Poster

1. High-pressure behavior of
Fe-bearing silicate garnets up to 100 GPa
Leyla Ismailova1
, Maxim Bykov1
, Valerio Cerantola1
, Elena Bykova1
, Denis Vasyukov1
, Andrei Bobrov2
,
Catherine McCammon1
, Natalia Dubrovinskaia1
and Leonid Dubrovinsky1
(1)University of Bayreuth, Bayreuth, Germany (2) Lomonosov Moscow State University, Petrology, Moscow, Russia
leyla.ismailova@uni-bayreuth.de
Introduction
Methods
Results
Summary
Bayerisches Geoinstitut , Bayreuth
• High-pressure high-temperature synthesis
• Diamond anvil cell
• Synchrotron radiation
Acknowledgments
Silicate garnets are key phases not only because of their petrological importance in
thermobarometry and oxybarometry, but also due to the intriguing relationship between
their structure and physico-chemical properties. Natural silicate garnets can
accommodate a variety of divalent and trivalent cations in their crystal structure and
form many solid solutions. While most major cations in garnet occur in only a single
oxidation state (Al3+
, Ca2+
, Mg2+
, Si4+
), iron occurs as both Fe2+
and Fe3+
.
Skiagite garnet (Fe2+
3Fe3+
2Si3O12) contains iron in two oxidation states and as a
component in peridotitic garnet, it can be used as a redox sensor to determine mantle
ƒO2 from Fe3+
/∑Fe. With increasing pressure/depth the Fe3+
/∑Fe ratio increases due to
the higher solubility of Fe3+
in garnet, hence expanding the stability field of skiagite. At
greater depth skiagite garnet is expected to accommodate an excess of Si, forming a
solid solution with the iron majorite endmember (Fe4Si4O12). Studying the high-pressure
behavior and redox relations of Fe-bearing silicate garnet can therefore provide
important insight into the chemical composition and physical properties of the Earth’s
mantle.
250 or 125 um diamond cullet size
Re gasket
Ne as a pressure transmitting medium
Ruby chip as a pressure marker
Multi-anvil press
18/11 assembly
LaCrO3 heater
Pt capsule
ESRF, Grenoble, France ID09A High-pressure beamline
λ= 0.4151 Å
ID18 Nuclear Resonance Beamline
DESY, Hamburg, Germany P02.2 Extreme Conditions Beamline
λ= 0.2903 Å ESRF, Grenoble
N P, GPa T, °C V0, Å3
Composition
S6073 9.5 1100 1608.30(3)
Fe2+
3(Fe2+
0.234(2)Fe3+
1.532(1)Si4+
0.234(2))Si3O12
Ski76.6Maj23.4
S6176 9.5 1300 1608.25(3)
Fe2+
3(Fe2+
0.31(2)Fe3+
1.39(2)Si4+
0.31(2))Si3O12
Ski69Maj31
S6177 9.5 1200
1603.81(9) Fe2+
3(Fe2+
0.46(2)Fe3+
1.08(2)Si4+
0.46(2))Si3O12
Ski54Maj46
S6160 7.5 1100 1596.99(3)
Fe2+
3(Fe2+
0.76(5)Fe3+
0.48(5)Si4+
0.76(5))Si3O12
Ski24Maj76
High-pressure single crystal X-Ray diffraction
High-pressure high-temperature synthesis
FeO6
SiO4
Fe2+
Crystal structure of
skiagite-rich garnet
Crystal system – cubic
Space group - Ia-3d
High Spin State
V0=1608.1(4)
K0=169(3)
Low Spin State
V58 =1264.2 Å3
(fixed)
K300, 58=452(12)
GPa
K´fixed to 4
V0=1606(2)
K0=172(1)
V0=1591.8(8)
K0=174(2)
V0=1603.9(8)
K0=171.8(3)
Data were fitted to a 3rd order Birch-Murnaghan equation of state
Synchrotron Mössbauer spectra
High spin Low spin
1. First synthesis of single crystals of skiagite-rich garnet
2. Skiagite-majorite solid solution was shown to extend up
to 76% of Fe majorite component
3. The equations of state of skiagite-rich garnet were
measured up to 100 GPa
4. The spin transition of Fe3+
at 50-60 GPa was observed
5. The pressure region of the spin transition has no
compositional dependence
FeO6
octahedron
↓V(overall) ~ 3%
↓V(FeO6) ~ 7%
All other polyhedra don’t
show any drastic changes upon compression
Bulk modili of octahedra don’t change with composition
Polyhedral volume
S6073
We would like to acknowledge H.P. Liermann and K.Glazyrin (Petra III) for
assistance in using synchrotron radiation at DESY. We would like to thank
M.Hahnfland, A.Chumakov and I.Kupenko from ESRF for support during the
measurements.
Pure skiagite K=170 GPa
(Woodland et al. 1999)