834 research outputs found
Multiple sources or late injection of short-lived r-nuclides in the early solar system?
Comparisons between the predicted abundances of short-lived r-nuclides
(107Pd, 129I, 182Hf, and 244Pu) in the interstellar medium (ISM) and the
observed abundances in the early solar system (ESS) conclusively showed that
these nuclides cannot simply be derived from galactic chemical evolution (GCE)
if synthesized in a unique stellar environment. It was thus suggested that two
di erent types of stars were responsible for the production of light and heavy
r-nuclides. Here, new constraints on the 244Pu=238U production ratio are used
in an open nonlinear GCE model. It is shown that the two r-process scenario
cannot explain the low abundance of 244Pu in the ESS and that this requires
either than actinides be produced at an additional site (A-events) or more
likely, that 129I and 244Pu be inherited from GCE and 107Pd and 182Hf be
injected in the ESS by the explosion of a nearby supernova.Comment: 4 pages, 1 figure, Nucl. Phys. A, in press (proceedings of NIC8
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 (2Ï). 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
Oxygen isotopic evidence for accretion of Earth's water before a high-energy Moon-forming giant impact
The Earth-Moon system likely formed as a result of a collision between two large planetary objects. Debate about their relative masses, the impact energy involved, and the extent of isotopic homogenization continues. We present the results of a high-precision oxygen isotope study of an extensive suite of lunar and terrestrial samples.We demonstrate that lunar rocks and terrestrial basalts show a 3 to 4 ppm (parts per million), statistically resolvable, difference in Î17O. Taking aubrite meteorites as a candidate impactor material, we show that the giant impact scenario involved nearly complete mixing between the target and impactor. Alternatively, the degree of similarity between the Î17O values of the impactor and the proto-Earth must have been significantly closer than that between Earth and aubrites. If the Earth-Moon system evolved from an initially highly vaporized and isotopically homogenized state, as indicated by recent dynamical models, then the terrestrial basalt-lunar oxygen isotope difference detected by our study may be a reflection of postâgiant impact additions to Earth. On the basis of this assumption, our data indicate that postâgiant impact additions to Earth could have contributed between 5 and 30% of Earthâs water, depending on global water estimates. Consequently, our data indicate that the bulk of Earthâs water was accreted before the giant impact and not later, as often proposed
The Dual Origin of the Terrestrial Atmosphere
The origin of the terrestrial atmosphere is one of the most puzzling enigmas
in the planetary sciences. It is suggested here that two sources contributed to
its formation, fractionated nebular gases and accreted cometary volatiles.
During terrestrial growth, a transient gas envelope was fractionated from
nebular composition. This transient atmosphere was mixed with cometary
material. The fractionation stage resulted in a high Xe/Kr ratio, with xenon
being more isotopically fractionated than krypton. Comets delivered volatiles
having low Xe/Kr ratios and solar isotopic compositions. The resulting
atmosphere had a near-solar Xe/Kr ratio, almost unfractionated krypton
delivered by comets, and fractionated xenon inherited from the fractionation
episode. The dual origin therefore provides an elegant solution to the
long-standing "missing xenon" paradox. It is demonstrated that such a model
could explain the isotopic and elemental abundances of Ne, Ar, Kr, and Xe in
the terrestrial atmosphere.Comment: Icarus, in press, 31 pages, 6 tables, and 6 figure
Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect
This study reports the bulk rare earth element (REEs, La-Lu) compositions of
41 chondrites, including 32 falls and 9 finds from carbonaceous (CI, CM, CO and
CV), enstatite (EH and EL) and ordinary (H, L and LL) groups, as well as 2
enstatite achondrites (aubrite). The CI-chondrite-normalized REE patterns and
Eu anomalies in ordinary and enstatite chondrites show more scatter in more
metamorphosed than in unequilibrated chondrites. This is due to parent-body
redistribution of the REEs in various carrier phases during metamorphism. The
dispersion in REE patterns of equilibrated ordinary chondrites is explained by
the nugget effect associated with concentration of REEs in minor phosphate
grains.
Terrestrial rocks and samples from ordinary and enstatite chondrites display
negative Tm anomalies of ~-4.5 % relative to ca chondrites. In contrast, CM, CO
and CV (except Allende) show no significant Tm anomalies. Allende CV chondrite
shows large excess Tm (~+10 %). These anomalies are similar to those found in
group II refractory inclusions in meteorites but of much smaller magnitude. The
presence of Tm anomalies in meteorites and terrestrial rocks suggests that
either (i) the material in the inner part of the solar system was formed from a
gas reservoir that had been depleted in refractory dust and carried positive Tm
anomalies or (ii) CI chondrites are enriched in refractory dust and are not
representative of solar composition for refractory elements. The observed Tm
anomalies in ordinary and enstatite chondrites and terrestrial rocks, relative
to carbonaceous chondrites, indicate that material akin to carbonaceous
chondrites must have represented a small fraction of the constituents of the
Earth.Comment: Geochimica et Cosmochimica Acta, in press, 58 pages, 6 tables, 14
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