Degassing of carbon dioxide from basaltic magma at spreading centers: I. Afar transitional basalts

Abstract

Abstract This study investigates the hypothesis that a significant fraction of the CO2 in basalt supplied to axial volcanic ranges of spreading centers in the Afar depression, escapes by degassing during residence in crustal magma reservoirs. The investigation employs volcanic gas data to test the degassing hypothesis. Volcanic gases emitted from source vents at Erta'Ale lava lake are used to represent volatiles present in the basalt supplied to magma reservoirs underlying Afar spreading centers. For comparison, volcanic gases from a large fissure eruption at Ardoukoba are used to represent volatiles in basalt after a period of storage in the Afar magma reservoirs. The results confirm the hypothesis. Gases from the lava lake and fissure eruption are the same except for CO2. They lie along a common CO2 control line. The fissure eruption gases are six-fold depleted in CO2 compared to gases from the continuously supplied lava lake. This difference corresponds to a loss of approximately 85% of the initial CO2. Moreover, gases from the fissure eruption are nearly identical to those emitted by Kilauea basalts that have lost CO2 by magma reservoir degassing. Mass balance modeling indicates an initial CO2 content for Afar basalt of 0.12 wt.% compared to CO2 concentrations as low as 0.02 wt.% after degassing at depth.