Differential Diffusion of Helium Isotopes in Glass, Quantum-tunneling He Enrichment, and Portable He/He Monitoring of Mantle Processes.

While studying the scientific and engineering aspects of a field-portable He/He ratio detector, we found elevated ratios at comparatively lower temperatures that appear to result from differential diffusion of these isotopes in pure quartz glass. The He enrichment relative to He in lab air, expressed as the ratio R and normalized to the accepted He/He ratio of 1.40 E-06 (R), ranges from peak values of about 200 to 600 in dry static samples. Even at the maximum classical He/He diffusivity ratio of 1.15, the expected R would be only 1.61 E-06. Within a narrow temperature window, the air value in our experimental set up with pure quartz glass can range from about 2.70 to 8.20 E-04, or nearly 1000 times the expected enrichment based upon classical fractionation. When plotted versus temperature, the narrow He net partial pressure peak reveals at least three sharper embedded peaks that may be quantized vibrational entrance states in quartz glass which are temperature specific. This discovery has implications for relatively low-energy industrial enrichment of scarce He from natural sources on Earth, and for radiogenic and cosmogenic helium dating assumptions in natural glasses. It also has bearing upon designs for field portable He/He ratio detectors aimed at earthquake and volcanic eruption studies, and monitoring of nuclear sites.