Editor’s Notes
Total Results: 1697
Volume 99 : May - June 2014 Issue
An assessment of the reliability of melt inclusions as recorders of the pre-eruptive volatile content of magmas
https://doi.org/10.2138/am.2014.4574
On page 976 of this issue, Esposito et al. examine associated melt inclusions, where petrographic evidence indicates simultaneous trapping. Such melt inclusions tend to be very consistent in terms of major oxide and H2O contents, but they contain highly variable amounts of CO2. Provided that these MI were indeed trapped at the same time, these results indicate that H2O-CO2 trends, where H2O contents are nearly invariant, might not represent a magma degassing path. Instead, such paths may reflect variations in CO2 during trapping, or post-crystallization loss of CO2 from the MI.
Beryllium mineral evolution
https://doi.org/10.2138/am.2014.4675
On page 999 of this issue, Grew and Hazen examine the possible evolution of terrestrial Be-bearing minerals. Their work indicates a near steady increase in diversity of Be minerals, interrupted in places by plateaus of negligible diversity increase, lasting 100 Ma or longer. However, the authors note that such plateaus may depend as much on chance rather than causative conditions, since nearly half of all Be minerals are found in just a few locations of unique chemistry.
Role of silica for the progress of serpentinization reactions: Constraints from successive changes in mineralogical textures of serpentinites from Iwanaidake ultramafic body, Japan
https://doi.org/10.2138/am.2014.4748
On page 1035 of this issue, Miyoshi et al. show that orthopyroxene is something like a keystone mineralogical species in serpentinization reactions. The breakdown of orthopyroxene provides excess silica that can then react with brucite to form magnetite. The formation of magnetite further contributes to the electro-magnetic characteristics of the bulk rock, and moreover release H that may be crucial for the development of microbial communities on the ocean floor.
Bubble formation during decompression of andesitic melts
https://doi.org/10.2138/am.2014.4719
On page 1052 of this issue, Fiege et al. conduct experiments to examine bubble growth and bubble number densities (BND) in andesitic melts. Their results show that, for a given pressure drop, bubbles nucleate more readily along single step or multi-step decompression paths compared to a continuous decompression path. They also find that melt composition, rather than temperature, plays an important role for bubble formation. For example, at a given decompression rate, andesitic melts yield much higher BNDs compared to rhyolitic melts. Such compositional dependencies are not described by existing models of bubble nucleation, and so may cause overestimates of magma ascent rates by as much as an order of magnitude.
Melting phase equilibria of model carbonated peridotite from 8 to 12 GPa in the system CaO-MgO-Al2O3-SiO2-CO2 and kimberlitic liquids in the Earth’s upper mantle
https://doi.org/10.2138/am.2014.4826
On page 1119 of this issue, Keshav and Gudfinnsson conduct partial melting experiments in the system CaO + MgO + Al2O3 + SiO2 + CO2. They find that at high pressures of 8-12 GPa, heating along an isobaric path produces a smooth continuum in liquid compositions that are in equilibrium with four phase assemblage of forsterite + orthopyroxene + clinopyroxene + garnet. Along an isobar, at low temperatures, the liquids are model carbonatites, and at high temperatures, liquids are model kimberlites. At the investigated pressure-temperature conditions, Keshav and Gudfinnsson also find that in terms of oxide ratios, individual isopleths are very close to each other than is the case at lower pressures of 3-8 GPa (Gudfinnsson and Presnall 2005). On this basis, the authors suggest that further increase in pressure will not have much effect on liquid compositions, and it might be that most primary kimberlites are just limited to upper mantle depths in Earth. In this manner, diamonds that are interpreted to have come from depths approaching those of the Transition Zone, or even deeper, might have come via mantle flow to shallower depths where kimberlites are primarily generated. From such regions in the mantle, later generation kimberlites bring diamonds to the Earths surface.
Volume 99 : April 2014 Issue
New data on lunar magmatic processes
https://doi.org/10.2138/am.2014.4803
We have two Highlights and Breakthroughs articles in this issue. On page 561, Lofgren provides perspective on a recent paper by Elardo et al. who find zoned pyroxenes in lunar basalts. Pyroxene grains exhibit oscillatory zoning that cannot be the result of interface-mediated kinetic processes, and so must reflect magma wide processes where pyroxenes are exposed to different environment as they grow. Then on page 562, Pasteris provides perspective on a recent paper by Rollin-Martinet et al., who apply thermodynamic laws to understand how bone material (apatite) matures in biological systems. This new work shows that the chemical reactions of maturation can be explained as thermodynamically driven and as the reason that old bone apatite needs to be replaced by new apatite to retain its biological functionality. The next step in such work is to evaluate the thermodynamic effects of carbonate constituents in bone material.
Effects of chemical composition and temperature on transport properties of silica-rich glasses and melts
https://doi.org/10.2138/am.2014.4683
On page 564 of this issue, Hofmeister et al. measure thermal diffusivity on a range of silicate liquid compositions. Their work shows that Ca and Fe both have a significant effect on liquid thermal diffusivities. The result is that at lower temperatures, Ca is the key control, where calc-alkaline magmas should cool more quickly than tholeiitic basalts and I-type granitic liquids should cool more quickly than lower-Ca A- and S-type magmas. At higher temperatures, the effects of Fe on diffusivity may become more important, which may reverse the order.
Solid phases of FeSi to 47 GPa and 2800 K: New data
https://doi.org/10.2138/am.2014.4612
On page 720 of this issue, Geballe and Jeanloz examine the phase transition of FeSi to a CsCl structure at 30 GPa; this phase transition is accompanied by a 5% increase in density relative to the low-pressure precursor, as Si increases from seven- to eightfold coordination. This increase in Si coordination has implications for the solubility of Si in the cores of Mars and Mercury. On Mars, for example, the CsCl phase of FeSi may be stable through the entire depth extent of the martian core. The implied increase in the effective radius of Si dissolved in metallic Fe suggests that Si may be even more soluble in the cores of these planets than previously thought.
In-situ infrared spectra of hydroxyl in wadsleyite and ringwoodite at high pressure and high temperature
https://doi.org/10.2138/am.2014.4634
On page 724 of this issue, Yang et al. examine H bonding strengths (through OH stretching frequencies) in ringwoodite and wadsleyite—two minerals that are key to understanding the storage of water in Earth's mantle. The low OH-stretching frequencies discovered for both these phases imply that H/D ratios may be strongly fractionated if either of the crystalline phases are in equilibrium with a fluid. If degassing of Earth's mantle was responsible for the formation of the oceans, then such fluid-mineral fractionation could explain the deuterium depletion of Earth's mantle relative to the oceans.
Experimental and infrared characterization of the miscibility gap along the tremolite-glaucophane join
https://doi.org/10.2138/am.2014.4590
On page 730 of this issue, Jenkins et al. perform experiments to examine the miscibility gap in amphiboles that fall along the tremolite-glaucophane join. As miscibility gaps are temperature sensitive, their work provides a new basis for estimating temperatures in medium- to high-grade metamorphic assemblages. This better-defined miscibility gap can also be used as a test of equilibrium. The authors show, for example, that in some reported cases, putative miscibility gaps are too wide to represent equilibrium, and so the amphiboles in question are more likely mechanical mixtures, rather than equilibrated dissolution pairs.
Qingsongite, natural cubic boron nitride: The first boron mineral from the Earth's mantle
https://doi.org/10.2138/am.2014.4714
On page 764 of this issue, Dobrzhinetskaya et al. present the first description of cubic BN, which they term qinsongite. The bonding in BN is much like that in diamond, and synthetic BN, with similar hardness, is known as the industrial material Borazon. These authors have found a natural sample of BN in Cr-rich oceanic crustal rocks of Tibet, which formed during the closing of the Tethys. This natural BN was apparently formed as the crust was subducted to >300 km depths.
Geothermometric study of Cr-spinels of peridotite mantle xenoliths from northern Victoria Land (Antarctica)
https://doi.org/10.2138/am.2014.4515
On page 839 of this issue, Perinelli et al show that the Fe3+ contents of spinels can greatly impact rates of intracrystalline cation exchange. They examine intracrystalline partitioning of Al and Cr in two different suites of peridotite xenoliths. One suite records temperatures appropriate for ambient mantle conditions, while a second suite records much cooler conditions. They find that the inferred T contrasts between are unlikely to reflect cooling rate, but rather that the cooler recorded temperatures are a result of higher Fe3+ contents, which facilitate more rapid cation ordering.
Quantitative models linking igneous amphibole composition with magma Cl and OH content
https://doi.org/10.2138/am.2014.4623
On page 852 of this issue, Giesting and Filiberto exploit experimental results on amphibole-volatile equilibria to provide a means for estimating OH-, Fe3+ in amphiboles, and if Cl is measured, to use these estimates and partitioning relationships to determine H2O contents for coexisting melts. As these authors point out, amphibole complexity provides igneous petrologists with great challenges, but also great potential, especially for assessing difficult-to-measure quantities such as the pre-eruptive water contents of co-existing liquids. On page 866 of this issue, Bindi et al. describe the first example of the mineral perovskite (MgSiO3) to show appreciable levels of Cr in solid solution, at the level of 7 wt%. Chromium was found to substitute for both Mg and Si in equal proportion, by the scheme Mg2+ + Si4+ = 2Cr3+, lengthening the ""Si-O"" and shortening the Mg-O bond lengths, and increasing the tilting of the octahedra with respect to pure MgSiO3. Perovksite may have the ability to store appreciable Cr in the lower mantle.
Volume 99 : February - March 2014 Issue
Entrapping CO2, while recovering methane
https://doi.org/10.2138/am.2014.4741
On page 253 of this issue, Jung provides a perspective on a new study on the trick of injecting CO2 into methane-bearing sediments, to both store CO2 and to release CH4. He argues that work by Hyodo et al., published last month in the American Mineralogist, provides an important first step for understanding the mechanical stability of methane-bearing sediments following CH4-CO2 exchange. A key problem that must be understood is the nature of volume changes, which can be as high as nearly 400%, if the un-exchanged sediments contain up to 50% methane. Such volume exchanges may allow fracturing, especially in low-porosity systems.
Sulfate-bearing deposits at Dalangtan Playa and their implication for the formation and preservation of martian salts
https://doi.org/10.2138/am.2014.4594
On page 283 of this issue, Kong et al. consider the formation conditions of kieserite, a hydrated sulfate that is widespread on the martian surface. Their work indicates that kieserite is a weathering product, rather than a primary aqueous precipitate. Rather than providing evidence for brines at the martian surface, kieserite instead forms as a dehydration product of hexahydrate. Weathering occurs over a span of 6 months—the length of a martian summer—as wind and sunlight drive dehydration. The resulting kieserite then appears to survive the colder, wetter martian winter.
Magma chamber dynamics recorded by oscillatory zoning in pyroxene and olivine phenocrysts in basaltic lunar meteorite Northwest Africa 032
https://doi.org/10.2138/am.2014.4552
On page 355 of this issue, Elardo and Shearer discuss the unusual occurrence of oscillatory-zoned minerals contained within a Moon-derived meteorite. Although common among terrestrial volcanic rocks, this is one of only a handful of reports of oscillatory-zoned lunar minerals. The authors hypothesize that the implied magmatic convective system that created such zoning was a result of the storage of magma within a relatively cold crust, even at 3 Ga. Their work thus implies that the lunar crust was loosing heat rapidly, or at least rapidly enough to maintain a strong thermal contrast between crust and stored magmas, to yield a vigorously convective system.
Evaluation of residual pressure in an inclusion-host system using negative frequency shift of quartz Raman spectra
https://doi.org/10.2138/am.2014.4427
On page 433 of this issue, Kouketsu et al. provide a novel method for estimating peak pressures in metamorphic systems. A key problem in the application of traditional compositionally based thermometers and barometers is that mineral compositions may be reset as rocks follow a retrograde metamorphic path en route to the surface. Kouketsu et al. get around this problem by examining shifts in peak positions in Raman spectra of quartz inclusions contained within garnet hosts. Elastic modeling shows that the magnitude of such peak shifts are a function of the pressures at which the quartz grains were originally included within the garnet, during garnet growth. Most importantly, this elastic information is retained even when surrounding matrix materials are thoroughly recrystallized, thus apparently preserving peak metamorphic pressure conditions.
Transformation of graphite to diamond via a topotactic mechanism
https://doi.org/10.2138/am.2014.4658
On page 531, Garvie et al. examine nano-sized grains of interstratified graphite and diamond from Gujba, an extraterrestrially shocked meteorite using HRTEM. This coexistence allows them to derive a topotactic mechanism for the transition, complete with the transition matrix. The findings provide new mechanistic insights into the interactions that control the transformation of graphite into diamond.
Subsolidus isothermal fractional crystallization
https://doi.org/10.2138/am.2014.4693
On page 543, David London shows that it is the extent of liquidus undercooling that determines whether or not zoned pegmatites or granites form. When a liquid is undercooled, the driving force for crystallization may differ substantially for each phase, even for eutectic compositions. Pegmatite formation occurs via the sequential (rather than simultaneous) crystallization of feldspars and quartz from liquids that possess near-minimum or eutectic compositions. This occurs via subsolidus isothermal fractional crystallization, which generates the characteristic zonation. Consequently, for natural liquid compositions that contain at least tenths of a weight percent of CaO, feldspars predominate over quartz, plagioclase precedes K-feldspar, and the most calcic plagioclase and mafic minerals, with their higher actual liquidus temperatures, crystallize first.
Merwinite in diamond from São Luiz, Brazil: A new mineral of the Ca-rich mantle environment
https://doi.org/10.2138/am.2014.4767
On page 547, Zedgenizov et al. report the first occurrence of merwinite as an inclusion in diamond originating from the Sao Luiz alluvial deposits, Juina, Brazil. Merwinite cannot form under eclogitic or peridotitic mantle conditions, and its presence is evidence of a quite different Ca-rich, Mg-poor, and Si-poor mantle composition. It is suggested that subduction-derived calcium carbonatite melt reacts with host peridotite to form merwinite, and that its presence may be an indicator of Ca-carbonatite metasomatism in the deep mantle.
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