  One of the major unresolved problems in cosmochemistry is the origin of
chondrules, once molten, spherical silicate droplets with diameters of 0.2 to 2
mm. Chondrules are an essential component of primitive meteorites and perhaps
of all early solar system materials including the terrestrial planets. Numerous
hypotheses have been proposed for their origin. Many carbonaceous chondrites
are composed of about equal amounts of chondrules and fine-grained matrix.
Recent data confirm that matrix in carbonaceous chondrites has high Si/Mg and
Fe/Mg ratios when compared to bulk carbonaceous chondrites with solar abundance
ratios. Chondrules have the opposite signature, low Si/Mg and Fe/Mg ratios. In
some carbonaceous chondrites chondrules have low Al/Ti ratios, matrix has the
opposite signature and the bulk is chondritic. It is shown in detail that these
complementary relationships cannot have evolved on the parent asteroid(s) of
carbonaceous chondrites. They reflect preaccretionary processes. Both
chondrules and matrix must have formed from a single, solar-like reservoir.
Consequences of complementarity for chondrule formation models are discussed.
An independent origin and/or random mixing of chondrules and matrix can be
excluded. Hence, complementarity is a strong constraint for all
astrophysical-cosmochemical models of chondrule formation. Although chondrules
and matrix formed from a single reservoir, the chondrule-matrix system was open
to the addition of oxygen and other gaseous components.
