Mathematics – Logic
Scientific paper
Dec 1976
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976e%26psl..33..185v&link_type=abstract
Earth and Planetary Science Letters, Volume 33, Issue 2, p. 185-207.
Mathematics
Logic
7
Scientific paper
The difference in magnetic anomaly amplitudes between the Juan de Fuca (JDF) and Gorda Ridges is examined to test the hypothesis of ``magnetic telechemistry'', according to which the JDF high-amplitude zone reflects more intensely magnetized ``FeTi'' basalts (here defined as FeO > 10% and TiO2 > 1.5%). A lesser high-amplitude zone with FeTi basalts is found near anomaly 13 west of central California. Maximum magnetic relief per 55 × 55 km square provides a rapid estimate of crustal magnetization for the Gorda-JDF Ridge area. Transverse and longitudinal anomaly profiles and contour charts of magnetic relief are compared with earlier published basalt compositions and newer microprobe results of basalt glasses. Total FeO, TiO2, SiO2, Na2O and K2O increase with magnetic relief, while Al2O3, MgO, and CaO decrease. Increased crystal fractionation from extensive volumes of partially molten mantle released by the JDF mantle plume may explain these compositional patterns. We postulate that the plume-derived partial melts flow below the JDF spreading axis until impeded at the Blanco fracture zone to the south, and possibly the Cobb fracture zone to the north. Ponding and fractionation of partially molten mantle at Mid-Oceanic Ridge fracture zone intersections may explain narrow high-amplitude zones, not associated with hot spots, along several northeast Pacific fracture zones. These fracture high-amplitude zones can only have formed continuously at the ridge-fracture intersection, not by leaking along the length of the fracture zone. The 10 m.y. B.P. beginning of the JDF high-amplitude zone, at least in this area, is linked with increased discharge from the JDF plume and the consequent development of the Blanco and Sovanco fracture zones. In summary, we find that (1) high-amplitude zones are probably due to FeTi-enriched basalt; (2) the FeTi enrichment is primarily caused by crystal fractionation at shallow depth, rather than by primary mantle plume enrichment in these elements; (3) nevertheless, high-amplitude zones and FeTi zones are associated with the JDF and some other mantle hot spots, although they may also occur in narrow bands along some transform fractures. Now with the U.S. Geological Survey, Reston, Va. 22092.
Byerly Gary R.
Vogt Peter R.
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