Gas chemistry and nitrogen isotope compositions of cold mantle gases from Rungwe Volcanic Province, southern Tanzania

Abstract

We report the first complete bulk gas chemistry and nitrogen isotope data for geothermal volatiles from the Rungwe Volcanic Province, located in the western branch of the East African Rift north of Lake Malawi. Temperatures of springs and gas emissions at Rungwe vary from 13°C to 65°C with the highest temperatures observed at the springs in the northern and southern lowlands. The vigorously degassing cold CO2 vents and springs have temperatures between 13°C and 36°C and are located at higher elevation than the hot springs. The gas compositions are ~99% CO2, 0.0008 to 0.0078mmol/mol H2, 0.0004 to 0.062mmol/mol He, 0.08 to 0.77mmol/mol Ar, 3.1 to 28.5mmol/mol N2, 0.4 to 3.73mmol/mol O2, <0.002 to 1.541mmol/mol CH4, <0.001 to 0.009mmol/mol CO, and are poor in H2S (0.045 to 0.201mmol/mol). The CO2 flux at a local gas collection plant is estimated to be 1.6×105 mol/year. Gas geothermometry indicates a range of equilibration temperatures from >250°C (from CO2–Ar) to ~60°C (from H2–Ar), which is interpreted to reflect deep equilibration with hot saline fluids and shallow re-equilibration of kinetically fast gas geothermometers with cold meteoric recharge from the highlands. N2–He–Ar systematics show that the gases fall on a well-defined mixing line between upper mantle or sub-continental lithospheric mantle and air saturated water endmembers. Details of an improved method for analyzing nitrogen isotope compositions in gas samples are presented. Nitrogen isotope compositions (δ15N values) range between +2‰ and −5.9‰, overlapping with the upper mantle range, with only one sample location displaying δ15N values greater than air (0‰). The results emphasize the importance of the East African Rift as a potential, but poorly constrained, contributor of sub-continental lithospheric mantle volatiles to the Earth's surface even in regions that are currently volcanically dormant, but are seismically active.