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U-Pb ages of angrites

Yuri Amelin1 and Tony Irving2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Deptartment of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA

 

Figure 1. Pb isotopic data for pyroxenes from Angrites AdoR and LEW 86010, and pyroxenes and whole rock fractions from D'Orbigny, plotted in a 207Pb/206Pb vs. 204Pb/206Pb isochron diagram, and the weighted average 207Pb*/206Pb* ages.

Angrites form a small but remarkable group of meteorites. These very old differentiated achondrites experienced much less secondary processing than most meteorites, and their peculiar mineralogy and geochemistry facilitates precise dating with several isotopic methods. The group includes a variety of the specimens, ranging from "basaltic" rocks with prominent vesicles that formed rapidly quenched melts, to plutonic igneous rocks with cumulate textures and annealed plutonic rocks. Recent findings of new angrites, mainly in the deserts of northwest Africa and in Antarctica, increased their number from one in 1970's (Angra dos Reis, or AdoR, originally the prototypical meteorite of this group, which is now considered an anomalous angrite) to 16 in 2008.

Angrites are singularly well suited to serve as benchmarks of the early Solar System chronology. The ages of some of these meteorites were determined in 1970's to early 90's, and AdoR has been long known as one of the oldest rocks in our Solar System, but precision and accuracy of these dates are insufficient for constructing a detailed timescale of the early Solar System events. The ages of recently found angrites were not known at all. Recent developments in Pb isotopic analysis of meteorites: using 202Pb-205Pb double spike optimized for isotopic dating, and a new procedure for efficient removal of common Pb, provides a background for revisiting and refining chondrite chronology.

Precise U-Pb ages for seven angrites are determined with double spike (202Pb-205Pb) thermal ionization mass spectrometry. The data for three angrites with well preserved U-Pb isotopic systems are shown in Fig. 1. The implications of these ages are threefold. First, they demonstrate that AdoR and LEW are not coeval, and the group of "slowly cooled" plutonic angrites is genetically diverse. Second, the new age of LEW suggests an upward revision of 53Mn-53Cr "absolute" ages by 0.7 Ma. Third, a precise age of D'Orbigny allows consistent linking of the 53Mn-53Cr and 26Al-26Mg extinct nuclide chronometers to the absolute lime scale.

Furthermore, these data show that the angrite parent body underwent prolonged high temperature igneous (both volcanic and plutonic) activity and metamorphism for at least 7 Ma during a period very early in solar system history. It is difficult to conceive of a thermal mechanism by which this could be accomplished unless the parent body was a relatively large, differentiated planet.