Editor’s Notes
Total Results: 1697
Volume 102 : December 2017 Issue
Eruption style and crystal size distributions: Crystallization of groundmass nanolites in the 2011 Shinmoedake eruption
https://doi.org/10.2138/am-2017-6052CCBYNCND
On page 2367 of the December 2017 (vol. 102, 12) issue, Mujin et al. investigate microlites and nanometer-scale crystals that they term nanolites, from the 2011 eruption at Shinmoedake volcano, whose crystal size distributions may reveal fragmentation and re-welding of magmatic materials during eruption. Their FE-SEM and TEM study allow them to study crystals at a stunningly small scale and identify gaps in growth patterns that would appear to record magmatic events at very, very short time scales. In their model, they attempt to describe nano-scale textural and mineralogical patterns as pressure-time paths, that in turn affect magma dehydration and temperature changes, which can affect (spur or inhibit) crystal nucleation and growth.
Trace element zoning in hornblende: Tracking and modeling the crystallization of a calc-alkaline arc pluton
https://doi.org/10.2138/am-2017-6063
On page 2390 of the December 2017 (vol. 102, 12) issue Barnes et al. present new major and trace element data involving zoning profiles of amphiboles from arc-related plutons. They find core-to-rim patterns that record monotonic cooling, and associated changes in trace elements that record the progressive saturation and in some cases the later dissolution, of both major and accessory phases. The key advance is the recognition that amphiboles are saturated over a wide enough T range to record the saturation of a wide range of phases, including zircon and biotite. And because of its complexity, amphiboles provide information about the P-T conditions and the changing magma compositions from which various minerals form as a pluton or batholith is assembled.
Mineralogical, geochemical, and textural indicators of crystal accumulation in the Adamello Batholith (Northern Italy)
https://doi.org/10.2138/am-2017-6026
On page 2467 of the December 2017 (vol. 102, 12) issue, Fiedrich et al. use a range of methods, including whole rock and mineral compositions, mass balance, and cathodoluminesence imaging, to quantify that amount of melt that is trapped between crystals, in various units of the Adamello batholith of northern Italy. Their goal is to calculate “crystallized liquid fractions” or CLFs, which should approximate the melt that is trapped when magmas reach a critical crystallinity so as to form a rigid (melt-trapping) framework. They find that the amounts of trapped liquid vary widely, ranging from 7-70%, and tend to be lowest for systems with strong CPO, which indicates that syn-magmatic deformation may be an efficient mechanism by which melts are expelled from an otherwise rigid crystal-liquid mush.
A qualitative and quantitative investigation of partitioning and local structure of arsenate in barite lattice during coprecipitation of barium, sulfate, and arsenate
https://doi.org/10.2138/am-2017-6148
On page 2512 of the December 2017 (vol. 102, 12) issue, Ma et al. investigate the conditions under which As(V) partitions into Barite. They find that As may be readily incorporated into barite and at pH>5 can from a stable Ba-Arsenate phase. Their work illustrates the conditions under which aqueous As may be fixed to a crystalline phase, depending upon the concentrations and pH conditions, that may be of crucial aid to hazard assessments and remediation efforts.
Volume 102 : November 2017 Issue
Rutile: A novel recorder of high-ƒO2fluids in subduction zones
https://doi.org/10.2138/am-2017-6261
On page 2153 of this issue, Alicia Cruz-Uribe provides an overview of Guo et al. (2017; p. 2268 of this issue), who investigate rutile grains that form at the rims of Fe-Ti oxides in greenschist facies metamorphic rocks. Associated mineral and inferred reactions indicate that rutile forms by the action of highly oxidized fluids, approaching nearly 4 log units above QFM, during retrograde metamorphism. As Cruz-Uribe notes, this study illustrates a how rutile may be a record of elevated fO2 fluids at subduction zones. The possibility is that rutile is forming as the rocks intercept high fO2 fluids that are driven off the slab, or are otherwise connected in some way to subduction zone magmatism. These rutile grains, though, would not be responsible for the high field strength element signatures of arc magmas.
Granites and rhyolites: Messages from Hong Kong, courtesy of zircon
https://doi.org/10.2138/am-2017-6260
On page 2154 of this issue, Calvin Miller reviews the granite controversy of the prior century, and new questions that have evolved since. The context involves new findings by Tang et al. (2017; p. 2190 of this issue) who use zircon compositions from volcanic and intrusive felsic rocks from Hong Kong, to test ideas of volcano-plutonic connections. Their work indicates that at least at Hong Kong, plutons are not the residues of felsic eruptions. Instead, felsic volcanic chambers are nearly completely evacuated, and intrusive rocks are evolutionary analogs intruded when conditions for eruption were unfavorable. Though closely related in space and time, the felsic rocks of this region are not co-magmatic, but generated independently and erupted, or not, as structural conditions allow.
Do Fe-Ti-oxide magmas exist? Probably not!
https://doi.org/10.2138/am-2017-6091
"On page 2157 of this issue, Lindsley and Epler present new experimental data to re-examine the genesis of massive Fe-Ti oxide bodies that occur mainly in association with anorthosites, often as dikes. At issue is whether such oxide-rich bodies are crystalline residues of a silicate melt, or were melts in and of themselves, possibly formed along an oxide-silicate melt solvus (as an immiscible melt). These authors conclude that such oxide bodies have bulk compositions that cannot occur as melts at geologically reasonable temperatures. More likely the oxide-rich bodies were intruded as crystal-rich mushes, perhaps lubricated by small amounts of a silicate melt. They also imply that some ""jotunite""-like rocks (having low Si and high Fe and Ti) are these very same lubricating silicate melts, apparently acquiring their low Si and high Fe and Ti by dissolving some of their otherwise mechanically associated Fe-Ti oxides."
Calcium (Ti,Zr) hexaorthophosphate bioceramics for electrically stimulated biomedical implant devices: A position paper
https://doi.org/10.2138/am-2017-6165
On page 2170 of this issue, Robert Heimann reviews a class of orthophosphates with NASICON structure, which nicely illustrate how fundamental concepts of mineralogy are pivotal in the search for superior bioceramics used in a variety of medical applications. Heimann proposes that Ca(Ti,Zr) hexaorthophosphates have bone growth-mediating characteristics that are particularly well suited for use as coatings on metallic implants (in knee or hip replacements) so as to aid bio-integration of the foreign materials, and, based on the solid-state ionic conductivity of these compounds, proposes a new device that is expected to electrically stimulate bone growth. While the biological uses are emphasized, the author suggests that these same structures should also be useful for storage of radioactive waste or as electrodes in molten Na-ion batteries, among other applications.
Parameterized lattice strain models for REE partitioning between amphibole and silicate melt
https://doi.org/10.2138/am-2017-6110
On page 2254 of this issue, Shimizu et al. develop parameterized lattice-strain models predictive of the partitioning of REE into amphibole. They find that REE partition coefficients are highly sensitive to the amphibole major element compositions--with order of magnitude variations accompanying arc magma genesis. Their new mineral-composition model allows one to reconstruct equilibrium liquid REE concentrations from amphibole compositions alone, provided that temperature is known, and a new thermometer is presented as well. Application of these new models reveals a greater role for amphibole fractionation of arc magmas, relative to clinopyroxene, and that REE-rich amphiboles are likely records of particularly low-T amphibole crystallization.
Dolomite dissociation indicates ultra-deep (>km) subduction of a garnet-bearing dunite block (the Sulu UHP terrane)
https://doi.org/10.2138/am-2017-5982
On page 2295 of this issue, Su et al. describe magnesite + aragonite intergrowths within the Sulu UHP terrane of eastern China, that form as breakdown products of dolomite, at >5 GPa. The authors infer that the precursor dolomite formed from a metasomatic melt, perhaps within the uppermost part of the mantle. In any case, because the breakdown reaction of dolomite is not sensitive to T, it seems that the carbonates were indeed formed at very high pressure, and the lack a retrograde reaction of magnesite + aragonite back to dolomite further indicates that the exhumation was rapid or dry or both. The authors infer that other dunites may harbor the very same clues of a UHP history, if similar textures and assemblages may be found.
Volume 102 : October 2017 Issue
Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland
https://doi.org/10.2138/am-2017-6015ccby
On page 2007 of this issue, Neave et al. use the textures and zoning patterns of plagioclase crystals to examine the disaggregation of a liquid-crystal mush prior to eruption. They find that most microcrysts grow just after macrocrysts of a mush are entrained on a final path to eruption. The result is that the seemingly homogeneously mixed magmas of the Laki eruptions are the result of multiple disaggregation and mixing events. The authors calculate, for example, that the 15 km3 of erupted material of the 1783-1784 extrusives were assembled in batches of eruptible magma no greater than about 1.5-2 km3, with magmas being assembled from mush systems about 10 days prior to eruption. These multiple mixing events also yielded similar magmas, perhaps indicating that the eruption triggering mechanisms are threshold dependent, although those thresholds remain undiscovered.
A new hydrothermal moissanite cell apparatus for optical in-situ observations at high pressure and high temperature, with applications to bubble nucleation in silicate melts
https://doi.org/10.2138/am-2017-6093
On page 2022 of this issue Masotta and Keppler present a new experimental assembly that allows bubble nucleation and growth experiments at elevated P-T conditions (up to 850oC and 2 kbar), along controlled P-T-time paths. Their preliminary experiments, on a haplogranite (or simple, synthetic granite) system appears to show that bubble nucleation in a closed system will end when the average distance between bubbles is equal or less than the mean diffusion distance of water molecules. (But then how does a water molecule at a distance greater than the mean know that system-wide the mean has been reached?) A potentially significant result is that magma fragmentation (e.g., vapor phases are >70% by volume) appears to occur at 0.2 kbar, regardless of decompression rate. In any case, their new experimental assembly should aid investigations of late-stage volcanic processes.
Experimental and thermodynamic investigations on the stability of Mg14Si5O24 anhydrous phase B with relevance to Mg2SiO4 forsterite, wadsleyite, and ringwoodite
https://doi.org/10.2138/am-2017-6115
On page 2032 of this issue, Kojitani et al. experimentally investigate the stability of Mg14Si5O24, the so-called “anhydrous phase B” found in some high P experiments in simple silicate systems. In this work, Kojitani et al. suggest that this phase can form, by reaction of forsterite and periclase, at pressures that might explain the seismic X-discontinuity, which occurs at depths of 250-350 km. Their work confirms earlier suggestions that the reaction to form anhydrous phase B should only occur in the presence of fluids, when such fluids dissolve Si so as to leave a relatively Si depleted residue. Such conditions furthermore allow this phase to be stable at lower pressures. If this hypothesis is valid, anhydrous phase B might then affect the fluid-rock and melt-rock partitioning of trace elements in subduction zones.
Model for the origin, ascent, and eruption of lunar picritic magmas
https://doi.org/10.2138/am-2017-5994ccbyncnd
On page 2045 of this issue, Rutherford et al. use the volatile contents of lunar orange picritic glasses to reconstruct their eruption history. That history begins with melt segregation from a melt-rich mantle source at 500 km and vapor (CO-rich, but including S and H) saturation at somewhere between 4 and 50 km below the surface (or possibly as deep as 500 km in some scenarios), which then accelerates magma ascent. By the time these magmas reach depths of 300-600 m, the residual magmas have lost nearly all their volatiles to the gas phase, and in the new model, these depths would also represent the point at which the magmas became fragmented (having >70% bubbles by volume).
Previously unknown mineral-nanomineral relationships with important environmental consequences: The case of chromium release from dissolving silicate minerals
https://doi.org/10.2138/am-2017-6170
On page 2142 of this issue, Schindler et al. identify a potentially new source of Cr in rock samples: Chromium (IIII) occurs as inclusions of chromite nanoparticles within silicates rather than being incorporated into their structures. Transport models of Cr in the environment may need to take into account the release and transport of these nanoparticles during weathering rather than transport of chromium as aqueous species, which could have important implications for environmental modelling and risk assessments in chromium-rich regions.
Making a fine-scale ruler for oxide inclusions
https://doi.org/10.2138/am-2017-6223
On page 1969 of this issue, Dongzhou Zhang provides an overview of Uenver-Thiele et al. (page 2054 of this issue) who explore the phase relationships of starting compositions having MgFe2O4 and Mg0.5Fe2+0.5Fe23+O4 stoichiometries. These experiments help delimit the precipitation conditions of magnetite inclusions in diamonds, by taking advantage of textural clues that might indicate the presence of certain precursor phases, such as ferropericlase or one of a few “unconventional oxides” that may be stable at high pressure. Their experimental work also shows that otherwise simple oxides might exhibit rather great stoichiometric variety at elevated P and T.
Volume 102 : September 2017 Issue
Looking for “missing” nitrogen in the deep Earth
https://doi.org/10.2138/am-2017-6218
On page 1769 of this issue, Zedgenizov and Litasov review Kaminsky and Wirth’s paper (in the August issue) regarding new data on nitride inclusions in diamonds. As noted in a prior summary, this work describes phases that are included in so-called “superdeep” diamonds; the nitrides inclusions may be derived by contamination of the diamond source region by metallic material from the core. Zedgenizov and Litasov here suggest that the contamination of the diamond source might have occurred at pressure conditions as low as 5-10 GPa, well short of the 120 GPa near the core-mantle boundary. But a metallic source for these inclusions is still implied; this leaves open the need for trace element and isotopic studies of such inclusions, to determine if any might represent mantle traces of an otherwise long-segregated core.
Element mobility during regional metamorphism in crustal and subduction zone environments with a focus on the rare earth elements (REE)
https://doi.org/10.2138/am-2017-6130
On page 1796 of this issue, Jay Ague examines the dissolution and transport of rare earths and other elements in the fluids generated in metamorphic systems. He finds that despite their trivalent character, REE are indeed quite mobile in high flux environments (more so than other high field strength elements) and are fractionated in the process; in contrast Th and Zr are relatively immobile. REE mobility appears to be enhanced in “extreme” environments, such as hydrothermal systems connected to magma emplacement or any system where supercritical fluids are developed. Ague proposes a diverse array of interesting and important implications, including how high field strength elements may be sufficiently mobile so as to sometimes negate certain tectonic discrimination diagrams; or using mass balance considerations to distinguish how CO2 is released by metamorphic reactions.
Subsolidus hydrogen partitioning between nominally anhydrous minerals in garnet-bearing peridotite
https://doi.org/10.2138/am-2017-6089
On page 1822 of this issue, Demouchy et al. present new data on how H is partitioned amongst the nominally anhydrous minerals in a garnet-lherzolite, experimentally equilibrated at 1100oC at 3 GPa. A key finding is that their partition coefficients for the pyroxenes relative to olivine tend to be much lower than those derived from studies where melt is present. The authors suggest that in some studies, high values for Pyx/Ol partition coefficients may be related to H loss in Ol. But clearly, new studies that show co-variations of H in pyroxenes and olivine are needed to better understand how water is stored, at least in the sub-solidus mantle, and perhaps under partial melting conditions as well.
Phase relations and formation of K-bearing Al-10 Å phase in the MORB+H2O system: Implications for H2O- and K-cycles in subduction zones
https://doi.org/10.2138/am-2017-6025
On page 1922 of this issue, Tao et al. compare the structures and compositions of a Al-10 Å phase and the K-bearing micas muscovite and phlogopite. Their worked is sparked by studies of the Mg-bearing 10 Å phase, found in many hydrous high pressure experimental studies, and thought to be an important carrier of water into the deep mantle via subduction. This new work indicates that an analogous K-bearing, Al-10 Å has at definite micro-solid solution relationships with more common K-bearing phyllosilicates. The results are important for two reasons. First the solid solution relationships provide a possible link between high- and low-P phase assemblages, and these linkages may act as steps on a downward ladder, so that water might eventually be partitioned into a high-P 10 Å phase, of one sort or another. The authors also point out that if this K-bearing downward-pointed ladder is operative, then the global K and water cycles are linked.
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