Volume 103, Issue 4

Oxygen fugacity ( f o 2 ) exerts first-order control on the geochemical evolution of planetary interiors, and the Fe 3+ /ΣFe ratios of silicate glasses provide a useful proxy for f O 2 . Fe K -edge micro-X-ray absorption near-edge structure (XANES) spectroscopy allows researchers to micro-analytically determine the Fe 3+ /ΣFe ratios of silicate glasses with high precision. In this study we characterize hydrous and anhydrous basalt glass standards with Mössbauer and XANES spectroscopy and show that synchrotron radiation causes progressive changes to the XANES spectra of hydrous glasses as a function of radiation dose (here defined as total photons delivered per square micrometer), water concentration, and initial Fe3+/ΣFe ratio. We report experiments from eight different radiation dose conditions and show that Fe in hydrous silicate glasses can undergo rapid oxidation upon exposure to radiation. The rate and degree of oxidation correlates with radiation dose and the product of water concentration and ferrous/ferric iron oxide ratio on a molar basis ( Φ = X HO 0.5 · X FeO / X FeO 1.5 ). For example, a basalt glass with 4.9 wt% dissolved H 2 O and Fe 3+ /ΣFe = 0.19 from its Mössbauer spectrum may appear to have Fe 3+ /ΣFe ≥ 0.35 when analyzed over several minutes at a nominal flux density of ~2 × 10 9 photons/s/μm 2 . This radiation-induced increase in Fe 3+ /ΣFe ratio would lead to overestimation of f O 2 by about two orders of magnitude, with dramatic consequences for the interpretation of geological processes. The sample area exposed to radiation shows measureable hydrogen loss, consistent with radiation-induced breaking of O–H bonds, associated H migration and loss, and oxidation of Fe 2+ . This mechanism is consistent with the observation that anhydrous glasses show no damage under any beam conditions. Cryogenic cooling does not mitigate, but rather accelerates, iron oxidation. The effects of beam damage appear to persist indefinitely. We detect beam damage at the lowest photon flux densities tested (3 × 10 6 photons/s/ μm 2 ); however, at flux densities ≤6 × 10 7 photons/s/µm 2 , the hydrous glass calibration curve defined by the centroid (derived from XANES spectra) and Fe 3+ /SFe ratios (derived from Mössbauer spectra) is indistinguishable from the anhydrous calibration curve within the accuracy achievable with Mössbauer spectroscopy. Thus, published Fe 3+ /ΣFe ratios from hydrous glasses measured at low photon flux densities are likely to be accurate within measurement uncertainty with respect to what would have been measured by Mössbauer spectroscopy. These new results demonstrate that to obtain accurate Fe 3+ /ΣFe ratios from hydrous, mafic, silicate glasses, it is first necessary to carefully monitor changes in the XANES spectra as a function of incident dose (e.g., fixed-energy scan). Defocusing and attenuating the beam may prevent significant oxidation of Fe in mafic water-bearing glasses.

The availability of terrestrial sites that are martian analogs allows researchers to investigate Mars using knowledge gained on Earth. Among the terrestrial analog sites for Mars is Craters of the Moon National Monument (COTM) in Idaho, U.S.A. Craters of the Moon National Monument is home to over 60 basalt lava flows, many of which have been dated from 2050 to 18 340 years before present (y.b.p.). Following previous authors, we examined the chemistry and petrogenesis of COTM basalts compared to basaltic martian rocks, martian meteorites, and meteorite clasts, and then examined the results of chemical weathering of the basaltic flows. Results of our comparative chemical analysis suggest COTM basalts are generally more evolved than the martian materials, with a few notable exceptions. Several COTM flow basalts, including rocks of the >18 000 year old Kimama flow, have high FeO, TiO 2 , and P 2 O 5 contents similar to the Wishstone and Watchtower class rocks analyzed at Gusev Crater, Mars, by the Mars Exploration Rover Spirit. The youngest basalts of COTM, such as those of the Minidoka (3890 y.b.p.) and Blue Dragon (2050 y.b.p.) flows have similarities in SiO 2 , alkali contents, and mineralogical norms with select clasts in meteorite Northwest Africa (NWA) 7034. These similarities over a range of flow ages therefore suggest that COTM basalts have the potential to shed important light on specific igneous processes occurring on Mars. Many of the basaltic rocks measured by rovers on Mars are thought to have experienced chemical weathering during aqueous interactions; however, few basalt weathering rates exist for terrestrial Marsrelevant field environments to help interpret these processes. COTM, which has important similarities to some martian rocks discussed above, also represents a basalt flow chronosequence, and therefore allows for the investigation of basalt weathering as a function of time. We measured the depth of developed porosity in a suite of basalt flows ranging from 2050 to 18 340 y.b.p., and compared field weathering relationships at COTM to weathering rinds developed on the Gusev Crater martian rocks Humphrey, Champagne, Mazatzal, and Wooly Patch. Our results indicate that depths of incipient weathering in COTM rocks increase with time at a rate of 2.32 × 10 –2 to 3.04 × 10 –2 μm/yr, which is comparable to other terrestrial advance rates. Interestingly, this rate also indicates that chemical weathering strongly outpaces physical weathering even in this arid to semi arid environment. Weathering primarily of the matrix glass indicates that glass may be functioning as the profile-controlling mineral, which may have implications for chemical weathering in glass-rich rocks on Mars. Weathering rates of glass and other minerals can also help constrain the conditions (pH, temperature) of alteration on Mars. Of the altered martian rocks we compared to COTM (Humphrey, Champagne, Mazatzal, and Wooly Patch), altered surfaces of Mazatzal rock at Gusev Crater show the most similarities to weathered surfaces at COTM. Comparisons of chemical weathering in COTM basalts with altered surfaces of rocks in Gusev Crater, Mars, indicate Gusev Crater martian rocks have undergone significantly more aqueous alteration than that experienced by basaltic flows at COTM.

Commercially produced vermiculite insulation from Libby, Montana, contains trace levels of asbestiform amphibole, which is known to cause asbestos-related diseases. When vermiculite insulation is found in a building, evaluation for its potential asbestos content traditionally involves collecting a sample from an attic or wall and submitting it for time-consuming analyses at an off-site laboratory. The goal of this study was to determine if in situ near-infrared reflectance measurements could be used to reliably identify the source of vermiculite ore and therefore its potential to contain asbestos. Spectra of 52 expanded ore samples, including attic insulation, commercial packing materials, and horticultural products from Libby, Montana; Louisa, Virginia; Enoree, South Carolina; Palabora, South Africa; and Jiangsu, China, were measured with a portable spectrometer. The mine sources for these vermiculite ores were identified based on collection location, when known, and on differences in elemental composition as measured by electron probe microanalysis. Reflectance spectra of the insulation samples show vibrational overtone and combination absorptions that vary in wavelength position and relative intensity depending on elemental composition and proportions of their constituent micas (i.e., vermiculite ore usually consists of a mixture of hydrobiotite and vermiculite mineral flakes). Band depth ratios of the 1.38/2.32, 1.40/1.42, and 2.24/2.38 μm absorptions allow determination of a vermiculite insulation’s source and detection of its potential to contain amphibole, talc, and/or serpentine impurities. Spectroscopy cannot distinguish asbestiform vs. non-asbestiform amphiboles. However, if the spectrally determined mica composition and mineralogy of an insulation sample is consistent with ore from Libby, then it is likely that some portion of the sodic-calcic amphibole it contains is asbestiform, given that all of the nearly two dozen Libby vermiculite insulation samples examined with scanning electron microscopy in this study contain amphiboles. One sample of expanded vermiculite ore from multiple sources was recognized as a limitation of the spectral method, therefore an additional test (i.e., 2.24 μm absorption position vs. 2.24/2.38 μm band depth ratio) was incorporated into the spectral method to eliminate misclassification caused by such mixtures. With portable field spectrometers, the methodology developed can be used to determine vermiculite insulation’s source and estimate its potential amphibole content, thereby providing low-cost analysis with onsite reporting to property owners.

Bone is a natural nanocomposite. Its mineral component is nanocrystalline calcium phosphate apatite, whose synthetic biomimetic analogs can be prepared by wet chemistry. The initially formed crystals, whether biological or synthetic, exhibit very peculiar physicochemical features. In particular, they are nanocrystalline, nonstoichiometric, and hydrated. The surface of the nanocrystals is covered by a non-apatitic hydrated layer containing mobile ions, which may explain their exceptional surface reactivity. For their precipitation in vivo or in vitro, for their evolution in solution, for the 3D organization of the nanocrystals, and for their consolidation to obtain bulk ceramic materials, water appears to be a central component that has not received much attention. In this mini-review, we explore these key roles of water on the basis of physicochemical and thermodynamic data obtained by complementary tools including FTIR, XRD, ion titrations, oxide melt solution calorimetry, and cryo-FEG-SEM. We also report new data obtained by DSC, aiming to explore the types of water molecules associated with the nanocrystals. These data support the existence of two main types of water molecules associated with the nanocrystals, with different characteristics and probably different roles and functions. These findings improve our understanding of the behavior of bioinspired apatite-based systems for biomedicine and also of biomineralization processes taking place in vivo, at present and in the geologic past. This paper is thus intended to give an overview of the specificities of apatite nanocrystals and their close relationship with water.

Time–composition relationships in eruptive sequences at composite volcanoes can show how the ongoing intrusion of magmas progressively affects the lithosphere at continental convergent margins. Here, new whole-rock and microanalytical major and trace element data from andesite-dacite lava flows are integrated with previous studies and existing isotopic data, and placed within the framework of a high-resolution chronostratigraphy for Ruapehu volcano (southern Taupo Volcanic Zone, New Zealand). The geochemical evolution of lavas erupted over the ~200 kyr lifetime of the exposed edifice reflects variable degrees of fractionation and systematic changes in the type of crustal assimilation in the Ruapehu magma system. Lavas erupted from ~200–150 ka have previously been distinguished from those erupted <150 ka based on Sr-Nd isotopic characteristics, which indicate that the oldest lavas were sourced from magmas that assimilated oceanic crust. Such source rocks underlie the regionally widespread Mesozoic meta-sedimentary greywacke-argillite basement, which was conversely assimilated by <150 ka magmas. New results from this work reveal that since 150 ka, an upper limit of magma differentiation occurred from ~50–35 ka. High K 2 O (~6 wt%) and Rb contents (~270 ppm) in melt inclusions, interstitial glass, and glass from in situ quenched melts of partially fused crustal xenoliths are reported for andesite-dacite lavas erupted during this period. In addition to crystal fractionation, selective partial melting and assimilation of K- and Rb-rich mineral phases (e.g., biotite, K-feldspar) that are significant components of the meta-sedimentary basement rocks is inferred to explain these geochemical characteristics. These processes coincided also with the effusion of high-MgO andesite-dacite lavas that display petrological evidence for mixing between andesite-dacite and more mafic magmas. An influx of hotter mafic magma into the system explains why the extent of crustal assimilation recorded by Ruapehu lavas peaked during the ~50–35 ka eruptive period. From 26 ka to the present, andesite lavas have reverted to more mafic compositions with less potassic melt inclusion and whole-rock compositions when compared to the ~50–35 ka lavas. We suggest that the younger lavas assimilated less-enriched melts because fertile phases had been preferentially extracted from the crustal column during earlier magmatism. This scenario of bottom-up heating of the lithosphere and exhaustion of fertile phases due to the progressive intrusion of magma explains the geochemical evolution of Ruapehu lavas. This model may be applicable to other long-lived composite volcanoes of the circum-Pacific continental arcs.

Arc magmas commonly are mixtures of newly arriving primitive melts, stored magmas at shallow levels, and xenolithic material added on ascent. Almost every eruption has a unique assembly of these components, which may record magmatic processes occurring in the plumbing system prior to an eruption. In this study, we focus on complexly zoned olivines (crustal xenocrysts) to obtain a better understanding of the magmatic processes and the assembly of the 1963–65 erupted magmas of Irazú volcano, one of the most voluminous active volcanoes in Costa Rica. We performed high-precision in situ Fe-Mg isotope analyses by femtosecond-LA-MC-ICP-MS on these olivines, to unravel the origin of their complex chemical zoning (growth, diffusion, or a combination of both processes). This information was used to establish a refined diffusion model to explore magma mixing-to-eruption timescales. Furthermore, trace element analyses using LA-ICP-MS were performed. Chromium displays a chemical zoning in the investigated olivine, which coincides spatially as well as in terms of length scale and geometry with Fe-Mg zoning and that was used to constrain Cr diffusivity in natural olivine. Our findings show that Fe-Mg zoning in Irazú olivine mainly results from Fe-Mg inter-diffusion after two crystal growth episodes as indicated by strongly coupled chemical and isotopic zoning. Simulations of this diffusive process indicate that mixing of these crystals into ascending primitive melts occurred <600 days before their eruption, consistent with a previously reported diffusion study based on Ni zonation in Mg-rich olivines. Trace element characteristics of olivine suggest that the complex-zoned olivine crystals originate from a crystal mush/cumulate in the middle or lower crust and deeper than the shallow magma chamber and were mobilized by mantle-derived magma bearing Mg-rich olivines. Finally, modeling of the observed Cr zoning in the Irazú olivines indicates that the diffusion coefficient for Cr in olivine ( D Cr ) is smaller than D Fe-Mg by a factor of 4.9 ± 2.9 at the conditions experienced by these crystals consistent with Cr diffusion experiments at high silica activity in the melt. Our results show that by combining elemental and isotope zoning studies in individual minerals we can refine the timing/assembly of magmatic eruptions and provide independent constraints on element diffusivities. Last, it confirms that primitive arc magmas at Irazú are not aphyric during ascent, but carry primitive phenocrysts from lower crust or Moho depth to the surface.

We calibrate the pressure-dependent Raman shift of feldspars by measuring spectra of 9 compositionally diverse plagioclase and alkali feldspars at pressures ranging between 0.1 MPa and 3.6 GPa using a diamond-anvil cell coupled with Raman spectroscopy. We observe up to 12 vibrational modes that are caused by deformation of Si(Al)O 4 tetrahedral chains. The most intense modes are v 22 , v 24 , and v 25 , which are produced by stretching and bending of four-membered Si(Al)-O-Si(Al) rings. Because modes are a product of lattice environments, feldspar composition may relate to mode frequency. We find that the frequencies of the v 25 mode correlates with composition, whereas the other intense bands do not correlate with composition. All feldspar compositions exhibit modes that shift linearly ( r 2 > 0.9) to higher frequencies with increasing pressure. Modes v 22 , v 24 , and v 25 shift to higher frequencies with slopes that range from 1.7 ± 0.5 to 5.5 ± 1.6 cm −1 GPa −1 , and provide the best combination of intensity and pressure-sensitivity. For all compositions the v 22 mode exhibits the most advantageous pressure-dependent ( P - T ) frequency shift. We use an elastic model, thermodynamic properties, and shear moduli to establish the pressure-temperature dependent sensitivity of feldspar inclusions hosted by garnet, clinopyroxene, and olivine. Raman shifts for all feldspars are <2 cm −1 for crustal and upper lithosphere conditions. Albitic plagioclase inclusions show the least temperature-sensitive inclusion pressures and provide the best barometers, followed by alkali feldspars and anorthitic plagioclase. Our new calibration allows Raman spectroscopy of feldspars to be used to quantify P - T conditions for crustal magmatic rocks, low- to high-grade metamorphic rocks, and the mantle.

In the Pan-African belt of the Dronning Maud Land, Antarctica, crystallized melt inclusions (nano-granitoids) occur in garnet from ultramafic granulites. The granulites contain the peak assemblage pargasite+garnet+clinopyroxene with rare relict orthopyroxene and biotite, and retrograde symplectites at contacts between garnet and amphibole. Garnet contains two generations of melt inclusions. Type 1 inclusions, interpreted as primary, are isolated, <10 μm in size, and generally have negative crystal shapes. They contain kokchetavite, kumdykolite, and phlogopite, with quartz and zoisite as minor phases, and undevitrified glass was identified in one inclusion. Type 2 inclusions are <30 μm in size, secondary, and contain amphibole, feldspars, and zoisite. Type 2 inclusions appear to be the crystallization products of a melt that coexisted with an immiscible CO 2 -rich fluid. The nanogranitoids were re-homogenized after heating in a piston-cylinder in a series of four experiments to investigate their composition. The conditions ranged between 900 and 950 °C at 1.5–2.4 GPa. Type 1 inclusions are trachytic and ultrapotassic, whereas type 2 melts are dacitic to rhyolitic. Thermodynamic modeling of the ultramafic composition in the MnNCKFMASHTO system shows that anatexis occurred at the end of the prograde P - T path, between the solidus (at ca. 860 °C–1.4 GPa) and the peak conditions (at ca. 960 °C–1.7 GPa). The model melt composition is felsic and similar to that of type 1 inclusions, particularly when the melting degree is low (<1 mol%), close to the solidus. However the modeling fails to reproduce the highly potassic signature of the melt and its low H 2 O content. The combination of petrology, melt inclusion study, and thermodynamic modeling supports the interpretation that melt was produced by anatexis of the ultramafic boudins near peak P - T conditions, and that type 1 inclusions contain the anatectic melt that was present during garnet growth. The felsic, ultrapotassic composition of the primary anatectic melts is compatible with low melting degrees in the presence of biotite and amphibole as reactants.

The potential use of a new family of synthetic swelling micas for cesium immobilization from aqueous solution was evaluated and the structural modifications after adsorption were analyzed. The results have revealed that they are good cesium adsorbents compared to natural clays and as the layer charge increases, the adsorption capacity and affinity increase. The cesium ions are adsorbed through a cation exchange mechanism, but an inner sphere complex with the basal O atoms of the tetrahedral sheet is favored. These findings imply that is possible to design minerals with improved environmental applications.

The garnierite ore at Campello Monti occurs as dark green colloform concretions covering surfaces, fractures, and filling veins in harzburgite rocks. The representative composition (Ni 2.45 Mg 0.14 Cu 0.12 Co 0.05 ) Σ2.76 Si 2.10 O 5 (OH) 4 is consistent with a 7 Å phase, namely pecoraite or népouite. Relevant chemical features are an exceptionally high Ni/Mg ratio, a significant level of Cu substituting for Ni, and a low content of S, possibly in tetrahedral sites. Olivine and orthopyroxene in the harzburgite host rock are only partially serpentinized, do not contain detectable Ni, and are almost iron free. The green coating probably originated from groundwater solutions that leached nearby weathered peridotites and sulfide ores, and deposited less-mobile elements along fractures and voids of the host peridotite, just outside their provenance area. Bulk techniques such as X-ray powder diffraction and infrared spectroscopy do not confidently distinguish between népouite and pecoraite, although the comparison with synthetic, implicitly pure polymorphs indicates népouite as the best matching phase. On the other hand, HRTEM clearly shows that garnierite is mostly constituted by plumose aggregates made of curved crystals with frayed tips, a few nanometers thick along the stacks and a few tens of nanometers long (nanoflakes). All known lizardite stacking sequences, namely 1 T , 2 H 1 , and 2H 2 , have been locally observed, even though most crystals show stacking disorder. The recorded nanostructure suggests possible explanations for the recurrent anomalies (low oxide totals, high IV T/ VI M cation ratios, etc.) found in EMP analyses of garnierites. The small grain size, the high density of defects, and the structural arrangement actually intermediate between lizardite and chrysotile probably explain the ambiguities that occurred during the characterization with bulk techniques. The results obtained in this study may have important implications for ore processing methods.