  Refractory inclusions in chondritic meteorites, namely amoeboid olivine
aggregates (AOAs) and Ca-Al-rich inclusions (CAIs), are among the first solids
to have formed in the solar system. The isotopic composition of CAIs is
distinct from bulk meteorites, which either results from extreme processing of
presolar carriers in the CAI-forming region, or reflects an inherited
heterogeneity from the Sun's parental molecular cloud. Amoeboid olivine
aggregates are less refractory than CAIs and provide a record of how the
isotopic composition of solid material in the disk may have changed in time and
space. However, the isotopic composition of AOAs and how this composition
relates to that of CAIs and later-formed solids is unknown. Here, using new O,
Ti, and Cr isotopic data for eight AOAs from the Allende CV3 chondrite, we show
that CAIs and AOAs share a common isotopic composition, indicating a close
genetic link and formation from the same isotopic reservoir. Because AOAs are
less refractory than CAIs, this observation is difficult to reconcile with a
thermal processing origin of the isotope anomalies. Instead, the common
isotopic composition of CAIs and AOAs is readily accounted for in a model in
which the isotopic composition of infalling material from the Sun's parental
molecular cloud changed over time. In this model, CAIs and AOAs record the
isotopic composition of the early infall, while later-formed solids contain a
larger fraction of the later, isotopically distinct infall. This model implies
that CAIs and AOAs record the isotopic composition of the Sun and suggests that
the nucleosynthetic isotope heterogeneity of the solar system is predominantly
produced by mixing of solar nebula condensates, which acquired their distinct
isotopic compositions as a result of time-varied infall from the protosolar
cloud.
