K.R. Bermingham^a,b, K. Mezger^a,c, S.J. Desch^d, E.E. Scherer^a, M. Horstmann^e

^aInstitut für Mineralogie, Westfälische Wilhelms-Universität, Corrensstraße 24, Münster, 48149, Germany
^bIsotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD-20742 USA
^cInstitut für Geologie, Universität Bern, Baltzerstrasse 1 + 3, 3012 Bern, Switzerland
^dSchool of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ-85287-1404 USA
^eInstitut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Straße 10, Münster, 48149, Germany

The abundance and distribution of isotopes throughout the Solar System can be used to constrain the number and type of nucleosynthetic events that contributed material to the early nebula. Barium is particularly well suited to quantifying the degree of isotope heterogeneity in the Solar System because it comprises seven stable isotopes that were synthesized by three different nucleosynthetic processes (s-, r-, and p-processes), all of which contributed material to the Solar System. There is also potential contribution to ¹³⁵Ba from short-lived radioisotope ¹³⁵Cs, conclusive evidence for which is yet to be reported. Four Allende (CV3) Ca,Al-rich inclusions (CAI 1, CAI 2, CAI 4, CAI 5) and one Allende dark inclusion (DI) were analyzed for Ba isotope variability. Two CAIs (CAI 2 and CAI 5) display ¹³⁵Ba excesses that are not accompanied by ¹³⁷Ba anomalies. Calcium-aluminium-rich inclusion 1 displays a ¹³⁵Ba excess that is possibly coupled with a ¹³⁷Ba excess, and the remaining refractory inclusions (CAI 2 and DI) have terrestrial Ba isotope compositions. These Ba isotope data are presented in conjunction with published whole rock Ba isotope data from individual Allende CAIs. The enrichment in ¹³⁵Ba and absence of coupled ¹³⁷Ba excesses in CAI 2 and CAI 5 is interpreted to indicate that the anomalies are not purely nucleosynthetic in origin but also contain contributions (16 – 48 ppm) from the decay of short-lived ¹³⁵Cs. The majority of Allende CAIs studied to date may also have similar contributions from ¹³⁵Cs on the basis of higher than expected ¹³⁵Ba excesses if the Ba isotope anomalies were purely nucleosynthetic in origin. The ¹³⁵Ba anomalies appear not to be coupled with superchondritic Cs/Ba, which may imply that the contribution to ¹³⁵Ba did not occur via in situ decay of live ¹³⁵Cs. However, it is feasible that the CAIs had a superchondritic Cs/Ba during decay of ¹³⁵Cs, but Cs was subsequently removed from the system during aqueous alteration on the parent body. An alternative scenario is the potential existence of a transient high-temperature reservoir having superchondritic Cs/Ba in the early Solar System while ¹³⁵Cs was extant, which enabled a radiogenic ¹³⁵Ba signature to develop in some early condensates. The nucleosynthetic source of ¹³⁵Cs can be determined by reconciling the predicted astrophysical ¹³⁵Cs abundance with its measured abundance in meteorites. The currently accepted initial¹³⁵Cs/¹³³Cs of the Solar System, [¹³⁵Cs/¹³³Cs]₀, may be underestimated because the spread of Cs/Ba among samples is small and the range of excess ¹³⁵Ba is limited thus leading to inaccuracies when estimating [¹³⁵Cs/¹³³Cs]₀. If the initial meteoritic abundance of ¹³⁵Cs was indeed higher than is currently thought, the most probable stellar source of short-lived radioisotopes was a nearby core-collapse supernova and/or the Wolf-Rayet wind driven by its progenitor.

Reference
Bermingham KR, Mezger K, Desch SJ, Scherer EE and Horstmann M (in press) Evidence for extinct 135Cs from Ba isotopes in Allende CAIs? Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.016]
Copyright Elsevier

Link to Article

A. Morlok^a,b, C. Lisse^c, A.B. Mason^d, E. Bullock^e, M.M. Grady^a,f

^aDepartment of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
^bCurrent address: Institut für Planetologie, WWU Münster, Wilhelm-Klemm-Straße 10, 48149, Münster, Germany
^cThe Johns Hopkins University Applied Physics Laboratory,11100 Johns Hopkins Road, Laurel, MD 20723, USA
^dFinnish Centre for Astronomy with ESO (FINCA), University of Turku, Tuorla Observatory, Väisäläntie 20, FI-21500 PIIKKIÖ, Finland
^eSmithsonian Institution PO Box 37012, MRC 119 Washington, DC 20013-7012, USA
^fDepartment of Physical Sciences, The Open University, Walton Hall, MK7 6AA Milton Keynes, UK

We obtained mid-infrared spectra of chondrules, matrix, CAIs and bulk material from primitive type 1-4 chondrites in order to compare them with the dust material in young, forming solar systems and around comets. Our aim is to investigate whether there are similarities between the first processed materials in our early Solar System and protoplanetary disks currently forming around other stars. Chondrule spectra can be divided into two groups. 1) Chondrules dominated by olivine features at ∼11.3 μm and ∼10.0 μm. 2) mesostasis rich chondrules that show main features at ∼10 μm. Bulk ordinary chondrites show similar features to both groups.
Fine-grained matrix is divided into three groups. 1) phyllosilicate-rich with a main band at ∼10μm, 2) olivine-rich with bands at 11.3 μm and ∼10 μm, 3) pyroxene–rich with several peaks between 9.3 μm and 11.2 μm. Impact shock processed matrix from Murchison (CM2) shows features from phyllosilicate-rich, amorphous and olivine–rich material. CAIs show melilite/spinel –rich features between 10.2 μm and 12.5 μm.
Astronomical spectra are divided into four groups based on their spectral characteristics – amorphous (group 1), pyroxene-rich (group 2), olivine–rich (group 3) and ‘complex’ (group 4). Group 2 is similar to enstatite-rich fine grained material like e.g. Kakangari (K3) matrix. Group 3 and 4 can be explained by a combination of varying concentrations of olivine and mesostasis-rich chondrules and fine-grained matrix, but also show very good agreement with shock processed material. Comparison of band ratios confirms the similarity with chondritic material e.g. for HD100546, while the inner disk of HD142527 show no sign of chondrule material.
Comparison between the laboratory infrared-red IR data and astronomical spectra indicate a general similarity between primitive solar system materials and circumstellar dust and comets, especially in the inner disks of young solar systems. However, other amorphous materials like IDP/GEMS have to be taken into account.

Reference
Morlok A, Liss C, Mason AB, Bullock E and Grady MM (in press) Mid-Infrared Spectroscopy of Components in Chondrites: Search for Processed Materials in Young Solar Systems and Comets. Icarus
[doi:10.1016/j.gca.2013.12.020]
Copyright Elsevier

Link to Article

Edward R. D. Scott

Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, HI, USA

This is an announcement without abstract.

Reference
Scott ERD (in press) Handbook of Iron Meteorites by Vagn F. Buchwald, 1975: Electronic edition. Meteoritics & Planetary Science 48:2608.
[doi:10.1111/maps.12232]
Published by arrangement with John Wiley & Sons

Link to Article