| Abstract: | Three-dimensional inversions of the magnetic field reveal the presence of zones of high magnetization over the propagating limb of large spreading center discontinuities. The presence of high-magnetization zones is confirmed by rock magnetic measurements where available. Magnetization highs are associated with basalts which tend to have high Fe contents and low Mg numbers. These data suggest that high-magnetization zones observed over large ridge axis discontinuities are associated with highly differentiated basalts enriched in iron. Following Christie and Sinton 1], such highly evolved basalts may be the result of shallow-level crystal fractionation in small magma bodies with a low supply rate. These small magma bodies are postulated to correspond to the first stages in the development of a sub-axial magmatic system as a result of the propagation of one of the limbs of the offset into older lithosphere. Because high-magnetization zones at large ridge axis discontinuities often correspond to gaps in the along-axis extent of a seismically detectable magma chamber, these magma bodies may be smaller than a few hundred meters. Rock magnetic measurements suggest that the enrichment in iron associated with increased differentiation may be accompanied, in a few cases, by an increase in the concentration of titanomagnetite within the basalts and in the magnetization of the rocks. However, the exact relationship between high magnetization intensities and iron enrichment is complex and unclear, and may be significantly affected by factors such as magnetic mineralogy and crystallization history. |
