Natural diamond formation by self-redox of barbonate - Drs. Jinfu Shu and Ho-Kwang Mao
MARCH 15, 2018
Intensive research on deep-Earth diamonds over the past half century has shown that the formation of most natural diamonds are via redox reactions involving carbon-bearing phase fluid or melt with external reductants. New study from a team of scientists co-led by HPSTAR director Dr. Ho-Kwang Mao found natural diamond could also form by a self-redox process without other reductant. This work is published in recent PNAS. Their study suggests that diamond could be a dominant host of carbon in the Earth’s lower mantle.
Natural diamonds mainly formed at depths of 87 to 120 miles in the Earth's. Besides forming from the static compression in the Earth, diamond also formed during the meteorite impact. A lot natural diamonds have been found in meteorite craters together with some natural minerals.
The research team including Drs. Ming Chen, Xiande Xie and Dayong Tan of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Dr. Jinfu Shu of HPSTAR, and Dr. Ho-Kwang Mao of HPSTAR and CIW used multiple microscopic analyses to identify the mineral components of the shocked gneiss rock at the Xiuyan impact crater, China. Their microscopic analyses show a mixture of composition—natural diamond together some other natural minerals.
What surprised the scientists is that they did not find any reducing sample or evidence of melting of carbonates. For most natural diamond formation, redox reaction is assumed a common mechanism which needs additional reductant (eg. metallic iron). While the natural diamond found in this work seemed forming in a different way—ankerite, a carbonate mineral containing calcium, iron, and magnesium decomposed during the impact event and reduced to diamond and simultaneously oxidized its ferrous iron to ferric iron to form a high-pressure mineral.
“The present diamond formation mechanism does not require melting or additional reductant—the carbonate self-reduced to form diamond and ferrous iron in carbonates can act as a reductant similar to metallic iron to reduce CO2 for the production of elemental carbon in the form of diamond”, explained Dr. Ming Chen, the lead author.
Dr. Ming Chen and co-workers has previously found some other lower mantle minerals in the same Crater. Thus the scientists estimated that the natural diamond in formed under the conditions corresponding to the Earth’s lower mantle from the formation conditions of co-existed low mantle minerals. “Ferropericlase (Fe, Mg)O is a major mineral in the lower mantle. It appears that CO2 reduced from carbonates might react with ferropericlase to form diamonds”, said Dr. Jinfu Shu.
“This suggests that diamonds may be rich in the Earth’s lower mantle where the carbonates are abundant and pressures and temperatures are sufficiently high”, added Dr. Mao. “Consequently, diamond would be a common host of carbon in the lower mantle without the need of another reductant and regardless of solid or molten states”.
媒体报道：
科学网： http://news.sciencenet.cn/htmlnews/2018/2/403975.shtm
Chemical & Engineering News (C&EN): https://cen.acs.org/articles/96/i10/Carbonate-mineral-forms-diamond-own.html
天然金刚石在高温高压条件下形成，主要途径包括星球撞击和巨大星球内部地质作用。地球内部的金刚石形成均涉及了流体相或熔体和另一种还原剂的存在，尚未发现碳酸盐在没有外部还原剂条件下发生亚固态分解形成金刚石的现象。广州地球化学研究所与北京高压科学研究中心通过对我国岫岩陨石撞击坑中岩石和矿物的冲击变质效应分析，发现铁白云石这种铁镁碳酸盐在撞击产生的高温高压下发生亚固态自氧化还原反应生成金刚石，同时伴随着二价铁氧化为三价铁。反应过程中既没有发生碳酸盐熔融，也没有流体和另一种还原剂的参与，该研究为了解地幔中碳的赋存形式提供了重要依据。