Workshop Report - Ridge 2000 Program

Related Studies
In addition to these strategies which relate directly to the
active ocean ridge system, there are two other areas of study
which should provide important constraints which cannot be
obtained in any other way. The first involves the study of
ophiolites, which may be appropriate structural analogues for
ocean crust and uppermost mantle, and are exposed in ways that
never occur in the oceans, providing a unique three dimensional
perspective. Detailed structural and geochemical investigations
of these bodies can provide important constraints on the nature
and scale of melt segregation processes in the uppermost
mantle, on the size and internal dynamics of magma chambers, and
on the critical interface between crustal magma bodies and
circulating water systems.
The second important objective is at least one deep crustal
drill hole which extends all the way to the mantle. To date,
the principal stratigraphic units in the ocean crust have been
defined on the basis of characteristic velocities of transmitted
seismic waves. Each of these layers is assumed to be composed
of rock which has a seismic velocity similar to that of the
layer, and analogous to the layers observed in ophiolites. But
the bulk seismic properties of a rock may be very different from
that of individual samples, since the bulk properties integrate
the effect of cracks, veins and zones of alteration. Thus the
assumption that ophiolites accurately represent the stratigraphy
and composition of ocean crust has yet to be tested by actual
observation and sampling of a deep section of ocean crust. Such
a hole also is essential to provide "ground truth" against which
we may test the correspondence between actual observed
lithologic boundaries and the boundaries obtained by
conventional seismic surveys in the vicinity of the hole.
Continuous core sampling through layer 3 gabbro can also
provide much mineralogical and geochemical information on the
nature and rates of crystallization within a crustal magma
chamber, and is the only way we can determine the bulk
composition of the ocean crust. Also, the presumed connection
between fractionated basalts I in layer 2) and crystallization
processes in the chamber can be tested. Finally, if the hole
does successfully penetrate the underlying mantle peridotite, we
would for the first time have a coherent set of samples with
which to trace the evolution of a melt from its origin as a
mantle partial melt, through the deep crustal accretion zone, to
its extrusion on the seafloor.
1.
out full
imaging
Data To Be Collected
Morphology and Sampling. It will be necessary to carry
coverage bathymetric mapping and side scan sonar
at a wide range of resolutions, from the regional, to
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