Isotopic constraints of mantle derived carbonatitic magma from Calatrava, Spain

Carbonatite volcanism is typically associated both spatially and temporally with alkaline, ultramafic volcanism (Woolley & Church, 2005). Recent discoveries in Calatrava, Spain illustrate the activity of carbonatite in the source melts of leucitite volcanism. Melilitite pyroclastic lapilli tuffs also show a clear association with volcanic carbonate. Carbonatitic activity has been shown to initiate at depths greater than 100km (Humphreys et al., 2010) despite a maximum estimate of lithospheric thickness of 80km. The presence of aragonite and abundant mantle xenoliths in many deposits are clear indication of the rapid emplacement rates of such magmas. Carbonatitic activity in the source of the leucitite melts is indicated by carbonate inclusions within olivine xenocrysts and the presence of occasional carbonatite lenses. The composition of lead and strontium isotope ratios in the bulk rock, and spatially resolved analysis of carbonate from the groundmass and from inclusions demonstrates a genetic affinity between the inclusions and the related bulk rock composition. Lead and strontium isotopic analysis suggest that such melts do not represent the composition of convecting asthenospheric mantle. 87Sr/86Sr (0.7055-0.7068) values are higher than those of MORB and most OIB. Lead isotope ratios show a trend displaced to higher 207Pb/204Pb relative to MORB and OIB. Carbonate inclusions have less radiogenic lead values than the more radiogenic bulk rocks. Our data indicate that carbonatitic activity in the mantle is intrinsic in the generation of the leucitite lava. However, petrographic and isotopic evidence suggest a complex melt history. Olivine xenocrysts are not in equilibrium with the host leucitite, despite inclusions within olivine showing an isotopic affinity to the bulk rock. We suggest that in this example, alkaline magmatism was induced by the presence of CO2 in the mantle source. Isotopic evidence shows that the mantle producing such melts was not asthenospheric mantle; we therefore infer that metasomatism has resulted in the thickening and enriching of the lithosphere. Carbonatitic and silicate metasomatism is reported from the mantle in this region and the presence of abundant igneous carbonate associated with other alkaline volcanic rocks in the province, leads us to conclude that the presence of CO2 in the mantle and the resulting carbonatitic melts may play a crucial role in the genesis of many intraplate alkaline volcanic rocks. Humphreys, E.R., Bailey, K., Hawkesworth, C.J., Wall, F., Najorka, J. & Rankin, A. (2010). Aragonite in olivine from Calatrava, Spain: Evidence for mantle carbonatitic melts from >100km depth. Geology, 38, 911-914. Woolley, A.R. & Church, A.A. (2005). Extrusive carbonatites: A brief review. Lithos, 85, 1-14.