Zr Diffusion and Zircon Dissolution in H₂O-Bearing Felsic Melts with and Without Halogens (F & Cl)

Aida Maria Conte (conte@axrma.uniroma1.it)¹, Don Read Baker (donb@eps.mcgill.ca)², Carmela Freda (lillif@hotmail.com)³ & Luisa Ottolini (ottolini@crystal.unipv.it)⁴

¹ CNR-C.S. Equilibri Sperimentali in Minerali e Rocce, c/o Dipartimento di Scienze della Terra, Università La Sapienza, 00185 Roma, Italy

² Earth and Planetary Sciences, McGill University 3450 rue University, Montreal, QC H3A 2A7, Canada

³ Dipartimento di Scienze della Terra, Università La Sapienza, 00185 Roma, Italy

⁴ CNR-C.S. per la Cristallochimica e la Cristallografia, Via Ferrata, 1, 27100 Pavia, Italy

The effect of halogens (F and Cl) on Zr diffusion and zircon solubility in hydrated metaluminous granitic melts has been investigated to complement earlier studies on zircon saturation (Harrison and Watson, 1983; Keppler, 1993). Zr diffusion has been measured at 1.0 GPa and temperatures between 1050 and 1400°C. Water, 0.5-5.0 wt%, and halogens, either 0.35 wt% Cl or 1.2 wt% F, were added to the melts. For comparison, Zr diffusion was measured also in a halogen-free hydrous granitic melt at the same conditions. Zircon solubility was determined from the diffusion profiles. This study demonstrates that in melts with water contents between 3.5-5.0 wt%, both Zr diffusion and zircon solubility are only slightly affected by halogens. Difference in Zr diffusion is only a factor of 8 between the hydrous metaluminous melt and either the fluorinated or chlorinated melts at 1200°C. Diffusivity varies from 2.85x10^-14 m²s^-1 at 1050°C to 2.63x10^-12 m²s^-1 at 1400°C. Solubilities range from 767 ppm at 1050°C to 11492 ppm at 1400°C. When 0.5 to 1.5 wt% H₂O is present in the melts, the effect of halogens remains minor. Zr solubility is lowered and diffusion at any temperature is approximately 1.5-2 orders of magnitude below that measured in melts with higher water content. The 2 wt% H₂O content, indeed, seems to represent a "critical" value: at 1200°C, diffusivity in the halogens-bearing compositions attains values comparable to those of the most hydrated melts, while that in the halogen-free ones remain lower (of a factor of 7-8). For most petrogenetic studies it appears that the effect of halogens on Zr diffusion and zircon solubility can be safely neglected.

Harrison TM & Watson EB, Contrib. Mineral. Petrol, 84, 66-72, (1983).

Keppler H, Contrib. Mineral. Petrol, 114, 479-488, (1983).

Magma Evolution of S. Antioco Volcanics (Sardinia, Italy) as Evidenced from Laboratory Experiments and Thermochemical Calculations

Aida Maria Conte (conte@axrma.uniroma1.it)¹, Emmanuelle Argenti², Danilo Mauro Palladino (dpalladino@axrma.uniroma1.it)³ & Raffaello Trigila (trigila@axrma.uniroma1.it)³

¹ CNR-C.S. Equilibri Sperimentali in Minerali e Rocce, c/o Dipartimento di Scienze della Terra, Università La Sapienza, P.le A. Moro, 5 00185 Roma, Italy

² Via L. Dominici, 30, 00062 Bracciano (Rm), Italy

³ Dipartimento di Scienze della Terra, Università La Sapienza, P.le A. Moro, 5 00185 Roma, Italy

Among S. Antioco volcanics, at the SW border of Sardinian Region, basalts, andesites and dacites of Miocene age outcrop as part of the Oligo-Miocene calc-alkaline magmatism of the European, anticlockwise rotated, compressive margin. To estimate phase relations at crustal pressures and magma ascent temperatures and pressures, pilot experiments coupled with thermochemical calculations have been performed on three powdered high-Al₂O₃ basaltic andesites characterized by different K₂O, Al₂O₃ and MgO amounts. Loop experiments performed at 100 KPa in the sub-liquidus region and oxygen fugacity corresponding to NNO buffer, show plagioclase (An₇₅-An₇₇) as the unique crystalline phase in equilibrium with the melt. At 1230°C, for example, in spite of the limited chemical variation among starting compositions, the relative amount of plagioclase varies from less than 5% in the MgO-rich compositions to more than 30% in the Al₂O₃-rich composition suggesting a plagioclase cumulus in the source melt. Thermochemical calculations aiming at estimating P, T and H₂O^liq of magma ascent show equilibrium conditions between experimental and rock phenocrysts plagioclase just for the more albitic compositions (An₇₄-An₇₈) indicating crystallization conditions of these evolved magmas at P in the range 50-250 MPa, T in excess of 1200°C and H₂O^liq between 2 and 3%.

Deformation Mechanisms of Stishovite at 14 GPa, 1300°C

Patrick Cordier (patrick.cordier@univ-lille1.fr)^1,2, Elina Thurel (elina.thurel@univ-lille1.fr)¹ & Jed Mosenfelder (jed.mosenfelder@uni-bayreuth.de)²

¹ LSPES, ESA 8008. Université des Sciences et Technologies de Lille, France

² Bayerisches Geoinstitut. Universität Bayreuth, Germany.

Stishovite is likely to be present in subducted oceanic crust from about 300 km throughout the Transition Zone of the mantle. The stability field of stishovite extends to P, T conditions prevailing in the lower mantle. The transition from low-pressure polymorphs of silica to stishovite involves a change in the coordination of silicon from four-fold to six-fold. Stishovite is therefore an ideal prototype structure to study the effect of coordination and bonding changes on plasticity.

We have synthesised stishovite with large grains in order to enhance plastic deformation by dislocation creep. Corning grade G silica glass was used as a starting material. Glass cylinders were embedded in a Re capsule and annealed in the multi-anvil apparatus at 14 GPa and 1300°C for 10 hours. The recovered material exhibits large needle-shaped grains.

Plastic deformation experiments have been performed with the multi-anvil apparatus in a two-step procedure. Samples were first synthesised as described above. The specimens were then recovered and placed again in a high-pressure assembly designed to generate high deviatoric stresses (the specimen is placed between two hard alumina rods). The deformation experiments were then performed under the same P, T conditions (14 GPa, 1300°C).

Two deformation mechanisms are competing at 14 GPa, 1300°C. Optical examination shows that some grains exhibit undulose extinction, suggesting that dislocation creep has been activated. This is confirmed by TEM which shows dislocations in glide configuration in almost every grains. c glide is dominant. One observes edge c dislocations gliding in {110} planes and screw c dislocations gliding in (100). Secondary a slip is also activated in highly deformed grains. The second deformation mode is deformation twinning on {011}. Shear associated with twinning is evidenced by TEM and many dislocations are present within the twin boundaries. A model of deformation twinning involving glide of partial a/2 dislocations in {011} planes is presented.