VARIABLE UPPER MANTLE CO2/NB AND CO2 ABUNDANCES: EVIDENCE FROM UNDEGASSED MELT INCLUSIONS FROM BORGARHRAUN, NORTHERN ICELAND

Magmas deliver carbon from the Earth’s upper mantle and release it to the atmosphere and oceans as CO₂. The presence of carbon in the mantle can influence the depth of melting within the mantle [1], and CO₂ bubbles can exert influence on the style of volcanic eruptions [2]. The release of CO₂ into the atmosphere affects long-term global climate and may provide a feedback mechanism [3] that influences the response of mid-ocean ridge magmatism to glaciation [4]. However, the solubility of carbon in silicate melt decreases strongly with decreasing pressure [5] and so most magmas arrive at the Earth’s surface having already lost most of their carbon via degassing. Silicate melt inclusions are tiny samples of magma (typically <200µm diameter) trapped in crystals that grow in the magma prior to eruption. By virtue of being enclosed by their crystal hosts, melt inclusions are prevented from degassing their volatiles during eruption and can be used to study the carbon content of magmas at the time the inclusions were trapped.

NAL709 is a tephra sample from Borgarhraun, a post-glacial monogenetic eruption in northern Iceland, containing melt inclusions trapped in phenocrysts of high-Fo olivine, Cr-diopside and Cr-spinel. The melt inclusions display well-correlated variations of CO₂ with non-volatile trace elements such as Nb, Rb and Ba. The CO₂/Nb ratio of the melt inclusion population (n=161) is precisely determined at 391±16 (95% confidence limit). These data, along with other published data on undegassed MORB, demonstrate that upper mantle CO₂/Nb is variable, and not a simple function of mantle depletion/enrichment. These results indicate that upper mantle CO₂ abundances are heterogeneous on a large geographic scale. Although precise data are sparse, our results suggest that CO₂ contents beneath the North Atlantic may be higher by a factor of two than those beneath the eastern Pacific Ocean. This observation is correlated with a similar distinction in H₂O/Ce [6]. We suggest that the upper mantle beneath the younger north Atlantic Ocean basin contains components of hydrated and carbonated subduction-modified mantle wedge from Iapetus subduction that were mixed into the upper mantle during opening of the Atlantic Ocean basin.