Of Mars and Australia’s opal fields… We already covered the formation of Aussie
Of Mars and Australia’s opal fields…We already covered the formation of Aussie opal in our article on opalised fossils (at http://tinyurl.com/nms2zmd), but new research published last month by Patrice Rey of the University of Sydney in the Australian Journal of Earth Sciences provides with our clearest view of the geological events that gave us this lovely glimmering stone, along with evidence that the Big Red in the island continent’s centre is the closet Earth analogue to the red landscape of Mars. The opal fields extend through a vast area of the centre, the Great Artesian Basin, the largest intra-continental basin on Earth. It was once occupied by the Eromanga, a shallow, cold and muddy inland sea covering some 60% of the country. Fossils such as Eric the plesiosaur and Australia’s famous precious opal shells attest to the marine environment. Rey, who has been studying the basin for 30 years started off by asking himself what was special about the geological history of the area that made the gems he loves. They formed in horizons close to the old land surface, and are now overlain by later sediments.Australia and Antarctica were the last chunks of Gondwana to rift apart, and the red continent started to drift northwards towards the warmer climes that it now lives under. As it drifted, the inland sea began to dry out and retreat as the whole continent dried up, heading towards the warmer conditions of today, which led to a very specific type of chemical weathering that is very similar to that undergone by Mars. The Eromanga sediments were of the right type to allow these reactions.The weathering was of a similar type to acid mine drainage (covered at http://tinyurl.com/kvtv9oo). as the sea retreated between 100 and 97 million years ago, and the marine mudstones it had deposited were exposed, the pyrite within them began to react with rain and groundwater producing sulphuric acid. This produced a strong episode of acidic and oxidising chemical erosion, that turned Australia into the red continent (by oxidising the iron in sandstones) and dissolved silica from quartz rich sandstones that later precipitated as precious opal.The underlying rock helped, as there were few carbonate rocks such as limestone, which would have neutralised the acid. Once the acid was used up, the conditions switched to alkaline, which allowed the precipitation of the opal. This process of chemical erosion is thought to be similar to that revealed by the Mars rovers currently trundling around the planet.This implies that conditions in the Artesian Basin are good proxies for Martian environments, where non precious opal was found in 2008, along with iron oxides (the Red Planet) and similar clays to those found Down Under.It also implies that the bacterial ecosystems in central Australia may be studied in order to explore the shape that possible Martian ecosystems may have had. Some primitive bacteria in Australia live in acidic conditions and feed off iron chemical energy gradients, and were present during the time of opalisation. They survived similar conditions to those any bacteria on Mars would have gone through, and exploring the shape they have evolved into today may give us a clue of what to look for on the next planet outwards.LozImage credit: Gemology onlinehttp://www.astrobio.net/exclusive/5521/precious-opals-shine-a-light-on-marshttp://www.australiangeographic.com.au/journal/uncovered-the-truth-about-opal-formation.htmhttp://www.asianscientist.com/in-the-lab/australias-opal-rich-red-center-shows-similar-geology-red-planet-2013/http://www.sci-news.com/geology/article01128-australia-opal.htmlhttp://sydney.edu.au/news/84.html?newsstoryid=11678 -- source link
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