Publications des scientifiques de l'IRD

Zerathe S., Braucher R., Lebourg T., Bourles D., Manetti M., Leanni Laetitia. (2013). Dating chert (diagenetic silica) using in-situ produced Be-10 : possible complications revealed through a comparison with Cl-36 applied to coexisting limestone. Quaternary Geochronology, 17, p. 81-93. ISSN 1871-1014.

Titre du document
Dating chert (diagenetic silica) using in-situ produced Be-10 : possible complications revealed through a comparison with Cl-36 applied to coexisting limestone
Année de publication
2013
Type de document
Article référencé dans le Web of Science WOS:000320826400007
Auteurs
Zerathe S., Braucher R., Lebourg T., Bourles D., Manetti M., Leanni Laetitia
Source
Quaternary Geochronology, 2013, 17, p. 81-93 ISSN 1871-1014
This paper highlights potential complications that may arise while using in situ produced Be-10 to date exposure or burial events using diagenetic silica (chert). The initiation and evolution of large gravitational collapses in sedimentary rocks were constrained using cosmic ray exposure dating. Because these collapses occurred in a stratigraphic level composed of chert (diagenetic silica) concretions interbedded in limestone layers, their development was studied by performing in situ-produced Cl-36 and Be-10 concentration measurements in both the limestone and coexisting diagenetic silica (chert), respectively. Following the routinely used decontamination and preparation protocols for Be-10 produced in diagenetic silica, large discrepancies were observed with exposure ages determined by Cl-36 within carbonate for samples originating from the same scarp. While Cl-36 exposure ages were clustered as expected for a unique single gravitational event, Be-10 exposure ages were scattered along the same studied scarps. To determine the origin of such a bias, petrological investigations were carried out for chert (diagenetic silica). Thin sections highlighted a complex mineralogical texture characterized by remnant silicified ooids showing calcitic cores, calcite inclusions and a dominant amorphous hydrated silica (grain > 20 mu m). To decipher and characterize the potential origins of the excess measured Be-10 within diagenetic silica, all samples were first reprocessed following the routine decontamination protocol (HCL-H2SiF6 leachings and three partial HF dissolutions) but starting from three different grain size fractions (GS1: 1000-500, GS2: 500 -250 and G53: 250-50 mu m). The resulting concentrations clearly showed a decreasing Be-10 content as a function of the grain size, but still yielded Be-10 exposure ages significantly higher than Cl-36 counterparts. Because potential adsorption of Be-10 at the surface of amorphous silica grains was suspected, partial dissolution steps following by a leaching step in hydroxylamine were investigated. Finally, it seems that an additional leaching in KOH allowed removal of the amorphous silica phase and the measured Be-10 concentrations yielded Be-10 exposure ages agreeing within uncertainties with the Cl-36 ones. This work suggests that measuring in situ produced Be-10 within chert (amongst other types of diagenetic silica, e.g. flint, hornstone, jasper, etc.) containing amorphous silica requires caution.
Plan de classement
Sciences fondamentales / Techniques d'analyse et de recherche [020] ; Géologie et formations superficielles [064]
Description Géographique
FRANCE
Localisation
Fonds IRD [F B010060454]
Identifiant IRD
fdi:010060454
Contact