Magnetic properties, opaque mineralogy and magnetic anisotropies of serpentinized peridotites from ODP Hole 670A near the Mid-Atlantic Ridge

Abstract

Abstract Magnetic mineralogy and magnetic properties of 16 samples of serpentinized peridotites (harzburgites) from ODP Hole 670A are studied. The degree of serpentinization was determined by a new method which combines saturation magnetization and whole-rock iron content. A unique Curie temperature ( T c ) of 580°C corresponding to pure magnetite was observed. Microprobe analyses, whole-rock metal content and Curie temperatures indicate the absence of awaruite (Ni 3 Fe) or native metals. Hysteresis parameters reveal the contribution of pseudo-single-domain (PSD) grains in samples with a low degree of serpentinization and large multi-domain (MD) grains in highly serpentinized samples with J rs J s ⩽ 0.08 . Natural remanent magnetization (NRM) is mainly unstable and possibly composite, particularly in the highly serpentinized samples. The intensity ( J a ) of anhysteretic remanent magnetization (ARM) in the geomagnetic field is much higher than J n (intensity of NRM), with J a J n averaging 2.4. The Koenigsberger ratio Q is low and susceptibility ( k ) depends on degree of serpentinization, ranging from 1.2 to 51.5 (×10 −6 m 3 kg −1 ). NRM inclinations are scattered, with some tendency to lie near the horizontal plane, which is close to the foliation plane. Anisotropies of susceptibility, of ARM and of saturation remanent magnetization (SRM) were studied and compared with spinel foliation determined for three less serpentinized samples. In these samples, magnetic anisotropies seem to be mimetic of high-temperature deformation before serpentinization. When the degree of serpentinization is high, later magnetic fabrics, probably related to magnetite veins, dominate previous anisotropies. The temperature dependence of susceptibility and of viscous remanent magnetization (VRM) were studied. The thermal variation of susceptibility shows a shallow Hopkinson effect at about 300–400°C. Time dependence of induced and remanent magnetization increases with temperature, reaching maximum values at about 300–400°C. Based on these observations, induced magnetization and VRM of lower crustal and upper-mantle rocks and their contribution to marine magnetic anomalies are discussed.