High-MgO melt inclusions trapped in olivine and plagioclase phenocrysts in
MORB have been
identified and proposed to be samples of mantle-derived liquids that were
trapped prior to
aggregation at shallower levels [ Sullivan, 1991; Sobolev and
Shimizu, 1993; Sinton et al., 1993]. The major, minor and trace
element compositions of the inclusions support hypotheses of polybaric,
near-fractional melting (melts segregate from their source residues in
small increments). In addition the melt inclusion data raise
interesting, new questions about the nature of melt segregation and
transportation through
the mantle, and aggregation in the upper mantle/lower oceanic crust. The
compositions of
the high-MgO melt inclusions observed thus far are depleted in terms of
their major and
minor elements (high CaO, low TiO
), and are interpreted to represent
the most
advanced stage of near-fractional melting at the shallowest level beneath
the ridge. An
interesting aspect of these high-MgO melt inclusions is that while a suite
of melt
inclusions sampled from a single phenocryst, for example, appears to have
fairly uniform
major element compositions, the minor and trace element compositions vary
widely. The
nature of the variation of the trace element abundances is consistent with that
anticipated for melts produced by near-fractional melting of peridotite;
however, why the
trace elements preserve the signature, while the major elements appear to have
homogenized, is an important question that remains to be addressed.