Workshop Report - Ridge 2000 Program

Critical Investigations
The specific physical and chemical characteristics and
processes which must be studied in order to answer these two
questions include:
1. Orientation of Upper Mantle Crystal F"bric. Olivine
crystals are strongly anisotropic in their elastic properties
and preferentially aligned by shear flow deformation. Thus,
spatial distribution of seismic velocity anisotropy can
constrain flow directions.
2. Three-dimensional Distribution of Melt within the
Mantle. The geometry of melt distribution will help constrain
the conditions of melt formation and pathways of melt migration.
3. Distribution of Temperatures within the Mantle.
Lateral variations in temperature reflect spatial variations in
hydrothermal circulation, flow of mantle materials, and melt
formation and transport.
4. Compositional Variability within the Mantle. Variations
in composition of upwelling mantle dictate the volume and
chemistry of melt. Variations in composition of the residual
mantle are introduced by the melt extraction process.
5. Physics and Chemistry of Melting within an Open System.
Nearly all laboratory and thermodynamics studies to date are
based on the assumption of a closed system, which is not valid
in the mantle because of differential movement of melt and
mantle materials.
6. Physical Properties of Crystal-melt Aggregates. Interpretation
of seismological, electromagnetic, and rheological
phenomena requires more complete knowledge of the physical
characteristics of rock-melt mixtures.
7. Physical Mechanisms for Melt Migration and Segregation
in a Deforming Crystalline Matrix. Whether melt moves from the
generation site to the surface by way of porous flow, diapirism.
or dike propagation will be critical to the delivery rate of
melt and the degree of mixing and chemical modification which
occurs during ascent.
8. Variations in Crustal Structure, Morphology. and
Composition. These variations. in the form of thickened crust.
seamounts. or changes in composition, provide our only record
of long-term changes in the spatial and temporal distribution
of the melting and melt transport processes.
15