Titanian andradite in a metapyroxenite layer from the Malenco ultramafics (Italy): implications for Ti-mobility and low oxygen fugacity |
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| Authors: | Othmar Müntener Jörg Hermann |
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| Institution: | (1) Institut für Mineralogie und Petrographie, ETH Zentrum, CH-8092 Zürich, Switzerland |
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| Abstract: | Ti-andradite (melanite) has been found in a metapyroxenite layer in the upper part of the Malenco ultramafics(Italy), coexisting with clinochlore, diopside and magnetite. Field observations, as well as major and trace elementbulk-rock composition, strongly suggest a cumulate origin for the layer. Textural relationships indicate thatTi-andradite formed during two different metamorphic stages. Under peak metamorphic conditions (400–450°C, 5±2 kbar)Ti-andradite grew in an assemblage of diopside, clinochlore, magnetite and rare ilmenite and perovskite. Later, retrograde brittle deformationinduced formation of veins containing the paragenesis Ti-andradite, vesuvianite, diopside, chlinochlore, magnetite and accessory perovskite.The Ti-andradite varies considerably in TiO2 (0.11–9.62 wt%), Fe2O3(14.3–30.5 wt%), Al2O3 (0.65–3.90 wt%), Cr2O3(>0.18–0.98 wt%) and SiO2 (32.1–36.1 wt%); this is mostly, but not entirely, due to distinct zoning.Ti-andradite contains 0.32 to 0.66 wt% H2O as determined by infrared spectroscopy and 0.83 to 1.76 wt% FeO. The CaO shows almost no variation (34.1±0.7 wt%) and Ca completely fills the dodecahedral site. Single crystal site refinements indicate that no tetrahedral Ti or Fe replaces Si. Titanium incorporation is attributed to similar degrees of substitution along the exchange vectors Ti3+ Fe3+, Ti4+ AlIV Al
-1
VI
Si-1 and (Fe2+, Mn2+, Mg2+)Ti4+ 2Fe
-1
3+
. The presence of mixed valence states of both Fe and Ti suggests a low oxygen fugacity during crystallization of Ti-andradite. Mass balance calculations indicate an isochemical origin of the first generation of Ti-andradite in the clinopyroxenite layer. Its occurrence is restricted to antigorite-free mineral assemblages containing clinochlore of 0.95X
Al>1.1. The hydrothermal crystallization of Ti-rich andradite in veins demonstrates Ti mobility in aqueous fluids under moderate P-T conditions. The zonation patterns indicate disequilibrium conditions during vein crystallization. As no fluorine-, carbonate- and phosphate-bearing minerals were found, OH- is most probably the ligand complexing Ti. |
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