HR: 0800h
AN: V51B-0563    [Abstracts]
TI: Ascension Island, South Atlantic: Deep Plume or Shallow Melting Anomaly?
AU: * Weaver, B
EM: bweaver@ou.edu
AF: Barry Weaver, Geology and Geophysics University of Oklahoma, Norman, OK 73071 United States
AB: Ascension Island ($7\deg$ 56' S, $14\deg$ 22' W) is the sub-aerial manifestation of a broad region of anomalous volcanism on, and adjacent to, the South Atlantic MAR between the Ascension Fracture Zone (AFZ; approx. $7\deg$ S) and the Bode Verde Fracture Zone (BVFC; approx. $11.5\deg$ S). Zero age MORB from the MAR in this region have Pb isotope compositions more radiogenic than N-MORB and which plot significantly above (positive $\Delta$8/4) the Northern Hemisphere Reference Line (NHRL), and have La$_{N}$/Sm$_{N}$ higher and Zr/Nb lower than N-MORB. The enriched character of the MORB volcanism has been suggested to be the product of a plume located near Circe Seamount, at approx. $9\deg$ S, $11.7\deg$ W, or of a plume located beneath two large on-axis seamounts at $9\deg$ 50' S on the MAR. The volcanic rocks of Ascension Island comprise a diverse basalt - hawaiite - mugearite - benmoreite - trachyte suite. There is significant chemical heterogeneity in the mafic lavas, with high Zr/Nb, intermediate Zr/Nb, low Zr/Nb, and Dark Slope Crater lava types reflecting significant source heterogeneity, as indicated by trace element and Sr-Nd-Pb isotope systematics. Data for samples from shallow boreholes and one deep borehole suggest that high Zr/Nb lavas dominate in the subsurface and that there has been a temporal trend toward eruption of increasingly enriched, and more diverse, lava compositions with growth of the volcanic edifice. Ascension Island volcanic rocks have Pb isotope compositions which plot significantly below (negative $\Delta$8/4) the NHRL and trend toward St. Helena HIMU compositions. In addition to Ascension Island, there are numerous seamounts (Circe, Grattan, Stvor, etc.) both on and off the MAR axis between the AFZ and the BVFZ. The locations of the seamounts are closely associated with fracture zones and do not reflect the directions of absolute motion of the South American and African plates (for example, the two large seamounts to the west of Ascension Island are on a flow line parallel to the AFZ). The nature of the distribution of Ascension Island and seamounts between the AFZ and BVFZ does not conform to the classic deep plume model. The geochemistry of Ascension Island and MORB lavas is best explained as the result of melting of shallow chemically heterogeneous mantle, with bursts of excess magmatism when "pods" of enriched (HIMU-type component) mantle pass into the sub-axial MAR melting zone, and with focussing of magma supply being controlled by fracture zone distribution.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting