|
University of Minnesota 116 Church St. SE, Minneapolis, Minnesota, 55455, U.S.A. |
3 Institut de Minéralogie, de
Physique des Matériaux et de Cosmochimie (IMPMC) UMR 7590 Sorbonne Université, CNRS 4 place Jussieu, 75252 Paris Cedex 05, France |
|
Universität Bayreuth, 95440 Bayreuth, Germany. |
Université Paris-Saclay CEA Saclay, 91191 Gif sur Yvette, CEDEX, France. |
| 5
Division
of Geological
and Planetary
Sciences California Institute of Technology Pasadena, CA 91125-2500, USA |
|
It
is well
known that
some
garnet compositions
can incorporate
hydrogen
and/or fluorine at
levels up to several
weight
percent.
However, accurate measurement
of these
elements
can
be difficult
at trace
to minor levels
of abundance
so they are
frequently ignored in routine
chemical analysis.
Furthermore,
the mechanisms
of H incorporation are still
under debate and only one mechanism
for F substitution is commonly considered.
We employed
infrared
spectroscopy (FTIR),
elastic
recoil
detection
analysis
(ERDA),
secondary ion
mass
spectrometry (SIMS), and electron
probe microanalysis
(EPMA) to
measure H
and F concentrations
and constrain
incorporation mechanisms
in ten
grossular garnets.
We also present
SIMS data
for 11
spessartine garnets
and two andradites.
ERDA on three of
the grossulars
was
used to obtain an
infrared integral molar
absorption coefficient (ei)
for H2O of
13,470 L · mol-1·cm-2.
H2O and F concentrations
of
the
grossulars
range from 0.017 to 0.133 wt% and 0.012 to
0.248 wt%, respectively.
Correlations between
16OH and 19F in SIMS
data and
interpretation of FTIR
spectra promt us
to consider various coupled
substitutions of H and F for
Si, which can
explain some high
frequency IR absorption bands that have
been previously attributed
to "hydrogrossular
clusters"
(variably-sized clusters
in
which 4H substitutes
for Si)
or to inclusions of hydrous minerals.
A strong
correlation
between 16OH
and 19F in
spessartines
implies
a
similar role
for H-
F
substitution in these garnets.
Coupled H-F substitution
is also
probably relevant to some andradite-rich garnets,
rare pyropes from the
Dora Maira massif, and
some synthetic garnets. Improvements in
analytical
methods
for trace to
minor H and F open up more possibilities
for using these elements
to calculate the activities of H2O and
F-species
in fluids that were in equilibrium
with garnet-bearing phase assemblages,
as well
as constraining the recycling
of
these elements
into the mantle
via study of xenoliths.