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Abstract Nitrogen has been proposed to be stored within planetary cores, but its effects on the structure and density of molten Fe–alloys have not been explored experimentally. Using energy‐dispersive X‐ray diffraction, we determined the structure of Fe–N(–C) liquids at core conditions (1–7 GPa and 1700–1900°C) within a Paris‐Edinburgh press. Variation of N up to 7 wt.% and C up to 1.5 wt.% results in near‐linear changes in Fe–Fe atom distances and structure factor with increasing light element content. We did not observe a significant pressure‐driven structural transition in Fe–N(–C) liquids. We model the expansion of the Fe–Fe bonds using a modified Birch‐Murnaghan equation of state. With this model, we demonstrate that N or C contamination could lead to an overestimation of the Fe–Fe distances of pure Fe. We observe that the incorporation of 1 wt.% N or C into Fe results in a change in Fe–Fe distances that is twice as significant as the effect of 1 GPa. By approximating the change in volume, we infer that N and C incorporated in liquid iron could contribute to the density deficit observed in the cores of terrestrial bodies. 

Online and hybrid instruction as a response to the coronavirus (COVID-19) pandemic presented specific challenges in geosciences due to the role of laboratory and field activities. We carried out a research study on student learning in undergraduate mineralogy at a large public research university in the United States over a 4-year period including pre-pandemic in-person instruction and during pandemic online and hybrid instruction. A total of 94 students participated in course activities and mixed-method surveys. Survey results indicated that during the pandemic, students entered the course with lower sense of belonging relative to pre-pandemic students. Despite challenges related to instruction and student learning since 2020, the pandemic did not necessarily produce worse learning outcomes for mineralogy students as measured by course assessments. Structured course delivery and opportunities for interaction can offset challenges resulting from global emergencies. 

Abstract Incorporation of ferric iron in mantle silicates stabilizes different crystal structures and changes phase transition conditions, thus impacting seismic wave speeds and discontinuities. In MgSiO3-Fe2O3 mixtures, recent experiments indicate the coexistence of fully oxidized iron-rich (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 with Fe-poor silicate (wadsleyite or bridgmanite) and stishovite at 15 to 27 GPa and 1773 to 2000 K, conditions relevant to subducted lithosphere in the Earth’s transition zone and uppermost lower mantle. X-ray diffraction measurements show that (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 recovered from these conditions adopts the R3c LiNbO3-type structure, which transforms to the bridgmanite structure again between 18.3 GPa and 24.7 GPa at 300 K. Diffraction observations are used to obtain the equation of state of the LiNbO3-type phase up to 18.3 GPa. These observations combined with multi-anvil experiments suggest that the stable phase of (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 is bridgmanite at 15-27 GPa, which transforms on decompression to LiNbO3-type structure. Our calculation revealed that ordering of the ferric ion reduces the kinetic energy barrier of the transition between (Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3 LiNbO3 structure and bridgmanite relative to the MgSiO3 akimotoite-bridgmanite system. Dense Fe3+-rich bridgmanite structure is thus stable at substantially shallower depths than MgSiO3 bridgmanite and would promote subduction. 

Abstract The stable forms of carbon in Earth’s deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions. However, current estimates of the distribution of carbon in Earth’s mantle are uncertain, due in part to limited understanding of the fate of carbonates through subduction, the main mechanism that transports carbon from Earth’s surface to its interior. Oxidized carbon carried by subduction has been found to reside in MgCO 3 throughout much of the mantle. Experiments in this study demonstrate that at deep mantle conditions MgCO 3 reacts with silicates to form CaCO 3 . In combination with previous work indicating that CaCO 3 is more stable than MgCO 3 under reducing conditions of Earth’s lowermost mantle, these observations allow us to predict that the signature of surface carbon reaching Earth’s lowermost mantle may include CaCO 3 . 

Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO3 bridgmanite, and (Mg,Fe)SiO3 post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg0.55Fe0.45)2SiO4 and (Mg0.28Fe0.72)2SiO4 at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO2-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO2-content at all mantle depths, an important finding for interpretation of deep lower mantle structures. 

Abstract Calcium carbonate (CaCO3) is one of the most abundant carbonates on Earth's surface and transports carbon to Earth's interior via subduction. Although some petrological observations support the preservation of CaCO3 in cold slabs to lower mantle depths, the geophysical properties and stability of CaCO3 at these depths are not known, due in part to complicated polymorphic phase transitions and lack of constraints on thermodynamic properties. Here we measured thermal equation of state of CaCO3-Pmmn, the stable polymorph of CaCO3 through much of the lower mantle, using synchrotron X-ray diffraction in a laser-heated diamond-anvil cell up to 75 GPa and 2200 K. The room-temperature compression data for CaCO3-Pmmn are fit with third-order Birch-Murnaghan equation of state, yielding KT0 = 146.7 (±1.9) GPa and K′0 = 3.4(±0.1) with V0 fixed to the value determined by ab initio calculation, 97.76 Å3. High-temperature compression data are consistent with zero-pressure thermal expansion αT = a0 + a1T with a0 = 4.3(±0.3)×10-5 K-1, a1 = 0.8(±0.2)×10-8 K-2, temperature derivative of the bulk modulus (∂KT/∂T)P = –0.021(±0.001) GPa/K; the Grüneisen parameter γ0 = 1.94(±0.02), and the volume independent constant q = 1.9(±0.3) at a fixed Debye temperature θ0 = 631 K predicted via ab initio calculation. Using these newly determined thermodynamic parameters, the density and bulk sound velocity of CaCO3-Pmmn and (Ca,Mg)-carbonate-bearing eclogite are quantitatively modeled from 30 to 80 GPa along a cold slab geotherm. With the assumption that carbonates are homogeneously mixed into the slab, the results indicate the presence of carbonates in the subducted slab is unlikely to be detected by seismic observations, and the buoyancy provided by carbonates has a negligible effect on slab dynamics. 

Abstract Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO₃–10 mol% Fe₂O₃system, rather than forming a homogeneous single phase. The Fe³⁺-rich bridgmanite has substantially lower velocities and a higherV_P/V_Sratio than MgSiO₃bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe³⁺-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe³⁺-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.