Editor’s Notes
Total Results: 1697
Volume 100 : October 2015 Issue
Oceanic lavas sampling the high-3He/4He mantle reservoir: Primitive, depleted, or re-enriched?
https://doi.org/10.2138/am-2015-5154
On page 2066 of this issue Garapic et al. present new analysis of high 3He/4He basalts from Hawaii, Samoa, Iceland and Galapagos. Their study shows that these lavas have high Ti contents and enriched Pb-isotopic signatures. The Ti contents are far too high to be obtained by even very low degree partial melting of a primitive, let alone a depleted mantel source. But the high Ti contents and enriched Pb-isotope signatures are plausible if at least a portion of the source regions are mixed with recycled MORB crust. But in their view, the high 3He/4He ratios are not related to the subducted MORB material. Instead, they argue that high 3He/4He is indeed derived from a primitive mantle source, but that such a source exists only in a form that has been mixed with recycled eclogites derived from subducted MORB.
An updated calibration of the plagioclase-liquid hygrometer-thermometer applicable to basalts through rhyolites
https://doi.org/10.2138/am-2015-5232
On page 2172 of this issue, Waters and Lange present a new calibration for a plagioclase-liquid hygrometer. What's especially important in the present update is that the authors present rigorous tests of the new model, using data from five different studies that are not used for calibration, including anhydrous, water-undersaturated, and highly viscous melt compositions. These tests indicate that when temperature is known, water in plagioclase-saturated liquids can be predicted to 0.5 wt%.
Decagonite, Al71Ni24Fe5, a quasicrystal with decagonal symmetry from the Khatyrka CV3 carbonaceous chondrite
https://doi.org/10.2138/am-2015-5423
On page 2340 of this issue, Bindi et al. report the first mineral with decagonal (10-fold rotational) symmetry, decagonite. This represents only the second quasicrystalline mineral to be recognized by the IMA, after icosahedrite, which instead exhibits a fivefold rotational symmetry. Synthetic quasicrystals are often found in three-component alloys, many of which contain Al, the dominant component of both quasicrystalline minerals. The existence of these quasicrystalline minerals through geological times is of strong relevance to discussions concerning the stability of quasicrystals in condensed matter physics.
Earth’s “missing” minerals
https://doi.org/10.2138/am-2015-5417
On page 2344 of this issue Robert Hazen et al. give a statistical analysis from data drawn from large mineralogical databases to predict that at >1500 mineral species remain to be discovered, representing about a quarter of the >6000 species so far described. Their model shows that the as-yet undiscovered minerals are not uniformly distributed across the periodic table, and that sociological aspects of mineral collection, e.g., perceptions of beauty, distinct color, etc., strongly affect the chance of reporting. Obvious implications are that a new emphasis on hypothesis-driven discovery is needed to provide a distribution of known species that more accurately reflects natural variation.
Volume 100 : August - September 2015 Issue
Data, ideas, and the nature of scientific progress
https://doi.org/10.2138/am-2015-Ed1008-94
Recently, it was suggested that data are eternal. On page 1657 of this issue, we consider to what extent data or ideas stand the test of time, and how such views impact what is, or should be, published.
Beyond the equilibrium paradigm: How consideration of kinetics enhances metamorphic interpretation
https://doi.org/10.2138/am-2015-5097
On page 1659 of this issue, Carlson et al. show that many metamorphic systems exhibit substantial evidence for disequilibrium reaction paths and phase assemblages. Carlson et al. acknowledge that much has been learned from using equilibrium states as a benchmark for understanding natural phase assemblages. But they also show, for example, that mapped metamorphic isograds can be displaced from expected positions by 1 or 2 km if equilibrium conditions are assumed. Similarly, solid-state mineral reactions and sequences, and mineral zoning, can lead to erroneous interpretations, unless kinetic factors, rather than equilibrium states, are used to analyze reaction paths. The authors demonstrate how disequilibrium effects can be used to better understand the response of metamorphic rocks to changing P-T conditions during metamorphism.
Field and model constraints on silicic melt segregation by compaction/hindered settling: The role of water and its effect on latent heat release
https://doi.org/10.2138/am-2015-5121
On page 1762 of this issue, Lee et al. present a field study that indicates that support the Bachmann and Bergantz (2004) model of granite melts being extracted by compaction/hindered settling processes. In their model, basaltic magmas differentiate by fractional crystallization to form intermediate composition daughter liquids. These daughter products differentiate further rising upwards together with mafic enclaves, but without significant crystal-liquid separation. Once emplaced into the middle or upper crust, compaction/hindered settling processes eventually lead to the expulsion of a felsic magma, which creates a cap on any undifferentiated intermediate composition parent liquids, or the mafic crystalline residues that form as a consequence of the melt segregation process. What is particularly interesting in their model is that if the expelled felsic melt fraction is small, and especially if some residual melt is retained, the restite portions of the pluton may not be greatly different in composition compared to the intermediate composition parent. This model thus explains why some plutons can be successfully modeled as a liquid, and yet contain the textural characteristics of a cumulate.
Formation of rhyolite at the Okataina Volcanic Complex, New Zealand: New insights from analysis of quartz clusters in plutonic lithics
https://doi.org/10.2138/am-2015-5135
On page 1778 of this issue, Graeter et al. examine the textures and compositions of quartz and feldspars within lithic fragments of the Kaharoa eruption of the Taupo Volcanic Zone, New Zealand. Their work shows that many different quartz grains, with different magmatic and resorption histories, are brought together in preferred orientations and in clusters. They interpret the development of such structures to represent crystal accumulation processes that occur by hindered settling and compaction, which shows that at least some high silica plutonic rocks form synchronously with the eruption of high-silica crystal poor rhyolites, through the settling and compaction of multi-stage quartz and feldspar crystals. Their data indicate total crystallinities of 25-60% at the stage of high silica melt extraction.
Experimental investigation of F, Cl, and OH partitioning between apatite and Fe-rich basaltic melt at 1.0–1.2 GPa and 950–1000 °C
https://doi.org/10.2138/am-2015-5233
On page 1790 of this issue, McCubbin et al. provide new experimental data that bring us a step closer to calibrating a magmatic hygrometer based on apatite-liquid equilibria. Their work shows that a key flaw in current hygrometry methods relate to very uncertain conditions of apatite saturation. The authors instead recommend that hygrometry may be more precisely accomplished through the use of OH-F exchange equilibria, and knowledge of the F content of the bulk system or the parental magma that was saturated with apatite. So in effect, precise apatite hygrometry would appear to require precise melt fluorometry, as a prerequisite.
Synthesis, characterization, and thermodynamics of arsenates forming in the Ca-Fe(III)-As(V)-NO3 system: Implications for the stability of Ca-Fe arsenates
https://doi.org/10.2138/am-2015-5199
On page 1803 of this issue, Paktunc et al. make a potentially important advance in understanding how to mitigate As releases upon weathering of mine waste. Arsenic is commonly held by scorodite, but in Ca-poor environments scorodite can dissolve, releasing As in the process. Paktunc et al. provide experimental evidence to show that in a Ca-rich environment, scordite dissolution can be accompanied by formation of arseniosiderite, which has a very low solubility at pH conditions of 4.5 to 10.5. Thus, the effect of lime neutralization of mine wastes should simultaneously help to stabilize arseniosiderite and minimize As release.
Planar microstructures in zircon from paleo-seismic zones
https://doi.org/10.2138/am-2015-5236
On page 1834 of this issue, Kovaleva et al. investigate zircon microstructures in association with pseudotachylite in the Ivrea-Verbano zone of the southern Alps. Their zircons are from ultramylonites that contain pseudotachylite veins, the latter of which have provided one of the few geologic means to identify a paleoseismic zone. Their work shows that planar fabrics in zircons are not restricted to meteorite impact sites. Planar fabrics can also be developed by earthquakes, provided that differential stresses and strain rates are sufficiently high. This association opens the possibility of directly identifying and dating paleoseismic events in the deep crust.
Ultra-deep subduction of Yematan eclogite in the North Qaidam UHP belt, NW China: Evidence from phengite exsolution in omphacite
https://doi.org/10.2138/am-2015-4899
On page 1848 of this issue, Han et al. examine the compositions and stabilities of phengite and the omphacite precursors from which they form. In many other issues of this journal, we have reported on structures synthesized in isolation at high pressure that may be candidates for containing water or K in the mantle. The obvious problem is that many such structures may be irrelevant to Earths mantle, as such investigations rarely examine the issue of thermodynamic stability within a peridotite or pyroxenite bulk composition. In this work, Fan et al. show that natural omphacite from the Qiadim UHP province of NW China, can contain >1 wt% K2O and as much as 10,000 ppm H2O at pressures exceeding 6 GPa. Subducted eclogites may thus provide a means of recycling water and K2O back into the convective mantle.
Decrease of hydrogen incorporation in forsterite from CO2-H2O-rich kimberlitic liquid
https://doi.org/10.2138/am-2015-5200
On page 1912 of this issue, Baptiste et al. conduct experiments to examine possible hydration reactions between synthetic olivine and a model kimberlite-like liquid. In their experiments olivines equilibrated with such liquids show no evidence of hydration. The authors suggest that the presence of CO2 in their model kimberlite liquid serves to catalyze olivine growth, but also decreases the fugacity of H such that less (or no) H is incorporated into the seed olivine crystals. They conclude that since diffusion of H into olivine appears to be quite limited, that high OH contents in mantle-derived olivines are likely to reflect water contents of the mantle source, rather than a more-shallow liquid-crystal reaction.
Resolving oxygen isotopic disturbance in zircon: A case study from the low δ18O Scourie dikes, NW Scotland
https://doi.org/10.2138/am-2015-5221
On page 1952 of this issue Davies et al. examine zircon crystals from Scourie dikes of NW Scotland. Their use of Raman spectroscopy and trace element analyses by electron microprobe, allow them to identify areas of zircons that have been altered by fluids. In this way, they are able to identify zircon areas that better reflect original magmatic compositions, which in this case reveal very low O18 values. They suggest that such non-destructive techniques should be regularly applied prior to SIMS analysis, so that unaltered zircons may be more easily targeted.
The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions
https://doi.org/10.2138/am-2015-5369
On page 2001 of this issue, Merlini et al. Provide the first single-crystal evidence of a tetrahedrally coordinated carbonate structure (Mg2Fe2C4O13, with C[IV] in C4O13 chains) containing trivalent ion (Fe3+) at P = 135 GPa and T = 2650 K. These conditions approximate the P-T conditions of the top of the D layer. Their experiments, involving the decomposition of magnesite-siderite mixtures, are important in that they reveal the effect of adding Fe to the mineralogy of the deep mantle. In the same experiments they detect a new ferric iron oxide, Fe13O19, with a new stoichiometry and a new structure. Due to their densities, these phases may sink to near the bottom of the mantle. The implications of these results are far-ranging, affecting our understanding of the mineralogy of the deep mantle, the D seismological anomaly, and deep Fe-C cycling and redox-equilbiria.
Volume 100 : July 2015 Issue
Going small: Nanoscale geochronology using atom probe tomography
https://doi.org/10.2138/am-2015-5331
On page 1333 of this issue, Steve Parman provides an overview of Valley et al. (p. 1355), where they use atom probe tomography (APT) to analyze isotope ratios in zircon crystals. As Parman explains, APT allows for nano-scale resolution of isotopic variations. For example, Valley et al. find that Pb and Y in some zircons occur in clusters, and that such inhomogeneity reflects diffusion into amorphous regions of the crystal formed by radiation damage. Such diffusion requires reheating, which can be dated using the isotope ratios of the cluster. APT thus provides a way to extract complex geologic heating and cooling histories from single crystals, as well as a means to confirm closed-system behavior at larger scales, relevant to the interpretation of bulk SIMS analyses.
Bubbles do matter!
https://doi.org/10.2138/am-2015-5288
On page 1335 of this issue, Claudia Cannatelli reviews the work of Wallace et al. (2015; April issue) who demonstrate the significance of shrinkage bubbles in estimating volatile contents from silicate melt inclusions. Their work shows that the vast majority of CO2 (up to 90%; 75% on average) that is contained in melt inclusions prior to quenching is lost to shrinkage bubbles. This post-entrapment lost impacts interpretations of pre-degassing volatile contents, as further means that pressures of entrapment are probably often much greater than may be inferred from H2O-CO2 contents derived from silicate contents alone. These findings may move us toward a resolution of the contrasts in P estimates derived from melt inclusions based on fluid saturation and the oftentimes greater pressures obtained by mineral-melt equilibria.
Assuring the future of mineralogy
https://doi.org/10.2138/am-2015-5164
We have two Invited Centennial Articles this issue: On page 1337 of this issue John Brady considers the future of the Mineral Sciences. Brady notes that while undergraduate enrollments in the geosciences has increased, graduate enrollments and degrees awarded have declined, as has inflation-adjusted total spending in Earth Sciences research. Brady urges members of MSA to better promote Mineralogy, to our students and colleagues, as a matter of advancement in the sciences, and as a mater of public good. Then on page 1341 of this issue, Kirkpatrick et al. provide a review of NMR and computation techniques that together reveal how cations and organic matter interact with fluids and mineral surfaces. Their review shows how computational methods provide valuable molecular-scale interpretations of NMR observations.
Pink color in Type I diamonds: Is deformation twinning the cause?
https://doi.org/10.2138/am-2015-5044
On page 1518 of this issue, Howell et al. examine the coloration of gemologically valuable pink Type 1 (N-bearing) diamonds. They note that pink diamonds have been found in association with deformation twins. So a key question is whether the twin-planes themselves are responsible for the diamonds hue. Their EBSD analyses of Group 1 (low N) and Group 2 (high N) pink diamonds, however, reveals a lack of twin structures in the former. These authors entertain the possibility that the pink Group 1 crystals were originally twinned, but then were later untwinned. If so, this implies that their pink colors results from defects that originate by the process of twinning, rather than from the twins themselves. Brown diamonds also lack twins and their color banding is thought to be generated by slip planes. Their analysis of N contents further constrains residence times, albeit quite broadly, since diffusion is highly sensitive to T. For example, Group 1 N concentrations are low, but highly aggregated, implying either long residence times (> 3 Ga) at 1200 oC or shorter residence times (200 Ma) at 1300 oC. In either thermal scenario, mantle residence times of Group 2 diamonds (higher N, but less aggregated) is considerably shorter, at <1.5 Ga at 1100 oC or <30 Ma at 1200 oC.
Silica polymorphs in lunar granite: Implications for granite petrogenesis on the Moon
https://doi.org/10.2138/am-2015-5058
On page 1533 of this issue, Seddio et al. examine the silica polymorphs from granitic samples from the Apollo 14 mission. Their investigation of hackle patterns is suggestive of decrease in molar volume on cooling, and hence the initial precipitation of a high molar volume polymorph. Their volume contraction calculations lead them to surmise that silica initially crystallized as tridymite or cristobalite, which appears to be confirmed by the presence of up to 0.7 wt% TiO2, which is more compatible in the high T polymorphs. These authors conclude that these granitic samples formed as volcanic rocks, and that similarly formed rock types may explain felsic domes elsewhere on the Lunar surface.
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