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

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.