Ocean crust geochemistry: Construction and hydrothermal alteration
of oceanic crust
Project Manager/Lead Scientist: Debra Stakes
This project is a renewal of the Ocean Crust
Geochemistry project. The goal of this effort is to constrain the
relationship of magmatism and hydrothermalism in the construction of ocean
crust. It includes field and geochemical studies of the upper crust around
mid-ocean ridges where new ROV observations suggest that combined
hydrothermalism and magmatism are present several kilometers from the
spreading axis. A new emphasis in this project will be to incorporate
isotopic and collaborative biological studies to search for clues of
the Mendocino Transform Zone (MTZ), intact slabs of ocean crust transferred from the Gorda Plate to
the Pacific Plate by the northward progression of the Mendocino Triple
Junction (MTJ) are exposed along the Gorda Escarpment. Rock age and
chemistry reveals both the tectonic and magmatic processes inherent in
ocean crust created near a triple point. The
uplift of the escarpment has also induced basin-wide flow of
methane-charged pore fluids from the organic rich Miocene-
sediments of the Pacific Plate. The northward flow and venting of these
warm pore fluids has produced methane seeps with two sparsely populated
chemosynthetic communities and authigenic carbonates with methane-hydrate
isotopic signatures. Continuing effort for the MTZ sample suite will be
detailed analyses of rock alteration
(hydrothermal and microbial) on samples already collected.
samples from the southern Guaymas basin will be analyzed for mineral
assemblages and growth history, complemented by bulk chemistry and stable
isotopic studies (oxygen, carbon, sulfur).
Ongoing study of the Cleft Segment of the southernmost Juan de Fuca
Ridge is addressing the construction and alteration of oceanic layer two.
Field observations from past ROV dives are the basis
of a model for the role of off-axis volcanism in the construction of the
uppermost layers of oceanic crust.
The Endeavour Segment of the
northern Juan de Fuca ridge is both seismically more active and
tectonically more dismembered than the Cleft Segment. There is evidence of
recent off-axis volcanic events that apparently reset the robust axial
hydrothermal system. Rocks collected there in 2002 will be subjected to geochemistry and geochronological studies. This area will offer an
important comparison to the Cleft segment in testing the hypothesis of the
importance off-axis volcanism. The geochemistry study will also provide a
useful complement to the Keck studies in documenting off-axis volcanism
and hydrothermal venting.