Chronology of planetesimal accretion and magmatism derived from U-Pb and Rb-Sr systematics of ungrouped achondrites

Abstract

Current meteoritic collections include about 73,000 rocks and represent approximately 100-150 asteroids with various compositions and histories of chemical differentiation. Only a few groups of meteorites, each group probably derived from a single asteroid, have been studied in detail so far. Ungrouped achondrites, magmatic meteorites that do not fall into established groups and, therefore, represent previously unsampled asteroids, provide valuable information about the diversity of accretional and magmatic processes in the early Solar System.

The main aim of this study is to better understand the history of accretion and magmatism of a few previously unsampled asteroids. For accretion chronology, we employed an initial-Sr chronometer based on the decay of 87Rb to 87Sr. We developed a high-precision multidynamic Sr isotope analysis using thermal ionization mass spectrometry (TIMS) and applied it to a suite of achondrites. To determine crystallization chronology, we conducted high-precision isotope dilution thermal ionization mass-spectrometry (ID-TIMS) U-Pb systematics of one fresh eucrite and three ungrouped achondrites.

Rb-Sr systematics of several achondrites revealed varying degrees of depletion in moderately volatile elements. The initial Sr chronometer indicates that planetesimal accretion began contemporaneously with CAI formation (around 4567.3 Ma) and peaked between 4564-4562 Ma. In most instances, the initial Sr chronology correlates with Pb-Pb crystallization ages, with the time of accretion and magmatism overlapping within uncertainties. This suggests a rapid transition from the separation of precursor material to the accretion, melting, differentiation, and crystallization of planetesimals. The regression line of such a correlation yields an initial 87Sr/86Sr ratio of 0.698917 +/- 0.000020 at 4567.3 Ma. Correspondingly, the slope of the regression line is related to an 87Rb/86Sr ratio of 1.14 +/- 0.52. Notably, this value aligns with the 87Rb/86Sr of the CI chondrite and Solar composition, suggesting that the precursor material of the parent bodies of the studied achondrites formed directly from the Solar nebula.

U-Pb is the only long-lived isotopic system that offers sufficient time resolution for magmatic processes. We successfully determined the absolute Pb-Pb age of three achondrites. Eucrite NWA 8661 has an age of 4565.2 +/- 1.5 Ma, which aligns with ages derived from short-lived isotopic systems. The ungrouped achondrite NWA 11119 is one of the oldest rocks with a Pb-Pb age of 4566.42 +/- 0.81 Ma. The most significant finding is the precise and accurate determination of the age of another exceptionally old ungrouped achondrite, Erg Chech 002, at 4565.56 +/- 0.12 Ma. By combining this age with the published initial 26Al/27Al for Erg Chech 002, we discovered a heterogeneous distribution of Al-26 in the inner Solar System. This implies that any chronology of the early Solar System based on the Al-Mg isotopic system must take this heterogeneity into consideration. Show more