Publication Date:
2022-05-04
Description:
This study reports the first crystal chemical database resulting from a detailed structural investigation of
trioctahedral micas found in xenolithic ejecta produced during the AD 1631, 1872 and 1944 eruptions,
three explosive episodes of recent volcanic period of Vesuvius volcano (Southern Italy). Three xenolith
types were selected: metamorphic/metasomatic skarns, pyrometamorphic/hydrothermally altered nodules
and mafic cumulates. They are related to different magma chemistry and effusive styles: from sub-plinian and
most evolved (AD 1631 eruption) to violent strombolian with medium evolution degree (AD 1872 eruption)
to vulcanian-effusive, least evolved (AD 1944 eruption) event, respectively. Both xenoliths and micas were
investigated employing multiple techniques: the xenoliths were characterized by X-ray fluorescence,
inductively-coupled plasma-mass spectrometry, optical microscopy, X-ray powder diffraction, and quantitative
energy-dispersive microanalysis; the micas were studied by electron probe microanalysis and single crystal
X-ray diffraction. The mica-bearing xenoliths showvariable texture and mineralogical assemblage, clearly related
to their different origin. Based on the major oxide chemistry, only one xenolithic sample falls in the skarn compositional
field fromthe Somma-Vesuvius literature, some fall close to the skarns and cumulate fields, others plot
close to the syenite/foidolite/essexite field. A subgroup of the selected ejecta does not fall or approach any of the
compositional fields. Trace and rare earth element patterns show some petrological affinity between studied
xenoliths and erupted magmas with typical Eu, Ta and Nb negative anomalies. Strongly depleted patterns
were detected for the 1631 metamorphic/metasomatic skarns xenoliths. Three distinct mica groups
were distinguished: 1) Mg-, Al-rich, low Ti-bearing, low to moderate F-bearing varieties (1631 xenolith),
2) Al-moderate, F- and Mg-rich, Ti-, Fe-poor varieties (1872 xenolith), and 3) Al-, Ti- and Fe-rich, F-poor phases
(1944 xenolith). All the analyzed mica crystals are 1Mpolytypes with the expected space group C2/m. Micas from
xenoliths of the 1631 Vesuvius eruption are phlogopites characterized by a combination of low extent of
oxy-type and variable extent OH−→F− substitutions, as testified by the range of F concentration (from ~0.20
to 0.80 apfu). Micas from xenoliths of the 1872 Vesuvius eruption exhibit structural peculiarities typical of
fluorophlogopites, i.e. OH−→F− substitution is predominant. Micas from the xenolith of the 1944 Vesuvius
eruption display features typical of oxy-substituted micas. The variability of the crystal chemical features of the
studied micas is consistentwith the remarkable variation of their host rocks. Micas from1631 nodules are related
to metasomatic, skarn-type environment, deriving from the metamorphosed wall-rocks hosting the magma
reservoir. The fluorophlogopites from the 1872 xenoliths testify for strongly dehydrated environmental conditions
compared to those of the 1631 and 1944 hosts. Finally, magma storage condition at depth, associated to a
decreasing aH2O may have promoted major oxy-type substitutions in 1944 biotites.
Description:
Published
Description:
84–97
Description:
3.5. Geologia e storia dei vulcani ed evoluzione dei magmi
Description:
5.3. TTC - Banche dati vulcanologiche
Description:
JCR Journal
Description:
restricted
Keywords:
phlogopite
;
Crystal chemistry
;
Vesuvius
;
petrogenesis
;
04. Solid Earth::04.01. Earth Interior::04.01.04. Mineral physics and properties of rocks
;
04. Solid Earth::04.04. Geology::04.04.05. Mineralogy and petrology
;
04. Solid Earth::04.04. Geology::04.04.07. Rock geochemistry
;
04. Solid Earth::04.08. Volcanology::04.08.05. Volcanic rocks
;
05. General::05.02. Data dissemination::05.02.01. Geochemical data
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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