@article{fdi:010016201,
  title = {40{A}r/39{A}r dating of {W}est {A}frican lateritic cryptomelanes},
  author = {{H}{\'e}nocque, {O}. and {R}uffet, {G}. and {C}olin, {F}abrice and {F}{\'e}raud, {G}.},
  editor = {},
  language = {{ENG}},
  abstract = {{D}ating weathering processes is an innovative approach to understanding climate-dependant modifications of the {E}arth's surface. {P}otassium-bearing manganese oxides are a ubiquitous feature of weathering mantles developed from {M}n-bearing parent rocks. {R}ecent studies have demonstrated the possibility of dating {K}-rich manganese oxides from the hollandite group, such as cryptomelane ({K}1-2{M}n8{O}16, n{H}2{O}), using 40{A}r/39{A}r argon-ion laser probe method. {T}his work applies that approach to cryptomelanes formed in the lateritic manganese ore deposits of {T}ambao (northern {B}urkina {F}aso), a present unexploited deposit among the cryptomelane-richest in the world. {T}wo samples of massive {M}n-rich concretions (10 mm and 4 mm in thickness) were collected from the massive 80-m high {M}n-rich hill at {T}ambao. {A} detailed petrographical study shows that these cortical formations are composed of successive {N}a-rich cryptomelane layers, with {N}a/({N}a+{K}+{B}a) up to 20%. {S}even and five subsamples from these two concretions were studied with a 40{A}r/39{A}r laser probe. {T}he age spectra indicate ages that are coherent with the general organization of the concretions, showing a regular decrease from cores towards rims. {W}ithin the two concretions, ages range from 49.8 + or - 0.4 {M}a to 48.1 + or - 0.1 {M}a, and from 47.5 + or - 0.3 {M}a to 44.5 + or - 0.4 {M}a, respectively, the youngest ages being obtained for the most external layers. {T}he integrated rates of cryptomelane precipitation are estimated at 1 and 5 mm/{M}a. {T}hese ages clusters show that weathering at {T}ambao was functioning during the early {E}ocene and thus suggest that there is a relationship between the formation of the bauxites of the {A}frican {S}urface and the {M}n-{F}e laterites. ({R}{\'e}sum{\'e} d'auteur)},
  keywords = {{GEOCHIMIE} ; {ALTERATION} {CHIMIQUE} ; {ALTERATION} {PHYSIQUE} ; {LATERITE} ; {MINERALOGIE} ; {PETROGRAPHIE} ; {DATATION} {ABSOLUE} ; {ISOTOPE} ; {ARGON} ; {BURKINA} {FASO} ; {TAMBAO} {REGION}},
  booktitle = {},
  journal = {{G}eochimica et {C}osmochimica {A}cta},
  volume = {62},
  numero = {16},
  pages = {2739--2756},
  ISSN = {0016-7037},
  year = {1998},
  DOI = {10.1016/{S}0016-7037(98)00185-9},
  URL = {https://www.documentation.ird.fr/hor/fdi:010016201},
}