Search for nucleosynthetic and radiogenic tellurium isotope anomalies in carbonaceous chondrites

Abstract

Tellurium isotope data acquired by multiple-collector inductively coupled plasma-mass spectrometry (MC-ICPMS) are presented for sequential acid leachates of the carbonaceous chondrites Orgueil, Murchison, and Allende. Tellurium isotopes are produced by a broad range of nucleosynthetic pathways and they are therefore of particular interest given the isotopic anomalies previously identified for other elements in these meteorites. In addition, the data provide new constraints on the initial solar system abundance of the r-process nuclide ¹²⁶Sn, which decays to ¹²⁶Te with a half-life of 234,500 years. The ¹²⁶Te/¹²⁸Te ratios of all leachates were found to be identical, within uncertainty, despite variations in ¹²⁴Sn/¹²⁸Te of between about 0.002 and 1.4. The data define a ¹²⁶Sn/¹²⁴Sn ratio of <7.7 × 10⁻⁵ at the time of last isotopic closure, consistent with the value of <18 × 10⁻⁵ previously reported for bulk carbonaceous chondrites. How close this is to the initial ¹²⁶Sn/¹²⁴Sn ratio of the solar system depends on when the investigated samples last experienced redistribution of Sn and Te. No clear evidence is found for nucleosynthetic anomalies in the abundances of p-, s-, and r-process nuclides. The largest effect detected in this study is a small excess of the r-process nuclide ¹³⁰Te in a nitric acid leachate of Murchison. This fraction displays an anomalous ε¹³⁰Te of +3.5 ± 2.5. Although barely resolvable given the analytical uncertainties, this is consistent with the presence of a small excess r-process component or an s-process deficit. The general absence of anomalies contrasts with previous results obtained for K, Cr, Zr, Mo, and Ba isotopes in similar leachates, which display nucleosynthetic anomalies of up to 3.8%. The reason for this discrepancy is unclear but it may reflect volatility and more efficient mixing of Te in the solar nebula.

Introduction

Nucleosynthetic isotope anomalies in ⁵⁴Cr (Rotaru et al., 1992, Podosek et al., 1997a), ⁴⁰K (Podosek et al., 1999), Mo (Dauphas et al., 2002a), Ba (Hidaka et al., 2003), and ⁹⁶Zr (Schönbächler et al., 2003, Schönbächler et al., 2005) were reported for acid leachates of ordinary and carbonaceous chondrites and it was suggested that these anomalies may be hosted in various types of presolar grains (Alexander, 2002, Dauphas et al., 2002a, Schönbächler et al., 2005) or in unidentified non-acid resistant presolar phases (Podosek et al., 1997a, Podosek et al., 1999, Alexander, 2002, Schönbächler et al., 2003, Schönbächler et al., 2005). Tellurium is another element that has considerable potential for the study of such nucleosynthetic isotope anomalies. It has eight stable nuclides, of which ¹²⁰Te is produced by the p-process, ^122,123,124Te by the s-process, and ^128,130Te by the r-process. Two isotopes, ¹²⁵Te and ¹²⁶Te, are formed by both the r- and the s-process. Presolar diamonds isolated from the Allende chondrite furthermore exhibit permil to percent level Te isotope anomalies of nucleosynthetic origin (Richter et al., 1998, Maas et al., 2001). In contrast, recent Te isotope ratio measurements by multiple-collector inductively coupled plasma-mass spectrometry (MC-ICPMS) were unable to identify any Te isotope variations in bulk chondrites and the sulfide and metal fractions of iron meteorites (Fehr et al., 2005).

The primary aim of the present study is to determine whether sequential acid leachates of carbonaceous chondrites exhibit nucleosynthetic Te isotope anomalies. A further goal is the investigation of potential radiogenic isotope effects from the short-lived radionuclide ¹²⁶Sn, which decays to ¹²⁶Te with a half-life of 234,500 years (Oberli et al., 1999). This decay system is of particular interest because ¹²⁶Sn is predominantly an r-process nuclide that most likely forms in supernova environments (Qian et al., 1998). The discovery of radiogenic effects from the decay of short-lived nuclides with half-lives of less than 1 Myr (e.g, ²⁶Al, ⁴¹Ca) in meteorites requires that these isotopes were produced either within the nascent solar system by spallation or in a late stellar nucleosynthetic event that took place just prior to the collapse of the protosolar cloud (Lee et al., 1998, Meyer and Clayton, 2000). As ¹²⁶Sn cannot be produced by spallation, the discovery of ¹²⁶Te excesses that correlate with Sn/Te ratios in meteorites would provide tight time constraints on the formation of the solar system. The presence of former live ¹²⁶Sn in the early solar system would be in agreement with a supernova trigger for formation of the solar system (Cameron and Truran, 1977) and with the aerogel-model (Ouellette et al., 2005), whereby short-lived radionuclides from a supernova are injected into an already-formed protoplanetary disk (Chevalier, 2000). To constrain the initial solar system abundance of ¹²⁶Sn, concentration data for Sn and Te have been acquired for all leachate fractions. These latter results are of additional interest, because they provide new information on the host phases of Sn and Te in carbonaceous chondrites.