[MSA-talk] Notable Papers in Am Min, Oct., 2017

Keith Putirka kputirka at csufresno.edu
Tue Oct 17 10:58:07 EDT 2017

Dear MSA Members,

Below are Editors' picks of notable articles, for this month’s issue
of the *American
Mineralogist: Journal of Earth and Planetary Materials*, and you can click
here for the full Table of Contents.

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 GeoScience World,
then from the *American Mineralogist *menu on http://www.minsocam.org/
<http://www.minsocam.org/MSA/Ammin/AM_Notable_Articles.html>, go to "Online
Access to MSA Publications <http://www.msapubs.org>" - look for and select
the "portal page" link, to access GeoScience World. Once at the portal
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Keith Putirka

Ian Swainson

*Editors Selections, October 2017*

*Highlights & Breakthroughs*

*Precipitation Conditions of Oxides in Diamonds*

On page 1969 <https://doi.org/10.2138/am-2017-6223> of this issue, Dongzhou
Zhang provides an overview of Uenver-Thiele et al. (page 2054
<https://doi.org/10.2138/am-2017-6119> of this issue) who explore the phase
relationships of starting compositions having MgFe2O4 and Mg0.5Fe2+0.5Fe23+O
4 stoichiometries. These experiments help delimit the precipitation
conditions of magnetite inclusions in diamonds, by taking advantage of
textural clues that might indicate the presence of certain precursor
phases, such as ferropericlase or one of a few “unconventional oxides” that
may be stable at high pressure. Their experimental work also shows that
otherwise simple oxides might exhibit rather great stoichiometric variety
at elevated P and T.


*Dynamics of Magmatic Processes*

*10 Days for Pre-Eruption Magma Assembly at Laki*
On page 2007 <https://doi.org/10.2138/am-2017-6015ccby> of this issue,
Neave et al. use the textures and zoning patterns of plagioclase crystals
to examine the disaggregation of a liquid-crystal mush prior to eruption.
They find that most microcrysts grow just after macrocrysts of a mush are
entrained on a final path to eruption. The result is that the seemingly
homogeneously mixed magmas of the Laki eruptions are the result of multiple
disaggregation and mixing events. The authors calculate, for example, that
the 15 km3 of erupted material of the 1783-1784 extrusives were assembled
in batches of eruptible magma no greater than about 1.5-2 km3, with magmas
being assembled from mush systems about 10 days prior to eruption. These
multiple mixing events also yielded similar magmas, perhaps indicating that
the eruption triggering mechanisms are threshold dependent, although those
thresholds remain undiscovered.

*Making Bubbles*

On page 2022 <https://doi.org/10.2138/am-2017-6093> of this issue Masotta
and Keppler present a new experimental assembly that allows bubble
nucleation and growth experiments at elevated *P-T* conditions (up to 850oC
and 2 kbar), along controlled *P-T*-time paths. Their preliminary
experiments, on a haplogranite (or simple, synthetic granite) system
appears to show that bubble nucleation in a closed system will end when the
average distance between bubbles is equal or less than the mean diffusion
distance of water molecules. (But then how does a water molecule at a
distance greater than the mean know that system-wide the mean has been
reached?) A potentially significant result is that magma fragmentation
(e.g., vapor phases are >70% by volume) appears to occur at 0.2 kbar,
regardless of decompression rate. In any case, their new experimental
assembly should aid investigations of late-stage volcanic processes.

*Does Anhydrous Phase B Exist in Earth’s Mantle?*

On page 2032 <https://doi.org/10.2138/am-2017-6115> of this issue, Kojitani
et al. experimentally investigate the stability of Mg14Si5O24, the
so-called “anhydrous phase B” found in some high P experiments in simple
silicate systems. In this work, Kojitani et al. suggest that this phase can
form, by reaction of forsterite and periclase, at pressures that might
explain the seismic X-discontinuity, which occurs at depths of 250-350 km.
Their work confirms earlier suggestions that the reaction to form anhydrous
phase B should only occur in the presence of fluids, when such fluids
dissolve Si so as to leave a relatively Si depleted residue. Such
conditions furthermore allow this phase to be stable at lower pressures. If
this hypothesis is valid, anhydrous phase B might then affect the
fluid-rock and melt-rock partitioning of trace elements in subduction

*More on Magma Fragmentation -- Lunar Eruptions*

On page 2045 <https://doi.org/10.2138/am-2017-5994> of this issue,
Rutherford et al. use the volatile contents of lunar orange picritic
glasses to reconstruct their eruption history. That history begins with
melt segregation from a melt-rich mantle source at 500 km and vapor
(CO-rich, but including S and H) saturation at somewhere between 4 and 50
km below the surface (or possibly as deep as 500 km in some scenarios),
which then accelerates magma ascent. By the time these magmas reach depths
of 300-600 m, the residual magmas have lost nearly all their volatiles to
the gas phase, and in the new model, these depths would also represent the
point at which the magmas became fragmented (having >70% bubbles by volume).


*Hidden chromium*
On page 2142 <https://doi.org/10.2138/am-2017-6170> of this issue,
Schindler et al. identify a potentially new source of Cr in rock samples:
Chromium (IIII) occurs as inclusions of chromite nanoparticles within
silicates rather than being incorporated into their structures. Transport
models of Cr in the environment may need to take into account the release
and transport of these nanoparticles during weathering rather than
transport of chromium as aqueous species, which could have important
implications for environmental modelling and risk assessments in
chromium-rich regions.

Keith Putirka
Dept. Earth & Env. Sciences
California State University, Fresno
2576 E. San Ramon Ave. M/S ST24

kputirka at csufresno.edu
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