Editor’s Notes
Total Results: 1697
Volume 105 : September 2020 Issue
Metasomatism-controlled hydrogen distribution in the Spitsbergen upper mantle
https://doi.org/10.2138/am-2020-7290
This work by Tang et al. reports the hydrogen concentrations in olivine and pyroxene of Spitsbergen mantle xenoliths. The correlations between hydrogen and incompatible trace elements in xenoliths suggest that hydrogen distribution in the Spitsbergen upper mantle is controlled by metasomatism. The hydration of mantle was achieved by two different processes in this metasomatic event. Furthermore, the metasomatic melt could have derived from an OIB-type source, consistent with the Sr-Nd isotope compositions of the xenoliths.
Phase transformation of hydrous ringwoodite to the lower-mantle phases and the formation of dense hydrous silica
https://doi.org/10.2138/am-2020-7261
Chen et al. performed new high-pressure experiments that showed that hydrous ringwoodite can break down to form bridgmanite, ferropericlase, and a dense hydrous silica phase. The new observation of hydrous silica formation may explain the coexistence of stishovite and ferropericlase inclusions in lower-mantle diamonds, which cannot be explained by the equilibrium phase equilibria of dry phases.
Density and sound velocity of liquid Fe-S alloys at Earth’s outer core P-T conditions
https://doi.org/10.2138/am-2020-7349
The Earth’s outer core is mostly composed of liquid iron (Fe) alloy with a small amount of light elements, such as S, O, C, Si, and H. Among these light elements, S stands as a key candidate element, but its concentration is very controversial. In order to constrain the S concentration, Fu et al. developed a thermal equation of state for liquid Fe-S alloys by first-principles molecular dynamics simulations. Comparing with Preliminary Reference Earth Model, they clarify that the S concentration range is from 10 ~ 14 wt%, assuming S is the only light element. Taking into account the geophysical and geochemical constraints, they propose the outer core contains no more than 3.5 wt% S, 2.5 wt% O, or 3.8 wt% Si.
Some geometrical properties of fission-track-surface intersections in apatite
https://doi.org/10.2138/am-2020-7271
Apatite fission-track dating conventionally relies on counting the etched damage trails from uranium fission in apatite prism faces, which are considered to have high counting efficiencies. This presents a severe practical limitation. The work by Jonckheere et al. aims to lay the basis for extending fission-track counts to other apatite faces. It presents a model calculation of the orientations, lengths, and widths of the intersections of etched fission tracks with the prism and non-prism faces of apatite. The model predictions are consistent with extensive measurements. Their implications section discusses future practical uses of such measurements. Their contribution is a step toward understanding which tracks are counted and which are not in fission-track dating, and toward using other faces than prism faces for dating.
Thermal equation of state of post-aragonite CaCO3-Pmmn
https://doi.org/10.2138/am-2020-7279
Lv et al. investigated the stability and physical properties of post-aragonite CaCO3-Pmmn, a high-pressure polymorph of CaCO3, at lower mantle pressure and temperature conditions. Using synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell, they confirm the stability of CaCO3-Pmmn at pressures and temperatures corresponding to the middle of the lower mantle, and provide the first characterization of the thermal equation of state of this phase. The newly determined thermodynamic parameters of CaCO3-Pmmn are used to quantitatively model the density and seismic velocity of CaCO3 and carbonated eclogite. With the assumption that carbonates are homogeneously mixed into the slab, this work determines that the presence of carbonates in the subducted slab is unlikely to be detectable through seismic observations, and the buoyancy of carbonates will have a negligible effect on slab dynamics.
Structure of NaFeSiO4, NaFeSi2O6, and NaFeSi3O8 glasses and glass-ceramics
https://doi.org/10.2138/am-2020-7285
This paper by Ahmadzadeh et al. studies the structure of melts of geologically relevant Na-Fe-silicates. Both as-quenched and heat-treated powders of three Na-Fe-silicates with compositions analogous to well-known aluminosilicate minerals were studied. The main focus of the paper is on the role of Fe and the effects of increasing Si/Fe ratio. It is found that most of the Fe occur as 4-coordinated Fe3+ in the glasses. The crystallization behavior upon both quenching and heat-treating varies depending on Si/Fe ratio. The correlation between Si/Fe ratio and Fe structural role, as well as its oxidation state in such glasses, is important for geosciences and nuclear waste management. The study of these simplified compositions can provide fundamental insight into the physical behavior of silicate melts as well as high-Fe nuclear waste glass crystallization, which is related to their aqueous alteration behavior.
Raman spectroscopic studies of O–H stretching vibration in Mn-rich apatites: A structural approach
https://doi.org/10.2138/am-2020-7324
In this manuscript, Pieczka et al. present results of their studies of the OH-stretching vibration band in Raman spectra of Mn-bearing apatites. Deconvolution of the complex band gives an opportunity to gain insight into the distribution of Mn among the M1 and M2 structural sites in the apatite structure. Thus, the data can, at least partly, verify (or complete) structural data obtained by single-crystal X-ray diffraction and structure refinement, and in the case of very small crystals or their zoned texture, the described method may be the only tool that is able to yield structural data.
Characterization of modified mineral waste material adsorbent as affected by thermal treatment for optimizing its adsorption of lead and methyl orange
https://doi.org/10.2138/am-2020-7227
In the paper by Su et al., physiochemical properties of mineral waste material (MMWM) can be modified by thermal treatment. Increase in the adsorption capacity of MMWM is attributed to the enlarged surface area and disappearance of -OH groups. Adsorption of Pb on MMWM could be assigned to monolayer coverage and chemisorption. Thermally treated MMWM is a good adsorbent for Pb in solution.
Morin-type transition in 5C pyrrhotite
https://doi.org/10.2138/am-2020-7266
Haines et al. report the discovery of a low-temperature spin-flop transition in 5C pyrrhotite at ~155 K that is similar to those seen in hematite at 260 K and FeS (troilite) at 440 K. The 5C crystal was produced by annealing a 4C pyrrhotite crystal at 875 K to produce a change in the vacancy-ordering scheme that developed during cooling. The 5C structure is confirmed by single-crystal x-ray diffraction and the stoichiometry and homogeneity by electron microprobe and SEM BSE mapping. Resonant ultrasound spectroscopy, heat capacity, and magnetization measurements from room temperature down to 2 K are reported. The transition is marked by a steep change in elastic properties at the transition temperature, a peak in the heat capacity, and weak anomalies in measurements of magnetization. Magnetic hysteresis loops and comparison with the magnetic properties of 4C pyrrhotite suggest that the transition involves a change in orientation of moments between two different antiferromagnetic structures, perpendicular to the crystallographic c-axis at high temperatures and parallel to the crystallographic c-axis at low temperatures. The proposed structures are consistent with a group theoretical treatment that also predicts a first-order transition between the magnetic structures.
The formation of marine red beds and iron cycling on the Mesoproterozoic North China Platform
https://doi.org/10.2138/am-2020-7406
Tang et al. makes three points in this paper. First, marine red beds (MRBs) have been identified in the 1.4 Ga Xiamaling Formation, North China. Second, continentally sourced iron reactivated by dissimilatory iron reduction and distal hydrothermal fluids have supplied Fe(II) for the MRBs Third, Xiamaling MRBs record a moderate oxygenation event and a long-lasting ferruginous deep water mass.
A multi-methodological study of kernite, a mineral commodity of boron
https://doi.org/10.2138/am-2020-7433
Gatta et al. investigated the chemical composition and the crystal structure of kernite [Na2B4O6(OH)2·3H2O], one of the most important mineral commodities of B, by a series of analytical techniques, including single-crystal neutron diffraction. The general experimental formula of the kernite sample used in this study is Na1.99B3.99O6(OH)2·3.01H2O; the fraction of other elements (measured for more than 50 elements) is, overall, insignificant (<100 wt ppm); excluding B, kernite does not act as geochemical trap of other technologically-relevant elements (e.g., Li, Be, or REE). The H-bonding network in the structure of kernite is complex, pervasive, and plays a primary role on its structural stability. The potential utilizations of kernite, as a source of B (B2O3 ~ 50 wt%), are discussed on the basis of the experimental findings of this study.
Si-rich Mg-sursassite Mg4Al5Si7O23(OH)5 with octahedrally coordinated Si: A new ultrahigh-pressure hydrous phase
https://doi.org/10.2138/am-2020-7533
The crystal structure of a new high-pressure hydrous phase, Si-rich Mg-sursassite, of composition Mg4Al5Si7O23(OH)5 that was produced by sub-solidus reaction at 24 GPa and 1400 degrees in an experiment using a model sedimentary bulk composition has been determined by Bindi et al. using single-crystal X-ray diffraction. The reactions defining the stability of Si-rich Mg-sursassite are unknown, but are likely to be fundamentally different from those of Mg-sursassite, and involve other ultra-high-pressure dense structures such as Phase D, rather than Phase A.
Inherited Eocene magmatic tourmaline captured by the Miocene Himalayan leucogranites
https://doi.org/10.2138/am-2020-7608
Tourmaline, which is very common in the Himalayan leucogranites and typically the dominant reservoir of B in the rocks, is stable in various P-T-X conditions and could record the physical and chemical conditions of its formation. Thus, tourmaline chemistry has been frequently used to investigate the petrogenesis of its host magmatic rocks. However, these studies relied on the assumption that the tourmalines formed cogenetically with their magmatic host rocks, as is widely interpreted in most of the global tourmaline occurrences. Han et al. applied the 40Ar/39Ar dating method to coarse-grained tourmalines from the Miocene Cuonadong leucogranite in the Tethyan Himalayan, Southern Tibet, which yielded Eocene ages. The results, together with textural observations and geochemical studies, clearly suggest that the coarse-grained tourmalines found in pegmatites were inherited. The identification of inherited tourmalines not only contributes new insights into the Himalayan collisional orogeny, but also provides constraints for the application of tourmaline chemistry to petrological studies. The capture of Eocene tourmaline by the Miocene leucogranites at Cuonadong suggests that the crust-derived Eocene magmatism may have occurred in the southern Tethyan Himalaya together. Identification of the inherited magmatic tourmaline, although not common, challenges the current application of tourmaline chemistry to the investigation of magmatic-hydrothermal systems.
Memorial of F. Donald Bloss 1920–2020
https://doi.org/10.2138/am-2020-m1002
Memorial of Don Bloss
BOOK REVIEW
https://doi.org/10.2138/am-2020-B105921
Book Review: Geology of the Lassen Country: The Geologic Story of Lassen Volcanic National Park and Vicinity. (2019)
Volume 105 : August 2020 Issue
Are quasicrystals really so rare in the Universe?
https://doi.org/10.2138/am-2020-7519
Until 2009, the only known quasicrystals were synthetic, formed in the laboratory under highly controlled conditions. Conceivably, the only quasicrystals in the Milky Way, perhaps even in the Universe, were the ones fabricated by humans, or so it seemed. Then came the report that a quasicrystal with icosahedral symmetry had been discovered inside a rock recovered from a remote stream in far eastern Russia, and later that the rock proved to be an extraterrestrial, a piece of a rare CV3 carbonaceous chondrite meteorite (known as Khatyrka) that formed 4.5 billion years ago in the presolar nebula. At present, the only known examples of natural quasicrystals are from the Khatyrka meteorite. Does that mean that quasicrystals must be extremely rare in the Universe? In this speculative essay, Bindi et al. present a number of reasons why the answer might be no. In fact, quasicrystals may prove to be among the most common minerals found in the Universe.
Reaction between Cu-bearing minerals and hydrothermal fluids at 800 °C and 200 MPa: Constraints from synthetic fluid inclusions
https://doi.org/10.2138/am-2020-7114
Qi et al. study the reaction between native Cu and NaCl solution that leads to the coexistence of fluid inclusions and Na-bearing silicate melt inclusions. Micrometer- to submicrometer-sized cuprite (Cu2O) crystals have been observed in both types of the inclusions, and they are formed most probably due to the dissociation of CuOH. When CuO reacts with HCl and CuCl solutions, or Cu+ reacts with NaCl solution, nantokite (CuCl) has been found in the fluid inclusions that were formed due to oversaturation. Cu solubility in pure water and in 1.5 m NaCl solutions are 0.004 ± 0.002 m and 0.16 ± 0.07 m, respectively. The main responsible Cu-bearing complexes are CuOH(H2O)x in water, NaCuCl2 in NaCl solutions, and HCuCl2 in alkali-free solutions.
Evaluation and application of the quartz-inclusions-in-epidote mineral barometer
https://doi.org/10.2138/am-2020-7379
Epidote is one of the most common minerals found in igneous and metamorphic rocks. This study by Cisneros et al. introduces a new barometer that has the potential to be widely used among petrologists and other geoscientists. The barometer utilizes elastic modeling of quartz inclusions in epidote to constrain the pressure conditions of epidote growth. Modeling, pressure estimates, and applications of the technique are discussed.
Let there be water: How hydration/dehydration reactions accompany key Earth and life processes
https://doi.org/10.2138/am-2020-7380
This paper by Brovarone et al. highlights some fundamental aspects of hydration and dehydration reactions in the solid Earth, in biology, and in modern society, as well as their connections to carbon cycling on our planet.Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India
Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India
https://doi.org/10.2138/am-2020-7108
Karmaker et al. describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from the Manavadi (MvAm) and Ayyarmalai (AyAm) localities contain anorthite lenses (An90-99) with euhedral to elliptical outline set in a finer grained matrix of calcic plagioclase (An85-90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic phenocrysts, and the matrix are both recrystallized under static conditions presumably during the regional high pressure (HP) metamorphism (~800 °C, 8 to 11 kbar) at ~2.45 Ga. Combined petrological data and computed phase relations are consistent with metasomatic growth of corundum in an open system during infiltration-driven regional metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultrabasic rocks in general, which should provide new insight into lower crustal processes.
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