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Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal
Journal of Volcanology and Geothermal Research, 41 (1990) 369-377 Elsevier Science Publishers B.V., Amsterdam--Printed in the Netherlands
369
Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal* M A J A H A A G 1, F R I E D R I C H H E L L E R 1, J U A N C A R L O S C A R R A C E D O 2 A N D V I C E N T E SOLER 2 l Institut fitr Geophysik, ETH-HSnggerberg, CH-8093 Zi~,rich, Switzerland 2 Estaci6n VolcanolSgica de Canarias, Instituto de Recursos Naturales, La Laguna, Tenerife, Spain
(Received December 1, 1989)
Abstract Haag, M., Heller, F., Carracedo, J.C. and Soler, V., 1990. Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal. In: S.N. Williams (Editor), Nevado del Ruiz Volcano, Colombia, I. J. Volcanol. Geotherm. Res., 41: 369-377. The natural remanent magnetization of andesitic pumice emitted during the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has a direction opposite to the present geomagnetic field. The selfreversing mechanism can be re-activated in the laboratory during cycles of heating and subsequent cooling in air and zero magnetic field. Laboratory-produced thermoremanent magnetization is dominated by the same self-reversal process in fields up to several mT. Microchemical, optical and Curie temperature analyses indicate that the ferromagnetic minerals are members of the magnetite-ulvSspinel and hematiteilmenite series with average compositions of Fe2.73Ti0.2704 and Fel.38Ti0.6203, respectively. In analogy with the magnetic behaviour of synthetically grown antiferromagnetic-ferromagnetic FeMn-FeNi films, the self-reversal can probably be interpreted in terms of an exchange field acting between a Ti-poor canted antiferromagnetic and a Ti-rich ferrimagnetic phase in the hemoilmenite grains.
Introduction D u r i n g the 1985 e r u p t i o n of N e v a d o del Ruiz b r o w n - g r e y andesitic a n d w h i t e - g r e y dacitic p u m i c e f r a g m e n t s were s p r e a d a r o u n d the emission c e n t r e in pyroclastic falls like those described by T h o u r e t et al. (1985). P r o g r e s s i v e m a g m a m i x i n g has caused the different chemical composition. * Contribution no. 603, Institut ffir Geophysik, ETH Zfirich, Switzerland
0377-0273/90/$03.50
The r e m a n e n t m a g n e t i z a t i o n of the two rock types differs completely. The directions of n a t u r a l r e m a n e n t m a g n e t i z a t i o n (NRM) of g r e y dacitic pumice bombs cluster n e a r the p r e s e n t g e o m a g n e t i c field in Colombia which points n o r t h w a r d w i t h a n inclination of +31 ° (Fig. la). T h e NRM of the b r o w n andesitic pyroclastics, however, is a l i g n e d a n t i p a r a l l e l to the p r e s e n t field due to a self-reversing m e c h a n i s m (Heller et al., 1986), The samples w e r e collected as o r i e n t e d h a n d samples. T h e i r m e a n N R M direction has been
© 1990 Elsevier Science Publishers B.V.
calculated after projection to lower hemisphere regardless of the actual polarity. The mean NRM has a declination D = 357.8 ° and inclination I = 36.0 ° with a 95% cone of confidence (a95--10"1°) t h a t includes the present-day
field direction. The close parallel or antiparallel alignment of the NRM vectors to the local geomagnetic field points to an emplacem ent of the pumice bombs at t e m p e r a t u r e s higher t h a n 400°C (maximum Curie temperature).
Remanent magnetization grey
p
~.~
,\
NRM
/
/
~'A\
/N"~eyp~iee
\'\
ib
,/
TRM
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. Normal directions (i.e. parallel to the external field) are measured in the grey dacitic pumice fragments, whereas reversed directions (i.e. antiparallel to the applied field) are observed in the brown andesitic pumice. Mean directions projected to lower hemisphere are very close to the external field. (a). Present earth magnetic field in Colombia (star): declination = 0 °, inclination = +31°; Laboratory field: declination = 45°E, inclination - +45 °.
The self-reversing mechanism of the NRM of the andesitic samples is still active during cycles of heating up to 165°C and subsequent cooling in air and zero magnetic field (Fig. 2ai. The reversed NRM changes to normal polarity at 120°C in the first heating cycle and returns back to a reversed direction at about the same t e m p e r a t u r e when cooling to room t e m p e r a t u r e (ll°C). The next heating cycle removes the reversed component completely at temperatures around 175°C. The normal NRM component is gradually lowered with furt her increasing t e m p e r a t u r e and falls below noise level of the measuring high-temperature spinner m a g n e t o m e t e r (Heiniger and Heller, 1976) at t e m p e r a t u r e s around 400°C. If t e m p e r a t u r e s of 175 °C are exceeded during the first heating cycle (Fig. 2b), then the reversed component is not reactivated upon cooling back to room temperature. The resulting normal NRM is relatively small compared to the initial reversed NRM intensity. Apparently the self-reversal mechanism is suppressed under these conditions. Some andesitic and dacitic samples were magnetized in a laboratory field of the same amplitude as the local field in Colombia (B = 0.04 mT) by cooling from 450°C to room t e m p e r a t u r e (Fig. lb). The acquired therm o r e m a n e n t magnetization (TRM) has exactly the same intensity as the original NRM. This result suggests t h a t the NRM is actually a TRM, acquired during cooling in situ and not affected by chemical alteration processes. The TRM directions group closely around the laboratory field direction applied (declinat i o n = 4 5 ° E , i n c l i n a t i o n = 4-45°). They are
371
REMANI~]NT MAGNETIZATION OF ANDESITE AND DACITIC PUMICEFROMNEVADO DEL RUIZ
parallel to the applied field in the grey dacitic material and antiparallel in the brown andesitic pyroclastics where self-reversal is observed again. 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 normal 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 normal polarity at an interpolated applied field
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 increasing field (up to 15 mT) although a linear trend cannot be recognized. Its stability against alternating fields is generally higher t h a n in the dacites. The reversed andesite TRM first increases 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 m i n i m u m at 40 mT during AF demagnetization indicates t h a t ~-~ +0.2 [ H e a
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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 selfreversal 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.
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Fig. 3. Alternating field demagnetization of TRM acquired in constant fields of (a) 0.04 mT (trianglest 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.
the self-reversing mechanism is still active, Coercivities of the reversed component approach 70 to 80 mT (Fig. 3b) whereas substantial parts of the normal component cannot be erased even at fields of 0.3 T (not shown in Fig. 3b). The higher stability of the self-reversing andesites is also demonstrated by AF demagnetization of isothermal r e m a n e n t magnetization (IRM) acquired in an applied field of 1 T. The coercivity spectra of the two rock types have been plotted in Figure 5. The dacite spectrum peaks at low fields and gradually drops toward high coercivities, as observed for many volcanic rocks. The low field peak of the andesite coer-
civity spectrum, however, is less pronounced. In addition, a second peak between 30 and 40 mT occurs, above which the spectrum slowly tails off, suggesting that the self-reversal mechanism might be connected to ~ highcoercivity mineral phase. Magnetic
mineralogy
High-field-magnetization measurements, microscopic examination and microchemica] analysis indicate the presence of at least two magnetic mineral phases in the andesitic pyroclasts. The dependence of high field magnetization on temperature (M~(T)) is
373
REMANENT MAGNETIZATION OF ANDESITE AND DACITIC PUMICE FROM NEVADO DEL RUIZ
c h a r a c t e r i z e d by t w o inflexions w h i c h reflect t h e C u r i e t e m p e r a t u r e s of f e r r o m a g n e t i c m i n e r a l phases• T h e Ms(T) c u r v e s a r e completely reversible between 600°C and room t e m p e r a t u r e w i t h o u t a n y sign of c h e m i c a l a l t e r a t i o n of t h e f e r r o m a g n e t i c m i n e r a l content. They are dominated by a mineral with a C u r i e t e m p e r a t u r e a r o u n d 3 8 0 ° C (Fig. 6). M i c r o p r o b e a n a l y s i s (Table 1) i n d i c a t e s t h e p r e s e n c e of a m e m b e r of t h e ulvSspinelm a g n e t i t e solid-solution series w i t h a n a v e r a g e c o m p o s i t i o n of Fe2.73Ti0.2704 . A s s u m i n g stoic h i o m e t r y a n d r e p l a c e m e n t of Fe by t h e measured impurities, a theoretical Curie t e m p e r a t u r e of 383 °C is c a l c u l a t e d a c c o r d i n g to R i c h a r d s et al. (1973). T h e r e f o r e , t h e abovem e a s u r e d C u r i e t e m p e r a t u r e is a t t r i b u t e d to a t i t a n o m a g n e t i t e p h a s e w h i c h also h a s b e e n
2s DACITE
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Fig. 6. High field magnetization Ms(T) of an andesite fragment versus temperature. Two Curie temperatures at 190°C and 380°C are observed. The magnetization is dominated by the higher Curie point phase and completely reversible during thermal cycling. Applied field: 0.1 T; heating rate: 18°C min 1.
71.73
Sum (wt%)
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Ti (wt%) Fe (wt%) Mn (wt%) Cr (wt%) A1 (wt%) Mg (wt%)
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Average
Composition and theoretically derived Curie t e m p e r a t u r e s ITc, after Richards et al., 1973) of magnetic ore grains fi'om andesitic fl'agments of the Nevado del Ruiz 1985 eruption
TABLE 1 c~
REMANENT MAGNETIZATION OF ANDESITE AND DACITIC PUMICE FROM NEVADO DEL RUIZ
identified under the ore microscope in grain sizes up to 150 ~m. The second ore mineral visible under the microscope belongs to the hematite-ilmenite series and locally contains exsolution lamellae structures (the composition of which could not be determined because of their small grain size (< 1 ~m)). The average composition (Fe1.38Ti0.6203) yields a theoretical Curie temperature of 190°C (Table 1). This compares well with the faint kink in the Ms(T) curve at about 190°C. Thus, both titanomagnetite and hemoilmenite contribute to the magnetic properties of the andesitic pumice. According to our magnetic and microchemical analyses the composition of the two mineral phases stays within narrow boundaries of the average (cf. Table 1). Similar microprobe results have been obtained by Fujiwara et al. (1987). D i s c u s s i o n of s e l f - r e v e r s a l m e c h a n i s m s
The Nevado del Ruiz 1985 andesitic pumice fragments prove for the first time without any doubt that self-reversal actually has controlled the NRM acquisition process (Figs. 1, 2). Selfreversing properties of NRM were discovered much earlier by Nagata et al. (1952), Ishikawa and Akimoto (1957), Uyeda (1958) and Ishikawa and Syono (1963) in dacitic pumice and pitchstone from Mount H a r u n a and Mount Asio (Japan). These authors used the interaction models of N~el (1951) and Meiklejohn and Bean (1957) to interpret this phenomenon. One-phase or two-phase models are at hand: N~el's N-type model with different temperature-dependent sublattice magnetization in one phase and different types of two-phase models with magnetostatic (i.e. dipole-dipole) or (super)exchange interaction. The energies involved in these interactions are usually assumed to be different by orders of magnitude. The N-type model requires an exchange-interaction field strength on the order of several Teslas. The one-phase model can be rejected as a possible source of the Nevado del Ruiz self-reversal
375
because: (1) the Ms(T) curve in Figure 6 decreases regularly between room temperature and 4O0°C with increasing temperature and vice versa during cooling: and (2) the applied fields of the order of 1 - 2 mT are capable to suppress the self-reversal completely (Fig. 3). It was observed during the heating and cooling experiments of Figure 2 that a critical temperature of ca. 175°C may not be exceeded in order to reproduce the NRM self-reversal in zero field. This implies that the titanomagnetite phase with its Curie temperature of approximately 380°C does not contribute to the self-reversal process. Instead, two phases of intermediate hemoilmenite with Curie temperatures near 200°C and possibly very similar composition may be interacting. The observed lamellae structures in some otherwise optically homogeneous hemoilmenites are potential candidates for two-phase interaction models. The earlier Japanese scientists already found evidence for an o r d e r disorder transition during rapid cooling of hemoilmenite which leads to a microstructure of two hemoilmenite phases with different titanium content and hence different Curie temperatures. Further evidence for cation-ordered grain domans with disordered boundaries has been given recently by Lawson et al. (1987). The self-reversing mechanism seems to be induced by cooling through the higher Curie temperature of a Ti-poor hemoilmenite phase with low spontaneous magnetization but high coercivity. This phase, which is magnetized parallel to the applied field, induces an antiparallel magnetic ordering because of negative interaction to a Ti-richer hemoilmenite with high spontaneous magnetization but low coercivity. This leads to a net negative magnetization, when the temperature drops below the lower Curie temperature. This antiparallel coupling must be due either to magnetostatic or exchange interaction between the Ti-poor, possibly canted antiferromagnetic hemoilmenite and the Ti-rich, possibly ferrimagnetic hemoilmenite. Magnetostatic in-
teraction works only under very restricted geometrical conditions (Uyeda, 1958) which are uncommon in rock-forming minerals. The weak critical field, however, needed to suppress the self-reversal mechanism (Fig. 4) apparently contradicts a simple exchange field model involving field strength on the order of several Tesla. A similar problem exists for synthetically grown antiferromagnetic-ferromagnetic FeMnFeNi films produced under ultra-high vacuum conditions, where no magnetostatic interaction is possible because of the purely antiferromagnetic FeMn phase (Tsang et al., 1981; Mauri et al., 1987a, b). In this system, the measured "exchange bias" field - which is identical to our "critical" field to suppress the self-reversal mechanism - is on the order of 1 - 2 0 mT (Meiklejohn and Bean, 1957), much less t h a n typical exchange fields. The apparent discrepancy can be solved by a domain-wall model (Westcott-Lewis and Parry, 1971; Mauri et al., 1987a, b). The strong exchange interaction between the two phases is not influenced by small external fields. Domain-wall nucleation and subsequent movement, however, is readily achieved by fields of 1 - 1 0 roT. The easy suppression of the self-reversal in the andesite may be explained by the creation of a domain in the ferrimagnetic hemoilmenite phase whose magnetization is antiparallel to
the external field before nucleation of a domain wall (Fig. 7, stage 2 to 3). With increasing ap~ plied field, the domain with parallel magnetization grows at the expense of the domain which is locked antiparallel by the exchange coupling across the boundary of the two phases. This finally leads to a change in polarity of the total magnetization and further increase of the netpositive magnetization with higher applied fields. Conclusion The andesitic pumice of the Nevado dei Ruiz 1985 eruption contains hemoilmenite whose NRM and low field TRM (acquired in fields of less t h a n 1 mT) are subject to self-reversal (Fig. 1, 2). In analogy to the magnetic behaviour of FeMn-FeNi thin films the self-reversal mechanism may be explained in terms of an exchange model of two, chemically different hemoilmenite phases with a domain-wall nucleation process superimposed. These phases carrying normal and reversed magnetization are characterized by high coercivities (Fig. 5). As the nucleated domain wall moves towards the phase boundal~y (Fig. 7~ stage 4), thereby acquiring higher magnetic stability (Westcott-Lewis and Parry, 1971), the coercivity spectrum of the normal TRM component shifts to higher values with increasing ap-
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Fig. 7. Model of the suggested self-reversal mechanism as function of increasing (stages 1 to 4) applied field (Hext). Negative exchange across the boundary (starred line) of two phases causes the net magnetic moment to be antiparallel to the applied field as long as this is small (stages I to 2). With increasing external field, domainwall (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 magnetization completely changed from reversed to normal.
REMANENT MAGNETIZATION OF ANDESITE AND DACITIC PUMICE FROM NEVADO DEL RUIZ
plied field (Fig. 3b). At high e n o u g h e x t e r n a l fields the n o r m a l l y m a g n e t i z e d d o m a i n s will o v e r w h e l m those r e v e r s e l y m a g n e t i z e d so t h a t t h e self-reversal effect is more and more suppressed.
References Fujiwara, Y., Gautam, P., Yoshida, M. and Katsui, Y., 1987. On the nature of remanence in the andesite pumice with self-reversed magnetization from the Nevado del Ruiz, Colombia. Rock Magnet. Paleogeophys., 14: 7-12. Heiniger, C. and Heller, F., 1976. A high temperature vector magnetometer. Geophys. J. R. Astron. Soc., 44: 281-288. Heller, F., Carracedo, J.C. and Soler, V., 1986. Reversed magnetization in pyroclastics from the 1985 eruption of Nevado del Ruiz, Colombia. Nature, 324: 241-242. Ishikawa, Y. and Akimoto, S., 1957. Magnetic properties of the FeTiO3-Fe203 solid solution series. J. Phys. Soc. Jpn, 12: 1083-1098. Ishikawa, Y. and Syono, Y., 1963. Order-disorder transformation and reverse thermo-remanent magnetism in the FeTiO3-Fe203 system. J. Phys. Chem. Solids, 24: 517-528. Lawson, C.A., Nord, G.L. and Champion, D.E., 1987. Fe-Ti oxide mineralogy and the origin of normal and reverse remanent magnetization in dacitic pumice blocks from Mt. Shasta, California. Phys. Earth Planet. Inter., 46: 270-288. Mauri, D., Kay., E. Scholl, D. and Howard, J.K., 1987a. Novel method for determining the
377
anisotropy constant of MnFe in a NiFe/MnFe sandwich. J. Appl. Phys., 62: 2929-2932. Mauri, D., Siegmann, H.C., Bagus, P.S. and Kay, E., 1987b. Simple model for thin ferromagnetic films and exchange coupled to an antiferromagnetic substrate. J. Appl. Phys., 62: 3047-3049. Meiklejohn, W.H. and Bean, C.P., 1957. New magnetic anisotropy. Phys. Rev., 105: 904-913. Nagata, T., Uyeda, S. and Akimoto, S., 1952. Selfreversal of thermoremanent magnetism of igneous rocks. J. Geomagn. Geoelectr., 4: 22-38. N~el, L., 1951. L'inversion de l'aimantation permanente des roches. Ann. G~ophys. 7: 90-102. Richards, J.C.W., O'Donovan, J.B., Hauptmam Z., O'Reilly, W. and Creer, K.M., 1973. A magnetic study of titanomagnetite substituted by magnesium and aluminium. Phys. Earth Planet. Inter., 7: 437-444. Thouret, J.C., Vatin-P~rignon, N., Cantagrel, J.M., Salinas, R. and Murcia, A., 1985. Le Nevado E1 Ruiz (Cordill~re Centrale des Andes de Colombie): stratigraphie, structures et dynamisme d'un appareil volcanique and~sitique, compos~ et polyg~nique. Rev. Geol. Dyn. Geogr. Phys., 26: 257 -271. Tsang, C., Heiman, N. and Lee, K., 1981. Exchange induced unidirectional anisotropy at FeMnNisoFe2o interfaces. J. Appl. Phys., 52: 2471-2473. Uyeda, S., 1958. Thermoremanent magnetism as a medium of palaeomagnetism, with special reference to reverse thermo-remanent magnetism. Japan. J. Geophys., 2: 1-123. Westcott-Lewis, M.F. and Parry, L.G., 1971. Thermoremanence in synthetic rhombohedral irontitanium oxides. Aust. J. Phys., 24: 735-742.
369
Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal* M A J A H A A G 1, F R I E D R I C H H E L L E R 1, J U A N C A R L O S C A R R A C E D O 2 A N D V I C E N T E SOLER 2 l Institut fitr Geophysik, ETH-HSnggerberg, CH-8093 Zi~,rich, Switzerland 2 Estaci6n VolcanolSgica de Canarias, Instituto de Recursos Naturales, La Laguna, Tenerife, Spain
(Received December 1, 1989)
Abstract Haag, M., Heller, F., Carracedo, J.C. and Soler, V., 1990. Remanent magnetization of andesitic and dacitic pumice from the 1985 eruption of Nevado del Ruiz (Colombia) reversed due to self-reversal. In: S.N. Williams (Editor), Nevado del Ruiz Volcano, Colombia, I. J. Volcanol. Geotherm. Res., 41: 369-377. The natural remanent magnetization of andesitic pumice emitted during the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has a direction opposite to the present geomagnetic field. The selfreversing mechanism can be re-activated in the laboratory during cycles of heating and subsequent cooling in air and zero magnetic field. Laboratory-produced thermoremanent magnetization is dominated by the same self-reversal process in fields up to several mT. Microchemical, optical and Curie temperature analyses indicate that the ferromagnetic minerals are members of the magnetite-ulvSspinel and hematiteilmenite series with average compositions of Fe2.73Ti0.2704 and Fel.38Ti0.6203, respectively. In analogy with the magnetic behaviour of synthetically grown antiferromagnetic-ferromagnetic FeMn-FeNi films, the self-reversal can probably be interpreted in terms of an exchange field acting between a Ti-poor canted antiferromagnetic and a Ti-rich ferrimagnetic phase in the hemoilmenite grains.
Introduction D u r i n g the 1985 e r u p t i o n of N e v a d o del Ruiz b r o w n - g r e y andesitic a n d w h i t e - g r e y dacitic p u m i c e f r a g m e n t s were s p r e a d a r o u n d the emission c e n t r e in pyroclastic falls like those described by T h o u r e t et al. (1985). P r o g r e s s i v e m a g m a m i x i n g has caused the different chemical composition. * Contribution no. 603, Institut ffir Geophysik, ETH Zfirich, Switzerland
0377-0273/90/$03.50
The r e m a n e n t m a g n e t i z a t i o n of the two rock types differs completely. The directions of n a t u r a l r e m a n e n t m a g n e t i z a t i o n (NRM) of g r e y dacitic pumice bombs cluster n e a r the p r e s e n t g e o m a g n e t i c field in Colombia which points n o r t h w a r d w i t h a n inclination of +31 ° (Fig. la). T h e NRM of the b r o w n andesitic pyroclastics, however, is a l i g n e d a n t i p a r a l l e l to the p r e s e n t field due to a self-reversing m e c h a n i s m (Heller et al., 1986), The samples w e r e collected as o r i e n t e d h a n d samples. T h e i r m e a n N R M direction has been
© 1990 Elsevier Science Publishers B.V.
calculated after projection to lower hemisphere regardless of the actual polarity. The mean NRM has a declination D = 357.8 ° and inclination I = 36.0 ° with a 95% cone of confidence (a95--10"1°) t h a t includes the present-day
field direction. The close parallel or antiparallel alignment of the NRM vectors to the local geomagnetic field points to an emplacem ent of the pumice bombs at t e m p e r a t u r e s higher t h a n 400°C (maximum Curie temperature).
Remanent magnetization grey
p
~.~
,\
NRM
/
/
~'A\
/N"~eyp~iee
\'\
ib
,/
TRM
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. Normal directions (i.e. parallel to the external field) are measured in the grey dacitic pumice fragments, whereas reversed directions (i.e. antiparallel to the applied field) are observed in the brown andesitic pumice. Mean directions projected to lower hemisphere are very close to the external field. (a). Present earth magnetic field in Colombia (star): declination = 0 °, inclination = +31°; Laboratory field: declination = 45°E, inclination - +45 °.
The self-reversing mechanism of the NRM of the andesitic samples is still active during cycles of heating up to 165°C and subsequent cooling in air and zero magnetic field (Fig. 2ai. The reversed NRM changes to normal polarity at 120°C in the first heating cycle and returns back to a reversed direction at about the same t e m p e r a t u r e when cooling to room t e m p e r a t u r e (ll°C). The next heating cycle removes the reversed component completely at temperatures around 175°C. The normal NRM component is gradually lowered with furt her increasing t e m p e r a t u r e and falls below noise level of the measuring high-temperature spinner m a g n e t o m e t e r (Heiniger and Heller, 1976) at t e m p e r a t u r e s around 400°C. If t e m p e r a t u r e s of 175 °C are exceeded during the first heating cycle (Fig. 2b), then the reversed component is not reactivated upon cooling back to room temperature. The resulting normal NRM is relatively small compared to the initial reversed NRM intensity. Apparently the self-reversal mechanism is suppressed under these conditions. Some andesitic and dacitic samples were magnetized in a laboratory field of the same amplitude as the local field in Colombia (B = 0.04 mT) by cooling from 450°C to room t e m p e r a t u r e (Fig. lb). The acquired therm o r e m a n e n t magnetization (TRM) has exactly the same intensity as the original NRM. This result suggests t h a t the NRM is actually a TRM, acquired during cooling in situ and not affected by chemical alteration processes. The TRM directions group closely around the laboratory field direction applied (declinat i o n = 4 5 ° E , i n c l i n a t i o n = 4-45°). They are
371
REMANI~]NT MAGNETIZATION OF ANDESITE AND DACITIC PUMICEFROMNEVADO DEL RUIZ
parallel to the applied field in the grey dacitic material and antiparallel in the brown andesitic pyroclastics where self-reversal is observed again. 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 normal 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 normal polarity at an interpolated applied field
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 increasing field (up to 15 mT) although a linear trend cannot be recognized. Its stability against alternating fields is generally higher t h a n in the dacites. The reversed andesite TRM first increases 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 m i n i m u m at 40 mT during AF demagnetization indicates t h a t ~-~ +0.2 [ H e a
~ , +0.2
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Temperature (°C) Heating : Cooling : Heating
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Temperature (°(5')
0
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189
100
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b
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 selfreversal 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.
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Fig. 3. Alternating field demagnetization of TRM acquired in constant fields of (a) 0.04 mT (trianglest 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.
the self-reversing mechanism is still active, Coercivities of the reversed component approach 70 to 80 mT (Fig. 3b) whereas substantial parts of the normal component cannot be erased even at fields of 0.3 T (not shown in Fig. 3b). The higher stability of the self-reversing andesites is also demonstrated by AF demagnetization of isothermal r e m a n e n t magnetization (IRM) acquired in an applied field of 1 T. The coercivity spectra of the two rock types have been plotted in Figure 5. The dacite spectrum peaks at low fields and gradually drops toward high coercivities, as observed for many volcanic rocks. The low field peak of the andesite coer-
civity spectrum, however, is less pronounced. In addition, a second peak between 30 and 40 mT occurs, above which the spectrum slowly tails off, suggesting that the self-reversal mechanism might be connected to ~ highcoercivity mineral phase. Magnetic
mineralogy
High-field-magnetization measurements, microscopic examination and microchemica] analysis indicate the presence of at least two magnetic mineral phases in the andesitic pyroclasts. The dependence of high field magnetization on temperature (M~(T)) is
373
REMANENT MAGNETIZATION OF ANDESITE AND DACITIC PUMICE FROM NEVADO DEL RUIZ
c h a r a c t e r i z e d by t w o inflexions w h i c h reflect t h e C u r i e t e m p e r a t u r e s of f e r r o m a g n e t i c m i n e r a l phases• T h e Ms(T) c u r v e s a r e completely reversible between 600°C and room t e m p e r a t u r e w i t h o u t a n y sign of c h e m i c a l a l t e r a t i o n of t h e f e r r o m a g n e t i c m i n e r a l content. They are dominated by a mineral with a C u r i e t e m p e r a t u r e a r o u n d 3 8 0 ° C (Fig. 6). M i c r o p r o b e a n a l y s i s (Table 1) i n d i c a t e s t h e p r e s e n c e of a m e m b e r of t h e ulvSspinelm a g n e t i t e solid-solution series w i t h a n a v e r a g e c o m p o s i t i o n of Fe2.73Ti0.2704 . A s s u m i n g stoic h i o m e t r y a n d r e p l a c e m e n t of Fe by t h e measured impurities, a theoretical Curie t e m p e r a t u r e of 383 °C is c a l c u l a t e d a c c o r d i n g to R i c h a r d s et al. (1973). T h e r e f o r e , t h e abovem e a s u r e d C u r i e t e m p e r a t u r e is a t t r i b u t e d to a t i t a n o m a g n e t i t e p h a s e w h i c h also h a s b e e n
2s DACITE
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Coercivity (mT) 4
Fig. 5. Coercivity spectra of a dacite and an andesite sample derived from alternating field demagnetization of IRM (given at I-T field strength).
3
•
•
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0.6
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Fig. 4. Total T R M as function of applied field. Initially the reversely polarized TRM increases with increasing DC field, but changes to normal polarity at a critical field of 1.2 mT.
i
L
i
100
200
300
" \ T(oc) 400
Fig. 6. High field magnetization Ms(T) of an andesite fragment versus temperature. Two Curie temperatures at 190°C and 380°C are observed. The magnetization is dominated by the higher Curie point phase and completely reversible during thermal cycling. Applied field: 0.1 T; heating rate: 18°C min 1.
71.73
Sum (wt%)
0.25 394
8.9 51.0 40.1
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Sum (wt%)
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71.30
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Rhombohedral structure (impurities replace Fei
20.60 45.75 0.08 0.04 0.58 1.74
Ti (wt%) Fe (wt%) Mn (wt%) Cr (wt%) At (wt%) Mg (wt°~)
NI4
T i t a n o h e m a t i t e (1 y)Fe203 • yFeTiO 3
x Tc (°C)
TiO 2 (wt~) FeO (wt%) Fe203 (wtC~)
Spinel structure (impurities replace Fe)
5.58 61.67 0.17 0.21 1.83 2.27
Ti (wt%) Fe (wt%) Mn (wt%) Cr (wt%) A1 (wt%) Mg (wt%)
NI2
T i t a n o m a g n e t i t e (1-x)Fe304 • xFe2TiO 4
0.63 183
33,0 40,6 26.4
69.07
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NI6
0.26 390
8.9 31.9 59.2
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MA1
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31.2 43.2 25.6
69.44
19.19 47.93 0.12 0.17 0.37 1.66
MA2
0.27 381
9.4 39.5 51.1
70.82
5.89 60.63 0.22 0.24 1.78 2.06
HE2
0.62 190
32.2 29.2 38.7
68.51
19.61 47.47 0.09 0.09 0 1.25
HE 1
0.26 388
9.0 39.2 51.8
70.82
5.67 60.79 0.24 0.25 1.73 2.14
HE3
0.62 190
32.5 38.1 29.6
68.96
19.93 46.96 0.11 0.09 0.24 2.25
Average
0.27 383
9.3 41.9 48.7
70.95
5.86 60.64 0.21 0.22 1.77 1.63
Average
Composition and theoretically derived Curie t e m p e r a t u r e s ITc, after Richards et al., 1973) of magnetic ore grains fi'om andesitic fl'agments of the Nevado del Ruiz 1985 eruption
TABLE 1 c~
REMANENT MAGNETIZATION OF ANDESITE AND DACITIC PUMICE FROM NEVADO DEL RUIZ
identified under the ore microscope in grain sizes up to 150 ~m. The second ore mineral visible under the microscope belongs to the hematite-ilmenite series and locally contains exsolution lamellae structures (the composition of which could not be determined because of their small grain size (< 1 ~m)). The average composition (Fe1.38Ti0.6203) yields a theoretical Curie temperature of 190°C (Table 1). This compares well with the faint kink in the Ms(T) curve at about 190°C. Thus, both titanomagnetite and hemoilmenite contribute to the magnetic properties of the andesitic pumice. According to our magnetic and microchemical analyses the composition of the two mineral phases stays within narrow boundaries of the average (cf. Table 1). Similar microprobe results have been obtained by Fujiwara et al. (1987). D i s c u s s i o n of s e l f - r e v e r s a l m e c h a n i s m s
The Nevado del Ruiz 1985 andesitic pumice fragments prove for the first time without any doubt that self-reversal actually has controlled the NRM acquisition process (Figs. 1, 2). Selfreversing properties of NRM were discovered much earlier by Nagata et al. (1952), Ishikawa and Akimoto (1957), Uyeda (1958) and Ishikawa and Syono (1963) in dacitic pumice and pitchstone from Mount H a r u n a and Mount Asio (Japan). These authors used the interaction models of N~el (1951) and Meiklejohn and Bean (1957) to interpret this phenomenon. One-phase or two-phase models are at hand: N~el's N-type model with different temperature-dependent sublattice magnetization in one phase and different types of two-phase models with magnetostatic (i.e. dipole-dipole) or (super)exchange interaction. The energies involved in these interactions are usually assumed to be different by orders of magnitude. The N-type model requires an exchange-interaction field strength on the order of several Teslas. The one-phase model can be rejected as a possible source of the Nevado del Ruiz self-reversal
375
because: (1) the Ms(T) curve in Figure 6 decreases regularly between room temperature and 4O0°C with increasing temperature and vice versa during cooling: and (2) the applied fields of the order of 1 - 2 mT are capable to suppress the self-reversal completely (Fig. 3). It was observed during the heating and cooling experiments of Figure 2 that a critical temperature of ca. 175°C may not be exceeded in order to reproduce the NRM self-reversal in zero field. This implies that the titanomagnetite phase with its Curie temperature of approximately 380°C does not contribute to the self-reversal process. Instead, two phases of intermediate hemoilmenite with Curie temperatures near 200°C and possibly very similar composition may be interacting. The observed lamellae structures in some otherwise optically homogeneous hemoilmenites are potential candidates for two-phase interaction models. The earlier Japanese scientists already found evidence for an o r d e r disorder transition during rapid cooling of hemoilmenite which leads to a microstructure of two hemoilmenite phases with different titanium content and hence different Curie temperatures. Further evidence for cation-ordered grain domans with disordered boundaries has been given recently by Lawson et al. (1987). The self-reversing mechanism seems to be induced by cooling through the higher Curie temperature of a Ti-poor hemoilmenite phase with low spontaneous magnetization but high coercivity. This phase, which is magnetized parallel to the applied field, induces an antiparallel magnetic ordering because of negative interaction to a Ti-richer hemoilmenite with high spontaneous magnetization but low coercivity. This leads to a net negative magnetization, when the temperature drops below the lower Curie temperature. This antiparallel coupling must be due either to magnetostatic or exchange interaction between the Ti-poor, possibly canted antiferromagnetic hemoilmenite and the Ti-rich, possibly ferrimagnetic hemoilmenite. Magnetostatic in-
teraction works only under very restricted geometrical conditions (Uyeda, 1958) which are uncommon in rock-forming minerals. The weak critical field, however, needed to suppress the self-reversal mechanism (Fig. 4) apparently contradicts a simple exchange field model involving field strength on the order of several Tesla. A similar problem exists for synthetically grown antiferromagnetic-ferromagnetic FeMnFeNi films produced under ultra-high vacuum conditions, where no magnetostatic interaction is possible because of the purely antiferromagnetic FeMn phase (Tsang et al., 1981; Mauri et al., 1987a, b). In this system, the measured "exchange bias" field - which is identical to our "critical" field to suppress the self-reversal mechanism - is on the order of 1 - 2 0 mT (Meiklejohn and Bean, 1957), much less t h a n typical exchange fields. The apparent discrepancy can be solved by a domain-wall model (Westcott-Lewis and Parry, 1971; Mauri et al., 1987a, b). The strong exchange interaction between the two phases is not influenced by small external fields. Domain-wall nucleation and subsequent movement, however, is readily achieved by fields of 1 - 1 0 roT. The easy suppression of the self-reversal in the andesite may be explained by the creation of a domain in the ferrimagnetic hemoilmenite phase whose magnetization is antiparallel to
the external field before nucleation of a domain wall (Fig. 7, stage 2 to 3). With increasing ap~ plied field, the domain with parallel magnetization grows at the expense of the domain which is locked antiparallel by the exchange coupling across the boundary of the two phases. This finally leads to a change in polarity of the total magnetization and further increase of the netpositive magnetization with higher applied fields. Conclusion The andesitic pumice of the Nevado dei Ruiz 1985 eruption contains hemoilmenite whose NRM and low field TRM (acquired in fields of less t h a n 1 mT) are subject to self-reversal (Fig. 1, 2). In analogy to the magnetic behaviour of FeMn-FeNi thin films the self-reversal mechanism may be explained in terms of an exchange model of two, chemically different hemoilmenite phases with a domain-wall nucleation process superimposed. These phases carrying normal and reversed magnetization are characterized by high coercivities (Fig. 5). As the nucleated domain wall moves towards the phase boundal~y (Fig. 7~ stage 4), thereby acquiring higher magnetic stability (Westcott-Lewis and Parry, 1971), the coercivity spectrum of the normal TRM component shifts to higher values with increasing ap-
®
@
t Hext
®
i
Hext
(
®
1
Hext
Hext
Fig. 7. Model of the suggested self-reversal mechanism as function of increasing (stages 1 to 4) applied field (Hext). Negative exchange across the boundary (starred line) of two phases causes the net magnetic moment to be antiparallel to the applied field as long as this is small (stages I to 2). With increasing external field, domainwall (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 magnetization completely changed from reversed to normal.
REMANENT MAGNETIZATION OF ANDESITE AND DACITIC PUMICE FROM NEVADO DEL RUIZ
plied field (Fig. 3b). At high e n o u g h e x t e r n a l fields the n o r m a l l y m a g n e t i z e d d o m a i n s will o v e r w h e l m those r e v e r s e l y m a g n e t i z e d so t h a t t h e self-reversal effect is more and more suppressed.
References Fujiwara, Y., Gautam, P., Yoshida, M. and Katsui, Y., 1987. On the nature of remanence in the andesite pumice with self-reversed magnetization from the Nevado del Ruiz, Colombia. Rock Magnet. Paleogeophys., 14: 7-12. Heiniger, C. and Heller, F., 1976. A high temperature vector magnetometer. Geophys. J. R. Astron. Soc., 44: 281-288. Heller, F., Carracedo, J.C. and Soler, V., 1986. Reversed magnetization in pyroclastics from the 1985 eruption of Nevado del Ruiz, Colombia. Nature, 324: 241-242. Ishikawa, Y. and Akimoto, S., 1957. Magnetic properties of the FeTiO3-Fe203 solid solution series. J. Phys. Soc. Jpn, 12: 1083-1098. Ishikawa, Y. and Syono, Y., 1963. Order-disorder transformation and reverse thermo-remanent magnetism in the FeTiO3-Fe203 system. J. Phys. Chem. Solids, 24: 517-528. Lawson, C.A., Nord, G.L. and Champion, D.E., 1987. Fe-Ti oxide mineralogy and the origin of normal and reverse remanent magnetization in dacitic pumice blocks from Mt. Shasta, California. Phys. Earth Planet. Inter., 46: 270-288. Mauri, D., Kay., E. Scholl, D. and Howard, J.K., 1987a. Novel method for determining the
377
anisotropy constant of MnFe in a NiFe/MnFe sandwich. J. Appl. Phys., 62: 2929-2932. Mauri, D., Siegmann, H.C., Bagus, P.S. and Kay, E., 1987b. Simple model for thin ferromagnetic films and exchange coupled to an antiferromagnetic substrate. J. Appl. Phys., 62: 3047-3049. Meiklejohn, W.H. and Bean, C.P., 1957. New magnetic anisotropy. Phys. Rev., 105: 904-913. Nagata, T., Uyeda, S. and Akimoto, S., 1952. Selfreversal of thermoremanent magnetism of igneous rocks. J. Geomagn. Geoelectr., 4: 22-38. N~el, L., 1951. L'inversion de l'aimantation permanente des roches. Ann. G~ophys. 7: 90-102. Richards, J.C.W., O'Donovan, J.B., Hauptmam Z., O'Reilly, W. and Creer, K.M., 1973. A magnetic study of titanomagnetite substituted by magnesium and aluminium. Phys. Earth Planet. Inter., 7: 437-444. Thouret, J.C., Vatin-P~rignon, N., Cantagrel, J.M., Salinas, R. and Murcia, A., 1985. Le Nevado E1 Ruiz (Cordill~re Centrale des Andes de Colombie): stratigraphie, structures et dynamisme d'un appareil volcanique and~sitique, compos~ et polyg~nique. Rev. Geol. Dyn. Geogr. Phys., 26: 257 -271. Tsang, C., Heiman, N. and Lee, K., 1981. Exchange induced unidirectional anisotropy at FeMnNisoFe2o interfaces. J. Appl. Phys., 52: 2471-2473. Uyeda, S., 1958. Thermoremanent magnetism as a medium of palaeomagnetism, with special reference to reverse thermo-remanent magnetism. Japan. J. Geophys., 2: 1-123. Westcott-Lewis, M.F. and Parry, L.G., 1971. Thermoremanence in synthetic rhombohedral irontitanium oxides. Aust. J. Phys., 24: 735-742.