Low-temperature alteration and magnetic changes of variably altered pillow basalts

Abstract

Pillow basalt fragments from the East Pacific Rise, dredged during the Phoenix expedition, often show discoloured rims due to alteration. A suite of nine pillow basalts with such discoloured rims and ranging in age between 200 and 820 ka has been characterized in terms of their Fe–Ti-oxide mineralogy and rock magnetic properties. These large pillow fragments show relatively unaltered grey interiors, surrounded by darker, concentric halos, which vary in thickness as measured from glassy pillow rims and surfaces caused by large cracks penetrating into the original pillow interior. The discoloured zones are characterized by precipitation of abundant secondary minerals, such as Fe 3+ -rich clays that filled vesicle spaces. Fe–Ti oxides in subsamples from discoloured rims and grey interiors have been investigated with electron microscopy and rock magnetic techniques. The subsamples come from traverses that are parallel to the outer glassy pillow rims, allowing us to study the low-temperature alteration effects and rock magnetic properties without having to take variable grain size into account. Not surprisingly, titanomaghemites in discoloured rims are, in a general sense, oxidized to a higher degree ( z typically >0.55) than those in the relatively unaltered grey interior ( z typically <0.55). However, exceptions are numerous and reveal that oxidation state of the Fe–Ti oxides and visible alteration in the discoloured rims are not directly correlated. Moreover, the alteration front of the discoloured rims does not generally coincide with a pronounced jump in z . The titanomaghemite within the discoloured rims appears to have oxidized relatively quickly, reaching z > 0.6 within 200 000 yr. The difference between the oxidation states of titanomaghemite within the grey pillow interior and the discoloured rims gradually diminishes with increasing age, so that for samples with ages of 800 ka the oxidation state of titanomaghemites in the grey interior approaches that of the discoloured rim. Our study demonstrates that visible discolouration of pillow basalts is not a suitable proxy for z . Because average Ti content can vary from sample to sample, Curie temperatures are also inaccurate proxies for z . If one wants to study possible correlations between z and rock magnetic parameters, the best technique is to determine z for each subsample by using transmission electron microscopy (TEM), electron microprobe, MÖssbauer or similar techniques. In agreement with many (but not all) previous observations on natural samples, we find that bulk coercivity ( H c ) , and high-field susceptibility (Χ hf ) increase, whereas low-field susceptibility (Χ l f ) , natural remanent magnetization intensity and saturation magnetization ( M s ) generally decrease with increasing oxidation state.