A paper of interest to the geo-metamorphism list titled
"Yttrium zoning in garnet:
Coupling of major and accessory phases during metamorphic reactions"
by Joseph M.Pyle and Frank S.Spear
has just been published in Geological Materials Research, the refereed
electronic journal of the Mineralogical Society of America
(http://gmr.minsocam.org). The paper is large (4700K) because it is
full of color photos of elemental maps of garnet crystals, but it is
worth the wait to download it.
I urge you to view the papers published in GMR and to consider how the
publication of your own research might be enhanced by the use of color,
animations, etc. The GMR editors welcome submissions in petrology,
mineralogy, crystallography, and related fields.
Sincerely,
John Brady
=====================
John B. Brady
Department of Geology
Smith College
Northampton, MA 01063
Voice: 413-585-3953
Fax: 413-585-3786
GMR: gmr.minsocam.org
=====================
Yttrium zoning in garnet:
Coupling of major and accessory phases during metamorphic reactions
Joseph M. Pyle1 and Frank S. Spear2
Department of Earth and Environmental Sciences, Rensselaer Polytechnic
Institute, Troy, NY 12180, USA
1<[log in to unmask]> 2<[log in to unmask]>
(Received:July 10, 1999; Published: December 14, 1999)
Abstract
The concentration of yttrium in pelitic garnets as a function of
metamorphic grade has been examined in relation to the distribution of
xenotime (YPO4) in samples from New England and British Columbia.
Samples with xenotime present only as inclusions in garnet generally
possess high-Y cores and concentrations that drop off discontinuously
along zoning shoulders of variable width to low-Y outboard regions.
Samples with matrix xenotime are restricted to the garnet zone; Y
concentration of these garnets generally decreases smoothly from core to
rim. Xenotime may also be present in reaction zones around garnet. In
xenotime-bearing samples, [Y]Grt is strongly temperature-dependent and
ranges from ~5000 ppm in the garnet zone to ~150 ppm in the sillimanite
zone. Measured yttrium zoning profiles in xenotime-absent samples are
reproduced with both Rayleigh fractionation and diffusion models, but
P-T histories of the samples examined favor the Rayleigh model, with
garnet volume, bulk-rock yttrium, and mode of (Y,HREE) accessory phases
controlling the profile shape. High-yttrium annuli in staurolite-zone
samples may form by garnet overgrowth of proximal matrix enriched in
yttrium due to garnet consumption during discontinuous
staurolite-forming reactions. An increase in [Y]Grt and [HREE]Grt in
garnet from anatectic samples is related to dissolution of phosphates in
vapor-absent, peraluminous melt, with partitioning of highly compatible
Y and HREE into garnet grown during anatexis; textural analysis reveals
that phosphates are absent from regions of garnet grown in equilibrium
with melt. A main result of this study is identification of an intimate
coupling between major pelite phases and accessory phases during
reaction progress. This coupling is of great advantage in that it may be
used to: (1) calibrate sensitive geothermometers and geobarometers, (2)
identify particular regions of garnet grown in different
garnet-producing reactions over a range of grades, and (3) reveal
portions of pelite reaction history invisible to major elements.
Keywords: garnet, pelite, xenotime, yttrium, trace element, zoning.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|