Crustal residence times of explosive phonolite magmas: U–Th ages of magmatic Ca-Garnets of Mt. Somma-Vesuvius (Italy)

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.

Alam, M.M., Kimura, M., 2004. Statistical analysis of time-distance relationship
between volcanic eruptions and great earthquakes in Japan. Earth Planets Space
56, 179–192.
Andronico, D., Cioni, R., Marianelli, P., Santacroce, R., Sbrana, A., Sulpizio, R., 1996.
Introduction to Somma-Vesuvius. Vesuvius Decade Volcano, Workshop Handbook.
Consiglio Nazionale delle Riccerche.
Auger, E., Gasparini, P., Virieux, J., Zollo, A., 2001. Seismic evidence of an extended
magmatic sill under Mt. Vesuvius. Science 294, 1510–1512.
Aulinas, M., Civetta, L., Di Vito, M., Orsi, G., Gimeno, D., Fernandez-Turiel, J.L., 2008. The
“Pomici di mercato” Plinian eruption of Somma-Vesuvius: magma chamber
processes and eruption dynamics. Bull. Volcanol. 70, 825–840.
Avanzinelli, R., Elliott, T., Tommasini, S., Conticelli, S., 2008. Constraints on the genesis of
potassium-rich Italian volcanic rocks from U/Th disequilibrium. J. Petrol. 49,195–223.
Ayuso, A.A., De Vivo, B., Rolandi, G., Seal II, R., Paone, A., 1998. Geochemical and isotope
(Nd-Pb-Sr-O) variations bearing on the genesis of volcanic rocks from Vesuvius,
Italy. J. Volcanol. Geotherm. Res. 82, 53–78.
Bourdon, B., Zindler, A.,Wörner, G., 1994. Evolution of the Laacher–See magma chamber—
evidence from SIMS and TIMS measurements of U–Th disequilibria in minerals and
glasses. Earth Planet. Sci. Lett. 126, 75–90.
Bourdon, B., Turner, S.P., Henderson, G.M., Lundstrom, C.C., 2003. Introduction to U-series
Geochemistry, in: Bourdon, B., Henderson, G.M., Lundstrom, C.C., Turner, S.P., (Eds),
Reviews in Mineralogy and Geochemistry Volume 52: Uranium-series geochemistry.
Geochemical Society and Mineralogical Society of America,Washington.
Caldwell, D.A., 1989. Physical and geochemical properties of summit flows and recent
volcanic ejecta from Mt. Erebus, Ross Island, Antartica. M.S. thesis, NewMexico Inst.
of Mining and Tech.
Carslaw, H.S., Jaeger, J.C., 1959. Conduction of Heat in Solids, first ed. Oxford Univ. Press,
London.
Chadwick, J.P., Troll, V.R., Ginibre, C., Morgan, D., Gertisser, R.,Waight, T.E., Davidson, J.P.,
2007. Carbonate assimilation at Merapi volcano, Java, Indonesia: insights from
crystal isotope stratigraphy. J. Petrol. 48, 1793–1812.
Charlier, B., Zellmer, G., 2000. Some remarks on U–Th mineral ages from igneous rocks
with prolonged crystallisation histories. Earth Planet. Sci. Lett. 183, 457–469.
Cheng, H., Edwards, R.L., Hoff, J., Gallup, C.D., Richards, D.A., Asmerom, Y., 2000. The
half-lives of uranium-234 and thorium-230. Chem. Geol. 169, 17–33.
Cioni, R., Civetta, L., Marianelli, P., Metrich, N., Santacroce, R., Sbrana, A., 1995.
Compositional layering and syn-eruptive mixing on a periodically refilled shallow
magma chamber: the AD 79 Plinian eruption of Vesuvius. J. Petrol. 36, 739–776.
Cioni, R., Marianelli, P., Santacroce, R., 1998. Thermal and compositional evolution of the
shallow magma chambers of Vesuvius: evidence from pyroxene phenocrysts and
melt inclusions. J. Geophys. Res. — Solid Earth 103, 18277–18297.
Cioni, R., Santacroce, R., Sbrana, A., 1999. Pyroclastic deposits as a guide for reconstructing
the multi-stage evolution of the Somma-Vesuvius Caldera. Bull. Volcanol.
60, 207–222.
Civetta, L., Orsi, G., Scandone, P., Pece, R.,1978. Eastwards migration of the Tuscan anatectic
magmatism due to anticlockwise rotation on the Apennines. Nature 279, 604–605.
Civetta, L., Galati, R., Santacroce, R., 1991. Magma mixing and convective compositional
layering within the Vesuvius magma chamber. Bull. Volcanol. 53, 287–300.
Civetta, L., D'Antonio, M., De Lorenzo, S., Di Rienzo, V., Gasparini, P., 2004. Thermal and
geochemical constrains on the “deep”magmatic structure of Mt. Vesuvius. J. Volcanol.
Geotherm. Res. 133, 1–12.
de Lorenzo, S., Di Renzo, V., Civetta, L., D'Antonio, M., Gasparini, P., 2006. Thermal model
of the Vesuvius magma chamber. Geophys. Res. Lett. 33, L17302.
De Natale, G., Troise, C., Pinque, F., De Gori, P., Chiarabba, C., 2001. Structure and
dynamics of the Somma-Vesuvius volcanic complex. Mineral. Petrol. 73, 5–22.
Di Renzo, V., Di Vito, M.A., Arienzo, I., Carandente, A., Civetta, L., D'Antonio, M., Giordano,
F., Orsi, G., Tonarini, S., 2007. Magmatic history of Somma-Vesuvius on the basis of
new geochemical and isotopic data from a deep borehole (Camaldoli della Torre).
J. Petrol. 48, 753–784.
Elter, P., Grasso, M., Parotto, M., Mezzani, L., 2003. Structural setting of the Apennine–
Maghrebian thrust belt. Episodes 26, 205–211.
Giordano, D., Romano, C., Papale, P., Dingwell, D.B., 2004. The viscosity of trachytes, and
comparision with basalts, phonolites, and rhyolites. Chem. Geol. 213, 49–61.
Goldstein, S.L., Deines, P., Oelkers, E.H., Rudnick, R.L., Walter, L.M., 2003. Standards for
publication of isotope ratio and chemical data in Chemical Geology. Chem. Geol. 202,
1–4.
Hawkesworth, C.J., Blake, S., Evans, P., Hughes, R., MacDonald, R., Thomas, L.E., Turner, S.P.,
2000. Time scales of crystal fractionation in magma chambers—integrating physical,
isotopic and geochemical perspectives. J. Petrol. 41, 991–1006.
Hawkesworth, C., George, R., Turner, S., Zellmer, G., 2004. Time scales of magmatic
processes. Earth Planet. Sci. Lett. 218, 1–16.
Heath, E., Turner, S.P., Macdonald, R., Hawkesworth, C.J., van Calsteren, P., 1998. Long
magma residence times at an island arc volcano (Soufriere, St. Vincent) in the Lesser
Antilles: evidence from 238U–230Th isochron dating. Earth Planet. Sci. Lett. 160,
49–63.
Heumann, A., 1999, Timescales of processes within silicic magma chambers. Phd theses,
Vrije Universiteit te Amsterdam.
Heumann, A., Davies, G.R., 2002. U–Th disequilibrium and Rb–Sr age contraints on the
magmatic evolution of peralkaline rhyolites from Kenya. J. Petrol. 43, 557–577.
Hill, D.O., Pollitz, F., Newhall, C., 2002. Earthquake-volcano interactions. Phys. Today 55,
41–47.
Hort, M., 1997. Cooling and crystallization in sheet-like magma bodies revisited.
J. Volcanol. Geotherm. Res. 76, 297–317.
Johansen, T.S., Hauff, F., Hoernle, K., Klügel, A., Kokfelt, T.E., 2005. Basanite to phonolite
differentiation within 1550–1750 years: U–Th–Ra isotopic evidence from the A.D.
1985 eruption on La Palma, Canary Islands. Geology 33, 897–900.
Lavecchia, G., Stoppa, F., 1996. The tectonic significance of Italian magmatism: an
alternative view to the popular interpretation. Terra Nova 8, 435–446.
Lima, A., Danyueshevsky, L.V., De Vivo, B., Fedele, L., 2003. A model for the evolution of
the Mt. Somma-Vesuvius magmatic system based on fluid and melt inclusions
investigations. In: De Vivo, B., Bodran, R.J. (Eds.), Melts inclusions in volcanic
systems: Methods, Applications and Problems. Series development in Volcanology,
pp. 227–249.
Linde, A.T., Sacks, I.S., 1998. Triggering of volcanic eruptions. Nature 395, 888–890.
Ludwig, K.R., 1999. Using Isoplot/Ex, Version 2.01, A Geochronological Toolkit for
Microsoft Excel. Berkeley Geochronology Center Spec. Pub. 1a.
Lundstrom, C.C., Hoernle, K., Gill, J., 2003. U-series disequilibria in volcanic rocks from
the Canary Islands: plume versus lithospheric melting. Geochim. Cosmochim. Acta
67, 4143–4177.
Marianelli, P., Metrich, N., Sbrana, A., 1999. Shallow and deep reservoirs in magma
supply of the 1944 eruption of Vesuvius. Bull. Volcanol. 61, 48–63.
Marsh, B.D., 2000. Magma chambers. In: Sigurdsson, H. (Ed.), Encyclopedia of Volcanoes.
Academic Press, New York, pp. 191–206.
Nostro, C., Stein, R.S., Cocco, M., Belardinelli, M.E., Marzocchi, W., 1998. Two-way
coupling between Vesuvius eruptions and southern Apennine earthquakes (Italy)
by elastic stress transfer. J. Geophys. Res. 103, 24487–24504.
Paone, A., 2004. Evidence of crustal contamination, sediment, and fluid components in
the campanian volcanic rocks. J. Volcanol. Geotherm. Res. 138, 1–26.
Peccerillo, A., 2005. Plio-Quaternary volcanism in Italy. Petroloy, Geochemistry,
Geodynamics, first ed. Springer, Heidelberg.
Pietruszka, A.J., Carlson, R.W., Hauri, E.H., 2002. Precise and accurate measurement of
226Ra–230Th–238U-disquilibria in volcanic rocks using plasma ionization multicollector
mass spectrometry. Chem. Geol. 188, 171–191.
Piochi, M., Ayuso, R.A., De Vivo, B., Somma, R., 2006. Crustal contamination and crystal
entrapment during polybaric magma evolution at Mt. Somma-Vesuvius volcano,
Italy: geochemical and Sr isotope evidence. Lithos 86, 303–329.
Rampino, M.R., Stothers, R.B., Self, S., 1995. Climatic effects of volcanic eruptions. Nature
313, 272.
Reagan,M.K., Volpe, A.M., Cashman, K.V.,1992. U-238 series and Th-232 series chronology
of phonolite fractionation atMount Erebus, Antarctica. Geochim. Cosmochim. Acta 56,
1401–1407.
Reid, M., 2003. Timescales of magma transfer and storage in the crust. In: Holland, H.D.,
Turekian, K.K (Eds.), Treatise on Geochemistry, Volume 3: The Crust. Elsevier, The
Netherlands, pp. 167–193.
Rolandi, G., Maraffi, S., Petrosino, P., Lirer, L., 1993. The Ottaviano eruption of Somma-
Vesuvio (8000 y B.P.): a magmatic alternating fall and flow-forming eruption.
J. Volcanol. Geotherm. Res. 58, 43–65.
Santacroce, R., 1983. A general model for the behaviour of the Somma Vesuvius volcanic
complex. J. Volcanol. Geotherm. Res. 17, 237–248.
Santacroce, R., Cioni, R., Marianelli, P., Sbrana, A., Sulpizio, R., Zanchetta, G., Donahue, D.J.,
Joron, J.L., 2008. Age andwhole rock-glass compositions of proximal pyroclastics from
the major explosive eruptions of Somma-Vesuvius: a review as a tool for distal
tephrostratigraphy. J. Volcanol. Goetherm. Res. 177, 1–18.
Scaillet, B., Pichavant, M., 2004. Crystallisation conditions of Vesuvius phonolites.
Geophys. Res. Abstr. 6, 03764.
Scaillet, B., Pichavant, M., Cioni, R., 2008. Upward migration of Vesuvius magma
chamber over the past 20,000 years. Nature 455, 216–219.
Scheibner, B., Wörner, G., Civetta, L., Simon, K., Kronz, A., 2007. Rare earth element
fractionation in magmatic Ca-rich garnets. Contrib. Mineral. Petrol. 154, 55–74.
Schmincke, H.-U., Park, C., Harms, E., 1999. Evolution and environmental impacts of the
eruption of Laacher See Volcano (Germany) 12,900 a BP. Quat. Int. 61, 61–72.
Schmitt, A.K., 2006. Laacher See revisited; high-spatial-resolution zircon dating
indicates rapid formation of a zoned magma chamber. Geology 34, 597–600.
Somma, R., Ayuso, R.A., De Vivo, B., Rolandi, G., 2001. Major, trace element and isotope
geochemistry (Sr–Nd–Pb) of interplinian magmas from Mt. Somma-Vesuvius
(Southern Italy). Mineral. Petrol. 73, 121–143.
Tommasini, S., Heumann, A., Avanzinelli, R., Francalanci, L., 2007. The fate of high-angle
dipping slabs in the subduction factory: an integrated trace element and radiogenic
isotope (U, Th, Sr, Nd. Pb) study of Stromboli Volcano, Aeolian Arc, Italy. J. Petrol. 48,
2407–2430.
Turner, S., Costa, F., 2007. Measuring timescales of magmatic evolution. Elements 3,
267–272.
Turner, S., George, R., Jerram, D.A., Carpenter, N., Hawkesworth, C., 2003. Case studies of
plagioclase growth and residence times in island arc lavas from Tonga and the
Lesser Antilles, and a model to reconcile discordant age information. Earth Planet.
Sci. Lett. 214, 279–294.
VanWestrenen,W., Blundy, J.,Wood, B., 1999. Crystal-chemical controls on trace element
partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847.
vanWestrenen,W., Blundy, J.D.,Wood, B.J., 2001. High field strength element/rare earth
element fractionation during partial melting in the presence of garnet: implications
for identification of mantle heterogeneities. Geochem. Geophys. Geosyst. 2 (7),
1039.
Walter, T.R., 2007. How a tectonic earthquake may wake up volcanoes: stress transfer
during the 1996 earthquake-eruption sequence at the Karymsky Volcanic Group,
Kamchatka. Earth Planet. Sci. Lett. 264, 347–359.
Wohletz, K., Civetta, L., Orsi, G., 1999. Thermal evolution of the Phlegraean magmatic
system. J. Volcanol. Geotherm. Res. 91, 381–414.
Zollo, A., et al., 1996. Seismic evidence for a low-velocity zone in the upper crust
beneath Mount Vesuvius. Science 274, 592–594.