HR: 1340h
AN: T53B-1311    [Abstracts]
TI: Magma Starvation, Extensive Development of Oceanic Core Complexes, and Evidence of High Degrees of Melting in a Region of Low Magmatic Production on the Mid-Atlantic Ridge at 13¡ã-14¡ãN
AU: * Casey, J F
EM: jfcasey@uh.edu
AF: Geosciences Department, University of Houston, Houston, TX 77204, United States
AU: Searle, R
EM: R.C.Searle@durham.ac.edu
AF: Geology Department, Durham University, Durham, DH13LE, United Kingdom
AU: MacLeod, C
EM: macleod@cardiff.ac.uk
AF: Geology Department, Cardiff University, Cardiff, cf103xq, United Kingdom
AU: Murton, B
AF: National Oceanography Centre, University of Southampton, Southampton, SO143ZH, United Kingdom
AU: Scientific Party, J
EM: macleod@cardiff.ac.uk
AF: Durham University, Durham, Durham, 000, United Kingdom
AB: The region between 12\textdegree{}N and 16\textdegree{}N on the Mid-Atlantic is characterized by at least 10 core complexes exposing ultramafic and gabbroic rocks. We have extended sampling programs from previously surveyed northern regions from 14\textdegree{} to 16\textdegree{}N to four newly discovered core complexes (Smith et. al., 2006) and adjacent ridge segments between 14 and 12\textdegree{}30'N on a cruise of the research vessel James Cook in March-April 2007. We examine the distribution of lithologies sampled in the entire region, the newly discovered core complexes, the ridge and core complex morphotectonics along the segments, and the results of geochemical analysis of basalts, gabbroic rocks and mantle assemblages. Geochemical results from these assemblages may help to explain why the basalt characteristics can show variations symptomatic of high degrees of melting and why the mantle compositions are among the most strongly depleted along the MOR (also indicating high degrees of melting), yet the evidence of mantle unroofing and a thin magmatic crust persist throughout such a broad area. Integrated studies of major element, trace element, and isotopic variations among basalts, gabbroic rocks and igneous and residual ultramafic rocks in the region indicate that 1) the enriched basalts have positive Ta-Nb anomalies, enriched relative to La and Th, 2) basalts have relatively high SiO₂ abundances compared to the global average, 3) basalts show a HIMU isotopic influence, and 4) bulk major element abundances and mineral chemistry in mantle rocks indicate that they are among the most depleted, although variably refertilized, residual mantle assemblages sampled to date along MORs. We suggest that much of the regional variation in major and trace element data, as well as isotopic data and the unusual regional geology (multiple core complexes and ridge morphotectonics) reflecting magma supply can be explained by melting of a sub-axial mantle that contains two end members, one highly depleted and the other enriched. These components appear to involve ancient recycled ocean crust and lithospheric mantle.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3035 Midocean ridge processes
DE: 8150 Plate boundary: general (3040)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting