Olivine pseudomorphs after serpentinized orthopyroxene record transient oceanic lithospheric mantle dehydration (Leka Ophiolite complex, Norway)

Abstract

We examine the partial survival of high-temperature mantle microstructures throughout multi-stage hydration- and dehydration-mediated pseudomorphism at differing pressure-temperature-fluid conditions. Throughout the harzburgitic mantle section of the Leka Ophiolite Complex (Norway), finite domains of parallel olivine encompassed by mesh-textured olivine resembling 'perfectly cleaved' olivine grains were identified. Crystallographic orientation mapping, combined with micro-computed tomography, reveals that the parallel olivine grains are highly misoriented (up to 90°) with no crystal-preferred orientation, despite remaining parallel in three dimensions. Parallel olivine grains exhibit free dislocations with low dislocation density, whereas within mesh-textured olivine dislocations are aligned into walls. MnO is enriched (up to 1·8 wt %) and NiO depleted (0·21 ± 0·24 wt %) within parallel olivine grains compared with mesh-textured olivine (0·29 ± 0·14 wt % MnO; 0·38 ± 0·19 wt % NiO). Clinopyroxene lamellae that are crystal-plastically deformed occur sandwiched in lizardite layers between every parallel olivine grain or fully enclosed within olivine. Al. 2O. 3 and Cr. 2O. 3 concentrations of clinopyroxene lamellae (2·09 ± 0·88 wt % Al. 2O. 3; 0·79 ± 0·27 wt % Cr. 2O. 3) overlap with those of primary clinopyroxene grains (2·43 ± 0·69 wt % Al. 2O. 3; 0·83 ± 0·36 wt % Cr. 2O. 3) and are distinctly different from those of secondary diopside found within the parallel olivine domains. Intragranular serpentine inclusions (X. Mg = 0·95 ± 0·01), displaying elevated Al. 2O. 3 (3·92 ± 4·10 wt %) and Cr. 2O. 3 (0·78 ± 0·82 wt %) concentrations, are exclusively found within parallel olivine grains. Lizardite (X. Mg = 0·92 ± 0·02) within the domains originates from hydration of parallel olivine and compositionally overlaps with mesh-texture lizardite. Antigorite (X. Mg = 0·95 ± 0·01) replaces both types of olivine grains. Whole-rock compositions indicate a harzburgitic composition; however, microstructural and chemical observations and the current absence of primary orthopyroxene suggest that the precursor silicate of every parallel olivine domain was a single orthopyroxene grain that was initially serpentinized and later dehydrated to result in the present microstructure. Although desilicification is necessary during the transformation of orthopyroxene to olivine via a bastite stage, calculations based on whole-rock compositions imply that the released SiO. 2(aq) was mobile only over micrometer to centimeter scales, reacting with the surrounding olivine directly to form serpentine. Crosscutting relationships and serpentine compositions imply that dehydration occurred prior to the now evident lizardite- and antigorite-serpentinization. Comparison with the regional geological setting indicates that dehydration may have occurred transiently within the oceanic lithosphere prior to obduction. © The Author 2012. Published by Oxford University Press. All rights reserved.