Editor’s Notes
Total Results: 1697
Volume 101 : September 2016 Issue
Modeling dislocation glide and lattice friction in Mg2SiO4 wadsleyite in conditions of the Earth’s transition zone
https://doi.org/10.2138/am-2016-5578CCBYNCND
On page 2085 of this issue, Ritterbex et al. develop a model for dislocation creep in wadsleyite that can be applied to the strain rates and pressure-temperature conditions relevant to Earths transition zone (410-660 km). The mineral wadsleyite, a polymorph of (Mg,Fe)2SiO4, is a dominant phase in Earths transition zone (410-520 km) with olivine stable at shallower depths, and ringwoodite at greater depths (520-660 km). In an earlier study, Ritterbex and others showed that even the easiest of dislocation slip mechanisms in ringwoodite were too inefficient to drive deformation at the base of the upper mantle. The authors extend this work to wadsleyite, obtaining a similar result. These results explain why some subducted slabs stall within or at the base of the transition zone: resistance to subduction may reflect a substantial increase in resistance to deformation as (Mg,Fe)2SiO4 transforms from olivine to wadsleyite, and eventually ringwoodite.
Volume 101 : August 2016 Issue
A new approach to the ionic model
https://doi.org/10.2138/am-2016-5782
On page 1717 of this issue, I. David Brown presents an overview of Wander and Bickmore (2016; page 1862 of this issue), who, as Brown explains, use of spherical harmonics expansion to calculate the energy of aluminosilicate mineral structures. Wander and Bickmore suggest that their approach, provided that bond-valence sums are small, leads to errors that are comparable to those obtained from the (vastly) more computationally intensive quantum mechanical approach. The challenge moving forward is the lack of experimental data that can be used to calibrate molecular models.
Na-P concentrations in high-pressure garnets: A potentially rich, but risky P-T repository
https://doi.org/10.2138/am-2016-5845
On page 1718 of this issue, William Glassley summarizes new experimental work by Konzett (2016; page 1756 of this issue), which provides a test of the usefulness of P in garnet as a geobarometer. Konzett argues that the lack of high phosphorus garnets in UHP metamorphic rocks reflects later re-equilibration: Konzett synthesized at high pressures, garnets that that are rich in Na-P components; these are then re-equilibrated in an eclogitic bulk composition at lower pressures, where the Na-P components exsolve to form apatite inclusions. This work suggests that a large fraction of apatite in eclogites might originate as a result of decompression-driven dissolution of phosphorus in garnet, and that the preservation of phosphorus-rich garnets may require very rapid upward transport. As Glassley further notes, though, to make use of such experimental results, it will be necessary to analyze Na and P contents at much higher than usual precision.
Crystal accumulation in a tilted arc batholith
https://doi.org/10.2138/am-2016-5404
On page 1719 of this issue, Barnes et al. provide a detailed analysis of mineral and whole rock compositions of the Wooley Creek Batholith in northern California. They find that the middle to lower structural sections of the batholith are largely crystal cumulates, with little residual melt. Only the uppermost portions of the batholith contain whole rocks that are plausible liquid compositions. Their results imply that plutonic whole-rock compositions are rarely frozen liquids, and thus that plutonic compositions are a poor proxy for arc liquid or arc volcanic compositions.
A tale of two garnets: The role of solid solution in the development toward a modern mineralogy
https://doi.org/10.2138/am-2016-5522
On page 1735 of this issue, Charles Geiger helps to celebrate our centennial by telling the story of mineralogy through the history of studies of garnet. That history stretches back to a time when mineralogy and chemistry were not distinct sciences, and includes: Hays idea of a smallest corpuscle; Goldschmidts ideas of atomic substitutions, controlled by atomic radii; the resulting classification schemes for garnet based on solid solution behavior; and very recent suggestions that, given extensive to complete solid solution, the pyralspite and ugrandite distinctions should be abandoned.
Origin and petrogenetic implications of anomalous olivine from a Cascade forearc basalt
https://doi.org/10.2138/am-2016-5651
On page 1807 of this issue, Rowe and Tepley analyze olivine grains from rare absarokite and shoshonite lavas from the Cascades arc. The cores from high forsterite olivine grains have very high Ni concentrations (>6000 ppm) and low Ca (850 ppm), while their crystal rims have low Ni (1000 ppm) and high Ca (>1500 ppm). The authors interpret such zoning as a result of a two-stage history; with the core compositions growing from a harzburgite source that had reacted with a Si-rich slab-derived melt (transforming some olivine to pyroxene in the process). Rim compositions might arise from one of a range of different processes, including late crystal growth from a peridotite-derived melt, or post-eruptive re-equilibration. In the former case, trace element systematics indicate no difference in incompatible element enrichments between the contrasting mineralogic sources, which implies that slab-derived fluids affect trace element concentrations over a broad region, but only locally affect source mineralogy.
Location and stability of europium in calcium sulfate and its relevance to rare earth recovery from phosphogypsum waste
https://doi.org/10.2138/am-2016-5684
On page 1854 of this issue, Radha Shivaramaiah et al. examine the means by which Rare Earth Elements (Eu, by way of example) are sequestered by phosphogypsym, which is a waste product related to the creation of phosphorous-based fertilizers. REEs are not the great concern they were just a few years ago, thanks to increased global production, but their extraction is nonetheless vital given their use in various technologies, including rechargeable batteries and phosphors in TV and computer screens. Shivaramaiah et al. find that REE are likely to be stored as a nanocrystalline precipitates on the crystal surfaces of phophogypsum, and so should be highly extractable from a material that otherwise fills solid waste landfills.
Fluid inclusion examination of the transition from magmatic to hydrothermal conditions in pegmatites from San Diego County, California
https://doi.org/10.2138/am-2016-5559
On page 1906 of this issue, Gammel and Nabelek examine the compositions and paleo-temperatures of fluid inclusions captured by various phases in granite-hosted pegmatites in southern California. Fluid inclusion temperatures are mostly less than 400 C, and range to as low as 70 C—well below the water-saturated solidus of host granite. Their work not only reveals the conditions by which pegmatite minerals precipitate, but observed phase relations and compositions track increases in acidity for hydrothermal fluids, due to precipitation of alkali-rich phases such as lepidolite; these evolutionary trends drive a fluid system towards kaolinite saturation. Clay minerals might thus not be a product of supergene reactions, but instead the very low-T end of a crystallization sequence that begins at magmatic temperatures.
Nanoscale gold clusters in arsenopyrite controlled by growth rate not concentration: Evidence from atom probe microscopy
https://doi.org/10.2138/am-2016-5781CCBYNCND
On page 1916, Fougerouse et al. present the first atom probe study of gold distribution in arsenopyrite to characterize the three-dimensional (3D) distribution of gold at the nanoscale and provide data to discriminate among competing models for gold incorporation in refractory ores. Gold incorporation is shown to be controlled by the rate of crystal growth, with slow growth-rate promoting the formation of gold clusters and rapid growth-rate leading to homogeneous gold distribution. This may have consequences on approaches to the economic recovery of Au from refractory ores.
Volume 101 : July 2016 Issue
New evidence for lunar basalt metasomatism by underlying regolith
https://doi.org/10.2138/am-2016-5790
On page 1497 of this issue, John Pernet-Fisher reviews new H-isotope analyses by Treiman et al. (p. 1596 of this issue), who suggest that very low dD values are not indicative of unique mantle reservoirs, but instead are related to regolith metasomatism. The lunar regolith has very low dD, due to solar wind and cosmic ray bombardment. In their new work, Treiman et al. recognize that lunar apatite samples having the lowest dD values co-exist with un-zoned olivine and pyroxene grains, and these unzoned grains may indicate long-term equilibration at very slow cooling rates, which may have allowed such basalts to react with underlying low dD regolith materials, that were in turn heated during basalt emplacement.
Alunite on Mars
https://doi.org/10.2138/am-2016-5802
On page 1499 of this issue Kathleen Benison reviews Ehlmann et al. (p. 1527 of this issue), who present compelling evidence for alunite on Mars. The mineral alunite is more than just a curiosity. As noted by Benison, we have long known that the ancient martian surface was wet. But less well understood are the compositions of those waters. The acidic nature of the martian surface has not been in doubt, given discoveries of the Fe-rich mineral jarosite. The discovery of alunite, though, not only further confirms that surface waters were locally acidic and S-rich, but Al-rich as well. Alunite is rare on Earth because of the unusual combination of Al- and acidic fluids. But what is rare on Earth might be abundant on Mars. We have yet to take full advantage of our knowledge of rare terrestrial minerals, to better understand the diversity of hydrologic environments on Earth (although work by Hazen and Ausubel, p. 1245 of this volume, is a needed step forward). But such work is perhaps all the more urgent, since as Benison so aptly notes, Mars hydrologic and mineral history is different from, but as complex as, that of Earth.
Detection of liquid H2O in vapor bubbles in reheated melt inclusions: Implications for magmatic fluid composition and volatile budgets of magmas?
https://doi.org/10.2138/am-2016-5689
"On page 1691 of this issue, Esposito et al. discover liquid H2O and H2O-CO2 vapor in bubbles of olivine-hosted melt inclusions. Their results show that water moves freely between the vapor and melt (glass) phases and that low water contents in what are necessarily CO2-rich bubbles are not evidence for a dry melt or a CO2-rich source. Instead, low water contents of bubbles may simply reflect post-trapping exsolution loss of water from a bubble into co-existing glass, or the precipitation of water as an annulus about the bubble wall. The annulus can be quite thin, and so optically undetectable, but compositionally molecularly significant. For example Esposito et al. estimate that this easily un-detected water might account for as much as 20-60% of the total water budget of a given melt inclusion, depending upon the bubble/melt-inclusion volume ratio, the size of the melt inclusion, and the initial H2O content of the trapped melt. Their data also show that S contents of melt inclusions might also be underestimated if bubbles and their annuli are not accounted for in the mass balance. Their results emphasize that great care must be taken to obtain accurate reconstruction of volatile contents of sub-surface melts based on analysis of melt inclusions."" "
Interface coupled dissolution-reprecipitation in garnet from subducted granulites and ultrahigh-pressure rocks revealed by phosphorous, sodium, and titanium zonation
https://doi.org/10.2138/am-2016-5707
On page 1696 of this issue Ague and Axler propose that minor elements in garnet, such as Na, Ti and especially P, provide a faithful record or prograde and retrograde metamorphic paths, compared to other minor or major elements that more easily diffuse through the garnet structure. In their study, they examined zoning patterns in garnets obtained from high pressure and ultra-high pressure (diamond-bearing) granulites. Their work reveals fine-scale zoning patterns for P that are invisible in Ca or Mg profiles. They suggest that the fine-scale growth patterns observed for P, Ti, and Na record dissolution-precipitation reactions that are driven by prograde and retrograde metamorphic reactions. We appear to have insufficient understanding of metamorphic equilibria to translate P or Ti zonation to specific estimates of temperature and pressure, but experimental studies of such might prove to be gainful employment.
Discreditation of diomignite and its petrologic implications
https://doi.org/10.2138/am-2016-5757
On page 1700 of this issue, Anderson reports on the recent discreditation by IMA of diomignite (Li2B4O7) as a valid mineral species. The lack of evidence for diomgnite's existence negates the inferred role of a Li2B4O7-flux-rich melt in the generation of primary pegmatite textures and rare element oxide mineralization
Accurate determination of ferric iron in garnets
https://doi.org/10.2138/am-2016-5695
On page 1704 of this issue, Quinn et al. show that the ferric contents of garnets might indeed be obtained by a combination of electron microprobe determinations of total Fe as FeO, and charge balance calculations. The literature is replete with compelling experimental data that show how measured ferric Fe contents are completely uncorrelated with ferric contents determined by charge balance, using electron microprobe analyses of major elements. This failure occurs for a wide range of minerals (e.g., Hawthorne, 1983, Can Min; Dyar et al. 1989, 1993, Am Min). However, Quinn et al. show that garnet may be an exception to the rule. Ferric Fe contents obtained from electronic microprobe analyses and mineral stoichiometry match Mossbauer determinations of the same and that errors of estimation are especially low at high total FeO contents (0.05 at >15 wt% FeO total). Although not specifically explored by Quinn et al., this work opens the possibility of estimating oxygen fugacity from microprobe analyses of metamorphic and igneous garnets.
Volume 101 : June 2016 Issue
On the nature and significance of rarity in mineralogy
https://doi.org/10.2138/am-2016-5601CCBY
On page 1245 of this issue, Robert Hazen and Jesse Ausubel consider the fact that of the >5000 known minerals, the vast majority are quite rare. A mere dozen or so make up >90% of Earth's crust, and probably all of Earth, while >99% of Earth consists of less than 100 mineral species. But rarity does not necessarily insinuate triviality; the occurrence of many rare terrestrial mineral species may be yet another unique characteristic in our solar system, along with massive granite-rich continents, abundant water, and life. Hazen suggests that mineral complexity may act as a proxy for other forms of evolutionary complexity. For example, given that 2/3 of known mineral species originate by biologically mediated processes, mineralogy complexity at some threshold value might allow us to identify planets that (necessarily?) harbor life.
Zircon saturation and Zr diffusion in rhyolitic melts, and zircon growth geospeedometer
https://doi.org/10.2138/am-2016-5462
On page 1252 of this issue, Zhang and Xu present the results of new experiments on the dissolution and diffusion characteristics of zircons in rhyolitic liquids. They find that zircon dissolution depends mostly on temperature, and less so on pressure or subtle contrasts between various rhyolitic liquid compositions. From existing, and their new data, the authors present a model of zircon growth, and find that for hydrous rhyolitic magmas, zircon growth rates range from 0.01 to 1.0 mm/yr, and suggest that zircon size can be used to infer pre-eruption storage times for such systems. Applied to the early Bishop Tuff, they infer that under ideal thermal conditions, zircon crystals, which range to 200 mm diameter in the early Bishop Tuff, experienced residence times of at least 1.4 to 2.2 ka.
On silica-rich granitoids and their eruptive equivalents
https://doi.org/10.2138/am-2016-5307
On page 1268 of this issue, Frost et al. show that the mineralogical similarity of silica-rich systems – mostly quartz + feldspars -- belies the myriad of processes by which they may be generated. They propose a new classification scheme for leucogranites, based on major and trace element compositions, which preserve a complexity that is not evident from mineral associations alone, although must certainly be reflected within mineral chemistry. Of course, the power of this method is that a geochemical approach provides a means to link mineral-poor rhyolitic systems to sub-solidus granites. Interestingly, these authors show that certain rhyolitic systems do indeed have plutonic equivalents – denied in prior studies because the rhyolites in question were compared to the wrong plutons. Their proposed six-fold (and further) divisions may add some considerable clarity with regard to source and petrogenesis.
From the Hadean to the Himalaya: 4.4 Ga of felsic terrestrial magmatism
https://doi.org/10.2138/am-2016-5516
On page 1348 of this issue, Harrison and Wielicki examine 4.4 Ga zircons that appear to have formed from nearly water-saturated conditions, at low temperatures of ca. 680oC. But they also show that Tertiary age granites in the Himalaya, formed by under-thrusting of the Indian plate beneath Asia, yield similar temperatures (ca. 660oC). Moreover, these granites share more than a thermal history – both the ancient and more recently formed zircons contain inclusions that yield similar pressures, or 5-15 kbar. Two aspects of these results are remarkable, and highly significant; the first is that if nothing else, the Hadean exhibited, at least locally and for a time, a modern-like continental geotherm, effectively equivalent to a Miocene case (at ca. 25oC/km). Second, to the extent that P-T conditions are indicative, a similar tectonic setting for both systems is implied; and if the association holds, then active plate tectonics – including subduction – occurred from almost the inception of Earths history.
Trace element thermometry of garnet-clinopyroxene pairs
https://doi.org/10.2138/am-2016-5427
On page 1438 of this issue, Pickles et al. conduct new experiments on equilibrated garnet + clinopyroxene pairs, using synthetic compositions. They find that the partitioning of rare earth elements (REE) is less selective at high temperatures, as might be expected from strain-partitioning relationships. This, and the tendency of trivalent REE to resist re-equilibration at low temperatures, means that REE contents can be used as a thermometer that better preserves peak temperatures of garnet + clinopyroxene co-saturation.
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