40K–40Ca isotopic constraints on the oceanic calcium cycle

摘要: The contributions of crustal silicates to the oceanic calcium cycle are investigated using high-precision ^(40)K–^(40)Ca measurements in Archean and Proterozoic carbonates, river waters, a series terrestrial extraterrestrial mafic/ultramafic samples. Using multidynamic data collection scheme with strict controls on instrumental mass fractionation, we show that reproducibility 0.35 e-units (2σ) is obtained for ^(40)Ca/^(44)Ca ratio ThermoFinnigan Triton. This represents an improvement by factor 3 compared previous generations data. Well-defined excesses ^(40)Ca from ^(40)K decay were found waters draining Himalayas, but not Mississippi Columbia Rivers. All marine carbonate samples ranging age recent no discernable effects within limit ± ^(40)Ca/^(44)Ca. Our results, therefore, indicate Ca isotopic composition seawater has remained constant indistinguishable mantle past 3.5 Ga, despite influx radiogenic delivered weathering high K/Ca components continental crust. Thus, over most geologic time, sources continents far below hydrothermal sources. Mass balance constraints unless present-day contribution much less than 8% flux, total flux must exceed input high-temperature vents at ridge axes least one order magnitude. requires [(4–15) × 10^(12) mol Ca/yr] low-temperature sites.