First steps of LA-ICPMS U-Pb magnetite geochronology

The density of metamorphic reactions occurring in subduction zones, due to intense fluid-melt activity, is very high. One of the minerals resulting from all these reactions, both in the downgoing slab and in the serpentinized mantle wedge, is magnetite. Its study is crucial to understanding geodynamic processes, as it gives information about oxidized fluids, temperature, trace elements mobility, etc... It encompasses not only metamorphic petrology but also tectonic processes of accretion, exhumation/obduction, as well as ore geology, as it is a ubiquitous mineral in numerous ore deposit types. Being able to date magnetite, providing a temporal framework to all these reactions, is to put in one more piece of this big puzzle.

Advances in the LA-ICPMS have made that currently, the U-Pb dating reaches far beyond the traditionally dated minerals (zircon, monazite, rutile, etc.). In this context, at the FIERCE laboratory of the Goethe University-Frankfurt, we have investigated the possibility of dating magnetite. Magnetite from several localities has been studied, resulting in a variety of U and Pb contents (up to a few µg/g for U) as well as a significant spread on the U/Pb ratios. This has allowed us to date the studied samples, with internal precisions as good as 1.5% in the best of the studied cases.

Recognising the possibility of dating such a mineral is only the first step in the implementation of the technique. Currently, we are comparing the ages obtained by different methods and different minerals, to search for a magnetite reference material.