Minimum Age of the Aluto Geothermal System (Ethiopia) from Fossil Temperatures Beneath a Deep Lateral Outflow

Abstract

Fossil temperatures of about 260 degC, obtained from a study of alteration minerals, are indicated for a deep layer of acidic rocks (1.05 depth) in an exploratory well outside the Aluto geothermal system. The layer is underlain by dense basalts. The present-day temperature of the layer is only The fossil and present-day temperature profiles can be explained in terms of a heating-cooling cycle associated with the build-up and decay of a deep sheet flow of hot water. A model is presented which reproduces the ancient temperature profile and which shows that the heating-cooling cycle at the bottom of the well lasted about 140,000 yr. Since the thermal, outflow originated from the Aluto thermal system, this period indicates a age for this system.