Editor’s Notes
Total Results: 1697
Volume 104 : January 2019 Issue
Nature of hydrogen defects in clinopyroxenes from room temperature up to 1000 °C: Implication for the preservation of hydrogen in the upper mantle and impact on electrical conductivity
https://doi.org/10.2138/am-2019-6661
Yang et al. (page 79) Investigated the behavior of hydrogen defects in 10 natural clinopyroxene crystals at temperatures up to 1000 °C using in situ and quenched experiments. The in situ high-T Fourier transform infrared (FTIR) spectra indicate no proton transfer between point defects, but the local environments of hydrogen defects vary. Dehydration rates at 1000 °C of the six samples are not only slightly site-specific but also increase with Fe and tetrahedrally coordinated Al contents. Near-FTIR spectra suggest that the dehydration of the studied samples involves oxidation of Fe2+. For two diopsides with a mantle affinity, the diffusivity is about 10–12 m2/s at 1000 °C. The results imply that the different local environments of hydrogen defects between high T and low T may be responsible for the different mechanism of water impact on electrical conductivity between high and low T experiments; and because hydrogen diffusivities are positively related to Fe and IVAl contents, more care is required for interpretation of measured water concentrations in clinopyroxenes with high Fe and IVAl contents. Based upon the hydrogen diffusivities of olivine, orthopyroxene, and clinopyroxene in mantle peridotite, clinopyroxene should be the most reliable recorder of water from a given depth.
Phase transition boundary between fcc and hcp structures in Fe-Si alloy and its implications for terrestrial planetary cores
https://doi.org/10.2138/am-2019-6636
Komabayashi et al. (page 94) examined the phase transition between a face-centered cubic (fcc) and hexagonal close-packed (hcp) structures in Fe-4wt% Si and Fe-6.5wt% Si alloys to 71 GPa and 2000 K by in situ synchrotron X-ray diffraction. The fcc-hcp phase boundaries in the Fe-Si alloys are located at higher temperatures than that in pure Fe, indicating that the addition of Si expands the hcp stability field. The dP/dT slope of the boundary of the fcc phase in Fe-4wt% Si is similar to that of pure Fe, but the two-phase region is observed over a temperature range that widens with increasing pressure, from 50 K at 15 GPa to 150 K at 40 GPa. The triple point, where the fcc, hcp, and liquid phases coexist in Fe-4wt% Si, is placed at 90–105 GPa and 3300–3600 K with the assumption that the melting curve is same as Fe. These results support the hypothesis that the hcp phase is stable at Earth's inner core conditions. The core of Mercury (well below the triple point), containing an Fe-Si alloy with a Si content up to 6.5 wt% would likely crystallize an inner core with an fcc structure. Both cores from Venus and Mars are currently believed to be totally molten. Upon secular cooling, Venus is expected to crystallize an inner core with an hcp structure, as the pressures are similar to those of the Earth's core (far higher than the triple point), whereas the Martian inner core will take an hcp or fcc structure depending on the actual Si content and temperature.
Cathodoluminescence features, trace elements, and oxygen isotopes of quartz in unidirectional solidification textures from the Sn-mineralized Heemskirk Granite, western Tasmania
https://doi.org/10.2138/am-2019-6534
Hong et al. (page 100) studied distinctive quartz-rich unidirectional solidification textures (USTs) in apical carapaces of the Sn-mineralized Heemskirk Granite in western Tasmania (SE Australia). Individual UST layers consist dominantly of hexagonal quartz (>95%) with minor K-feldspar, plagioclase, biotite, muscovite, and magnetite. Multiple UST-quartz layers are intercalated with aplitic layers, and can locally extend for hundreds of meters. The Ti-in-quartz geothermometer yields temperatures of 545 ± 40 and 580 ± 20 °C (at 130 MPa) for the UST and aplitic quartz, respectively. The UST-quartz have higher Al/Ti values and Ge/Ti values than quartz phenocrysts in aplite layers, which is consistent with crystallization from a highly evolved fluid. LA-ICP-MS analyses show that UST-quartz has lower Ti, Li, and Sn than aplitic quartz, but higher Al, Li, Na, K, Mn, Fe, Ge, Rb, and Cs concentrations. The O-isotopic compositions (+5.1 to +10.2‰) of UST and aplitic quartz are consistent with magmatic source circulated by minor meteoric and/or formation waters. Scanning electron microscope-cathodoluminescence (SEM-CL) reveals that aplitic quartz is homogeneous and CL-bright with minor CL-dark patches. The bases of the UST quartz crystals are homogeneous and CL-bright with minor thin CL-dark fractures, whereas the trigonal apexes of the UST-quartz display CL-oscillatory growth zones. The results show that the UST layers in the Heemskirk Granite precipitated from magmatic-hydrothermal aqueous fluid exsolved from granitic melt during emplacement into the shallow crust (6–10 km). Such UST layers are characteristics of mineralized intrusions, and therefore provide significant indications for mineral exploration.
Controls on cassiterite (SnO2) crystallization: Evidence from cathodoluminescence, trace-element chemistry, and geochronology at the Gejiu Tin District
https://doi.org/10.2138/am-2019-6466
Cheng et al. (page 118) evaluate controls on cassiterite crystallization under hydrothermal conditions based upon the texture and geochemistry of cassiterite from a traverse from close to the host granitic pluton out into the mineralized country rock (Gejiu tin district, southwest China). The cassiterite samples feature diverse internal textures, as revealed by cathodoluminescence (CL) imaging, and contain a range of trivalent (Ga, Sc, Fe, Sb), quadrivalent (W, U, Ti, Zr, Hf), and pentavalent (Nb, Ta, V) trace elements, with Fe, Ti, and W being the most abundant trace elements. Cassiterite Ti/Zr ratios tend to decrease with distance away from the granite intrusion, and potentially can be used as a tool for vectoring toward a mineralized intrusive system. Elemental mapping of cassiterite grains reveals that trace-element concentration variations correspond closely to CL zoning patterns. The exceptions are distinct irregular domains that sharply cut across the primary oscillatory zoning, as defined by the concentrations of W, U, Sb, and Fe. Zones with low W and U (and Sb) and high Fe are interpreted to have formed during interaction with relatively oxidized fluids in which W and U are stripped from cassiterite due to cation exchange with Fe3+. Systematics of W, U, Sb, and Fe partitioning into cassiterite can, therefore, be used as a monitor of the relative oxidation state of the hydrothermal fluid from which cassiterite precipitates. Cassiterite U-Pb ages determined on zones of dissolution-reprecipitation are similar to ages for primary cassiterite growth and demonstrate a short (<3 m.y.) timespan of hydrothermal activity, indicating the potential of U-Pb dating of cassiterite for constraining the timing of Sn deposition.
Anomalous elastic behavior of phase egg, AlSiO3(OH), at high pressures
https://doi.org/10.2138/am-2019-6694
Mookherjee et al. (page 130) performed high-pressure, high-temperature experiments on lithological compositions resembling hydrated sedimentary layers in subducting slabs and found that the phase egg, AlSiO3(OH), is stable to pressures of 20–30 GPa or depths equivalent to the transition zone to lower mantle. Thus, phase egg is a potential candidate for transporting water into the Earth's mantle transition zone. First-principles simulations based on density functional theory explored the pressure dependence of crystal structure and its influence on energetics and elasticity. The phase egg exhibits anomalous behavior of the pressure dependence of the elasticity at mantle transition zone depths (~15 GPa). The anomalous behavior is related to changes in the hydrogen bonding O-H···O configurations, which were delineated as a transition from a low-pressure to a high-pressure structure of phase egg. Full elastic constant tensors indicate that phase egg is anisotropic, resulting in a maximum anisotropy of compressional wave velocity, AvP ≈ 30% and of shear wave velocity, AvS ≈ 17% at zero pressure. Results indicate that the phase egg has one of the fastest bulk sound velocities (vP and vS) compared to other hydrous aluminous phases in the Al2O3-SiO2-H2O ternary, which include topaz-OH, phase Pi, and δ-AlOOH. At depths corresponding to the base of the mantle transition zone, phase egg decomposes to a mixture of δ-AlOOH and stishovite. The changes in compressional ΔvP and shear ΔvS velocity associated with the decomposition is ~0.42% and –1.23%, respectively. Although phase egg may be limited to subducted sediments, it could hold several weight percentages of water along a normal mantle geotherm.
“Kamchatite” diamond aggregate from northern Kamchatka, Russia: New find of diamond formed by gas phase condensation or chemical vapor deposition
https://doi.org/10.2138/am-2019-6708
Kaminsky et al. (page 140) found polycrystalline diamond grains within the Valizhgen Peninsula in Koryakia, northern Kamchatka, Russia. One grain from the Aynyn River area studied by TEM contained diamond crystallites, 2–40 μm in size, that are twinned and have a high dislocation density. The crystallites are cemented by tilleyite Ca5(Si2O7)(CO3)2, SiC, Fe-Ni-Mn-Cr silicides, native silicon, graphite, calcite, and amorphous material. Three polymorphs of SiC were discriminated: hexagonal 4H and 6H and cubic C3 (β-SiC). Silicides have variable stoichiometry with (Fe,Ni,Mn,Cr)/Si = 0.505–1.925. Native silicon is an open-framework allotrope of silicon S24, which appears to be a new natural mineral phase. Three types of amorphous material were distinguished: a Ca-Si-C-O material, similar in composition to tilleyite; amorphous carbon and amorphous SiO2. Diamond crystallites and moissanite are intensively twinned, which is characteristic when these minerals formed by gas phase condensation or chemical vapor deposition (CVD) processes. The synthetic analogs of all other cementing compounds (β-SiC, silicides, and native silicon) are typical products of CVD processes. This confirms the earlier suggested CVD mechanism for the formation of Avacha diamond aggregates. Both Avacha and Aynyn diamond aggregates are not related to “classic” diamond locations within stable cratons, but to areas of active and Holocene volcanic belts. The studied diamond aggregates from Aynyn and Avacha, by their mineralogical features and by their origin during the course of volcanic eruptions via a gas phase condensation or CVD mechanism, may be considered a new variety of polycrystalline diamond and may be called “kamchatite.” Kamchatite extends the number of unusual diamond localities. It increases the potential sources of diamond and indicates the polygenetic character of diamond.
An example of high-T, high-symmetry crystallization: Spherical (Mg,Fe)-oxides formed by particle attachment in the shocked martian meteorite Northwest Africa 7755
https://doi.org/10.2138/am-2019-6597
Zhang et al. (page 150) describe spherical (Mg,Fe)-oxides with a protrusion surface in a shock-induced melt pocket from the Martian meteorite Northwest Africa 7755. Transmission electron microscopic observations demonstrate that the (Mg,Fe)-oxides are structure-coherent intergrowth of ferropericlase and magnesioferrite. The magnesioferrite is mainly present adjacent to the interface between (Mg,Fe)-oxides spherules and surrounding silicate glass, but not in direct contact with the silicate glass. Thermodynamic and kinetic considerations suggest that development of the spherical (Mg,Fe)-oxides can be best interpreted with crystallization by particle attachment and subsequent Ostwald ripening. This indicates that crystallization by particle attachment (previously hypothesized to occur in low-temperature aqueous natural and synthetic systems) can take place in high-temperature melts and has potential implications for understanding the nucleation and growth of early-stage crystals in high-temperature melts, such as chondrules in the solar nebula, erupted volcanic melts, and probably even intrusive magmas.
Zinc transport in hydrothermal fluids: On the roles of pressure and sulfur vs. chlorine complexing
https://doi.org/10.2138/am-2019-6719
Etschmann et al. (page 158) provide an experimental confirmation of the suggestion, based on thermodynamic simulations and extrapolations (Zhong et al. 2015), that Zn is transported in the form of chloride complexes in most acidic, shallow hydrothermal systems; while bisulfide complexes become increasingly important in deep, pH neutral to basic hydrothermal systems. We used in situ X-ray absorption spectroscopy (XAS) diamond-anvil cell experiments to determine Zn(II) speciation in a 1 m NaHS + 0.2 m HCl solution in contact with sphalerite. XANES data indicate that Zn coordinates to oxy/hydroxyl/chloride ligands from room temperature up to and including 200 °C, and then at higher temperatures (≥300 °C) and pressures (>200 MPa) it changes to complexing with sulfur. Our data confirm that bisulfide complexes become increasingly important in neutral-alkaline solutions at high pressure and temperature, due to an increase in sulfur solubility and to favorable entropy contributions for bisulfide vs. chloride complexes.
Book Review
https://doi.org/10.2138/am-2019-685
Elimi (page 162) reviews the book: Infrared and Raman Spectroscopies of Clay Minerals, Volume 8, Developments in Clay Science, 1st Edition, by Will Gates, J. Theo Kloprogge, Jana Madejova, and Faïza Bergaya. (2017) Elsevier, pp. 620.
Volume 103 : December 2018 Issue
The role of magma mixing, identification of mafic magma inputs, and structure of the underlying magmatic system at Mount St. Helens
https://doi.org/10.2138/am-2018-6555
"On page 1925 of the December issue, Leeman and Smith use whole rock and mineral compositions from Mount St. Helens (MSH) to examine the variety of mantle-derived liquids that are delivered to crustal reservoirs. They conclude that the sub-MSH mantle delivers two types of intraplate basaltic magmas, a low-K tholeiite and an ocean-island basalt variant, neither of which resemble common calcalkalic arc basalts. These magmas follow distinct ascent paths, rising from depths of 80 and 60 km respectively, though their main vents are separated laterally by only a few kilometers. These magmas then differentiate and mix with resident magmas to yield the full array of erupted products. The resulting model adds mantle inputs to the ""trans-crustal magmatic mush"" model of Spark and Cashman (2017). Outstanding questions concern why the more primitive mafic magmas erupted only briefly (between ca. 1800-2000 yrs BP) during the 40 ka lifetime of Mt. St. Helens, and what triggered this unusual eruptive behavior?"
Structural variations along the apatite F-OH join
https://doi.org/10.2138/am-2018-6608
On page 1981 of the December issue, Hughes et al. address the crystallographic mystery of how F is incorporated into fluor-hydroxylapatite solid solutions—the basis of fluoridation of human teeth. Fluorine and (OH)- occur in a column defined by a triangle of Ca (Ca2) atoms. They find that F- is consistently underbonded to Ca and the degree of underbonding is positively correlated with total F-. This underbonding is compensated by overbonded O(H), and matched by increasing Ca-OH bond bond strengths. Thus, it is not the incorporation of F- into hydroxylapatite per se that affects the strength of the resulting structure, but rather the downstream effect of a strengthened anionic column; this further hints at the possibility that there is some maximum F- content beyond which the increased F- might decrease bond strengths.
Rapid solid-state sintering in volcanic systems
https://doi.org/10.2138/am-2018-6714
On page 2028 of the December issue, Ryan et al. conduct experiments to examine sintering in volcanic rocks. This work addresses an issue long-debated in petrology classes: how is it that various phases come to be glued together to make rocks. The authors find that in shallow volcanic conduits, solid-state sintering can close porosity and densify bulk materials within a matter of days, and continue over time scales of weeks to months, thereby sealing pathways for fluid/vapor loss. Such loss of permeability allows pressure to re-build, and so lead to an eruptive event: the authors suggest that solid-state sintering may indeed control eruptive intervals. This is a new view of the controls on critical stresses needed to induce eruption, where the critical stresses are often thought to be controlled by ambient crust, rather than a cooling magmatic plug.
How geometry and anisotropy affect residual strain in host-inclusion systems: Coupling experimental and numerical approaches
https://doi.org/10.2138/am-2018-6700CCBY
On page 2032 of the December issue, Campomenosi et al. provide a detailed analysis of strain mapping via Raman spectroscopy of coesite and zircon inclusions inside garnets as a function of step-by-step systematic polishing of sections. They show that the external form of the inclusions is important in the heterogeneity of the retained residual strain fields, which may be due to the sharp edges and corners acting as stress concentrators, and discuss the importance of the differential elastic properties between host and inclusion crystals. They give guidance on the use of Raman measurements for the use of such techniques and their interpretation.
Volume 103 : November 2018 Issue
Probing planetary core structure and dynamics using density and sound velocity
https://doi.org/10.2138/am-2018-6775
On page 1717 of the November issue, Anne Pommier provides a review of a new paper by Morard et al. (see note below) and the implications for understanding planetary metallic cores. As noted by both Pommier and Morard et al., the tentative finding, that the Moon may have twice the prior estimated amount of S in its core, depends on the assumed T for the core/mantle boundary. A lower T required a greater amount of S to explain core density. Pommier also notes that the Moon may have T profile that allows for top-down, rather than bottom-up crystallization, and that such an “iron snow regime” might explain and early shutdown of a lunar dynamo.
Geochemical constraints on residual metal and sulfide in the sources of lunar mare basalts
https://doi.org/10.2138/am-2018-6368
On page 1734 of the November issue, James Day examines lunar volcanic samples for their variations in elements of contrasting character (e.g., lithophile, chalcophile, siderophile). He finds that their co-variation is consistent with partial melting of a source that is not saturated in either sulfide or metal phases, despite very low oxygen fugacities estimated for the lunar interior. This means that estimates for siderophile element abundances for the bulk silicate moon are probably not under-estimated, and that the bulk silicate moon has significantly (>20) less highly siderophile elements as bulk silicate Earth. Day suggests that this contrast means that the Moon was less affected by late-stage accretionary materials that presumably increased siderophile element contents of bulk silicate Earth.
Liquid properties in the Fe-FeS system under moderate pressure: Tool box to model small planetary cores
https://doi.org/10.2138/am-2018-6405
On page 1770 of the November issue, Morard et al. present new, in situ, X-ray diffraction data and ab initio calculations to deduce the physical properties of liquid Fe-S alloys at high pressures, which allows them to translate seismic properties of the Lunar core density to core S content. They find that at a core/mantle boundary temperature of 1750 K, their new equation of state requires twice the amount of S in the lunar core compared to prior estimates, regardless of core radius. This greater S content does not affect interpretations of moment of inertia but has a significant impact on the thermal evolution of the lunar core and dynamo.
High-pressure granulite facies metamorphism (~1.8 GPa) revealed in silica-undersaturated garnet-spinel-corundum gneiss, Central Maine Terrane, Connecticut, U.S.A.
https://doi.org/10.2138/am-2018-6543
On page 1851 of this issue, Keller and Ague provide a detailed look at the compositions and metamorphic conditions of high-pressure granulate rocks from the Central Maine Terrane of CT. They suggest that such rocks are ideal for estimating maximum temperature-pressure (T-P) conditions of metamorphism and determine such conditions to be ca. 1040oC and 1.8 GPa, the latter representing the highest P yet recorded for such granulites in the U.S. These same rocks yield syenite leucosomes that appear to have formed by partial melting of the host rock, which upon extraction may leave a residue that has a greater density than underlying peridotite, and so may help drive lithosphere delamination.
Volume 103 : October 2018 Issue
Positively oriented trigons on diamonds from the Snap Lake kimberlite dike, Canada: Implications for fluids and kimberlite cooling rates
https://doi.org/10.2138/am-2018-6496
On page 1634 of this issue, Li et al. find that diamonds from the Snap Lake kimberlite dike in Canada are covered in “positive” (in relief) trigons that cover the surfaces of most diamonds, including {111} growth faces and resorbed surfaces. Some overprint negative trigons. By comparison with experimental studies, the authors infer that the trigons are related to fluid-drive resorption events that occur after dike-like emplacement, at temperatures of 800-1000 °C. By contrast, the more common absence of positive trigons on diamonds from other Canadian kimberlite localities relates to rapid cooling below 800 °C. Their conclusion is that surface textures may provide records of kimberlitic fluids and emplacement conditions.
Fe-Ni ideality during core formation on Earth
https://doi.org/10.2138/am-2018-6651
On page 1707 of this issue, Huang and Badro test whether large amounts of Ni might affect the partition of Ni (or Cr or V) between and Fe-rich metal alloy and silicate melt at the very high P-T conditions that would describe core formation. They find that partition coefficients of Ni, Cr, and V are not dependent upon the total Ni content dissolved into Fe metal, even as Ni contents vary from 3.5 to 49 wt% of the metal alloy.
Volume 102 : December 2017 Issue
Multiple-reaction geobarometry for olivine-bearing igneous rocks
https://doi.org/10.2138/am-2017-6154
On page 2349 of the December 2017 (vol. 102, 12) issue Ziberna et al. present a new calibration of several equilibria whose intersections are sensitive to pressure. The value in these findings is that pressure is such a crucial geologic parameter to determine, and yet among condensed phases volume changes are so small across most reactions that viable barometers are quite rare. The new models presented in this work yield pressure estimates that have errors of ±1-2 kbar, which is about at the limit of precision for condensed phase equilibria and is certainly more than precise enough to place the target rock types (mafic and ultramafic rocks) to differentiate whether a given set of mineral assemblages form in the upper, middle, or lower crust, or upper mantle.
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