Viscous magnetization at 300 K in a profile through Troodos type oceanic crust

Abstract This study examines the question of the relative importance of viscous magnetism, (VM), at 300 K, to the total magnetization of older oceanic crust. A section through the Troodos, Cyprus, ophiolite, now recognized as a good proxy for in-situ oceanic crust, has been used as the source of samples. The section extends downwards continuously from little altered submarine extrusives through greenschist facies sheeted dikes to mafic and variously serpentinized ultramafic cumulate intrusives. The principal result of the investigation is that VM is not expected to be the dominant magnetization, and is often relatively negligible, at all crustal levels. VM acquisition varies irregularly with depth, with predicted maximum values equivalent to about one-third of the total magnetization, in the extrusives and sheeted dikes. The contribution of VM in two samples of serpentinized ultramafics is insignificant. VM acquisition does not show any simple relationships with primary lithology, alteration history, or magnetic properties or history. The occurrence of two-stage VM growth mechanisms is widespread. The second stage typically has an onset time of about 10 3 min and an acquisition rate three times that of the initial growth mechanism. The physical origin of this two-stage mechanism, and its significance in predicting VM growth over geologic time intervals, are considered to be important problems for future work.