A comparison of the magnetic properties of synthetic titanomaghemites and some oceanic basalts

Abstract

The variation of some magnetic properties of (1) synthetic titanomaghemites and (2) oceanic basalts from DSDP site 417 D as a function of grain size and oxidation is presented. The synthesis of the initial titanomagnetite (x = 0.6) was carried out, using controlled fugacity and self-buffering techniques. Singlephase titanomaghemites were produced by heating the titanomagnetite in air at low temperatures (<350øC). The magnetic properties studied on the synthetic titanomagnetite (grain size --, 1/•m) show the following variations with increasing degrees of oxidation (0 _< z <_ 0.6): (1) Curie temperature (Tc) increases, (2) saturation magnetization (Js) at room temperature increases, (3) J, measured at 77øK decreases for z •< 0.3 and then increases for 0.3 < z • 0.6, (4) bulk and reinanent coercivities (He and Hr) increase slightly initially (z < 0.2) and then decrease at both room temperature and 77•'K, (5) susceptibility (Xo) decreases for z • 0.3 and then increases, (6) the magnetic viscosity acquisition and decay coefficients (Sa and Sa) increase and (7) the median destructive fields for an anhystretic reinanent magnetization and a viscous reinanent magnetization increase for z •< 0.3 and then decrease. These magnetic data also suggest that the synthetic titanomaghemites are pseudo-single domained (PSD) and with increasing degrees of oxidation these PSD grains become more SD-like. The basalt samples from site 417D are divided into fine-grained pillow basalts and coarse-grained massive flows. Most of the magnetic properties measured are distinctly different between these two groups. The magnetic data indicate that the domain states of these basalts are probably PSD. Although it is difficult to separate the effects of grain size and oxidation on the magnetic properties of these basalts, it appears that grain size effects predominate. A preliminary investigation of the magnetic viscosity of the synthetic and oceanic titanomaghemites suggests that a combination of thermally activated and diffusive magnetic viscosity is responsible for their viscous behavior. Finally, we will discuss the roles of grain size and oxidation variation in oceanic basalts and their separate and combined effects on their magnetic behavior. vasiveness of low-temperature oxidation and its effect on their magnetic properties [Marshall and Cox, 1972; Johnson and Hall, 1978; Marshall, 1978]. A degradation of the magnetic signature of the oceanic crust with distance from the ridge seems to be associated with this low-temperature (lt) oxidation. Stoichiometric titanomagnetite (Fe3_xTixO4) with a restricted range of composition around x --0.6 -0.65 is ubiquitous in almost all young (<1 m.y.) submarine basalts [Johnson and Hall, 1978; Marshall, 1978; Pr•vot et al., 1979]. Subsequent to formation, the titanomagnetite is subjected to various degrees of It oxidation, producing single-phase titanomaghemite [Marshall and Cox, 1972; Ozima et aL, 1974; Marshall, 1978; Pr•vot et al., 1979]. Many studies indicate that oxidation of submarine basalt occurs re...

Moessbauer spectra of titanomagnetite: A reappraisal.

“…NISHITANI and KONO (1983) suggested that the differences in cation distribution of original titanomagnetite samples might be one of the causes for the inconsistencies within the results of the low temperature oxidization experiments reported by four groups: OZIMA and SAKAMOTO (1971), READMAN and O'REILLY (1972), KEEFER andSHIVE (1981), andKONO (1983), These discrepancies cause a severe problem when oxidized oceanic basalts are examined (MosKowlTz and BANERJEE, 1981). NISHITANI and KONO (1982) indicated that the main reason for the difference between the data of OzIMA and SAKAMOTO (1971) and those of READMAN and O'REILLY (1972) was in the average size of the ferromagnetic grains, although no significant grain size differences were reported between the samples of other authors. Errors in oxidation parameter (z) will occur if oxidation does not proceed along the constant Fe/ Ti ratio line (FURUTA et al, 1985;AKIMOTO et al, 1984), because z is determined by assuming a constant ratio for Fe/Ti in the two methods that were employed-weight change analysis during heating and wet chemical analysis.…”

“…The second model, proposed by NEEL (1955) andCHEVALLIER et al (1955), assumes the preference of Fe3+ ions for the A site.…”

Viscous magnetization of loess/palaeosol samples from Bulgaria

“…2 should be considered as an underestimation, as far as we have plotted the total VRM ZFS (having in mind that the samples are non-oriented), but we have only used the VRM acquired along the (+Z ) direction in order to minimize some effects of changes in horizontal component of the laboratory magnetic field. As a result, we have the following situation: 1) equal amounts of stable remanences, remained after ZFS and double equal-time exposure to the laboratory field, and 2) significantly higher VRM +−z in comparison with VRM ZFS in spite of the suppressing effect of ZFS on subsequent viscous acquisition (Tivey and Johnson, 1984;Moskowitz and Banerjee, 1981). The applied procedure of ZFS for a period of 30 days will not destroy all the laboratory acquired VRM since the non-oriented pieces of the material have been stored before the experiments for about 6-7 months in the laboratory.…”

“…Recent soil So shows mainly a maghemite presence, while older palaeosols are thought to contain partially oxidized magnetite grains of predominantly SP/SD sizes, probably with Fe 3 O 4 -core/γ Fe 2 O 3 -shell structure. On the basis of this ferromagnetic mineralogy of the samples, we would expect some differences in the viscosity parameters among different palaeosol horizons, having in mind that the low temperature (LT) oxidation can influence magnetic viscosity (Ozdemir and Banerjee, 1981;Moskowitz and Banerjee, 1981). Experimental procedures designed for samples of set 2 were intended to reveal whether this LT oxidation will influence the shape of the VRM acquisition curves.…”

Effect of low-temperature oxidation on the remanence properties of titanomagnetites.

“…They concluded that an unmixing correspond to the true effect of low-temperature oxidation taking place in nature. MosxowlTZ and BANERJEE (1981) synthesized titanomagnetite of composition x=0.6 and measured magnetic properties as a function of the oxidation parameter. They presented magnetic properties of oceanic basalts from DSDP Site 417D as a function of grain size and oxidation.…”