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"Radiometric chronometers and time scales for the Galaxy, early Solar system, and terrestrial planets"

I. Tolstikhin (Geological Institute, Kola Science Center

Abstract:

Here we discuss evolutionary processes of the Galaxy and the early Solar system, radioactive isotope systematics allowing their dating, and respective time scales. Because half-lifes of the isotopes vary from tenths of milliards of years till almost zero, radio-chronology can be applied to a great number of natural phenomena of different duration.  This knowledge on the early evolution is in some way complementary to the present-day studies of astrophysical objects. 

Comparison of 232Th/Eu ratios derived from r-process models and those actually observed in the most ancient stars (with as low metallicities as -3) gives 14 2 Gyr, indistinguishable from the age of the Universe, 14.4 0.4 Gyr.  238U/232Th chronometer yields a slightly younger age 12 3 Gyr for the early r-process.  According to these estimates massive starts (and probably host Galaxies) were formed soon after the Big Bang.  Solar 232Th 235U 238U abundances can be reconciled with exponentially decreasing rate of the r-process in our Galaxy (by a factor of ~2 during 10 Gyr). 

The earliest objects of the Solar system, refractory Ca-Al-rich inclusions (CAI) in chondritic meteorites, were probably formed in the innermost Solar nebula from a material composed by solar gas + fully vaporized dust.  This specific vapor was cooled down to produce melts, which crystallization then gave rise to the refractory minerals of CAI. High formation temperature resulted in very high U/Pb ratios, which allow precise determination of 207Pb/206Pb model ages of the inclusions, 4567.2 0.6 Myr (Efremovka CV chondrite).  Chondrules and matrix of chondrites were generally formed 2 to 3 Myr later, as recorded by both long-life U-Pb and short-life 26Al 26Mg systematic.  Non-chondritic high-U/Pb material originated via magmatic fractionation of planetesimals as early as e.g. 4557.8 0.5 Myr ago (LEW-86010 and Angra dos Reis meteorites).  A similar 10-Myr-long interval (after CAI) results from 107Pd - 107Ag dating of silicate metal fractionation.  

The most impressive event of the earth accretion, the giant impact and impact-triggered segregation of the earth core, had occurred not earlier then 30 Myr after CAI, as it follows from 182Hf-182W systematic. Terrestrial xenology gives longer time scale for Xe loss from the atmosphere (also driven by impacts), more than 70 Myr (129I 129Xe(I)) and could be as long as a few hundredth millions of years (244Pu - 136Xe(Pu)).  The most ancient terrestrial minerals are zircons, which precise U-Pb isochron age approaches 4404 10 Myr; their parental granitic melts probably indicate presence of the continental crust and water on the earth surface. 

These time scales are compared with early chronology of the Moon and Mars.