Composición isotópica (δ¹⁸ O y δD) de espeleotemas silíceos en las cuevas volcánicas de Algar do Carvão y Branca Opala (Isla Terceira, Azores, Portugal)

The volcanic caves of Algar do Carvao (a pit with a complex genesis) and of Branca Opala (a lava-tube cave), differ from other caves on the island of Terceira by having speleothems composed only by opal. This kind of speleothems, in volcanic caves, suggests that the silica origin can be related to hydrothermal activity. The objective of this study is to determine the speleothem isotopic composition in δ¹⁸O SMOW and δD SMOW to determine their geochemical signature and conditions of formation. The studied samples from Algar do Carvao pit are stalactites and flowstones/gours that have been formed in subaerial conditions, while those from the Branca Opala lava tube have been defined as underwater silica stromatolites. dD data of speleothems from both caves are relatively similar (−102.0 ± 10.5‰ in Branca Opala and −103.7 ± 11.1‰ in Algar do Carvao) and it can be assumed that the fluids involved in the silica deposition may be similar. Moreover the δ¹⁸O SMOW data show that the Algar do Carvao speleothems have higher values (δ¹⁸O SMOW 39,4 ± 0,3‰), relatively to the Branca Opala stromatolites (δ¹⁸O SMOW 35,2 ± 0,8‰). The diagenesis that affected the opal A may explain, in part, the observed differences. However, other genetic factors are also influencing such as the fact that Algar do Carvao stalactites are formed in sub aerial conditions, under the influence of evaporation processes, while Branca Opala speleothems were formed in underwater conditions. However, it is also possible, that the water that originated the Branca Opala speleothems was 10,7 °C warmer than the water that formed the Algar do Carvao stalactites.