F. Pineau

Hydrothermal vent waters at 13°N on the East Pacific Rise: isotopic composition and gas concentration

Abstract Gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13°N on the East Pacific Rise. In the most Mg-depleted samples (≌ 5 × 10 −3 moles/kg) the gas concentrations are: 3–4.5 × 10 −5 cm 3 STP/kg helium, 0.62–1.24 cm 3 STP/kg CH 4 , 10.80–16.71 × 10 −3 moles/kg CO 2 . The samples contain large quantities (95–126 cm 3 /kg) of H 2 and some carbon monoxide (0.26–0.36 cm 3 /kg) which result from reaction with the titanium sampling bottles. δ 13 C in methane and CO 2 (−16.6 to −19.5 and −4.1 to −5.5 respectively) indicate temperatures between 475 and 550°C, whereas δ 13 C CO is compatible with formation by reduction of CO 2 on Ti at 350°C close to the sampling temperature. 3 He/ 4 He are very homogeneous at (7.5 ± 0.1)R A ( 3 He/ 4 He = 1.0 × 10 −5 ) and very similar to already published data as well as CH 4 / 3 He ratios between 1.4 and 2.1 × 10 6 . 18 O and D in water show enrichments from 0.39 to 0.69‰ and from 0.62 to 1.49‰ respectively. These values correspond to W/R ratios of 0.4–7. The distinct 18 O enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations. The 3 He-enthalpy relationship is discussed in terms of both hydrothermal heat flux and 3 He mantle flux.

Fluid inclusion and mineral isotopic compositions (HCO) in eclogitic rocks as tracers of local fluid migration during high-pressure metamorphism

Eclogite facies metagabbros from the Monviso ophiolitic complex (Italian Western Alps) provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of > 40 km. n nWe have determined the omphacites δ18O and the abundance, δD and δ13C of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction. Prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90% fluid loss. The remaining fluid was trapped as primary water-rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene. Deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone. Oxygen isotopes of omphacite (omp) and hydrogen isotopes of water in the fluid inclusions (FI), analyzed in low-strain rocks, mylonites and undeformed/deformed veins show marked variations, δ18Oomp and δDFI values ranging from +3.0 to +5.3‰ and from −31 to −93‰, respectively. Detailed isotopic analysis of several individual vein-wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter. Therefore, the eclogitic minerals and fluids filling the veins are concluded to be locally derived. Our results argue against recent models which suggest large-scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism. n nOn average, the H2O contents and δDFI value are within the upper mantle range. The carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean δ13C value of −24.2 ± 1.2‰. Two carbonaceous components can be recognized, condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions. n nδ18Oomp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole-rock values from hydrothermally altered oceanic crust. It is suggested that the isotopic imprint of the Monviso eclogitic minerals and fluids represents the signature of mid-ocean ridge hydrothermal alteration and subduction zone eclogitization processes. The rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths. In addition, the range in δ18Oomp values recorded in the different microstructural domains covers most of the δ18O values reported in type A, B and C eclogitic xenoliths, implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths.

Carbon and oxygen isotopes in eclogites, amphibolites, veins and marbles from the Western Gneiss region, Norway

The 13C12C and 18O16O ratios have been measured in carbonates and silicates from pods of eclogite, garnet-amphibolite and marbles and also some veins which outcrop in the Western Gneiss Region, Norway, in an attempt to clarify the origin of these rocks. n nThe eclogite carbonates (magnesite or dolomite) display carbon and oxygen isotope values (−9‰ < δ 13C < −2.4‰ PDB; +6.8‰ < δ 18O < +10.4‰ SMOW) which lie within the field of mantle-effluxed carbonate as represented by primary igneous carbonatites. Two garnet-amphibolites have heavier oxygen in the same δ 13C range. Two marbles have heavier carbon and differing δ 18O. Three types of carbonate-bearing vein show different isotopic relationships to the carbonate of their host rocks. n nOxygen isotope fractionations amongst minerals indicate preservation of a high-temperature equilibration (650–850°C) comparable to the temperature previously deduced from Feue5f8Mg partitioning amongst silicates (700–850°C). n nThe eclogite carbonate carbon and oxygen isotope compositions may have been produced either by metamorphism of crustal carbonates or by carbonation of carbonate-free rocks by mantle-effluxed CO2 before or during the eclogitisation processes. The whole-rock δ 18O-values (4–8‰ SMOW) are within the range of crustal basalts and gabbros thus supporting a crustal origin for these eclogites. Several other lines of petrological and geochemical evidence also support the model of crustal protoliths of these rocks. n nThese data may contribute to the geodynamic modelling of the carbon cycle in indicating that subducted carbon reinjected into the mantle could have essentially the same isotope composition as that of carbon effluxed from the mantle and that a crustal protolith origin for some erupted xenoliths gains in credibility.