Refractory as well as insoluble nature of zirconium (Zr) dictates that its behavior mostly is driven by magmatic fractionation whereby Zr-rich phases, such as zircon or baddeleyite, and to a lesser amount pyroxene, can control stable Zr isotope systematics of silicate systems. In carbonatites, HFSE often are carried by pyrochlore, garnet and/or pyroxene, and scavenging of these phases during magmatic evolution of carbonatite liquids may result in significant depletions, particularly apparent for Ti, Zr and Hf. Therefore, fractional crystallization of HFSE-bearing phases may bear on the understanding of the role of carbonatites for HFSE distribution in the mantle.

We have analyzed Zr stable isotope systematics (δ^94/90Zr_IPGP-Zr) of several carbonatites from various geotectonic positions to further constrain their petrogenesis. The preliminary data shows ~0.4‰ variation which is not easily related to major element chemistry of carbonatites, nor emplacement age. Samples of unmodified carbonatites from continental rifts and hot-spots plot above the mantle value (δ^94/90Zr = 0.04 ± 0.04‰), with δ^94/90Zr of up to ~0.35‰ whereas carbonatites from shear zones display resolvedly lower δ^94/90Zr. Carbonatites overprinted by F-rich fluids carry distinctly low δ^94/90Zr, associated with high Nb/Ta, suggesting high mobility of HFSE in F-rich fluids. In contrast, δ^94/90Zr of carbonatites carrying sulfide mineralization does not deviate from that of unmodified carbonatites. These cumulative observations indicate stable Zr isotope fractionation between silicate and carbonate melts in the mantle. Besides, they also indicate strong mineralogical control of HFSE-bearing phases on the stable Zr isotope systematics of carbonatites.

Carbonatitic melts are subject to different processes during their ascent (e.g., fractional crystallization and crustal contamination), which may cause a strong change in their composition. Their original composition has not been definitively determined. In order to find indications of the primitive composition, it appears reasonable to investigate the deepest known carbonatite occurrences. One of the deepest known carbonatites is the Palabora complex in South Africa. Most of the complex is represented by varieties of pyroxenite, while the center of the complex comprises a multiple calcite carbonatite intrusion (called Loolekop). A new discovery of a second carbonatite in the southern part of the complex, reveals a head section of a stuck carbonatite intrusion, which is reflected by isolated veins on the surface and a more extensive abundance with depth. This carbonatite shows a strong enrichment in Ti (>10 wt.% TiO₂) with up to 20 modal% ilmenite. Fenitising fluids exsolved from the carbonatitic melt have even titanitised the surrounding pyroxenite. Additionally, previous investigations of the Loolekop show that the Ti content of the carbonatite increases systematically from the center of the intrusion to the margin. This could indicate a noticeably higher Ti content in the more primitive melt that crystallized in the marginal areas in comparison to a slightly more evolved melt that formed the center. Could this be an indication that primitive carbonatite melts are rich in titanium and lose the titanium rapidly during their evolution?

Carbonatites are relatively rare rocks with only about 600 occurrences world-wide. While dominated by intrusive carbonatites the rock record only shows about 50 occurrences of extrusive carbonatites. The geochemical link between intrusive and extrusive equivalents is essentially unstudied. To shed light on this topic the interface between fine-grained dolomite-carbonatite dykes and associated diatremes of the Gross Brukkaros in central Namibia was investigated. Whole rock geochemistry and petrography provide evidence that the carbonatite dykes contain significant amounts of Si and Al, which is assigned to assimilation of crustal xenoliths. At the transition from carbonatite dyke to diatreme formation, the carbonatite melt degassed (release of CO₂ and other volatiles to the atmosphere), while the Si, Ca, Mg and Fe load together with a high amount of trace elements became precipitated from a hydrothermal “magmatic-provenance-dominated” fluid during rapid temperature drop in the course of decompression as a mixture of micro- and cryptocrystalline quartz (quartz I) and aegirine-augite (plus minor magnetite). This mineral assemblage forms the matrix of the diatreme breccia. A second quartz generation (quartz II) is formed in the post-eruption environment by precipitation from a fluid resulted by a mixture of remaining fluids and an influx of meteoric waters. All measured trace elements show significantly higher contents in quartz I compared to quartz II (with exception of Li). This study provides the first holistic dataset that show how a carbonatite geochemically behave during eruption.

The Miocene Kaiserstuhl Volcanic Complex in the Upper Rhine Graben is known for simultaneously exposing both intrusive and pyroclastic calciocarbonatites. This makes Kaiserstuhl a promising candidate for studying the field and genetic relations between intrusive calciocarbonatite and its eruptive equivalent, and the processes enabling eruption of the calciocarbonatite at the surface in particular. Eruptive calciocarbonatites in Kaiserstuhl are represented by carbonatite tuff and lapilli-stone beds covering a agglomerate fan on the western flank of the volcano. The debrites represent lahar (debris flow) and possibly also debris avalanche deposits. Based on observed textures, the debris flows were most likely derived by water-dilution from debris avalanches resulting from edifice failure, which occurred in the central part of the Kaiserstuhl Volcanic Complex and ultimately exposed the intrusive system. The carbonatite pyroclasts (lapilli and ash) were ejected from narrow vents represented by open framework tuff-breccias aligned along the detachment scarp. Since the Ca-carbonates break down rapidly at high temperatures and low pressures, calciocarbonatites are unlikely to form surface lavas. On the other hand, the presence of the calciocarbonatite pyroclastic deposits suggests that some geological process faster than the high-temperature break-down of Ca-carbonate may facilitate calciocarbonatite eruption. Prompt exposure of a suprasolidus high-level carbonatite intrusion by edifice collapse may be a suitable scenario enabling calciocarbonatite eruption. The absence of edifice failures on alkaline volcanoes, where carbonatite intrusion is either supposed or exposed, may explain overall scarcity of erupted calciocarbonatites.