Editor’s Notes
Total Results: 1697
Volume 101 : February 2016 Issue
Cancrinite-group minerals: Crystal-chemical description and properties under non-ambient conditions—A review
https://doi.org/10.2138/am-2016-5282
On page 253 of this issue, Gatta and Lotti review the crystal chemistry and structural properties of cancrinite minerals, which are a part of the feldspathoid group. Studies of cancrinite minerals are motivated by their microporous behavior, and their ability to house very large amounts of H, C, S and Cl, due to their characteristic structural incorporation of such components. Here, Gatta and Lotti review experimental studies of how cancrinite minerals respond to increases in T and P. They find that unlike other microporous minerals, cancrinite minerals retain their crystallinity even after being dehydrated. But their review also shows that we still have an incomplete understanding of how the crystal chemistry of this mineral group responds to such changes, which is important for understanding the potential of cancrinite minerals for the storage of solid and radioactive waste, and perhaps also for understanding H, Cl, and S budgets within the crust.
Mafic replenishments into floored silicic magma chambers
https://doi.org/10.2138/am-2016-5429
On page 297 of this issue, Bob Wiebe traces the history of our understanding of mafic replenishments into silicic magma chambers. Wiebe applies the term Mafic-Silicic Layered Intrusions , or MASLI, to the field localities that expose mafic-felsic magma interactions. The history itself is fascinating, but there are also important implications to his review. The so-called volcanic-plutonic connection has been a topic of long-standing and recent interest, and as Wiebe notes, MASLI provide a valuable opportunity to explore such. For example, MASLI exhibit stratigraphic sequences that apparently are constructed on a timescale comparable to the time scales of volcanic stratigraphic sections. This being the case, MASLI may provide an even more important target than volcanic rocks themselves for understanding the storage/chamber-specific processes that trigger volcanic eruptions, as well as an examination of any possible dichotomy between the compositions of erupted and un-erupted volumes.
Deciphering magmatic processes in calc-alkaline plutons using trace element zoning in hornblende
https://doi.org/10.2138/am-2016-5383
On page 328 of this issue, Barnes et al. examine hornblende compositions from the Kuna Crest Lobe (KCL) of the Tuolumne Intrusive Complex, and from the Wooley Creek Batholith (WCB), both in California. Their work shows that the wide T range over which amphiboles precipitate allows these crystals to capture much of the magmatic history of granitic magma bodies. Trace element analyses of these crystals reveal near homogeneity amongst the WCB hornblendes but by contrast, KCL hornblendes vary from sample to sample. These results seem to imply that the KCL was built from numerous small batches of magma, while the WCL batholith, at least syn- and post-hornblende saturation, is comprised of a relatively large and homogenized magma body. The diversity among the KCL amphibole trace elements, despite similar major element compositions, shows that even lacking precise U-Pb age dating, it is possible to demonstrate that a number of magma batches were brought together to from the KCL and that these batches maintained their mineralogical distinctiveness. Moreover, these results imply that a single large magma body of the size of the entire lobe was never available to be erupted from the KCL, but such a body could have crystallized to form the WCB. Amphibole compositions thus provide a potentially powerful means to assess how large eruptible felsic magma bodies are assembled.
The Lassen hydrothermal system
https://doi.org/10.2138/am-2016-5456
On page 343 of this issue, Ingebritsen et al. assess the energetic discharge from the Cascades largest hydrothermal system, at the Lassen Volcanic Center in northern California. Their assessment indicates that to produced a steady energy output at the measured value of 140 MW, that this would require the combined cooling (from 800 C to 300 C) and crystallization of a silicic magma intruded at a rate of 2400 km3/year. They calculate that heat transfer occurs over an area of about 5 km2 at a depth fo 4-5 km. Similar mass rates of basaltic magma intrusion are derived from observed CO2 output rates. These long-term intrusion rates can also be compared to observed mappable volcanic output rate of 340 km3/Ma, and leads to a magmatic intrusion:eruption ratio of 7:1. The authors note, however, that there is still much work to be done to understand transients in heat flow and how they are modulated by climate, earthquakes, and the transition from intrusive to extrusive magmatic activity.
In defense of magnetite-ilmenite thermometry in the Bishop Tuff and its implication for gradients in silicic magma reservoirs
https://doi.org/10.2138/am-2016-5367
"In page 469 of this issue, Evans et al. argue that Fe-Ti oxides reliably capture at least the late thermal maturation of the Bishop Tuff magmatic system. Prior work cast doubt on earlier T estimates, but as Evans et al. show, the compositions of ilmenites, magnetites, and co-existing glasses track one another rather closely. Roozeboom diagrams illustrate these characteristics, and so provide a compelling, although by no means certain, test of inter-phase equilibrium. These results, together with correlations between compositional parameters and Fe-Ti oxide temperatures, indicate that even if the oxide pairs do not record the correct absolute T estimates from the Bishop Tuff, they probably at least capture the magnitude of temperature changes that occurred post-Fe-Ti oxide saturation. The 100 C interval recorded by such oxides thus represent a minimum T interval over which the Bishop Tuff felsic magmas crystallized, tracking fractionation as T decreases. We still lack a fully quantitative comparison of independent thermometers to test both the absolute values and their relative T ranges; such work, and an analogous assessment of crystal heterogeneity as provided by Barnes et al. (see preceding note) may provide rich rewards regarding the evolution of this very well-known and important ""super"" (eruption) magmatic system."
Volume 101 : January 2016 Issue
A spin on lower mantle mineralogy
https://doi.org/10.2138/am-2016-5497
On page 1, Jeffrey Pigott reviews the issue of whether or not bridgmanite undergoes a high-spin to low-spin transition. The issue is consequent to estimates of mineralogy of the lowermost mantle, based on elasticity and density measurements. Dorfman et al. in the Oct. 2015 issue, appear to settle the issue by showing that in bridgmanite, Fe2+ remains in a high spin state (with unpaired 3d electrons in unfilled orbitals), at reasonable pyrolite-like Fe contents (ca. 10 wt% FeSiO3), but should undergo a transition to a low-spin state (3d orbitals contained paired electrons, leaving some orbitals empty) at higher Fe contents, at 50-70 GPa.
Safe long-term immobilization of heavy metals: Looking at natural rocks
https://doi.org/10.2138/am-2016-5548
"On page 3 of this issue, Associate Editor Maarten Broekman discusses a new study by Khoury et al. in this issue, of natural (Ca,Cd)O, a mineral that occurs only rarely in nature, but as CaO it represents a common accessory in Ordinary Portland Cement (OPC) clinker. Their study of natural (Ca,Cd)O formed by ""combustion metamorphism"" through ignition of intercalated bituminous lithologies and that was weathered for >100 ka, which formed hydroxides also related to those found in hydrated OPC—and the phases retain their Cd. The work thus implies a reliable means to lithologically sequester heavy metals."
Spinel in planetary systems
https://doi.org/10.2138/am-2016-5554
On page 5 of this issue, Stephen Haggerty reviews a study of spinel compositions by Papike et al. (2015). New experiments from Papike et al. reveal greater detail about how V3+/V4+, Fe2+/Fe3+, and Cr partition between spinel and coexisting melt. This new work quantifies the effect of increasing fO2 relative to converting normal spinel to an inverse structure. The gradual change in spinel structure (from >90% normal spinel at IW-1, to <75% normal spinel at QFM+2) means that different kinds of crystallographic sites are available to cations in the melt, and so fO2 has a profound influence on the nature of spinel/melt partitioning. This interplay between temperature, fO2, and redox-sensitive element partitioning means that spinels offer much promise for reconstructing parental magmas and spinel + melt intensive parameters.
Pathways for nitrogen cycling in Earth’s crust and upper mantle: A review and new results for microporous beryl and cordierite
https://doi.org/10.2138/am-2016-5363
On page 7 of this issue, Bebout et al. note that most N is not housed in the atmosphere, but rather a vast majority—ca. 70% (perhaps even more)—resides in the crust and mantle. In their new contribution, they consider various pathways through which N passes from one reservoir to another. Perhaps most interesting is that crucial role of biological processes, in extracting N from the atmosphere, as NH3+ that can later be fixed into mineral phases, thus allowing N to be stored in the crust and subducted into the mantle. Layer silicates are a key means of transferring N from the fluid to the solid state, mostly as NH4+, which substitutes quite effectively for K+ in micas, which are then the main solid-state N reservoirs in the subsurface. Nitrogen is returned to a fluid phase during prograde metamorphism and the decomposition of micas. At least some of these reactions occur with minimal N isotope fractionation, but new isotope fractionation data are needed to better understand these processes.
Metamorphic chronology comes of age: Past achievements and future prospects
https://doi.org/10.2138/am-2016-5146
On page 25 of this issue, Matt Kohn reviews the marriage of chronology and thermobarometry in metamorphic systems. Having an advantage in phase complexity, metamorphic petrology has long held the lead over igneous petrology in regard to deciphering pressure-temperature histories, combining these to obtain P-T-t paths. These paths have been crucial in characterizing metamorphic reactions and the tectonic processes that control them. Kohn reviews the most recent means by which P-T-t knowledge is being advanced. Among the many new developments, direct combined thermometry and geochronology appears most compelling. The minerals zircon, titanite, and rutile each provide avenues for simultaneous age dating and thermometry. By obtaining T-t information from each spot analyzed on a given grain, it should be possible to measure crystal growth rates and the energetic potentials that control them, which could add much nuance to interpretations of the P-T-t paths that catalyze recrystallization.
K-bentonites: A review
https://doi.org/10.2138/am-2016-5339
On page 43 of this issue, Warren Huff provides a review of global distributions of K-bentonite clays, the altered remains of volcanic tephra. As Huff shows, ancient volcanic tephra provide essential tie points for stratigraphic correlation. K-bentonite-bearing ash layers range from Proterozoic to Cenozoic in age and are shown to preserve high field strength elements or HFSE (which are expected to be immobile during weathering), and these HFSE may provide a tectonic context for the parent volcanoes. Most of the units described by Huff appear to derive from arc-related systems. The global record of arc systems as preserved by K-bentonites may provide yet-to-be exploited and independent record of the waxing and waning of arc activity, related to plate collisional events. As noted by Huff, these may be invaluable for understanding global geodynamics.
Volume 100 : November - December 2015 Issue
On understanding the structure and composition of crystals
https://doi.org/10.2138/am-2015-5391
"On page 2365 of this issue, I. David Brown provides an overview of Hawthorne's contribution ""Toward Theoretical Mineralogy"", which appeared on page 696 of this volume. As yet, we still do not have a ""theory of mineralogy"", at least not in the same way that we have a theory of gravity or of biological evolution. As mineralogists well know, thousands of mineralogical structures have been identified, and many more have been synthesized, but just a few dominate Earth and its solar system. But why? As Brown explains, Hawthorne addresses the issue with a concept referred to as the ""valence matching rule"". This is a variation on Pauling's rules, where we define the bonding strength of an atom, a quantity closely related to Paulings electrostatic valence bond strength, from the standpoint of both the anion and the cation: given their charge and a typical coordination number a characteristic bonding strength can be predicted. Bonds are most stable when the bonding strength is the same for both the cation and anion. If the bonding strengths differ by more than a factor of two, the bond is unstable, and so unlikely to form; the concept can be taken further, where bonding strength is proportional to Lewis acid/base strength. As Brown states, Hawthorne's new work ""elevates the Bond Valence Model to the level of a theory of structural chemistry""."
When was the Earth’s conveyor belt set in motion?
https://doi.org/10.2138/am-2015-5468
On page 2367 of this issue, Bernard Charlier reviews the experimental work of Hou and Veksler (2015), which appeared on page 1304 of this volume. In the latter work, experimental data are presented that indicate that immiscible ferrobasaltic liquid may be stable at high temperatures—above 1150oC. As Charlier explains, the high T experiments yield silicate liquids that are andesitic, with SiO2 contents in the range of 53-56% SiO2 (and 14-18%FeOt). The experiments thus indicate that high SiO2 contents, as well as elevated FeOt, may be hallmarks of silicate compositions that can reach immiscibility at high T. This then raises the issue of whether tholeiitic liquids can reach immiscibility at high T, since sufficiently high SiO2 contents may occur due to Fe-Ti oxides, but then decreasing FeOt would drive liquids away from the two-liquid solvus. But the new experiments may support recent work indicating liquid immiscibility in the Bushveld. On page 2369 of this issue, Igor Puchtel provides an overview of Blichert-Toft et al.s geochemical results on komatiites from the Barberton Greenstone Belt in South Africa, which appears on page 2387 of this issue. In earlier work, Puchtel and his co-workers also examined 3.5 Ga komatiites, and discovered a de-coupling of the Sm-Nd and Lu-Hf isotopic systems. The decoupling can be explained by internal mantle differentiation, i.e., partial crystallization of a magma ocean. However, as Puchtel explains, Blichert-Toft et al. find even greater degrees of de-coupling at 3.5 Ga, to an extent that cannot be explained by internal mantle differentiation alone. One possibility is that subducted pelagic sediments contribute to the isotopic signature of some 3.5 Ga komatiites. Such sediments would have little zircon, and so contain high Lu/Hf, which over time would develop high eHf isotopic signatures. This implies that a modern-like form of plate tectonics was operative before 3.5 Ga."
Trace element partitioning into sulfide: How lithophile elements become chalcophile and vice versa
https://doi.org/10.2138/am-2015-5358CCBYNCND
On page 2371 of this issue, Wood and Kiseeva examine how elements under certain conditions are lithophile, but under other conditions become chalcophile, and vice versa. They show that the sulfide/silicate liquid partitioning of chalcophile behavior, while generally linear with respect to element valency (slope) and T and P (intercept), that the linear coefficients depend upon how a given element interacts with oxygen. This interaction can be quantified as the difference in lithophile and chalcophile properties of a given element, and FeO. Their new experiments indicate that lithophile or chalcophile behavior can depend on the FeO content of a silicate liquid, and that elements that normally behave as lithophile, may become chalcophile at either very low or very high FeO contents of coexisting silicate melts. Such behavior implies that elements such as U or Th, under reducing conditions and with the addition of sufficient S, might partition into a growing metallic core, rather than a silicate mantle, which in turn could affect the powering of a geodynamo.
Petrology on Mars
https://doi.org/10.2138/am-2015-5257
On page 2380 of this issue, McSween reviews the petrology of Mars. This review reveals some interesting and important features: First, and unsurprisingly, the martian surface is dominated by lavas, volcaniclastics, and ultramafic cumulates. But among these, alkalic rocks are common in the more ancient terranes, but are mostly absent from younger terranes that are dominated by tholeiitic compositions, which suggest some manner of temporal evolution. But highly evolved compositions, such as granites, are effectively absent. Some silica rich sediments have been observed that were probably created by hydrothermal processes. There is some evidence for metamorphism, in the form of metabasalts and serpentinites, indicating low-P hydrothermal processes. But until such rocks are analyzed directly at the surface, metamorphic processes are still speculative.
The accretion and differentiation of Earth under oxidizing conditions
https://doi.org/10.2138/am-2015-5153
On page 2739 of this issue, Georg and Shahar present geochemical models of accretion and simultaneous core formation on Earth, and examine the implications of oxidizing conditions and its effects on metal-silicate partitioning. They find that greater FeOt contents in the Bulk Silicate Earth (BSE) imply greater amounts of FeO, and thus more O, in the resulting core. This O content in turn affects the partitioning of Si into the core. With initial FeO in the BSE of 11 wt%, Si has a maximum concentration of ~2 wt% in the core; this maximum Si content increases to ~3% when initial FeO of the BSE increases to 15 wt%. An implication of oxidizing accretion is that Si partitioning into the core is too weak to greatly affect Si isotope contrasts between the core and mantle. So under oxidizing conditions, the mantle should have a near-chondritic Si isotope signature, unless pressure or temperature play some role in affecting isotopic partitioning. A tentative conclusion, then, is that solar nebula processes may be responsible for generating Si isotope contrasts in the inner solar system.
Volume 100 : October 2015 Issue
Reaching new boundaries for in-situ U-Th geochronology
https://doi.org/10.2138/am-2015-5393
On page 2017 of this issue, Bernal provides an overview of a new approach to U-Th geochronology, as provided by Wu et al., beginning on page 2082. The new study provides a means to date young, silica undersaturated volcanic rocks, through the analysis of baddeleyite, which appears to crystallize largely in vesicles in such rocks. The age dates, obtained for lavas from Campi Flegrei, near Naples, Italy, are similar to K-Ar ages, and are thought to -- at least mostly -- reflect eruption ages, although some age dates may reflect late-stage pre-eruptive crystal growth. As Bernal notes, this advances open up new avenues of research and possibly much better understanding of the timescales of crystallization and eruptive emplacement of mafic volcanic systems.
Normal to inverse transition in martian spinel: Understanding the interplay between chromium, vanadium, and iron valence state partitioning through a crystal-chemical lens
https://doi.org/10.2138/am-2015-5208
On page 2018 of this issue, Papike et al. use partial melting experiments to examine the crystal chemistry of spinels, and the effects of valence state on the partitioning of V, Fe, and Cr. They show that increasing fO2 from IW – 1 to FMQ + 2, produces an increase from a mostly normal spinel structure (>90%), to a structure containing up to 25% inverse spinel. This structural change affects the relative compatibilities of V3+ and V4+, since the inverse structure can incorporate larger fractions of the more oxidized species. Their work allows the use of V4+/V3+ ratios to be more quantitatively related to parental parentage, since V4+/V3+ ratios vary strongly between terrestrial, martian, and lunar samples and so can be used to reconstruct contrasting fO2 histories.
Accuracy of timescales retrieved from diffusion modeling in olivine: A 3D perspective
https://doi.org/10.2138/am-2015-5163
On page 2026 of this issue, Shea et al. measure diffusion profiles in olivine, to assess the timescales of magmatic processes. Their work shows that results obtained from random 1D profiles can yield timescales that differ from the 3D case by factors that range from 0.1 to 25. Moreover, 1D profiles, even when corrected for mineral anisotropy, still differ from 3D results by factors ranging from 0.2 to 10. The authors find that a set of selection criteria, including size, crystal shape and diffusion plateaus can be used to increase accuracy to 5% and decrease precision to 15-20% relative, when results from as many as 20 traverses from such selections are averaged.
Diffusion of phosphorus in olivine and molten basalt
https://doi.org/10.2138/am-2015-5416
"This month, two papers in the Special Section: Olivine, feature studies of P in this nesosilicate. On page 2053 of this issue, Watson et al. investigate P diffusion in olivine and molten melt, concluding that anomalously high P concentrations can develop in olivine when P is built up in a melt boundary layer during growth, but this mechanism requires growth speeds approaching those relevant to dendrite formation. Alternatively, high P contents of olivine may be a consequence of ""growth entrapment"" of a high P near-surface region in the olivine lattice, which can occur at more modest growth speeds. Either way, cooling must occur on a time scale of months to preserve delicate P zoning features. Then on page 2043 of this issue Fowler-Gerace and Tait show that very high P concentrations are present in olivine from a pallasite meteorite (a class of meteorites where olivines are entrained in a metallic matrix). Their textural and compositional analysis indicates that high P regions in these olivine grains formed before complete cooling of the metal matrix but after the olivines were rounded. They relate the high P overgrowths to very rapid crystal growth, recording an impact event on the pallasite parent body."
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