Genesis of ilmenite rods and palisades along humite-type defects in olivine from Alpe Arami

Autor: Volkmar Trommsdorff, Anne-Chantal Risold, B. Grobéty
Rok vydání: 2001
Předmět:
Zdroj: Contributions to Mineralogy and Petrology. 140:619-628
ISSN: 1432-0967
0010-7999
Popis: Rod-shaped ilmenite inclusions in olivine have been observed in numerous peridotite lenses of the Adula–Cima Lunga unit, Central Alps. The rods are elongated parallel to [010]ol and identical in size, shape and concentration to those reported from garnet peridotite at Alpe Arami (AA). TEM investigation of the rods at AA, Cima di Gagnone (CDG) and Monte Duria (MD) demonstrates that the titanate inclusions correspond to a Mg-bearing ilmenite with exclusively a trigonal crystal structure. No other FeTiO3 polymorphs have been identified. The shape of the ilmenite inclusions concurs with minimum topotactic misfits between the lattices of olivine and Mg-bearing ilmenite. Optical microscopy reveals that the ilmenite rods are not randomly distributed within olivine but have, at all localities, a preferred alignment parallel to (001)ol, forming palisades. TEM images of the ilmenite palisades at AA and CDG show that they are controlled by the presence of (001) planar defects in olivine along which the rods nucleated. The defects have been characterised by high-resolution TEM and identified as 4.4 A-wide humite-type layers, i.e. (Mg2SiO4)·Mg(OH,F)2, intergrown within olivine. In such layers substitution of up to 50 mol% MgH2 by Ti4+ is possible. Assuming Ti-saturated humite slabs, the maximum density of (001) faults observed at AA, approximately 1900 mm–1, can accommodate about 170 ppm TiO2 in olivine. This represents about half the values obtained by UV-laser ablation ICP-MS and EMPA analysis. Evidence for annealing of the defects, however, suggests that only parts of the defects are still preserved. The formation of ilmenite rods is considered to be a consequence of the breakdown of isolated humite layers in olivine. The apparent TiO2 content in olivine at AA, CDG and MD is regarded as a function of the Ti-humite defect density.
Databáze: OpenAIRE
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