The Influence of Different H₂O-CO₂ Fluid Compositions and Na-component on the P-T-Stability of Fe-cordierite

Manfred Scheikl & Peter W. Mirwald (peter.mirwald@uibk.ac.at)

Institut f. Mineralogie und Petrographie, Universität Innsbruck, A-6020 Innsbruck, Austria

Cordierite Na_<0.1(Mg,Fe)₂Al₄Si₅O₁₈n(H₂O,CO₂ etc.) is a characteristic mineral phase of metapelites. In connection with Na incorporation experiments into Mg-cordierite (Knop et al., 1998) the influence of H₂O+CO₂+Na-compound (ab) on the P-T-stability of the two endmembers has been studied by Scheikl and Mirwald (1999). The there reported prelimenary data on Fe-cordierite have been substantiated and extended.

Fe-cord+(ab-glass) and its breakdown products alm+sil+qtz +(ab) were used as starting materials in our QFI-buffered experiments which were performed in cold seal pressure vessels (P-T-range: 600 to 750°C and up to 0.3 GPa).

The conflicting data on the P-T-stability of Fe-cordierite at P_tot.= P_H2O produced by different workers have been reviewed by (Mirwald und Knop, 1995). The present reinvestigation at P_tot.= P_H2O reconfirms the data by (Mukhopadhyay and Holdaway, 1994). The results (600 to 750°C) may be represented by equation (1):

P = -0.14 + 0.0005 T (GPa; °C)

In presence of albite and at P_tot.= P_H2O the stability boundary changes to a negative dP/dT-slope (-1 MPa/K) and is confined by partial melting at 700° C. The data yield equation (2):

P = 0.93 - 0.001 T (GPa; °C)

Further experiments without ab-component but in presence of different H₂O-CO₂ fluids (C₂H₄O₄.2H₂O + H₂O and Ag₂C₂O₄) reveal that Fe-cordierite is only stable below X_CO2 = 0.23. Above spl+sil+qtz is formed. The again negatively sloped stability boundary between 640 and 760°C follows the equation (3):

P = 0.75 - 0.0007 T (GPa; °C)

By comparison with the results on Mg-cordierite of the recent study by Scheikl and Mirwald (1999) the correspondent stability boundary for Fe-cordierite in presence of ab-component may be allocated in an intermediate position between the boundaries (2) and (3). The geobarometric potential of this stability boundary is obvious.

Knop E, Scheikl M & Mirwald PW, Terra, Abstr. Suppl. 1, 10, 10, (1998).

Mirwald PW & Knop E, Geol. -Paleont. Mitt. Innsbruck, 29, 153-164, (1995).

Mukhopadhyay B & Holdaway MJ, Contrib. Mineral. Petrol, 116, 462-475, (1994).

Scheikl M & Mirwald PW, Ber. Dtsch. Mineral. Ges., Beih. 1, 198, (1999).

P-T Stability of the Lazulite-Scorzalite Solid-Solution Series

Peter Schmid-Beurmann (psb@min.uni-kiel.de)¹, Stefani Knitter (knitter@mbox.aca.uni-hannover.de)² & Lado Cemic (lc@min.uni-kiel.de)¹

¹ Institut für Geowissenschaften, Universität Kiel, Olshausenstr.40, D-24098 Kiel, Germany

² Institut für Anorganische Chemie, Universität Hannover, Callinstr. 9, D-30167 Hannover, Germany

The isotypic minerals lazulite (MgAl₂(PO₄)₂(OH)₂) and scorzalite (FeAl₂(PO₄)₂(OH)₂) form a solid solution series which is common as accessory phases in variety of geological environments (Pecory & Fahey, 1950). Starting from the P-T-stability of the end members (Cemic & Schmid-Beurmann, 1995; Schmid-Beurmann et al., 1997) the thermodynamic stability of the solid - solutions was investigated in the P-T range between 505 to 675°C and 0.1 to 0.3 GPa in hydrothermal synthesis experiments (Ni/NiO-buffer). Starting from the end - member lazulite the stability of the solid - solutions was found to be strongly decreased with increasing content of (FeAl₂(PO₄)₂(OH)₂) (scorzalite) component. At a 0.2 GPa pure lazulite decomposes at about 660°C whereas at the same pressure a solid - solution with about 80% by mole of (MgAl₂(PO₄)₂(OH)₂) (lazulite) component is only stable up to 590°C under the oxygen fugacity of the Ni/NiO buffer. The members of the lazulite - scorzalite solid - solution series with limiting composition coexist with an Fe-richer member of the (Mg,Fe)Al(PO₄)O series and berlinite according to the formal equation:

Lazulite_ss (Mg,Fe)Al₂(OH)₂(PO₄)₂ <=>

(Fe,Mg)Al(PO₄)O + berlinite (AlPO₄) + H₂O

The mixing behaviour of the lazulite-scorzalite and the (Mg,Fe)Al(PO₄)O solid - solution series was interpreted in terms of a model on the basis of a simple mixture for the lazulite - scorzalite system and of an ideal mixture for the (Mg,Fe)Al(PO₄)O series. With this model the interaction parameter W^G(Laz-Sco) which expresses the non-ideality of the lazulite - scorzalite solid - solution series amounts to W^G (Laz-Sco) = 3.8(9) kJ/mole. This treatment neglects the influence of ferric iron present. Our previous results (Schmid-Beurmann et al., 1999) have shown that about 2% of the individual Me²⁺-sites in lazulite are occupied by Fe³⁺ or vacancies. The remaining sites (98%) are occupied by Fe²⁺ and Mg²⁺.

Pecora WT & Fahey JJ, Am Mineral, 35, 1-18, (1950).

Cemic L & Schmid-Beurmann P, Eur Journal Miner, 7, 921-929, (1995).

Schmid-Beurmann P et al., Mineral and Petrol, 61, 211-222, (1997).

Schmid-Beurmann P et al., Phys Chem Miner, 26, 496-505, (1999).