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| Titel |
Can subducted eclogite be the petrologic explanation for the X-discontinuity? |
| VerfasserIn |
Alan Woodland, Nadia Knapp, Kevin Klimm |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094844
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| Publikation (Nr.) |
 EGU/EGU2014-10277.pdf |
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| Zusammenfassung |
| The X-discontinuity (X-disc) is a seismic feature observed at some locations around 300-350
km depth (e.g. Revenaugh & Jordan 1991; Deuss 2009). A number of petrologic
explanations have been proposed for this discontinuity, but most recently Pushcharovsky &
Pushcharovsky (2012) consider it to be due to the formation of stishovite in eclogitic bodies,
based upon the suggestion of Williams & Revenaugh (2005). This explanation
considers that stishovite will appear in an eclogitic assemblage either through the
transformation of previously existing free coesite or by exsolution of "excess" SiO2 from
Ca-Eskola-bearing clinopyroxene (Ca0.5AlSi2O6). An essential aspect to this model
is whether enough free SiO2 would be present to produce the observed seismic
impedance contrast or not. Our previous experimental study (Knapp et al. 2013)
indicated that the exsolution of stishovite from Ca-Eskola-bearing clinopyroxene is
probably not a feasible mechanism. Here we report the results of further experiments to
investigate if the amount of free SiO2 in eclogite could be adequate to explain the
X-disc.
A series of experiments was performed with 3 "natural" analog eclogite compositions in
the K2O-Na2O-CaO-MgO-FeO-Al2O3-SiO2 system over a range of pressures from 4 to 10
GPa at 1000-1200°C. The compositions were chosen to simulate i) altered MORB with no
melting (cold subduction), ii) eclogite after shallow melting at 2.5 GPa (slow, flat subduction)
and iii) eclogite after deep melting at 5 GPa (fast, deep subduction). All experiments
produced the typical eclogitic assemblage of clinopyroxene + garnet ± SiO2. Compositions
of clinopyroxene confirm that changes in the Ca-eskola content cannot play a role in
producing the X-disc. In fact the Ca-eskola contents are very small (< 4 mol %).
As expected, the amount of free SiO2 depends on initial bulk composition. The
shallow melting residue (comp. ii) produced no free SiO2 at 4 or 6 GPa. The deep
melting residue (comp. iii) produced < 4 wt % coesite at 6 and 8 GPa with no jump
in amount at 10 GPa where stishovite is stable. These amounts are too small to
produce the impedance contrast observed for the X-disc (≥ 5 wt % SiO2, Williams &
Revenaugh 2005). The unmelted altered MORB (comp. i) yielded 6-7 wt % free SiO2 in
the mineral assemblage, even at 10 GPa. Thus, only subducted oceanic crust that
never underwent any partial melting (cold subduction) would contain enough free
SiO2 to explain the X-disc as it transforms to stishovite at depths of about 300
km.
Deuss A (2009) Surv Geophys, 30, 301-326.
Knapp et al. (2013) Eur J Mineral, 25, 579-596.
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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