Volume 103, Issue 2

On the contemporary Earth, distinct plate tectonic settings are characterized by differences in heat flow that are recorded in metamorphic rocks as differences in apparent thermal gradients. In this study we compile thermal gradients [defined as temperature/pressure ( T/P ) at the metamorphic peak] and ages of metamorphism (defined as the timing of the metamorphic peak) for 456 localities from the Eoarchean to Cenozoic Eras to test the null hypothesis that thermal gradients of metamorphism through time did not vary outside of the range expected for each of these distinct plate tectonic settings. Based on thermal gradients, metamorphic rocks are classified into three natural groups: high d T /d P [>775 °C/GPa, mean ~1110 °C/GPa ( n = 199) rates], intermediate d T /d P [775–375 °C/GPa, mean ~575 °C/GPa ( n = 127)], and low d T /d P [<375 °C/GPa, mean ~255 °C/GPa ( n = 130)] metamorphism. Plots of T , P , and T / P against age demonstrate the widespread occurrence of two contrasting types of metamorphism—high d T /d P and intermediate d T /d P —in the rock record by the Neoarchean, the widespread occurrence of low d T /d P metamorphism in the rock record by the end of the Neoproterozoic, and a maximum in the thermal gradients for high d T /d P metamorphism during the period 2.3 to 0.85 Ga. These observations falsify the null hypothesis and support the alternative hypothesis that changes in thermal gradients evident in the metamorphic rock record were related to changes in geodynamic regime. Based on the observed secular changes, we postulate that the Earth has evolved through three geodynamic cycles since the Mesoarchean and has just entered a fourth. Cycle I began with the widespread appearance of paired metamorphism in the rock record, which was coeval with the amalgamation of widely dispersed blocks of protocontinental lithosphere into supercratons, and was terminated by the progressive fragmentation of the supercratons into protocontinents during the Siderian–Rhyacian (2.5 to 2.05 Ga). Cycle II commenced with the progressive reamalgamation of these protocontinents into the supercontinent Columbia and extended until the breakup of the supercontinent Rodinia in the Tonian (1.0 to 0.72 Ga). Thermal gradients of high d T /d P metamorphism rose around 2.3 Ga leading to a thermal maximum in the mid-Mesoproterozoic, reflecting insulation of the mantle beneath the quasi-integral continental lithosphere of Columbia, prior to the geographical reorganization of Columbia into Rodinia. This cycle coincides with the age span of most anorogenic magmatism on Earth and a scarcity of passive margins in the geological record. Intriguingly, the volume of preserved continental crust of Mesoproterozoic age is low relative to the Paleoproterozoic and Neoproterozoic Eras. These features are consistent with a relatively stable association of continental lithosphere between the assembly of Columbia and the breakup of Rodinia. The transition to Cycle III during the Tonian is marked by a steep decline in the thermal gradients of high d T /d P metamorphism to their lowest value and the appearance of low d T /d P metamorphism in the rock record. Again, thermal gradients for high d T /d P metamorphism show a rise to a peak at the end of the Variscides during the formation of Pangea, before another steep decline associated with the breakup of Pangea and the start of a fourth cycle at ca. 0.175 Ga. Although the mechanism by which subduction started and plate boundaries evolved remains uncertain, based on the widespread record of paired metamorphism in the Neoarchean we posit that plate tectonics was established globally during the late Mesoarchean. During the Neoproterozoic there was a change to deep subduction and colder thermal gradients, features characteristic of the modern plate tectonic regime.

Large amounts of nitrate salts occur in very specific environments and somewhat rare hyper-arid conditions, which may provide clues to fundamentally different nitrogen cycling and life survival mechanisms. Remote detection of ancient and modern nitrates on Earth and on other planetary bodies where they may occur requires a detailed understanding of their visible to near infrared (VNIR) spectral signatures. This study explores the VNIR spectral characteristics of several synthetic nitrate salts, sulfate minerals, and nitrate-bearing field samples from the Atacama Desert, Chile, to identify diagnostic spectral features of nitrate and possible interferences from other coexisting minerals. Results indicated that most of the nitrate salts have characteristic absorptions around 1.81, 1.94, 2.06, 2.21, and 2.42 μm. A significant positive correlation exists between the continuum-removed band depths of the 2.42 μm absorption and nitrate contents for the Atacama regolith samples, especially for samples with >10 wt% nitrate. The five absorption features of nitrate in the field spectra collected from multiple nitrate-rich regions in the Atacama Desert were then evaluated to determine the variabilities in these features in natural settings, while the band depths of 2.42 μm absorption were further calculated on the continuum-removed field spectra to estimate the nitrate abundances at the study sites. This work will supplement spectral libraries where nitrate spectra are lacking and have implications for future comparisons to planetary spectra to search for potentially life-related nitrate on Mars.

The high-pressure behavior of monovalent-cation-exchanged chabazites was investigated by means of in situ synchrotron X-ray powder diffraction with a diamond-anvil cell, and using water as penetrating pressure-transmitting medium, up to 5.5 GPa at room temperature. In all cases, except for Na-containing chabazites, a phase transition from the original rhombohedral ( R 3 m) to triclinic symmetry (likely P 1 ) was observed in the range between 3.0 GPa and 5.0 GPa. The phase transition is accompanied by an abrupt decrease of the unit-cell volume by up to 10%. Evidence of pressure-induced hydration (PIH), i.e., P -induced penetration of H 2 O molecules through the zeolitic cavities, was observed, as reflected by the incompressibility of the cation-exchanged chabazites, which is governed by the distribution of the extra-framework cations. The reversibility of the PIH and P -induced phase transitions in the high-pressure behavior of the cation-exchanged chabazites are discussed in the context of the role played by the chemical nature and bonding configuration of the extra-framework cations, along with that of the H 2 O content at room conditions.

Occurrences of high-Mg andesite (HMA) in modern volcanic arcs raise the possibility that significant volumes of continental crust could be directly derived from Earth’s mantle. Such rocks are commonly associated with subduction of young, warm oceanic lithosphere or occur in areas heated by mantle convection. A relatively rare occurrence near Mt. Shasta in the Cascades volcanic arc has been considered to represent one such primary mantle-derived magma type, from which more evolved andesitic and dacitic magmas are derived. Recognition that the Shasta area HMA is actually a hybrid mixed magma, calls into question this notion as well as the criteria upon which it is based. We report new chemical and mineralogical data for samples of the Shasta HMA that bear on the components and processes involved in its formation. Several generations of pyroxenes and olivines are present along with different generations of oxide minerals and melt inclusions. The most magnesian olivines (Fo 93 ) exhibit disequilibria textures, exotic melt inclusions, and reaction rims of Fo 87 composition; these crystals along with spongy, ~Mg# 87 orthopyroxene crystals are interpreted to be xenocrystic and do not signify a primitive mantle derivation. The groundmass is andesitic with moderate MgO content, and melt inclusions of similar compositions are hosted by equilibrium olivine (~Fo 87 ). The bulk magma (whole rock) is more magnesian, but primarily due to incorporation of mafic minerals and ultramafic xenolith debris. We propose that the exotic crystal and lithic debris in these rocks is derived from (1) dacitic magmas of possible crustal derivation, (2) prograded ultramafic rocks in the underlying crust, and (3) random lithic debris and crystals derived from conduit wall rocks and earlier intruded magmas within the feeder plexus beneath Shasta. The HMA is inferred to represent a mixture between evolved dacitic and primitive basaltic magmas as well as incorporation of xenolithic crystal cargo. There is no compelling evidence that HMA is present in large volumes, and it is not considered to be an important parental liquid to more evolved magmas at Shasta.

Understanding clay mineral transformation is of fundamental importance to grasping phyllosilicate crystal chemistry and unraveling geochemical processes. In this study, hydrothermal experiments were conducted on lizardite and antigorite, to investigate the possibility of the transformation from serpentine to smectite, the effect of precursor minerals’ structure on the transformation and the transformation mechanism involved. The reaction products were characterized using XRD, TG, HRTEM, and 27 Al MAS NMR. The results show that both lizardite and antigorite can be converted to smectite, but such conversion is much more difficult than that of kaolinite group minerals. The successful transformation is mainly evidenced by the occurrence of the characteristic (001) reflection of smectite at 1.2–1.3 nm in the XRD patterns and smectite layers with a thickness of 1.2–1.3 nm in HRTEM images of hydrothermal products as well as the dehydroxylation of the newly formed smectite at a higher temperature in comparison to that of the starting minerals. The difficulty for the transformation of serpentine to smectite may be due to the lack of enough available Al in the reaction system, in which the substitution of Al 3+ for Si 4+ in the neo-formed tetrahedral sheet is critical to control the size matching between the neo-formed tetrahedral sheet and octahedral sheet in starting minerals. Since the neighboring layers in antigorite are linked by the strong Si-O covalent bonds, the transformation only takes place at the edges of an antigorite layer rather than the whole layer, and the neo-formed smectite is non-swelling due to the inheritance of such Si-O covalent bonds. The conversion of lizardite to smectite is more feasible than that of antigorite, accompanied by exfoliation. This leads to a prominent decrease of the particle size in the hydrothermal products and the number of phyllosilicate layers contained therein. Two dominant pathways were observed for the transformation of lizardite and antigorite into smectite, i.e., conversion of one serpentine layer to one smectite layer via attachment of Si-O tetrahedra onto the octahedral sheet surface of the starting minerals and two adjacent serpentine layers merging into one smectite layer. In the case of the latter, dissolution of octahedra and inversion of tetrahedral sheets took place during the transformation. Besides these two dominant pathways, precipitation and epitaxial growth of smectite were also observed in the cases of lizardite and antigorite, respectively. The present study suggests that solid-state transformation is the main mechanism for conversion of serpentine minerals to smectite, similar to the transformation of kaolinite group minerals to beidellite.

Witherite [BaCO 3 ] and norsethite [BaMg(CO 3 ) 2 ] are perceived as chemical and structural analogs of aragonite [CaCO 3 ] and dolomite [CaMg(CO 3 ) 2 ], respectively. However, norsethite, unlike dolomite, readily precipitates from aqueous solutions at ambient conditions. This is of special interest as the dehydration barrier of Mg 2+ may be a likely cause of the dolomite growth inhibition. The easiness of norsethite growth shows that the problem of dolomite formation is more complex. To attain a comprehensive understanding of the analog BaCO 3 -MgCO 3 system and of the formation of ordered anhydrous Mg-bearing double carbonates, we investigated the fate and behavior of aqueous magnesium during growth of witherite. Growth experiments were conducted on witherite seeds in mixed-flow reactors at 50 °C and various Mg-concentrations (0.25–2 mM Ba 2+ , 0–20 mM Mg 2+ , pH 7.8–8.5, ionic strength 0.1 M). At Mg:Ba ratios in solution smaller than 6:1, Mg 2+ did not affect witherite growth kinetics. No significant amount of Mg 2+ was incorporated. The rate constant k and reaction order n for witherite growth were determined for the first time (k = 0.65 ± 0.05 × 10 -7 mol m -2 s -1 ; n = 1.3 ± 0.1; supersaturation Ω = IAP/ K s = 1–4, where IAP is the ionic activity product and K s the solubility constant). The insensitivity of witherite growth kinetics to these levels of Mg is analogous to aragonite growth. The general absence of the formation of solid solutions in the entire BaCO 3 -MgCO 3 system, however, is not shared by the CaCO 3 -MgCO 3 system, for which it is well known that substitution in the sixfold-coordinated cation sites occurs extensively. Mg:Ba ratios in solution larger than 12:1 led to a replacement of witherite by norsethite. This replacement also is in strong contrast to the CaCO 3 -MgCO 3 system, where higher temperatures and/or much longer timescales are necessary to obtain dolomite. The replacement rate of witherite at 50 °C was estimated to be ~200 times faster than the analogous replacement of aragonite by dolomite observed over 7 years at even 60 °C (Usdowski 1989). We speculate that the preferential formation of ordered norsethite over a solid solution is facilitated by the large difference in Mg 2+ and Ba 2+ ionic radii. Due to the presumably very high free energy of formation of the solid solution, ordering into distinct Ba- and Mg-layers is the only way to combine both cations within one phase. In the CaCO 3 -MgCO 3 system, solid solution occurrence is common and effectively contributes to the inhibition of the formation of the ordered double carbonate dolomite over a wide range of conditions (cf. Arvidson and Mackenzie 1999).

We present the first integrated study of carbonate, hydroxyl, fluoride, and chloride ion partitioning in the apatite-melt system. We determined volatile partitioning behavior between apatite and silicate melt for both haplobasaltic andesite and trachyte bulk compositions at 0.5–1 GPa and 1250 °C using the piston-cylinder apparatus. All volatile species were analyzed directly in both apatite and glass using secondary ion mass spectrometry (SIMS) and electron probe microanalysis. Distribution coefficients for OH-halogen exchange are similar to those from previous studies, and together with literature data, reveal a significant log-linear relationship with temperature, while the effects of pressure and melt composition are minimal. Meanwhile, halogen-free experiments generate very high C contents (up to 5000 ppm) in apatite. Stoichiometry calculations and infrared spectra indicate that this C is mainly incorporated onto the channel volatile site together with hydroxyl. In halogen-bearing experiments, apatite crystals contain significantly lower C (≤500 ppm), which may be partly incorporated onto the phosphate site while the channel volatile site is filled by OH+F+Cl+C. Our experiments give the first constraints on H 2 O-CO 2 exchange between apatite and silicate melt, with a K D of 0.355 ± 0.05 for the trachyte and 0.629 ± 0.08 for the haplobasaltic andesite. The new constraints on the temperature-dependence of partitioning will enable quantitative modeling of apatite-volatile exchange in igneous systems, while this new partitioning data and method for direct, in situ analysis of C in apatite mark a significant advance that will permit future studies of magmatic C and other volatiles. This has a broad range of potential applications including magmatic differentiation, fractionation, and degassing; quantification of volatile budgets in extraterrestrial and deep earth environments; and mineralization processes.

An understanding of the formation of toxic hexavalent chromium (Cr 6+ ) in Cr-containing mine tailings and associated soils and sediments, requires an understanding of the underlying dissolution mechanisms of chromitite, a common chromite-bearing rock in both ophiolites suites and ultramafic intrusions. This study will examine dissolution mechanisms of chromitite in various acidic, neutral, and alkaline solutions containing cultivated bacteria, manganese oxides, sulfates, and phosphates. Dissolution of chromitite is non-stoichiometric under acidic, near-neutral, and alkaline pH conditions and involves the release of chromite nanoparticles and complex dissolution/re-precipitation reactions. Chromitite samples are obtained from the Black Thor chromite deposit in Northern Ontario, Canada; part of the “Ring of Fire” intrusive complex. The examined chromitite is composed of chromite, (Fe 0.5 Mg 0.5 ) (Al 0.6 Cr 1.4 )O 4 and clinochlore; Mg 3 [Si 4 O 10 (OH) 2 ]·[MgAl 1.33 (OH) 6 ], the latter phase contains ~3 wt% Cr in the form of chromite nanoparticles. Bulk dissolution data are collected after dissolution experiments with chromitite powders, and the chemical and mineralogical composition of treated chromitite surfaces is characterized with a combination of surface analytical techniques (X-ray photoelectron spectroscopy) and nano- to micro-analytical techniques (scanning electron microscopy, transmission electron microscopy, and focused ion beam technology). In the chromitite systems studied here, the non-stoichiometric dissolution of clinochlore is the dominant reaction, which results in the formation of a hydrous and porous silica precipitate that is depleted in chromite nanoparticles relative to untreated clinochlore. Complete replacement of clinochlore by hydrous silica on the surface of chromitite under acidic conditions promotes the release of chromite nanoparticles and results in higher Cr:Si in solutions and in higher proportions of secondary Cr species on its surface (secondary Cr species are defined as surface terminations that do not occur on an untreated chromite surface, such as -Cr 3+ -OH 2 and -Cr 6+ -OH). Cultivated bacteria from a sulfide-bearing acid-mine drainage system affect neither the degree of dissolution nor the formation of secondary Cr species, whereas pyrolusite (MnO 2 ) particles, and adsorbed or precipitated Fe- and Al-bearing hydroxide, -sulfate, and -phosphate species, affect release and re-adsorption of chromite nanoparticles and Cr-bearing species during dissolution of chromitite under acidic, neutral, and alkaline conditions. These results show that weathering of chromitite and the release of Cr into the environment are strongly controlled by factors such as dissolution rates of Cr-bearing silicates and chromite, the release of chromite nanoparticles, re-precipitation of amorphous silica, the presence of particles in solution, and the pH-dependence adsorption (or precipitation) of Fe- and Al-bearing hydroxides and sulfates.

Basalt fragment 71597 is the sole high-titanium mare basalt showing evidence for olivine accumulation during formation. The petrogenesis of this unique sample was investigated using quantitative textural analysis and major- and trace-element mineral geochemistry. Crystal size distribution analysis identified two size populations of olivine, which we separate into cumulate and matrix olivine. The spatial distribution of olivine also supports clustering of olivine crystals, likely during accumulation. Observed mineral chemistry was consistent with an origin through olivine accumulation, although where this occurred cannot be discerned (e.g., in ponded melts at the base of or in the lunar crust, or within a thick high-Ti basalt flow). Attempts to place 71597 within a geochemical group were inconclusive both using subtraction of cumulate olivine from bulk composition, and by modal recombination of major phases. However, equilibrium liquid compositions of augite and plagioclase are determined to be consistent with an origin by fractionation from the Type B2 chemical suite of Apollo 17 high-Ti basalts. This method of classification has potential for placing other Type U (“Unclassified”) basalts into chemical suites.

Tourmalines form the most important boron rock-forming minerals on Earth. They belong to the cyclosilicates with a structure that may be regarded as a three-dimensional framework of octahedra Z O 6 that encompass columns of structural “islands” made of X O 9 , Y O 6 , B O 3 , and T O 4 polyhedra. The overall structure of tourmaline is a result of short-range and long-range constraints resulting, respectively on the charge and size of ions. In this study, published data are reviewed and analyzed to achieve a synthesis of relevant experimental results and to construct a crystal-chemical model for describing tourmalines and their compositional miscibility over different length scales. Order-disorder substitution reactions involving cations and anions are controlled by short-range structural constraints, whereas order-disorder intracrystalline reaction involving only cations are controlled by long-range structural constraints. The chemical affinity of a certain cation to a specific structural site of the tourmaline structure has been established on the basis of structural data and crystal-chemical considerations. This has direct implications for the tourmaline nomenclature, as well as on petrogenetic and provenance information. Some assumptions behind the classification scheme of tourmaline have been reformulated, revealing major agreement and significant improvements compared to earlier proposed scheme.

Here, we report the first occurrence of two (Ca-Y)-phosphate phases in apatite crystals from ancient rocks from both the Barberton greenstone belt and the Pilbara Craton. First, a cubic Ca 3 Y(PO 4 ) 3 phase was observed in a sample of silicified tuff from the Mendon Formation from the Barberton greenstone belt. A second phase, corresponding to a synthetic compound with the formula CaYP 7 O 20 , was observed in a sample of black banded chert from the Hooggenoeg Formation of the Onverwacht Group and in a sample of chert from the Strelley Pool Chert Formation (East Pilbara Terrane). Based on the presence of these phosphates and specific textures revealed by transmission electron microscopy, we argue for the importance of dissolution-reprecipitation processes in the formation of these phosphate phases. Temperature was likely not the primary parameter controlling the crystallization of the Ca 3 Y(PO 4 ) 3 and CaYP 7 O 20 phases. Instead, the REE-F complexes in an H 2 O solution and the specific budget of REEs and Y in apatite were likely responsible for the nucleation and formation of the (Ca-Y)-phosphate phases in the Archean rocks of the Barberton greenstone belt and Pilbara Craton.

For the purpose of geological carbon storage, it is necessary to understand the long-term effects of introducing CO 2 and sulfur-species into saline aquifers. CO 2 stripped from the flue gas during the carbon capture process may contain trace SO 2 and H 2 S and it may be economically beneficial to inject S-bearing CO 2 rather than costly purified CO 2 . Furthermore, reactions between the S-CO 2 -bearing formation brines and formation minerals will increase pH and promote further dissolution and precipitation reactions. To investigate this we model reactions in a natural analog where CO 2 - and SO 4 -H 2 S bearing fluids have reacted with clay-rich siltstones. In the Mid-Jurassic Carmel formation in a cap rock to a natural CO 2 -bearing reservoir at Green River, Utah, a 3.1 mm wide bleached alteration zone is observed at the uppermost contact between a primary gypsum bed and red siltstone. Gypsum at the contact is ~1 mm thick and shows elongate fibers perpendicular to the siltstone surface, suggesting fluid flow along the contact. Mineralogical concentrations, analyzed by Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN), show the altered siltstone region comprises two main zones: a 0.8 mm wide, hematite-poor, dolomite-poor, and illite-rich region adjacent to the gypsum bed; and a 2.3 mm wide, hematite-poor, dolomite-poor, and illite-poor region adjacent to the hematite alteration front. A one-component analytical solution to reactive-diffusive transport for the bleached zone implies it took less than 20 yr to form before the fluid self-sealed, and that literature hematite dissolution rates between 10 –8 and 10 –7 mol/m 2 /s are valid for likely diffusivities. Multi-component reactive-diffusive transport equilibrium modeling for the full phase assemblage, conducted with PHREEQC, suggests dissolution of hematite and dolomite and precipitation of illite over similar short timescales. Reaction progress with CO 2 -bearing, SO 4 -rich, and minor H 2 S-bearing fluids is shown to be much faster than with CO 2 -poor, SO 4 -rich with minor H 2 S-bearing fluids. The substantial buffering capacity of mineral reactions demonstrated by the S- and CO 2 -related alteration of hematite-bearing siltstones at the Green River CO 2 accumulation implies that corrosion of such a cap rock are, at worst, comparable to the 10 000 yr timescales needed for carbon storage.

Bones mostly consist of composite materials based on almost equivalent volume fractions of mineral (apatite) and organic (collagen) components. Accordingly, their infrared spectroscopic properties should reflect this composite nature. In this letter, we show by theory and experiment that the variability of the strong phosphate bands in the ATR-FTIR spectra of a series of modern and archeological bone samples can be related to electrostatic interactions affecting apatite particles and depending on the bone collagen content. Key parameters controlling the shape of these bands are the mineral volume fraction and the dielectric constant of the embedding matrix. The magnitude of these effects is larger than the one related to microscopic changes of the apatite structure. Consequently, the interplay of microscopic and macroscopic parameters should be considered when using FTIR spectroscopy to monitor the preservation state of bioapatite during diagenetic and fossilization processes, especially during the degradation of the organic fraction of bone.