Dating chert (diagenetic silica) using in-situ produced ¹⁰Be: Possible complications revealed through a comparison with ³⁶Cl applied to coexisting limestone

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• 作者：Swann Zerathe^a ; ^{zerathe@geoazur.unice.fr} ; Ré ; gis Braucher^b ; Thomas Lebourg^a ; Didier Bourlè ; s^b ; Michel Manetti^c ; Laé ; titia Lé ; anni^b
• 关键词：Cosmic ray exposure dating ; 10Be ; 36Cl ; Chert ; Diagenetic silica ; Limestone
• 刊名：Quaternary Geochronology
• 出版年：2013
• 出版时间：June, 2013
• 年：2013
• 卷：17
• 期：Complete
• 页码：81-93
• 全文大小：3863 K

文摘

This paper highlights potential complications that may arise while using in situ produced ¹⁰Be 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 ³⁶Cl and ¹⁰Be concentration measurements in both the limestone and coexisting diagenetic silica (chert), respectively. Following the routinely used decontamination and preparation protocols for ¹⁰Be produced in diagenetic silica, large discrepancies were observed with exposure ages determined by ³⁶Cl within carbonate for samples originating from the same scarp. While ³⁶Cl exposure ages were clustered as expected for a unique single gravitational event, ¹⁰Be 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?¦Ìm). To decipher and characterize the potential origins of the excess measured ¹⁰Be 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 GS3: 250-50?¦Ìm). The resulting concentrations clearly showed a decreasing ¹⁰Be content as a function of the grain size, but still yielded ¹⁰Be exposure ages significantly higher than ³⁶Cl counterparts. Because potential adsorption of ¹⁰Be 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 ¹⁰Be concentrations yielded ¹⁰Be exposure ages agreeing within uncertainties with the ³⁶Cl ones. This work suggests that measuring in situ produced ¹⁰Be within chert (amongst other types of diagenetic silica, e.g. flint, hornstone, jasper, etc.) containing amorphous silica requires caution.