Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v22c..03c&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V22C-03
Other
1040 Radiogenic Isotope Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1213 Earth'S Interior: Dynamics (1507, 7207, 7208, 8115, 8120), 8125 Evolution Of The Earth (0325)
Scientific paper
The 40K-40Ca (T1/2=1.25 Gyr) systematics were investigated in marine carbonates to assess the potential of this system as a tracer of crustal evolution. The use of 40K-40Ca stems from the fact that the upper crust is characterized by a high K/Ca ratio (~0.9) and thus increases 40Ca several epsilon units (ɛ) above the initial solar system value. This radiogenic calcium is transferred to the oceans where the average 40Ca/40Ca ratio of weathering (and hydrothermal) fluxes are recorded in carbonate sediments. Due to their extremely low K/Ca ratio, carbonate rocks generally experience little post- deposition decay of 40K. We have measured Ca in two meteorites: A K free Angrite pyroxene (ADOR) and bulk Guarena (H6). ADOR results are taken as the initial abundances in the early solar system and are used as the isotopic reference standard. Guarena shows a radiogenic excess of 0.7±0.25 ɛ-units compared with ADOR as a result of its higher K/Ca ratio. Radiogenic 40Ca excesses in marine carbonates do not exceed 1 to 2 ɛ-units and their detection requires precise determination of normalized Ca isotopic ratios. Using new generation thermal- ionization mass spectrometry, we have improved the reproducibility of Ca isotopic measurements by a factor of 3 to 10 compared with previous studies, allowing the determination of the 40Ca/40Ca ratio with a reproducible resolution of 25 ppm (2σ). Using this technique we determined epsilon Ca to reconstruct the Calcium isotopic evolution of the ocean by analyzing a series of carbonate sediments. Modern Sea water and a cretaceous carbonate gave ɛ40Ca = 2.0±0.25 above the solar system initial which correspond to a mean crustal age of ~2.0 Gyr. A sample of a ~600 Myr carbonate (Namibia) gives ɛ40Ca = 2.26±0.18. These data demonstrate the predominance of continental crustal sources in the oceanic budget. Archean carbonates (Ontario, Kaapvaal, Hammersley) with ages between 2.2- 2.5 Gyr give low ɛ40Ca = 0.7±0.25 corresponding to a mean crustal age of 300 Myr at the time of deposition. More ancient carbonates from the Barberton (South Africa) fm. (3.3-3.5 Gyr) give ɛ40Ca = 1.4±0.25 and also reflect a continental source with mean age of 300 Myr. This high radiogenic excess at early times indicates that the isotopic composition of Archean oceans was dominated by a continental flux from very early formed crust. These observations suggest that there was a transition between sources from an early, short-lived Archean crust followed by a long-lived crust. whose formation started 2.7 Gyr ago.
Caro Guillaume
Papanastassiou Dimitri Anastassios
Schopf William J.
Wasserburg Gerald J.
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