The role of carbonatitic volcanism in the degassing of mantle CO2

It has long been known that regions of central Europe, such as the Eifel, Germany and Massif central, France are areas where significant volumes of CO2 are liberated. Many of these areas are exploited to produce bottled, carbonated mineral water. Areas with a high CO2 output are not just confined to Europe, but occur in many continental regions around the world. One characteristic association is that of alkaline ultramafic magmatism, which in many European provinces is of Miocene to quaternary age, and although volcanism is no longer active, carbon dioxide emissions are still prolific. The recent discovery of extrusive carbonatitic activity in alkaline volcanic provinces adds to their complexity, but also hints at the role of CO2 in the genesis of such melts. Carbonatites are now documented from Spain, France, Italy and Germany, with potential examples in many other regions. Recent discoveries by Humphreys et al. (2010) from Calatrava, Spain, show that volcanism is derived from below the depth of the lithospheric mantle (~80km) and that CO2 is an essential component of the mantle melt. The presence of aragonite, abundant mantle xenolithic debris and the physical morphology of the typical diatremic volcanoes are evidence of deep-derived, rapidly erupted mantle melts. In all volcanic provinces where carbon isotopes have been measured from CO2 emissions, they are indicative of a mantle origin. Together with evidence for CO2 in the source of many volcanics and our recent evidence of their sublithospheric mantle origin, we suggest that CO2 in these provinces represents an important component of direct mantle degassing. The volumes of CO2 and rates of emission are yet to be quantified in most regions, but importantly outlast volcanic activity by millions of years in some cases. We suggest that intraplate alkaline-ultramafic volcanic areas represent a long-term flux of CO2 derived directly from the mantle. Humphreys et al., 2010. Aragonite in olivine from Calatrava, Spain: Evidence for mantle carbonatite melts from >100 km depth. Geology, v.38 p. 911-914.