- Email: [email protected]
The lawsonite paradox: Mineral equilibria modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O
Goldschmidt Conference Abstract 2006
The lawsonite paradox: Mineral equilibria modelling in the system Na2O–CaO–K2O– FeO–MgO–Al2O3–SiO2–H2O 1
1
G.L. CLARKE , J.A. FITZHERBERT , R. POWELL
Using the chemistry of apatite to track fluids in Fe-oxide Cu–Au systems J.S. CLEVERLEY
2
1
School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia (geoff[email protected]) 2 School of Earth Sciences, University of Melbourne, Parkville, Vic. 3010, Australia Lawsonite equilibria are predicted to occur over a broad PT spectrum developed during subduction, yet lawsonite-bearing assemblages are rare. In the context of mafic mineral equilibria modelled for the range of common crustal metamorphism(P = 4–23 kbar, T = 400–750 °C) using the system Na2O– CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O and software THERMOCALC, unusually high water contents are demanded by lawsonite assemblages. As a consequence, lawsonite assemblages are predicted to have difficulty forming and lawsonite equilibria to be uncommon. Metabasalt undergoing cooler subduction may experience substantial periods involving the metastable persistence of mineral assemblages due to water under-saturation and recrystallization not occurring. If formed, lawsonite-bearing assemblages are observed to be highly unstable; their preservation requires that exhumation be accompanied by substantial cooling. The amount of structurally bound H2O plays a critical role in the formation and preservation of mineral assemblages, controlling key changes in rocks undergoing shallow subduction. doi:10.1016/j.gca.2006.06.123
A105
pmd*CRC and Economic Geology Research Unit, James Cook University, Australia ([email protected]) Apatite has long been recognised as a suitable mineral for determining the HF/H2O and HCl/H2O compositions of fluids in fluid–rock systems (Yardley, 1985; Zhu and Sverjensky, 1991). It has also been recognised as an important indicator mineral for the evolution of S in magmatic systems (Streck and Dilles, 1998). This work presents results from the microprobe analysis of apatite associated with regional un-mineralised and mineralised Fe-oxide Cu–Au hydrothermal breccia systems from the Mount Isa Eastern Succession. The apatites are dominantly F-rich with subordinate amounts of Cl and little or no calculated OH. The F is stoichiometrically enriched relative to the normal 2 afu which, aside from some analytical complexities, may indicate the presence of CO2 in the structure (Binder and Troll, 1989). In the mineralised system (Ernest Henry) there is strong zoning in the apatite with S-rich cores (0.5 wt% SO3) and As-rich rims (5 wt% As2O5). These components are measured in apatite from the regional breccia systems but never in such abundances. Changes in the bulk mineralogy of the regional hydrothermal breccias from magnetite-calcite to Ti-mangnetite-titanite are linked to changes in the S/As and F/ Cl/(CO2) chemistry of the apatite. Assuming that the apatite does not cause in-situ oxidation during the process of substituting S and As for P these elements record the relative activities of the oxidised components (S6+ and As5+) in solution.
References Yardley, B.W.D., 1985. Mineral. Mag. 49, 77–79. Zhu, C., Sverjensky, D.A., 1991. Geochim. Cosmochim. Acta 55, 1837– 1858. Streck, M.J., Dilles, J.H., 1998. Geology 26 (6), 523–526. Binder, G., Troll, G., 1989. Contrib. Mineral. Petrol. 101, 394–401. doi:10.1016/j.gca.2006.06.124
The lawsonite paradox: Mineral equilibria modelling in the system Na2O–CaO–K2O– FeO–MgO–Al2O3–SiO2–H2O 1
1
G.L. CLARKE , J.A. FITZHERBERT , R. POWELL
Using the chemistry of apatite to track fluids in Fe-oxide Cu–Au systems J.S. CLEVERLEY
2
1
School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia (geoff[email protected]) 2 School of Earth Sciences, University of Melbourne, Parkville, Vic. 3010, Australia Lawsonite equilibria are predicted to occur over a broad PT spectrum developed during subduction, yet lawsonite-bearing assemblages are rare. In the context of mafic mineral equilibria modelled for the range of common crustal metamorphism(P = 4–23 kbar, T = 400–750 °C) using the system Na2O– CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O and software THERMOCALC, unusually high water contents are demanded by lawsonite assemblages. As a consequence, lawsonite assemblages are predicted to have difficulty forming and lawsonite equilibria to be uncommon. Metabasalt undergoing cooler subduction may experience substantial periods involving the metastable persistence of mineral assemblages due to water under-saturation and recrystallization not occurring. If formed, lawsonite-bearing assemblages are observed to be highly unstable; their preservation requires that exhumation be accompanied by substantial cooling. The amount of structurally bound H2O plays a critical role in the formation and preservation of mineral assemblages, controlling key changes in rocks undergoing shallow subduction. doi:10.1016/j.gca.2006.06.123
A105
pmd*CRC and Economic Geology Research Unit, James Cook University, Australia ([email protected]) Apatite has long been recognised as a suitable mineral for determining the HF/H2O and HCl/H2O compositions of fluids in fluid–rock systems (Yardley, 1985; Zhu and Sverjensky, 1991). It has also been recognised as an important indicator mineral for the evolution of S in magmatic systems (Streck and Dilles, 1998). This work presents results from the microprobe analysis of apatite associated with regional un-mineralised and mineralised Fe-oxide Cu–Au hydrothermal breccia systems from the Mount Isa Eastern Succession. The apatites are dominantly F-rich with subordinate amounts of Cl and little or no calculated OH. The F is stoichiometrically enriched relative to the normal 2 afu which, aside from some analytical complexities, may indicate the presence of CO2 in the structure (Binder and Troll, 1989). In the mineralised system (Ernest Henry) there is strong zoning in the apatite with S-rich cores (0.5 wt% SO3) and As-rich rims (5 wt% As2O5). These components are measured in apatite from the regional breccia systems but never in such abundances. Changes in the bulk mineralogy of the regional hydrothermal breccias from magnetite-calcite to Ti-mangnetite-titanite are linked to changes in the S/As and F/ Cl/(CO2) chemistry of the apatite. Assuming that the apatite does not cause in-situ oxidation during the process of substituting S and As for P these elements record the relative activities of the oxidised components (S6+ and As5+) in solution.
References Yardley, B.W.D., 1985. Mineral. Mag. 49, 77–79. Zhu, C., Sverjensky, D.A., 1991. Geochim. Cosmochim. Acta 55, 1837– 1858. Streck, M.J., Dilles, J.H., 1998. Geology 26 (6), 523–526. Binder, G., Troll, G., 1989. Contrib. Mineral. Petrol. 101, 394–401. doi:10.1016/j.gca.2006.06.124