8 research outputs found
Evolution of oxygen isotopic composition in the inner solar nebula
Changes in the chemical and isotopic composition of the solar nebula with
time are reflected in the properties of different constituents that are
preserved in chondritic meteorites. CR carbonaceous chondrites are among the
most primitive of all chondrite types and must have preserved solar nebula
records largely unchanged. We have analyzed the oxygen and magnesium isotopes
in a range of the CR constituents of different formation temperatures and ages,
including refractory inclusions and chondrules of various types. The results
provide new constraints on the time variation of the oxygen isotopic
composition of the inner (<5 AU) solar nebula - the region where refractory
inclusions and chondrules most likely formed. A chronology based on the decay
of short-lived 26Al (t1/2 ~ 0.73 Ma) indicates that the inner solar nebula gas
was 16O-rich when refractory inclusions formed, but less than 0.8 Ma later, gas
in the inner solar nebula became 16O-poor and this state persisted at least
until CR chondrules formed ~1-2 Myr later. We suggest that the inner solar
nebula became 16O-poor because meter-size icy bodies, which were enriched in
17,18O due to isotopic self-shielding during the ultraviolet photo dissociation
of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated
outside the snowline, drifted rapidly towards the Sun, and evaporated at the
snowline. This led to significant enrichment in 16O-depleted water, which then
spread through the inner solar system. Astronomical studies of the spatial
and/or temporal variations of water abundance in protoplanetary disks may
clarify these processes.Comment: 27 pages, 5 figure
A Critical Examination of the X-Wind Model for Chondrule and Calcium-rich, Aluminum-rich Inclusion Formation and Radionuclide Production
Meteoritic data, especially regarding chondrules and calcium-rich,
aluminum-rich inclusions (CAIs), and isotopic evidence for short-lived
radionuclides (SLRs) in the solar nebula, potentially can constrain how
planetary systems form. Intepretation of these data demands an astrophysical
model, and the "X-wind" model of Shu et al. (1996) and collaborators has been
advanced to explain the origin of chondrules, CAIs and SLRs. It posits that
chondrules and CAIs were thermally processed < 0.1 AU from the protostar, then
flung by a magnetocentrifugal outflow to the 2-3 AU region to be incorporated
into chondrites. Here we critically examine key assumptions and predictions of
the X-wind model. We find a number of internal inconsistencies: theory and
observation show no solid material exists at 0.1 AU; particles at 0.1 AU cannot
escape being accreted into the star; particles at 0.1 AU will collide at speeds
high enough to destroy them; thermal sputtering will prevent growth of
particles; and launching of particles in magnetocentrifugal outflows is not
modeled, and may not be possible. We also identify a number of incorrect
predictions of the X-wind model: the oxygen fugacity where CAIs form is orders
of magnitude too oxidizing; chondrule cooling rates are orders of magnitude
lower than those experienced by barred olivine chondrules; chondrule-matrix
complementarity is not predicted; and the SLRs are not produced in their
observed proportions. We conclude that the X-wind model is not relevant to
chondrule and CAI formation and SLR production. We discuss more plausible
models for chondrule and CAI formation and SLR production.Comment: Accepted for publication in The Astrophysical Journa
Thermal Processing of Silicate Dust in the Solar Nebula: Clues from Primitive Chondrite Matrices
The most abundant matrix minerals in chondritic meteorites, hydrated
phyllosilicates and ferrous olivine crystals, formed predominantly in asteroids
during fluid-assisted metamorphism. We infer that they formed from minerals
present in three less altered carbonaceous chondrites that have silicate
matrices composed largely of micrometer- and nanometer-sized grains of
crystalline forsterite, Mg2SiO4, and enstatite MgSiO3, and amorphous,
ferromagnesian silicate. Compositional and structural features of enstatite and
forsterite suggest that they formed as condensates that cooled below 1300 K at
\~1000 K/h. Most amorphous silicates are likely to be solar nebula condensates
also, as matrix, which is approximately solar in composition, is unlikely to be
a mixture of genetically unrelated materials with different compositions. Since
chondrules cooled at 10-1000 K/h, and matrix and chondrules are chemically
complementary, most matrix silicates probably formed close to chondrules in
transient heating events. Shock heating is favored as nebular shocks capable of
melting millimeter-sized aggregates vaporize dust. The crystalline and
amorphous silicates in the primitive chondrite matrices share many
characteristic features with silicates in chondritic interplanetary dust
particles suggesting that most of the crystalline silicates and possibly some
amorphous silicates in the interplanetary dust particles are also nebular
condensates. Except for small amounts of refractory oxides that formed with
Ca-Al-rich inclusions at the inner edge of the disk and presolar dust, most of
the crystalline silicate dust that accreted into chondritic asteroids and
long-period comets appears to have formed from shock heating at ~2-10 AU.
Forsterite crystals around young stars may have a similar origin.Comment: 16 page