Mantle minerals offer clues to deep Earth's composition - Dr. Yonggang Wang

MARCH 31, 2018

Through high-pressure experiment studies on bridgmanite, the most abundant mineral in the Earth’s mantle, a group of scientists include Dr. Yonggang Wang of HPSTAR, determined the influence of spin state and valence state of Fe on bridgmanite density, bulk sound velocity, and electrical conductivity. Based on their high-pressure experimental data on Fe3+-only bridgmanite together with previous results on Fe2+-domain bridgmanite, they found that neither valence state nor spin state of Fe in bridgmanite would cause significant seismic signature in the mantle. The discovery is published by Nature Communications.

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging.

The team including collaborators from HPSTAR, Michigan State University, University of Michigan, University of Hawaii, Geophysical Laboratory of Carnegie Institution of Washington, Argonne National Laboratory, the University of Illinois, studied the compression behavior, spin state and electrical conductivity of ferric-iron-only (Mg0.46Fe3+0.53)(Si0.49Fe3+0.51)O3 Bdg by using diamond anvil cells coupled with a range of synchrotron techniques. Their experimental results constrain the pressure range of spin transition of Fe3+ in the octahedral site and its influence on the geophysical properties of bridgmanite.

The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions,the valence state of iron has minor effects on density and sound velocities relative to major cation composition.

"The deep mantle is a weird place with mysterious features that may be residues of Earth's formation, graveyards for piles of sunken tectonic plates, sources for hotspot volcanoes like Hawaii or the processes that shaped the atmosphere. Anything we can detect about the composition of features at the base of the mantle may help us solve these mysteries." Dorfman said, one of the two lead authors of this study.

“Our study disentangles valence and spin effects on the elastic and electrical properties of bridgmanite. It provides high-resolution mineral physics data for interpreting seismic anomalies.” Jiachao Liu said, the first author of this study.

“The electrical conductivity measurements present here clarifies the influence of spin transition of Fe in lower-mantle bridgmanite, which had been controversial for almost 10 years” said Yonggang Wang.

Caption: The electrical conductivity of bridgmanites across the spin state transition.

媒体报道：

Phys.Org： https://phys.org/news/2018-04-mantle-minerals-clues-deep-earth.html

ScienceDaily: https://www.sciencedaily.com/releases/2018/04/180403111048.htm

布里奇曼石作为地幔中含量最高的矿物，它的物理化学性质对地球地幔的物理与化学演化具有重要意义。铁是地球中含量最高的元素，同时铁能够以多种价态以及自旋态存在于布里奇曼石中，因而可能造成地幔地震波异常。在由北京高压科学研究中心的王永刚研究员参与的一项研究中，采用金刚石压砧高压技术与多种同步辐射技术对铁价态和自旋态对布里奇曼石的密度，弹性性质以及电导率的影响进行了一系列的研究。该研究表明对于同样铁含量的布里奇曼石，铁的价态或者自旋态对其地球物理性质的影响并没有之前研究认为的那么明显。因而下地幔的地震波异常带或许是由于整体铁含量的不均一性造成的，而不能用铁的价态或者自旋态来解释。