Magma-Carbonate Interaction and CO2 Release: A Case Study from Carlingford Igneous Centre, Ireland

Sammanfattning: Magma which intrudes into carbonate rich crust, interacts with the carbonate in several ways, for example by contact metamorphism and formation of marble or by metasomatism resulting in calc-silicate skarn. These processes release volatiles, such as CO2, from the carbonate and might thus cause climate change. One volcanic complex where the intrusions and their surrounding metamorphic aureole are well exposed and therefore convenient for investigation of magma-carbonate interaction is the Carlingford Igneous Centre in NE of Ireland. The complex is dominated by a gabbro lopolith and a microgranite ring dyke, which intruded into limestone and meta-siltstone at around 62-60 Ma. The purpose of this study is to investigate the extent of magma-carbonate interaction and assess the amount of CO2 which could have been released from the aureole at Carlingford. This is done by analysing major and trace elements as well as carbon and sulphur content in skarn and marble samples from a transect along the calc-silicate metamorphic aureole. The analytical methods used are SEM- EDX, XRF, LA-ICP-MS and carbon and sulphur analyses. The CO2 release is calculated by comparing the carbon and CO2 content in the skarn and marble samples with that in the unmetamorphosed limestone. The results show that the skarn has experienced a strong interaction with the magma, as it contains several high-grade minerals, such as wollastonite, vesuvianite and pyrope, and has lost almost all its carbon. The extent of interaction and amount of CO2 release is generally much lower in the marble, even though a few marble samples show a strong interaction and a high degree of degassing. On the other hand, the amount of marble in the aureole turns out to be significantly higher than the amount of skarn (70-90% compared to 10-30%). The total mass of CO2 release from the calc-silicate aureole is calculated to 0.64-9.06 Gt, where 1.30-2.67 Gt being the most realistic amount. This released CO2 has probably not caused any significant climate change on its own but may have had a small contribution to global warming together with other volcanoes that were active during the same period of time.