Editor’s Notes
Total Results: 1697
Volume 98 : October 2013 Issue
Petrology and geochemistry of lunar granite 12032,366-19 and implications for lunar granite petrogenesis
https://doi.org/10.2138/am.2013.4330
On page 1697 of this issue Seddio et al. examine a granitic rock fragment from a lunar soil sample recovered from the Apollo 12 mission. Their work shows that the granitic composition is not easily obtained by fractionation of any known lunar basaltic composition, but that a Na-rich KREEP basalt (with 1.2 to 1.4 wt% Na2O in the bulk liquid) could work as a parent magma, or as a protolith of partial melting.
Aluminum speeds up the hydrothermal alteration of olivine
https://doi.org/10.2138/am.2013.4469
On page 1738 of this issue, Andreani et al. show that the presence of aluminum greatly enhances the rates of dissolution of olivine and concomitant precipitation of serpentine. This is not only of great geological interest since it affects the dynamics of plate tectonics but could potentially have social implications since the serpentinization reaction is a low-energy and carbon-free way of producing H2 and binding CO2.
Crystal structure and hydration/dehydration behavior of Na2Mg(SO4)2·16H2O: A new hydrate phase observed under Mars-relevant conditions
https://doi.org/10.2138/am.2013.4509
On page 1772 of this issue Leftwich, Bish and Chen examine the reversible hydration-dehydration behavior of Na2Mg(SO4)2nH2O at Mars-relevant conditions. Their work shows that at equatorial latitudes, evaporates of this type may have the ability to absorb H2O from the Martian atmosphere at night (n = 16), and then dehydrate (n = 4) H2O back to the Martian atmosphere during the Martian day, provided that daytime surface temperatures exceed 10 C. At polar latitudes the Na2Mg(SO4)2nH2O phase would remain in the more hydrated (n = 16) state. Their work implies that at equatorial latitudes, such hydration/dehydration reactions could greatly impact the Martian water cycle.
The diffusion behavior of hydrogen in plagioclase feldspar at 800–1000 °C: Implications for re-equilibration of hydroxyl in volcanic phenocrysts
https://doi.org/10.2138/am.2013.4521
"On page 1779 of this issue Johnson and Rossman examine diffusion rates of H through plagioclase. They find that H diffusion rates are sufficiently fast so as to record only the last few hours of plagioclase growth. This means that deep-seated H contents of a host magma can only be recorded by plagioclase crystals if such crystals are transported at very rapid ascent rates, exceeding 66m/hr. Slower magma ascent rates allow for re-equilibration of plagioclase H contents with ambient magmatic H contents during the last few hours of sub-surface residence, prior to quenching at the surface. On page 1803 of this issue Lyubutin et al. report on a high-pressure Mossbauer study of the (Fe,Mg)2SiO4 polymorph Ringwoodite, believed to be a major constituent in the Earth's mantle between 520 and 670 km. They report the first experimental evidence of an electronic high-spin to low-spin transition for the Fe2+ contained in ringwoodites crystal structure. This has important implications on the physical properties of the Earth's mantle where ringwoodite is stable. Since the high-spin to low-spin-transition affects the volume and thus sound velocities of ringwoodite, this finding has important consequences on the seismic properties of this important mantle mineral.
Phosphorus partitioning between olivine and melt: An experimental study in the system Mg2SiO4-Ca2Al2Si2O9-NaAlSi3O8-Mg3(PO4)2
https://doi.org/10.2138/am.2013.4237
On page 1860 of this issue, Grant and Kohn examine the partitioning behavior of P between olivine and silicate liquids (DP). They find that DP is much higher in highly polymerized melts (with low ratios of non-bridging oxygens relative to tetrahedral cations, or NBO/T). Thus, P contents in olivine may change even if P contents in a liquid are constant, as DP responds to changing NBO/T in an evolving melt. Importantly, they also find that olivine P and Al contents are uncorrelated. This finding points the way towards distinguishing rapidly grown crystals (with high P and high Al) from crystals (with high P and low Al) that are grown at near-equilibrium conditions, but from highly polymerized melts.
Volume 98 : August - September 2013 Issue
Why you should publish your best papers in American Mineralogist: An International Journal of Earth and Planetary Materials
https://doi.org/10.2138/am.2013.626
Editorial: Why You Should Publish Your Best Papers in Am Min
New constraints on the size of chondrite parent bodies
https://doi.org/10.2138/am.2013.4555
On page 1379 of this issue, Kevin Righter offers a perspective in a Highlights and Breakthroughs article regarding the recent work by Kimura et al. (2013) on the discovery of an eclogite mineralogy within an unaltered CR Chondrite meteorite. Righter notes that this is at least the second description of an eclogite mineralogy in a chondrite meteorite. Such mineralogies require a parent body radius of at least 1500 km, and as Righter notes, such discoveries may lead to new insights regarding the roles played not just of P and T, but also bulk composition for eclogite formation.
Behavior of gold in a magma at sulfide-sulfate transition: Revisited
https://doi.org/10.2138/am.2013.4502
On page 1459 of this issue, Botcharnikov et al. re-examine the effects of temperature, oxygen fugacity, and the identity of sulfide phases on the partition coefficients of Au (DAu) between basalt melts and sulfide phases. New data are presented that indicate an order-of-magnitude difference between the DAu values for monosulfide solid solution/silicate-liquid and sulfide-liquid/silicate-liquid emphasizing the importance of the nature of the sulfide phase present. Temperature, P and fO2 have strong indirect effects on Au partitioning by controlling the identity of the condensed sulfide phase. These authors show that the presence of a monosulfide solid solution (mss) or oxidizing conditions (fO2 close to the sulfide-sulfate transition where sulfur mobility is much higher) favor elevated Au concentrations in magmas.
NanoSIMS Pb/Pb dating of tranquillityite in high-Ti lunar basalts: Implications for the chronology of high-Ti volcanism on the Moon
https://doi.org/10.2138/am.2013.4467
On page 1477 of this issue, Tartese et al. use NanoSIMS to obtain highly precise Pb/Pb dates on ca. 3.7-3.8 Ga high Ti lunar basalts from the Apollo 11 and Apollo 17 missions. Their work shows that the mineral tranquillityite provides an ideal target for highly precise Pb/Pb dating by ion microprobe. Precision on age dates compared to Rb-Sr and Sm-Nd methods is lower by a factor of 3 to 5, and so provide a more precise timescale for the eruption of high Ti basalts. As these basalts have been used to calibrate chronometers for planet surface ages based on crater density, this work has the potential to add precision to the understanding of surface ages for planetary bodies beyond the Moon.
Experimental determination of siderite stability at high pressure
https://doi.org/10.2138/am.2013.4351
On page 1565 of this issue, Tao et al. examine the decomposition of siderite at upper mantle pressures. Magnesite-siderite solid solutions are expected to be the stable form of carbonate as C is subducted into the mantle, but the stability of the Fe end-member of the series had yet to be studied at mantle pressures. Tao et al. find that the decomposition of siderite into magnetite and either graphite or diamond occurs at temperatures that are close to the average mantle geotherm; the cooler subduction geotherms should still retain C in the form of a magnesite-siderite solid solution, but as these carbonates are cycled back into the mantle, siderite decomposition may allow the release of C to form graphite or diamond.
The system K2CO3-MgCO3 at 6 GPa and 900–1450 °C
https://doi.org/10.2138/am.2013.4407
On page 1593 et al. Shatskiy et al. examine melting relationships in the K2CO3-MgCO3H2O system. Their results indicate that if K and C are both held in the form of magnesite and K2Mg(CO3)2 in the upper mantle, then this phase assemblage could control the solidus of carbonated peridotite in the upper mantle. Near-solidus melting of such phase assemblages could explain the origin of ultrapotassic carbonatitic melts that are found as microinclusions in fibrous diamonds from kimberlites worldwide.
Volume 98 : July 2013 Issue
A step closer to predicting the bonding geometry of crystals
https://doi.org/10.2138/am.2013.4514
On page 1093 of this issue, Brown provides a perspective on a new analysis of bond valence by Bickmore et al. (2013; American Mineralogist, v. 98, p. 340349). This new work provides a means by which bond angles can be at least qualitatively predicted from bond-valence vectors. They key observation is that bond-valence vectors do not always sum to zero, and these sums can be used to infer bond angles in cases where the bonding is asymmetric. This work marks a major advance of the bond valence bond model, and introduces the prospect of our being able to predict bond geometries for even highly complex mineral structures.
Microprobe analysis and dating of monazite from the Potsdam Formation, New York: A progressive record of chemical reaction and fluid interaction
https://doi.org/10.2138/am.2013.4304
On page 1106 of this issue Allaz et al. obtain in situ age dates and trace element concentrations of detrital monazite grains. They are able to document dissolution and re-growth of monazite rims during diagenesis, which implies that monazite rim compositions can be used for age dating sedimentary processes, and as might be expected, that whole-grain monazite analyses might not be useful for differentiating sedimentary source areas. Their results also raise cautionary notes about monazite as a storehouse for radioactive waste, given the potential for monazites to dissolve and re-precipitate.
A micro-reflectance IR spectroscopy method for analyzing volatile species in basaltic, andesitic, phonolitic, and rhyolitic glasses
https://doi.org/10.2138/am.2013.4277
On page 1162 of this issue, King and Larson present a new calibration for micro-reflectance IR spectra for the estimation of water and CO2 contents in volcanic glass samples. The new method allows water contents to be determined for a wide range of glass compositions and volatile contents for samples that are difficult to prepare, either because of fragility, high water contents, or samples that are too small to be doubly polished for transmitted IR measurements.
Magnesite formation from MgO and CO2 at the pressures and temperatures of Earth’s mantle
https://doi.org/10.2138/am.2013.4260
On page 1211 of this issue diamond anvil experiments conducted by Scott et al. show that magnesite readily forms from MgO and CO2 at 540 GPa and 14001800 K. A key result is that magnesite was able to form even at the highest pressures, whereas CO2 was in a very low partial molar volume form (CO2-V). This work confirms earlier studies that indicate that the volume relationships may allow magnesite to act as the key storehouse of oxidized C in the lower mantle.
Volume 98 : May - June 2013 Issue
A carbonate-fluoride defect model for carbonate-rich fluorapatite
https://doi.org/10.2138/am.2013.4445
On page 1066 of this issue, Yi et al. examine carbonate-phosphate substitutions in flourapatite. Much of the world's phosphorus resources are in the form of francolite, a carbonate-rich apatite; yet, how these carbonate groups are incorporated has been a long-standing puzzle. Yi et al. examine the problem using state-of-the-art NMR, FTIR, and first-principles calculations. They show that while a small fraction of carbonate groups do substitute in the hexagonal channels (A-type carbonate), a charge-compensating composite tetrahedron, composed of a planar carbonate group lying on one sloping face and a F ion at the remaining vertex, can replace the PO4 unit (B-type carbonate). They suggest that a particular IR Raman band may prove useful in determining the extent of recrystallization of apatite from bone and teeth.
High-pressure aragonite phenocrysts in carbonatite and carbonated syenite xenoliths within an alkali basalt
https://doi.org/10.2138/am.2013.4410
On page 1074 of this issue, Hurai et al describe primary magmatic aragonite. It occurs in a carbonatite and carbonated syenite near the HungarySlovakia border. The host rocks contain calcite that is interpreted to represent quenched carbonatite liquid; isotopic ratios indicate that the aragonite formed prior to carbonate-liquid degassing. Aragonite is expected to be stable at pressures >1.22 GPa, i.e., below the Moho in this region. But Mg contents of the carbonate phases are too low to allow equilibration with mantle peridotite. The authors hypothesize that aragonite was stabilized by a transient pressure within the crust (equal to about 186% of lithostatic pressure), generated by degassing. Regardless of the crystallization conditions, isotope ratios of the carbonate phases are consistent with its derivation by metasomatism of mantle wedge materials, due to subducted fluid inputs.
Kangite, (Sc,Ti,Al,Zr,Mg,Ca,ï¯)2O3, a new ultra-refractory scandia mineral from the Allende meteorite: Synchrotron micro-Laue diffraction and electron backscatter diffraction
https://doi.org/10.2138/am.2013.4290
On page 870, Ma et al. describe a new mineral, kangite, found in the Allende meteorite. Kangite is proposed to have formed during an oxidation event from a reduced precursor Sc-Ti oxide, which if discovered, would also constitute a new mineral. Kangite has a very high ratio of heavy to light rare earth elements, providing evidence that the precursor material was ultrarefractory, and so formed from as an early condensate at very high temperatures. The existence of such high temperature condensates has been predicted on thermodynamic grounds (Boynton 1975) and their formation as condensation (rather than evaporation) products is supported by isotopic evidence (Smith et al. 1996).
Solidification of trapped liquid in rocks and crystals
https://doi.org/10.2138/am.2013.4327
On page 888 of this issue, Morse shows that the lever rule can be used to define the solidification history of trapped liquid in an igneous cumulate, or in a melt inclusion inside a crystal. In his model, the two processes differ because the cumulate system closes by failure of communication with the main parent magma, whereas the melt inclusion originates in an evolved sheath surrounding the crystal. For this reason, Morse argues that the melt inclusion cannot represent the parent magma and is destined to evolve further during crystal growth. In either case, the range of the anorthite fraction in plagioclase serves to define the amount of liquid trapped.
Using the chemical composition of carbonate rocks on Mars as a record of secondary interaction with liquid water
https://doi.org/10.2138/am.2013.4187
On page 897 of this issue, Hausrath and Olsen use reactive transport modeling to examine aqueous alteration of carbonate (magnesite + siderite) protoliths found on Mars. This work illustrates scenarios of chemical weathering and enrichment in various elements, and shows that it should be possible to detect evidence for near-surface, water-mediated alteration of martian carbonates, since the dissolution-precipitation reactions will leave telltale mineralogical signatures. Moreover, those signatures may also indicate pH or oxidation levels in the reacting fluids. For example, under acidic, low-oxidation state conditions, carbonate protoliths should yield siderite-bearing crusts, while under oxidizing conditions, the surface is preferentially enriched in magnesite and ferric oxide. Acknowledging the complexity of the problem, the authors suggest that future profiles through martian carbonates may be used to test for evidence of aqueous alteration.
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