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In the context of combined raw material and energy production, public attention focusses increasingly on geothermal power plants. The Upper Rhine Graben (URG) in southwest Germany and eastern France is of special interest as it represents a possible geothermal reservoir with raw material potential. From around 1300 different oil as well as geothermal wells and thermal water wells, knowledge can be acquired about the reservoir rocks of different depths. In the URG, the Triassic sandstones and Variscan crystalline rocks have the greatest geothermal potential. Furthermore, from these wells, the geochemical properties of the fluids can be studied and related to the reservoir rocks. Up to now, only limited knowledge is available about the recent fluid-rock interactions releasing trace elements to the fluid. There are several hypotheses about the reaction kinetics and transport mechanisms of the elements and water. One of the hypotheses is that trace elements are released by the alteration of feldspars and mica of the Variscan crystalline basement. To corroborate the hypothesis, initial mineralogical and geochemical analyses were carried out on partially altered monzogranite core samples which were extracted from different depths while constructing the geothermal power plant in Soultz-sous-Forêts (France). The first results of geochemical and mineralogic analysis revealed a need to concretize the hypothesis. Based on the direct comparison between the unaltered mineral and the alteration product, it was determined that trace elements are released but can also be fixed. Thus, the question arises whether these alteration processes lead to the present fluid composition.

Based on U-Pb geochronology, strongly SiO2-undersaturated foidites and melilitites in the southern Central European Volcanic Province (SW Germany, E France) were shown to have formed during two distinct periods in different regions. An Upper Cretaceous to Lower Eocene group (~72–47 Ma) is characterized by melilite-free nephelinites, nepheline basanites, and haüynites, whereas an Oligocene to Miocene group (27–9 Ma) is represented by olivine melilitites and melilite-bearing nephelinites. Both rock series were generated by extremely low degrees of partial melting of the metasomatically overprinted sub-lithospheric mantle, but exhibit a number of distinct mineralogical, mineral chemical, and whole-rock geochemical features. However, thermodynamic modelling did not reveal significant differences for melt temperature, oxygen fugacity, and silica activity. Accordingly, the foiditic and melilititic melts formed at temperatures of 1100–1350 °C and their polybaric crystallization started at ~14 kbar. The silica activities vary for both age cohorts between ~0.4 and 0.9 with a ΔFMQ of +1 to +4. Thus, it is likely that the differences between the rock series reflect a temporally and spatially heterogeneous sub-lithospheric mantle source beneath central Europe and are coupled to different depths of melt formation, various initial melt compositions, and/or different degrees of partial melting. Besides, an influence of crustal contamination and fractionation cannot be completely excluded. Magmatism in both periods is associated with asthenospheric doming and surface uplift causing erosion and isostatic compensation in the Alpine foreland. These processes indicate decompression and are interrupted by a period of subsidence resulting in a ~20 Myr volcanic hiatus.

Foidites and melilitites are strongly SiO2-undersaturated rocks that form by extremely low degrees of partial melting of the metasomatically overprinted lithospheric mantle. In Central Europe, they occur in volcanic fields, dike swarms or as isolated stocks and diatremes. Our detailed study on foidites from SW Germany indicates two distinct age groups with marked differences in mineralogy and mineral chemistry: Based on in-situ U Pb age data (apatite, perovskite, zircon) a Miocene cohort (~ 9–19 Ma) of predominantly olivine melilitites and melilite-bearing nephelinites can be distinguished from a much older Upper Cretaceous to Lower Eocene group (~ 48–68 Ma) of melilite-free nephelinites and nepheline basanites. This contrasts with previous K-Ar whole-rock and mineral data suggesting continuous magmatism between 90 and 6 Ma. The older group is characterized by the frequent occurrence of green core pyroxenes, hydroxyapatite, and minor feldspar, whereas the younger group contains melilite, late magmatic fluorapatite, Ba- and F-rich mica and occasionally perovskite, but no feldspar. It crops out in the Freiburger Bucht and the Bonndorf Graben, the Vosges (France), the Odenwald and Kraichgau region, in the Taunus and the Lower Main Plain, whereas the younger group is represented by occurrences in the Hegau, the Urach region and the Central Upper Rhine Graben including the Kaiserstuhl. As part of the Central European Volcanic Province, the spatial distribution and age of these rocks reflect regional tectonic events, while the petrologic contrasts between the two age groups indicate heterogeneous crystallization conditions and/or magma source variations such as different formation depths.

Contamination of carbonatite melts is often neglected due to a fast magma ascent and low liquidus temperatures. However, increased silicate mineral formation observed in numerous carbonatite occurrences world-wide requires an external Si introduction. Our study demonstrates that carbonatite dykes penetrating different lithologies of Palabora (South Africa) shows different modes of mineralogical modification. In particular Al and Si-rich lithologies show the most significant effects. Besides silicate mineral formation Si introduction may cause directly and indirectly variations of the REE mineralization at different stages of the carbonatite emplacement. While Si introduction during apatite formation causes an increased REE incorporation into apatite due to the britholite substitution accompanied by an early consumption of REE from the melt, an REE enrichment in the melt and related specific REE mineral formation in late magmatic stages become inhibited. A Ti-rich carbonatite magma additionally experiences the formation of titanite at the expense of ilmenite. Although REE consumption by titanite is less important as for apatite, specific REE consumption can influences REE patterns of subsequent mineralizations. On the other hand, magma wall rock interactions in a carbonatitic systems may furthermore directly influence the type of REE mineralization reflected by discrete REE minerals. In this way contamination can directly control the formation of either allanite, britholite, chevkenite or monazite and hence influences the economic processibility of a REE deposit. Furthermore, the stability of HFSE minerals such as baddeleyite or thorianite can be suppressed by the predominance of their Si-bearing counterparts (e.g., zircon and thorite).

The Gross Brukkaros (Namibia) reflects a broad dome structure showing a crater-shaped depression with numerous peripheral beforsitic carbonatite dykes. These dykes frequently contain an extreme load of basement (Nama-group) xenoliths (> 60 vol.%) including shales, quartzites, granites and gneisses. While xenoliths of exposed country rocks (mainly shales) show an angular habit, a pronounced rounding of xenoliths from other lithologies proves a wide transport and strong abrasion. This consumption of xenolithic material may result in remarkable contamination of the carbonatitic magma. MicroXRF mapping and optical microscopy provides first evidence that the corrosion and alteration of crustal xenoliths is controlled primarily by the xenoliths´ mineralogy and geochemistry. While some xenoliths exhibit distinct zoning reflecting a progressive leaching, others appear to be relatively inert. This proves the Gross Brukkaros of being an ideal natural laboratory to study the influence of crustal contamination in the carbonatitic system, particularly at the subvolcanic-volcanic depth transition. On the other hand, cross-cutting carbonatite dykes generated diatremes almost completely composed of quartz. A closer proximity to the diatreme yields an increase of the Si content in the dykes. In some cases, dykes occur with an extremely high proportion of microscopic xenolith fragments (>95 vol.%) and only subordinate proportions of carbonate. This indicates evaporation of the carbonatite melt during eruption, while the inherent Si remains as a residue along with the xenolith fragments and is precipitated in the diatreme breccia. Combined with C and O isotope systematics, carbonate crystallization is suspected to have proceeded under super-cooled conditions at ~150 °C.

The Gardiner (E-Greenland) and Kovdor (Russia) alkaline complexes display a similar succession of rock types comprising dunites-pyroxenites, ijolite series rocks, melilitolites and carbonatites. Although similar melanephelinitic parental magmas are suggested for both complexes, they display enrichment in HFSE at strikingly different evolutionary stages: At Kovdor, melilitolites are barren but carbonatites are mineralized with HFSE. In contrast, melilitolites at Gardiner contain ore-grade accumulates of perovskite having wt.%-level contents of Nb, Ta and REE, while associated carbonatites are barren. Previous studies suggested that HFSE-poor carbonatites at Gardiner were formed by liquid immiscibility while Kovdor carbonatites result from fractional crystallization and retained high HFSE contents. These two evolutionary trends were explained by a different CO2-dependent stability of melilite vs. clinopyroxene+nepheline+calcite during the ijolite stage [1]. However, it is poorly investigated how the HFSE budget is affected by the crystallization of Ti-phases during different stages of the magmatic evolution, which are stabilized depending on magma composition (i.e. aTiO2, aSiO2) but also on intensive parameters such as P, T, and fO2 [2]. Preliminary results suggest that, in contrast to Kovdor, magmas at Gardiner had physiochemical conditions which favoured abundant crystallization of Ti-phases along with co-precipitation of HFSE earlier in the sequence. This is supported by (1) pyroxenites with abundant Ti-magnetite and ilmenite, (2) titanite-rich ijolites and (3) perovskite-rich melilitolites. Possibly, Ti-rich melts reflect a distinct mantle regime beneath E-Greenland, which also produced anomalously Ti-enriched flood basalts ~6-10 Ma before. [1] Veksler et al. (1998) J.Pet. 39, 2015-2031; [2] Marks et al. (2008) CG 257, 153-172

This session thematizes the formation, evolution and economic importance of carbonatites and alkaline rocks. These play an important role for our society, as they contain exceptional high levels of critical raw materials, such as REE, Nb, P and F, to name a few. However, to understand how such deposits form, we need to understand how their rocks and mineral assemblages formed in space and time. Although our knowledge about the formation and mineralization as well as the magmatic and post-magmatic modification of carbonatites and alkaline rocks strongly improved over the last decades and the extraction of related raw materials could be steadily refined, there are still many open questions that need to be addressed. We encourage participants to submit and present their studies relating to carbonatites and alkaline rocks in this sub-session and to discuss and network their research. The scope of this session covers the formation, the ascent and the emplacement, but also the mineralization, differentiation and alteration of these unique rocks as well as their exploration, exploitation and processing. Experiments and numerical calculations are as welcome as field work studies and analytical investigations.