Editor’s Notes
Total Results: 1697
Volume 106 : March 2021 Issue
Tungsten mineralization during the evolution of a magmatic- hydrothermal system: Mineralogical evidence from the Xihuashan rare-metal granite in South China
https://doi.org/10.2138/am-2020-7514
Micas can record the magmatic-hydrothermal evolution of tungsten granite. Li et al. demonstrate that the geochemical variations and textures of zoned micas indicate magmatic fluids, rather than external fluids, were involved in greisenization. The siderite present is related to a Fe, Mn, and CO2-rich fluid under reducing conditions. The greisenization process plays a critical role in tungsten mineralization. The reducing environment and the mixture of a W-rich solution and a Fe-, Mn-rich external fluid facilitated tungsten mineralization.
Crystallization and melt extraction of a garnet-bearing charnockite from South China: Constraints from petrography, geochemistry, mineral thermometry, and rhyolite-MELTS modeling
https://doi.org/10.2138/am-2020-7335
"Zhang et al. investigated the Yunlu garnet-bearing charnockite as an example of the very few peraluminous magmatic charnockites around the world. The magmatic pressure-temperature-melt H2O content and associated crystallization of the charnockite was constrained quantitatively by the integration of petrography, geochemistry, fluid inclusion investigations, mineral thermo-barometry, and thermodynamic modeling. The Yunlu magma solidified at ""wet"" (H2O-saturated) and ""cold"" (~630 °C) conditions, which is different from metaluminous charnockites that solidified at ""dry"" (H2O-unsaturated) and ""hot"" (>800 °C) conditions. This study indicates that the peraluminous charnockites may experience a distinct crystallization process compared to metaluminous charnockites. Meanwhile, the temperature discrepancies between mineral thermometer results and the magmatic solidus were interpreted by the ""melt extraction"" model. The study sheds new light on the interpretations of granite thermometry."
Reducing epistemic and model uncertainty in ionic inter-diffusion chronology: A 3D observation and dynamic modeling approach using olivine from Piton de la Fournaise, La Réunion
https://doi.org/10.2138/am-2021-7296CCBY
Modeling of Mg-Fe zonation in olivine crystals from mafic ejecta and deposits from volcanic eruptions is an often-used tool for calculating magmatic timescales, but sub-perfect diffusion profiles are often rejected. This is a bias that Couperthwaite et al. suggest should, and can, be addressed as a community. The results in this open access paper by Couperthwaite et al. unlock the majority of these profiles and, in doing so, reveal a richer view of magmatic processes than previously could be seen.
Volume 106 : February 2021 Issue
Effect of water on carbonate-silicate liquid immiscibility in the system KAlSi3O8-CaMgSi2O6-NaAlSi2O6-CaMg(CO3)2 at 6 GPa: Implications for diamond-forming melts
https://doi.org/10.2138/am-2020-7551
Partial melting of recycled sediments (metapelites) subducted to mantle depths is essentially controlled by the phengite/K-feldspar + clinopyroxene + carbonate assemblage. Here Shatskiy et al. show that at 6 GPa and nominally dry conditions incipient melting of this assembly occurs at 1050-1100 °C and yields a K-dolomitic melt. By contrast, in the presence of H2O two immiscible phonolite-like and K-dolomitic melts appear. The established melts resemble the carbonatitic and silicic HDFs established in diamonds worldwide.
Jasonsmithite, a new phosphate mineral with a complex microporous framework, from the Foote mine, North Carolina, U.S.A.
https://doi.org/10.2138/am-2020-7582
Kampf et al. describe a new mineral, jasonsmithite, from the Foote mine in North Carolina. It has a complex framework structure containing large channels. Its 70% void space makes it one of the most porous mineral structures known and suggests that it may have technological applications.
Titanium in calcium amphibole: Behavior and thermometry
https://doi.org/10.2138/am-2020-7409
Thermometry of high-grade metamorphic rocks is difficult due to fast cationic diffusion during slow cooling. Liao et al. propose to use Ti content of amphibole (Ti-Amp) as thermometric tools for igneous and high-grade metamorphic rocks.
Phase relationships in the system ZnS-CuInS2: Insights from a nanoscale study of indium-bearing sphalerite
https://doi.org/10.2138/am-2020-7488
Xu et al. conducted micrometer- to nano-scale characterization on sphalerite containing 17-49 mol% CuInS2 that occurs in a pyrrhotite-dominant matrix. Results provide new insights into phase relationships in the system ZnS-CuInS2. Metal ordering is modeled as mixed sites in a sphalerite-type structure, [(Cu,In,Zn)3(Zn0.5Fe0.5)]4S4, with P4-3m symmetry. This modification is distinct from the reported cubic-tetragonal phase transition. Different degrees of fluid percolation explain the textures shown.
Major and trace element composition of olivine from magnesian skarns and silicate marbles
https://doi.org/10.2138/am-2020-7566
Nekrylov et al. analyzed olivine from magnesian skarns and silicate marbles, which could be easily distinguished from olivine formed in other processes by unusually low contents of Ni, Co, and Cr and high content of B. These features are linked to the composition of its formation environment -- dolomites and their contacts with intermediate-acidic magmatic rocks.
Decompression experiments for sulfur-bearing hydrous rhyolite magma: Redox evolution during magma decompression
https://doi.org/10.2138/am-2020-7535
Understanding redox evolution during magma ascent is important for exploring oxygen fugacity of magma and mantle and for modeling and predicting the chemical species of volcanic gases emitted to the surface. Okumura et al. experimentally investigated redox evolution during magma ascent to the surface. Results showed that sulfur-bearing hydrous rhyolite magma is slightly reduced during the ascent with decompression rates corresponding to explosive and effusive volcanic eruptions.
On the crystal chemistry of sulfur-rich lazurite, ideally Na7Ca(Al6Si6O24)(SO4)(S3)–·nH2O
https://doi.org/10.2138/am-2020-7317
Sapozhnikov et al. report that dark-blue lazurite from Malo-Bystrinskoe deposit (Russia) contains high sulfur. In addition to SO42-, the S3- radical-ion occupies beta-cages within the structure. The idealized formula of the mineral is Na7Ca[Al6Si6O24](SO4)2–(S3)–·H2O. The structure of studied lazurite contains incommensurate modulations.
Experimental evaluation of a new H2O-independent thermometer based on olivine-melt Ni partitioning at crustal pressure
https://doi.org/10.2138/am-2020-7014
Pu et al. demonstrate that the partitioning of Ni between olivine and basaltic melt (DNi) is independent of dissolved water (up to 4.3 wt%), unlike the partitioning of Mg (DMg). Olivine-melt equilibrium experiments were performed at 1 bar (anhydrous) and 0.5 GPa (anhydrous and hydrous). The results confirm that an olivine-melt thermometer based on DNi can be applied to hydrous natural basalts from subduction zones without any correction for H2O content.
Contrasting compositions between phenocrystic and xenocrystic olivines in the Cenozoic basalts from central Mongolia: Constraints on source lithology and regional uplift
https://doi.org/10.2138/am-2020-7431
Two Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their causes and relationship remain unclear. To solve these issues, Zhang et al. conducted analyses of major and trace element compositions for olivine phenocrysts and xenocrysts in the Cenozoic basalts. They conclude that mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic.
The composition of garnet in granite and pegmatite from the Gangdese orogen in southeastern Tibet: Constraints on pegmatite petrogenesis
https://doi.org/10.2138/am-2020-7388
Yu et al. report two stages of garnet growth in granite and pegmatite from the Gandese orogen in Tibet. The first generation of garnet (Grt-I) grew in the pegmatite from early evolved magmatic-hydrothermal fluids, and the second generation (Grt-II) crystallized after dissolution of the preexisting pegmatite garnet in the presence of a granitic magma. Both granite and pegmatite originate from partial melting of the same juvenile crust in the Mesozoic continental arc prior to the Cenozoic continental collision forming the Himalayan orogen.
Formation of metasomatic tourmalinites in reduced schists during the Black Hills Orogeny, South Dakota
https://doi.org/10.2138/am-2020-7405
Boron is an important component of granites, pegmatites, and metamorphic rocks in many geologic settings, including collisional orogens. Boron is a highly fluid-mobile element and thus it exchanges easily between these geologic materials. Nabelek describes mineral chemistry of tourmalinite in the Black Hills orogen and proposes ion-exchange reactions that led to its formation. The tourmalinite places constraints on the chemistry of the fluid that caused replacement of a schist by tourmaline and graphite.
New insights into the crystal chemistry of sauconite (Zn-smectite) from the Skorpion zinc deposit (Namibia) via a multi-methodological approach
https://doi.org/10.2138/am-2020-7460
Schingaro et al. conducted a multi-methodical characterization of a sauconite (Zn-bearing trioctahedral smectite) specimen from the Skorpion ore deposit (Namibia) using X-ray diffraction, cation exchange capacity analysis, differential thermal analysis, thermogravimetry, infrared spectroscopy and transmission electron microscopy. The results have implications not only for economic geology/recovery of critical metals but also, more generally, in the field of environmental sciences.
The new mineral crowningshieldite: A high-temperature NiS polymorph found in a type IIa diamond from the Letseng mine, Lesotho
https://doi.org/10.2138/am-2020-7567
Smith et al. describes crowningshieldite, a new nickel monosulfide mineral that is the high-temperature polymorph of millerite. It was discovered within a fine-grained multi-phase inclusion in a gem quality diamond from the Letseng Mine, Lesotho. The mineral name recognizes G.R. Crowningshield, who was a central figure in research at the Gemological Institute of America for over fifty years.
Elucidating the natural–synthetic mismatch of Pb2+Te4+O3: The redefinition of fairbankite to Pb2+ 12(Te4+O3)11(SO4)
https://doi.org/10.2138/am-2020-7536
The determination of fairbankite's structure by Missen et al. has solved the long-standing uncertainty around the natural Pb2+Te4+O3 formula. Additionally, the fairbankite structure is unique and contains a trimeric tellurite anion new to both synthetic and natural structures.
Are the thermodynamic properties of natural and synthetic Mg2SiO4-Fe2SiO4 olivines the same?
https://doi.org/10.2138/am-2021-7764CCBY
It is unclear whether the thermodynamic properties of some rock-forming minerals and their synthetic analogues are quantitatively the same. Olivine is an important substitutional solid-solution consisting of the two end-members forsterite, Mg2SiO4, and fayalite, Fe2SiO4. Gieger et al. undertook first low-temperature CP measurements on two natural olivines between 2 and 300 K; nearly end-member fayalite and a forsterite-rich crystal Fo0.904Fa0.096. They show that the CP behavior of the natural and synthetic crystals is similar.
Volume 106 : January 2021 Issue
P-V-T equation of state of hydrous phase A up to 10.5 GPa
https://doi.org/10.2138/am-2020-7132
Yang et al. report the precise equation of state of Mg7Si2O8(OH)6 phase A by using the pressure-volume-temperature (P-V-T) data of synthetic pure phase A up to ~10.5 GPa and ~900 K by in situ X-ray diffraction study at the Photon Factory-Advanced Ring (PF-AR) in Japan. Because phase A is one of the important hydrous phases and the water carriers into the Earth's mantle, the precise equation of state of phase A is very important and valuable to investigate the precise thermoelastic properties (e.g., temperature dependence of bulk modulus, thermal expansion, etc.), for discussing the state of the Earth interiors.
Elastic properties and structures of pyrope glass under high pressures
https://doi.org/10.2138/am-2020-7410
Hisano et al. indicate that the structural densification of pyrope glass is closely correlated to the change in elastic properties. Compared to other silicate glasses, the variation of chemical composition has a significant effect on the properties and their pressure dependences. Especially, the influence of the magnesium cation on the aluminosilicate glass is larger than that of other cations. Considering the melting of Earth's mantle, the magnesium component is abundant in the melt at the deeper region. Therefore, the knowledge of magnesium-bearing aluminosilicate glass can help us to understand the behavior of deep magmas.
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