Retrograde fluid inclusions in eclogitic metagabbros from the Ligurian Western Alps
European Journal of Mineralogy Volume 4 Number 5 (1992), p. 1097 - 1112
published: Oct 14, 1992
manuscript accepted: Dec 6, 1991
manuscript received: Jun 11, 1991
ArtNo. ESP147050405011, Price: 29.00 €
Abstract Quartz veins and apatites of the eclogitic Fe-Ti metagabbros from the Ligurian Western Alps contain a variety of aqueous fluid inclusions. The quartz veins are related to fluid flow occurred at different stages of the metamorphic history and formed on a pressure - temperature span ranging from high pressure to greenschist facies conditions. The microstructures and microthermometry allow to relate all the analyzed inclusions to the greenschist retrogression of the eclogitic mineral assemblages. Two main types of inclusions can be distinguished: CO2-free aqueous inclusions and CO-H2O-rich inclusions with a few moles percent N2 mixed with the CO2. No clear relationships have been observed between the above two groups, but the textural and microthermometric features allow to relate the aqueous fluid inclusions in apatite to the earliest episodes of syn-greenschist fluid infiltration. Fluid/rock interactions during water consuming reactions may have the major effect to concentrate salts and CO2 (when present) in the metamorphic brine. The intersection between the experimental curves defining the stability field of the greenschist assemblages and the calculated isochores of inclusions in apatite, give an approximate trapping estimate of about 3 kbars and 500 °C. Such evaluation consents to derive anomalously high geothermal gradients, related to high uplift rates occurring at the brittle-ductile transition, and preventing the thermal reequilibration of rocks at depth.