文档介绍：CLBE001-ESS2E November 14, 2006 3:54
274 Encyclopedia of the Solar System
consist of refractory silicate and oxide mineral assemblages
rimmed by thin multilayered bands of minerals. Major-
positions of CAI agree with calculations by
equilibrium vapor-deposition evaporation models to repre-
sent the first 5% of condensable nebular matter solidify-
ing at ≥1400 K from a gas of cosmic (solar photospheric)
composition at a pressure of 10−3 atm or at atm, if the
dust/gas ratio is 40-fold enriched. Most individual CAI con-
tain tiny particles (usually <50 μm) very rich in refractory
siderophiles (Re, W, Mo, Pt, Pd, Os, Ir, and Rh) and, oc-
casionally, refractory lithophiles like Zr and Sc. Sometimes,
even smaller (micrometer-sized) refractory metal nuggets
are found consisting of single-phase pure noble metals or
their alloys.
FIGURE 16 Stable Ba isotopes in a SiC separate from the The textural and plexities of CAI in-
Murchison CM chondrite normalized to those in normal dicate a variety of formation and alteration processes in
terrestrial Ba. Letters indicate nucleosynthetic processes by their history. Undoubtedly, CAI formed at high temper-
which individual isotopes are produced. The presolar atures; properties of some suggest vapor condensation as
neutron-capture isotopes (on slow, s, and rapid, r, timescales that crystalline solids, whereas others seemingly reflect liquid
formed in presupernova and supernova stages, respectively) are or amorphous intermediates. Volatilization, melting, solid-
×
anomalously high, by up to 4 . state metamorphism, and/or alteration in the nebula or af-
ter accretion may also have affected some to many CAI.
derive from outside our solar system. These grains were Clearly, CAI plicated histories that obscured their
incorporated into the solar nebula with intact memories primary textural properties but left their chemical and iso-
of their individual nucleosynthetic sources, accreted in