irradiated, and standard glasses. Methods of determining uranium concentrations in homogenized whole rock samples have been described by Fisher (1970). Again, in this work, standard flux monitor glasses of known uranium content were used to calibrate each sample. The absolute accuracy of results obtained by this method is approximately 10% (Fisher, 1972), and since only fission of 235U takes place, the total amount of uranium present is calculated assuming a constant 235^/238^ ratio. Microscopic examination of thin rock slices from these Leg 34 samples proved useful in distinguishin g fresh and altered basalts. The uranium concentrations in the basalts could thus be compared to their state of alteration as established by microscopic examination.