HR: 1340h
AN: V13B-1486    [Abstracts]
TI: Determining the Source for the Magmas of Monte Amiata (Central Italy) using Strontium, Neodymium, and Lead Isotope Analysis
AU: * James, K E
EM: kristen.james@gmail.com
AF: Carleton College, 300 N College St, Northfield, MN 55057 United States
AB: The volcanoes of the Tuscan Magmatic Province (TMP) and the Roman Magmatic Province (RMP), situated on the western flank of Italy, record an extraordinarily complex subduction history. Although clearly subduction related, the volcanoes of the more northern TMP exhibit an abnormal degree of crustal influence. New isotope and trace element data may shed light on magma genesis of lavas at Monte Amiata, situated on the border between the TMP and RMP. The Monte Amiata lavas range in major element composition from trachydacites to olivine latites and form tight isotopic groupings. $^{87}$Sr/$^{86}$Sr ratios range from 0.712447 to 0.713068 and $^{143}$Nd/$^{144}$Nd ratios from 0.512094 to 0.512203. $^{207}$Pb/$^{204}$Pb ratios range from 15.6694 to 16.2330 and $^{206}$Pb/$^{204}$Pb ratios from 18.7125 to 19.0882. The isotopic data are remarkably homogeneous, revealing neither geographic nor temporal trends. The unusual isotope and trace element compositions of the Monte Amiata volcanics may have been produced by one of several different processes: 1) crustal contamination of the magma during ascension and emplacement, 2) magma mixing between the more silicic TMP magmas and a high-K series of the RMP, and 3) crustal contamination of the upper asthenosphere by marine sediments being carried down with the subducting slab under central Italy. The data appear to be closest in isotopic affinity to marine sediments from adjacent off-shore areas. The tightly constrained compositional range for both isotopic ratios and trace elements, including LREE would suggest a single magma source for these volcanics. We tentatively conclude that although some upper crustal contamination and magma mixing may be involved in the evolution of the Monte Amiata lavas, the high degree of isotopic similarity between samples of different ages and different bulk compositions suggests magma derivation from a metasomatized suprasubduction zone where the upper mantle has been altered by infiltration of fluids and partial melts from subducting marine sediments.
DE: 8499 General or miscellaneous
DE: 3670 Minor and trace element composition
DE: 3022 Marine sediments--processes and transport
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting