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TitleReconstructing the oxygen isotope composition of late Cambrian and Cretaceous hydrothermal vent fluid
AuthorTurchyn, A V; Alt, J C; Brown, S T; DePaolo, D J; Coggon, R M; Chi, G; Bédard, J H; Skulski, T
SourceGeochimica et Cosmochimica Acta vol. 123, 2013 p. 440-458,
Alt SeriesEarth Sciences Sector, Contribution Series 20130207
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf; html
ProvinceNewfoundland and Labrador; Quebec
NTS12H/16; 21I/01; 21L/03
AreaBaie Verte Peninsula; Thetford Mines; Troodos; Canada; Cyprus
Lat/Long WENS-56.5000 -55.6667 49.8667 49.6667
Lat/Long WENS-71.5000 -71.0000 46.2500 46.0000
Lat/Long WENS 32.0000 32.0000 34.2500 34.0000
Subjectsgeochemistry; isotopes; oxygen isotopes; radiometric dating; analytical methods; modelling; ophiolites; bedrock geology; Mesozoic; Cretaceous; Paleozoic; Cambrian
Illustrationslocation maps; plots; tables
ProgramTargeted Geoscience Initiative (TGI-3), 2005-2010
AbstractOxygen isotope analyses (d18O) of 16 quartz - epidote pairs from late Cambrian (Betts Cove and Mings Bight, Newfoundland), Ordovician (Thetford Mines, Québec, Canada) and Cretaceous (Troodos, Cyprus) ophiolites are used to calculate the d18O of the hydrothermal fluids from which they crystallized. We combine these with 3 quartz-fluid inclusion measurements and 3 quartz - magnetite measurements from the Cambrian ophiolites to explore how the range in the d18O of submarine hydrothermal vent fluid has varied between the late Cambrian, Cretaceous and today. The range of calculated d18O values of vent fluid (-4 to +7.4) is larger than that of modern seafloor hydrothermal vent fluid (0 to +4). We employ two numerical models to ascertain whether this range is most consistent with changes in paleo-seawater d18O or with changes in the reactive flow path in ancient hydrothermal systems. A static calculation of the vent fluid oxygen isotope composition as a function of the water - rock ratio suggests that in an ocean with a lower d18O than today, the range of vent fluid d18O should be larger. Our data, however, show little evidence that the d18O of the ocean was much lower than the global ice-free value of -1.2. A dual porosity model for reactive flow through fractured and porous media is used to model the relative evolution of the 87Sr/86Sr and d18O of vent fluid in contact with rock. Our 87Sr/86Sr and d18O for Cretaceous epidotes suggest the strontium concentration
of the Cretaceous oceans may have been much higher than at present. The 87Sr/86Sr and d18O data from Cambrian epidotes are strikingly different from the younger samples, and are difficult to model unless fluid-rock interaction in the Cambrian hydrothermal systems was substantially different. It is also possible that some of the quartz - epidote veins have been reset by obduction-related metamorphism. Our data suggest that the high calcium-to-sulfate ratio in early (and Cretaceous) seawater may have affected the degree of strontium isotope exchange, causing hydrothermal fluids to have 87Sr/86Sr closer to that of seawater than in modern systems.
Summary(Plain Language Summary, not published)
Oxygen and strontium isotope data on minerals in hydrothermal veins from Ordovician ophiolites show that Ordovician seawater had a different composition from that of modern seawater.

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