correlated O And Mg isotopic anomalies in Allende Inclusions: II. Magnesium

Abstract

Abstract. Mg in two Allende Ca-AI rich inclusions shows large isotopic, mass-dependent fractionation which enriched the heavier isotopes. After normalization, Mg in these inclusions shows negative 6 16 Mg which appears to require the presence of nuclear effects m Mg distinct from 16 AI decay. The Mg mass fractionation is correlated with distinct but smaller fractionation effects for 0 reported by Clayton and Mayeda for the same inclusions (see companion paper). The observation of distinctive but uniform Mg isotopic composition in different phases within single Allende inclusions indicates that nuclear effects in 0 and Mg are not due to the entrapment of interstellar carrier grains as discrete entities, which are preserved as remnants, but are instead due to a homogenized mixture of components of extraordinary isotopic composition mixed with a component of ordinary solar system material and subjected to isotopic fractionation. The distinct 0 isotopic composition of different phases Within a single inclusion is believed to be due to incomplete back-reaction of the higher temperature condensates with a cooler solar nebula of "normal" composition. The processes responsible for the 0 and Mg nuclear effects and the astrophysical site of their occurrence remain undefined.This note presents the observation in three Allende inclusions of large isotopic fractionation effects in Mg; two of the samples show a correlation between Mg and 0 isotopic anomalies and indicate possible nuclear effects in Mg. In general, the effect of small fractionation on the isotopic ratios of an element is given approximately by (1) where R, 1 is the measured ratio of the isotopes of mass Mi and Mi, R3 is the "normal unfractionated" ratio, and a is the fractionation factor per unit of mass difference which includes the fractionation intrinsic in the sample and that produced by the analytical procedures. A key issue is the ability to distinguish between fractionation in the sample and that produced by analytical procedures. We define any variation in Isotopic ratios which does not satisfy equation (I) as a non-linear isotopic effect. Catanzaro and Murphy [ 1966] showed that the Mg fractionation is less than l%o per mu (mass unit) in terrestrial samples. In a search for effects due to 2 6 AI, Schramm, Tera and Wasserburg [STW, 1970] studied Mg in a variety of samples using a technique with high precision and sensitivity. These workers normalized their data to a standard value of 2 5 Mgj2 4 Mg to determine a and corrected 2 6 Mg/ 2 4 Mg for fractionation using eq. (1) to obtain deviations in 2 6 Mg/ 2 4 Mg in permil relative to the standard. These arbitrarily normalized deviations are defined as 6 2 6 Mg. STW showed that there were no variations in 6 2 6 Mg within ± 0.2°bo and thereby demonstrated the absence of any non-linear effects in the samples investigated. In 1974 two independent groups [Gray and Compston, 1974; Lee and Papanastassiou (L & P), 1974] reported non-linear isotopic effects in Mg extracted from inclusions in the Allende...

A corundum-rich inclusion in the Murchison carbonaceous chondrite

“…The (26Mg/24Mg), ratio for the first area, 6,26M g = 3.1 f 6.5%0, lies within 117 of normal Mg, while the (26Mg/24Mg),v ratio for the second area, 6,v26 Mg = 5.0 * 4.7%~~ lies just outside of normal Mg in the 2a limit. The raw isotopic ratios from both areas lie well within the range of fractionation observed for terrestrial samples and, together with the raw isotopic ratios for hibonite, provide no evidence for mass-fractionated Mg like that found in Allende FUN inclusions (Wasserburg et al, 1977) or in Murchison hibonite MH-8 (Macdougall and Phinney, 1979). Both of the corundum 27A1/24Mg ratios calculated from the ion probe data are consistent with the upper limit on Mg content (-200 ppm) established with the electron probe.…”

182 Hf– 182 W age dating of a 26 Al-poor inclusion and implications for the origin of short-lived radioisotopes in the early Solar System

“…STP-1 contains only minor amounts of secondary minerals (nepheline, sodalite, grossular, and monticellite), indicating that it largely avoided secondary alteration processes. Similar to most previously identified FUN CAIs (10,12), STP-1 shows massdependent enrichment in the heaviest isotopes of magnesium, as well as deficits in the mass-independent components of 26 Mg ( 26 Mg*) and 54 Cr of ∼300 and ∼3,500 ppm, respectively ( Table 1). Trace element analysis demonstrates that STP-1 is characterized by a Group II rare-earth element (REE) pattern (Fig.…”

“…Such heterogeneity could result from selective thermal processing of presolar carriers, including the carrier(s) of 26 Al, thereby generating reservoirs enriched or depleted in presolar components (25). However, 26 Al-poor objects such as FUN CAIs, PLACs, and BAGs show large-scale nucleosynthetic heterogeneity in the stable 48 Ca and 50 Ti nuclides, including both enrichments and depletions (8,10,12,26,27), implying that the heterogeneity preserved in these objects is unrelated to 26 Al. Moreover, the mineralogy of STP-1, coupled with its group II REE pattern and 16 O-rich composition, suggest that the precursor material of this inclusion formed by condensation from a gas of solar composition depleted in the most refractory REEs, similar to the majority of fine-grained CAIs.…”

“…This environment existed in the innermost part of the protoplanetary disk during the early stage of its evolution characterized by high mass accretion rates (∼10 −5 M ☉ y -1 ) to the proto-Sun (3). Formation of CAIs near the proto-Sun is also indicated by the presence in these objects of the shortlived radioisotope 10 Be formed by solar energetic particle irradiation (4). Some of the CAIs subsequently experienced melting and evaporation to form distinct coarser igneous inclusions, such as the compact Type A and Type B CAIs commonly observed in CV meteorites (carbonaceous chondrite of the Vigarano type) (5).…”

THE ELUSIVE60Fe IN THE SOLAR NEBULA

“…For some time, however, it has been recognized that, in the inner solar system, mixing was incomplete, whether considering isotopes or the elements themselves (Birck 2004). Since the 1970s, isotopic anomalies have been observed at the mineral scale for many elements, particularly in certain types of Ca-Al-rich inclusions (CAIs) from the Allende meteorite known as fractionated and unknown nuclear effect inclusions (Wasserburg 1977;Clayton 1978). Although a chemical origin has also been proposed (Fujii et al 2006), these mineral-scale anomalies are usually ascribed to incomplete mixing of the products of stellar nucleosynthesis in the early solar system (Birck 2004).…”