Mathematics – Logic
Scientific paper
Mar 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.1331g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 3, pp.1331-1341
Mathematics
Logic
2
Scientific paper
At the southern end of the Gregory Rift Valley, Lake Magadi occupies the bottom of a relatively large drainage basin. It is presently covered by a thick trona crust, which overlies two silty-clay units deposited during Late Pleistocene high lake stands. These units consist of a mixture of detrital grains (anorthoclase, amphiboles, quartz), clays (illite, authigenic zeolites), phosphates, and sedimentary sodium silicates and cherts. A late diagenetic calcite is occasionally observed. The authigenic and/or diagenetic mineralogical phases were extracted and analyzed for their uranium and thorium isotope contents. All yielded highly correlated ( 234 U / 232 Th vs. 238 U / 232 Th ) and ( 230 Th / 232 Th vs. 234 U / 232 Th ) isochrons defining two-component mixing systems (detrital and authigenic phases). The detrital component is characterized by large excesses of 230 Th (over 234 U) and by 230 Th / 232 Th ratios carrying an imprint of the source rocks (e.g., Precambrian basement vs. volcanics) and indicating efficient uranium-leaching processes during the previous pedologic cycle. The slope of the isochrons defines the 234 U / 238 U and 230 Th / 234 U ratios of the authigenic component, i.e., the age of the uranium-uptake episode. Zeolites yield an age of 10.4 ± 0.6 ka. This age is in agreement with the 14 C chronology already established for the most recent high lake level episode in the basin (10,000-12,000 yr Bp). Sodium-silicates and cherts yield distinct ages of 98.5 ± 20 and 6 ± 3 ka , respectively, for the lower and upper lacustrine units. These ages allow the conclusion that (1) sedimentary silicates are of late diagenetic origin, and (2) that the lower lacustrine unit was deposited during a former high Lake Magadi level, possibly during the lacustrine episode dated at 135 ± 10 ka from uranium-series measurements on littoral stromatolites. Finally, the late diagenetic calcite, which yields an age of about 5 ka, indicates a significant change in the sediment interstitial water chemistry. This is also shown by the occurrence of calcitic oncolites in flat deposits assigned to a mid-Holocene minor oscillation of Lake Magadi level on the basis of their 14 C ages. It is concluded that uranium-series measurements in diagenetic minerals may give access to the age of their formation.
Goetz Christian
Hillaire-Marcel Claude
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