Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions

Abstract

Primitive or undifferentiated meteorites (chondrites) date back to the origin of the Solar System, and thus preserve a record of the physical and chemical processes that occurred during the earliest evolution of the accretion disk surrounding the young Sun. The oldest Solar System materials present within these meteorites are millimetre- to centimetre-sized calcium-aluminium-rich inclusions (CAIs) and ferromagnesian silicate spherules (chondrules), which probably originated by thermal processing of pre-existing nebula solids. Chondrules are currently believed to have formed approximately 2-3 million years (Myr) after CAIs (refs 5-10)--a timescale inconsistent with the dynamical lifespan of small particles in the early Solar System. Here, we report the presence of excess (26)Mg resulting from in situ decay of the short-lived (26)Al nuclide in CAIs and chondrules from the Allende meteorite. Six CAIs define an isochron corresponding to an initial (26)Al/(27)Al ratio of (5.25 +/- 0.10) x 10(-5), and individual model ages with uncertainties as low as +/- 30,000 years, suggesting that these objects possibly formed over a period as short as 50,000 years. In contrast, the chondrules record a range of initial (26)Al/(27)Al ratios from (5.66 +/- 0.80) to (1.36 +/- 0.52) x 10(-5), indicating that Allende chondrule formation began contemporaneously with the formation of CAIs, and continued for at least 1.4 Myr. Chondrule formation processes recorded by Allende and other chondrites may have persisted for at least 2-3 Myr in the young Solar System.

Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths

“…All whole-rock chondrites measured plot within ±0.01‰ Δ 25 Mg of the 334 terrestrial fractionation line, in keeping with other studies (Bizzarro et al, 2004;335 Young and Galy, 2004;Baker et al, 2005;Teng et al, 2010 (Catanzaro et al, 1966) and further 339 "external" normalisation of samples to bracketing standards (Table 1). These 340 chondritic mass independent Mg isotope ratios, average δ 26 Mg* = 0.0029 ± 0.017‰, 341 are indistinguishable from our terrestrial values, average = 0.0044 ± 0.022‰ (Thrane 342 et al, 2008).…”

Chondrule formation and evolution of the early solar system

“…Al ratios close to the canonical value, suggesting that such chondrules formed at the same time as CAIs (Bizzarro et al, 2004). Considering that these initial 26 Al/ 27 Al ratios were not obtained for Al rich phases, the initial 26 Al/ 27 Al ratios obtained for bulk chondrules by ICP MS may not record the time of the last melting event of chondrules, but represent the time of chondrule precursor (silicate dust) formation (see also Fig.…”

Short-lived chlorine-36 in a Ca- and Al-rich inclusion from the Ningqiang carbonaceous chondrite

“…According to such a stellar origin, shortlived radionuclides were injected homogeneously in the solar nebula, hence they may be used as chronometers. This idea is supported by the measurement of U-Pb absolute ages of Caand Al-rich inclusions (CAIs) and chondrules with a Ϸ1-million year (My) resolution (8), which yields a time interval between formation of CAIs and chondrules similar to that inferred by many 26 Al- 26 Mg measurements, although a recent study reported that chondrule formation began contemporaneously with the formation of CAIs (9). On the other hand, the same short-lived radionuclides may be produced locally by intense irradiation of nebular materials by the proto-sun (10)(11)(12).…”