Publication Date:
2019-10-18
Description:
to evaluate how the exposure to thermal and redox conditions close to that of active craters affect the texture
and composition of juvenile pyroclasts. Selected samples were placed within a quartz tube, in presence of air
or under vacuum, and kept at T between 700 and 1,130 °C, for variable time (40 min to 12 h). Results show
that reheating reactivates the melt, which, through processes of chemical and thermal diffusion, reaches new
equilibrium conditions. In all the experiments performed at T = 700–750 °C, a large number of crystal nuclei
and spherulites grows in the groundmass, suggesting conditions of high undercooling. This process creates
textural heterogeneities at the scale of few microns and only limited changes of groundmass composition,
which remains clustered around that of the natural glasses. Reheating at T = 1,000–1,050 °C promotes massive
groundmass crystallization, with a different mineral assemblage as a function of the redox conditions.
Morphological modifications of clasts, from softening to sintering as temperature increases, occur under these
conditions, accompanied by progressive smoothing of external surfaces, and a reduction in size and abundance
of vesicles, until the complete obliteration of the pre-existing vesicularity. The transition from sintering to
welding, characteristic of high temperature, is influenced by redox conditions. Experiments at T = 1,100–
1,130 °C and under vacuum produce groundmass textures and glass compositions similar to that of the
respective starting material. Collapse and welding of the clasts cause significant densification of the whole
charge. At the same temperature, but in presence of air, experimental products at least result sintered and
show holocrystalline groundmass. In all experiments, sublimates grow on the external surfaces of the clasts
or form a lining on the bubble walls. Their shape and composition is a function of temperature and f O2 and
the abundance of sublimates shows a peak at 1,000 °C. The identification of the features recorded by pyroclasts
during complex heating–cooling cycles allows reconstructing the complete clasts history before their final
emplacement, during weakly explosive volcanic activity. This has a strong implication on the characterization
of primary juvenile material and on the interpretation of eruption dynamics.
Description:
Published
Description:
3V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici
Description:
JCR Journal
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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