Fe-Ti oxide inclusions and magnetism of oceanic gabbro

Paleomagnetic research aims to reconstruct the Earths magnetic field and, through the determination of apparent pole wander paths, to unravel global-scale tectonics. It is based on determining the ancient geomagnetic field from the remanent magnetization engraved in the magnetic minerals of rocks of different geological age. The large-scale magnetic patterns of alternating polarity flanking mid-ocean ridges have provided crucial insight into reversals of the geomagnetic field and sea-floor spreading. In the basalts and gabbros of the ocean floor iron- titanium oxide minerals are the main carriers of rock magnetism. As compared to basalts, ocean floor gabbros experience relatively slow cooling and undergo a complex petrogenetic evolution. Their magnetic signature has contributions from different magnetic minerals with their remanent magnetization attained at different times and under different geomagnetic fields. The resulting bulk magnetic signature is complex, and its deconvolution requires that magnetic measurements at different scales be combined with mineralogical and petrological information on the formation-conditions and formation-pathways of the Fe-Ti oxide minerals and their subsequent evolution. In gabbros, Fe-Ti oxide minerals that are present as micrometre to sub- micrometre sized inclusions in the rock-forming silicates are particularly stable carriers of rock magnetism. Despite of their small size and the negligible volume fraction they represent, the magnetic properties of the silicatehosted Fe-Ti oxide micro-inclusions determine the magnetic signatures of their host minerals and ultimately the bulk rock remanent magnetization. Based on advances in mineralogical and petrophysical analytical techniques the full continuum from the atomic scale to the hand-specimen scale is now amenable to detailed material characterization. In this project, mineralogical, microstructural, textural and magnetic information will we collected from gabbros dredged from the Mid Atlantic Ridge to link their petrogenetic history with their magnetic signature on all length scales. To this end, state of the art electron beam micro-analytical techniques will be combined with systematic magnetic measurements on bulk- rock samples, separated grains of the Fe-Ti-oxide inclusion bearing silicates, and of the internal magnetic structure of individual Fe-Ti oxide micro-inclusion by electron holography. Understanding the remanence of the silicate-hosted Fe-Ti oxide micro-inclusions in the light of their primary formation and subsequent evolution is of pivotal importance for evaluating magnetic measurements of slowly cooled magmatic rocks. A unique sample collection of gabbros from the ocean floor is available for investigation, and a consortium of project partners from Austria, Russia, Slovenia and Germany with largely complementary expertise teams up for accomplishing the multidisciplinary research tasks. The outcomes of the intended research will extend our ability to interpret magnetic data from ocean floor gabbro and to better link the magnetic inventory to their petrogenetic evolution. 1

The Earth's magnetic field is related to convection in the metallic core and it shields our plant from high energy particles arriving from space. Over the geological times, the Earth's magnetic field has undergone substantial changes. Understanding how the Earth's magnetic field has varied in the past is of key interest and depends on the recording fidelity of the remanent magnetization held within the magnetic minerals in rocks, with magnetite being the most common. Magnetite may occur as mineral grain at the same size as the other constituent minerals of the rock. Alternatively, they may occur as tiny inclusions in common rock-forming minerals such as plagioclase feldspar. When they do, they are particularly robust magnetic recorders due to their small size and are preferred for paleomagnetic reconstructions as compared to the large magnetite grains. Plagioclase from mafic plutonic rocks often contains needle- or lath-shaped magnetite micro-inclusions whose orientations are fixed along specific crystallographic directions of the plagioclase. The uneven orientation distribution of the elongated magnetite micro-inclusions leads to pronounced magnetic anisotropy of the plagioclase-magnetite host-inclusion assembly. This anisotropy can significantly bias magnetic recording by deflecting the magnetization direction into the magnetic foliation plane or the lineation direction, which may be at high angles from the magnetic field direction. We used correlated optical microscopy as well as high-resolution Scanning- and Transmission-Eelectron Microscopy to unravel the systematics of the shape- and crystallographic orientation relationships of elongated magnetite micro-inclusions to the plagioclase host. We then combine optical and electron microscopy with magnetic measurements of individual magnetite bearing plagioclase grains, and show that different types of crystallographic twinning in plagioclase re-enforces the uneven shape orientation distribution of the magnetite inclusions; the resultant anisotropic distribution of magnetite crystals in turn controls the direction of the magnetic remanence.