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Fig. 6. High field magnetization M(T) of an andesite fragment versus temperature. Two Curie temperatu- res at 190°C and 380°C are observed. The magneti- zation is dominated by the higher Curie point phase and completely reversible during thermal cycling. Applied field: 0.1 T; heating rate: 18°C min71.

Figure 6 High field magnetization M(T) of an andesite fragment versus temperature. Two Curie temperatu- res at 190°C and 380°C are observed. The magneti- zation is dominated by the higher Curie point phase and completely reversible during thermal cycling. Applied field: 0.1 T; heating rate: 18°C min71.

Related Figures (7)

Fig. 1. Stereographic projection of directions of (a) natural remanent magnetization (NRM) and (b) laboratory-induced thermoremanent magnetization (TRM) of the Nevado del Ruiz 1985 pyroclastics. Nor- mal directions (i.e. parallel to the external field) are measured in the grey dacitic pumice fragments, whe- reas reversed directions (i.e. antiparallel to the applied field) are observed in the brown andesitic pumice. Mean directions projected to lower hemi- sphere are very close to the external field. (a). Pre- sent earth magnetic field in Colombia (star): declina- tion = 0°, inclination = +31°; Laboratory field: declination = 45°E, inclination = +45°.
Fig. 1. Stereographic projection of directions of (a) natural remanent magnetization (NRM) and (b) laboratory-induced thermoremanent magnetization (TRM) of the Nevado del Ruiz 1985 pyroclastics. Nor- mal directions (i.e. parallel to the external field) are measured in the grey dacitic pumice fragments, whe- reas reversed directions (i.e. antiparallel to the applied field) are observed in the brown andesitic pumice. Mean directions projected to lower hemi- sphere are very close to the external field. (a). Pre- sent earth magnetic field in Colombia (star): declina- tion = 0°, inclination = +31°; Laboratory field: declination = 45°E, inclination = +45°.
Fig. 2. Inclination, declination and intensity of NRM of two andesitic samples during thermal treatment in air and zero magnetic field. Heating rate: 10°C min~1. The maximum temperature during the first heating cycle is critical for re-activating the self-reversal effect under laboratory conditions. (a). The reversed NRM polarity is recovered during cooling after an initial heating cycle to 165°C where normal polarity is observed. The self. reversal process is still active. (b). The reversed polarity is not recovered during cooling after heating to 189°C. The self-reversal property has been lost. value of about 1.2 mT (Fig. 4), but the normal and reversed TRM components of these rocks do not show a simple relation to the applied field strength (Fig. 3b). The normal low-coercivity TRM component generally increases with in- creasing field (up to 15 mT) although a linear trend cannot be recognized. Its stability against alternating fields is generally higher than in the dacites. The reversed andesite TRM first in- creases with increasing field, but is gradually suppressed in higher fields. The TRM acquired in 15 mT is dominated completely by normal polarity. The slight intensity minimum at 40 mT during AF demagnetization indicates that The TRM self-reversal process is dependent on the inducing field strength (Fig. 3). A linear relationship between applied field (maximum amplitude: 0.5 mT) and TRM intensity of nor- mal polarity is observed in the grey dacites (Fig. 3a). This remanence is not very stable, with about 90% being destroyed by alternating fields (AF) up to 20 mT. The total TRM of the brown andesites changes from reversed to nor- mal polarity at an interpolated applied field polarity. The slight intensity minimum at 40
Fig. 2. Inclination, declination and intensity of NRM of two andesitic samples during thermal treatment in air and zero magnetic field. Heating rate: 10°C min~1. The maximum temperature during the first heating cycle is critical for re-activating the self-reversal effect under laboratory conditions. (a). The reversed NRM polarity is recovered during cooling after an initial heating cycle to 165°C where normal polarity is observed. The self. reversal process is still active. (b). The reversed polarity is not recovered during cooling after heating to 189°C. The self-reversal property has been lost. value of about 1.2 mT (Fig. 4), but the normal and reversed TRM components of these rocks do not show a simple relation to the applied field strength (Fig. 3b). The normal low-coercivity TRM component generally increases with in- creasing field (up to 15 mT) although a linear trend cannot be recognized. Its stability against alternating fields is generally higher than in the dacites. The reversed andesite TRM first in- creases with increasing field, but is gradually suppressed in higher fields. The TRM acquired in 15 mT is dominated completely by normal polarity. The slight intensity minimum at 40 mT during AF demagnetization indicates that The TRM self-reversal process is dependent on the inducing field strength (Fig. 3). A linear relationship between applied field (maximum amplitude: 0.5 mT) and TRM intensity of nor- mal polarity is observed in the grey dacites (Fig. 3a). This remanence is not very stable, with about 90% being destroyed by alternating fields (AF) up to 20 mT. The total TRM of the brown andesites changes from reversed to nor- mal polarity at an interpolated applied field polarity. The slight intensity minimum at 40
Fig. 3. Alternating field demagnetization of TRM acquired in constant fields of (a) 0.04 mT (triangles) and 0.5 mT (dots) for dacitic and (b) of 0.04 mT (triangles), 0.5 mT (dots), 3 mT (circles) and 15 mT (stars) for andesitic samples.
Fig. 3. Alternating field demagnetization of TRM acquired in constant fields of (a) 0.04 mT (triangles) and 0.5 mT (dots) for dacitic and (b) of 0.04 mT (triangles), 0.5 mT (dots), 3 mT (circles) and 15 mT (stars) for andesitic samples.
Fig. 4. Total TRM as function of applied field. Ini- tially the reversely polarized TRM increases with increasing DC field, but changes to normal polarity at a critical field of 1.2 mT.
Fig. 4. Total TRM as function of applied field. Ini- tially the reversely polarized TRM increases with increasing DC field, but changes to normal polarity at a critical field of 1.2 mT.
Fig. 5. Coercivity spectra of a dacite and an andesite sample derived from alternating field demagnetiza- tion of IRM (given at 1-T field strength).
Fig. 5. Coercivity spectra of a dacite and an andesite sample derived from alternating field demagnetiza- tion of IRM (given at 1-T field strength).
Fig. 7. Model of the suggested self-reversal mechanism as function of increasing (stages 1 to 4) applied fielc (H,,,). Negative exchange across the boundary (starred line) of two phases causes the net magnetic moment tc be antiparallel to the applied field as long as this is small (stages 1 to 2). With increasing external field, domain wall (thin straight line) nucleation takes place reducing the size of the reversely magnetized phases (stage 3). Finally, domain-wall movement towards the phase boundary (stage 4) leads to a polarity of the net magnetiza tion completely changed from reversed to normal. nent shifts to higher values with increasing ap- These phases carrying normal and reversed magnetization are characterized by high coer- civities (Fig. 5). As the nucleated domain wall moves towards the phase boundary (Fig. 7. stage 4), thereby acquiring higher magnetic stability (Westcott-Lewis and Parry, 1971), the coercivity spectrum of the normal TRM compo- nent shifts to higher values with increasing ap-
Fig. 7. Model of the suggested self-reversal mechanism as function of increasing (stages 1 to 4) applied fielc (H,,,). Negative exchange across the boundary (starred line) of two phases causes the net magnetic moment tc be antiparallel to the applied field as long as this is small (stages 1 to 2). With increasing external field, domain wall (thin straight line) nucleation takes place reducing the size of the reversely magnetized phases (stage 3). Finally, domain-wall movement towards the phase boundary (stage 4) leads to a polarity of the net magnetiza tion completely changed from reversed to normal. nent shifts to higher values with increasing ap- These phases carrying normal and reversed magnetization are characterized by high coer- civities (Fig. 5). As the nucleated domain wall moves towards the phase boundary (Fig. 7. stage 4), thereby acquiring higher magnetic stability (Westcott-Lewis and Parry, 1971), the coercivity spectrum of the normal TRM compo- nent shifts to higher values with increasing ap-
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