Magnetic telechemistry and magmatic segmentation on the southern east Pacific rise

Results from axial dredges and a profile inversion of magnetic anomaly data along the axis of the East Pacific Rise (EPR) at 13-23 degrees S provide a n estimate of the average degree of fractionation for the extrusive layer a t this ultrafast-spreading (similar to 145 mm/yr full rate) ridge. We find a high correlation (R = 0.81) between dredge mean FeO* (total iron as FeO) and natural remanence for 34 axial dredges with multiple samples having coi ncident geochemical and magnetic data. We attribute this good correlation t o detailed sampling spanning the full range of cooling-related magnetizatio n changes within a flow and to the young age (0-6 ka) of these axial sample s, which effectively minimizes time-dependent magnetization changes due to geomagnetic intensity or alteration. A composite axial magnetic anomaly pro file shows large amplitude (up to 400 nT) fluctuations with wavelengths of 50-200 km, which theoretical considerations suggest can reliably be related to the magnetization directly beneath the ship. For much of the southern E PR, seismic data provide independent limits on the axial thickness (259 +/- 55 m) and the pattern of off-axis thickening of the extrusive magnetic sou rce layer. These data also provide evidence for an axial magma lens that ef fectively eliminates anomaly contributions from deeper magnetic sources. In version of the axial magnetic anomaly data utilizing these geophysical cons traints yields a magnetization solution which, through use of the regressio n relating FeO* and natural remanence, may be related to the average degree of differentiation of the extrusive source layer. The magnetic data reveal a pattern of magmatic segmentation that closely parallels the tectonic seg mentation of the ridge, suggesting that magma supply may be an important co ntrol on the average degree of differentiation of the extrusive layer. (C) 1998 Elsevier Science B.V. All rights reserved.