U/Pb fractionation

The reason why nearly all such approaches yield similar apparent ages that postdate the start of the solar system by a few tens of millions of years is that there was a very strong U/Pb fractionation that took place during the protracted history of accretion. The U-Pb model age of the Earth can only be young if U/Pb is fractionated at a late stage. This fractionation was of far greater magnitude than that associated with any later processes. Thus it has left a clear and irreversible imprint on the and isotope... 

Some authors have suggested this requires a major U-Pb fractionation event during the early Archaean or Hadean (Vervoort et al., 1994 Kamber et al., 2003). 

One other highly useful chronometer is measurement of °Po- b disequilibria. °Po has a half-life of 138.4 days making the chronometer active for 2 yrs. °Po- °Pb fractionation is based on Po but not Pb partitioning into volatiles during degassing (Gill et al. 1985). Ey repeat analysis of °Po, Rubin et al. (1994) constrained the time of eruption of several samples from 9°N EPR to windows of-100 days. These dates are consistent with eruption windows based on submersible observation. Thus, this technique can provide critical age constraints for other U-series parent-daughter pairs but requires that samples be collected and analyzed as soon as possible after eruption. 

A much younger intrusive event is associated with the BM2 kimberlite, where Early Paleocene (6-fraction U-Pb perovskite weighted average of 63.5 0.7 Ma) hypabyssal kimberlite intruded into Albian and possibly Cenomanian host rocks. This interpretation is supported by... 

Villemant B, Flehoc C (1989) U-Th fractionation by fluids in K-rich magma genesis the Vico volcano, Central Italy. Earth Planet Sci Lett 91 312-326 Vollmer R (1976) Rb-Sr and U-Th-Pb systematics of alkaline rocks the alkaline rocks from Italy. Geochim Cosmochim Acta 40 283-295 Vollmer R (1977) Isotopic evidence for genetic relations between acid and alkaline rocks in Italy. Contrib Mineral Petrol 60 109-118 Vollmer R (1989) On the origin of the Italian potassic magmas. A discussion contribution. Chem Geol 74 229-239... 

Fig. 6. Modelling U/Pb and Sr/Y fractionation by intracrustal melting of amphibolite and ecloglte sources adapted from Luais Hawkesworth (1994). Sr/Y parameter reflects difference in the ratio Plagioclase/Gamet, i.e. upper-crust v. lower-crust source regions. The residue assemblages are as given by Luais Hawkesworth...

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