No abstract

SUMMARY39 dacitic pumice and lithic samples from the 1991 eruption of Mount Pinatubo were investigated through both magnetic and mineralogical means. As in a previous study, natural remanent magnetization (NRM) is found to be reversed for most of the samples, with respect to the direction of the actual geomagnetic field direction. A few samples, amongst them ancient lithics transported by pyroclastic flows, show scattered NRM directions. From thermal demagnetization of these particular samples it is concluded that their orientation changed after emplacement. The emplacement temperature is estimated to be more than 460°C from thermal demagnetization of lithic samples.Two magnetic minerals with large grain sizes are observed under the optical microscope: titanomagnetite (TM) and haemo-ilmenite ( hem-ilm). Microprobe analyses yield x#0.10 for TM and y#0.52 and y#0.54 for two hem-ilm phases, in agreement with the observed Curie temperatures (~480°C for TM and~250°C for hem-ilm). The hem-ilm particles display chemical zonation, which seems to be correlated with a change of the domain structure: typically, a ferrimagnetic (FM) phase with slightly higher titanium content is observed in the central part whilst the crystal margin, which is weakly ferromagnetic (WF, due to spin-canted antiferromagnetism) is slightly poorer in titanium. Two different mechanisms for the origin and formation of the two observed phases are discussed: (1) chemical zonation of hem-ilm crystals due to a change in conditions in the magma chamber shortly before eruption; (2) similar to the microstructures observed from synthetic samples, this zonation in the large natural hem-ilm could be the result of migration of the WF phase towards the grain boundary during residence below the order-disorder transition temperature in the magmatic chamber. The room temperature hysteresis loop, which seems to be dominated by TM, provides multidomain (MD)-like parameters: J rs /J s =0.01 and H cr /H c =20. The large coercivity of remanence (15-40 mT), which is attributed to hem-ilm, may be intrinsic or due to interactions between WF and FM phases. The field dependence of the magnitude of the thermoremanent magnetization (TRM) is not linear: it increases first, reaches a maximum (negative) value for an applied field H close to 0.5 mT, then decreases steadily. By extrapolation, it is estimated that the TRM should be zero for a field of about 12 mT and become positive beyond. This total TRM is in fact the sum of several components. AF demagnetization of TRMs acquired in different fields shows the presence of at least three components: a self-reversed (SR) component that contains both hard and moderately hard components and a soft normal component. Independently of the value of H, the median destructive field of the SR component is of the order of 159

We report on the discovery of magnetite nanoparticles ranging in size from 2 to 14 nm in the mineralized zones of the Peña Colorada iron-ore deposit, southern Mexico. Micrometric scale magnetite was magnetically reduced and divided into distinct size ranges: 85-56 μm, 56-30 μm, 30-22 μm, 22-15 μm, 15-10 μm, 10-7 μm and 7-2 μm. Nanometric-scale magnetite in the size range 2-14 nm was identified. The magnetite was characterized by X-ray diffraction, transmitted and reflected light microscope, high-resolution transmission electron microscopy (TEM), high angle annular dark field, Mössbauer spectroscopy and its magnetic properties. Crystallographic identification of nanostructures was performed using high-resolution TEM. Characteristic changes were observed when the particles make the size transition from micro-to nanometric sizes, as follows: (1) frequency-dependent magnetic susceptibility percentage (χ FD %) measurements show high values (13%) for the 2-14 nm fractions attributed to dominant fractions of superparamagnetic particles; (2) variations of χ FD % < 4.5% in fractions of 56-0.2 μm occur in association with the presence of microparticles formed by magnetite aggregates of nanoparticles (<15 nm) embedded in berthierine; (3) Mössbauer spectroscopy results identified a superparamagnetic fraction; (4) nanometric and 0.2-7 μm grain size magnetite particles require a magnetic field up to 152 mT to reach saturation during the isothermal remanent magnetization experiment; (5) coercivity and remanent magnetization of the magnetite increase when the particle size decreases, probably due to parallel coupling effects; (6) twomagnetic susceptibility versus temperature experiments of the same 2-14 nm sample show that the reversibility during the second heating is due to the formation of new magnetite nanoparticles and growth of those already present during the first heating process.

No abstract