Transcript McCammon_Earth`s Interior

Applications of Mössbauer Spectroscopy
to Studies of the Earth’s Interior
C. McCammon
Bayerisches Geoinstitut
Universität Bayreuth
D-95440 Bayreuth, Germany
E-mail: [email protected]
http://www.bgi.uni-bayreuth.de
Introduction
The major mineralogy within the Earth’s mantle has
primarily been determined through (1) laboratory
synthesis and characterisation of high-pressure phases
for the relevant chemical systems (e.g., MgO-FeOSiO2), (2) measurement of their elastic properties, and
(3) comparison of these properties with seismic data
(figure right). Although iron is only ~ 6 atom % of the
mantle, it is the most abundant transition element, and
changes in oxidation and spin state influence a broad
spectrum of physical and chemical properties. 57Fe
Mössbauer spectroscopy is one of the most important
tools for studying iron in mantle phases, particularly
because it can be applied at conditions of both high
pressure and high temperature.
For recent reviews of iron behaviour in the Earth’s mantle see:
- C. A. McCammon, Science 308, 807-808 (2005).
- C. McCammon, in Earth's Deep Mantle: Structure, Composition, and Evolution R.D. van der
Hilst, J. Bass, J. Matas, J. Trampert, Eds. (American Geophysical Union, Washington D.C., 2005)
pp. 221-242.
- C. McCammon, J. Mineral. Petrol. Sci. 101, 130-144 (2006).
- D.J. Frost, C.A. McCammon, Ann. Rev. Earth. Planet. Sci. 36, 389-420 (2008).
Several examples of 57Fe Mössbauer investigations relating to the Earth’s interior are
given in the next slides.
Oxygen fugacity during metasomatism
Mössbauer spectra taken of different
zones of metasomatised garnet
peridotite (figure right) show
differences in Fe3+ concentration
(figure far right) that can be
converted to oxygen fugacity values
using the olivine-orthopyroxenegarnet oxybarometer. Results (figure
bottom) show a progressive increase
in oxygen fugacity during
metasomatism to nearly reach the
breakdown curve between
graphite/diamond and carbonate.
Diamond is therefore unlikely to
survive extended metasomatising
events in the upper mantle.
metasomatism
2
C.A. McCammon, W.L. Griffin, S.H. Shee, H.S.C. O'Neill, Contrib. Mineral. Petrol. 141, 287-296 (2001).
FMQ = fayalite-magnetite-quartz
OG = olivine-graphite
OD = olivine-diamond
IW = iron-wüstite
EM = enstatite-magnesite
Depth profile of oxygen fugacity in the upper mantle
Numerous studies in the past two decades have shown that the dominant assemblage
in the top part of the upper mantle, spinel peridotite, equilibrated at oxygen fugacities
ranging from -2 to +2 FMQ, depending on factors such as tectonic environment and
metasomatism. Oxygen fugacity at greater depths has been determined through
measurement of Fe3+ in garnet using Mössbauer spectroscopy (figure left) combined
with the olivine-orthopyroxene-garnet oxybarometer. Results show a strong decrease
in oxygen fugacity with depth to nearly Fe metal equilibrium (figure right).
subduction
continent
spinel peridotite
garnet 25-4
pressure (kbar)
depth (km)
MORB
garnet
peridotite
Dlog fO2 (FMQ)
MORB = Mid-ocean rich basalt
A.B. Woodland, M. Koch, Earth Planet. Sci. Lett. 214, 295-310 (2003).
C.A. McCammon, M.G. Kopylova, Contrib. Mineral. Petrol. 148, 55-68 (2004).
Iron oxidation state in transition zone phases
The dominant phases of the transition zone can be
synthesised at high P,T in the laboratory and quenched
to ambient conditions for study using Mössbauer
spectroscopy (figure bottom). When these phases are
synthesised at their minimum oxygen fugacity stability
limit (i.e., in equilibrium with SiO2 and Fe metal),
Mössbauer spectra (figure right) show the presence of
measureable Fe3+ in all phases, implying elevated Fe3+
in the transition zone despite relatively low oxygen
fugacity conditions.
ringwoodite
wadsleyite
ringwoodite
wadsleyite
majorite
DJ Frost
majorite
H.S.C. O'Neill, C.A. McCammon, D.C. Canil, D.C. Rubie, C.R. Ross II, F. Seifert, Amer. Mineral. 78, 456-460 (1993).
Iron oxidation state in (Mg,Fe)(Si,Al)O3 perovskite
(Mg,Fe)(Si,Al)O3 perovskite can be synthesised at
high P,T in the laboratory (figure bottom left). In the
absence of Al, the phase incorporates 15-20 % Fe3+
(figure top right), while the presence of Al stabilises
much higher concentrations (figure bottom right and
bottom centre). To balance charge in the lower
mantle, the following reaction takes place:
3 Fe2+ → Fe23+ + Fe0.
200 µm
C.A. McCammon, Nature 387, 694-696 (1997).
S. Lauterbach, C.A. McCammon, P. van Aken, F. Langenhorst, F. Seifert, Contrib. Mineral. Petrol. 138, 17-26 (2000).
D.J. Frost, C. Liebske, F. Langenhorst, C.A. McCammon, R. Trønnes, D.C. Rubie, Nature 428, 409-411 (2004).
Diamonds from the lower mantle
Mössbauer spectra of inclusions in diamonds
from the lower mantle (figure below) show
that (Mg,Fe)(Si,Al)O3 perovskite contains a
large fraction of Fe3+, while (Mg,Fe)O
contains only a small amount (figure right).
This is consistent with a low oxygen fugacity
in the lower mantle, and the stability of large
amounts of Fe3+ in the perovskite phase, even
under reducing conditions.
(Mg,Fe)(Si,Al)O3
(Mg,Fe)O
C.A. McCammon, M. Hutchison, J. Harris, Science 278, 434-436 (1997).
Iron spin state in (Mg,Fe)O
Mössbauer spectra (figure right) of (Mg,Fe)O
subjected to high pressure in a diamond anvil cell
show the appearance of a new component (red singlet)
which corresponds to Fe2+ in the low-spin state. Spin
crossover is expected to occur in (Mg,Fe)O over a
broad region in the lower part of the lower mantle
(figures below).
S. Speziale, A. Milner, V.E. Lee, S.M. Clark, M.P. Pasternak, R. Jeanloz, Proc. Natl. Acad. Sci. 102, 17918-17922 (2005).
I.Y. Kantor, L.S. Dubrovinsky, C.A. McCammon, Phys. Rev. B 73, 100101 (2006).
Iron spin state in (Mg,Fe)(Si,Al)O3 perovskite
Pressures given in GPa
Pressures given in GPa
Mössbauer spectroscopy
(figure right) and NFS (figure
far right) data of
(Mg,Fe)(Si,Al)O3 perovskite
subjected to high pressure and
temperature in an externally
heated diamond anvil cell
show the appearance of a new
component (red doublet)
which corresponds to Fe2+ in
the intermediate-spin (IS)
state. IS Fe2+ is observed to be
stabilised by high
temperature; hence lower
mantle perovskite contains
Fe2+ predominantly in the IS
state.
C. McCammon, I. Kantor, O. Narygina, J. Rouquette, U. Ponkratz, I. Sergueev, M. Mezouar, V. Prakapenka, L. Dubrovinsky, Nature Geoscience 1, 684-687 (2008).