Ion microprobe analysis of a potassium self-diffusion experiment in biotite

Self-diffusion of potassium in biotite has been measured by inducing isotopic exchange between the potassium in the biotite and potassium enriched in ⁴¹K in a hydrothermal alkali chloride solution at 650°C and 2000 bars pressure, and then analyzing the resulting distribution of the K isotopes in the mica using an ion microprobe mass analyzer. The distribution of K isotopes in one mica flake was studied by three-dimensional mapping. The results show the diffusivity in the a or b directions to be greater than in the c direction by 2 to 4 orders of magnitude. In the c direction, the isotopic profile yields a diffusion coefficient of D = 1 × 10^?17cm²sec^?1 with an estimated error of a factor of ten. These results are in good agreement with bulk isotopic transport measurements on the same experiment reported by A.W. Hofmann and B.J. Giletti (1970). It is thus shown that diffusivities involving very slow net transport can be measured using an isotopic exchange technique and measuring the effect using an ion microprobe mass analyzer.     

Quantitative study of the distributary braidplain of the Preboreal ice-contact Gardermoen delta complex, southeastern Norway

This raised delta structure is an ice-contact deltaic complex with a volume of c. 4.4.10⁹ m³, deposited c . 9500 yr BP in a shallow wide 'fjord' during the retreat of the Scandinavian ice cap. The delta plain lies at an altitude of 200–223 m. It aggraded c . 20 m above the contemporaneous sea level during a regional marine regression. The braidplain palaeochannel characteristics indicate a peak meltwater discharge of 7–9 10³ m³/s. Calculations based on a glacial ablation model indicate a mid-summer discharge of c . 5.5 10³ m³/s. However, the fluvial topset of the delta has an erosive base whose altitude decreases upstream and indicates stream incision by more the 6 m below the contemporaneous sea level. The deep scour is ascribed to episodic floods over the relatively short delta plain, which exceeded direct ablation-associated discharges. The depositional time-span of the delta is assessed to have been 70 years, calculated from coastal gradient and shoreline displacement curves. The average sedimentation rate of the delta is thereby inferred to have been extremely high, c . 6. 10⁷ m³/yr. The sedimentation is thought to reflect 'extreme' ice-margin conditions, where the sediment and water discharge was maximized by full-scale ablation, with simultaneous subglacial, englacial and supraglacial sediment and water supply. These conditions might further coincide with an abundant rainfall in the catchment area or the drainage of dammed waters, initiating episodic floods which eroded deep beneath sea level. As a whole, the study illustrates the hydrological conditions of proglacial sedimentation at the front of the rapidly retreating last Scandinavian ice cap.