Rock magnetic properties of recent and historical Etna ashes: relationships with the explosive activity
Abstract
A detailed rock-magnetic study was carried out for characterizing different explosive events occurring at Etna volcano during the last 18 ka, with the aim to test the results as proxies for discriminating different eruptive cycles. Pyroclastic deposits, representing the plinian Biancavilla Ignimbrite (Y1 tephra collected in a core, ET97-70, from the Jonian Sea; age 16-18 ka) and the sub-plinian basaltic eruptions of late Holocene age (TV, FS, FL, FG layers + other proximal deposits), were combined with ashes representing recent explosive activity, including products ejected during the year 2002. To better discriminate the magnetic properties of the explosive disintegration of newly erupted magma, the two end members of the juvenile ash, tachylite fragments (microlite-rich) and sideromelane (glassy), were separated from recent ashes (2002 eruption) and from the old FG tephra (122 BC Plinian eruption).
The samples are characterized by coercivity of the remanence (Bcr) to coercivity (Bc) ratios ranging from 2 to 6, defining three main groups according to the position in their Day-Dunlop plot. These groups represent variable magnetic grain sizes corresponding to the old, historical and recent activity. The young ashes (including the tachylites) show wasp-waisted hysteresis loops that can depend on two magnetic components with contrasting coercivities or grain sizes. The latter is supported by the shapes of their FORC diagrams, indicating the prevalence of a mixture of MD+SP grains. The presence of a harder magnetic fraction is suggested by the Bc distribution spreading up to 0.2T, even if Mossbauer data do not show the presence of antiferromagnetic minerals. Among the old tephras, only the samples from the plinian FG tephra exhibit similar waisp-waisted loops. Thermomagnetic curves indicate dominating Ti-rich titanomagnetites, with Curie temperatures <300°C in most of the studied samples. Magnetite or Ti-poor titanomagnetite (Tc ~ 540-570°C) was well identified only in a few historical tephras (TV, FS, FL), suggesting minor or missing Ti-substitution for this class of samples. In low temperature experiments, the Verwey transition was not observed, confirming that titanomagnetite is the main magnetic carrier.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP42A..04V
- Keywords:
-
- 1599 General or miscellaneous;
- GEOMAGNETISM AND PALEOMAGNETISM