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

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 degrees C to 65 degrees 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 degrees C and 36 degrees C and are located at higher elevation than the hot springs. The gas compositions are similar to 99% CO2, 0.0008 to 0.0078 mmol/mol H-2, 0.0004 to 0.062 mmol/mol He, 0.08 to 0.77 mmol/mol Ar, 3.1 to 28.5 mmol/mol N-2, 0.4 to 3.73 mmol/mol O-2, <0.002 to 1.541 mmol/mol CH4, <0.001 to 0.009 mmol/mol CO, and are poor in H2S (0.045 to 0201 mmol/mol). The CO2 flux at a local gas collection plant is estimated to be 1.6x 10(5) mol/year. Gas geothermometry indicates a range of equilibration temperatures from >250 degrees C (from CO2-Ar) to similar to 60 degrees C (from H-2-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. N-2-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 (delta N-15 values) range between +2 parts per thousand and 5.9 parts per thousand, overlapping with the upper mantle range, with only one sample location displaying delta N-15 values greater than air (0 parts per thousand). 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. (C) 2012 Elsevier B.V. All rights reserved.