Olivine–Spinel Diffusivity Patterns in Chromitites and Dunites from the Finero Phlogopite-Peridotite (Ivrea-Verbano Zone, Southern Alps): Implications for the Thermal History of the Massif
MINERALS(2019)
摘要
The study of Mg-Fe2+ subsolidus exchange between olivine and spinel is a powerful tool to unravel the thermal history of ultramafic rocks. We have implemented such a study using olivine-spinel diffusivity patterns in fresh mineralogical samples from the Finero mafic-ultramafic Complex in the Ivrea-Verbano zone of Northern Italy. Our analytical suite includes chromitites and dunites of the Phlogopite-Peridotite Unit from the core of the Complex. Primary and re-equilibrated olivine and spinel compositions were derived from diffusivity curves calculated by fitting data via an exponential function. Resulting XMg (Mg/(Mg + Fe2+) values were then used for geothermometry. Samples are found to demonstrate a maximum temperature of 849 degrees C and a minimum temperature of 656 degrees C; these temperatures coincide with the limits of elemental exchange in this mineralogic system. We were unable to identify primary olivine/spinel compositions related to the original formation of the dunite-chromitite suite during Early Permian metasomatic activity. Temperature of 849 degrees C is ascribed to the Late Triassic re-heating event dated at 208 +/- 2 Ma. Continuous cooling followed this event at rates of 10(-4) and 10(-2) degrees C/yr until cessation of elemental exchange activity at -656 degrees C. A rapid cooling rate is associated with the uplift and subsequent decrease of geothermal gradient during the early stages of the opening of the Piemont ocean basin.
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关键词
Finero complex,chromitite,olivine-spinel equilibrium,geothermometry,diffusivity patterns,cooling rates
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