Science: Brown diamond gives up its secret

日期:2019-03-02 08:01:02 作者:晏豆搛 阅读:

Curiosity led a student from Hebrew University in Jerusalem to buy an unusual brown diamond and investigate where its colour came from. His surprising discovery, that it was solid carbon dioxide that turned it brown, sheds new light on the origin of the diamond and on what happens in the rocks of the Earth’s mantle. The student, Marcus Schrauder, together with Oded Navon of the Institute of Earth Sciences at the university investigated the gem. In its absorption spectrum they found lines that matched those of solid carbon dioxide at 5 gigapascals, 50 000 times atmospheric pressure. Schrauder and Navon report in Nature (vol 365, p 42) that the carbon dioxide must have been trapped at even higher pressures of 7 to 8.5 gigapascals when the diamond formed – corresponding to a depth of 220 to 270 kilometres from the Earth’s surface, within the mantle. But how did the carbon dioxide get there? Geologists had not expected it to exist at such a depth, because it reacts with the mantle’s hot silicate rocks. One possibility is that it could have formed when large chunks of carbonate rock (such as limestone) from the surface were pulled to much deeper layers. Another theory is that molten silicate rocks rising from great depths may have released it. Such rising melts form deposits called kimberlite, which are a major source of diamond. Other evidence indicates that some diamonds form from subducted carbon. Carbonates could provide the carbon if they survive deep in the mantle,