陨石中林伍德石的形成条件对寻找地球产状林伍德石的启示

The formation conditions of ringwoodite in meteorites: Clues to finding terrestrial ringwoodite.

橄榄石高压多形林伍德石被认为是地幔过渡带的主要矿物。天然产状林伍德石主要在发生强烈冲击变质的球粒陨石冲击脉体中出现。目前还没有在地球岩石中发现林伍德石的报告。陨石冲击脉体的温度压力历史和矿物组合特征研究表明,林伍德石形成后,高压下淬火是使林伍德石不发生退变作用的重要条件。陨石中有利于林伍德石保存的淬火时间仅为数秒到十多秒。在地球上任何地质事件中,均难以实现在如此短的时间内使位于地幔过渡带的林伍德石被带往地球表层。寻找地球产状的林伍德石,关键是要在岩石和矿物中存在有利于林伍德石保存的条件,特别是当这些岩石和矿物仍处于高温的环境时。

Ringwoodite, a high-pressure polymorph of olivine, is an important mineral predominating in the transition zone of the Earth mantle. Natural ringwoodite has been found only in the shock-produced veins of chondritic meteorites. No terrestrial ringwoodite has yet ever been found. The pressure and temperature history of shock veins in meteorites and their mineral assemblages indicate that quenching of shock veins under pressure plays a key role in preserving ringwoodite from a reverse transformation to olivine. Time duration of up to ten seconds was estimated to be necessary for the quenching of ringwoodite. No geological process on the Earth could bring ringwoodite from the transition zone of the mantle to the surface of the Earth in such a short time period. To look for terrestrial ringwoodite, one should find those rocks or minerals available for keeping ringwoodite under high pressure, especially when the rocks and minerals are still under high temperature.