There was much interest in the 1980's in the ``phylogeny'' of
plumes, i.e., defining ``species'' and 
members and
deducing their origin. The focus in the last few years has been on
a possible common component in plumes. Hart et al. [1992]
applied principle component analysis to a subset of the oceanic
basalt isotope data and found that three eigenvectors can account for
97.5% of the variance, confirming that mantle plumes can be modeled
as mixtures of just a few components. Most of the data thus plot within
a tetrahedron in
Sr/
Sr--
Nd/
Nd--
Pb/
Pb
space. Viewing the data in this way, Hart et al. [1992]
concluded that most arrays converge on a region they referred to as
the ``Focus Zone'' or ``FOZO'' at the base of the tetrahedron (Figure
1). FOZO has an isotopic composition similar to that of MORB, except
that the Pb is more radiogenic than MORB. FOZO, the PHEM component of
Farley et al. [1992], and the ``C'' (common) component of
Hanan and Graham [1994] may well be one and the same.
Conceptually, all refer to a component that is common to many plumes
and that has intermediate Pb isotope ratios and
high 
He/
He.
There is some merit to the idea of a common component to many
plumes. Many plumes, as well as MORB, define arrays that extend
towards intermediate compositions on isotope-isotope plots. This
is evident, for example, in the plot of 
Sr/
Sr
vs. 
Pb/
Pb shown in Figure 2. Except for the
Cook-Australs (which may be the products of several plumes), no
volcanic island chain appears to cross this middle region, suggesting
at least the possibility that a composition in the middle is actually
an end-member.
Hart et al. [1992] argued that FOZO represents the composition of the lower mantle and is viscously entrained by plumes rising from the core-mantle boundary. In the Societies and Hawaii, main shield phase lavas have the most extreme isotopic compositions; late state lavas have isotopic compositions closer to that of the common component. Since the main shield phase magmas are higher degree melts, they are probably derived from the hot plume core, with late stage magmas derived from the entrained sheath. This supports the contention of Hart et al. [1992] that the common component is entrained. It seems unlikely, however, that it could occupy the entire lower mantle, as Hart et al. [1992] suggest, because this leads to very serious mass balance problems for Pb.
Recognition of the common component may have been slow because it has intermediate Sr, Nd, and Pb isotope ratios, and because it seems to be characterized by a range of isotopic compositions rather than single values. If this common component exists, it has some very important implications for the origin of plumes and the structure of the mantle. Verifying the existence of this common component, specifying its composition, and understanding its implications are likely to be priorities for mantle geochemists over the next 4 years.