Geochemical and petrological characterization of gem opals from Wegel Tena, Wollo, Ethiopia: opal formation in an Oligocene soil

Research areas:

• Surfaces planétaires (2011 - 2016)

Year:

2012

Authors:

• Benjamin Rondeau
• Benedicte Cenki-Tok
• Emmanuel Fritsch
• Francesco Mazzero
• Jean-Pierre Gauthier
• Yves Bodeur
• Eyassu Bekele
• Eloise Gaillou
• Dereje Ayalew

Journal:

GEOCHEMISTRY-EXPLORATION ENVIRONMENT ANALYSIS

Volume:

12

Number:

2

Pages:

93-104

Month:

May

ISSN:

1467-7873

BibTex:

@article{WOS:000307119000002, author = "Benjamin Rondeau and Benedicte Cenki-Tok and Emmanuel Fritsch and Francesco Mazzero and Jean-Pierre Gauthier and Yves Bodeur and Eyassu Bekele and Eloise Gaillou and Dereje Ayalew", abstract = "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.", doi = "10.1144/1467-7873/10-MINDEP-058", extra = "PICL", issn = "1467-7873", journal = "GEOCHEMISTRY-EXPLORATION ENVIRONMENT ANALYSIS", month = "MAY", number = "2", pages = "93-104", title = "{G}eochemical and petrological characterization of gem opals from {W}egel {T}ena, {W}ollo, {E}thiopia: opal formation in an {O}ligocene soil", volume = "12", year = "2012", }

Abstract:

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.