Multi-phase saturation experiments on the Oldoinyo Lengai natrocarbonatite lavas: derived from a sodic-potassic calciocarbonatite through fractionation

Abstract:

Typical natrocarbonatite eruption temperatures are 490-595°C and are at least 200-300°C lower than temperatures for any suitable silicic liquid that could be conjugate across a miscibility gap. In particular, the 2007 Oldoinyo Lengai (OL) eruption delivered a mix of nephelinitic and carbonatite melts, supporting the commonly accepted supposition that carbonatites are conjugate to spatially associated peralkaline nephelinites. We constrain possible parental melt compositions of the natrocarbonatites by saturation with calcite, apatite, cpx, wollastonite, nepheline, combeite or nyerereite (at 1 kbar, 650-1200°C, fast-quench cold-seal vessels); all occurring in the silicate or carbonatite rocks of OL The results show that the natrocarbonatite is highly undersaturated in calcite and apatite. At 1200°C, calcite saturation results in decreasing Na₂O+K₂O from initially 41.5 to 10.3 wt% while the CaCO₃-component is increased to 80 wt%. In the complex system, there is no thermal (nyerereite-fairchildite) maximum, hence fractionation of calcite+apatite may proceed from a parent melt with 15 wt% alkali and 70% Ca-component to the observed OL natrocarbonatites. The observed melt inclusions in phenocrysts in the nephelinites at Keramasi (Guzmics et al. 2012, CMP) would serve as ideal parents, these melt compositions correspond to 1050^oC. The modelled liquid line of descent along the calcite surface requires a total fractionation of ~48% calcite and ~9 wt% apatite. SiO₂ solubility only increases from 0.2-2.9 wt% at 750-1200°C, leaving little leeway for reaction with silicates. A peritectic reaction among the above silicates conserving the Si-content in the carbonatite could not be identified. At >950°C cpx yields, through peritectic melting, an immiscible peralkaline silicate melt + extensive wollastonite (as observed in OL cumulates). This experimental silicate melt resembles the unusual silicate ash compositions from the 2007 eruption. The natrocarbonatite lavas from OL are not conjugate liquids to spatially associated nephelinite or phonolite melt. Saturation of the carbonatite with the minerals saturated in these silicate melts at the time of immiscibility yields a Na-K-rich calciocarbonatite parent, while the observed natrocarbonatite would be several hundred degrees above its liquidus.