Presolar isotopic and chemical signatures in hibonite-bearing refractory inclusions from the Murchison carbonaceous chondrite

Abstract

Magnesium-, calcium-, and titanium-isotopic compositions, as well as trace-element abundances, have been determined by ion microprobe mass spectrometry for 37 hibonite-bearing inclusions from the Murchison carbonaceous chondrite. These and previously analyzed hibonite-bearing inclusions can be divided into three main groups based on morphological and mineralogical criteria. PLAty Crystal fragments (PLACs) generally have depletions in the relatively volatile refractory trace elements (Eu, Yb, Mb, V, Ba), little or no evidence of 26Al, I.E., little or no radiogenic 26Mg ( 26Mg∗), and widely variable Ca- and Ti-isotopic compositions. One PLAC, 13-13, has excess 48Ca of 104%. and 50Ti of 273%., the largest effects yet measured in these elements. Four Blue AGgregates (BAGs) are distinctive in having low overall trace-element abundances with large relative depletions in the ultrarefractory elements Gd-Er, Lu, Zr, Hf, Y, Sc, and mass-fractionated Mg-isotopic compositions enriched in the heavy isotopes. Large 50Ti deficits have been measured in three of the four BAGs; the only BAG analyzed for its Ca-isotopic composition also shows a deficit in 48Ca. Spinel HIBonite inclusions (SHIBs) show a wide variety of morphologies ranging from bladed hibonite crystals within spinel, to spherules of hibonite and spinel, to large uniform hibonite crystals within spinel. SHIBs consisting of bladed hibonite in spinel or hibonite-spinel spherules typically have ultrarefractory-depleted trace-element patterns, although uncommon ultrarefractory-enriched patterns were also found, 26Mg∗ at a level consistent with 5 × 10 −5 × 27Al , and generally small (<10%.) Ca- and Ti-isotopic anomalies. One of the SHIB spherules, 7–170, has highly unusual systematics with a large 50Ti deficit, only a small 48Ca deficit, and no 26Mg∗. The large hibonite crystals within spinel have variable trace-element patterns, commonly with anomalies in the relatively volatile elements Ce, Eu, and Yb. 26Mg∗ is present in some of these inclusions but none of this type of SHIB has large Ca- or Ti-isotopic anomalies. While at least four distinct components are required to model the Ti-isotopic compositions, three components are sufficient to describe the variations in Ca-isotopic compositions measured thus far. The systematics of PLAC 13-13 with large excesses of 48Ca and 50Ti match the predictions of the multiple-zone-mixing model for nucleosynthesis in supernova ejecta to a maximum neutron excess of 0.159. Compositions depleted in 48Ca and 50Ti are unlikely to represent discrete nucleosynthetic components but represent material with a shortfall of neutron-rich material relative to the solar system abundances. Additional components must be present but are not easily resolved. Within the morphological groups of hibonite-bearing inclusions, a few have exceptional chemical and isotopic characteristics such as SHIB spherule 7–170. However, independent of the morphological groups, there are correlations in hibonite isotopic systematics and chemistry that hold for all inclusions studied: (1) no inclusion has both a large 50Ti anomaly and 26Mg∗; and (2) inclusions depleted in the ultrarefractory elements have 26Mg∗, with the exception of those which have a large 50Ti anomaly. These features clearly establish a relationship between chemical and isotopic characteristics of the inclusions which must predate their formation. Several components were combined, probably as mixtures of dust grains or crystals, and were melted to form refractory inclusions. The nucleosynthetic anomalies must be inherited from stellar environments; it is possible that the depletions in ultrarefractory elements are also a memory of stellar condensation and are not necessarily related to a condensation event in the solar nebula as has generally been assumed.