Uranium isotope compositions of the basaltic angrite meteorites and the chronological implications for the early Solar System

Abstract

Events occurring within the first 10 million years of the Solar System's approximately 4.5 billion-year history, such as formation of the first solids, accretion, and differentiation of protoplanetary bodies, have determined the evolutionary course of our Solar System and the planetary bodies within it. The application of high-resolution chronometers based on short-lived radionuclides is critical to our understanding of the temporal sequence of these critical events. However, to map the relative ages from such chronometers onto the absolute time scale, they must be "anchored" to absolute ages of appropriate meteoritic materials using the high-precision lead-lead (Pb-Pb) chronometer. Previously reported Pb-Pb dates of the basaltic angrite meteorites, some of which have been used extensively as time anchors, assumed a constant 238 U∕ 235 U ratio (¼137.88). In this work, we report measurements of 238 U∕ 235 U ratios in several angrites that are distinct from the previously assumed value, resulting in corrections to the Pb-Pb ages of ≥1 million years. There is no resolvable variation in the 238 U∕ 235 U ratio among the angrite bulk samples or mineral separates, suggesting homogeneity in the U isotopic composition of the angrite parent body. Based on these measurements, we recalculated the Pb-Pb age for the commonly used anchor, the D'Orbigny angrite, to be 4563.37 AE 0.25 Ma. An adjustment to the Pb-Pb age of a time anchor (such as D'Orbigny) requires a corresponding correction to the "model ages" of all materials dated using that anchor and a shortlived chronometer. This, in turn, has consequences for accurately defining the absolute timeline of early Solar System events.anchor | geochronology T he time from the formation of the first solids in the Solar System to the accretion and differentiation of protoplanetary embryos is less than approximately 10 million years (Ma) (ref. 1 and references therein), and these events have determined the evolutionary course of our Solar System and the planetary bodies within it. Knowledge of the precise timing of events during this period is critical to a broader understanding of how star systems and planetary bodies form and evolve. As such, precise and accurate geochronology, allowing the resolution of events occurring within this critical approximately 10 Ma interval, is required to understand this earliest sequence of events in the Solar System. A very limited number of chronometers can provide the sub-Ma precision necessary to resolve early Solar System events. These include the long-lived lead-lead (Pb-Pb) chronometer, and the short-lived chronometers such as 26 Al-26 Mg (t 1∕2 approx. 0.72 Ma), 53 Mn-53 Cr (t 1∕2 approx. 3.7 Ma), and 182 Hf-182 W (t 1∕2 approx. 9 Ma). The Pb-Pb chronometer is based on two radioactive isotopes of uranium, utilizing the distinct decay schemes of 235 U → 207 Pb (t 1∕2 approx. 704 Ma) and 238 U → 206 Pb (t 1∕2 approx. 4.47 Ga) to calculate an absolute age of the sample. The short-lived or extinct radionuclide chronometers have parent isotopes ...

Early aqueous activity on the ordinary and carbonaceous chondrite parent bodies recorded by fayalite

“…Owing to the presence of isotopically anomalous chromium, absence of primary minerals with Mn/Cr 4 41 and evidence for a late-stage disturbance of 53 Mn- 53 Cr systematics in CV CAIs 20 , the initial 53 Mn/ 55 Mn ratio of the Solar System is not known. In this paper, relative 53 Mn- 53 Cr ages obtained in fayalites as well as those in calcites reported in literature 8 are anchored to the D'Orbigny angrite for which ( 53 Mn/ 55 Mn) 0 and U-corrected Pb-Pb absolute ages are known 21,22 .…”

The Isotope Geochemistry of Ni

“…So a reassuringly consistent value has emerged from these various TIMS/MC-ICPMS studies which span several decades of work. The data from d'Orbigny provides the best constrained value and using an age of 3.9Ma post CAI (Amelin 2008a;Brennecka and Wadhwa, 2012;Connelly et al, 2012) we calculate 60 Fe/ 56 Fe°S SI =(9.8±4.5)x10 -9 . We note this value is lower than the equivalent cited by Tang and Dauphas (2012) as a consequence of our using the Pb-Pb ages rather than Mn-Cr chronometry.…”

“…6a & b). Moreover, the welldefined ages for these samples determined using extant isotope chronometry (Amelin 2008a,b;Connolly et al, 2008, Brennecka andWadhwa, 2012), potentially provide more accurate decay correction in calculating 60 Fe/ 56 Fe°S SI . Three independent studies (Quitté et al, 2010;Spivak-Birndorf et al, 2011;Tang and Dauphas 2012) obtained consistent values for internal isochrons of d'Orbigny (e.g.…”

“…Brown et al 1987) given its functional presence in the enzyme urease (Dixon et al, 1975). However, Ni abundances as low as 90ng/g can be sufficient to maintain this function (Dixon et al, 1975) …”

Rapid cooling of planetesimal core-mantle reaction zones from Mn-Cr isotopes in pallasites

“…Isolation of late silico-phosphate melt before ~2.5 to 4 Myr after Solar System formation implies that it represents one of the earliest silicate differentiation products, potentially predating almost all basaltic eucrites and angrites (Wadhwa et al, 2009;Schiller et al, 2010;Brennecka and Wadhwa, 2012;McKibbin et al, 2015;Schiller et al, 2015). Early cooling of the Main-group pallasite parent body to near solidus temperatures provides further evidence for early accretion of iron meteorite-type parent bodies, distinct from later forming ureilite-and chondrite-type parent bodies (e.g., Budde et al, 2015).…”