The East Pacific Rise (EPR) north of the Orozco transform fault provides unique
opportunities to further our understanding of two important classes of problems
relating to the origin of the ocean crust nature of the upper mantle. First,
in recent years it has become clear that the geochemical systematics of the
EPR differ in fundamental ways from those of the Atlantic. Whereas in the
Atlantic enriched basalts are associated with hot spots, and normal sections
of ridge are relatively homogeneous and depleted, along most of the EPR enriched
and depleted basalts commonly occur in close proximity, and with no apparent
association with hot spots. In addition, the ñlocal vectorî of petrological
variability contrasts between the fast-spreading EPR and slow-spreading ridges.
One model to account for the intermixed occurrence of enriched and depleted
magmas is that the sub-Pacific mantle possesses ambient and ubiquitous small-scale
heterogeneities that are not related to hot spots, possibly resulting from recycled
subducted materials. Alternatively, it has been suggested that small hot spots
are the source of this heterogeneity, and are diluted and dispersed by the more
active upper mantle circulation caused by rapid spreading. There are also
computing models for the fast-spreading local vector. Some say it reflects small
changes in mantle potential temperature, while others call upon changes in
mantle composition associated with hot spot influence. To distinguish between
these models we need a ñsmoking gunî - a small hot spot whose effects on the EPR
can be calibrated. An ideal candidate occurs in the region north of Orozco.
The EPR in this region is the shallowest of the northern EPR, contains a
preponderance of enriched basalts and off-axis there is a possible hot spot trace.
The chemical effects of the hot spot can be traced in space and time because
basalts from this region have a distinctive esotopic composition that can be
used as a chemical fingerprint. Mapping, dating and sampling the possible
off-axis trace can test whether this region is influenced by a hot spot.
Sampling the axis, abyssal hills and off-axis seamounts can investigate the
systematics of the Orozco chemical anomaly in space and time, and test whether
a small hot spot can cause the commonly observed intimate association of normal
and enriched MORB. Together these results will substantially further our
understanding of the origin of the characteristic petrological signature of
the EPR, and of possible contrasts in the mantle beneath the Pacific and
Atlantic ocean basins.
Second, the width of the active volcanic zone, the extent of off-axis volcanism
and the petrologic changes that take place over short time intervals at the
EPR axis are poorly known. Published results from 12ÁN is "starved" while 9ÁN
is "robust" in terms of axial morphology. The EPR N. of Orozco extends the
range of axial morphology that can be investigated by a factor of two, and
recent multichannel seismic results confirm that the region has an exceptional
magmatic budget. Investigating this region will clarify how basalt composition
and volcanic activity change as the magmatic budget increases and will aid in
the design and interpretation of investigation of the new frontier of temporal
variability. We propose, in collaboration with Mexican colleagues, to sample
the N. Orozco region both on and off-axis and carry out a comprehensive
petrological and geochemical study of the recovered samples. Multibeam
mapping will be essential to map the off-axis terrain and put the sampling
in a rigorous context. A complete geochemical investigation of the samples
will be undertaken, including major elements, trace elements, isotopes and
collaborative dating of samples using Th-U-Pa and 40Ar/39Ar.