Noble gases are of interest because they provide information about the Earth's degassing history. In MORB, the geochemical effect of near-ridge plumes is clearly demonstrated in He isotope ratios [e.g., Graham et al., 1992; Poreda et al., 1993]. Among oceanic islands, the temporal trends observed in Mauna Loa [ Kurz and Kammer, 1991] contrast with temporal homogeneity on Reunion [ Graham et al., 1990]. In the Juan Fernandez Islands, basalts from the shield-building stage have more primitive He than post-shield alkalic lavas [ Farley et al., 1993], a pattern reminiscent of Hawaii.
For the other noble gases, there is now evidence that
atmospheric contamination may be more of a problem than had been
previously supposed. Farley and Poreda [1993] found that
all oceanic islands as well as MORB define
Ne/
Ne--
Ne/
Ne arrays that trend
toward atmospheric ratios. They suggested that the entire mantle
has 
Ne/
N near the solar value, but plumes and depleted
mantle have different 
Ne/
Ne (
Ne is produced in
the mantle by 
O(
,n)
Ne and
Mg(n,
)
Ne reactions). Contamination
with atmospheric Ne during or after eruption then produces the
observed arrays. Farley and Craig [1992] concluded Ar
isotope variablity in olivine phenocrysts from Juan Fernandez was due
to dilution of radiogenic mantle Ar with assimilated atmospheric Ar.
Other studies have begun to resolve the relationship of He
isotope ratios to other isotope ratios. For example, it is now clear
that the St. Helena, or HIMU, ``species'' is characterized by
low 
He/
He [ Graham et al., 1991]. In Samoa and
the Galapagos, the highest 
He/
He ratios are associated
with intermediate Sr, Nd, and Pb isotope ratios [ Graham et al.,
1993; Farley et al. 1992]. Farley et al. [1992]
proposed that high 
He/
He ratios were associated with a
component they called PHEM (``primitive helium mantle'').