The timing of alkali metasomatism in paleosols.
We have measured the concentrations of rubidium and strontium and 87Sr/86Sr values of whole-rock samples from three paleosols of different ages.  The oldest of the three weathering horizons, the 2,760 Ma Mt. Roe #1 paleosol in the Fortescue Group of Western Australia, experienced addition of Rb, and probably Sr, at 2,168 +/- 10 Ma.  The intermediate paleosol, developed on the Hekpoort Basalt in South Africa, is estimated to have formed at 2,200 Ma, and yields a Rb-Sr isochron age of 1,925 +/- 32 Ma.  The youngest of the three paleosols, developed on the  Ongeluk basalt in Griqualand West, South Africa ca. 1,900 Ma, yielded a Rb-Sr age of 1,257 +/- 11 Ma.  The Rb-Sr systematics of all three paleosols were reset during post-weathering metasomatism related to local or regional thermal disturbances.  The Rb-Sr systematics of the paleosols were not subsequently disturbed.  The near-complete removal of the alkali and alkaline earth elements from these paleosols during weathering made them particularly susceptible to resetting of their Rb-Sr systematics.  Paleosols of this type are therefore sensitive indicators of the timing of thermal disturbances.
MacFarlane
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Department of Geology, Florida International University, Miami 33199, USA.
Holland
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HD
D000468:Alkalies / Q000032:analysis*; D001272:Atmosphere; D055593:Geological Phenomena; D005844:Geology; D008903:Minerals / Q000737:chemistry*; D010163:Paleontology; D012413:Rubidium / Q000032:analysis; D017640:Silicates / Q000737:chemistry*; D012987:Soil / Q000032:analysis*; D013019:South Africa; D013324:Strontium / Q000032:analysis; D013325:Strontium Isotopes; D014887:Weather; D014904:Western Australia
D000468:Alkalies; D008903:Minerals; D017640:Silicates; D012987:Soil; D013325:Strontium Isotopes; C060346:basalt; D012413:Rubidium; D013324:Strontium