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9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Evidence for oxygenation of Fe-Mg oxides at mid-mantle conditions and the rise of deep oxygen

Abstract As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D” layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+δ (0 < δ < 1, denoted as ‘OE-phase’). It forms at pressures greater than 40 gigapascal when (Mg, Fe)-bearing hydrous materials are heated over 1500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2Hx which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1000 km in the Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial or subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.

58 GEOSCIENCES↗
DOE OSTI2016

Thermal stability comparison of nanocrystalline Fe-based binary alloy pairs

Here, the widely recognized property improvements of nanocrystalline (NC) materials have generated significant interest, yet have been difficult to realize in engineering applications due to the propensity for grain growth in these interface-dense systems. While traditional pathways to thermal stabilization can slow the mobility of grain boundaries, recent theories suggest that solute segregation in NC alloy can reduce the grain boundary energy such that thermodynamic stabilization is achieved. Following the predictions of Murdock et al., here we compare for the first time the thermal stability of a predicted NC stable alloy (Fe-10at.% Mg) with a predicted non-NC stable alloy (Fe-10at.% Cu) using the same processing and characterization methodologies. Results indicate improved thermal stability of the Fe-Mg alloy in comparison to the Fe-Cu, and observed microstructures are consistent with those predicted by Monte Carlo simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Fe-bearing phases in modern lacustrine microbialites from Mexico

Transition metal ions impurities in microbialites have been previously suggested as paleoenvironmental and/or paleobiological proxies. However, how metals are incorporated into microbialites remains poorly known. Here, in order to assess the distribution and speciation of Fe in modern microbialites, we conducted bulk X-ray diffraction, infrared spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, electron microscopy and X-ray microscopy analyses on samples collected from shallow depths in five alkaline lakes in Mexico: Lake Alchichica, Lake La Preciosa, Lake Atexcac, Lake La Alberca de Los Espinos and Lake Pátzcuaro. A range of Fe contents were measured in these microbialites, from low (~0.12 wt.%) to relatively high (~2.2 wt.%). Fe was distributed heterogeneously in microbialites, mostly localized as hotspots or sometimes arranged as discrete laminae. Fe was mostly trivalent in all microbialites and was incorporated into diverse authigenic phases, the proportion of which varied among microbialites. Authigenic phases included Fe-bearing Mg-phyllosilicates such as kerolite and/or stevensite, i.e. (Mg,Fe(II),Fe(III)) 3 Si 4 O 10 (OH) 2 ·nH 2 O and Fe-Mg layered double hydroxides (LDH), i.e. pyroaurite [Mg(II) 6 Fe(III) 2 (CO 3 )(OH) 16 ·4H 2 O] and/or iowaite [Mg(II) 6 Fe(III) 2 Cl 2 (OH) 16 ·4H 2 O], together with Fe-(oxyhydr)oxides. Carbonate phases were negligible carriers of Fe in all lakes. Iowaite/pyroaurite phases, which have often been found as low temperature alteration products in serpentinites, were surprisingly observed in Lake Alchichica, whereas Fe-bearing kerolite/stevensite was present in Lake La Preciosa, Lake Atexcac, Lake La Alberca de Los Espinos and Lake Pátzcuaro. Fe present in these shallow water microbialites may be originated from groundwater seepage derived from adjacent Fe-rich volcanic rocks. We suggest that the occurence of iowaite/pyroaurite vs authigenic Fe-bearing kerolite/stevensite depends on the orthosilicic acid concentration in the lakes. Pyroaurite/iowaite may form and stay preserved in Lake Alchichica because of the low [H 4 SiO 4 ] as well as high alkalinity, [Mg 2+ ], [CO 3 2- ], [Cl-] and pH (~9) prevailing in this lake, while in other lakes, where [H 4 SiO 4 ] is higher (above ~0.4 mM), Fe-bearing kerolite or smectite phases are formed. Overall, the message carried by the Fe-bearing mineral phases in these modern microbialites is multifold: Fe-bearing phases within microbialites may contain some information on environmental conditions (e.g., [H 4 SiO 4 ]) as well as on geochemical processes implicated during their formation (e.g., seepage of anoxic Fe-bearing groundwater). Considering the reactivity of pyroaurite/iowaite and kerolite/stevensite and their possible transformation into diverse mineral phases upon microbial activity, burial and diagenesis, a careful appraisal of Fe speciation using a similar combination of bulk and microscopy analyses is required in order to better assess the origin of Fe in ancient microbialites.

58 GEOSCIENCES↗
DOE OSTI2025

Equilibrium Fe isotope fractionation between olivine, pyroxene, spinel and MORB glass: Implications for mantle partial melting to generate MORBs

Primitive mid-ocean ridge basalts (MORBs) exhibit Fe isotopic compositions heavier than the upper mantle by +0.074 ± 0.028 ‰ for δ 56 Fe. The processes responsible for this isotopic difference remain unclear. Modeling of Fe isotope fractionation during mantle partial melting requires reliable equilibrium Fe isotope fractionation factors between minerals and melts, for which consistent data are still lacking. Here, in this study, we used Nuclear Resonant Inelastic X-ray Scattering (NRIXS) technique to measure Fe force constants for a MORB glass (ALV 519-4-1) and natural mantle minerals (olivine, orthopyroxene, clinopyroxene, and spinel) to determine the equilibrium Fe isotope fractionation factors between them. The force constants determined in this study, in increasing order, are 167 ± 26 N/m for spinel, 175 ± 17 N/m for olivine, 176 ± 20 N/m for MORB glass, 205 ± 26 N/m for clinopyroxene, and 219 ± 36 N/m for orthopyroxene. We evaluated the previously proposed mechanisms for the heavy Fe isotopic composition of MORBs, including (i) mantle partial melting, (ii) mantle lithological heterogeneity, with pyroxenite in the source, (iii) mantle metasomatism by low-degree melts, and (iv) fractional crystallization of olivine from melts. For (i), we used the pMELTS program to simulate adiabatic decompression melting of mantle peridotites, and calculated Fe isotope fractionation based on Fe 3+ –Fe 2+ equilibrium-controlled fractionation, where Fe 3+ forms stronger bonds and is more incompatible than Fe 2+ . At 10 wt% peridotite melting, corresponding to MORB generation, only +0.03 ‰ Fe isotope fractionation between the melt and the original bulk composition (Δ 56 Fe = δ 56 Fe melt - δ 56 Fe 0 ) was produced, insufficient to account for the observed MORB-upper mantle difference. For (ii), melting of pyroxenites yields smaller Fe isotope fractionation than melting of peridotites, making it unlikely the cause for the MORB-upper mantle isotopic difference. For (iii), both the Fe 3+ /ΣFe ratio and the δ 56 Fe of melts increase with the degree of partial melting, indicating that low-degree melts are not isotopically heavy enough to significantly alter the isotopic composition of lithospheric mantle through metasomatism. For (iv), equilibrium isotope fractionation between olivine and melt is near zero. These results suggest that equilibrium Fe isotope fractionation alone cannot explain the MORB isotopic signature, highlighting the potential role of kinetic isotope fractionation. Using a diffusion model, we calculated kinetic Fe and Mg isotope fractionations associated with (iv) olivine crystallization from a melt, and found that the predicted Fe and Mg isotope fractionations were inconsistent with observations in MORBs. Qualitatively, two processes could have induced kinetic Fe isotope fractionation during MORB generation: (a) Fe-Mg interdiffusion between melt and solid during melt migration and (b) reactive melt-rock interactions during melt focusing. However, a quantitative understanding of their role in modifying the melt isotopic composition remains limited and requires further investigation.

Fe isotopes↗
DOE OSTI2022

Nano-FTIR Investigation of the CM Chondrite Allan Hills 83100

Mid-infrared (MIR) spectroscopy has been used with great success to quantitatively determine the mineralogy of geologic samples. It has been employed in a variety of contexts from determining bulk composition of powdered samples to spectroscopic imaging of rock thin sections via micro-Fourier transform infrared (micro-FTIR) imaging spectroscopy. Recent advances allow for IR measurements at the nanoscale. Near field nanoscale infrared imaging and spectroscopy with a broadband source (nano-FTIR) enable understanding of the spatial relationships between compositionally distinct materials within a sample. This will be of particular use when analyzing returned samples from Bennu and Ryugu, which are thought to be compositionally like CI or CM1/2 carbonaceous chondrites. Returned samples will likely contain olivine/pyroxene chondrules that have been transformed into hydrous phyllosilicates, sulfides, carbonates, and other alteration phases. The use of near-field infrared techniques to probe the boundaries between once pristine chondrules and alteration phases at the nanoscale is a novel approach to furthering our understanding of the compositional evolution of carbonaceous asteroids and the processes that drive their evolution. Here we report the results of nano-FTIR spectroscopy and imaging measurements performed on the carbonaceous chondrite Allan Hills (ALH) 83100 (CM1/2). We show with nanoscale resolution that spatially resolved Fe-Mg variations exist within the phyllosilicates around a chondrule rim. We also present effects of crystal orientation on the nano-FTIR spectra to account for the spectral differences between the meteorite and mineral spectra.

58 GEOSCIENCES↗
DOE OSTI2022

Askival: An altered feldspathic cumulate sample in Gale crater

Askival is a light-toned, coarsely crystalline float rock, which was identified near the base of Vera Rubin Ridge in Gale crater. We have studied Askival, principally with the ChemCam instrument but also using APXS compositional data and MAHLI images. Askival and an earlier identified sample, Bindi, represent two rare examples of feldspathic cumulate float rocks in Gale crater with >65% relict plagioclase. Bindi appears unaltered whereas Askival shows textural and compositional signatures of silicification, along with alkali remobilization and hydration. Askival likely experienced multiple stages of alteration, occurring first through acidic hydrolysis of metal cations, followed by deposition of silica and possible phyllosilicates at low T and neutral-alkaline pH. Through laser-induced breakdown spectroscopy compositional analyses and normative calculations, we suggest that an assemblage of Fe-Mg silicates including amphibole and pyroxene, Fe phases, and possibly Mg-rich phyllosilicate are present. Thermodynamic modeling of the more pristine Bindi composition predicts that amphibole and feldspar are stable within an upper crustal setting. This is consistent with the presence of amphibole in the parent igneous rocks of Askival and suggests that the paucity of amphiboles in other known Martian samples reflects the lack of representative samples of the Martian crust rather than their absence on Mars.

58 GEOSCIENCES↗
DOE OSTI2020

Materials Data on Mg2Fe by Materials Project

Mg2Fe crystallizes in the orthorhombic Cmcm space group. The structure is two-dimensional and consists of two Mg2Fe sheets oriented in the (0, 1, 0) direction. there are two inequivalent Mg sites. In the first Mg site, Mg is bonded in a 4-coordinate geometry to four equivalent Fe atoms. All Mg–Fe bond lengths are 2.84 Å. In the second Mg site, Mg is bonded in a 12-coordinate geometry to two equivalent Fe atoms. Both Mg–Fe bond lengths are 2.80 Å. Fe is bonded in a 8-coordinate geometry to six Mg and two equivalent Fe atoms. Both Fe–Fe bond lengths are 2.32 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Materials Data on Mg149Fe by Materials Project

Mg149Fe is alpha Samarium-derived structured and crystallizes in the hexagonal P-6m2 space group. The structure is three-dimensional. there are twenty-seven inequivalent Mg sites. In the first Mg site, Mg is bonded to twelve Mg atoms to form MgMg12 cuboctahedra that share a cornercorner with one FeMg12 cuboctahedra, corners with seventeen MgMg12 cuboctahedra, edges with sixteen MgMg11Fe cuboctahedra, and faces with twenty MgMg12 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 3.16–3.31 Å. In the second Mg site, Mg is bonded to eleven Mg and one Fe atom to form distorted MgMg11Fe cuboctahedra that share corners with eighteen MgMg12 cuboctahedra, edges with sixteen MgMg12 cuboctahedra, a faceface with one FeMg12 cuboctahedra, and faces with fifteen MgMg12 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 2.99–3.26 Å. The Mg–Fe bond length is 3.02 Å. In the third Mg site, Mg is bonded to twelve Mg atoms to form MgMg12 cuboctahedra that share corners with eighteen MgMg11Fe cuboctahedra, an edgeedge with one FeMg12 cuboctahedra, edges with thirteen MgMg12 cuboctahedra, and faces with twenty MgMg12 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 3.17–3.22 Å. In the fourth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are four shorter (3.19 Å) and four longer (3.20 Å) Mg–Mg bond lengths. In the fifth Mg site, Mg is bonded to twelve Mg atoms to form MgMg12 cuboctahedra that share corners with eighteen MgMg11Fe cuboctahedra, an edgeedge with one FeMg12 cuboctahedra, edges with seventeen MgMg12 cuboctahedra, and faces with eighteen MgMg12 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 3.17–3.22 Å. In the sixth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are two shorter (3.19 Å) and six longer (3.20 Å) Mg–Mg bond lengths. In the seventh Mg site, Mg is bonded in a distorted single-bond geometry to eleven Mg and one Fe atom. There are a spread of Mg–Mg bond distances ranging from 3.03–3.31 Å. The Mg–Fe bond length is 3.02 Å. In the eighth Mg site, Mg is bonded to twelve Mg atoms to form MgMg12 cuboctahedra that share corners with sixteen MgMg12 cuboctahedra, an edgeedge with one FeMg12 cuboctahedra, edges with sixteen MgMg12 cuboctahedra, and faces with nineteen MgMg12 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 3.16–3.22 Å. In the ninth Mg site, Mg is bonded to twelve Mg atoms to form MgMg12 cuboctahedra that share corners with sixteen MgMg12 cuboctahedra, edges with eighteen MgMg11Fe cuboctahedra, and faces with twenty MgMg12 cuboctahedra. There are four shorter (3.19 Å) and three longer (3.20 Å) Mg–Mg bond lengths. In the tenth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 3.17–3.20 Å. In the eleventh Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. All Mg–Mg bond lengths are 3.20 Å. In the twelfth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are three shorter (3.19 Å) and two longer (3.20 Å) Mg–Mg bond lengths. In the thirteenth Mg site, Mg is bonded to twelve Mg atoms to form MgMg12 cuboctahedra that share corners with sixteen MgMg12 cuboctahedra, edges with seventeen MgMg12 cuboctahedra, a faceface with one FeMg12 cuboctahedra, and faces with seventeen MgMg12 cuboctahedra. There are two shorter (3.17 Å) and one longer (3.18 Å) Mg–Mg bond lengths. In the fourteenth Mg site, Mg is bonded to twelve Mg atoms to form MgMg12 cuboctahedra that share a cornercorner with one FeMg12 cuboctahedra, corners with seventeen MgMg12 cuboctahedra, edges with seventeen MgMg12 cuboctahedra, and faces with eighteen MgMg12 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 3.16–3.23 Å. In the fifteenth Mg site, Mg is bonded to twelve Mg atoms to form MgMg12 cuboctahedra that share corners with eighteen MgMg12 cuboctahedra, edges with eighteen MgMg12 cuboctahedra, a faceface with one FeMg12 cuboctahedra, and faces with sixteen MgMg12 cuboctahedra. All Mg–Mg bond lengths are 3.22 Å. In the sixteenth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 3.18–3.20 Å. In the seventeenth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are four shorter (3.19 Å) and three longer (3.20 Å) Mg–Mg bond lengths. In the eighteenth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are three shorter (3.19 Å) and two longer (3.20 Å) Mg–Mg bond lengths. In the nineteenth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are one shorter (3.19 Å) and two longer (3.20 Å) Mg–Mg bond lengths. In the twentieth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are four shorter (3.19 Å) and one longer (3.20 Å) Mg–Mg bond lengths. In the twenty-first Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are two shorter (3.18 Å) and four longer (3.19 Å) Mg–Mg bond lengths. In the twenty-second Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are two shorter (3.18 Å) and two longer (3.19 Å) Mg–Mg bond lengths. In the twenty-third Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. There are four shorter (3.18 Å) and one longer (3.19 Å) Mg–Mg bond lengths. In the twenty-fourth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. All Mg–Mg bond lengths are 3.19 Å. In the twenty-fifth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. In the twenty-sixth Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. Both Mg–Mg bond lengths are 3.20 Å. In the twenty-seventh Mg site, Mg is bonded to twelve Mg atoms to form a mixture of edge, corner, and face-sharing MgMg12 cuboctahedra. Both Mg–Mg bond lengths are 3.19 Å. Fe is bonded to twelve Mg atoms to form FeMg12 cuboctahedra that share corners with eighteen MgMg12 cuboctahedra, edges with eighteen MgMg12 cuboctahedra, and faces with fourteen MgMg11Fe cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg3Fe by Materials Project

Mg3Fe is beta Cu3Ti-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Mg is bonded to eight equivalent Mg and four equivalent Fe atoms to form distorted MgMg8Fe4 cuboctahedra that share corners with four equivalent FeMg12 cuboctahedra, corners with fourteen equivalent MgMg8Fe4 cuboctahedra, edges with six equivalent FeMg12 cuboctahedra, edges with twelve equivalent MgMg8Fe4 cuboctahedra, faces with four equivalent FeMg12 cuboctahedra, and faces with sixteen equivalent MgMg8Fe4 cuboctahedra. There are a spread of Mg–Mg bond distances ranging from 2.85–3.08 Å. There are two shorter (2.93 Å) and two longer (2.97 Å) Mg–Fe bond lengths. Fe is bonded to twelve equivalent Mg atoms to form FeMg12 cuboctahedra that share corners with six equivalent FeMg12 cuboctahedra, corners with twelve equivalent MgMg8Fe4 cuboctahedra, edges with eighteen equivalent MgMg8Fe4 cuboctahedra, faces with eight equivalent FeMg12 cuboctahedra, and faces with twelve equivalent MgMg8Fe4 cuboctahedra.

36 MATERIALS SCIENCE↗