Radiogenic helium isotope fractionation: The role of tritium as ~3He precursor in geochemical applications

摘要

Reduced ~4He/~3He ratios, e.g., down to approx = 1/100 times those expected from radiogenic production, were observed in sedimentary rocks. Formation and history of these rocks eliminate a contribution of mantle ~3He-bearing fluid. To explain the difference between the observed and the calculated production ~4He/~3He ratios Loosli et al. (1995) and Tolstikhin et al. (1996) suggested a different behaviour of helium and tritium in damage tracks produced by emission of these nuclides. Generally, the tracks cross grain boundaries or some imperfections within a rock or mineral allowing a fast loss of noble ~4He and ~3He atoms. However, radiogenic ~3He has the precursor ~3H, generated in the exothermic ~6Li(n_t, a)~3H+4.5 MeV reaction. The energetic tritons produce damage tracks comparable with those from #alpha#-decay of U and Th series. If ~3H is chemically bound within a track, and the track is able to recover via some diagenetic process before the ~3H decay, then ~3H and daughter ~3He atoms are trapped within the recovered track. This mechanism would explain the shorter residence time of ~4He in the rocks/minerals than of ~3He, therefore, ~4He/~3He ratios could decrease through time. To check this mechanism ~4H, ~3H, and ~3He (from ~3H-decay) were produced by the above reaction in special targets, consisting of layered composites of thin sections of quartz, sample, Li-bearing cover, sample, and quartz. The samples were the same rocks in which reduced ~4He/~3He ratios have been previously observed. Each target was placed in a quartz ampoule, which was then pumped out, sealed off, and then exposed to the flux of thermal neutrons in a reactor. After irradiation and cooling down (total duration 145 days), the nuclides produced during (~3H, ~3He, ~4He) and after (~3He) irradiation were measured in the gas phase above the targets and compared with their total quantities expected from the Li abundance and the integrated neutron flux. The ratios obtained were ~3H(gas)/~3H(total)<0.05 and ~3He(gas)/~3He(total) varying from 0.2 to 0.9. The average residence times #tau# of ~3H and ~3He, respectively, were estimated to be approx = 16 and approx = 0.25 yr for this first time interval, which included the irradiation of the targets. After these first measurements, the targets were kept in a vacuum system under room temperature for 210 days and the amounts of ~3H and ~3He, which accumulated above the targets during this second time interval under fully controlled conditions, were also measured. Much slower rates of gas loss from the same targets with average residence times of #tau#(~3H) approx 600 yr and #tau#(He) approx 1.6 yr resulted for this second time interval. Probably these longer residence times are closer to those in the relevant natural environments, the ~3H residence time being much longer than the ~3H half-life. In all cases the inequality #tau#(~3He) #tau#(~3H) is valid. This confirms the proposed scenario envisaging longer retention of ~3H than He in damage tracks. Within the frame of this scenario the life-time of ~3H gives a time constraint on diagenetic processes; at least one to several newly formed atomic layers should appear during approx 10 yr to recover the tracks.