11:45 AM - 12:00 PM
[SGL39-11] D/H and Sr isotopic ratios of apatite inclusions from Archaean zircons to study Earth's evolution
Keywords:apatite, U-Pb dating, hydrogen isotopic ratio, strontium isotopic ratio, Archaean
U-Pb dating using NanoSIMS50 has been carried out on zircon crystals separated from tonalitic gneisses of the Eoarchaean Nuvvuagittuq supracrustal belt, Superior province, Canada. The estimated age, 3638±19Ma, using NanoSIMS50 is consistent with previous reported age of 3661±4Ma, using LA-MC-ICP-MS. Some zircons show discordant ages suggesting that a certain amount of Pb was lost by a thermal alteration event at 980±330Ma. We excluded such discordant zircon crystals because the isotopic composition of H and Sr could have been also altered by this thermal event.
Measurements have been made by peak jumping analysis using single collector with Cs+ primary ion beam. H and D were collected for hydrogen isotopic analyses, and 86Sr, 87Sr, and 88Sr were collected for strontium isotopic analyses.
Obtained D/H ratios from 7 apatites suggest that δD values range from -223 to +54 ‰ (vs. SMOW). Considering the correlation between measured D/H and O/H ratios, it turned out that these values were affected by contamination with a D-poor, H-rich component. As a result, +17±40 ‰ was estimated as the D/H true value for these apatite inclusions. This value is heavier than that of reported one by Pope et al (2012) which estimated the hydrogen isotopic ratio of Archaean sea water as -25 ‰ targeting hydrogen contained in serpentine from ca. 3700 Ma Isua supra crustal belt.