Tadhak alkaline ring-complex (Mali): existence of U-Pb isochrons and ``Dupal'' signature 270 Ma ago

Details Tadhak alkaline ring-complex (Mali): existence of U-Pb isochrons and ``Dupal'' signature 270 Ma ago Tadhak alkaline ring-complex (Mali): existence of U-Pb isochrons and ``Dupal'' signature 270 Ma ago

Apr 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987e%26psl..82..316w&link_type=abstract

Earth and Planetary Science Letters, Volume 82, Issue 3-4, p. 316-322.

Computer Science

4

Scientific paper

The Tadhak alkaline ring-complex of Permian age provides two whole rock U-Pb isochrons giving concordant ages in agreement within relative errors with the Rb-Sr isochron age: 235U-207Pb isochron: 271 +/- 32 Ma (MSWD = 0.3); 238U-206Pb isochron: 254 +/- 18 Ma (MSWD = 7.8), both on 8 whole-rock samples. The existence of these isochrons indicates that in favorable conditions U (and Pb) can be immobile. This can be due either to the lack of hard oxidizing conditions and/or to the location of U, in very low concentrations, in weathering-resistant minerals. The initial ratios (206Pb/204Pb = 18.714 +/- 70 and 207Pb/204Pb = 15.589 +/- 16), corrected for their Permian age, lie in the range observed for oceanic island basalts or continental alkali basalts and indicate an origin in a similar mantle, without any significant crustal contamination. This was also suggested by the initial 87Sr/86Sr ratio of 0.70457 +/- 4. Moreover, these Sr and Pb isotopic characteristics belong to the field of the so-called ``Dupal'' anomaly and indicate that it existed already 270 Ma ago. This study shows the potential interest of isotopic investigations of within-plate alkaline ring-complexes to characterize subcontinental mantle compositions, particularly in the past.

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