Editor’s Notes
Total Results: 1697
Volume 101 : May 2016 Issue
(FeH)1–xTixO2: A new water carrier to the mantle transition zone
https://doi.org/10.2138/am-2016-5643
On page 1021 of this issue, Tatsuhiko Kawamoto provides an overview of new work on hydrated oxides, by Nishihara and Matsukage (April issue), which may be stable at high pressures. Kawamoto notes that we have now identified a number of different phases that are potential carriers to of water from the surface to depths of at least 250 km. But we expect that amphiboles and phyllosilicates breakdown by dehydration reactions that release water to the mantle wedge, and so drive arc magmatism. Nishihara and Matsukage (2016) show that Fe-Ti oxides are stable at greater P-T conditions, ranging to 15 GPa and 1500oC, especially for those oxides rich in Ti. As Kawamoto implies, Fe-Ti oxides might actually take on much of the water released during lawsonite breakdown, decreasing the amounts of water going into the mantle wedge, and so increasing that delivered to the mantle transition zone.
Dissecting a volcano
https://doi.org/10.2138/am-2016-5705
On page 1023 of this issue, Corliss Kin I Sio discusses new results from Viccaro et al. (2016), who show how deep-seated intrusions may have triggered the March–April 2010 eruption at Eyjafjallajkull. These authors measured diffusion time scales that record episodes of magma mixing. Interestingly, they find that olivine samples with higher Fo content cores exhibit longer inferred time scales, based on Fe-Mg diffusion at their rims. Their highest Fo olivine samples (Fo88) yield times scales of about 30 days, while their lowest Fo content core compositions (Fo77) have rims that yield times scales of about two weeks. Viccaro et al. infer that the higher Fo core compositions are derived from greater depths in the volcanic conduit, and so the longer diffusion time scales represent the greater distances that such olivine samples had to travel. Their idea is that the March-April 2010 events were triggered by mantle-derived melts that were fed into the lower crust, with recharge cascading upwards. There is a nice attempt to link magma storage zones using seismic events, but as Corliss points out, the deeper seismic events occur later, not earlier, in the eruptive sequence.
W-WO joins the deep Earth electrochemical series
https://doi.org/10.2138/am-2016-5587
On page 1025 of this issue, Abby Kavner reviews our understanding of metal-metal oxide systems, and the contribution of Shofner et al. (2016) in providing the energetics of W-WO. In this work it is shown that W becomes less siderophile as pressure-temperature conditions approach the core-mantle boundary. Kavner plots this behavior as a function of oxygen fugacity, showing that W-WO equilibrium is established at nearly the same fO2 conditions as Fe-FeO at CMB conditions. Shofner et al. also find a new WO2 phase that is stable at high pressures. It is not clear that such a phase is stable, since W does not exist in isolation in the mantle, but in equilibrium with various phases that can dissolve W. So as noted by Kavner, there is still much work to be done to understand how nominally siderophile elements might be partitioned between natural phases at high P and T conditions.
Time’s arrow in the trees of life and minerals
https://doi.org/10.2138/am-2016-5419
On page 1027 of this issue, Peter Heaney discusses two common misconceptions of mineralogy: one, that mineral classification schemes lack a Linnean-like taxonomy, when indeed the chemical systems of Berzelius and Dana exhibit a Linnean tree-like structure; two, that there is not an intrinsic chronology to minerals themselves, an idea now apparently overturned by the mineral evolution work of Robert Hazen and collaborators. Heaney further argues, however, that only the chemical systems of Berzelius and Dana, not their structural or form-based predecessors, could provide a mineral classification tree that could also act as a map of evolutionary chronology. Only the chemical or electromagnetic classifications could provide an effective link to mineral evolution. Heaney uses phosphates as an example of the cladistic nature of a mineral classification tree—a tree that lacks extinction, but involves increasing competition for elements on a planet with increasingly specialized environments. These arguments and the Hazen hypotheses provide a clear path for mineralogists describing new species, as each discovery provides a possible test of ideas that now appear to underpin all of mineralogy.
A century of mineral structures: How well do we know them?
https://doi.org/10.2138/am-2016-5473
On page 1036 of this issue, Angel and Nestola review this history of investigations into crystal structures. Although Heaney suggests that the Berzelius and Dana systems are more useful for classification, Angel and Nestola provide a reminder that mineral structures are fundamental to understanding physical properties and bonding characteristics. Moreover, new advancements in structure analysis provide considerable promise for better understanding everything from geothermometery to deep-mantle water contents. The article focuses however, on the fascinating history of crystallography. The development of X-ray diffraction methods at the beginning of the 20th century was truly one of the most important scientific developments in all the sciences. Through the 19th century Daltons atomic hypothesis was found to be increasingly useful in chemistry, and especially thermodynamics, but it was still no more than a hypothesis, that is until X-ray diffraction demonstrated that atoms were real. The authors further note that analytical methods have sufficiently advanced so that structural interpretations are no longer limited by data, but rather by the assumptions used in common refinement models, e.g., that atoms diffract as un-bonded spherical shells. Development of the theory of bonding, published elsewhere on the pages of Am Min, will be a critical source of future advances.
Petrographic investigation of smithing slag of the Hellenistic to Byzantine city of Sagalassos (SW-Turkey)
https://doi.org/10.2138/am-2016-5390
On page 1072 of this issue, Eekelers et al. conduct a petrographic study of iron slag from blacksmith (smithing) sites that date from the 1st to 7th century C.E. The sites are within the ancient city Sagalassos, in southwest Turkey, which is an especially important location as it contains a near-continuous record of technological development of Roman smithing techniques. These authors discovered textural contrasts between two types of smithing techniques—contrasts that would go undetected using only geochemical means. In one smithing process, metals are deformed only at high-temperatures, to make items such as hammers and anvils. But in others, metals are deformed over a range of temperatures and utilize lime or other flux agents to limit oxidation; this latter process produces cutting tools or more finely shaped instruments. The authors also provide a textural index that can be used to decipher which iron deposits in the archeological record have undergone human intervention.
Oxygen isotope thermometry reveals high magmatic temperatures and short residence times in Yellowstone and other hot-dry rhyolites compared to cold-wet systems
https://doi.org/10.2138/am-2016-5591
"On page 1222 of this issue, Loewen and Bindeman use oxygen isotope thermometry to compare temperature estimates from nominally hot and cold rhyolites. The ""hot"" rhyolites are non-water saturated samples from Yellowstone and Iceland, while the ""cold"" rhyolites are from the Bishop Tuff and elsewhere. The authors employ a single-crystal laser fluorination technique that they find particularly useful for rhyolites, which quench rapidly, at relatively low temperatures compared to basalts. They find that minerals, including near-liquids phases such as magnetite and Cpx, from wet rhyolite magmas rarely yield temperatures that exceed 800oC. Such results are consistent with zircon age dates that indicate prolonged storage at near-solidus conditions prior to eruption allowing for equilibration of oxygen isotope values between minerals. In contrast, hotter rhyolites preserve near-liquidus magnetite and clinopyroxene temperatures at 900-1050oC; zircons record cooler temperatures, but still much higher than for the wetter rhyolites, at >800oC. Loewen and Bindeman show, based on diffusion characteristics, that these hotter systems require pre-eruption residence times of <1000 y, vastly less than the ca. 105 y time spans commonly inferred for highly felsic systems."
Volume 101 : April 2016 Issue
Glass structure, melt structure, and dynamics: Some concepts for petrology
https://doi.org/10.2138/am-2016-5386
On page 753 of this issue, Jonathan Stebbins reviews the structural properties of aluminosilicate melts, with a view to re-engaging petrologists with fundamental thermodynamic properties. Stebbins warns, perhaps rightfully, that the success and widespread use of various petrologic models may dull our curiosity of physio-chemical controls on phase equilibria and diffusion. But an understanding of such can be extraordinarily useful. For example, Stebbins shows how the solution of Al into a silicate melt converts concentrated negative charges on non-bridging oxygen atoms to more dispersed charges on bridging oxygen atoms; this leads to less clustering of atomic units, with the anticipated result that liquid immiscibility is inhibited by Al-enrichment, as observed by Charlier et al. (2013). In another example, Stebbins illustrates how Si-activity is affected by the mean field strength of dissolved network modifier cations, i.e., Si activity should decrease as field strength increases (e.g., K+ < Na+ < Ca2+ < Mg2+)—a result that as Stebbins shows is mimicked by a(Si4O8) values obtained from MELTs (Ghiorso et al. 2002). These kinds of insights are assuredly useful, not only in guiding future experiments, or predicting behavior in unexplored systems, but also for informing us when our petrologic models are leading us astray.
The validity of plagioclase-melt geothermometry for degassing-driven magma crystallization
https://doi.org/10.2138/am-2016-5314
On page 769 of this issue, Humphreys et al. compare two plagioclase-melt thermometers and the MELTS model, when applied to systems that are known to be at disequilibrium. Building on prior work on forced disequilibrium systems (e.g., decompression or cooling rate experiments), Humphreys et al. demonstrate that thermometers calibrated at equilibrium conditions yield systematically high T estimates at disequilibrium. Their suggestion is that T estimates in natural systems must be considered maxima in the absence of some check on equilibrium. A better test still would be to compare plagioclase + liquid T estimates to an independent thermometer, which should yield T estimates that, if are not the same (for minerals that co-precipitate), then are at least qualitatively consistent with known phase equilibria. In spite of such challenges, however, Humphreys et al. suggest that heating of magmas, through the latent heat of crystallization during decompression (with H2O loss) crystal growth may be a common phenomenon in erupted arc magmas.
A petrological assessment of diamond as a recorder of the mantle nitrogen cycle
https://doi.org/10.2138/am-2016-5464
On page 780 of this issue, Mikhail and Howell examine N speciation and the incorporation of N into diamond. A key observation is that N, having a similar size and charge as C, is incorporated mono-atomically in diamond, rather than as N2 or NH4+, the latter two of which are the most common N species in the mantle. They conclude that it is at least possible that N-rich diamonds may precipitate from a N-poor environment, if the form of N that is available is a monatomic species. Similarly, it is at least possible that N-poor diamonds may form from a N-rich environment, when that environment is rich in N2 and/or NH4+. This implies that N-contents in diamond do not serve as a simple proxy for N contents in the mantle as a whole, and that to understand the N cycle in the mantle, an understanding of more complex N-bearing equilibria is required.
Contrasting P-T paths within the Barchi-Kol UHP terrain (Kokchetav Complex): Implications for subduction and exhumation of continental crust
https://doi.org/10.2138/am-2016-5454
On page 788 of this issue, Stepanov et al. examine P-T paths of metamorphism in the Barchi-Kol terrain of Kazakhstan. They find that peak metamorphism, near 50 kbar and 1000 C, is recorded by a diamond-bearing gneiss, while other samples, from tens to thousands of meters away, yield different and lower peak P-T conditions (24-30 kbar, 710-950 C), comparable to those determined for other samples from the metamorphic belt. More importantly, though, these samples were metamorphosed within a <7 Ma time interval. Thus, their prograde-to-peak-to-retrogade paths provide something of a snapshot of the thermal structure of a subducted and resurrected slab. The authors find a low-T, high-P path typical of subducted slabs, but they also find a near-isobaric heating at 24 kbar (ca. 80 km), that they attribute to the slab coming in contact with overlying asthenosphere. As discussed by these authors, subduction-related metamorphic terranes that do not yield abrupt heating may indicate the subduction of a thicker lithosphere (or perhaps shallow-angle subduction).
Experimental formation of pyroxenite veins by reactions between olivine and Si, Al, Ca, Na, and Cl-rich fluids at 800 ºC and 800 MPa: Implications for fluid metasomatism in the mantle wedge
https://doi.org/10.2138/am-2016-5441
On page 808 of this issue Grant et al. conduct experiments where slab-like fluids are reacted with olivine. They find that such fluids yield orthyopyroxene +/- clinopyroxene veins within the olivine hosts. This work shows that metasomatic pyroxenite veins can form over a very wide range of pressures, between temperatures of 750-950 C, and that such reactions explain orthopyroxenite veins found in mantle xenoliths. In this interpretation, these veins may yield our most direct evidence for the evolution and metasomatic effects of subducted-slab inputs into the mantle wedge above subduction zones.
Crystal/melt partitioning of water and other volatiles during the near-solidus melting of mantle peridotite: Comparisons with non-volatile incompatible elements and implications for the generation of intraplate magmatism
https://doi.org/10.2138/am-2016-5437
On page 876 of this issue, Adam et al. conduct experiments on the mineral/melt partitioning of water for mantle minerals. They find that DH2O is dependent on H2O concentrations in melts as well as tetrahedral Al in pyroxenes. They also find that the partitioning ratio DH2O/DCe varies inversely with the size of M2 site radii in pyroxenes. Since M2 site radii should decrease with increased pressure, then Ce should be relatively more enriched in partial melts formed at greater depths, while H2O would be enriched over Ce at shallower depths of partial melting. These authors conclude that such results support the model of Michael (1995), who found regional similarities between proximal MORB and OIB, but global contrasts between various regions. The authors concur that water contents and incompatible elements in OIB reflect a link between OIB sources and small degree partial-melts derived from local MORB-source materials.
Carbon mineral ecology: Predicting the undiscovered minerals of carbon
https://doi.org/10.2138/am-2016-5546
On page 889 of this issue, Hazen et al. use the statistical Large Number of Rare Events (LNRE) model to estimate the total number of terrestrial C-bearing minerals. They find that at least 548 such minerals should be present, leaving about 150 yet to be discovered; the authors provide 432 possible species that might eventually be described. Such estimates serve more than idle curiosity. Earth-like exo-planets might be characterized by similar assemblages, which may in turn impact biological evolution. Such predictions also play into the question of whether Earth-like planets form by chance, or are a necessary consequence of early solar system assimilative processes and planetary evolution (if the right bulk chemistry is given). Is Earth mineralogically unique? If so, might it be unique in important or merely trivial ways? The Hazen et al. hypothesis might also implicitly predict the rates at which such minerals may be discovered. These vital questions are in the hands of those mineralogists and crystallographers who study rare and new species—and whose efforts will undoubtedly soon grace the pages of American Mineralogist.
Volume 101 : March 2016 Issue
The most-cited journal in mineralogy and petrology (and what scientists can learn from baseball)
https://doi.org/10.2138/am-2016-Ed10115
"We would like to re-advertise our Editorial on page 497 (on why H-index, and impact factor dont mean what you think they mean, and what Scientists can learn from Baseball), with apologies to the Journal of Crystal Growth. This and a few other crystallographic journals received higher total citations in 2014. We maintain our broader argument, though, that commonly used indices, garnered without context, yield over-simplified, if not meaningless proxies of scientific quality. On page 497 of this issue, we present an Editorial titled, The most-cited journal in mineralogy and petrology (and what scientists can learn from baseball).
Fluids in the crust during regional metamorphism: Forty years in the Waterville limestone
https://doi.org/10.2138/am-2016-5118
On page 500 of this issue, John Ferry presents an overview of fluid-rock interactions, gained from four decades of research on the Waterville Limestone in Maine, U.S.A. The work provides a fascinating perspective on the lifecycle of geologic hypotheses. In this case, the outcrops in question have been central to the recognition of how fluid compositions are controlled by metamorphic devolatilization reactions, and also to the quantification of fluid flow as time-integrated fluxes, which can be determined by estimating reaction progress in a suite of metamorphic rocks. Early studies though, led to a conundrum: hydrodynamic models indicate vertical, upward fluid flow, but petrologic observations indicated fluid flow in a horizontal, up-temperature direction. Further study of the Waterville Limestone, however, revealed that the observed spatial distribution of reaction progress can be reconciled with upward, down-T flow by explicit consideration of the interplay among solid solution, spatially variable rock composition, and a kilometer-scale gradient in fluid composition during coupled metamorphic fluid flow and mineral reaction. As noted by Ferry, problems related to the up-T-flow-direction model might not have been as readily resolved, had not the problems been identified within the very system that inspired the model in the first place.
Remanent magnetization, magnetic coupling, and interface ionic configurations of intergrown rhombohedral and cubic Fe-Ti oxides: A short survey
https://doi.org/10.2138/am-2016-5519
"On page 518 of this issue, Robinson et al. discuss magnetic substructures that form at the interfaces of cubic and rhombohedral Fe-Ti oxides. The key interfaces are contacts of exsolution lamellae, which deleted yield ""intense"" remanent magnetization in phases that might otherwise be only weakly magnetic. Such exsolution may result from the well-known oxy-exsolution of ilmenite from titanomagnetite, or reduction exsolution of magnetite from ferri-ilmenite or hematite documented here. Their TEM work shows that the magnetically active regions are focused on compositionally distinct octahedral layers that join the cubic and rhombohedral phases. The authors present mineralogical models to explain these sources of magnetism, with an emphasis on how the interfacial octahedral layers attach to adjacent tetrahedral or octahedral-tetrahedral layers of the adjacent crystals. The implication of this work is that these lamellar structures are the source of significant remanent magnetism in some rock types."
Are covalent bonds really directed?
https://doi.org/10.2138/am-2016-5299
On page 531 of this issue, I. David Brown asks whether bonds that are strongly covalent are necessarily strongly directional. Central to the analysis is flux theory of bonding, which describes bonding strength as a function of charge divided by coordination number and posits that in a state of static equilibrium, the highest possible symmetry state of a chemical species is preferred (having the lowest potential energy). Brown shows a series of simple equations that allow bond lengths and bond angles to be determined, in the absence of any (nominally artificial) distinction between covalent or ionic bond types. While the model allows one to predict a chemical geometry from knowledge of the chemical formula alone, a remaining challenge is to determine a larger three-dimensional (macroscopic) structure from the individual chemical geometric shapes. The buildup of a 3D structure may cause bond-lengths and bond-angles to diverge from their ideal calculated values, and the induced strains in the molecular building blocks may be relaxed by lowering the symmetry of the macroscopic structure.
Constraints on the early delivery and fractionation of Earth’s major volatiles from C/H, C/N, and C/S ratios
https://doi.org/10.2138/am-2016-5452
"On page 540 of this issue, Marc Hirschmann examines bulk silicate Earth (BSE) ratios of C/H, C/N, and C/S as a signal of combined core-formation, mantle differentiation, late veneer processes, and atmospheric ""blow off"". Hirschmann shows that it is much more insightful to deal with the three ratios simultaneously; this suggested approach stems in part from their contrasting values compared to carbonaceous chondrite ratios: C/H is lower, C/N is higher, and C/S is nearly equal to that of carbonaceous chondrites. As Hirschmann explains, high C/N and chondritic C/S make it less likely that a low C/H can due to C being lost to space or partitioned into the core. Instead it is suggested that the non-chondritic C/H and C/N ratios of BSE observed today reflect in part the compositions of the materials that accreted to form Earth, rather than reflecting post-accretion, post-late veneer fractionation processes of originally chondritic ratios. And the modern chondritic ratio of BSE C/S? Possibly a chance result of multiple processes."
Volume 101 : February 2016 Issue
The deep continental crust has a larger Mg isotopic variation than previously thought
https://doi.org/10.2138/am-2016-5483
On page 241 of this issue, Zhang provides a review of Yang et al. (page 243 of this issue, also in Invited Centennial Article) who show that parts of the lower and middle crust exhibit rather remarkable ranges in Mg isotope ratios, despite the fact that shallow crust granitic rocks from the same region are much more homogeneous. The unexpected nature of this result stems from expectations that Mg isotopes hardly budge at high temperatures, but are strongly fractionated by low T processes, such as weathering and carbonate deposition—processes that do not top the list of thoughts on middle or lower crust formation and evolution. Isotopic homogeneity at higher structure levels indicate that processes that lead to granite formation may drive homogenization. But if high-T processes cannot fractionate Mg isotopes, then heterogeneity must result from mixing between the isotopically disparate components that comprise the deeper crust; there may be a great story of middle and lower crust formation, if we can identify what these components are, and how they come together.
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