THE GEOCHEMISTRY OF HOT SPRINGS OF THE LUANGWA RIFT VALLEY, ZAMBIA SHOW NO EVIDENCE OF MANTLE PROCESSES

We investigated the geochemistry of hot/cold springs and streams within the amagmatic Luangwa Rift Valley and the surrounding Proterozoic basement in Zambia. We aimed to determine continental scale links between the mantle and crustal processes by determining if shallow crustal waters can be used as a proxy of mantle tectonism. We assessed the hot springs for mantle derived components (water and carbon) and evaluated the source of heat for the hot springs and compared the geochemistry data to heat flow estimates obtained from Curie Point Depths determined from aeromagnetic data. We also compared the chemical and isotopic results for the hot springs with stream water and cold springs. The temperature of the hot springs ranged from 31 to 85ºC. The dissolved inorganic carbon (DIC) concentrations in the hot springs were similar to stream water, although they were lower than cold spring samples. The stable carbon isotopic composition (δ¹³C_DIC) of the hot springs ranged between -20 to -4‰ which can be ascribed to CO_2(g) from the soil zone and the dissolution of carbonates in fractures and faults. The δD vs. δ¹⁸O for the hot springs and cold springs lie along the trend of the local meteoric water line, suggesting that the source of water for the hot springs is meteoric. In addition, the hot springs did not show any evidence of extensive water-rock interactions that could change the δ¹⁸O. Estimates of the maximum reservoir temperatures of 150ºC indicated a local geothermal gradient of 23ºC/km was responsible for heating the water in the hot springs. We conclude that the border faults extend to depths of at least 5 km and their high permeability allows for deep penetration of meteoric water which is then heated by the normal crustal geothermal gradient. The permeability of theses faults allows for rapid circulation with limited host rock interactions. This interpretation is consistent with the heat flow data that show no anomalous elevated heat flow beneath the rift, suggesting no near surface magmatic bodies.