The sup 36 Cl ages of the brines in the Magadi-Natron basin, east Africa

Journal Article · Mon Oct 01 00:00:00 EDT 1990 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(90)90017-F· OSTI ID:6106358

Kaufman, A; Margaritz, M A; Hollos, G ^[1]; Paul, M; Boaretto, E ^[2]; Hillaire-Marcel, C ^[3]; Taieb, M ^[4]

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1. Weizmann Institute of Science, Rehovot (Israel)
2. Hebrew Univ., Jerusalem (Israel)
3. Universite du Quebec, Montreal (Canada)
4. Universite-Luminy, Marseille (France)

The depression in the East African Rift which includes both Lake Magadi and Lake Natron forms a closed basin within which almost all the dissolved chloride originates in precipitation, since there is no important source of very ancient sedimentary chloride. This provides an ideal setting for the evaluation of the {sup 36}Cl methodology as a geochemical and hydrological tracer. The main source of recent water, as represented by the most dilute samples measured, is characterized by a {sup 36}Cl/Cl ratio of 2.5 {times} 10{sup {minus}14}, in agreement with the calculated value expected in precipitation. Surface evaporation increases the chlorinity of the local freshwater inflow by about a factor of 110 without changing the isotopic ratio, indicating that little chloride enters the system in the form of sediment leachate. A second type of brine found in the basin occurs in a hot deep groundwater reservoir and is characterized by lower {sup 36}Cl/Cl ratios (<1.2 {times} 10{sup {minus}14}). By comparing this value with the 2.5 {times} 10{sup {minus}14} in recent recharge, one obtains an approximate salt accumulation age of 760 Ka which is consistent with thee time of the first appearance of the lake. These older brines also have lower {sup 18}O and {sup 2}H values which indicate that they were recharged during a climatically different era. The {sup 36}Cl/Cl ratios in the inflowing waters and in the accumulated brine, together with the known age of the Lake Magadi basin, may be used to estimate the importance of the hypogene and epigene, as opposed to the meteoric, mode of {sup 36}Cl production. Such a calculation shows that the hypogene and epigene processes together contribute less than 6% of the total {sup 36}Cl present in the lake.

• OSTI ID: 6106358
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Vol. 54:10; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
BRINES
GEOCHEMISTRY
ISOTOPE DATING
SALT DEPOSITS
DEPOSITION
CHLORIDES
CHLORINE 36
KENYA
LAKES
ORIGIN
RIFT ZONES
TANZANIA
TRACER TECHNIQUES
WATER CHEMISTRY
AFRICA
AGE ESTIMATION
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
CHEMISTRY
CHLORINE COMPOUNDS
CHLORINE ISOTOPES
DEVELOPING COUNTRIES
ELECTRON CAPTURE RADIOISOTOPES
GEOLOGIC DEPOSITS
GEOLOGIC STRUCTURES
HALIDES
HALOGEN COMPOUNDS
ISOTOPE APPLICATIONS
ISOTOPES
LIGHT NUCLEI
NUCLEI
ODD-ODD NUCLEI
RADIOISOTOPES
SURFACE WATERS
YEARS LIVING RADIOISOTOPES
580000* - Geosciences