Chemical and isotopic characteristics of hot springs along the along the Neogene Malawi rift.

Abstract:

We measured the concentrations of major ions and dissolved inorganic carbon (DIC) and the stable isotopes of carbon (δ¹³C_DIC), hydrogen (δD) and oxygen (δ¹⁸O) of hot springs along the Neogene Malawi rift. We compared the results with those of streams and a cold spring. We aimed to assess the hot springs for evidence of addition of mantle mass, specifically water and carbon and (2) determine the processes that control the chemical and isotopic evolution of the hot springs. Understanding the source(s) of heat for the springs and if the chemical and isotopic characteristics show evidence of mantle processes is an important goal of the Project for Rift Initiation, Development and Evolution (PRIDE). The temperature of the hot springs ranged from 35 to 80 ºC. High temperature anomalies are observed between latitudes 10 to 11, 12 to 13 and 15 to 16 degrees south along the rift axis. The δD and δ¹⁸O for the cold spring, hot springs and streams had a similar range, were positively correlated and lie on the trend of the local meteoric water line. We suggest negligible contribution of water from a connate or magmatic source and limited oxygen exchange from water-rock interaction or CO₂ exchange from deep sedimentary carbonates. The DIC concentrations of the hot springs are higher (5 to 61 mg C/L) than those of streams (2 to 28 mg C/L) indicating addition of carbon to the DIC pool during the circulation of some springs. The range in the δ¹³C_DIC of the hot springs (-17 to -8‰) is broader and lower compared to streams (-12 to -5‰) due to addition of carbon with a δ¹³C_DIC of -15‰ to the spring water during circulation. Our results indicate that (1) the source of water for the hot springs is meteoric, (2) the hot springs have not experienced extensive water-rock interaction due to fast circulation suggesting highly permeable fault zones, (3) the source of carbon in the DIC of the hot springs is mostly CO_2(g) from the soil zone and (4) the springs are heated by normal geothermal gradients by deep circulation along the faults.