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| Titel |
Partial cordierite breakdown during post-seismic recovery: the significance of plastic deformation for cation diffusion and metamorphic equilibrium |
| VerfasserIn |
Steffen H. Büttner, Gelu Costin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034121
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| Zusammenfassung |
| Brittle intra-crystal fracturing occurred during a microseismic event in migmatites of the
Ordovician Sierras Pampeanas (NW Argentina), forming micro-shear zones and brittle
fragments in cordierite. The seismic event occurred at amphibolite facies P-T conditions
under high strain rates (-¥ 10-7 s-1). During post-seismic recovery and coarsening of crystal
fragments, primary cordierite (XMg=0.65) underwent partial breakdown along the
deformation zone, forming a secondary mineral assemblage in an alteration zone along grain
boundaries of coarsened crystal fragments. The secondary assemblage is restricted to the
recovery zone. The breakdown of primary cordierite (CrdP) is accompanied by the formation
of secondary sillimanite, magnetite, staurolite (XMg=0.24, ~0.5 wt% ZnO), quartz,
and secondary cordierite (CrdS; XMg=0.70-0.80). CrdS, volumetrically the most
important secondary phase, forms by diffusion of Mg and Fe, altering CrdP by
Fe loss and uptake of Mg. All other secondary phases form by nucleation. Two
simultaneous cordierite breakdown reactions have been balanced using CSpace
1.01:
100 CrdP (XMg 0.65) = 21.8 Sil +12.8 Mag + 33.5 Qtz + 5.6 H2O + 89.1 CrdS (XMg
0.75)
100 CrdP (XMg0.65) = 8.1 Mag + 53.6 Qtz + 4.5 H2O + 8.1 St (XMg0.24) + 83.3 CrdS
(XMg 0.75)
The bulk chemical major element composition of the alteration zone is nearly identical to
the composition of primary cordierite, suggesting that elemental exchange between the
alteration zone and the cordierite matrix is limited. However, minor fluid influx, supplying
Zn, K, Si, and O is indicated by the composition of staurolite, minor formation of biotite
and quartz, and by the oxidation of Fe2+ within the alteration zone. The modal
composition of the alteration zone has been determined by point counting, which yields
similar results like CSpace results (converted into vol%), and MODAN calculations,
which calculates modes based on the average alteration zone composition, and the
compositions of secondary phases. The average modal composition of the alteration zone
is:
2.3 Sil + 2.0 Mag + 4.3 Qtz + 3.9 St+ 87.5 CrdS (vol%)
Thermodynamic modelling of primary cordierite breakdown using Theriak Domino
shows that the observed breakdown is possible only in a small P -T window around P =450
MPa and T =555 Ë C, which is in good agreement with the retrograde P - T path of the
Sierra de Quilmes migmatites. Modes calculated using Theriak Domino are similar to results
using descriptive methods (point counting), or methods based on chemistry and
petrography (MODAN, CSpace). Since modes predicted on the assumption of petrological
equilibrium are close to the observed modes, the breakdown reaction seen in the
alteration zone most likely represents conditions of, or close to, thermodynamic
equilibrium.
The formation of the secondary mineral assemblage in the alteration zone depends upon
the efficient supply of cations, essentially Si, Al, Fe and Mg. The bulk composition of new
secondary minerals (Qtz, St, Mag, Sil) is enriched in Fe compared to CrdP, whereas CrdS is
Fe depleted. The provision of Si and Al required for Sil, Qtz, and St can be assigned
to partial cordierite breakdown. The excess Fe needed for Mag and St, and the
removal of surplus Mg from CrdP breakdown, depends on Fe-Mg diffusion within
CrdS. Since CrdS forms exclusively in the post-seismic recovery zone, we interpret
dislocation creep, and hence cation diffusion related to plastic deformation, as the key
process for the formation of reaction products reflecting thermodynamic equilibrium. |
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