CO2-dependent fractional crystallization of alkaline silicate magmas and unmixing of carbonatites within the intrusive complexes of Brava Island (Cape Verde)

Abstract:

Intrusive carbonatites often occur in intimate association with SiO₂-undersaturated rocks such as melilitites, nephelinites, syenites and phonolites. The occurrence of carbonatites on five of the 10 main islands of the Cape Verde hotspot argues for a CO₂-enriched mantle source. Whether alkali-poor carbonatites on the Cape Verdes directly represent small mantle melt fractions or form by extreme fractionation and/or liquid immiscibility from a CO₂-rich silicate magma remains a matter of debate. This study focuses on the pyroxenites, nephelinites, ijolites, syenites, phonolites and carbonatites of the intrusive unit of Brava Island. This relative complete series allows for the deduction of a CO₂-dependent fractionation pathway from the most primitive basanitic dikes towards phonolitic compositions through an ijolitic series. Major and trace element whole rock and mineral composition trends can be reproduced by fractionating a sequence of olivine, augite, perovskite, biotite, apatite, sodalite and FeTi-oxides, present as phenocrysts in the rocks corresponding to their fractionation interval. To reproduce the observed chemistry of the alkaline silicate rocks a total fractionation of ~87% is required. The melts evolve towards the carbonatite-silicate miscibility gap, an initial CO₂ of 0.5 wt% would be sufficient to maintain CO₂-saturation in the more evolved compositions. The modelled carbonatite compositions, conjugate to nepheline-syenites to phonolites, correspond well to the observed ones except for an alkali-enrichment with respect to the natural samples. The alkali-depleted nature of the small carbonatite intrusions and dikes on Brava is likely a consequence of fluid-release to the surrounding wall-rocks during crystallization, where fenitization can be observed. The trace element chemistry of primary carbonates and also cpx within both, the carbonatites and the associated silicate rocks, substantiates our fractionation model. Furthermore, carbonatite and silicate domains on a hand-specimen scale may be interpreted to document the beginning of unmixing. Altogether, there is ample evidence that carbonatites on Brava Island represent conjugate liquids, generated during multiple late stage liquid immiscibility events from highly fractionated alkaline SiO₂-undersaturated magmas.