P-Process W anomalies in iron meteorites
- Author(s)
- T. Schulz, C. Münker, Stefan T.M. Peters
- Abstract
Heavy, proton-rich stable isotopes belong to the least abundant isotopes in the solar system. Their formation mechanisms and their stellar sources are most likely different from those of neutron-capture generated r- and s-process nuclei that comprise the majority of nuclides heavier than iron. Heavy p-nuclide abundances in meteorites are therefore potentially useful in deciphering distinct stellar contributions to the nascent solar system. We therefore conducted the first high-precision measurements of the heavy p-process isotope W, achieving a typical precision of ±0.7 ε-units for ca. 300ngW. Measured samples comprise metals from magmatic- and non-magmatic iron meteorites, as well as metal from one H4 chondrite (NWA 926) and two reduced terrestrial basalts (from Disko Island, Greenland and the Dzheltul'ski massif from Eastern-Siberia, Russia).The analyzed iron meteorites show clearly resolvable W anomalies of up to +6 ε-units. Conversely, the chondritic metal and both terrestrial samples exhibit W abundances indistinguishable from the standard value. As cosmogenic effects during space exposure of the meteoroids may have affected the W budget, cosmic-ray exposure of the meteorites has to be critically evaluated. We therefore propose a method to approximate cosmogenic contributions to the W signatures in order to unravel nucleosynthetic W abundance anomalies. Our study reveals significant cosmogenic effects only for the longest exposed meteorites, shifting W anomalies always to lower values (average cosmic-ray correction-factors can be estimated to lie between 0.01 and 0.30 εW-units per 100Myr of exposure). Cosmogenic effects for most of the analyzed meteorites therefore appear to be negligible with respect to the analytical precision achieved for iron meteorites. In addition to cosmic-ray exposure, radiogenic effects can be caused by putative decay of Os or by decay of Ta in its ground state. Whereas potential alpha decay of Os could shift W anomalies to higher values (but only up to levels that are within the analytical error of ~0.5 ε-units for most samples), no significant production of W could have occurred from Ta decay.Notably, we identified significant and systematic abundance variations in W between different iron meteorite groups, indicating that these isotope anomalies are characteristic for their entire parent asteroids. Our finding of decreasing excesses in W from early formed magmatic iron meteorites (+3.8±1.2 e{open}-units) towards later formed non-magmatic iron meteorites (+0.6±0.5 e{open}-units), the analyzed chondrite and both terrestrial rocks (-0.3±0.7 ε-units) may thus mirror progressive homogenization of W in the early solar nebula. This overall trend is also supported by a co-variation between W and metal segregation ages for the different iron meteorite groups as well as by a co-variation between W deviations and the respective asteroidal accretion ages. Such an interpretation would suggest progressive homogenization of the solar nebula within about ~2.5 to ~6Myr.
- Organisation(s)
- Department of Lithospheric Research
- External organisation(s)
- Universität zu Köln, Steinmann-Institut für Geologie, Mineralogie und Paläontologie
- Journal
- Earth and Planetary Science Letters
- Volume
- 362
- Pages
- 246-257
- No. of pages
- 12
- ISSN
- 0012-821X
- DOI
- https://doi.org/10.1016/j.epsl.2012.11.009
- Publication date
- 01-2013
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 105105 Geochemistry
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/f356ce02-eb3e-45d2-b3d1-7cbf5c7be357