[MSA-talk] Notable Papers in Am Min, May-June, 2014

Keith Putirka kputirka at csufresno.edu
Tue May 27 15:09:36 EDT 2014

Dear MSA Members, 

Below are the Editors' picks of notable articles published this month in the American Mineralogist: A Journal of Earth and Planetary Materials . 

You may view the American Mineralogist Noted Papers at http://www.minsocam.org/MSA/Ammin/AM_Notable_Articles.html , or click on the page numbers below. If you are not already logged into GeoScienceWorld via your institution, then from the American Mineralogist menu on http://www.minsocam.org/ , go to "Subscriber Access" - look for and select the "portal page" link, to access GeoScience World. Once at the portal page, enter your user name (e-mail address), and your password (MSA membership number). 

Keith Putirka 
Ian Swainson 

Editors Selections, May-June 2014 

Highlights and Breakthroughs 
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. 

Apatite: the (nearly) ideal mineral 
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. 

Plagioclase Crystallization Kinetics 
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. 

Degassing Paths from Melt Inclusions: A Caveat 
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 H 2 O contents, but they contain highly variable amounts of CO 2 . Provided that these MI were indeed trapped at the same time, these results indicate that H 2 O-CO 2 trends, where H 2 O contents are nearly invariant, might not represent a magma degassing path. Instead, such paths may reflect variations in CO 2 during trapping, or post-crystallization loss of CO 2 from the MI. 

Evolution of Be-bearing minerals 
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. 

Keystone minerals during Serpentinization 
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. 

Magma ascent rates 
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. 

Carbonatite and Kimberlite Genesis 
On page 1119 of this issue, Keshav and Gudfinnsson conduct partial melting experiments in the system CaO + MgO + Al 2 O 3 + SiO 2 + CO 2 . 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 Earth’s surface. 

Nuggets of Truth 
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. 

Keith Putirka 
Editor, American Mineralogist: An International 
Journal of Earth and Planetary Materials 
Dept. Earth & Env. Sciences 
California State University, Fresno 
2576 E. San Ramon Ave. M/S ST24 

kputirka at csufresno.edu 
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