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<title lang=en>Geochemical and petrological characterization of gem opals from Wegel Tena, Wollo, Ethiopia: opal formation in an Oligocene soil</title>
<creator>Rondeau, Benjamin</creator>
<creator>Cenki-Tok, Bénédicte</creator>
<creator>Fritsch, Emmanuel</creator>
<creator>Mazzero, Francesco</creator>
<creator>Gauthier, Jean-Pierre</creator>
<creator>Bodeur, Yves</creator>
<creator>Bekele, Eyassu</creator>
<creator>Gaillou, Eloise</creator>
<creator>Ayalew, Dereje</creator>
<contributor>Laboratoire de Planétologie et Géodynamique de Nantes (LPGN) ; Université de Nantes (UN) - Institut national des sciences de l'Univers (INSU - CNRS) - Centre National de la Recherche Scientifique (CNRS)</contributor>
<contributor>Dynamique de la Lithosphere ; Géosciences Montpellier ; Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS) - Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS)</contributor>
<contributor>Institut des matériaux Jean Rouxel (IMN) ; Université de Nantes (UN) - Centre National de la Recherche Scientifique (CNRS)</contributor>
<contributor>Eyaopal, Addis Ababa ; Université du Québec</contributor>
<contributor>Department of Mineral Sciences, Smithsonian Institution, Washington ; Université du Québec</contributor>
<contributor>Department of Earth Sciences, Addis Ababa University ; Université du Québec</contributor>
<source>ISSN: 1467-7873</source>
<source>Geochemistry: Exploration, Environment, Analysis</source>
<publisher>Geological Society</publisher>
<identifier>hal-00767555</identifier>
<identifier>https://hal.archives-ouvertes.fr/hal-00767555</identifier>
<source>https://hal.archives-ouvertes.fr/hal-00767555</source>
<source>Geochemistry: Exploration, Environment, Analysis, Geological Society, 2012, 12 (2), pp.93-104. 〈10.1144/1467-7873/10-MINDEP-058〉</source>
<identifier>DOI : 10.1144/1467-7873/10-MINDEP-058</identifier>
<relation>info:eu-repo/semantics/altIdentifier/doi/10.1144/1467-7873/10-MINDEP-058</relation>
<language>en</language>
<subject lang=en>opal formation</subject>
<subject lang=en>Ethiopia</subject>
<subject lang=en>pedogenesis</subject>
<subject lang=en>plant fossils</subject>
<subject lang=en>chemical impurities</subject>
<subject>[SDU.STU.MI] Sciences of the Universe [physics]/Earth Sciences/Mineralogy</subject>
<type>info:eu-repo/semantics/article</type>
<type>Journal articles</type>
<description lang=en>Gem opals from Wegel Tena, Wollo Province, Ethiopia, occur in Oligocene rhyolitic ignimbrites. They display a unique geochemistry, with some samples yielding the highest Ba concentrations ever recorded. They are generally much richer in chemical impurities than opals from other localities. For example, the sum Al+Fe or the sum Na+Mg+Ca+K+Ba are often higher. These geochemical features make them easy to distinguish from other opals worldwide. We observed strong geochemical variations and some good positive correlations in our samples, such as Al+Fe vs. Na+Mg+Ca+K+Ba, Al vs Ca, or Ba vs Ca. This shows that the crystallography of opal has controlled, at least in part, the incorporation of chemical impurities, although opal is not well-crystallized. In addition, the multimodal distributions of several chemical impurities (e.g. U vs Sr, Al vs Ca, Ba vs Ca, etc.) suggest at least two origins of silica: weathering of feldspars and weathering of volcanic glass. In addition, opals from Wegel Tena contain numerous well-preserved microscopic plant fossils. Moreover, their host rock exhibits features typical of pedogenesis (abundant clays, desiccation cracks, and grain size sorting). We propose that the opals at Wegel Tena formed during the Oligocene period when volcanic emissions stopped for a time long enough to allow weathering of ingimbrites and therefore liberation of silica. This accompanied the formation of soil and development of plant life, and some plants were trapped in opal.</description>
<date>2012-05</date>
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