A detailed magnetic and mineralogical study of self-reversed dacitic pumices from the 1991 Pinatubo eruption (Philippines)

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

Magnetic properties of hemoilmenite single crystals in Haruna dacite pumice revealed by the Bitter technique, with special reference to self-reversal of thermoremanent magnetization

“…In order to check their results, we have studied our Pinatubo samples with polished sections using microprobe analysis and the Bitter technique. Our results are consistent with those by Hoffmann and Fehr (1996) and Bina et al (1999) that hemoilmenite has a shell structure of two phases with different compositions and magnetism. Figure 8 shows two examples of hemoilmenite crystals in our Pinatubo samples in which the shell structure is commonly observed.…”

“…Samples with compositions in the range 0.50 < X ilm < 0.75 are ferrimagnetic at room temperature, with the net magnetic moments aligned parallel to the c-plane ((0001) plane). Hoffmann and Fehr's (1996) and Bina et al's (1999) descriptions on the spin-alignment of this series contradicts the above-mentioned Brown et al's (1993) result. In our case of Haruna samples, the results obtained from the Bitter pattern of hemoilmenite crystals support the arguments by Brown et al (1993) and Goguitchaichvili and Prévot (2000).…”

“…6(f)) showed small patches of the ferrimagnetic parallel wall region at the rim. In all these cases, neither zoning-like shell structures of two phases nor WF structures (Bina et al, 1999) were observed.…”

“…According to Hoffmann and Fehr (1996) and Bina et al (1999), hemoilmenite crystals of dacite pumice of the 1991 eruption of Mt. Pinatubo, which are responsible for SRTRM, have a shell structure of two phases with different compositions, one is ordered ferrimagnetic and the other is disordered weak ferromagnetic (associated with anti-ferromagnetism).…”

“…Figure 7 shows the Bitter pattern of a section of these crystals. In the three tilted crystals, the contour line-like WF structure (Bina et al, 1999) is mixed with the irregular stain-like or streak-like pattern. On the other hand, in the three correctlyoriented crystals, the typical contour line-like WF structure was spread.…”