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| Titel |
Is the X-discontinuity really related to the presence of eclogite bodies in the mantle? |
| VerfasserIn |
Alan Woodland, Nadia Knapp, Kevin Klimm |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250081355
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| Zusammenfassung |
| A local seismic feature observed at ~300 km depth is referred to as the X-discontinuity
(X-disc, e.g. Revenaugh & Jordan 1991). Several petrological explanations have been
proposed for this discontinuity, but Pushcharovsky & Pushcharovsky (2012) attribute it to the
formation of stishovite in eclogitic bodies, based upon the suggestion of Williams &
Revenaugh (2005). If this link between the X-disc and the presence of eclogite is
valid, it could have important geodynamic implications. In their model, stishovite
appears in the eclogitic assemblage either through the transformation of previously
existing free coesite or by exsolution of "excess" SiO2 from Ca-Eskola-bearing
clinopyroxene (Ca0.50.5Si2O6). Essential to this model is if the amount of free
SiO2 is enough to produce the observed seismic impedance contrast or not. To test
whether exsolution of stishovite from Ca-Eskola-bearing clinopyroxene is a feasible
mechanism, we have undertaken high-pressure experiments to determine the maximum
Ca-Eskola component that can be incorporated in clinopyroxene over a range of P-T
conditions, both shallower and deeper than that corresponding to the position of the
X-disc.
One series of experiments were performed in the simplified CaO-MgO-Al2O3-SiO2±Na2O system
and one with 3 "natural" analog eclogite compositions (K2O-Na2O-CaO-MgO-FeO-Al2O3-SiO2).
For the CMAS-experiments, all samples have the typical eclogitic assemblage of
clinopyroxene + garnet ± SiO2 ± kyanite. With increasing pressure, the amount of garnet
increases at the expense of clinopyroxene. Maximizing the Ca-Eskola content of
clinopyroxene requires coexistence with a free SiO2 phase and an elevated Al2O3 content,
but not necessarily the presence of kyanite. Ca-Eskola contents of ~20 mol % are obtained at
4 GPa, but decrease steadily with increasing pressure so that -¤ 4 mol % is present at
pressures corresponding to the depth of the X-disc. Experiments in natural analog eclogite
compositions produced even less Ca-Eskola component in clinopyroxene. Thus no sharp
change in Ca-Eskola content occurs as a function of pressure and at depths corresponding to
the X-discontinuity exsolution of all Ca-Eskola component will yield only < 1 wt% free
SiO2. This amount is insufficient to produce a large enough impedance contrast to explain the
X-disc.
If the X-disc is related to the appearance of stishovite in eclogite as proposed
by Williams & Revenaugh (2005), then free SiO2 must be already present in the
mineral assemblage. However, our preliminary results suggest that in an unmodified
MORB-type eclogite only minor amounts of free SiO2 will be present. Greater
amounts of free SiO2 can only be reached in eclogite residues after melt extraction
at high pressures of ~5 GPa, where as residues from melting at lower pressures
(i.e. 2.5 GPa) do not produce any free SiO2. Therefore, if at all, only subducted
oceanic crust that first experienced melting at high pressures can contain enough free
SiO2 to produce the observed impedance contrast of the X-disc as it transforms to
stishovite.
Pushcharovsky DY & Pushcharovsky YM (2012) Earth-Sci Revs, 113, 94-109.
Revenaugh J & Jordan TH (1991) J Geophys Res, 96, 19,781- 19,810,
Williams Q & Revenaugh J (2005) Geology, 33, 1-4. |
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