Mantle volatiles in spring gases in the Basin and Range Province on the west of Beijing, China: Constraints from helium and carbon isotopes

Abstract

The mantle degassing observed at the Earth surface demonstrates both a provenance of fluids in the mantle and a pathway to the surface. Quantities of this process are discovered on the plate boundaries, where there are plenty of active volcanoes and active faults, releasing plenty of mantle volatiles. However, in intraplate tectonic settings without obvious mantle plume, the work for mantle degassing observed in spring gasses seems comparatively limited. We selected the Basin and Range Province on the west of Beijing, an area in the inner part of North China Craton, to discuss the mantle degassing based on the helium and carbon isotopes of spring gasses, and the previous works on seismic tomography and fault slip rate. The spring gas helium and carbon (CO2) isotopes indicate the mixture of crustal and mantle materials. The helium ratios (reported as RC/RA, air-corrected 3He/4He ratio, RA=1.4×10−6; RA is the air ratio) vary in the range of 0.33–2.08. The calculated mantle helium contributes 4%~26% of helium in spring gasses, and the remaining is generated in the crust by radiogenic decay of U–Th series with tiny air mixture. CO2 acquires analytical δ13CV-PDB values in the range from −20.3‰ to −10.2‰, affected by carbonate precipitation. The unaffected values are calculated to be −8.5~5.1 ‰ by temperature-dependent isotope fractionation, indicating the mixture of mantle and crustal (limestone) materials. The mantle volatiles are possibly generated in the upwelling asthenosphere, in that, the 3He/4He ratio corresponds well with the negative anomaly of P-wave velocity at the depth of 70km imaged by seismic tomography. The 3He/4He ratio also correlates with time-averaged fault slip rate, suggesting higher slip rate renders more permeable mantle vent. These consequences help to construct a conceptual model for intraplate mantle degassing, that the mantle volatiles generate in the upwelling asthenosphere and uprise through faults and fractures whose permeabilities are controlled by slip rates.