Previous observations by the Spirit rover in Gusev crater revealed a suite of rocks dubbed Wishstone and Watchtower Class in which the parent lithology and daughter products of a distinctive style of aqueous alteration are evident. Results from Spirit’s Miniature Thermal Emission Spectrometer (Mini-TES; ~2000–340 cm −1 ) were compromised by dust contamination of one of the instrument’s mirrors, for which a correction has since been developed. Now we have documented nearly 200 examples of rocks encompassing the span of alteration from Wishstone Class, which spectrally resemble minimally altered plagioclase-phyric basalt, to the most altered Watchtower Class. Among them is a rock dubbed Bruce that may be a previously unrecognized alteration spectral end-member. We employed factor analysis/target transformation and linear least-squares modeling to investigate the spectral characteristics and mineralogy of these rocks. Our results amplify those of a prior preliminary analysis showing that alteration produced a material resembling basaltic glass that masks the spectral features of plagioclase. The association of this amorphous silicate component with a ferric iron nanophase oxide phase identified via Spirit’s Mössbauer spectrometer is now clearly shown by our data, further characterizing the distinctive mineralogic expression of the alteration. These components and the absence of any recognizable secondary silicates or opaline silica may be an expression of alteration in the extreme aridity and cold of the martian environment. Similar mineralogic characteristics of soil measured with the CheMin X-ray diffraction instrument on the Curiosity rover in Gale crater may be an indication that this alteration process is widespread on Mars.
Special collection: geology and geobiology of lassen volcanic national park
Previously reported whole-rock δ 18 O values (5.6–7.8‰) for primitive quaternary mafic lavas from the southernmost Cascades (SMC) are often elevated (up to 1‰) relative to δ 18 O values expected for mafic magmas in equilibrium with mantle peridotite. Olivine, clinopyroxene, and plagioclase crystals were separated from 29 geochemically well-characterized mafic lavas for δ 18 O measurements by laser fluorination to assess modification of the mantle sources by ancient and modern subducted components. Oxygen isotope values of olivine phenocrysts in calc-alkaline lavas and contemporaneous high alumina olivine tholeiitic (HAOT) lavas generally exceed depleted mantle olivine values (~4.9–5.3‰). Modern addition of up to 6 wt% slab-derived fluid from Gorda serpentinized peridotite dehydration (~15‰) or chlorite dehydration (~10‰) within the serpentinized peridotite can provide the 18 O enrichment detected in olivine phenocrysts (δ 18 O olivine = 5.3–6.3‰) in calc-alkaline mafic lavas, and elevate 18 O in overlying mantle lithosphere, as well. Specifically, although HAOT δ 18 O olivine values (5.5–5.7‰) may refect partial melting in heterogeneous 18 O enriched mantle source domains that developed during multiple subduction events associated with terrane accretion (e.g., <1 wt% of ~15‰ materials), an additional 18 O enrichment of up to 2 wt% of 10–15‰ slab-derived hydrous fluids might be accommodated. The calc-alkaline primitive magmas appear to have experienced a continuous range of open system processes, which operate in the mantle and during rapid magma ascent to eruption, and occasionally post quench. Textural relationships and geochemistry of these lava samples are consistent with blends of mafic phenocrysts and degassed melts in varying states of 18 O disequilibrium. In lenses of accumulated melt within peridotite near the base of the crust, coexisting olivine and clinopyroxene δ 18 O values probably are not at isotopic equilibrium because fluids introduced into the system perturbed the δ 18 O melt values. A “sudden” melt extraction event interrupts 18 O equilibration in phenocrysts and poorly mixed melt(s). Rapid ascent of volatile oversaturated primitive mafic magma through the crust appears to be accompanied by devolatilization and crystallization of anorthite-rich plagioclase with elevated δ 18 O plag values. The (Sr/P) N values for the whole rock geochemistry are consistent with a 87 Sr/ 86 Sr ~0.7027 slab-derived fluid addition into the infertile peridotite source of magmas, and melt devolatilization is recorded in the mixture of disequilibrium δ 18 O values for the constituent phases of lavas. Morbidity of the Gorda Plate as it undergoes intense deformation from the spreading ridge to the trench is likely a key factor to developing the carrying capacity of hydrous fluids and mineral phases in the slab subducting into the SMC mantle.
Special collection: Dynamics of magmatic processes
The active volcano of Ischia, an island off-shore the city of Naples, Southern Italy, has a discontinuous volcanic activity characterized by caldera-forming paroxysmal eruptions, lava flows, and lava domes, and thus offers the opportunity to study the complexity of magma storage, differentiation, and extraction mechanisms in a long-lived magma reservoir. The overall geochemical composition of erupted magmas varies from shoshonite to latite and trachyte/trachyphonolite. Their Sr and Nd, isotope composition variation is typical of subduction-related magmas, akin to other potassic magmas of the Neapolitan District, and there is a complete overlap of radiogenic isotope composition among shoshonite, latite, and trachyte/trachyphonolite. The lack of systematic radiogenic isotope covariation during differentiation suggests that the radiogenic isotope variability could be a signature of each magma pulse that subsequently evolved in a closed-system environment. Erupted magmas record a recurrent evolutionary process consisting of two-step fractional crystallization along similar liquid lines of descent for each magma pulse, suggesting near steady-state magma chamber conditions with balanced alternating periods of replenishment, differentiation, and eruption. The dominant role of fractionating feldspars determines a significant depletion of Sr (<10 ppm) coupled with high Rb/Sr (>200) in the residual trachyte magma. Several more-evolved trachytes have anomalous radiogenic 87 Sr/ 86 Sr i (>0.707) coupled with high 87 Rb/ 86 Sr (>50), all other geochemical and isotopic characteristics being similar to normal 87 Sr/ 86 Sr i trachytes at the same degree of evolution. This radiogenic Sr isotope signature is not consistent with assimilation of crustal material and demands for a time-related in-growth of 87 Sr during storage within the magma chamber. Rb-Sr isochrons on separated mineral-groundmass pairs provide robust constraints on a prolonged pre-eruptive history ranging from a few tens to hundreds of thousands of years at relatively low temperature (~750 °C). Remarkably, also normal trachytes with high 87 Rb/ 86 Sr (>200) yield a magma residence time from some 4 to 27 kyr, implying that the long-lived history of Ischia magmas is not limited to the anomalous 87 Sr/ 86 Sr i trachytes. This long-lived history could be a characteristic feature of the magma chamber reservoir of this active volcano, which other volcanic products (i.e., shoshonite and latite) cannot disclose due to their lower Rb/Sr (i.e., low 87 Sr in-growth rate) and higher magma storage temperature (>900 °C) (i.e., rapid Sr isotope homogenization via diffusion). The magma chamber dynamics of the active volcano of Ischia, probed on the basis of geochemical and radiogenic isotope tools, is consistent with recent models of complex magma chamber reservoirs made up of multiple discrete melt pockets, isolated by largely crystalline mush portions, maintained in a steady-state thermal flux regime with no mass exchange, and with reactivation shortly before eruption.
Synthetic fluid inclusions formed in high P-T experiments, which are subsequently analyzed with LA-ICP-MS, enable us to collect thermodynamic data to constrain metal transport in aqueous fluids as well as partitioning of metals between coexisting phases. The most essential prerequisite for such studies is to ensure that equilibrium conditions between liquid and solid phases are reached prior to the formation of synthetic fluid inclusions in the host mineral. Various methods have been proposed by different authors to achieve this goal, but to this point our knowledge on the best approach to synthesize equilibrated fluid inclusions under constrained pressure, temperature, and compositional (P, T, and X ) conditions remains poor. In addition, information on the time needed to reach equilibrium metal concentrations in the fluid as well as on the timing of the onset of fluid inclusion formation in the host mineral are scarce. The latter has been tested in a series of time-dependent experiments at 800 °C and 200 MPa using scheelite (CaWO 4 ), molybdenite (MoS 2 ) and metallic gold as dissolving phases and using different approaches to optimize the formation of equilibrated fluid inclusions. Both f O 2 and f S 2 were fixed during all experiments using the pyrite-pyrrhotite-magnetite buffer (PPM). As an intermediate in situ quenching of the sample charge plays an important role in the synthesis of fluid inclusions, we further tested the efficiency of such an intermediate quench for re-opening fluid inclusions formed at 600 °C and 200 MPa. Our results reveal that fluid inclusions start forming almost instantaneously and that equilibrium between fluid and solid phases occurs in the timescale of less than two hours for molybdenite and gold up to ca. 10 h for scheelite. The best approach to synthesize equilibrated fluid inclusions at 800 °C was obtained by using an intermediate quench on a previously unfractured quartz host. Experiments at 600 °C showed similar results and illustrate that this should be the method of choice down to this temperature. Below 600 °C pre-treatment of the quartz host (HF etching and/or thermal fracturing) becomes important to produce large enough fluid inclusions for the analyses via LA-ICP-MS and special care must be taken to prevent premature entrapment of the fluid. Fluids with 8 wt% NaCl in equilibrium with scheelite, molybdenite and gold at 800 °C and 200 MPa have concentrations of ca. 7300 ppm W, 1300 ppm Mo, and 300 ppm Au, respectively, which is in good agreement with results from other studies or extrapolation from lower temperatures. It can be concluded that the formation of synthetic fluid inclusions from an equilibrated fluid is possible, but different experimental designs are required, depending on the investigated temperature. In general, dissolution of solid phases seems to be much faster than previously assumed, so that experimental run durations can be designed considerably shorter, which is of great advantage when using fast-consuming mineral buffers.
Special collection: Earth analogs for martian geological materials and processes
The discovery of Fe, Mg, and Al phyllosilicates on Mars using visible and short-wave infrared (VSWIR) spectroscopy from orbit indicates aqueous alteration of basaltic rocks. Analyses at Gusev Crater by the Spirit rover and Gale Crater by the Curiosity rover have discovered alkaline basaltic rocks. In this work, multiple methods—VSWIR spectroscopy, X-ray diffraction (XRD), and chemical analyses—were used to study a suite of alkaline basalts from San Carlos, Arizona, which have been altered by water in an oxidative, semi-arid environment. As an analog for the weathering of alkaline basaltic rocks on Mars, a suite of rocks visually identified to have different degrees of alteration were characterized to understand the spectral, mineralogical, and chemical trends in alteration as sensed by multiple techniques. Samples with strong 1.9 µm H 2 O-related absorptions in VSWIR commonly exhibited absorption bands at 1.4, 2.2, and/or 2.3 µm, indicating the presence of clay minerals or silica as well as features at 0.5–0.9 mm indicative of ferric iron oxides. Primary mineralogy for all samples, as determined by point analyses with the microprobe and XRD, consisted of olivine, plagioclase, nepheline, augite, and titanomagnetite. Compositional imaging and spot analyses with the microprobe revealed distinct alteration textures and phases, suggesting weathering pathways involving the oxidation of iron in olivine and primary Fe 2+ oxides to form Fe 3+ oxides as well as the formation of aluminum phyllosilicates and magnesium phyllosilicates from feldspars and olivines, respectively, while pyroxene remained relatively unaltered. Bivariate plots of major oxides both from bulk-chemical analysis and microprobe measurements also revealed trends in alkali and silica depletion and calcium enrichment, but there was little chemical fractionation in most of the major oxides. The strength of the 1.9 µm H 2 O absorption, loss on ignition, and depletion in silica and sodium, correlated with increasing alteration. The data sets provide an analog for understanding possible weathering pathways in martian alkaline basalts and thresholds for the detection of aqueous alteration in multiple data sets.
Special collection: Water in nominally hydrous and anhydrous minerals
Hydrogen incorporation in olivine involves many OH defects, which will control the hydrogen solubility at mantle conditions. Several of these OH defects are identified from the investigation of forsterite (the olivine Mg end-member). We study here the effect of Fe 2+ , Fe 3+ , Al 3+ , and Cr 3+ on OH defects to improve our understanding of the hydrogen speciation in natural olivine. Low-temperature infrared spectra (−194 °C) are collected on synthetic and natural olivines. These spectra are then interpreted in the light of the theoretical determination of the structural, vibrational, and infrared spectroscopic properties of Fe-related OH defects, using first-principles calculations based on density functional theory. The presence of Fe 2+ changes the cationic environment around the fully protonated vacancies in forsterite, leading to a slight modification of their infrared signatures. In particular, the presence of Fe 2 + in an octahedral site adjacent to a hydrogarnet-type defect is likely responsible for the additional bands observed at 3599 cm −1 and around 3520–3550 cm −1 in Fe-doped olivines. Results show that the OH bands between 3310 and 3380 cm −1 are associated with the presence of trivalent cations. Specifically, two bands at 3323 and 3358 cm −1 , commonly observed in natural olivine, are associated with the substitution of Mg 2+ by Cr 3+ while two similar bands at 3328 and 3353 cm −1 are associated with the substitution of Mg 2+ by Fe 3+ . The presence of these defects and the “titanoclinohumite” defect in natural olivine clearly underlines the prominent role of trace elements on the hydrogen incorporation in lithospheric olivine.
Sic and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a Sic-dominated ultra-reduced mineral assemblage, including Sic, Tic, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. Sic is 20–50 μm in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of Sic polytypes were identified, which are dominated by β-SiC (3 C polytype) and 4 H polytype followed by 15 R and 6 H . These Sic are featured by 13 C-depleted isotopic compositions (δ 13 C = –13.2 to –22.8‰, average = –17.7‰) with obvious spatial variation. We provided a numerical modeling method to prove that the C isotopic composition of the Dalihu SiC can be well-yielded by degassing. Our modeling results showed that degassing reaction between graphite and silicate can readily produce the low δ 13 C value of SiC, and the spatial variations in C isotopic composition could have been formed in the progressive growth process of SiC. The detailed in situ occurring information is beneficial for our understanding of the preservation mechanism of the Dalihu ultra-reduced phase. The predominant occurrence of SiC in micro-cavities implies that exsolution and filling of CO 2 and/or CO in the micro-cavities during the diapir rising process of carbonatitic melt could have buffered the reducing environment and separated SiC from the surrounding oxidizing phases. The fast cooling of host rock, which would leave insufficient time for the complete elimination of SiC, could have also contributed to the preservation of SiC.
Earth’s mantle convection is powered in part by the radiogenic heat released by the decay of 238 U, 235 U, 232 Th, and 40 K. We present ab initio calculations of uranium and thorium incorporation in CaSiO 3 -perovskite with and without aluminum, and propose that aluminous calcium silicate perovskite is the likely host of uranium and thorium in the lower mantle. At 15 GPa, the enthalpies of solution into aluminum-free CaSiO 3 -perovskite are 10.34 kJ/mol for U 4+ and 12.52 kJ/mol for Th 4+ in SiO 2 saturated systems, while the enthalpies are 17.09 kJ/mol and 19.27 kJ/mol, respectively, in CaO saturated systems. Coupled substitution of U 4+ and Th 4+ with aluminum is thermodynamically favored, with the enthalpies of solution negative for U 4+ and near 0 kJ/mol for Th 4+ throughout the stability field of CaSiO 3 -perovskite. Therefore, U incorporation into CaSiO 3 -perovskite is spontaneous in the presence of aluminum while Th forms a near ideal solid solution, implying these elements are potentially compatible with respect to partial melting in the transition zone and lower mantle. Furthermore, the solid solution reactions of U 4+ and Th 4+ are broadly similar to each other, suggesting a restriction on the fractionation of these actinides between the upper and lower mantle. U and Th compatibility in the presence of Al has implications regarding actinide transport into the deep mantle within subducting slabs and the geochemical content of seismic anomalies at the core-mantle boundary.
Spinels renaissance: The past, present, and future of those ubiquitous minerals and materials
Natural Cr-spinels previously characterized by X-ray single-crystal diffraction and electron microprobe have been analyzed by Raman spectroscopy. The results we report show that there is a strong correlation between the Cr/(Cr+Al) ratio (Cr#) and the A 1g mode for the studied spinels. A strong correlation of this mode with Mg/(Mg+Fe 2 +) (Mg#) can be seen only for spinels with Mg# higher than 0.60. Other modes can increase, decrease or disappear depending on the Cr#. Among the spinels with low Cr# it is possible to define their order/disorder degree. In fact, spinels with an inversion degree lower than 0.14 show an E g mode at about 400–410 cm −1 , while spinels with Cr# higher than 0.20 register the appearance of a peak in the region 150–200 cm −1 , while other peaks are substituted by smooth curves. The results show that the use of Raman applied to spinel in provenance studies cannot yield a 100% confidence because of the uncertainties in the relation between Mg# and the different modes.
Spinels Renaissance: The Past, Present, and Future of Those Ubiquitous Minerals and Materials
Flux-grown spinel crystals belonging to the MgAl 2 O 4 -MgCr 2 O 4 spinel series were investigated to reveal the effects of Cr substituting for Al on cation distribution and their influence on Mg-Al intra-crystalline exchange. Samples were structurally and chemically characterized by single-crystal X-ray diffraction and electron microprobe, and cation distribution was obtained with a tested optimization model for site populations. The results evidenced that the contribution of the tetrahedral bond distance to the unit-cell parameter is smaller than that of the octahedral bond distance, which is driven by the substitution of Cr for Al. Moreover, the influence that Cr exerts on Mg-Al order-disorder intersite exchange is non-linear along the whole series. The comparison between the cation distributions derived from crystal-chemical data and the O’Neill-Navrotsky thermodynamic model (with α Mg-Al = 23 kJ/mol and β Mg-Al = 13 kJ/mol) shows large discrepancies, which can be reconciled assuming α Mg-Al values variable from 23 to 100 kJ/mol as a function of Cr. This suggests that, irrespective of temperature, the Al ordering at the octahedrally coordinated site increases with increasing Cr substitution for Al. The geothermometric implications of the present study point out that closure temperatures, calculated from a well-tested intersite geothermometer, are reliable for spinels with magnesiochromite component smaller than 85%, i.e., Cr/(Cr+Al) < 0.85, whereas spinels with larger magnesiochromite component yield unreliable closure temperature.
Secondary orthopyroxenes occur as veinlets (<0.1 mm thick) cutting an olivine grain in a two-pyroxene peridotite xenolith from the Shiribeshi Seamount in the Sea of Japan. These orthopyroxenes are characterized by low Al 2 O 3 (0.4–1.7 wt%), Cr 2 O 3 (<0.1 wt%), and CaO (0.3–0.4 wt%) contents, which are the same signatures of the secondary orthopyroxenes in peridotite xenoliths from island arcs. The trace-element patterns of the melts in equilibrium with the secondary orthopyroxenes show enrichment in light rare earth elements and Sr and depletion in heavy rare earth elements, Nb and Ti. These trace-element characteristics are highly consistent with those of slab-derived adakites. The involvement of slab-derived melts in the mantle beneath the Sea of Japan has been inferred from the geochemical characteristics of the volcanic rocks formed during opening of the Sea of Japan. The source mantle of the enriched basalts in the Sea of Japan is likely to have been metasomatized by adakitic melts in the same manner as the peridotite-hosted veinlet. The secondary orthopyroxenes in the peridotite xenolith from the Shiribeshi Seamount provide direct evidence for the metasomatic infux of adakitic melts into the back-arc mantle beneath the Sea of Japan. Adakitic metasomatism, as documented in the Sea of Japan, potentially plays an important role in mantle evolution and magma generation beneath global back-arc basins.
In this work, we modeled the structure, the compressional behavior and the physical properties of topaz over six different fluorine contents and a wide range of pressure, using a quantum mechanical approach based on periodic boundary conditions. We adopted the density functional theory using the B3LYP functional and all-electron Gaussian-type orbitals basis sets. An atomic level description of the athermal ( T = 0 K ) pressure-induced structural modification of topaz is provided. From the compression results we obtained the athermal bulk modulus ( K T0 ), its first derivative ( K ′) and the athermal volume at zero pressure ( V 0 ) by a third-order Birch-Murnaghan equation fit. The results show that K T0 increases with fluorine content. The compressional pattern is anisotropic, as observed by the axial compressibility and second-order elastic constants calculations. We observed that the compression involves three different mechanism, polyhedral contraction, polyhedral tilting and hydrogen bonding, all of them influenced, with different extent, by the fluorine content in topaz. Recent experimental results obtained by single-crystal X-ray and neutron diffraction of specific topaz compositions are in very good agreement with our simulations, which further extend the knowledge of the structural and elastic properties of topaz over a wider range of fluorine content.
In this study, we performed synchrotron X-ray diffraction (XRD) and Mössbauer spectroscopy (SMS) measurements on two single-crystal bridgmanite samples [Mg 0.94 Fe0.042+Fe0.023+$\rm {Fe^{2+}_{0.04}}{Fe^{3+}_{0.02}}$Al 0.01 Si 0.99 O 3 (Bm6) and Mg 0.89 Fe0.0242+Fe0.0963+$\rm {Fe^{2+}_{0.024}}{Fe^{3+}_{0.096}}$Al 0.11 Si 0.89 O 3 (Al-Bm11)] to investigate the combined effect of Fe and Al on the hyperfine parameters, lattice parameters, and equation of state (EoS) of bridgmanite up to 130 GPa. Our SMS results show that Fe 2+ and Fe 3+ in Bm6 and Al-Bm11 are predominantly located in the large pseudo-dodecahedral sites (A-site) at lower-mantle pressures. The observed drastic increase in the hyperfine quadrupole splitting (QS) between 13 and 32 GPa can be associated with an enhanced local distortion of the A-site Fe 2+ in Bm6. In contrast to Bm6, the enhanced lattice distortion and the presence of extremely high QS values of Fe 2+ are not observed in Al-Bm11 at high pressures. Our results here support the notion that the occurrence of the extremely high QS component of approximately 4 mm/s in bridgmanite is due to the lattice distortion in the high-spin (HS) A-site Fe 2+ , instead of the occurrence of the intermediate-spin state. Both A-site Fe 2+ and Fe 3+ in Bm6 and Al-Bm11 remain in the HS state at lower-mantle pressures. Together with XRD results, we present the first experimental evidence that the enhanced lattice distortion of A-site Fe 2+ does not cause any detectable variation in the EoS parameters, but is associated with anomalous variations in the bond length, tilting angle, and shear strain in the octahedra of Bm6. Analysis of the obtained EoS parameters of bridgmanite at lower-mantle pressures indicates that the substitution of Fe in bridgmanite will cause an enhanced density and a reduced bulk sound velocity ( V Φ ), whereas the Al and Fe substitution has a reduced effect on density and a negligible effect on V Φ . These experimental results provide new insight into the correlation between lattice, hyperfine, and EoS parameters of bridgmanite in the Earth’s lower mantle.
Micro-X-ray absorption near-edge structure (m-XANES) spectroscopy has been used by several recent studies to determine the oxidation state and coordination of iron in silicate glasses. Here, we present new results from Fe m-XANES analyses on a set of 19 Fe-bearing felsic glasses and 9 basaltic glasses with known, independently determined, iron oxidation state. Some of these glasses were measured previously via Fe XANES (7 rhyolitic, 9 basaltic glasses; Cottrell et al. 2009), while most felsic reference glasses (12) were analyzed for the first time. The main purpose of this study was to understand how small changes in glass composition, especially at the evolved end of silicate melt compositions occurring in nature, may affect a calibration of the Fe m-XANES method. We performed Fe m-XANES analyses at different synchrotron radiation sources [Advanced Photon Source (APS), Argonne, U.S.A., and Angströmquelle Karlsruhe (ANKA), Germany] and compared our results to existing calibrations obtained at other synchrotron radiation sources worldwide. The compiled results revealed that changes in instrumentation have a negligible effect on the correlation between the centroid energy of the Fe pre-edge peak and the Fe oxidation state in the glasses. Oxidation of the glasses during extended exposure (up to 50 min) to the X-ray beam was not observed. Based on the new results and literature data we determined a set of equations for different glass compositions, which can be applied for the calculation of the iron valence ratio (Fe 3+ /ΣFe) in glasses by using XANES spectra collected at different synchrotron beamlines. For instance, the compiled felsic reference material data demonstrated that the correlation between the centroid energy of the Fe pre-edge peak C Fe (eV) and the Fe 3+ /ΣFe ratio of felsic glasses containing 60.9 to 77.5 wt% SiO 2 and 1.3 to 5.7 wt% FeO tot can be accurately described by a single linear trend, if the spectra were collected at 13-ID-E beamline at APS and for 0.3 ≤ Fe 3+ /ΣFe ≤ 0.85: C Fe [eV] = 0.012395 (±0.00026217) × Fe 3+ /ΣFe + 7112.1 (±0.014525); R 2 = 0.987. Based on this equation, the Fe oxidation state of felsic glasses can be estimated at an absolute uncertainty of ±2.4% Fe 3+ /ΣFe. In general, the differences between the calibrations for felsic and mafic glasses were small and the compiled data set (i.e., results collected at four different beamlines on 79 reference glass materials) is well described by a single second-order polynomial equation.
Bacterially mediated struvite usually crystallizes as unusual morphologies. To better understand the relationship between growth habit of struvite and bacterial activity in struvite biomineralization process, Shewanella oneidensis MR-1 was selected as a model microbe to induce struvite mineralizationin the synthetic sludge liquor. A combination of bacterial and biomimetic mineralization strategies was adopted. Different bacterial components were isolated from the cultures by a set of separation techniques, and used to influence struvite crystallization and growth. The identification and characterization of the mineralized products were done using XRD, FTIR, FESEM, TG-DTA, XPS, and elemental analysis. Bacterial mineralization experiments demonstrated that S. oneidensis MR-1 cannot only trigger mineralization and growth of struvite, but also mediate the specific morphogenesis of struvite. Biomimetic mineralization experiments revealed that different bacterial components had different effects on struvite morphology, and low molecular-weight peptides secreted by the bacteria played a dominant role. Current results can provide a deeper insight into bacterially mediated struvite morphogenesis, and be potentially applied to phosphorus and nitrogen recovery from various eutrophic wastewaters.
The amount of insoluble macromolecular organic matter in the Earth’s crust, commonly referred to as kerogen, far exceeds the mass of living organic matter. The fraction of kerogen in sediments subducted into the mantle remains poorly constrained and will vary depending on the physical-chemical properties of kerogen along different slab geotherms. We studied the pressure-temperature evolution of carbon vibrational frequencies in isolated kerogen, previously not subjected to metamorphism, using Raman spectroscopy in a sapphire optical cell up to 3.2 GPa and 450 °C, correspondingto colder subduction geotherms. For blue-green laser excitation, we find optical irradiance exceeding ~3 kW/cm 2 induces changes in spectral features of the primary graphitic (G-band) and two main disordered modes (D1 and D2) that might otherwise be mistaken for thermal maturation. Whereas previous in situ studies have investigated the changes in these molecular vibrations of kerogen at high temperature or high pressure, we collected Raman spectra of isolated kerogen at simultaneous high P-T conditions. Although instantaneous and irreversible changes in band ratios of isolated kerogen were observed above ~350°C at room-pressure, long-duration (2–8 h) heating experiments at 450 °C and 2.7–3.0 GPa reveal no permanent change in band structure. The reduction in vibrational frequencies of the disordered carbon modes with temperature (d v /d T ) at pressures >1 GPa is slightly less than found at room pressure, further indicating that pressure effectively increases the thermal stability of kerogen. Our results suggest that kerogen reaching depths of 60 km where the temperature is below ~450 °C may subduct into the mantle, providing a potential source for the organic-rich component of carbon recently detected in certain lower-mantle diamonds.
Due to the presence of additional volatiles and/or electrolytes in CO 2 -H 2 O fluids, the total pressure of many natural aqueo-carbonic fluid inclusions at high temperatures as determined using microthermometry is usually made with considerable uncertainty. In this paper, we present the results of our high P-T in situ Raman scattering study of high-density aqueo-carbonic fluids, with and without a small amount of CH 4 and NaCl, whose objective is to derive a new method for pressure determination in aqueo-carbonic fluid inclusions at high temperatures. The measurement of the Fermi dyad bands at temperatures up to 400°C and pressures up to 1200 MPa is described. The manner in which the frequency shifts and intensity of Raman bands are governed by pressure, temperature, presence of CH 4 in carbonic and NaCl in aqueous fluids is discussed. From the monotonic dependence of the frequency shifts of the lower Fermi dyad band v – and the Fermi resonant splitting D ( D = v + – v – ) with pressure and temperature, the pressure (in MPa) in aqueo-carbonic fluid inclusions at elevated temperatures can be determined directly by using the following two polynomial equations: P(MPa)=− 16+1.232× T− 53.72× (Δ ν − )− 1.83× 10− 3× T2+24.46× (Δ ν − )2− 0.292× T× (Δ ν − ),P(MPa)=− 26+1.501× T+193.24× (Δ D)− 1.61× 10− 3× T2+5.436× (Δ D)2+0.158× T× (Δ D), $$\begin{array}{} P \,\text{(MPa)} =-16+1.232\times T-53.72\times(\Delta \nu_{-})-1.83\times 10^{-3}\times T^{2}+24.46\times(\Delta \nu_{-})^{2}-0.292\times T\times(\Delta \nu_{-}),\\ P\, \text{(MPa)} =-26+1.501\times T+193.24\times(\Delta D)-1.61\times 10^{-3}\times T^{2}+5.436\times(\Delta D)^{2}+0.158\times T\times(\Delta D) , \end{array} $$ where T is in °C, Δ ν – and ΔD represent frequency shifts (in cm −1 ) of the lower band and the resonant splitting relative to the reference values measured at 23 °C and 6 MPa, respectively. Based on the attainable accuracy of the fitted peak positions and the results from fitting of Raman frequency shifts’ dependence with pressure and temperature, the uncertainty in pressure determination is about 50 MPa for pressures determined from ν – and 40 MPa from that determined from D .
The melting mechanism of Na 2 SiO 3 , a crystal with pyroxene structure, includes three distinct reactions. All are driven by heating with each reaction commencing at a different temperature. The first two reactions proceed within the crystal at temperatures well below the melting point and are expressed by distinctive crystallographic, calorimetric, and Raman spectroscopic changes to the crystal. With the reactions identified and explained for Na 2 SiO 3 (c) and the melting mechanism elucidated, the Na 2 SiO 3 system becomes the “Rosetta Stone” by which to decipher the melting mechanisms of all pyroxenes and other silicate minerals. The first reaction produces itinerant Na + within the crystal. Itinerancy results from dissociation of some NBO-Na bonds due to heating, with dissociation commencing at ~770 K. The reaction proceeds according to: Si− O− Na→ Si− O− +Na+. $${\rm{Si - O - Na }} \to {\rm{Si - }}{{\rm{O}}^ - }\;{\rm{ + }}\;{\rm{N}}{{\rm{a}}^ + }. $$ The Si-O − moiety remains attached to its SiO 3 chain and it is charged because one of its NBO atoms has no associated Na ion. The second reaction is characterized by the appearance of a Q 3 band in Raman spectra of the crystal at temperatures >770 K. It is produced via a polymerization reaction involving the Si-O − species, a product of the first reaction, and a Q 2 species of an adjacent SiO 3 chain according to: 1Na2− Q2+1(Na1− Q2)− → 2Na1− Q3+Na++O2− . $${\rm{1N}}{{\rm{a}}_{\rm{2}}}{\rm{ - }}{{\rm{Q}}^{\rm{2}}}\;{\rm{ + }}\;{\rm{ 1(N}}{{\rm{a}}_{\rm{1}}}{\rm{ - }}{{\rm{Q}}^{\rm{2}}}{\rm{)}}^{\rm{- }}\; \to \;2{\rm{N}}{{\rm{a}}_1}{\rm{ - }}{{\rm{Q}}^{\rm{3}}}{\rm{ }}\;{\rm{ + }}\;{\rm{N}}{{\rm{a}}^ + }{\rm{ }}\;{\rm{ + }}\;{{\rm{O}}^{2 - }}. $$ Na atoms are included with each Q species to preserve mass balances and (Na 1 −Q 2 ) − is equivalent to the Si-O − species. The produced Q 3 species form cross-chain linkages that affect the crystallographic properties of the crystal. They are responsible for the cessation of thermal expansion of the Na 2 SiO 3 unit cell in the a-b axial plane at T >770 K, and the near-constancy of a and b unit-cell parameters between ~770 and ~1300 K. The presence of Q 3 species in Raman spectra and the inhibited expansion in the a-b axial plane provide exceedingly strong evidence for this reaction. The third reaction commences at ~1200 K where Q 1 Raman band first appears. It can be produced only through depolymerization of Q 2 chains according to: 2Na2− Q2+2Na++O2− → 2Na3− Q1 $${\rm{2N}}{{\rm{a}}_{\rm{2}}}{\rm{ - }}{{\rm{Q}}^{\rm{2}}}\,{\rm{ +\, 2N}}{{\rm{a}}^{\rm{ + }}}{\rm\,{ + \, }}{{\rm{O}}^{2 - }} \to {\rm{2N}}{{\rm{a}}_3}{\rm{ - }}{{\rm{Q}}^1}{\rm{ }} $$ where Na + and O 2− are itinerant species produced by the second reaction. With conversion of Q 2 to Q 1 species, SiO 3 chains are ruptured, long-range order is lost, and melt is produced at 1362 K. The last two reactions proceed by nucleophilic substitution where Si centers are attacked to form fivefold-coordinated activated complexes. Si-O − acts as nucleophile in the second reaction (producing Q 3 species), and O 2− acts as nucleophile in the third reaction (producing Q 1 ). Taken in reverse, these mechanisms describe the formation of nuclei in crystallizing melts and in addition provide insight into the elusive changes that occur at the glass transition. Elucidation of the melting mechanism could thus provides a unified framework within which melting, crystallization, and the glass transition can be understood.
Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity ( f O 2 ) of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe 3+ /∑Fe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe 3+ /∑Fe ratios that relies on a reported correlation between the difference in Fe 3+ /∑Fe ratio by Mössbauer spectroscopy and by electron microprobe (ΔFe 3+ /∑Fe Möss-EPMA ) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mössbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe 3+ /∑Fe Möss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe 3+ /ZFe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe 3+ /∑Fe Möss-EPMA = 0.031 and corrected measurements have a mean ΔFe 3+ /∑Fe Möss-EPMA = −0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe 3+ /ZFe ratios despite the absence of a correlation between ΔFe 3+ /∑Fe Möss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe 3+ /∑Fe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive f O 2 . Because there is a logarithmic relationship between the activity of magnetite and log f O 2 , the uncertainty in f O 2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/−0.4 log units, 1σ, at Fe 3+ /∑Fe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe 3+ /EFe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative f O 2 . We also document and correct errors that appear in the literature when formulating f O 2 that, combined, could yield errors in absolute f O 2 of greater than 0.75 log units—even with perfectly accurate Fe 3+ /∑Fe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe 3+ /∑Fe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of f O 2 from spinel peridotite mineral compositions.
The Half Dome Granodiorite, Yosemite National Park, California, is recognized in the field by euhedral, fresh-looking, black hornblende phenocrysts up to 2 cm in length. This variety of granodiorite typifies intermediate-age hornblende-phyric units of Cretaceous nested plutonic suites in the Sierra Nevada batholith. Although only inclusions of feldspar are evident in hand samples, the phenocrysts are riddled with up to 50% inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling. Amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt% Al 2 O 3 to magnesiohornblende with over 8 wt%. Elemental zoning within the amphibole is highly irregular on the micrometer scale, showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions. The chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite, suggesting that the non-biotite inclusions are the result of metamorphism of the phenocrysts. Backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite. Pressure calculations using the Al-in-hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization. These hornblende phenocrysts would thus be unsuitable for geobarometry, and caution must be used to avoid similarly zoned phenocrysts in the application of the Al-in-hornblende geobarometer.
The intense heat and pressure resulting from the detonation of the world’s first nuclear device in the New Mexico desert, July 16, 1945, severely altered the arkosic sand, producing the fused, glassy material referred to as Trinitite. The study of Trinitite is key to the development of nuclear forensic techniques that can provide crucial information about a nuclear event, such as device composition and radionuclide distribution. Moreover, nuclear blasts are often considered analogs to catastrophic natural events such as meteorite impacts, and it is well-documented that with increasing impact severity, zircon and quartz grains deform systematically. In Trinitite, a sufficient number of primary quartz and zircon grains remain identifiable. Here, a multi-technique approach (focused ion beam, scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy) is employed to study the micrometer-to-nanometer-scale deformation features in altered grains of zircon and quartz to constrain blast pressure and temperature conditions. Trinitite zircon grains consistently show an outer halo of fibrous baddeleyite, radiating from a relatively unaltered core; HRTEM images show complex twinning, likely originating from an intermediate, tetragonal zirconia precursor. Trinitite quartz grains show various states of melting that appear to vary predictably with depth below the surface of the desert sand. Grains occurring deeper than ~1.5 cm are crystalline, with occasional planar fractures at the optical scale. At shallower depths, a systematic increase in quartz vitrification is observed. Considered together, these data suggest maximal temperatures in excess of 1500 °C and pressures of <10 GPa, the latter being considerably less than for any natural impact event. Taken in a broader context, the implications of this work extend toward exploiting the use of advanced imaging techniques to improve our understanding of mineral processes in extreme, non-equilibrium environments at the near-atomic scale.
Kegginite, Pb 3 Ca 3 [AsV 12 O 40 (VO)]·20H 2 O, is a new mineral species from the Packrat mine, near Gateway, Mesa County, Colorado, U.S.A. It is a secondary mineral found on asphaltum in a montroseite- and corvusite-bearing sandstone. Other secondary minerals found in close association with kegginite are ansermetite, gypsum, mesaite, and sherwoodite. Crystals of kegginite are orange-red simple hexagonal tablets. The streak is pinkish-orange, the luster is vitreous, the Mohs hardness is about 2, the tenacity is brittle, fracture is irregular, cleavage is good on {001}, and the calculated density is 2.69 g/cm 3 . Kegginite is optically uniaxial (–) with pleochroism: O orange-red and E red-orange; E < O . Electron microprobe analyses yielded the empirical formula Pb 2.98 Ca 2.39 Mg 0.56 V 13.05 As 0.95 O 61 H 40.15 . Kegginite is trigonal, P3¯ , $P\bar 3,$ with a = 14.936(5), c = 15.846(5) Å, V = 3061(2) Å 3 , and Z = 2. The crystal structure of kegginite ( R 1 = 0.064 for 1356 F o > 4σ F reflections) contains a [As5+V125+O40(VO)]12− ${{\rm{[A}}{{\rm{s}}^{{\rm{5 + }}}}{\rm{V}}_{12}^{5 + }{{\rm{O}}_{{\rm{40}}}}{\rm{(VO)]}}^{12 - }}$ polyoxometalate cluster, which is a mono-capped Keggin ε-isomer.
