10:30 AM - 10:45 AM
[BPT23-07] A chronological constraint on ancient hydrothermal activity using zircon inclusion ages
Keywords:Hadean, zircon, Ar-Ar dating, inclusion, Laser fusion
Regardless of such importance of inclusion of zircon, skepsis on the formation processes and origin of such inclusions have recently raised. Recent discoveries of Fe-hydroxides as the remarkable low temperature phase that could not form under or survive through the igneous activity are one reason. Further, the monazite inclusions having younger ages are also challenges the usability of zircon inclusions. Thus, the reliable ages of zircon and thermal history estimation are essential to solve such arguments. In this study, the local analysis performed as laser fused Ar-Ar dating with step-heating technique were applied. Muscovite and feldspar were chosen as the important index mineral of in situ component inclusions.
The Hadean zircon grains have some muscovite and fluid inclusion as an in-situ material. We separated the flesh zircon grains from host rock according to the comparison among non-irradiated grains. The samples have been irradiated in the KUR (Kyoto University Research Reactor Institute), and have been analyzed using the custom made mass spectrometer in Okayama University of Sciences. It was quite difficult to determine the procedure of the localized inclusion analysis in the zircon, the result of Ar analysis indicated the significant excess Ar in it some cases.
As the result of Ar measurement, plateau-like spectra were obtained. One of which indicated 1) the formation age of zircon (approx. 4.4 Ga) and, other two patterns of age spectra were 2) inherited 40Ar from the deep Earth (e.g excess 40Ar derived from magma) and 3) the 40Ar accumulation (= 40K decayed to radiogenic 40Ar) in inclusion itself. The second case shows significant releases of 40Ar in high temperature fraction (∼ the 1000 ℃) regardless of the little release of 39Ar transformed from 39K by (n, p) reaction. Then, the apparent ages becomes unlikely old ones regardless of the less-disturbed age spectrum. In the third case, the calculated mean age was possible to be older than the alteration age at the formation of inclusions. On the other hand, the U-Pb age of zircon is also altered to be younger by Pb-loss, or to be older by disturbance of Pb diffusion in the zircon. Thus we plan further sample analyses and investigations on the formation ages coupled with the geological settings.