Séminaire
Germanium (Ge) has been characterized as a “pseudo-isotope” of Silicon (Si) since 1958, supported by a constant Ge/Si value in seawater. It has been suggested as a good tracer of the Si cycle and biological activity. However, its behaviour remains largely unknown and it has infrequently been used as a tracer so far. Recently, Ge isotope (d74/70Ge) analytical development has facilitated improved characterization of Ge behaviour, providing a useful tool for understanding recent and ancient environments.
Here, I will present the first Ge/Si and d74/70Ge systematics of both hydrothermal fluids and deposits for low and high temperature hydrothermal vents (Loihi Seamount, Pacific Ocean, 18°54’N, 155°15’W and East Pacific Rise (EPR) at 9°50'N respectively). Hydrothermal fluids show an enrichment in Ge and heavy Ge isotopes relative to the host basaltic rock, suggesting precipitation of minerals such as quartz and Fe-sulfides during high-temperature seawater-rock interactions at depth. Modifications of the initial fluid signature reveal that Ge isotope ratios can also provide a record of subsurface reactions. In addition to the information obtained from the hydrothermal fluids, analysis of hydrothermal deposits directly associated with the fluids show a drastic fractionation during Ge precipitation in secondary minerals; Ge isotope signatures (d74/70Ge) have been estimated at around -2.0 ± 0.6 ‰ (2sd) and -5.6 ± 0.6 ‰ (2sd) during Ge precipitation with Fe-oxyhydroxides and sulfides respectively.
Using this knowledge, I will demonstrate the potential impact of combined Ge/Si and Ge isotope analyses on our understanding of terrestrial and coastal environments, and a preliminary oceanic budget of the Ge cycle using sinks and sources d74/70Ge in seawater. I will finish with a brief discussion of the exciting potential applications of this new geochemical tracer.