Editor’s Notes
Total Results: 1697
Volume 101 : December 2016 Issue
Radionuclide removal by apatite
https://doi.org/10.2138/am-2016-5769
On page 2611 of this issue, Rigali et al. review the various ways in which apatite can be used to isolate a wide range of radionuclides from the near-surface environment. These means of radionuclide neutralization include the familiar modes of surface adsorption and partitioning of species into apatite structures. Rigali et al. also review what may be less familiar mechanisms, such as dissolution/(re-)precipitation reactions that are now being used to remediate contaminated groundwater or act as semi-permeable membranes. For example, some recent studies have shown that apatite can dissolve in the presence of U-bearing fluids to re-precipitate as U-phosphate or U-carbonate, and that the addition of hydroxyapatite to contaminated soils may reduce U concentrations in pore waters to levels deemed safe for drinking.
Electron diffraction determination of 11.5 Å and HySo structures: Candidate water carriers to the Upper Mantle
https://doi.org/10.2138/am-2016-5722
On page 2645 of this issue Gemmi et al. employ cutting edge analytical techniques to determine the structures of two important candidates for carriers of water into the deep mantle: the 11.5 angstrom phase, Mg6Al(OH)7(SiO4)2, and the HySo phase, Mg3Al(OH)3(Si2O7). These phases can form by the breakdown of chlinochlore and so may carry water to depths beyond clinochlore and chlorite breakdown. These phases lack the H-bonded, infinite tetrahedral sheets structure of precursor silicates. The authors find structures with reduced Si-O-Si interconnections and much higher density. Thus, these high-density phases, which can contain between 8-13 wt% H2O, are expected to be stable to much greater depths.
Collapsing minerals: Crackling noise of sandstone and coal, and the predictability of mining accidents
https://doi.org/10.2138/am-2016-5809CCBY
On page 2751 of this issue Jiang et al. provide an update of prior work that indicated that acoustic emissions presage mine collapse. In this new work, the authors present experimental results that confirm that acoustic emissions increase just prior to the collapse of cavities in sandstone and coal. The energy released by such acoustic emissions can be described by a power law, with slightly different exponents for different materials, but the exponents also change with time. A key result then is that collapse of a mine shaft, or bridge or building, may be presaged by both acoustic emissions and their energies. Another fascinating result is that cavity collapse yields a power law with an exponent greater than that associated with crack propagation, the latter being associated with micro-faults and earthquakes.
Statistical petrology reveals a link between supercontinents cycle and mantle global climate
https://doi.org/10.2138/am-2016-5868
On page 2768 of this issue, Ganne et al. present an analysis of global magmatic temperatures from published data that span the temporal range of 600 Ma to present. Their most dramatic finding is that magmatic temperatures, as measured from whole rock and mineral compositions, record a maximum that falls between 325-125 Ma; these ages are the bookends of the lifespan of Pangea. This time period also coincides with a peak in mantle potential temperature. The authors suggest that these findings support numerical models (e.g., Coltice et al. 2009; Van Avendonk et al. 2016) whereby supercontinent formation results in both thermal insulation, and a disruption of mantle convection, such that increased temperatures temporarily influence supercontinent volcanism. Coltice et al. predict that supercontinent-induced heating should be <100 C; Ganne et al. identify some key targets for high precision thermometry, as a test of the Coltice et al. model.
An improved clinopyroxene-based hygrometer for Etnean magmas and implications for eruption triggering mechanisms
https://doi.org/10.2138/am-2016-5916
On page 2774 of this issue, in a Letter, Perinelli et al. re-calibrate their clinopyroxene-based hygrometer. The original, and new model, are applicable to trachyte or hawaiiite-type basalts. But while magmatically restrictive, the model predicts water contents without precise knowledge of liquid composition, relying on pyroxene components and the P-T conditions of crystallization. They find that at Mt Etna, magmas begin to dehydrate mostly at a <400 MPa and lose most of their water at pressure of <100 MPa. This result corroborates inferences form melt inclusions, and it indicates that eruption triggering, and magma transport acceleration due to dehydration, are mostly relatively shallow processes, at least in the Etnean plumbing system.
Volume 101 : November 2016 Issue
Melts, mush, and more: Evidence for the state of intermediate-to-silicic arc magmatic systems
https://doi.org/10.2138/am-2016-5914
On pages 2365-2366 of this issue, Erik Klemetti provides an overview of Paterson et al. (pages 2176-2198 of the October issue), first by outlining the growing consensus that large liquid magma bodies are rare and that long-lived bodies spend most of their time as crystalline mush. Paterson et al., in their study of the Tuolumne Intrusive Complex (TIC), show evidence of massive erosion and re-deposition of early magmatic materials by later magma intrusions. Their field and geochemical evidence indicate that as much as half of an original magma mush may be eroded and either incorporated into a later intrusion, or erupted (if thermally rejuvenated) or migrate downwards, being replaced by more buoyant magmas. This presents challenges for interpreting everything from emplacement dates (if most zircon crystals are recycled from earlier magma batches) to pluton volume growth rates. Klemetti end by noting that at least some numerical models apparently indicate that massive mush erosion and mixing are unlikely; field evidence shows that the unlikely is possible.
Nucleation rates of spherulites in natural rhyolitic lava
https://doi.org/10.2138/am-2016-5624
On page 2367 of this issue, Gardner et al. examine geochemical gradients in glass adjoining spherulites in rhyolitic obsidian, to estimate spherulite saturation conditions and growth rates, and by extension, the nucleation rates of spherulites and cooling rates of rhyolite flows. Their modeling of spherulites indicates that nearly anhydrous Yellowstone Plateau rhyolite lavas exhibit post-emplacement (sub-solidus) cooling rates of 0.3 to 1.2 C/day, in the temperature rages of 710 to 430C. This work interestingly complements a paper we highlighted earlier, by Seaman (2013; page 304, v. 98), who examined spherulites in hydrous systems, and showed that they exhibit either blade-like or needle shapes that vary as a function of cooling rate. Together, these studies provide means to evaluate rhyolite cooling rates from spherulite growth ranging from slightly above-solidus to sub-solidus conditions.
Silicic magma reservoirs in the Earth’s crust
https://doi.org/10.2138/am-2016-5675
"On page 2377 of this issue (open access), Bachmann and Huber review the genesis of silicic magma bodies. These authors develop a ""mush model"" to explain silicic systems, both plutonic and volcanic. The model embodies a counterintuitive view that crystal-liquid separation can be more effective at intermediate crystallinities (~50-70% crystals). This is a foreign concept to those studying basaltic systems, as olivine crystals, for example, readily separate from parent liquids at even the smallest of crystal fractions. But these authors argue that for silicic systems, intermediate to high crystallinities (>40-70%) inhibit convective stirring and small pockets of liquid contained within a larger crystal-rich mush can segregate to form eruptible cupolas. A driving force for this model is its apparent ability to simultaneously explain geophysical observations and the incremental growth of large magma reservoirs."
Petrogenesis of antecryst-bearing arc basalts from the Trans-Mexican Volcanic Belt: Insights into along-arc variations in magma-mush ponding depths, H2O contents, and surface heat flux
https://doi.org/10.2138/am-2016-5701
On page 2405 of this issue, Zellmer et al. apply MELTS to calculate maximum An contents of plagioclase for nominally aphyric lavas from the Trans-Mexican Volcanic Belt. They find that for water saturated mafic melts, An contents (predicted by MELTS) first increase with increasing pressure, to about 1.0 kbar, decreasing thereafter as pressure increases (to at least 9 kbar). The authors suggest that maximum An contents reflect the depths at which magmas stall prior to eruption. In their model, nearly aphyric magmas leave their mantle source region with varying amounts of dissolved water. Those liquids with the greatest water contents reach vapor saturation at the greatest depths and partially crystallize as the magma dehydrates. Both crystallization and dehydration serve to increase viscosity, causing magmas to stall and cool further, reaching plagioclase saturation. From this outlook, maximum An contents are a proxy for depths of vapor saturation and pre-eruption staging depths, or the final depths of crystallization for un-erupted fractions.
Phosphate minerals in the H group of ordinary chondrites, and fluid activity recorded by apatite heterogeneity in the Zag H3-6 regolith breccia
https://doi.org/10.2138/am-2016-5728
On page 2452 of this issue, Jones et al. examine the water and halogen contents of merrillite and apatite crystals from various chondritic meteorites. They find that while volatile contents vary across meteorite sub-types, and even within individual samples, they do not vary systematically with metamorphic grade. It would seem, then, that fluid evolution on chondrites can be quite localized, and that phosphate compositions record a range of processes related to regolith development. One hypothesis is that F-rich apatite grains form within a regolith by interaction with F-rich vapors released from impact-degassed melts near the chondrite parent body surface. To the extent that regolith development is an early solar system process, this mode of genesis implies that chondrites might be strongly degassed prior to their accretion into planet-sized objects. In such a case, Earths inventory of volatiles may be less than inferred from the compositions of non-brecciated chondrite fractions.
Volume 101 : October 2016 Issue
Temporal histories of Cordilleran continental arcs: Testing models for magmatic episodicity
https://doi.org/10.2138/am-2016-5718
On page 2133 of this issue, Kirsch et al. compare zircon ages along several arc segments that span nearly the entire North and South American Cordillera, from 400 to 80 Ma. They find that age distributions, both within and between arcs, are non-uniform: peaks and lulls are often separated by intervals that fall within a 50 to 80 Ma range. Moreover, while some flare-ups are localized, these inferred (from zircon age dates) magmatic maxima and minima are nearly synchronous for thousands of kilometers in many arcs. They suggest that plate tectonic factors (rather than intra-crustal magmatic differentiation, cooling or transport) may control magmatic input. Some authors have rejected this idea, but perhaps too quickly, having examined smaller-scale spatial and temporal patterns. Rather, by accounting for a lag time between tectonic forces and magmatic responses, correlation coefficients between the two are increased.
Graphite-diamond relations in mantle rocks: Evidence from an eclogitic xenolith from the Udachnaya kimberlite (Siberian Craton)
https://doi.org/10.2138/am-2016-5657
On page 2155 of this issue, Mikhailenko et al. combine Raman and FTIR spectroscopic studies to investigate graphite inclusions in diamond, from eclogite xenoliths in the Udcahnaya kimberlite. Their Raman data show graphite inclusions retain a large residual stress, both within the graphite and the immediately adjacent diamond. This observation, combined with petrographic observations and existing experimental work, leads these authors to conclude that the graphite did not form from diamond on a retrograde path, but rather that graphite existed metastably in the diamond stability field. These authors also find that N in diamond is highly aggregated, which may mean that diamond hosts for highly stressed graphite grains are quite old. This work implies that metastable graphite might persist within at least the immediate reaches of the diamond stability field for perhaps billions of years.
Experimental hydration of natural volcanic clinopyroxene phenocrysts under hydrothermal pressures (0.5–3 kbar)
https://doi.org/10.2138/am-2016-5711CCBYNCND
On page 2233 of this issue, Weis et al. examine the re-hydration of clinopyroxene phenocrysts from various volcanic systems. As with prior studies, they find that hydration and de-hydration reactions are controlled by an equilibrium involving ferric Fe: (OH)- + Fe2+ = O2- + Fe3+ + 0.5H2. Their work indicates that clinopyroxenes phenocrysts may often dehydrate significantly during magma transport and eruption, and so untreated, can only be used to estimate minimum water contents in a magmatic system. These authors argue that pre-eruption magmatic water contents can be obtained using their methods to re-hydrate clinopyroxenes, with the implied argument that ferric Fe contents in clinopyroxene place a maximum on rehydrated water contents. However, ferric iron itself is not a good proxy for magmatic water contents, as it is not always linked to hydrogen associated defects, but may be part of other charge balancing processes (e.g., Na+ Fe3+ vs. 2 Ca2+).
Silicic lunar volcanism: Testing the crustal melting model
https://doi.org/10.2138/am-2016-5619
On page 2312 of this issue, Gullikson et al. conduct partial crystallization experiments to test hypotheses of granite genesis under lunar conditions. They show that lunar-like granites can be obtained by partial melting at low pressures of monzogabbro and alkali gabbro crustal rocks, at least when equilibration temperatures are <1000 oC; but only the monzogabbro parent yields granite magmas in sufficient quantities to segregate from a solid residue. Some of their experiments also yield Fe- and Si-rich immiscible liquids, at T = 1000-1050 oC. But liquid immiscibility, at least for the examined starting compositions, can be rejected as a means to make Th-rich Lunar granite, since Th partitions strongly into the low-Si, Fe-rich liquid. Granites appear to be uniquely abundant on Earth—this work is a step towards understanding their rarity elsewhere and thus for understanding what makes Earth exceptional among neighboring planetary objects.
Transition metals in the transition zone: Crystal chemistry of minor element substitution in wadsleyite
https://doi.org/10.2138/am-2016-5681
On page 2322 of this issue, Zhang et al. explore the crystallography of transition elements and intra-crystalline partitioning of these into Wadsleyite. The inside-baseball result: Ni, Co, and Zn avoid the M2 site (in favor of M1 and M3), and do so in proportion to expected Crystal Field Stabilization Energies. Perhaps more significant, though, is that trivalent cations Cr and V (which strongly prefer M3 over M1 and M2), and tetravalent cations, such as Ti, appear to be much more soluble in wadsleyite tetrahedral sites compared to T sites in olivine, apparently because T sites in wadsleyite are larger. The authors suggest that the wadsleyite-bearing mantle (410-525 km) has the potential to absorb more of these elements compared to shallower mantle materials, and plumes that transit the wadsleyite stability field may be affected by this contrast in wadsleyite and olivine solubility.
Volume 101 : September 2016 Issue
Styles of aqueous alteration on Mars
https://doi.org/10.2138/am-2016-5866
On page 1925 of this issue, Edward Cloutis provides a review of Farrand et al. (2016; page 2005 of this issue), who show that it is possible to identify minerals by remote sensing, that have been altered by reaction with water. The technique makes use of visible and near infra-red (VNIR) spectra. Farrand et al. found evidence of clay minerals, hematite, hydrated sulfates, hydrated silica, and perhaps even free water in the pore spaces of some soils. As noted by Cloutis, a key value of this study is the demonstration that a coarse spectral resolution can be used to identify a wide range of hydrous alteration products on the martian surface, or other planetary surfaces.
Study on nanophase iron oxyhydroxides in freshwater ferromanganese nodules from Green Bay, Lake Michigan
https://doi.org/10.2138/am-2016-5893
On page 1927 of this issue, Michael Schindler reviews the work of Lee et al. (2016; page 1986 of this issue), who use a range of techniques to determine the nature of adsorption of arsenates on Fe-hydroxide mineral surfaces. As Schindler notes, Fe-hydroxides are common in soils and mining waste, and arsenic oxides have a high affinity for Fe-hydroxide surfaces over a wide range of conditions. Lee et al. are the first to analyze an important contaminant that occurs on a highly relevant mineral surface, and in the process are able to develop a model for the reactions that allow contaminants to be sequestered by mineral surfaces.
Redox variations in the inner solar system with new constraints from vanadium XANES in spinels
https://doi.org/10.2138/am-2016-5638
On page 1928 of this issue, Righter et al. review micro-X-ray absorption near-edge structure (micro-XANES) analyses of V in spinel to estimate oxygen fugacities (fO2) for a wide range of primitive planetary materials. They find that fO2 values are mostly more oxidizing than the sun, and range from 8 log units below to 8 log units above the iron-wustite (IW) buffer. Mars and Earth are more oxidized than chondrites and other meteorite groups, with Earth ranging to much higher fO2 values, presumably because ocean waters and an O-rich atmosphere partially react with subducted materials which are later partially melted. But a more complete understanding awaits samples from Venus and Mercury, which the authors suggest –- with certain legitimacy –- may revolutionize our understanding of planetary evolution.
Experimental simulation of bubble nucleation and magma ascent in basaltic systems: Implications for Stromboli volcano
https://doi.org/10.2138/am-2016-5639
On page 1967 of this issue, Le Gall and Pichavant use new experimental results to show that average bubble sizes (D), and bubble nucleation densities (BND) vary systematically with magma depressurization (ascent) rates. They find that varying ascent rates, and processes of bubble nucleation, growth, coalescence and outgassing, all yield different trajectories in on a chart of D vs. BND. For example, trajectories are negative for ascent rates of <1.5 m/s, but positive for ascent rates of 1.5 to 3.0 m/s. Their results show that D and BND values from Stromboli Volcano can only be reproduced for ascent rates > 1m/s, followed by ascent-independent bubble growth or coalescence. These results provide a new and potentially powerful tool for uncovering the final storage conditions and changes in magma pressure as a system reaches an eruptible state.
The effects of shear deformation on planetesimal core segregation: Results from in-situ X-ray micro-tomography
https://doi.org/10.2138/am-2016-5474
On page 1996 of this issue Todd et al. present experimental results that show that silicate + FeS mixtures, like their purely silicate counterparts, yield very high permeabilities when under a state of shear stress. The authors hypothesize that small planetesimals may momentarily experience high shear stress states when they collide with other planetesimal bodies in the early solar system. In this way, a small planetary body, too cool to support a magma ocean, might still undergo rapid silicate-melt fractionation, so as to form a metallic core within the 3-5 m.y. as is apparently required by W-Hf isotopic data. Presumably, then, collisions are the driving force of core formation on small planetary objects.
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