Editor’s Notes
Total Results: 1697
Volume 99 : October 2014 Issue
In-situ U-Th/Pb geochronology of (urano)thorite
https://doi.org/10.2138/am-2014-4920
On page 1985 of this issue, Cottle presents new LA-MC-ICPMS data from thorite and uranothorite [(Th,U)SiO4] crystals that range in age from 13 to 500 Ma. These single-crystal age determinations are concordant in the U-Th-Pb system, and so appear to reveal accurate dates of crystallization, with little loss of Pb despite the potential for radiation damage. The method can provide spatial resolution as fine as 5 micrometers, and thus age-dating of actinide-rich minerals should provide a valuable age dating tool that complements single-crystal zircon chronometry.
Phase diagram and P-V-T equation of state of Al-bearing seifertite at lowermost mantle conditions
https://doi.org/10.2138/am-2014-4697
On page 2035 of this issue, Andrault et al. show the results of experiments on Al-bearing SiO2, at high pressures and temperatures. Their results show that in SiO2 phases with 4-6 wt% Al2O3 there is a ~2-3% density increase for the transition from a CaCl2-structured phase, to the a-PbO2 structure (seifertite) at 113-119 GPa and 300 to 2000 K. Because of the small amount of Si in a subducted MORB, the net effect of this phase transition should be negligible. But since overlying sediments might have considerable SiO2, this CaCl2-seifertite phase transition may have a significant impact on the negative buoyancy of subducted sediments, and perhaps facilitate the descent of the sedimentary material to the lowermost part of the mantle.
Densified glasses as structural proxies for high-pressure melts: Configurational compressibility of silicate melts retained in quenched and decompressed glasses
https://doi.org/10.2138/am-2014-5019
On page 2142 of this issue, Malfait et al. show that measured compressibilities of glasses can be used to accurately calculate in-situ, high pressure-temperature densities and that any structural changes during decompression must be small. Therefore, ambient-pressure spectroscopic measurements on quenched glasses reflect those of the high-pressure structures. This work gives reassurance that various published work on high-P glasses is relevant to melt structure issues of most interest to geologists and geophysicists.
Geobarometry from host-inclusion systems: The role of elastic relaxation
https://doi.org/10.2138/am-2014-5047
On page 2146 of this issue, Angel et al. re-examine a key issue of geobarometers based on residual stresses that can be observed between an inclusion and a host phase. In their new approach, they do not assume that elastic properties of the inclusion and host are linear with respect to changes in P and T. Instead, they make use of isomekes, paths in P and T along which the fractional volume changes of host and inclusion are identical. Their approach is independent of the type of equation of state used for any given system and provides a more detailed physical basis to the barometry problem, which in turn allows for a more accurate determination of entrapment pressures and temperatures from residual stress measurements.
Volume 99 : August - September 2014 Issue
In-situ high-pressure transmission electron microscopy for Earth and materials sciences
https://doi.org/10.2138/am.2014.4857
On page 1521 of this issue, Wu and Buseck use carbon nanocontainers, examined using TEM, to study Earth materials at high pressures and high spatial resolution. The advantage of their approach is the possibility of observing the progress of high P-T reactions at nearly the atomic scale. This technique may be especially important in the investigation of unquenchable phases and reactions at a spatial resolution that is not possible with other techniques. A preliminary application of the technique allowed the authors to discover C concentrations in stacking faults in TiO2 at 8 GPa, which provides a new means for storing C in the deep mantle.
Evidence for multiple diamondite-forming events in the mantle
https://doi.org/10.2138/am.2014.4778
On page 1537 of this issue, Mikhail et al. investigate N contents and aggregation states in diamondites (polycrystalline diamonds) using IR spectra; the samples are inferred to derive from various parts of Southern Africa. Nitrogen contents vs. aggregation states of diamond provides constraints on the temperature history and mantle residence time. Their work indicates that residence times among different diamondite samples vary significantly. Another possible interpretation is that diamondites have similar residence times but form over a large depth interval, an hypothesis that is rejected since diamondites are expected to form at low melt fractions (which are thought unlikely to exist over a large depth range at any given time), and the composition of their syngenetic garnet inter-growths do not show compositional evidence to support variable depths of formation. The variable residence time interpretation requires multiple growth events, which is consistent with what is observed for the formation of monocrystalline diamonds. This work contributes to the question of why some kimberlites yield diamondites and monocrystalline diamonds, but some only yield monocrystalline diamonds. This question is still open, and requires further work to decipher if the diamondites share a genetic link with their monocrystalline gem-bearing counterparts, or not. But for now these data show for certain that diamondites are not related to coated diamonds and likely have little to do with proto-kimberlitic fluids, as previously thought.
Experimental determination of melting in the systems enstatite-magnesite and magnesite-calcite from 15 to 80 GPa
https://doi.org/10.2138/am.2014.4735
On page 1544 issue Thomson et al. conduct high P partial melting experiments in the system MgSiO3-MgCO3. They find that low-degree partial melts of subducted carbonates may be widespread in the convective lower mantle. They hypothesize that, during subduction, partial melting of carbonates is initiated near the upper/lower mantle boundary and that this may explain so-called superdeep diamonds, which are thought to derive from this depth interval. Their results also indicate that the slope of melting curves in this system (dT/dP) approach zero, or perhaps even become negative, at high pressures such that if S is positive as expected, carbonate-rich partial melts may be equally or more dense than bulk equilibrium solids. Such a characteristic would allow carbonate to be trapped in the lower mantle in the liquid state, providing the possibility of a carbon-rich reservoir in the deep mantle.
Redox effects on calcite-portlandite-fluid equilibria at forearc conditions: Carbon mobility, methanogenesis, and reduction melting of calcite
https://doi.org/10.2138/am.2014.4696
On page 1604 of this issue—in our Fluids in the Crust Special Section—Lazar et al. investigate the effects of fO2 on the dissolution and mobility of C in silicate systems. They find that at fO2 values of about 2 log units below QFM or lower, C mobility may be at a maximum as calcite dissolves and C migrates into fluids, mostly in the form of methane. The authors indicate that such reducing conditions may be found in a number of geologic environments—but may be especially important during serpentinization, where fO2 may be as low as 7 log units below QFM. The authors suggest that C mobility in subduction zones may be slightly limited by graphite saturation, but may even then still be quite mobile, with or without graphite. Their work also implies that calcite reduction may also be important in abiotic methanogenesis, a process that may be ubiquitous in subduction or astrobiological environments.
Constraints on the mobilization of Zr in magmatic-hydrothermal processes in subduction zones from in situ fluid-melt partitioning experiments
https://doi.org/10.2138/am.2014.4799
On page 1616 of this issue Louvel et al. conduct high P-T experiments to examine the partitioning of Zr between aqueous fluids and hydrous, F-bearing haplogranitic melts, using in situ synchrotron X-ray fluorescence. Their experiments show that Zr is not quantitatively excluded from fluids and that fluid-melt partition coefficients are >0.1, with or without F in the melt. Their work, and related studies, indicates that the HFSE (high field strength element) depletions characteristic of arc magmas might not be controlled by partial melting within or above the slab, but rather that fluid-rock interactions near the slab interface sequester HFSE, as fluids migrate from the slab into the overlying mantle wedge. Upon such reaction, hydrated peridotites just above the slab/wedge interface would then precipitate HFSE-bearing phases (rutile, clinopyroxene, garnet), which are presumed to be carried into the transition zone.
Volume 99 : July 2014 Issue
An alternative to alteration and melting processes in the Earth: Reaction between hydrogen (H2) and oxide components in the Earth in space and time
https://doi.org/10.2138/am.2014.4912
We have three Highlights and Breakthroughs articles in this issue. On page 1193, Mysen provides an overview of experiments by Shinozaki et al. (p. 1265 of this issue), which show that Mg/Si ratios of the mantle may be reduced if metasomatized by high H2+H2O fluids, compared to metasomatism by H2O alone. The relevant reactions create potentially novel Si-hydride species, increase the oxygen fugacity of the mantle, and may affect the SiO2 content of mantle partial melts. On page 1195 of this issue, Hatert reviews a recent paper in our April issue by Chukanov et al. (2014), who describe a new phosphate; as explained by Hatert, this mineral is one of an inexhaustible store of new structures used by materials scientists, and may provide new thermometers applicable to pegmatitic systems. Finally, on page 1197 of this issue, Topper provides a review of Proyer et al. (p. 1429 of this issue) who establish one of the first petrogenetic grids for subduction related rocks based on Ti- and Zr-bearing accessory minerals. These thermodynamic constraints allow the authors to reconstruct a retrograde P-T path in a continental collisional zone at UHP conditions.
Dissolution rates of amorphous Al- and Fe-phosphates and their relevance to phosphate mobility on Mars
https://doi.org/10.2138/am.2014.4613
On page 1206 of this issue, Tu et al. examine the dissolution rates of amorphous Al- and Fe-phosphates. These authors show that both phases dissolve rapidly under acidic conditions and that amorphous Al-phosphates dissolve approximately three orders of magnitude faster than their Fe-bearing counterparts. This work is important because P is an essential element for life as we understand it. These experiments thus show that amorphous phosphates could have provided P to near surface fluids, so as to support biological processes on Mars.
Mantle-derived guyanaite in a Cr-omphacitite xenolith from Moses Rock diatreme, Utah
https://doi.org/10.2138/am.2014.4771
On page 1277 of this issue, Schulze et al. report the discovery of guyanaite (CrOOH) in a pyroxenite xenolith from the Navajo Volcanic Field. The pyroxenites are similar to jadeite blocks from convergent margin serpentinites, and thus the authors interpret these xenoliths as remnants of the subducted Farallon plate. Here, guyanite is thought to result from Na-alteration of a serpentinite during subduction and evolved on a prograde metamorphic path involving dehydration. This work thus appears to confirm that oxy-hydroxide minerals (e.g., AlOOH, FeOOH, etc.) can transport water into the deep mantle, and lends support to the idea that breakdown reactions may control mantle rehydration, which in turn may influence volcanic activity and in this case, uplift of the Colorado Plateau.
A new UHP metamorphic complex in the ~1.8 Ga Nagssugtoqidian Orogen of West Greenland
https://doi.org/10.2138/am.2014.4726
On page 1315 of this issue, Glassley et al. report a series of mineralogical clues that indicate UHP metamorphism in 1.8 Ga rocks from West Greenland. These are the oldest UHP rocks reported to date, and nearly double the age for outcrops of such terranes. Among the observations are quartz needles in fayalite, which they interpret as silica that has been exsolved from b-spinel that incorporated excess silica. Reaction relationships reveal a remarkably low T of equilibration at high P: 975 C at 7 GPa. If the P-T conditions are real, then a key unresolved issue relates to whether the very cold conditions at such depths are unusual, or if such conditions are repeatable, and can be obtained when subduction is sufficiently rapid.
Volatile abundances of coexisting merrillite and apatite in the martian meteorite Shergotty: Implications for merrillite in hydrous magmas
https://doi.org/10.2138/am.2014.4782
On page 1347 of this issue, McCubbin et al. verify the presence of merrillite with OH-rich apatite in the martian Shergotty meteorite. Unlike whitlockite, merrillite does not contain water as an essential structural constituent. The lack of whitlockite in meteorite samples has thus been interpreted to mean that such samples were dry at the time of formation. McCubbin et al.s discovery indicates that the absence of whitlockite is not a reliable indicator of anhydrous conditions, and that the absence of whitlockite and the presence of merrillite might instead indicate not so much dry conditions but rather temperatures of equilibration in excess of 1050 C.
Sb5+Êand Sb3+Êsubstitution in segnitite: A new sink for As and Sb in the environment and implications for acid mine drainage
https://doi.org/10.2138/am.2014.4834
On page 1355 of this issue Mills et al. investigate Sb substitution mechanisms in segnitite, using a sample from the Black Pine mine, Montana, U.S.A. Antimony is a toxic heavy metal that is often found associated with weathering products of mine waste and tailings. Clearly, a means to remove Sb from mine drainage sites is an urgent need. Mills et al. show that at very low pH (>2) the mineral segnitite, which strongly partitions Sb, could be extremely important for the removal of not just Sb, but also Pb, As, and Fe from within the supergene zone.
Exploring the effect of lithium on pegmatitic textures: An experimental study
https://doi.org/10.2138/am.2014.4556
On page 1383 of this issue, Maneta and Baker conduct experiments to explore the effect of Li as a catalyst for the growth of quartz and feldspar from hydrous silicate melts. Their study shows that with the addition of 3700 ppm Li, graphic and granophyric intergrowths of quartz and feldspar occur at about half the supercooling (~100 C) required in the absence of Li (~200 C or more). Moreover, growth rates for feldspar (13 cm/yr) are approximately an order of magnitude greater than in the Li-free system. The crystallization interval is also greatly reduced, implying that crystal growth rates must be rapid for all phases, for a given isobaric loss of thermal energy. Lithium is thus perhaps the most efficient of possible fluxing agents.
Volume 99 : May - June 2014 Issue
Microporous gold: Comparison of textures from Nature and experiments
https://doi.org/10.2138/am.2014.4792
On page 1171 of this issue Brugger et al. examine the origins of microscopic gold. Also known as mustard gold, it is a common weathering product of Au-Te ores, and it can account for a large portion of the gold in economic deposits. The authors present compelling laboratory evidence that Au-Ag-tellurides can be transformed to form a mixture of microporous gold and tellurite or Te-associated iron oxides, which bear a striking resemblance to the textures seen in the natural deposits.
Absence of pressure-induced electron spin-state transition of iron in silicate glasses upon compression
https://doi.org/10.2138/am.2014.4864
We have two Highlights and Breakthroughs articles in this issue. On page 877 of this issue Lee provides an overview of a paper published by Mao et al. on the spin state of Fe in silicate glasses under compression. Their work shows that Fe, whether 3+ or 2+, retains a high spin state at pressures up to 120 GPa. Thus, to the extent that glasses might represent an appropriate analogue for liquids, spin transitions for Fe within the liquid phase are unlikely to explain their macroscopic properties or partitioning behavior in the deep Earth. Then on page 879 of this issue, Poli provides perspective on dolomite solid solutions, as investigated by Li et al., who describe oscillatory zoning in dolomite that describes in detail a prograde metamorphic path within a subduction zone. As noted by Poli, oscillatory zoning may be driven by changes in fluid compositions during the prograde decomposition of amphibole and lawsonite, and that the trace element profiles of carbonates may provide a yet-to-be tapped treasure trove of information on subduction-related metamorphism.
A novel technique for fluorapatite synthesis and the thermodynamic mixing behavior of F-OH apatite crystalline solutions
https://doi.org/10.2138/am.2014.4750
On page 890 of this issue, Hovis et al. provide unit cell and solution calorimetric measurements that show that mixing along the binary fluorapatite–hydroxylapatite is nearly ideal, with no excess volume of mixing, and only minimal negative excess heats of mixing. Their work just shows that there is no thermodynamic barrier to a complete solid solution between these end-members, which means that the F/OH ratios of natural apatite compositions will very directly reflect the F/OH ratios of the fluids or magmas from which they precipitate.
The cooling kinetics of plagioclase feldspar as revealed by electron-microprobe mapping
https://doi.org/10.2138/am.2014.4626
On page 898 of this issue, Iezzi et al. conduct cooling rate experiments on an andesite bulk composition. Their work shows that at higher cooling rates (>12.5 C/min) plagioclase grains tend to be enriched in An content relative to equilibrium values. Perhaps more interesting is that at intermediate cooling rates (3 C/min) two populations of plagioclase are evident, one that is at disequilibrium and enriched in An, and another population that approaches an An-poor composition that mimics equilibrium conditions. Apparently, such intermediate cooling rates are fast enough to force disequilibrium growth of high-An plagioclase, but still low enough to allow low An plagioclase to nucleate in the residual melt. This result may directly apply to lava flows that have thicknesses of a few centimeters to a few meters, where cooling rates will be within a range that may allow the nucleation and growth of both near-equilibrium and disequilibrium plagioclase compositions.
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