Tubular textures in pillow lavas from a Caledonian west Norwegian ophiolite: A combined TEM, LA-ICP-MS, and STXM study

Abstract

Tubular alteration textures, mineralized by titanite, in glassy rims of pillow lavas from a Norwegian ophiolite (Solund-Stavfjord ophiolite complex (SSOC)) are described and characterized by a multimethod approach. Tubular alteration textures, mineralized by titanite, have been previously proposed to result from bioalteration. The microstructure of the titanite and the tubes is investigated using focused ion beam milling in combination with transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). These indicate an assemblage of submicrometer-sized (about 500 nm) titanite single crystals with no organic film or residue in between the grains. In situ U-Pb radiometric dating of the titanite, using laser ablation-multicollector- inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS), yielded a metamorphic age of 442 ± 13 Ma. An isotope dilution-thermal ionization mass spectrometric age obtained previously for zircons from the SSOC plutonic rocks yielded a magmatic age of 443 ± 3 Ma. The overlap in ages indicates that subseafloor metamorphism, responsible for titanite formation, occurred during seafloor or subseafloor formation of the tubular alteration textures. The rare earth element contents of the titanite were determined using LA-ICP-MS and chondrite-normalized patterns are similar to those of the SSOC volcanics; hence these do not reflect hydrothermal or seawater influence. The Y/Ho ratio of ∼20-30 in the titanite is also consistent with an upper mantle-derived origin. The sum of all of the spatial resolved data reported here neither supports nor refutes a biogenic origin for the tubular textures. Copyright 2011 by the American Geophysical Union.