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<jats:title>Abstract</jats:title> <jats:p>Iceland’s Námafjall geothermal area exhibits a range of alteration environments. Geochemical and mineralogical analyses of fumaroles and hot springs interacting with Holocene basaltic lavas at Hverir, and with Pleistocene hyaloclastites atop nearby Námaskar∂, reveal different patterns of alteration depending on the water/rock ratio, degree of oxidation, and substrate composition and age. The focus of this study is a transect of a Hverir fumarole that has formed a bullseye pattern of alteration of a Holocene basaltic lava flow. Surface samples and samples collected from shallow pits were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) to constrain changes in mineral assemblage and major elemental composition with both distance and depth. Elemental sulfur is concentrated near the vent, with leached deposits with amorphous silica and anatase nearby and kaolinite, hematite, and jarosite/alunite-group sulfate minerals farther out, with smectites and less altered material at the margins, though smaller-scale mineralogical diversity complicates this pattern.</jats:p> <jats:p>Silica phases include amorphous silica (most samples), cristobalite (some samples in the leached part of the apron), and quartz (minor constituent of a few samples). The silica was concentrated through residual enrichment caused by leaching and is accompanied by a significant enrichment in TiO2 (in anatase). The presence of abundant cristobalite in a surface fumarole-altered Holocene basaltic lava flow most likely reflects cristobalite formed during the devitrification of volcanic glass or precipitation from fumarolic vapors, rather than high-temperature processes. Minor, localized quartz likely reflects diagenetic maturation of earlier-formed amorphous silica, under surface hydrothermal conditions. Natroalunite, natrojarosite, and jarosite are all present and even exhibit compositional zonation within individual crystals, showing that under surface hydrothermal conditions, these minerals can form a significant solid solution.</jats:p> <jats:p>The high iron content of the substrate basalt and the prevalence of Fe-sulfates and Fe-oxide spherules among the alteration products makes this geothermal area an especially useful analog for potential martian hydrothermal environments. The residual enrichment of silica in the leached deposits of the Hverir fumarole apron could serve as an acid-sulfate leaching model in which amorphous silica forms without appreciable sulfur-bearing phases in many samples, a possible analog for silica-rich soils in the Columbia Hills on Mars. The coexistence of hematite spherules and jarosite-group minerals serves as an intriguing analog for a volcanic/hydrothermal model for hematite and jarosite occurrences at Meridiani Planum.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The solid solution between CaSiO3 and MgSiO3 perovskites is an important control on the properties of the lower mantle but the effect of one of the most important impurity elements (iron) on this solution is largely unknown. Using density functional theory (DFT), ferrous iron’s influence on the reciprocal solubility of MgSiO3 and CaSiO3 perovskite (forming a single Ca-Mg mixed perovskite phase) was calculated under pressures and temperatures of 25–125 GPa and 0–3000 K, respectively. Except at iron-rich conditions, ferrous iron preferentially partitions into the mixed perovskite phase over bridgmanite. This is a small effect (partitioning coefficient KD ~0.25–1), however, when compared to the partitioning of ferrous iron to ferropericlase, which rules out perovskite phase mixing as a mechanism for creating iron-rich regions in the mantle. Iron increases the miscibility of Ca and Mg perovskite phases and reduces the temperature at which the two perovskite phases mix but this effect is highly nonlinear. We find that for a pyrolytic mantle [Ca% = 12.5 where Ca% = Ca/(Ca+Mg)] a perovskite ferrous iron concentration of ~13% leads to the lowest mixing temperature and the highest miscibility. With this composition, 1% ferrous iron in a pyrolytic composition would lead to mixing at ~120 GPa along the geothermal gradient, and 6.25% ferrous iron leads to mixing at ~115 GPa and 13% ~110 GPa. At high iron concentrations, Fe starts to impair miscibility, with 25% ferrous iron leading to mixing at ~120 GPa. Thus, in normal pyrolytic mantle, iron could induce a small amount of Ca-pv and Mg-pv mixing near the D″ layer but it generally partitions to ferropericlase instead and does not impact mixing. Extremely iron rich parts of the lower mantle such as ULVZs or the CMB (potentially) are also not a likely source of phase mixed perovskites due to the nonlinear effect of ferrous iron on phase mixing.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Cubic boron nitride (c-BN) has the same structure as diamond, and it shows very inert reaction activity in different chemical environments, even under high-pressure (P) and high-temperature (T) conditions. Furthermore, the P- and T-dependent Raman shift of c-BN (e.g., TO mode) can be distinguished from that of the diamond anvil (c-BN at ~1054 cm–1 vs. diamond at ~1331 cm–1 at ambient conditions), making c-BN a potential P-T sensor for diamond-anvil cell (DAC) experiments. However, the Raman shift of c-BN has not been well studied at high P-T conditions, especially at temperatures above 700 K. In this study, we systematically calibrated the Raman shift of the TO mode (νTO) for synthetic c-BN grains at high-P and high-T conditions up to 15 GPa and 1300 K. Both ruby (Mao et al. 1986) and Sm2+:SrB4O7 (Datchi et al. 2007) were used as internally consistent standards for calibration of c-BN P-T sensor. Our results show that the Raman shift of c-BN is negatively correlated with temperature [∂νTO/∂T = –0.02206(71)] but positively correlated with pressure [∂νTO/∂P = –3.35(2)]. More importantly, we found that the P-T cross derivative for the Raman shift of c-BN [∂2νTO/∂P∂T = 0.00105(7)] cannot be ignored, as it was assumed in previous studies. Finally, we calibrated a Raman shift P-T sensor of c-BN up to 15 GPa and 1300 K as follows:</jats:p> <jats:p>P = A ( T ) − A ( T ) 2 + 0.2194 B ( T , Δ v ) 0.1097</jats:p> <jats:p>where A(T) = 3.47(6) + 0.00105(7)T, B(T, ΔνTO) = 2.81(51) – 0.0053(16)T – 1.78(11) × 10–5T2 – ΔνTO. The c-BN Raman shift P-T sensor in this study fills the P-T gap ranging from previously performed externally resistance-heated to laser-heated DAC experiments. The effect of c-BN grain size and Raman system laser power on the calibration were also tested for the P-T sensor. In addition, we conducted three sets of high-P-T experiments to test the practicability of c-BN P-T sensor for water-rock interaction experiments in DAC. Testing experiments showed c-BN has very stable chemical activity in water and clear Raman signal at high-P-T conditions in comparison with other P-T sensors (e.g., ruby, Sm2+:SrB4O7, and quartz). Hence, the Raman shifts of c-BN may serve as an ideal P-T sensor for studying water-rock interactions in a DAC, especially at high-P and high-T conditions relevant to subduction zones.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Bastnäsite contains considerable amounts of U and Th and has been widely used for U-Th-Pb dating. Hydrothermal alteration of bastnäsite is common in nature but its effects on U-Th-Pb dating are not currently well constrained. Hence the significance of U-Th-Pb ages obtained from altered bastnäsite cannot be evaluated. Here, we present a detailed geochronologic as well as micro- and nano-scale mineralogical study of altered bastnäsite in a Mo-REE deposit, Central China. The original bastnäsite grains were confirmed to have crystalized at 208 Ma but were variably overprinted by a hydrothermal event at 150 Ma. They commonly exhibit typical replacement textures that appear to have formed from a coupled dissolution-reprecipitation process, i.e., a primary unaltered domain surrounded by a porous altered domain. Micro- and nano-scale mineralogical observations strongly suggest that during the coupled dissolution-reprecipitation process, non-radiogenic (common) Pb was incorporated into the altered domains in the form of nanoscale galena inclusions. Such incorporation (even minor) has significantly affected the 206Pb/238U and 207Pb/206Pb ratios due to the low contents of U and its daughter isotopes in bastnäsite, resulting in highly variable, discordant U-Pb dates for the altered domains. In contrast, incorporation of the non-radiogenic Pb has very limited effects (&amp;lt;5%) on the Th-Pb system due to the remarkably high contents of Th and radiogenic 208Pb in bastnäsite. Instead, the scattered 208Pb/232Th ages (208 to 150 Ma) of the altered domains were essentially affected by incomplete replacement, and thus can be used to approximate the lower age limit of the primary hydrothermal activity or the upper age limit of the secondary hydrothermal activity. The results from this study highlight that because of the different orders of magnitude between the U and Th contents in bastnäsite, the mobilization of radiogenic and non-radiogenic Pb during alteration may have significantly different impacts on the U-Pb and Th-Pb systems. Therefore, the two systems should be treated separately during the dating of bastnäsite resulting from secondary hydrothermal events.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Hydrated sulfates have been identified and studied in a wide variety of environments on Earth, Mars, and the icy satellites of the solar system. The subsurface presence of hydrous sulfur-bearing phases to any extent necessitates a better understanding of their thermodynamic and elastic properties at pressure. End-member experimental and computational data are lacking and are needed to accurately model hydrous, sulfur-bearing planetary interiors. In this work, high-pressure X-ray diffraction (XRD) and synchrotron Fourier-transform infrared (FTIR) measurements were conducted on szomolnokite (FeSO4·H2O) up to ~83 and 24 GPa, respectively. This study finds a monoclinic-triclinic (C2/c to P1) structural phase transition occurring in szomolnokite between 5.0(1) and 6.6(1) GPa and a previously unknown triclinic-monoclinic (P1 to P21) structural transition occurring between 12.7(3) and 16.8(3) GPa. The high-pressure transition was identified by the appearance of distinct reflections in the XRD patterns that cannot be attributed to a second phase related to the dissociation of the P1 phase, and it is further characterized by increased H2O bonding within the structure. We fit third-order Birch-Murnaghan equations of state for each of the three phases identified in our data and refit published data to compare the elastic parameters of szomolnokite, kieserite (MgSO4·H2O), and blödite (Na2Mg(SO4)2·4H2O). At ambient pressure, szomolnokite is less compressible than blödite and more than kieserite, but by 7 GPa both szomolnokite and kieserite have approximately the same bulk modulus, while blödite’s remains lower than both phases up to 20 GPa. These results indicate the stability of szomolnokite’s high-pressure monoclinic phase and the retention of water within the structure up to pressures found in planetary deep interiors.</jats:p>