The continental deep drilling project KTB is located in Windischeschenbach, within the Variscian orogen at the plate boundary between the Saxothuringikum and the Moldanubikum. The drill hole reached a depth of 9109m and a temperature of 260°C, giving the opportunity to gain samples under deep crustal in-situ conditions.
Since noble gases are rare and inert, they are ideal tracers for exchange and transport processes on different scales. Isotopic variations give new insight to fluid-rock-interaction and crustal degassing processes.
All stable noble gas isotopes (3He-136Xe) have been analyzed by mass spectrometry from 50 whole rock samples. Different treatment of samples (gas extraction by crushing or by fusion) allows to distinguish between noble gases trapped in fluid inclusions and those dissolved inside the mineral crystals. Additionally, gas samples from a free fluid gained in a pumping test at a depth of 4000m have been analyzed.
The concentrations of noble gases in thermally degassed samples are comparable to concentrations measured in granites and granitoids (Allègre et al., 1987). The helium isotopic ratios show variable admixtures of radiogenic dominated crustal helium with small amounts of mantle helium (3He/4He = 2*10-8- 3*10-6).
The isotopic composition of neon reveals contributions of nucleogenic 21Ne, produced by (a,n) reactions on 18O. Argon shows variable 40Ar/36Ar ratios due to varying
K-concentrations and different extend of Ar loss since the last metamorphic event. In most of the samples a spontaneous-fission-produced 134Xe and 136Xe component is present.
The measured noble gas data and the known age of the last metamorphic event gave the opportunity to estimate loss rates and crustal fluxes for the radiogenic and nucleogenic noble gases and to compare these with earlier investigations (Allègre et al., 1987; O'Nions and Oxburgh, 1983; Mamyrin and Tolstikhin, 1984; Torgersen and Ivey, 1985; Torgersen et al., 1989) (Table 1). Since 3He/4He ratios in fluid inclusions and free fluids are generally higher than in whole rock samples, the fluids seem to be the carrier of the mantle helium contributions. This is supported by the fact that the mantle helium contributions in the fluids decrease with increasing depth. Long time accumulation is required to explain the high 4He and 40Ar concentrations in the 4000m fluid (Weise et al., 1995). Thus fluids serve as a storage reservoir and a transport medium in terms of crustal degassing processes.
Allègre, C.J., Staudacher, Th. & Sarda, Ph., Earth Planet. Sci. Lett. 81, 127 (1987).
O'Nions, R.K. & Oxburgh, E.R., Nature 306, 429-431 (1983).
Mamyrin, B.A. & Tolstikhin, I.N., Helium isotopes in nature, 273 pp. (Elsevier, Amsterdam, 1984).
Torgersen, T. & Ivey, G.N., Geochim. et Cosmochim. Acta 49, 2445-2452 (1985).
Torgersen, T., Kennedy, B.M., Hiyagon H., Chiou, K.Y., Reynolds, J.H. & Clake, W.B., Earth Planet. Sci. Lett. 92, 43-56 (1989).
Weise, S.M., Drescher, J., Schäfer, K., Kirsten, T., Hansmann, J., Kamm, Chr., Kamm, H. & Machon, L., Scientific Drilling 5, 111-118 (1995).
Table 1: Calculated loss rates * (% of produced since last metamorphic event 365 Ma ago) and estimated fluxes * of radiogenic 4He, 40Ar and nucleogenic 21Ne in comparison with literature data.
4He 21Ne 40Ar
Loss rates * from KTB [%] 85 ± 15 65 ± 20 58 ± 20
Flux * [atoms m-2s-1] from KTB (3.5 ± 0.7) * 1010 500 - 4500 (2.1± 1.0) * 109
Earlier estimates for * of
whole cont. crust 1-5 [atoms m-2s-1] (0.9 - 3.2) * 1010 (1.8 - 3.2) * 109