133 research outputs found
A search for ^(70)Zn anomalies in meteorites
No ^(70)Zn isotopic anomalies have been detected in primitive meteorites to a level of precision of less than
40 parts per million (2Ď). Any pre-existing nucleosynthetic anomaly on ^(70)Zn was averaged out by mixing in
the solar nebula before planetary accretion in the solar system. Because neutron-rich nuclides ^(70)Zn and ^(60)Fe
are produced by similar nucleosynthetic processes in core-collapse supernovae, the homogeneity of ^(70)Zn in
meteorites limits the possible heterogeneity of extinct 60Fe radioactivity in the early solar system. Assuming
that Fe and Zn have not been decoupled during incorporation into the solar system, the homogeneity of the
^(70)Zn/^(64)Zn ratio measured here implies that the ^(60)Fe/^(56)Fe ratio was homogenized to less than 15% dispersion
before the formation of planetary bodies. The lack (Zn, Ni, Fe) or presence (Ti, Cr) of neutron-rich isotopic
anomalies in the iron mass region may be controlled by the volatility of presolar carriers in the nebula
Low 60Fe abundance in Semarkona and Sahara 99555
Iron-60 (t1/2=2.62 Myr) is a short-lived nuclide that can help constrain the
astrophysical context of solar system formation and date early solar system
events. A high abundance of 60Fe (60Fe/56Fe= 4x10-7) was reported by in situ
techniques in some chondrules from the LL3.00 Semarkona meteorite, which was
taken as evidence that a supernova exploded in the vicinity of the birthplace
of the Sun. However, our previous MC-ICPMS measurements of a wide range of
meteoritic materials, including chondrules, showed that 60Fe was present in the
early solar system at a much lower level (60Fe/56Fe=10-8). The reason for the
discrepancy is unknown but only two Semarkona chondrules were measured by
MC-ICPMS and these had Fe/Ni ratios below ~2x chondritic. Here, we show that
the initial 60Fe/56Fe ratio in Semarkona chondrules with Fe/Ni ratios up to
~24x chondritic is 5.4x10-9. We also establish the initial 60Fe/56Fe ratio at
the time of crystallization of the Sahara 99555 angrite, a chronological
anchor, to be 1.97x10-9. These results demonstrate that the initial abundance
of 60Fe at solar system birth was low, corresponding to an initial 60Fe/56Fe
ratio of 1.01x10-8.Comment: The Astrophysical Journal, in press. 28 pages, 2 tables, 3 figure
Origin of uranium isotope variations in early solar nebula condensates
High-temperature condensates found in meteorites display uranium isotopic variations (^(235)U/^(238)U), which complicate dating the solar systemâs formation and whose origin remains mysterious. It is possible that these variations are due to the decay of the short-lived radionuclide ^(247)Cm (t_(1/2) = 15.6 My) into ^(235)U, but they could also be due to uranium kinetic isotopic fractionation during condensation. We report uranium isotope measurements of meteoritic refractory inclusions that reveal excesses of ^(235)U reaching ~+6% relative to average solar system composition, which can only be due to the decay of ^(247)Cm. This allows us to constrain the ^(247)Cm/^(235)U ratio at solar system formation to (7.0 Âą 1.6) Ă 10^(â5). This value provides new clues on the universality of the nucleosynthetic r-process of rapid neutron capture
Distinct ^(238)U/^(235)U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes
Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar Systemformation, when several short-lived nuclides (e.g., ^(26)Al-^(26)Mg, ^(53)Mn-^(53)Cr, ^(182)Hf-^(182)W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an âassumedâ constant ^(238)U/^(235)U ratio (i.e., Pb-Pb ages published before 2009â2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, DâOrbigny, and Sahara 99555) using multi-collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from â0.17 to â1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (^(53)Mn/^(55)Mn)_0 ratio in the Early Solar System (ESS) is recalculated as being (7 Âą 1) Ă 10^(â6) at the formation of the Solar System (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions â CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs.
A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17â°. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The δ^(238)U values are consistent with fractionationduring magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction
Oxygen and Aluminum-Magnesium Isotopic Systematics of Presolar Nanospinel Grains from CI Chondrite Orgueil
Presolar oxide grains have been previously divided into several groups (Group
1 to 4) based on their isotopic compositions, which can be tied to several
stellar sources. Much of available data was acquired on large grains, which may
not be fully representative of the presolar grain population present in
meteorites. We present here new O isotopic data for 74 small presolar oxide
grains (~200 nm in diameter on average) from Orgueil and Al-Mg isotopic
systematics for 25 of the grains. Based on data-model comparisons, we show that
(i) Group 1 and Group 2 grains more likely originated in low-mass first-ascent
(red giant branch; RGB) and/or second-ascent (asymptotic giant branch; AGB) red
giant stars and (ii) Group 1 grains with (26Al/27Al)0 >= 5x10^-3 and Group 2
grains with (26Al/27Al)0 <= 1x10^-2 all likely experienced extra circulation
processes in their parent low-mass stars but under different conditions,
resulting in proton-capture reactions occurring at enhanced temperatures. We do
not find any large 25Mg excess in Group 1 oxide grains with large 17O
enrichments, which provides evidence that 25Mg is not abundantly produced in
low-mass stars. We also find that our samples contain a larger proportion of
Group 4 grains than so far suggested in the literature for larger presolar
oxide grains (~400 nm). We also discuss our observations in the light of
stellar dust production mechanisms
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