Paleomagnetic study of juvenile clasts of basaltic andesite pyroclastic flows
Abstract
To improve geomagnetic models of secular variation during the Holocene, it is necessary to bring new data, especially from the southern hemisphere. In most of the Andean volcanoes from Ecuador to the Chilean central volcanic zone, very well dated lava flows are not numerous. In contrast, pyroclastic flows (PF) often contain charcoal facilitating their age determination. Here we report paleomagnetic results from three pyroclastic flows. Instead of trying to use lithic clasts, we sampled only the juvenile clasts with size varying from about 10 cm to 1 meter. The 2006 PF from the Tungurahua volcano (Ecuador) was sampled to test the reliability of the paleomagnetic results. We also sampled in Chile the Pucon ignimbrite (PF41, 3,661±91 years 14C BP) from the Villarica volcano (Silva et al., 2010, Bull. Volc.) and one PF of the Osorno volcano (PF86) with a C14 age of 1250±40 BP. Samples of the PFs have strong natural remanent magnetization (NRM) and relatively low magnetic susceptibility χ (PF2006 : NRM 14.9 Am-1, χ: 0.015; PF41: NRM 19 Am-1 , χ: 0.003 SI; PF86: 9.5 Am-1 , χ: 0.004 SI). Hysteresis parameters (Jrs/Js ratio from 0.32 up to 0.58 and Hcr values up to 140mT), FORC data, and thermomagnetic experiments indicate that the magnetic carriers are mainly single domain titanomagnetites. Curie points and unblocking temperatures varies from one PF to the other with either a single magnetic phase with sharp unblocking temperatures around 300°C for PF2006 to two magnetic phases of titanomagnetite with unblocking temperatures around 250°C and the other close to magnetite for the PF of the Osorno volcano. The characteristic direction recorded by PF2006 (Declination: 3.3°, Inclination, 20.6°, α95:4.1°) is in good agreement with the expected direction from the IGRF model (Declination: -1.5°, Inclination, 20.3°). The dispersion in paleomagnetic directions between the clasts is also not significant for the Pucon ignimbrite, (D=358.7, I= -56.5, α95:1.3, k=626) and for the Osorno PF (D=3.5, I= -70.8, α95: 2.1,k=249). These results confirm that there is no relative displacement within the PF while cooling below the Curie temperatures. The juvenile clasts with strong NRM carried by single domain titanomagnetites are also very stable during paleointensity determination with the Thellier method providing a higher success rate (>75%) than paleointensity determination with samples from massive lava flows. For the PF2006 a paleofield strenght of 33.2±2.1μT was obtained, close to the expected one of 29.4 μT With improving models of geomagnetic secular variation, the paleomagnetic study of the Pyroclastic flows combined with the classic paleomagnetic study of lava flows is going to be a very good complementary method to further constrain volcanic stratigraphy and eruption rates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP23A1019C
- Keywords:
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- 1527 GEOMAGNETISM AND PALEOMAGNETISM / Paleomagnetism applied to geologic processes;
- 1540 GEOMAGNETISM AND PALEOMAGNETISM / Rock and mineral magnetism;
- 1560 GEOMAGNETISM AND PALEOMAGNETISM / Time variations: secular and longer