Editor’s Notes
Total Results: 1697
Volume 104 : February 2019 Issue
Dating mantle peridotites using Re-Os isotopes: The complex message from whole rocks, base metal sulfides, and platinum group minerals
https://doi.org/10.2138/am-2019-6557
This issue of American Mineralogist starts with an extensive review by Luguet and Pearson (pages 165-189) on Re-Os isotopic dating of mantle peridotites using the main Re-Os host minerals—base metal sulfides (BMS) and platinum group minerals (PGM)—versus whole-rock peridotites. Comparison of the results obtained at the two scales indicates that (1) BMS may provide a record of much older partial melting event, pushing back in time the age of the lithospheric mantle stabilization (BMS±PGM are considered as the mantle equivalents of crustal zircons), (2) if only whole-rock peridotite Re-Os analyses are possible, the best targets for constraining the timing of lithospheric stabilization are BMS-free/poor ultra-refractory spinel-bearing peridotites with very minimal metasomatic overprint, (3) while lherzolites are “fertile” in terms of their geochemical composition, they do not have a “primitive,” unmodified composition, and (4) the combined Re-Os isotopic investigations of BMS and whole-rock in BMS-rich mantle peridotites would provide a complementary view on the timing and nature of the petrological events responsible for the chemical and isotopic evolution and destruction of the lithospheric mantle. In addition, the 187Os/188Os composition of the BMS±PGM within any single peridotite may define several age clusters—in contrast to the single whole-rock value—and thus provide more accurate information on the formation and evolution of the lithospheric mantle.
In situ measurements of lead and other trace elements in abyssal peridotite sulfides
https://doi.org/10.2138/am-2019-6516
D’Errico et al. (pages 190-206) conducted in situ measurements of Pb and other trace elements in 150 abyssal peridotite sulfide grains (predominantly pentlandite) from the Gakkel and Southwest Indian ridges using the Sensitive High-Resolution Ion Microprobe with Reverse Geometry (SHRIMP-RG). The goal was to provide constraints on the storage of Pb and associated elements in the mantle. The authors developed a model for sulfide petrogenesis to explore the implication of assuming that all mantle Pb is hosted in mantle sulfides prior to melting. The results indicate that the measured average Pb concentration of 4 ppm (varying from 0.1 to 36 ppm) can be reproduced by >90% fractional crystallization from a sulfide melt. The remaining sulfide melt, which is modeled to contain 800 ppm Pb, will dissolve into silicate melt, as it rises through the mantle due to the increasing solubility of sulfur in silicate melt as pressure decreases. However, the amount of sulfide melt that remains after fractional crystallization is too low to contribute a significant amount of Pb to mid-ocean ridge basalts. Therefore, sulfides are not the main host for mantle Pb, even prior to the onset of any melting, and that the majority of mantle Pb is stored in silicate phases.
A terrestrial magmatic hibonite-grossite-vanadium assemblage: Desilication and extreme reduction in a volcanic plumbing system, Mount Carmel, Israel
https://doi.org/10.2138/am-2019-6733
Griffin et al. (pages 207-219) reported a unique occurrence of magmatic hibonite-grossite-spinel assemblages, crystallized from highly reduced Ca-Al-rich silicate melts trapped within aggregates of hopper/skeletal corundum, found as ejecta from Cretaceous pyroclastic deposits on Mt Carmel, N. Israel. The crystallization conditions were comparable to those of their meteoritic counterparts. Coarse-grained aggregates of hibonite + grossite + spinel +fluorite ± krotite ± perovskite ± Ca4Al6O12F2 represent a further evolution of the silicate melts. Native vanadium occurs as rounded inclusions in the hibonite, grossite, and spinel of the coarse aggregates. The presence of V0 requires fO2 ≤ ΔIW-9, suggesting a decline in fO2 by ~3 log units during the crystallization of this assemblage. The late crystallization of Ca4Al6O12F2 together with fluorite in the hibonite-grossite-spinel aggregates suggests that crystallization of the aggregates began at T >1400 °C, cooled to the pseudo-eutectic grossite + fluorite + Ca4Al6O12F2 + liquid at ca. 1375 °C, and remained at T >1150 °C until crystallization was terminated by the volcanic eruption. This study reported the first terrestrial example of the crystallization of hibonite and grossite from high-T silicate melts, the first terrestrial occurrence of krotite, and the first occurrence of native vanadium melts.
Intragranular plasticity vs. grain boundary sliding (GBS) in forsterite: Microstructural evidence at high pressures (3.5–5.GPa)
https://doi.org/10.2138/am-2019-6629
Bollinger et al. (pages 220-231) performed deformation experiments of polycrystalline forsterite at pressures of 3.5–5.0 GPa, temperatures of 1000–1200 °C, and a strain rate of ˜2 × 10–5s–1 at various applied strains in a 6-axis Mavo press to determine the contribution of individual mechanism (dislocation creep, diffusion creep, grain boundary sliding) to olivine deformation. They developed a methodology that allows the usage of the split-cylinder technique to extract information of the deformation mechanisms from an internal surface (with engraved strain markers) in the sample before and after the deformation experiments. The results suggest the dominance of intragranular deformation, in agreement with the fact that the samples have been deformed in the dislocation creep regime. Moreover, from strain markers and out-of-plane displacements of grains, the authors obtained the first microstructural evidence for a contribution of grain boundary sliding to plastic deformation at upper mantle pressure. Electron backscatter diffraction data indicate that grain boundary processes become increasingly relevant at temperatures above 1100 °C and ensure homogenous plastic strain distribution in the aggregate. Since olivine is the major component of the upper mantle, this study sheds important light on the plasticity and rheological behavior of the mantle.
Excess functions of chlorite solid solutions and neoformation of Fe-chlorites: Some implications of recent thermochemical measurements
https://doi.org/10.2138/am-2019-6794
Aja (pages 232-243) reported the thermodynamic properties of two natural chlorites—a magnesian chamosite and a ferroan clinochlore )—derived from calorimetric and low-temperature hydrothermal measurements. Moreover, the author developed a molecular modeling approach to calculate the excess thermodynamic properties of chlorite solid solutions. The obtained excess entropy of mixing in the ternary Al-rich and Si-rich system exhibits a curvilinear dependence on composition, and at 25 °C, the excess Gibbs energy of mixing varies from about –72 to 413 kJ/mol, implying a significant deviation from ideality. The molecular solid solution model further reveals significant deficiencies in the available database of standard state thermodynamic properties of chlorites. Application of the results to examine the neoformation of authigenic iron chlorites in green rusts suggests that green rusts will readily transform to berthierine and Fe-chlorites except under oxidizing conditions atypical of aquatic environments and ferrugineous sediments.
Nanoscale study of lamellar exsolutions in clinopyroxene from olivine gabbro: Recording crystallization sequences in iron-rich layered intrusions
https://doi.org/10.2138/am-2019-6764
Gao et al. (pages 244-261) investigated the pyroxene lamellar exsolutions and associated Fe–Ti oxides and spinels in clinopyroxne of an olivine gabbro sample from the Panzhihua intrusion, Southwest China, using high-angle annular dark-field scanning transmission electron microscopy, electron diffraction, and energy dispersive spectroscopy. The results indicate a sequence of nanoscale processes: from higher-T (~1030–1100 °C): (1) (clino)enstatite exsolutions in low-Ca diopside, followed by (2) slightly Ca-richer diopside overgrowths and high-T titanomagnetite exsolution in diopside; to lower-T (<450 °C) (3) titanomagnetite exsolutions into ulvöspinel + magnetite; followed by (4) sub-solidus re-equilibration in clinopyroxenes and among Fe–Ti oxides + hercynite. Using the exact phase boundary theory, the authors estimated the pressures of lamellar exsolution within the host diopside to be ~2 GPa. This study demonstrates that a nanoscale approach can help constrain the petrogenetic evolution during formation of layered intrusions.
Elasticity of single-crystal periclase at high pressure and temperature: The effect of iron on the elasticity and seismic parameters of ferropericlase in the lower mantle
https://doi.org/10.2138/am-2019-6656
Fan et al. (pages 262-275) measured the acoustic wave velocities and density of a periclase single-crystal by Brillouin light scattering combined with in situ synchrotron X ray diffraction up to ~30 GPa and 900 K in an externally heated diamond-anvil cell. Based on a comparison of the obtained elastic moduli of periclase with those of ferropericlase reported in the literature, the authors developed a comprehensive thermoelastic model for ferropericlase with up to ˜20 mol% FeO to evaluate the effect of Fe-Mg substitution on the elasticity and seismic parameters of ferropericlase at the lower mantle P-T conditions. The modeling results indicate that both the increase of the Fe content in ferropericlase and the increasing depth could change the compres¬sional wave anisotropy and shear wave splitting anisotropy of ferropericlase in the upper parts of the lower mantle. Furthermore, the authors conclude that Fe-induced lateral heterogeneities can significantly contribute to the observed seismic lateral heterogeneities in the lower mantle.
Elastic plastic self-consistent (EPSC) modeling of San Carlos olivine deformed in a D-DIA apparatus
https://doi.org/10.2138/am-2019-6666
Burnley and Kaboli (pages 276-281) conducted a suite of low strain deformation experiments on polycrystalline San Carlos olivine using a deformation DIA apparatus combined with in situ synchrotron X-ray diffraction at temperatures of 440–1106 °C and pressures of 3.8–4.6 GPa. The obtained data were fitted using elastic plastic self-consistent (EPSC) models, which incorporate an isotropic deformation mechanism that permits a small amount of non-elastic defor¬mation during the initial elastic portion of the experiment. This deformation mechanism mimics the observed reduction in the elastic modulus as a function of temperature and allows for better modeling of the remainder of the stress-strain curve. The critical resolved shear stresses (CRSS) for slip obtained from these models are in good agreement with those measured in single-crystal deformation experiments. Hence, polycrystalline deformation experiments analyzed with an EPSC model may be a viable approach to measure CRSS under conditions where single-crystal deformation experiments are more challenging.
Kinetics of antigorite dehydration: Rapid dehydration as a trigger for lower-plane seismicity in subduction zones
https://doi.org/10.2138/am-2019-6805
Liu et al. (pages 282-290) studied the dehydration kinetics of antigorite by thermogravimetric analysis using different heating rates of 10, 15, 20, and 25 K/min at temperatures up to 1260 K. The data were best fitted with the double-Gaussian distribution activation energy model (2-DAEM), in which a compensation effect exists between the pre-exponential factor and the average activation energy. The determined activation energy of the first step of antigorite dehydration stretches over a wide interval, whereas the second step has a significantly higher activation energy, distributed over a narrower interval. The release rate of water is 8.0×10–5 and 2.1×10–3 m3fluidm3rocks–1 at 893 and 973 K, respectively, which are near the onset temperature for the isothermal dehydration reaction. The results indicate that antigorite dehydration is fast enough to induce mechanical instabilities that may trigger seismicity in the lower plane of the double seismic zone.
Sound wave velocities of Fe5Si at high-pressure and high-temperature conditions: Implications to lunar and planetary cores
https://doi.org/10.2138/am-2019-6564
Deng et al. (pages 291-299) measured the sound wave velocities and density (ρ) of the Fe5Si (9 wt% Si) alloy that possesses a body-centered cubic (bcc) structure using ultrasonic technique and synchrotron X ray radiography combined with a Paris-Edinburgh press at pressures of 2.6–7.5 GPa and temperatures of 300–1173 K. The results show that at room temperature, the addition of Si to bcc-Fe increases the compressional wave velocity (vP) but decreases the shear wave velocity (vS). At high temperatures, a pronounced effect of pressure on the vS-T relations is observed. In the studied P-T range, the vP-ρ relationship follows the Birch’s law, whereas the vS-ρ relation exhibits complex behavior. Combined with planetary/spacecraft observations, these results have important implications in constraining the compositions of the lunar and Mercurian cores.
Evidence for syngenetic micro-inclusions of As3+- and As5+-containing Cu sulfides in hydrothermal pyrite
https://doi.org/10.2138/am-2019-6807
Merkulova et al. (pages 300-306) reported the first observation of the incorporation of As3+ in goldfieldite [Cu12(As,Sb,Bi)2Te2S13] and As5+ in colusite [Cu26V2(As,Sb)4Sn2S32] inclusions in pyrite (which contains up to 50 ppm As1-) from high-sulfidation deposits in Peru, using electron probe microanalysis, synchrotron X-ray fluorescence and absorption spectroscopy. The two Cu sulfide inclusions range from several to one hundred micrometers in size, and the As3+/As5+ concentration varies from a few ppm to 17.33 wt%. The results indicate that oxidizing hydrothermal conditions prevailed during the late stage of the mineralization process in the ore deposits, and provide new insights into the substitutional mechanisms of As3+ and As5+ in copper sulfosalts. From an environmental perspective, high concentrations of potentially toxic As contained in pyrite may pose a heretofore unrecognized threat to ecosystems in acid mine drainage settings.
The oxidation state of sulfur in lunar apatite
https://doi.org/10.2138/am-2019-6804
Brounce et al. (pages 307-312) performed measurements of the oxidation state of S in lunar apatites and associated mesostasis glass using synchrotron X‑ray absorption near edge structure spectroscopy. The results show that lunar apatites and glass contain dominantly S2–, whereas Earth apatites are only known to contain S6+. It is likely that many terrestrial and martian igneous rocks contain apatites with mixed sulfur oxidation states. The S6+/S2– ratios of such apatites could be used to quantify the fO2 values at which they crystallized, given information on the portioning of S6+ and S2– between apatite and melt and on the S6+/S2– ratios of melts as functions of fO2 and melt composition. Such a S-in-apatite oxybarometer could be developed and applied to igneous rocks from various planetary bodies in our solar system. The issue ends with a list of the 2018 reviewers for American Mineralogist, who are thanked for their invaluable services to the journal.
Volume 104 : January 2019 Issue
Iron carbide in the core
https://doi.org/10.2138/am-2019-6835
The January issue of American Mineralogist starts with a “Highlights and Breakthroughs” by Su and Liu (page 1). In their short contribution, they discuss the importance of the study by Lai et al. (published in last October's issue of American Mineralogist) on the thermoelastic properties of Fe7C3, a candidate component for the Earth's inner core.
Reconstruction of residual melts from the zeolitized explosive products of alkaline-mafic volcanoes
https://doi.org/10.2138/am-2019-6673
On page 2, Cambell et al. demonstrate that the combination of zeolitized proxy-glass signatures in alkaline-mafic pyroclastic deposits and Rhyolite-MELTS can provide new insights into the magmatic evolution of mafic alkaline systems. The predictive capability of the novel procedure is demonstrated in the case of a major caldera-forming eruption, the 355 ka Villa Senni event of the quiescent Colli Albani volcano, Rome, Italy, and its pervasively zeolitized Tufo Lionato deposit (>50 km3). The key finding is that a more-evolved residual melt fraction has been revealed, based on a reconstructed SiO2/Al2O3 ratio of 2.05 relative to that of the parent magma at 2.68, with implications for a reappraisal of pre-eruptive conditions and eruption mechanisms, and potentially for similar patterns across the volcanic stratigraphy and for other alkaline volcanoes.
Inefficient high-temperature metamorphism in orthogneiss
https://doi.org/10.2138/am-2019-6503
Chapman et al. (page 17) used large-scale large-scale electron backscatter diffraction (EBSD) and microbeam analysis to investigate crystallographic orientation and mineral chemistry data and quantify the proportion of relict igneous and neoblastic minerals forming a variably deformed, Cretaceous orthogneiss from Fiordland, New Zealand. Distinct metamorphic stages can be identified by texture and chemistry and were at least partially controlled by strain magnitude. At the grain-scale, the coupling of metamorphism and crystal plastic deformation appears to have permitted efficient transformation of an originally igneous assemblage. The effective distinction between igneous and metamorphic paragenesis and their links to deformation history enables greater clarity in interpretations of the makeup of the crust and their causal influence on lithospheric scale processes.
Nitrogen incorporation in silicates and metals: Results from SIMS, EPMA, FTIR, and laser-extraction mass spectrometry
https://doi.org/10.2138/am-2019-6533
Mosefelder et al. (page 31) investigate nitrogen incorporation in Earth materials by a combination of chemical (SIMS, EPMA, and laser-extraction mass spectrometry) and spectroscopic (FTIR) observations to study nitrogen contents and speciation mechanisms in silicate glasses, metal alloys, and an N-bearing silicate mineral (hyalophane). They demonstrate the general veracity of EPMA analysis of N in these samples and using SIMS show that the N content determined by EPMA (or laser extraction) are best fit with exponential functions rather than the linear regressions that are most commonly applied to SIMS data. They infer that under reducing conditions at high pressure and temperature N is dissolved in basaltic melts chiefly as NH−2 and NH2–, with N2 and/or nitride (X-N3–) complexes becoming increasingly important at low fO2, increasing N content, and decreasing H content. Our results have implications for future studies seeking to accurately measure N by SIMS and for studies of N partitioning at high pressure relevant to planetary accretion and differentiation.
Activation of [100](001) slip system by water incorporation in olivine and the cause of seismic anisotropy decrease with depth in the asthenosphere
https://doi.org/10.2138/am-2019-6574
Wang et al. (page 47) measured deformation mechanisms in anhydrous and hydrated (4-60 ppm H2O) olivine. The hydrated and dehydrated olivines were sheared in the [100] direction on the (001) plane at pressures of 2 to 5 GPa and temperatures of 1473 or 1573 K then observed by transmission electron microscopy on the (001) plane to determine whether the [100](001) slip system was activated or not. Only c-elongated [100] dislocations were observed for the anhydrous samples, while [100](001) dislocations dominated in the hydrous samples. These results support the idea that E-type fabrics can exist under hydrous conditions and that a transition to this fabric may be the cause of seismic anisotropy decrease with depth in the asthenosphere.
In-situ high-temperature vibrational spectra for synthetic and natural clinohumite: Implications for dense hydrous magnesium silicates in subduction zones
https://doi.org/10.2138/am-2019-6604
Liu et al. (page 53) collected in-situ high-temperature Raman and Fourier transform infrared (FTIR) spectra for both a synthetic [Mg9Si4O16(OH)2] and a natural, F-bearing, [Mg7.84Fe0.58Mn0.01Ti0.25(SiO4)4O0.5(OH)1.30F0.20], clinohumite sample up to 1243 K. Three OH bands above 3450 cm–1 are detected for both the natural and synthetic samples with negative temperature dependence, due to neighboring H-H repulsion in the crystal structure. Additional OH peaks are detected for the natural sample below 3450 cm–1 with positive temperature dependence, indicating that F- substitution significantly changes the high-temperature behavior of hydrogen bonds in the humite-group minerals. The mode Grüneisen parameters (γiP, γiT), as well as the intrinsic anharmonic parameters (ai) for clinohumite, chondrodite, and phase A, the dense hydrous magnesium silicate (DHMS) phases along the brucite–forsterite join were also evaluated. The averaged anharmonic parameters for the DHMS phases are systematically smaller (no more than 2% at 2000 K) than those of olivine and suggest that quasi-harmonic approximations are valid for clinohumite at subduction zone temperatures. Hence, the classic Debye model can reasonably simulate the thermodynamic properties (e.g., heat capacity) of these DHMS phases in subduction zones.
Stability of the hydrous phases of Al-rich phase D and Al-rich phase H in deep subducted oceanic crust
https://doi.org/10.2138/am-2019-6559
Liu et al. (page 64) experimentally investigated the stability of hydrous phases in mafic oceanic crust under deep subduction conditions by high-pressure and high-temperature experiments at 17–26 GPa and 800–1200 °C. In contrast to previous studies, three hydrous phases, including Fe-Ti oxyhydroxide, Al-rich phase D and Al-rich phase H, were present at the investigated P-T conditions. These results, in combination with published data on the stability of hydrous phases at lower pressures, suggest that a continuous chain of hydrous phases may exist in subducting, cold, oceanic crust (≤1000 °C): lawsonite (0–8 GPa), Fe-Ti oxyhydroxide (8–17 GPa), Al-rich phase D (18–23 GPa), and Al-rich phase H (>23 GPa). Therefore, in cold subduction zones, mafic oceanic crust, in addition to peridotite, may also carry a substantial amount of water into the mantle transition zone and the lower mantle.
Minerals in cement chemistry: A single-crystal neutron diffraction study of ettringite, Ca6Al2(SO4)3(OH)12·27H2O
https://doi.org/10.2138/am-2019-6783
Diego Gatta et al. (page 73) investigated ettringite, (Ca6Al2(SO4)3(OH)12·26H2O), a secondary-alteration mineral with more than 40 wt.% H2O and an important crystalline constituent of Portland cements. The crystal structure and crystal chemistry of ettringite were investigated by electron microprobe analysis, infrared spectroscopy, and single-crystal neutron diffraction at 20 K. Anisotropic neutron structure refinement allowed the location of (22+2) independent H sites, the description of their anisotropic vibrational regime and the complex hydrogen-bonding schemes. Analysis of the difference-Fourier maps of the nuclear density showed a disordered distribution of the inter-column (“free”) H2O molecules of the ettringite structure. Because disorder is still preserved down to 20 K, the authors are inclined to consider that as a “static disorder.” The structure of ettringite is largely held together by hydrogen bonding: the building units (i.e., SO4 tetrahedra, Al(OH)6 octahedra, and Ca(OH)4(H2O)4 polyhedra) are interconnected through an extensive network of hydrogen bonds. The effect of the low-temperature stability of ettringite and thaumasite on the pronounced “Sulfate Attack” of Portland cements, observed in cold regions, is discussed.
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