Publication Date:
2021-06-16
Description:
Assessing the residence times of phonolite magmas in the shallow crust contributes to the understanding of
explosive volcanic systems. Estimations of that by dating the residence time of a mineral in a melt was
difficult in the past, because e.g. of the lack of evidence for the co-genetic character of the crystals dated. Here
we present an estimate for the residence time of a phonolite magma feeding the Pomici di Mercato Plinian
eruption (8890±90 cal years BP) of Mt. Somma-Vesuvius (Southern Italy), employing U–Th disequilibrium
dating of unzoned Ca-rich phenocrystic magmatic garnets. Based on combined textural, geochemical, and Sr-
O isotope evidence, these garnets can be identified as co-genetic with their host phonolites. Furthermore,
experimental and petrological data suggest that Ca-garnets can be a liquidus phase in highly differentiated
phonolite magmas of Mercato. A whole-rock–glass–garnet U–Th isochron gives a crystallisation age for the
Ca-rich garnets of 14,400±1100 a (2σ). This implies a Ca-garnet residence time of 5510±1100 years (2σ) in
the Mercato phonolite melt prior to eruption and provides one of the first robust estimates of how long
explosive phonolite magma has resided in the shallow crust before eruption. Calculations of magma cooling
rates and settling velocities of the Ca-garnets confirm that garnet-bearing phonolite can remain liquid and
the garnets remain suspended in a magma chamber for as long as 5510 years before the time of eruption.
Processes which may have disturbed the U–Th isotope systematic of the samples, such as assimilation,
recharge or surface alteration can be ruled out.
Description:
Published
Description:
293-301
Description:
2.3. TTC - Laboratori di chimica e fisica delle rocce
Description:
3.5. Geologia e storia dei sistemi vulcanici
Description:
3.6. Fisica del vulcanismo
Description:
JCR Journal
Description:
reserved
Keywords:
Uranium
;
Thorium
;
U–Th isotopes
;
Somma-Vesuvius
;
04. Solid Earth::04.08. Volcanology::04.08.99. General or miscellaneous
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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