Volume 108, Issue 5

Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity ( f O2 ; iron-wüstite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 °C. Apatite-melt partitioning data for REE 3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing f O2 . Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: (Eu3+∑Eu)Apatite =11+10−0.10±0.01×log(fo2)−1.63±0.16. $${{\left( \frac{\text{E}{{\text{u}}^{3+}}}{\sum \text{Eu}} \right)}_{\text{Apatite }\!\!~\!\!\text{ }}}=\frac{1}{1+{{10}^{-0.10\pm 0.01\times \log \left( {{f}_{{{\text{o}}_{2}}}} \right)-1.63\pm 0.16}}}.$$ The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: (EuEu∗)DSm×Gd=11+10−0.15±0.03×log(fo2)−2.46±0.41. $$\left( \frac{\text{Eu}}{\text{E}{{\text{u}}^{*}}} \right)_{D}^{\sqrt{\text{Sm }\!\!\times\!\!\text{ Gd}}}=\frac{1}{1+{{10}^{-0.15\pm 0.03\times \log {{\left( {{f}_{{{\text{o}}_{2}}}} \right)}^{-2.46\pm 0.41}}}}}.$$ The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by (EuEu∗)Dlattice strain =11+10−0.20±0.03×log(fo2)−3.03±0.42. $$\left( \frac{\text{Eu}}{\text{E}{{\text{u}}^{*}}} \right)_{D}^{\text{lattice strain }\!\!~\!\!\text{ }}=\frac{1}{1+{{10}^{-0.20\pm 0.03\times \log {{\left( {{f}_{{{\text{o}}_{2}}}} \right)}^{-3.03\pm 0.42}}}}}.$$ The Eu valence-state partitioning techniques based on (Sm×Gd) $(\sqrt{\text{Sm}\times \text{Gd}})$and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or Sm×Gd $\sqrt{\text{Sm}\times \text{Gd}}$techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or (Sm×Gd) $\sqrt{(\text{Sm}\times \text{Gd})}$techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum D Eu3+ = 1.67 ± 0.25 (as determined by lattice strain) to D Lu3+ = 0.69 ± 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu 3+ ) is also the polyvalent element used to monitor f O2 . These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts.

We have performed experiments at 1.5 GPa and 1400 °C on 25 different bulk compositions to determine the effects of major element compositions on the Cl contents of silicate melts at known fugacities of Cl 2 and O 2 . The experimental method involved mixing a “sliding” Cl buffer, a mixture of AgCl, AgI, and Ag with the silicate bulk composition and performing the experiment in a graphite capsule together with a source of CO 2 (AgCO 3 ). The graphite capsules were sealed inside welded Pt tubes to maintain a CO 2 -CO atmosphere with oxygen fugacity fixed at the C-CO-CO 2 (CCO) buffer. During the experiment, the Cl buffer segregates leaving a Cl-bearing melt, which quenches to a glass. We used the results to define chloride capacity C Cl for each melt at the pressure and temperature of the experiment: C C l = C l ( w t f C l 2 × f O 2 4 $$ \begin{equation} C_{\mathrm{Cl}}=\frac{\mathrm{Cl}(\mathrm{wt} \%)}{\sqrt{f\left(\mathrm{Cl}_{2}\right)}} \times \sqrt[4]{f_{\mathrm{O}_{2}}} \end{equation}$$ Chloride capacity was found to correlate positively with optical basicity and NBO/T and negatively with ionic porosity and the Larsen index. We combined our new data with the results of Thomas and Wood (2021) to derive an equation describing the composition, pressure and temperature dependence of the chloride capacity: log C C l = 1.601 + 4470 X C a − 3430 X S i + 2592 X F e − 4092 X K − 894 P / T . $$ \begin{equation} \log \mathrm{C}_{\mathrm{Cl}}=1.601+\left(4470 \mathrm{X}_{\mathrm{Ca}}-3430 \mathrm{X}_{\mathrm{Si}}+2592 \mathrm{X}_{\mathrm{Fe}}-4092 \mathrm{X}_{\mathrm{K}}-894 P\right) / T. \end{equation}$$ In this equation, X Ca , X Si , and so on refer to the oxide mole fractions on a single-cation basis, P is in GPa and T in K. The equation reproduces 58 data points with an r 2 of 0.96 and a standard error of 0.089. The addition of literature data on hydrous experiments indicates that the effects of <4.3 wt% H 2 O are small enough to be ignored. We also performed experiments aimed at determining the conditions of NaCl saturation in melts. When combined with literature data we obtained: log C l − = log a N a C l + 0.06 − 2431 X C a + 3430 X S i − 2592 X F e + 3484 X N a + 4092 X K − 2417 / T $$ \begin{equation} \log \left(\mathrm{Cl}^{-}\right)=\log \left(a_{\mathrm{NaCl}}\right)+0.06-\left(2431 \mathrm{X}_{\mathrm{Ca}}+3430 \mathrm{X}_{\mathrm{Si}}-2592 \mathrm{X}_{\mathrm{Fe}}+3484 \mathrm{X}_{\mathrm{Na}}+4092 \mathrm{X}_{\mathrm{K}}-2417\right) / T \end{equation}$$ where (Cl – ) is the Cl content of the melt in wt% a NaCl is the activity of NaCl (liquid) and the other symbols are the same as before. The results indicate that basalt dissolves ~8 times more Cl than rhyolite at a given NaCl activity i.e., Cl is ~8 times more soluble in basalt than in rhyolite.

The stabilities of the minerals that can hold water are important for understanding water behavior in the Earth’s deep interior. Recent experimental studies have shown that the incorporation of aluminum enhances the thermal stabilities of hydrous minerals significantly. In this study, the phase relations of hydrous aluminosilicates in the AlOOH-AlSiO 3 OH system were investigated at 22 GPa and 1400–2275 K using a multi-anvil apparatus. Based on the X-ray difraction measurements and composition analysis of the recovered samples, we found that the AlSiO 4 H phase Egg forms a solid solution with δ-AlOOH above 1500 K. Additionally, at temperatures above 1800 K, two unknown hydrous aluminosilicates with compositions Al 2.03 Si 0.97 O 6 H 2.03 and Al 2.11 Si 0.88 O 6 H 2.11 appeared, depending on the bulk composition of the starting materials. Both phases can host large amounts of water, at least up to 2275 K, exceeding the typical mantle geotherm. The extreme thermal stability of hydrous aluminosilicates suggests that deep-subducted crustal rocks could be a possible reservoir of water in the mantle transition zone and the uppermost lower mantle.

Spinel minerals occur as inclusions in both silicates and sulfides in the Kalatongke magmatic Ni-Cu deposit in NW. China, showing textural and compositional variations. The spinel enclosed in olivine and other silicates (orthopyroxene, clinopyroxene, and hornblende) is predominantly Cr-magnetite with minor Cr-spinel, having wide variations in MgO (0.1–8.0 wt%), Al 2 O 3 (1–25 wt%), Cr 2 O 3 (3–20 wt%), and TiO 2 (0.5–6.2 wt%) contents. Such continuous variations suggest that Cr-magnetite in silicates was crystallized from residual melts and experienced extensive reaction with trapped liquid undergoing a typical tholeiitic trend of increasing Fe and Ti concentrations. Crystals of Cr-magnetite enclosed in disseminated sulfides have similar Mg, Al, Cr, Ti, V, Sc, Ga, Mo, Zr, and Nb concentrations to the Cr-magnetite in silicates. Such compositional similarity, which is explained by the simultaneous equilibrium crystallization of Cr-magnetite from the silicate and sulfide melts, shows that the Kalatongke deposit is a typical example of where the same mineral phase is formed from two coexisting immiscible liquids. However, the Cr-magnetite in disseminated sulfide and that in silicates show distinctly different crystal size distribution patterns, illustrating that the chemical equilibrium was attained despite contrasting growth rates. Nevertheless, the Cr-magnetite in disseminated sulfides shows significantly lower Ni, Co, and Zn contents (median value of 845, 22, and 319 ppm) than that in silicates (median value of 1428, 160, and 1039 ppm). This cannot be the result of sulfide fractionation because there is little compositional variation between Cr-magnetite included in pyrrhotite (early crystallized phase) and that immersed in chalcopyrite (late crystallized phase). Such Ni, Co, and Zn depletions, combined with the relatively constrained Fe/Ni, Fe/Co, and Fe/Zn ratios in those Cr-magnetite, are attributed to postcumulus reactions between Cr-magnetite and sulfide melts. The spinel hosted by massive sulfides is magnetite, which has distinctly different compositional variations and crystal size distribution patterns compared with those of the silicate-hosted Cr-magnetite, although the magnetite in massive ore generally has similar contents in some lithophile elements (Zr, Ta, Mo, Sn, Mn) to the silicate-hosted Cr-magnetite. This could be taken as evidence for a mixture of early accumulated sulfide pools with a component of drained sulfide from the cumulates above. This study shows a detailed textural and compositional investigation of spinel is useful to decode the sulfide evolution processes during the formation of magmatic Ni-Cu deposits and highlights that equilibrium crystallization and postcumulus reactions play critical roles in controlling the spinel/magnetite composition.

X-ray absorption spectroscopy (XAS) offers great potential to identify and quantify Mn species in surface environments by means of linear combination fit (LCF), fingerprint, and shell-fit analyses of bulk Mn XAS spectra. However, these approaches are complicated by the lack of a comprehensive and accessible spectrum library. Additionally, molecular-level information on Mn coordination in some potentially important Mn species occurring in soils and sediments is missing. Therefore, we investigated a suite of 32 natural and synthetic Mn reference compounds, including Mn oxide, oxyhydroxide, carbonate, phosphate, and silicate minerals, as well as organic and adsorbed Mn species, by Mn K -edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The ability of XAS to infer the average oxidation state (AOS) of Mn was assessed by comparing XANES-derived AOS with the AOS obtained from redox titrations. All reference compounds were studied for their local (<5 Å) Mn coordination environment using EXAFS shell-fit analysis. Statistical analyses were employed to clarify how well and to what extent individual Mn species (groups) can be distinguished by XAS based on spectral uniqueness. Our results show that LCF analysis of normalized XANES spectra can reliably quantify the Mn AOS within ~0.1 v.u. in the range +2 to +4. These spectra are diagnostic for most Mn species investigated, but unsuitable to identify and quantify members of the manganate and Mn(III)-oxyhydroxide groups. First-derivative XANES fingerprinting allows the unique identification of pyrolusite, ramsdellite, and potentially lithiophorite within the manganate group. However, XANES spectra of individual Mn compounds can vary significantly depending on chemical composition and/or crystallinity, which limits the accuracy of XANES-based speciation analyses. In contrast, EXAFS spectra provide a much better discriminatory power to identify and quantify Mn species. Principal component and cluster analyses of k 2 -weighted EXAFS spectra of Mn reference compounds implied that EXAFS LCF analysis of environmental samples can identify and quantify at least the following primary Mn species groups: (1) Phyllo- and tectomanganates with large tunnel sizes (2 × 2 and larger; hollandite sensu stricto, romanèchite, todorokite); (2) tectomanganates with small tunnel sizes (2 × 2 and smaller; cryptomelane, pyrolusite, ramsdellite); (3) Mn(III)-dominated species (nesosilicates, oxyhydroxides, organic compounds, spinels); (4) Mn(II) species (carbonate, phosphate, and phyllosilicate minerals, adsorbed and organic species); and (5) manganosite. All Mn compounds, except for members of the manganate group (excluding pyrolusite) and adsorbed Mn(II) species, exhibit unique EXAFS spectra that would allow their identification and quantification in mixtures. Therefore, our results highlight the potential of Mn K -edge EXAFS spectroscopy to assess bulk Mn speciation in soils and sediments. A complete XAS-based speciation analysis of bulk Mn in environmental samples should preferably include the determination of Mn valences following the “Combo” method of Manceau et al. (2012), EXAFS LCF analyses based on principal component and target transformation results, as well as EXAFS shell-fit analyses for the validation of LCF results. For this purpose, all 32 XAS reference spectra are provided in the Online Materials Deposit item AM-23-58236, Online Materials. Deposit items are free to all readers and found on the MSA website, via the specific issue’s Table of Contents (go to http://www.minsocam.org/MSA/AmMin/TOC/2023/May2023_data/May2023_data.html). for further use by the scientific community.

The formation of heteroaggregates is critical to controlling the stabilization and transformation of nanominerals and mineral nanoparticles (NMMNs) in nature, but the underlying mechanisms remain to be deciphered. In this work, we study the effect of surface interactions between ferrihydrite (Fh) and montmorillonite (Mnt) within their heteroaggregates on the transformation behaviors of Fh. A series of heteroaggregates composed of Fh and Mnt were synthesized by modulating their mass ratios and synthesis methods, i.e., directly complexing Fh with Mnt (Fh-Mnt) or in situ growing Fh on Mnt (Fh/Mnt). Structural characterization using XRD, TG-DSC, TEM, and FTIR indicated that Fh particles coated more evenly on the Mnt surface within the heteroaggregates synthesized by in situ growing Fh on Mnt and with lower Fh to Mnt ratio, and accordingly these heteroaggregates showed stronger surface interactions between Fh and Mnt. The phase transformation of Fh to hematite (Hem) on the heteroaggregates can be significantly affected during the heating treatment. Compared with that of pure Fh, the transformation of Fh on all of the heteroaggregates was retarded (e.g., slower transformation rate and smaller produced Hem particles), particularly for the samples with stronger surface interactions (e.g., Fh/Mnt with lower Fh to Mnt ratio). Noticeably, the heated heteroaggregates may simultaneously contain pristine Fh, intermediate maghemite, and transformed Hem, showing a heterogeneous transformation behavior of Fh. The strong interactions between Fh and Mnt will enhance the dispersion of Fh and restrict the structural rearrangement of Fh (particularly those at the interface) during the phase transformation process, resulting in retarded and heterogenous transformation of Fh on these heteroaggregates. These findings not only enrich our knowledge of the phase transformation characteristics of Fh but also advance our understanding of the important role of mineral surface interactions in stabilizing NMMNs in nature.

Calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites are composed of refractory minerals thought to be the first solids to have formed in the solar nebula. Among them, hibonite, nominally CaAl 12 O 19 , holds particular interest because it can incorporate significant amounts of Ti into its crystal structure in both Ti 3+ and Ti 4+ oxidation states. The relative amounts of these cations that are incorporated reflect the redox conditions under which the grain formed or last equilibrated and their measurement can provide insight into the thermodynamic landscape of the early solar nebula. Here we develop a new method for the quantification of Ti oxidation states using electron energy-loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscope (STEM) to apply it to hibonite. Using a series of Ti-bearing oxides, we find that the onset intensity of the Ti L 2,3 edge decreases with increasing Ti-oxidation state, which is corroborated by simulated Ti-oxide spectra using first-principles density-functional theory. We test the relationship on a set of synthetic hibonite grains with known Ti 4+ /ΣTi values and apply the developed method on a hibonite grain from a compact type A inclusion in the Northwest Africa (NWA) 5028 CR2 carbonaceous chondrite. The STEM-EELS data show that the chondritic hibonite grain is zoned with a Ti 4+ /ΣTi ratio ranging from 0.78 ± 0.04 to 0.93 ± 0.04 over a scale of 100 nm between the core and edge of the grain, respectively. The Ti substitution sites are characterized by experimental and calculated high-angle annular-dark-field (HAADF) images and atomic-level EEL spectrum imaging. Simulated HAADF images reveal that Ti is distributed between the M2 and M4 sites while Mg sits on the M3 site. Quantitative energy-dispersive X-ray spectroscopy shows that this grain is also zoned in Al and Ti. The Mg distribution is not well correlated with that of Ti and Ti 4+ /ΣTi at the nanoscale. The spatial decoupling of the element composition and Ti-oxidation states suggests a multistage evolution for this hibonite grain. We hypothesize that Ti and Mg were incorporated into the structure during condensation at high temperature through multiple reactions. Transient heating, presumably in the solar nebula, adds complexity to the crystal chemistry and potentially redistributed Ti and Mg. Concurrently, the formation of oxygen vacancies as a result of a reducing gas, led to the reduction of Ti 4+ to Ti 3+ . The multiple defect reactions occurring in this single hibonite crystal preclude a simple relationship between the Ti 4+ /ΣTi and the f O2 of formation. However, moving forward, these measurements are fundamental inputs for modeling of the thermodynamic conditions under which hibonite formed in the early solar nebula.

The high-pressure structure and stability of the calcic amphibole tremolite [Ca 2 Mg 5 Si 8 O 22 (OH) 2 ] was investigated to ~40 GPa at 300 K by single-crystal X-ray diffraction using synchrotron radiation. C 2/ m symmetry tremolite displays a broader metastability range than previously studied clinoamphiboles, exhibiting no first-order phase transition up to 40 GPa. Axial parameter ratios a / b and a / c , in conjunction with finite strain vs. normalized pressure trends, indicate that changes in compressional behavior occur at pressures of ~5 and ~20 GPa. An analysis of the finite strain trends, using third-order Birch-Murnaghan equations of state, resulted in bulk moduli ( K 0 T ) of 72(7), 77(2), and 61(1) GPa for the compressional regimes from 0–5 GPa (regime I), 5–20 GPa (II), and above 20 GPa (III), respectively, and accompanying pressure-derivatives of the bulk moduli ( K′ 0 T ) of 8.6(42), 6.0(3), and 10.0(2). The results are consistent with first-principle theoretical calculations of tremolite elasticity. The axial compressibility ratios of tremolite, determined as β a :β b :β c = 2.22:1.0:0.78 (regime I), 2.12:1.0:0.96 (II), and 1.03:1.0:0.75 (III), demonstrate a substantial reduction of the compressional anisotropy of tremolite at high pressures, which is a notable contrast with the increasingly anisotropic compressibility observed in the high-pressure polymorphs of the clinoamphibole grunerite. The shift in compression-regime at 5 GPa (I–II) transition is ascribed to stifening along the crystallographic a -axis corresponding to closure of the vacant A-site in the structure, and a shift in the topology of the a- oriented surfaces of the structural I-beam from concave to convex. The II–III regime shift at 20 GPa corresponds to an increasing rate of compaction of the Ca-polyhedra and increased distortion of the Mg-octahedral sites, processes which dictate compaction in both high-pressure compression-regimes. Bond-valence analyses of the tremolite structure under pressure show dramatic overbonding of the Ca-cations (75% at 30 GPa), with significant Mg-cation overbonding as well (40%). These imply that tremolite’s notable metastability range hinges on the calcium cation’s bonding environment. The eightfold-coordinated Ca-polyhedron accommodates significant compaction under pressure, while the geometry of the Ca-O polyhedron becomes increasingly regular and inhibits the reorientation of the tetrahedral chains that generate phase transitions observed in other clinoamphiboles. Peak/background ratio of difraction data collected above 40 GPa and our equation of state determination of bulk moduli and compressibilities of tremolite in regime III, in concert with the results of our previous Raman study, suggest that C 2/ m tremolite may be approaching the limit of its metastability above 40 GPa. Our results have relevance for both the metastable compaction of tremolite during impact events, and for possible metastable persistence of tremolite within cold subduction zones within the Earth.

Raman spectroscopy is widely used to identify mineral and fluid inclusions in host crystals, as well as to calculate pressure-temperature ( P - T ) conditions with mineral inclusion elastic thermobarometry, for example quartz-in-garnet barometry (QuiG) and zircon-in-garnet thermometry (ZiG). For thermobarometric applications, P - T precision and accuracy depend crucially on the reproducibility of Raman peak position measurements. In this study, we monitored long-term instrument stability and varied analytical parameters to quantify peak position reproducibility for Raman spectra from quartz and zircon inclusions and reference crystals. Our ultimate goal was to determine the reproducibility of calculated inclusion pressures (“ P inc ”) and entrapment pressures (“ P trap ”) or temperatures (“ T trap ”) by quantifying diverse analytical errors, as well as to identify optimal measurement conditions and provide a baseline for interlaboratory comparisons. Most tests emphasized 442 nm (blue) and 532 nm (green) laser sources, although repeated analysis of a quartz inclusion in garnet additionally used a 632.8 nm (red) laser. Power density was varied from <1 to >100 mW and acquisition time from 3 to 270s. A correction is proposed to suppress interference on the ~206 cm –1 peak in quartz spectra by a broad nearby (~220 cm –1 ) peak in garnet spectra. Rapid peak drift up to 1 cm –1 /h occurred after powering the laser source, followed by minimal drift (<0.2 cm –1 /h) for several hours thereafter. However, abrupt shifts in peak positions as large as 2–3 cm –1 sometimes occurred within periods of minutes, commonly either positively or negatively correlated to changes in room temperature. An external Hg-emission line (fluorescent light) can be observed in spectra collected with the green laser and shows highly correlated but attenuated directional shifts compared to quartz and zircon peaks. Varying power density and acquisition time did not affect Raman peak positions of either quartz or zircon grains, possibly because power densities at the levels of inclusions were low. However, some zircon inclusions were damaged at higher power levels of the blue laser source, likely because of laser-induced heating. Using a combination of 1, 2, or 3 peak positions for the ~128, ~206, and ~464 cm –1 peaks in quartz to calculate P inc and P trap showed that use of the blue laser source results in the most reproducible P trap values for all methods (0.59 to 0.68 GPa at an assumed temperature of 450 °C), with precisions for a single method as small as ±0.03 GPa (2σ). Using the green and red lasers, some methods of calculating P trap produce nearly identical estimates as the blue laser with similarly good precision (±0.02 GPa for green laser, ±0.03 GPa for red laser). However, using 1- and 2-peak methods to calculate P trap can yield values that range from 0.52 ± 0.06 to 0.93 ± 0.16 GPa for the green laser, and 0.53 ± 0.08 GPa to 1.00 ± 0.45 GPa for the red laser. Semiquantitative calculations for zircon, assuming a typical error of ±0.25 cm –1 in the position of the ~1008 cm –1 peak, imply reproducibility in temperature (at an assumed pressure) of approximately ±65 °C. For optimal applications to elastic thermobarometry, analysts should: (1) delay data collection approximately one hour after laser startup, or leave lasers on; (2) collect a Hg-emission line simultaneously with Raman spectra when using a green laser to correct for externally induced shifts in peak positions; (3) correct for garnet interference on the quartz 206 cm –1 peak; and either (4a) use a short wavelength (blue) laser for quartz and zircon crystals for P - T calculations, but use very low-laser power (<12 mW) to avoid overheating and damage or (4b) use either the intermediate wavelength (green; quartz and zircon) or long wavelength (red; zircon) laser for P - T calculations, but restrict calculations to specific methods. Implementation of our recommendations should optimize reproducibility for elastic geothermobarometry, especially QuiG barometry and ZiG thermometry.

Trace concentrations of H 2 O in olivine strongly affect diverse mantle and magmatic processes. H 2 O in olivine has been difficult to accurately quantify due to challenges in sample preparation and measurement, as well as significant uncertainties in standard calibrations. Here we directly compare secondary-ion mass spectrometry (SIMS) measurements of the olivine standards of Bell et al. (2003, hereafter Bell03) and Withers et al. (2012, hereafter Withers12) upon which most SIMS and Fourier transform infrared (FTIR) spectroscopy analyses are based. In the same SIMS session, we find that the olivine standards from the two studies are offset by ~50%, forming lines of different slope when comparing SIMS measurements to the independent nuclear reaction analysis (NRA) in Bell03 and elastic recoil detection analysis (ERDA) in Withers12. This offset is similar to the ~40% offset that exists in the FTIR absorption coefficients determined by those two studies, and points to the NRAERDA data as the cause for the offset more than different IR absorption characteristics of the different olivines. We find that the Withers12 olivine standards form the most precise calibration line, and that the measured Bell03 olivine standards have issues of reproducibility and accuracy due to the presence of hydrous inclusions (as documented previously by Mosenfelder et al. 2011). Owing to the limited availability of the Withers12 olivine standards, however, we recommend using orthopyroxene standards (Kumamoto et al. 2017) to calibrate H 2 O in olivine by SIMS due to similar calibration slopes. We revise the reference values of current orthopyroxene standards to account for uncertainties in the Bell et al. (1995) manometry data. With these revised values, the orthopyroxene calibration line is within 12% of the Withers12 olivine line, which is within the long-term uncertainty of the SIMS olivine measurements. We apply our SIMS calibration protocol to revise estimates of the partition coefficients for H 2 O between olivine and melt, resulting in a value of 0.0009 ± 0.0003 at pressures ~0.2–2 GPa. This brings into closer agreement between the partition coefficients determined from experimental studies and those based on natural studies of olivine-hosted melt inclusions.

Arsenian pyrite is known to have a strong association with gold in most auriferous refractory deposits, and thus understanding the chemical speciation of arsenic in localized environments in arsenian pyrite provides an important basis for determining its reactivity and mobility. However, arsenic is fast-oxidizing among elements in the Fe-As-S system and hence it may exist in various chemical states, which renders it dificult to establish arsenic nature under pristine conditions, particularly in arsenian pyrite. Herein, arsenian pyrite samples were analyzed on a synchrotron soft X-ray spectroscopy beamline under ultrahigh vacuum conditions, and As-3d as well as S-2p spectra were collected. A comparison between the spectrum of bulk As-3d in the samples with its bulk counterpart in arsenopyrite revealed a 0.6 eV shift toward lower binding energies. This observation was similar to loellingite (FeAs2), where the binding energy shift was attributed to high electron density on As of the As-As dimer. Formation of As clusters resulting in comparable binding energy shifts was also proposed from the spectroscopic studies. The experiments were complemented by a series of first-principles calculations simulating four experimentally observed pyrite surfaces where surficial S atoms were randomly substituted by As. As such, six arsenian pyrite crystal surfaces were modeled, two of which constituted surficial As clusters replacing both S and Fe atoms. The surfaces were geometrically optimized, and surface energies were calculated along with the corresponding electronic structure providing a detailed distribution of partial charges for surficial atoms obtained from Löwdin population analysis. The calculated partial charges of atoms located at the surface arsenian pyrite indicated that while the electron density on the As atom of As-S dimers in arsenian pyrite is less negative than the As in bulk arsenopyrite, it is more negative for the As atom of As-As dimers, which were only seen in the surficial As clusters. This validated the description of As presence in arsenian pyrite as local clusters inducing localized lattice strain due to increased bond distances. Our findings ofer a good background for future studies into the reactive sites in arsenian pyrite and how that compares with associated minerals, arsenopyrite, and pyrite.

The electrical conductivity and elasticity of deep hydrous phases are essential to constraining water distribution, as well as deciphering the origins of conductivity anomalies in the lower mantle. To uncover the impact of iron-bearing δ-AlOOH on the geophysical properties of the lower mantle, we carried out synchrotron X‑ray diffraction and electrical conductivity measurements on δ-(Al 0.52 Fe 0.48 )OOH and (Al 0.95 Fe 0.05 )OOH in diamond-anvil cells at pressures up to 75 GPa at room temperature. A sharp volume reduction of ~6.5% was observed in δ-(Al 0.52 Fe 0.48 )OOH across the spin transition at 40.8–43.3 GPa, where its electrical conductivity increases steadily without abrupt changes. The electrical conductiv‑ ity of δ-(Al 0.52 Fe 0.48 )OOH is greater than that of pure δ-AlOOH at high pressure, suggesting that both small polaron and proton conduction mechanisms dominate in iron-bearing δ-AlOOH. Furthermore, the high-pressure electrical conductivity profiles are comparable between δ-(Al 0.95 Fe 0.05 )OOH and δ-(Al 0.52 Fe 0.48 )OOH, indicating that high-iron content only marginally influences the conductivity of iron-bearing δ-AlOOH. Notably, the electrical conductivity of iron-bearing δ-AlOOH along the North Philippine geotherm is greater than the average 1D electrical conductivity profile in the mantle (Ohta et al. 2010a). This result suggests that δ-(Al,Fe)OOH is a promising candidate to account for high conductivity in some subducting slabs.

First-principles calculations based on density functional theory (DFT) using the generalized gradient approximation (GGA) were performed to assess the energetic barriers separating different topological configurations of the ( 4 H ) S i X $\begin{equation}(4 H)_{\mathrm{Si}}^{\mathrm{X}} \end{equation}$hydrogarnet type defect in Mg 2 SiO 4 forsterite with the climbing image nudged elastic band (CI-NEB) method. Barrier heights are low (<0.6 eV) with respect to typical activation energies observed for H-diffusion but more comparable to those for electrical conductivity of H 2 O-bearing nominally anhydrous minerals. As can be expected, hydrogen bonding to O atoms both within the defect and belonging to adjacent tetrahedra plays a fundamental role in the stability of each configuration. Saddle points along the minimum energy path (MEP) typically correspond to the transition of one hydrogen bond breaking to form a new hydrogen bond such that one or more OH bonds have shifted in direction without themselves breaking. MEPs show that slightly out-of-plane torsional hopping from one configuration to another can reduce the height of the barrier. We illustrate several different reaction coordinates between symmetry equivalent pairs of configurations and non-symmetry related pairs that can result in an effective means of local charge transport by shifting the center of mass of the (4H) 4+ cluster within the defect site without proton transfer to an interstitial site. Especially at low temperatures in the absence of thermally activated processes that result in the breaking of stronger chemical bonds, these types of configurational transformation mechanisms are likely to be important contributors to the dielectric behavior of nominally anhydrous silicate minerals and also affect both electrical conductivity and electrical conductivity anisotropy when investigated by AC methods such as impedance spectroscopy. The NEB method can also be used to examine more effective charge and mass transport processes that involve the dissociation of the hydrogarnet defect into more complex chemical species, which might involve similar hydrogen bond breaking and forming processes observed in this study along with more significant atomic displacements.

Goethite and modified goethites have been found as good photocatalysts because their conduction band can mediate electron transfer in various redox processes. Many kinds of metal elements can be incorporated into the structure of goethite to form solid solutions in nature, but their optoelectronic properties have not been well disclosed. Mn-substituted goethite is one of the potential photocatalysts, which can exhibit high-photocatalytic activity in many Earth’s surface processes. Based on the first-principles calculation, pairwise interaction energies and static lattice energies of goethitegroutite solid solution were computed, and the most thermodynamically stable configurations of Mn-substituted goethite were determined. The results indicate that Mn 3+ ion tends to distribute within the cation layer parallel to the (001) plane. Phase relations of goethite-groutite solid solution were derived by subsequent configurational statistics with energies of all 2 32 configurations of a 2 × 1 × 4 supercell with 32 exchangeable cations. The phase diagram shows that no more than 3 mol% Fe of goethite can be substituted by Mn ions. Therefore, Mn-substituted goethite is thermodynamically metastable or bears groutite-like clusters/lamellae. Furthermore, the effects of Mn substitutions on the band gap were experimentally and theoretically investigated. It is found that a small amount of Mn-substitution can reduce the band gap of goethite significantly, and the decrease ceases when the Mn content is higher than 3–4 mol%. Such a decrease in band gap causes red-shift to the photo response wavelength of goethite and improves the responding capability. This improvement was confirmed in the experiments of photocatalytic degradation of methylene blue (MB). Such kind of photocatalytic reaction probably can happen widely in natural environments. Therefore, the contribution of photocatalysis of natural goethites to geochemical processes on Earth’s surface should be considered.

Estimates of the oxidation states of magmas are important to current investigations of the geochemical characteristics of their source regions and of evolved magmatic series created during differentiation. One means of achieving such estimates is to capitalize on compositions of coexisting cubic and rhombohedral Fe-Ti oxides determined by electron microprobe. A combination of experimental calibration points and thermodynamic modeling provides a basis for translating such compositions into T -ƒ O2 values. This has been done until recently by estimating Fe 3+ /ΣFe on the basis of charge balance and stoichiometry by the method of Droop (1987), after matrix corrections of X-ray intensity data have been performed, as EPMA cannot be used routinely to distinguish different elemental valence states, much less accurately quantify abundances of Fe 3+ and Fe 2+ . The traditional approach of undertaking post-data-reduction calculations falls short of attaining the best possible quantitative results. The tactical choice of not accounting for light elements that have not been explicitly analyzed prior to matrix corrections of X-ray intensity data leads to systematic errors in reported oxide abundances for measured elements. This article addresses one such issue, the oxygen associated with Fe 3+ (hereafter “excess oxygen”), on the basis of coexisting Fe-Ti oxides from Andean lavas. A new software routine in probe for EPMA (PFE) uses an iterative calculation scheme to calculate amounts of excess oxygen that would not be considered if all iron were assumed to be ferrous and then applies this excess oxygen during matrix corrections. The PFE approach reveals that Fe-concentrations have been underestimated, universally, in these minerals because O atoms absorb Fe K α radiation: discrepancies increase as total Fe and Fe 3+ /Fe 2+ , hence excess oxygen, increase. Analyses of the most Fe-rich cubic oxide compositions in this data set have ~6 wt% excess oxygen and ~1 wt% more FeO+Fe 2 O 3 than would be reported without incorporating the impact of excess oxygen in matrix corrections. Minor to negligible differences in other elements are also observed. These effects are not because excess oxygen is directly attributed to these elements, although some may be present in multiple valence states, as matrix corrections are undertaken on the basis of the conventional assumptions that they occur as Cr 3+ , V 3+ , Mn 2+ , Mg 2+ , Ca 2+ , and Si 4+ . Rather, variably small increases in total Fe propagate through the matrix corrections for other elements, and these differences may be recorded as minor increases or decreases in some concentrations, depending on the particular element and the amount of change in Fe-concentration. Fe 3+ /ΣFe in analyses produced with the PFE routine are essentially identical to those determined in the traditional mode, as cation proportions calculated on the basis of charge balance and stoichiometry, with the method of Droop (1987), is a necessary step. The new method: (1) provides more accurate concentrations, mainly for Fe and Ti; (2) is applicable to any mineral containing ferric iron (subject to stoichiometric constraints); (3) provides more accurate analytical totals, which can be advantageous for evaluating analytical quality; and (4) does not impact estimates of oxidation state. Oxygen fugacities and temperatures determined with the model of Ghiorso and Evans (2008) are essentially unchanged.

Kamafugites are strongly silica-undersaturated melts that are difficult to produce by partial melting of volatile-free peridotites but can be produced experimentally in the presence of CO 2 . Nevertheless, there is not yet direct evidence for a CO 2 -rich mantle source and the possible presence of recycled carbonates in the source of natural kamafugites. Marine carbonates have a heavier zinc isotopic composition (δ 66 Zn) than that of the mantle by up to 1.0‰, making zinc isotopes a sensitive tracer for recycled carbonates in the sources of mantle-derived magmas. Here we take Cenozoic kamafugites from the West Qinling orogen in China as an example to address the origin of this rare volcanic rock. The West Qinling kamafugites are strongly silica-undersaturated (SiO 2 = 37.0 to 43.0 wt%) and have significantly higher δ 66 Zn (0.30‰ to 0.47‰) than that of the normal mantle (0.18 ± 0.05‰). No correlation between δ 66 Zn and MgO or SiO 2 contents is observed, indicating that the high δ 66 Zn was not a result of magmatic differentiation. Modeling of melting indicates that even at extremely low degree (~0.5%), partial melting of a normal peridotitic source is still unlikely to produce silicate melts with δ 66 Zn values exceeding 0.30‰. Thus, the elevated δ 66 Zn of the West Qinling kamafugites demonstrates the presence of recycled carbonates in their mantle sources. Binary-mixing modeling suggests that the source contains ~5 to 15% recycled carbonates, which is supported by the positive correlation between δ 66 Zn and CaO/Al 2 O 3 . Overall, the West Qinling kamafugites represent the products of low-degree partial melting of a recycled carbonate-bearing peridotite source, which provides evidence for an important role of recycled carbonates in the origin of natural kamafugite suites.

Both environmental and physiological factors cause carbonate ion structural disorder in biogenic Mg-calcites. A major component of this disorder is driven by the incorporation of Mg through environmental forcing and growth rate kinetics although non-Mg factors (e.g., other cation/anion impurities, organic molecules) also contribute. Understanding the drivers of Mg content in biogenic calcite and its effects on disorder has implications for octocoral Mg paleo-proxies and the stability and diagenetic alteration of their calcitic skeletons. However, prior studies of biogenic Mg-calcites have often been complicated by sampling inconsistencies over space and time and potential intra-sample Mg variability. This study aims to analyze the relative contributing factors of octocoral Mg-calcite structural disorder along gradients of both depth and growth rate. Calcitic octocorals (Coralliidae and Keratoisididae, N = 28) were collected from 221–823 m depths across a natural gradient in biogeochemical parameters (pH: 7.4–7.9, T : 5–16 °C) off the Kona coast of Hawai‘i Island and were analyzed using Raman spectroscopy. Samples were collected during the same month, controlling for potential seasonal variability. Raman spectral parameters from the ν 1 peak quantified total carbonate ion structural disorder (full-width at half maximum height [FWHM] of ν 1 ) and Mg content (ν 1 position, Raman shift). The total structural disorder was then partitioned into Mg-driven and non-Mg driven components (residual ν 1 FWHM). The total structural disorder and Mg content decreased significantly with increasing depth, correlating with temperature and carbonate system parameters. The Mg-temperature relationships from this study were also consistent with prior studies. Non-Mg structural disorder did not correlate to any environmental parameters. When measured across an intra-sample gradient of ontogenetic growth rate, total structural disorder, Mg content, and non-Mg structural disorder increased with growth rate for all but one taxon, demonstrating the kinetic effect of growth rate as well as potential taxon-specific physiological effects. These results provide insight into how environmental and growth rate kinetic effects independently afect different components of carbonate ion structural disorder (Mg content and non-Mg factors). These findings also suggest that Raman spectroscopy may be helpful in quantifying solubility within biogenic calcites.