Amplitudes of oceanic magnetic anomalies and the chemistry of oceanic crust: Synthesis and review of 'magnetic telechemistry'

Abstract

A growing body of evidence suggests that certain areas of high-amplitude (H) sea-floor spreading-type magnetic anomalies reflect FeTi-enriched basalts of high remanent magnetization. A worldwide tabulation of these 'H-zones' is presented, together with a review of pertinent geochemical, rock magnetic, and deep-tow data relevant to the hypothesis of magnetic telechemistry.' H-zones are found in two tectonic settings: (1) along 102–103 km long sections of spreading axis close to hot spots; and (2) in narrow bands extending a few hundred kilometres along the edges of some fracture zones. Amplitudes in both provinces are 1.5 to 5, typically 2 to 3 times normal, and the hot spot H-zones are known from spreading half-rates of 0.6 to 3.7 cm yr−1 The highest amplitudes, magnetizations, and FeTi enrichment (up to 15–18% FeOT and 2–3% TiO2) seem to occur where both provinces overlap, i.e., where fracture zones occur near hot spots, for example along the Blanco Fracture Zone south of the Juan de Fuca hot spot and along the Inca Fracture Zone east of the Galapagos hot spot. The FeTi enrichment appears to reflect shallow-depth crystal fractionation (plagioclase, augite, and olivine), which is more extensive near hot spots, and more generally for fast-spreading ridges. H-zones presently affect at least 2.6 × 103 km, or 6.5% of the Mid-Ocean Ridge axis. However, the total known H-area of 8.5 × 105 km2 represents only 0.3% of oceanic crust. This suggests that older H-zones remain to be discovered, or/and that conditions favoring the formation of FeTi basalt and H-anomalies are more prevalent now than they have been on the average for the last 108 years. Evidence for the latter is provided by the known expansion of the magnetically well surveyed Juan de Fuca, Galapagos, and Yermak (Arctic) H-zones in the last 5 million years.