Peking University, April 10, 2017: The internal composition and structure of the earth is at the forefront of geoscience research. Among 125 of the most challenging and critical scientific issues that Science published which human beings will strive to solve in the future, "How Does Earth’s Interior work?" ranks 10th, and it also ranks first in the field of earth science. Currently scientists have very limited understanding of the interior of the Earth. While radius of earth is about 6400 km, the deepest position scientists can reach by drilling is only 12 km deep. As a consequence, scientists can only speculate on the structure and composition of the earth through the rocks formed in the geological processes and exposed on the surface.
The mantle xenoliths are rock inclusions of the mantle brought to earth surface by deep magma such as kimberlite, fossil carbonatites, basalts, etc. They are the window of nature to study deep mantle. The magma originating from different depths brings mantle xenoliths with different material compositions and information. Carbonatite is one of the main rocks of surface carbon, and its magma origin has been controversial. Experimental rock studies have confirmed that the carbonatite magma is formed at the depth of 700km in the mantle, while geochemical evidence shows that carbonatite originates deep in the lithosphere. The information about the deep mantle is mainly obtained by high temperature and high pressure experiments and ultrahigh pressure mineral inclusions in mantle-derived diamonds. It is difficult to obtain the mantle precursors from below the soft pool, which greatly constrains our understanding of carbonatite magma origin and deep carbon cycle.
Researcher Xu Cheng from School of Earth and Space Sciences (SESS) found eclogite xenoliths in carbonatites for the first time in the North China Craton, together with Professor Zhang Lifei and FeiYingwei. The diatomite is mainly composed of omphaciteand garnet, garnet coated with apatite, rutile, kyanite, quartz and other minerals. More importantly, they found in the eclogite garnet ultra-silica garnet (Pyr90-93Alm3-7Gro2-4) inclusions with a clear excess of Si [3.15 to 3.22 per formulaunit (pfu)]. Ultra-silica garnet is a typical ultrahigh-pressure mineral, mainly distributed in the mantle below 200 km. At present, such minerals are only found in meteorites and a small amount of diamond body with less trivalent iron. The discovery of the rich trivalent iron ultrapure garnet reveals that there is an oxygen-rich component at the bottom of the mantle or the mantle transition layer, which is closely related to the sedimentary carbonatite.It also indicates that the carbonatite magma may be formed in the mantle transition zone. This study is of great significance for further exploring the dynamic evolution of the mantle transition zone and the origin of carbonatite magma.
Figure: Majoritic inclusions in garnet with Raman spectra
The study was published in Science Advances (Xu et al., Sci. Adv. 2017; 3: e1601589) on April 7, 2017. Professor Xu Cheng is the first author, Professor Zhang Lifei and Professor Fei Yingwei co-communication authors. This research was sponsored by the Ministry of Science and Technology "973" Project, the National Natural Science Foundation and the Carnegie Institute.
Written by: Lu Linlin
Edited by: Yan Shengnan
Source: School of Earth and Space Sciences
