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Composition and structure of titanian andradite from magmatic and hydrothermal environments Hilton, Elizabeth

Abstract

Titanian andradite provides a wealth of information about the environments in which they form. Zippa Mountain pluton and the Crowsnest volcanic rocks provide examples of titanian andradite formed indifferent environments (e.g., magmatic, skarn, volcanic, and hydrothermal). Electron-microprobe, petrographic, and geochemical analysis, coupled with X-ray techniques were used to determine the composition, structure, site occupancies, and to discriminate between titanian andradite formed in different environments. Site occupancies, determined from the study samples, are as follows: Ca²⁺ and Na⁺ are always assigned to the X site; Mn²⁺ is preferentially assigned to the X site, but may also be at the Y site; Al²³⁺, Mg²⁺, Cr³⁺, V³⁺, Ti⁴⁺, and Fe²⁺ are always assigned to the Y site; Fe3 + may be assigned into the Y site or the Z site; and Si⁴⁺, Zr⁴⁺, and H 4 + are always assigned to the Z site. The titanium substitution mechanism may be via the TiMgFe₂[sub -]³⁺ exchange component, indicative of octahedral Ti substitution and octahedrally controlled cell volume. Chemical zoning of magmatic and skarn titanian andradite is irregular. Garnet from volcanic samples have irregularly zoned cores and regularly zoned rims as does the magmatic cumulus sample. Hydrothermal samples show regular chemical zonation. EPMA data reveals titanian andradite zoning patterns within different rock types and different formation environments. Dark zones in the volcanic garnet contain more TiO₂ and Al₂O₃ and less Fe₂O₃ than lighter zones. Melasyenite samples show positive correlation between TiO₂ and Fe₂O₃ . Conversely, pyroxenite samples show regular zoning with darker zones having more Al₂O₃ and less Fe₂O₃ than lighter areas. In these samples, TiO₂ and Fe₂O₃ are negatively correlated. In terms of Thompson components, magmatic samples have small norms, negative MgCa-, very small H4Si- component, and positive FeMg-, whereas hydrothermal samples have positive H4Si- components, zero to slightly positive FeMg-, and large norms. On average, skarn samples have equal amounts of both T i S i - and TiMgFe₂[sub -]³⁺ components and are therefore intermediate between magmatic and hydrothermal samples. Oxygen fugacify and activity of silica are correlated by TiMgFe₂[sub -]³⁺ and TiSi- components and indicate that the magmatic samples formed under low f₀₂ conditions, skarn samples inherited the f₀₂ signature by interaction with early magmatic fluids, whereas the hydrothermal sample crystallised from a more evolved fluid which had a higher f₀₂ and a[sub SiO₂].

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