EEPS Colloquium: Tomáš Magna

1) One group but two tales? HFSE isotope message of carbonatites

Despite their close geochemical proximity and generally similar behavior in magmatic systems, Ti and Zr, IV.A group elements belonging to HFSE, appear to return distincitvely different information about the genesis and post-emplacement evolution of carbonatites, peculiar Ca-rich derivative mantle melts. Stable isotope systematics of Ti and Zr reveal fundamental differences in HFSE behavior in carbonatites and associated alkaline silicate systems. Zirconium shows a notably elevated mobility in F- and OH-bearing secondary fluids which also is reflected in stable isotope compositions. Variations in 94Zr/90Zr could also be imparted to mantle segments percolated by carbonatite-derived metasomatic liquids. In contrast, titanium appears to hold a robust signature over the entire existence of carbonatites from ca. 3 Ga until the last 300 Myr. Global oxygenation of mantle ahead of this time may be responsible for a systematic departure from the mantle value, supporting a redox-controlled Ti isotope variation in Phanerozoic carbonatites.

2) Gravitational collapse as a trigger for explosive calciocarbonatite: Example from Kaiserstuhl, Germany

Calcium-rich carbonates break down rapidly at high temperatures and low pressures, and calciocarbonatites are thus unlikely to form surface lavas. Conversely, the presence of calciocarbonatite pyroclastic deposits indicates that a geological process faster than high-temperature breakdown of Ca-carbonate may facilitate calciocarbonatite eruption. The sudden failure of volcanic edifice exposes suprasolidus high-level carbonatite intrusion and may, therefore, represent a suitable scenario for calciocarbonatite eruption.