Pd-Ag chronometry of iron meteorites and the accretion and thermal history of differentiated protoplanets
Zusammenfassung der Projektergebnisse
The main goal of this study was the investigation of the processes and timescales involved in the formation and cooling history of differentiated protoplanets in the early solar system by applying the short-lived 107Pd-107Ag chronometer to different groups of magmatic iron meteorites, which represent the remnant fragments of planetary cores. The interpretation of the Pd-Ag results obtained during the course of this study have changed and broadened the picture of the evolution and the chronology of early-formed and differentiated planetary bodies. Three major obstacles restricting the chronological application of the Pd-Ag system to iron meteorites have been successfully tackled: (i) the development of a method for quantification and correction of secondary neutron capture-induced modifications of the Ag isotopic composition; (ii) constraining the cooling histories of the cores of differentiated planetary bodies using Pd-Ag isochrons and relative Pd-Ag ages obtained for members of the IIAB, IID, IIIAB, IVA, and IVB iron meteorites; and (iii) the determination of the solar system initial 107Pd/108Pd. The latter, however, is currently hampered by uncertainties in the Pb-Pb age of the IVA iron meteorite Muonionalusta because this age was used to calculate the intial 107Pd/108Pd value at the time of CAI formation. The quantification of cosmic ray induced neutron capture effects was achieved by developing a method that combines a model of neutron capture reactions on Pd and Ag isotopes in iron meteorites with the Pt isotope composition of each sample, which serves as a direct neutron dose monitor. The application of this correction method enabled to deduce meaningful Pd-Ag isochrons even for some of the most strongly irradiated iron meteorites. The initial 107Pd/108Pd values obtained from Pd-Ag isochrons for members of the IIAB, IID, IIIAB, IVA, and IVB iron meteorites were used to calculate relative cooling ages and to constrain the thermal histories of these cores. The indistinguishable Pd-Ag ages of early and late crystallized IIIAB iron meteorites imply that the IIIAB core cooled isothermally below the Pd-Ag closure temperature at ~11 Ma or ~5 Ma after CAI formation, depending on which solar system initial 107Pd/108Pd value is used. The Pd-Ag age and the rapid isothermal cooling of the IIIAB core can be reconciled with a scenario where the insulating mantle of the IIIAB parent body was removed by collision. Other magmatic iron meteorite groups (IIAB, IID, IVA) show similar Pd-Ag ages and seem to share the same rapid cooling history. In order to place Pd-Ag ages onto an absolute timescale of solar system chronology we obtained a precise Pd-Ag isochron for the IVA iron meteorite Muonionalusta. The Pd-Ag data for Muonionalusta in conjunction with the Pb-Pb age reported for this meteorite indicate that the initial 107Pd/108Pd of the solar system was between (3.5±0.1) × 10^-5 and (6.6±0.4) × 10^-5. The large uncertainty on the initial value reflects uncertainties in the U isotopic composition of Muonionalusta, which in turn translate into uncertainties on the cooling timescale of the IVA core. Thus, depending on the exact absolute age of Muonionalusta, cooling of the IVA core below the Pd-Ag closure occurred between ~3 and ~9 Ma after CAI formation or between ~1.5 and ~7.5 Ma after core formation. Regardless of the exact Pd-Ag age of the IVA core, the Pd-Ag results constrain that Muonionalusta cooled with >500K/Ma through the Pd-Ag closure temperature of ~900 K. Thus, the Pd-Ag data are consistent with the rapid cooling as inferred from metallographic cooling rates, which would be consistent with a scenario where the IVA core cooled with only little or no insulating mantle.
Projektbezogene Publikationen (Auswahl)
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(2013). Palladium-silver ages for IIIAB iron meteorites. Paneth Kolloquium, Nördlingen, Germany
Matthes M., Fischer-Gödde M. and Kleine T.
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(2013). Palladium-silver isotope systematics of IIIAB iron meteorites. Lunar Planet. Sci. Conf. 44, The Woodlands, Texas, USA
Matthes M., Fischer-Gödde M. and Kleine T.
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(2014). Pd-Ag evidence for rapid cooling of iron meteorite parent bodies. International interdisciplinary workshop on “Accretion and Early Differentiation of the Terrestrial Planets”, Nice, France
Matthes M., Fischer-Gödde M., Kruijer T. S., Leya I. and Kleine T.
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(2014). Rapid cooling of the IIIAB iron meteorite parent body inferred from Pd-Ag chronometry. Lunar Planet. Sci. Conf. 45, The Woodlands, Texas, USA
Matthes M., Fischer-Gödde M., Kruijer T.S., Leya I. and Kleine T.
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(2015). Accretion and cooling history of the IIIAB iron meteorite parent body. Lunar Planet. Sci. Conf. 46, The Woodlands, Texas, USA
Matthes M., Fischer-Gödde M., Kruijer T. S., Leya I. and Kleine T.
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(2015). Neutron dosimetry and neutron capture model for palladium-silver chronometry of iron meteorites. Lunar Planet. Sci. Conf. 46, The Woodlands, Texas, USA
Matthes M., Fischer-Gödde M., Kruijer T. S., Leya I. and Kleine T.
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(2015). Pd-Ag chronometry of iron meteorites: Correction of neutron capture-effects and application to the cooling history of differentiated protoplanets. Geochim. Cosmochim. Acta 169, 45-62
Matthes M., Fischer-Gödde M., Kruijer T. S., Leya I. and Kleine T.
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(2015). Pd-Ag evidence for rapid cooling of the IIIAB iron meteorite parent body. 25th Goldschmidt Conference, Prague, Czech Republic
Matthes M., Fischer-Gödde M., Kruijer T., Leya I. and Kleine T.
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(2016). Palladium-Silver Chronology of Differentiated Protoplanets. PHD Thesis, University of Münster, Germany
Matthes M.
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(2017). Pd-Ag chronometry of IVA iron meteorites and the crystallization, compaction and cooling of a protoplanetary core. Geochim. Cosmochim. Acta
Matthes M., Fischer-Gödde M., Kruijer T. and Kleine T.
