Journal of Conference Abstracts

Comparison of Gravity, Bathymetry, Mantle Tomography and Geochemistry of Basalts along the Northern Mid-Atlantic Ridge

J. F. Casey Department of Geosciences, University of Houston, Houston, TX 77204, USA

jfcasey@uh.edu

H. Zhou Department of Geosciences, University of Houston, Houston, TX 77204, USA

E. Pavlenko Department of Geosciences, University of Houston, Houston, TX 77204, USA

C. Xia Department of Geosciences, University of Houston, Houston, TX 77204, USA

V. Golod Department of Geosciences, University of Houston, Houston, TX 77204, USA

W. B. Bryan Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution,

Woods Hole, MA 02542, USA

Comparison of multibeam bathymetry, satellite altimetry, marine gravity, and shallow to deep mantle tomography with basalt geochemistry provide a powerful set of tools in understanding mid-ocean ridge processes. Bathymetrically elevated regions (Iceland and the Azores) and large-scale segmentation along the Northern Mid-Atlantic Ridge (MAR) generally correlate with long-wavelength gravity anomalies and slowness anomalies in mantle. Iceland and the Azores also correspond to regions of higher extents of mantle depletion (lower Na_8.0) and larger crustal thicknesses. They are also characterized by significant mantle source anomalies based on basalt geochemistry. Basaltic rocks from Iceland and the Azores are interpreted as indications of regions of LREE/HREE enriched mantle sources and high temperatures. In general, decay in crustal thicknesses, ridge elevation, changes to more LREE/HREE depleted mantle sources and lower average extents of mantle melting (high Na_8.0) and mantle slowness anomalies are observed away from these anomalies. Another anomalous LREE/HREE enriched region occurs near the 15°20'N Transform region in the Central Atlantic. It shows similar basalt geochemical anomalies (low Na_8.0) to that of Iceland and the Azores, but is not characterized by the similar gravity, bathymetric and mantle slowness anomalies. It occupies a bathymetrically depressed region along the MAR. Although computed crustal thicknesses based on basalt geochemistry would tend to indicate a thick crust similar to the Azores, ocean bottom video and sampling show that the region is characterized by widespread mantle exposures indicating a thin crustal component of the lithosphere. The geochemical anomaly at 1.7°N is characterized by elevated LREE/HREE, low Na_8.0, as well as an elevated, complexly segmented ridge axis and mantle Bouguer anomalies that show that crustal thicknesses are normal to somewhat elevated in the region. The mantle section would not appear significantly colder in this region of the Equatorial Atlantic, as has been previously suggested. As the long-offset Romanche Transform is approached Na_8.0 becomes highly elevated and somewhat extreme when compared with the remainder of the MAR. We infer from these results that the geochemical anomalous LREE/HREE enriched and low Na_8.0 basalts along the MAR may not always be associated with a high temperature mantle regime, fertile mantle, excess volcanism, or high extents of present-day melting.