IRON-BEARING PEROVSKITE IN THE MANTLE
Mantle
I RON -B EARING P EROVSKITE
IN THE
M ANTLE Illustration by Aaron Ashley
A
lthough we are confined to Earth's crust, we live every day with the impacts that the large-scale motions of subterranean tectonic plates can have on our planet, such as changes to terrain or weather. Under the crust, Earth's mantle convects, transporting heat from the core outward, uplifting continental plates at their boundaries, recycling oceanic plates, and creating new crust and upper mantle rocks. Understanding the convection of the mantle gives us insight into processes that affect us locally. But Earth's mantle is a 2900-km-thick layer that makes up the majority of Earth by volume. Its temperature ranges from 773 K to 4273 K and its pressure soars as high as 140 GPa. What tools do we have to measure its characteristics? One proven method is replicating in a laboratory those temperatures and pressures, applying them to samples of relevant materials, and studying them with high-brightness x-rays from synchrotron light sources such as the APS. One team of researchers employed a GSECARS beamline at the APS to study an iron-bearing perovskite thought to exist in the lower Earth mantle. Their results indicate that the mantle could indeed contain an iron-bearing perovskite, adding another potential piece to the puzzle of the Earth’s composition.
126 APS S CIENCE 2012
additional iron does not affect Seismic probing of shear-wave properties. the mantle maps out hot But can this iron-bearing and cold masses where form of perovskite postulated compressional and to appear in the mantle proshear waves traveling duce the measured seismothrough the rock slow logical profile? The team input down and speed up, retheir derived moduli data into spectively. Seismic a standard bulk silicate Earth waves reflect and remodel and calculated the refract off mineral, thersulting shear and compresmal, and composition sional wave velocities. Even boundaries, indicating with the presence of where changes occur. Mg0.95Fe2+0.04Fe3+0.01SiO3, the From volcanic eruptions; subducting slabs velocity estimates were conthat meet, driving one of sistent with models based on them down into the seismological data. The team mantle; and inferences concluded that the mantle based on the composicould indeed contain an irontions of meteorites, gebearing perovskite, based on ologists postulate the their measured characteristics lower mantle comprises for Mg0.95Fe2+0.04Fe3+0.01 SiO3. Fig. 1. Experimental setup for ultrasonic interferometry measurements and x-ray (Mg,Fe)SiO3, or per— Mary Alexandra Agner diffraction. (A) X-ray radiography. (B) Energy dispersive x-ray diffraction. (C) ovskite, and MgO, periSound wave velocities. clase. But without See: Julien Chantel1,2*, Daniel 1 accurate information about how these J. Frost , Catherine A. McCammon1, beamline of GSECARS at the APS. rocks deform when stressed and then Zhicheng Jing3, and Yanbin Wang3, Measurements were taken at a range return to their original shape, geody“Acoustic velocities of pure and ironof pressures and temperatures reprenamicists cannot accurately model the bearing magnesium silicate perovskite sentative of the mantle to the upper mantle. measured to 25 GPa and 1200 K,” limit of the lower mantle. Research published in 2003 and Geophys. Res. Lett. 39, L19307 (2012). They measured the travel times of 2004 showed unexpected iron partitionDOI:10.1029/2012GL053075, 2012 both shear and compressional waves ing in (Mg,Fe)SiO3 under lower mantle Author affiliations: 1University Bayreuth, through the silicate samples with ultra2European Synchrotron Radiation Facilsound. Using simultaneous in situ x-ray conditions, raising questions about the radiographic imagery, they measured effect of additional iron in the structure ity, 3The University of Chicago the length of each sample. From these of perovskite silicates. These silicates Correspondence: data (Fig. 1) they calculated the bulk are crystal structures with alternating *[email protected] and shear moduli—the measure of how layers of SiO3 and some combination of This work was funded through the support of the minerals return to their original aluminum, iron, and magnesium. These a European Research Council (ERC) adshape after experiencing compression three elements all fit into the same lovanced grant 227893 “DEEP.” GSECARS is and shear, respectively—for each samcation in the crystal lattice, potentially supported by the National Science Foundaple at each temperature and pressure. changing the characteristics of the mintion - Earth Sciences (EAR-1128799) and The team found that the bulk moderal. An iron-bearing perovskite such as U.S. Department of Energy (DOE) - Geo2+ 3+ 2+ 3+ ulus of Mg0.95Fe 0.04Fe 0.01SiO3 was Mg0.95Fe 0.04Fe 0.01SiO3 may form sciences (DE-FG02-94ER14466). Use of the within subducting oceanic crust as it approximately 2% lower than that of Advanced Photon Source at Argonne Nasinks through the mantle and would beMgSiO3, implying that the iron-bearing tional Laboratory was supported by the U.S. have differently than MgSiO3 in the mineral does not compact as much as DOE Office of Science under Contract No. MgSiO3 due to compressional waves. same environment. DE-AC02-06CH11357. A team of researchers from UniverTo check this conclusion, the team calsity Bayreuth (Germany), the European culated the bulk modulus using a pro13-ID-C,D • GSECARS • Geoscience, environmental science • Inelastic x-ray scattering, Synchrotron Radiation Facility cedure that does not require x-ray micro x-ray absorbtion fine structure, microd(France), and The University of density data. While the value for iffraction, x-ray absorption fine structure, mi2+ Chicago characterized Mg0.95Fe 0.04 MgSiO3 remained consistent with the crofluorescence (hard x-ray), high-pressure scientific literature, the Fe3+0.01SiO3 and MgSiO3 under presdiamond anvil cell, high-pressure multi-anvil press • 4-45 keV • On-site • Accepting general Mg0.95Fe2+0.04Fe3+0.01SiO3 value was sures as high as 25 GPa and temperausers • tures of 1200 K. They measured the still lower. The shear modulus values compressional and shear wave velocifor both minerals agreed with previties and densities using the 13-ID-C,D ously published work, indicating that A RGONNE N ATIONAL L ABORATORY 127
I RON -B EARING P EROVSKITE
IN THE
M ANTLE Illustration by Aaron Ashley
A
lthough we are confined to Earth's crust, we live every day with the impacts that the large-scale motions of subterranean tectonic plates can have on our planet, such as changes to terrain or weather. Under the crust, Earth's mantle convects, transporting heat from the core outward, uplifting continental plates at their boundaries, recycling oceanic plates, and creating new crust and upper mantle rocks. Understanding the convection of the mantle gives us insight into processes that affect us locally. But Earth's mantle is a 2900-km-thick layer that makes up the majority of Earth by volume. Its temperature ranges from 773 K to 4273 K and its pressure soars as high as 140 GPa. What tools do we have to measure its characteristics? One proven method is replicating in a laboratory those temperatures and pressures, applying them to samples of relevant materials, and studying them with high-brightness x-rays from synchrotron light sources such as the APS. One team of researchers employed a GSECARS beamline at the APS to study an iron-bearing perovskite thought to exist in the lower Earth mantle. Their results indicate that the mantle could indeed contain an iron-bearing perovskite, adding another potential piece to the puzzle of the Earth’s composition.
126 APS S CIENCE 2012
additional iron does not affect Seismic probing of shear-wave properties. the mantle maps out hot But can this iron-bearing and cold masses where form of perovskite postulated compressional and to appear in the mantle proshear waves traveling duce the measured seismothrough the rock slow logical profile? The team input down and speed up, retheir derived moduli data into spectively. Seismic a standard bulk silicate Earth waves reflect and remodel and calculated the refract off mineral, thersulting shear and compresmal, and composition sional wave velocities. Even boundaries, indicating with the presence of where changes occur. Mg0.95Fe2+0.04Fe3+0.01SiO3, the From volcanic eruptions; subducting slabs velocity estimates were conthat meet, driving one of sistent with models based on them down into the seismological data. The team mantle; and inferences concluded that the mantle based on the composicould indeed contain an irontions of meteorites, gebearing perovskite, based on ologists postulate the their measured characteristics lower mantle comprises for Mg0.95Fe2+0.04Fe3+0.01 SiO3. Fig. 1. Experimental setup for ultrasonic interferometry measurements and x-ray (Mg,Fe)SiO3, or per— Mary Alexandra Agner diffraction. (A) X-ray radiography. (B) Energy dispersive x-ray diffraction. (C) ovskite, and MgO, periSound wave velocities. clase. But without See: Julien Chantel1,2*, Daniel 1 accurate information about how these J. Frost , Catherine A. McCammon1, beamline of GSECARS at the APS. rocks deform when stressed and then Zhicheng Jing3, and Yanbin Wang3, Measurements were taken at a range return to their original shape, geody“Acoustic velocities of pure and ironof pressures and temperatures reprenamicists cannot accurately model the bearing magnesium silicate perovskite sentative of the mantle to the upper mantle. measured to 25 GPa and 1200 K,” limit of the lower mantle. Research published in 2003 and Geophys. Res. Lett. 39, L19307 (2012). They measured the travel times of 2004 showed unexpected iron partitionDOI:10.1029/2012GL053075, 2012 both shear and compressional waves ing in (Mg,Fe)SiO3 under lower mantle Author affiliations: 1University Bayreuth, through the silicate samples with ultra2European Synchrotron Radiation Facilsound. Using simultaneous in situ x-ray conditions, raising questions about the radiographic imagery, they measured effect of additional iron in the structure ity, 3The University of Chicago the length of each sample. From these of perovskite silicates. These silicates Correspondence: data (Fig. 1) they calculated the bulk are crystal structures with alternating *[email protected] and shear moduli—the measure of how layers of SiO3 and some combination of This work was funded through the support of the minerals return to their original aluminum, iron, and magnesium. These a European Research Council (ERC) adshape after experiencing compression three elements all fit into the same lovanced grant 227893 “DEEP.” GSECARS is and shear, respectively—for each samcation in the crystal lattice, potentially supported by the National Science Foundaple at each temperature and pressure. changing the characteristics of the mintion - Earth Sciences (EAR-1128799) and The team found that the bulk moderal. An iron-bearing perovskite such as U.S. Department of Energy (DOE) - Geo2+ 3+ 2+ 3+ ulus of Mg0.95Fe 0.04Fe 0.01SiO3 was Mg0.95Fe 0.04Fe 0.01SiO3 may form sciences (DE-FG02-94ER14466). Use of the within subducting oceanic crust as it approximately 2% lower than that of Advanced Photon Source at Argonne Nasinks through the mantle and would beMgSiO3, implying that the iron-bearing tional Laboratory was supported by the U.S. have differently than MgSiO3 in the mineral does not compact as much as DOE Office of Science under Contract No. MgSiO3 due to compressional waves. same environment. DE-AC02-06CH11357. A team of researchers from UniverTo check this conclusion, the team calsity Bayreuth (Germany), the European culated the bulk modulus using a pro13-ID-C,D • GSECARS • Geoscience, environmental science • Inelastic x-ray scattering, Synchrotron Radiation Facility cedure that does not require x-ray micro x-ray absorbtion fine structure, microd(France), and The University of density data. While the value for iffraction, x-ray absorption fine structure, mi2+ Chicago characterized Mg0.95Fe 0.04 MgSiO3 remained consistent with the crofluorescence (hard x-ray), high-pressure scientific literature, the Fe3+0.01SiO3 and MgSiO3 under presdiamond anvil cell, high-pressure multi-anvil press • 4-45 keV • On-site • Accepting general Mg0.95Fe2+0.04Fe3+0.01SiO3 value was sures as high as 25 GPa and temperausers • tures of 1200 K. They measured the still lower. The shear modulus values compressional and shear wave velocifor both minerals agreed with previties and densities using the 13-ID-C,D ously published work, indicating that A RGONNE N ATIONAL L ABORATORY 127
