Hélène Horen

Magnetic properties of variably serpentinized abyssal peridotites

We have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven Deep Sea Drilling Project and Ocean Drilling Program sites. The magnetic susceptibility (K) of these samples does not increase linearly with the degree of serpentinization (S). Instead, K remains modest in partially serpentinized samples (S 75%) peridotites in the oceanic crust. Moderately serpentinized peridotites (S < 75%) have NRM values <5 A/m and K < 0.05 SI. Low-temperature oxidation of magnetite is found to lower the NRM and K values of seafloor samples and of cataclastically deformed drilled samples.

The magnetic signature of ultramafic-hosted hydrothermal sites

Unlike basalt-hosted hydrothermal sites, characterized by a lack of magnetization, the magnetic signature of ultramafic-hosted hydrothermal sites remains poorly known, despite their wide occurrence at slow-spreading ridges and their strong mineral potential. The first high-resolution magnetic surveys of such ultramafic-hosted sites, achieved by deep-sea submersible on four sites of the Mid-Atlantic Ridge, reveal positive magnetic anomalies, and therefore a strong magnetization at the largest sites. This observation reflects the presence of a wide mineralized zone beneath these sites, the stockwork, where several chemical processes concur to create and preserve strongly magnetized magnetite. Beyond pointing out the importance of subsurface chemical processes in hydrothermal activity, the aging of oceanic lithosphere, and the ocean chemical budget, our results have immediate application for detecting and characterizing economically valuable deep-sea mineral deposits.

Obduction‐related remagnetization at the base of an ophiolite: Paleomagnetism of the Samail nappe lower sequence and of its continental substratum, southeast Oman Mountains

This paper considers the effect of obduction tectonics on the magnetic properties of a fast spreading ridge ophiolite. The paleomagnetism of the ultramafics (seven sites) and layered gabbros (two sites) of the Samail, Wadi Tayin-Ibra, and Muscat massifs of Oman has been investigated, along with that of the Permian metabasites from the subophiolitic Saih Hatat continental window (five sites). Of the 202 studied samples, 183 exhibit stable components with a dominant NNW trend and a shallow, positive or, less frequently, negative inclination. Superposition of normal and reverse polarities was observed in one gabbro, three peridotite, and two metabasite sites. The in situ characteristic directions are similar in both the nappe and metamorphic window, except for a lone, low-grade metabasite with a component (D = 168°, I = 35°) referred to a preserved Permian magnetization. The moderate directional scatter can be explained by postnappe tilting along the NNE trending axis. The high-grade metabasites in the window were completely remagnetized during the metamorphism associated with nappe emplacement and subsequent exhumation tectonics between 80 and 70 Ma. Since the overlying ophiolites have similar, normal, or reverse directions, they were remagnetized contemporaneously. The magnetic mineralogy of the ultramafic-layered gabbro sequence was likely modified extensively at that time by serpentinization from the upward metamorphic fluid advection in the faulted ophiolite. Therefore the magnetic properties of the lower part of the southern Samail ophiolite cannot be used for modeling magnetic anomalies of oceanic lithosphere.

Highly magnetised and differentiated basalts at the 18–19° S propagating spreading centre in the North Fiji Basin

Geochemical and magnetic properties of basalts dredged along a propagating spreading centre (central part of the North Fiji Basin) have been analysed. Two phenomena appear to be important in controlling variations of natural remanent magnetisation (NRM): 1) an extensive magma differentiation to Fe-Ti basalts increases with the intensity of NRM; and both increase towards the propagator tip, 2) low temperature oxidation (maghemitisation) seems to have already occurred along the ridge axis for the zero age basalts. This paper shows that despite the apparent lack of correlation between NRM and the maghemitisation process (masked by the effect of extensive magma differentiation), the latter involves a change of the domain state of magnetic carriers, from pseudo single-domain to single-domain. We separated the NRM into 4 partial NRMs (pNRM) depending on the coercivity of grains. This analysis showed that the contribution to the remanence of grains with high coercivity increases with maghemitisation, whereas the contribution of grains with weak coercivity decreases.Despite the relatively high degree of maghemitisation, the variations of natural remanent magnetisation intensity principally reflect the variations of magmatic processes. This joint work on magnetic and magmatic properties of basalts collected on the NS-propagating spreading centre from the North Fiji Basin reinforces the magnetic telechemistry hypothesis of Vogt (1979).