Magnetite microstructure in oceanic serpentinites

Magnetite frequently forms during low-temperature serpentinization of mantle rocks and is hence a common mineral in serpentinites at oceanic core complexes. Deformation of serpentinites is considered a crucial ingredient in many geodynamic processes and yet poorly understood. Here we explore magnetite microstructures in order to test their suitability to track deformation in serpentinites.

The Atlantis Massif at the mid-Atlantic ridge is an oceanic core complex where a ~1.25 km long core (U1601C) was drilled during IODP Expedition 399. Shipboard data of recovered samples show strong variations in macroscopic parameters such as alignment, thickness and spatial distribution of magnetite aggregates. Using electron backscatter diffraction (EBSD), we analyse how microstructural parameters such as grain size, grain shape and alignment and crystallographic preferred orientation (CPO) link to the macroscopic parameters .

Four representative samples with a distinct macroscopic foliation intensity have been chosen and large area ESBD/EDX maps produced at 0.2/0.5 μm step size. Grain size is mostly <10 μm equivalent diameter. Long axes of individual grains tend to be (sub)parallel to the long axes of larger magnetite aggregates, irrespective of whether aggregates contribute to the main foliation or not. Magnetite CPO strength is variably strong and dominated by CSL3 twinning.

We will discuss the relation between macroscopic and microstructural parameters with regard to the development of the magnetite foliation and its possible relation to deformation.