Helium transfer from water into quartz crystals: A new approach for porewater dating

• Published in: Earth and planetary science letters 2005-09, Vol.238 (1), p.31-41
• Main Authors: Tolstikhin, I., Gannibal, M., Tarakanov, S., Pevzner, B., Lehmann, B., Ihly, B., Waber, H.N.
• Format: Article
• Language: English
• Subjects: concentration equilibrium; groundwater flow rate; helium; isotope; mineral; nano-geochemistry; porewater; quartz; residence time
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2005.07.017

Several important fundamental and applied problems require a quantification of slow rates of groundwater flow. To resolve these problems helium appears to be a promising tracer. In this contribution we discuss a new approach, which gives the helium inventory in a rock – pore water system by using the relevant mineral record, i.e., without extraction and investigation of the porewater samples. Some U- and Th-poor minerals such as quartz (quartz separates from Permo-Carboniferous Formation, sandstone–shale interlayering, Molasses Basin, Northern Switzerland, hereafter PCF, are used in this study) contain excessive helium having migrated into their internal helium-accessible volume (HAV) from the surrounding porewater [I.N. Tolstikhin, B.E. Lehmann, H.H. Loosli, A. Gautschi, Helium and argon isotopes in rocks, minerals and related groundwaters: a case study in Northern Switzerland, Geochim. Cosmochim. Acta 60 (1996) 1497–1514]. These volumes are estimated by using helium as a nano-size penetrating tool, i.e., by saturation of the minerals with helium under controlled pressure–temperature conditions and subsequent measurements of the helium-saturated concentrations. In the quartz separates HAV/total volume ratios vary from 0.017% to 0.16%; along with the measured initial (unsaturated) He concentration the HAV gives the internal helium pressure, the mean value obtained for 7 samples (25 sample aliquots) is P = 0.45 ± 0.15 atm (1 σ). The product of helium pressure and solubility (7.35 × 10 − 3 cc STP He/cc H 2O for the temperature and salinity of PCF aquifers reported in [F.J. Pearson, W. Balderer, H.H. Loosli, B.E. Lehmann, A. Matter, T. Peters, H. Schmassmann, A. Gautschi, Applied Isotope Hydrogeology–A Case Study in Northern Switzerland, Elsevier Amsterdam, 1991, 439 pp.]) is the mineral-derived He concentration in the respective porewater, C PW = 0.0035 ± 0.0017 cc He/cc H 2O. This value is in full accord with measured He concentrations in PCF aquifers, C PCF, varying from 0.0045 to 0.0016 cc He/cc H 2O. This agreement validates the proposed approach and also shows that the mineral–porewater helium–concentration equilibrium has been established. Indeed, estimates of the He-migration rates through our quartz samples show that in ∼6000 years the internal pressure should equilibrate with He-concentration in related porewater of PCF, and this time interval is short compared to independent estimates [I.N. Tolstikhin, B.E. Lehmann, H.H. Loosli, A. Gautschi, Helium