Candidate: Loago N. Molwalefhe

Degree of: Doctor of Philosophy

Department: Geosciences

Title: Geochemical and Isotopic Characterization of Shallow Basinal Brines From the Makgadikgadi Pans Complex of Northeastern Botswana: Determination of the Source of Salinity

Committee:
Dr. R. V. Krishnamurthy , Chair
Dr. Eliot A. Atekwana
Dr. C. Korestky

Date: Thursday, November 6, 2003 1:30 pm - 3:00 pm 1122 Rood Hall

Abstract: Isotopes and chemical tracers have been used to constrain the sources and causes of salinity in shallow groundwater brine from the Makgadikgadi Basin, northeastern Botswana. The brine is massively rich in chloride and sodium, and moderately rich in carbonates, sulphate and potassium, and is distinctively poor in divalent cationic species. Various models that have been advanced to explain the origin of the brines include evaporative enrichment at the surface and density-induced infiltration, dissolution of evaporites, and trapped connate water. This study presents the compositional variability and behaviors of the stable isotopes of carbon (d13C) and oxygen (d18O), dissolved inorganic carbon (DIC) content, chloride, bromide and sodium in the brine from the northern part of Sua Pan, one of the Makgadikgadi Pans Complex. DIC concentrations in the brine reach 5 grams of carbon per liter and d13CDIC compositions average
-1.6±0.2‰. The values are compatible with dissolution and accumulation of carbon dioxide of geothermal origin. The halogen content of the brine (Cl/Br ratio) is consistent with studies of fluid-inclusion brines from magmatic rocks, and suggests significant magmatic influence in the formation of the brine. Chloride mass balance indicates that up to 52% of the water derives from a magmatic source. The sodium inventory on the other hand suggests additional sodium comes from intense weathering of silicate rocks. Spatial patterns in DIC concentrations and fluid conductivity show a linear anomaly of higher conductivity and DIC to the northeast of the study area. The anomaly may correlate with subsurface structural terminus in the bedrock where saline magmatic fluids are forced towards the surface along faults. Stable hydrogen and oxygen isotope relationships show that the brine is a mixture of local meteoric water and magmatic water that has been expelled during the cooling of the basalt magma. These results are somewhat at variance with a climate-driven scenario currently used to explain the high salinities, and the conclusion of a deeper source is consistent with the geology and recent seismic evidence that indicates the area is structurally dynamic.

Related Topics

Main List of Archives:
Dissertation Defenses

Current Dissertation Defenses