HR: 0800h
AN: V41D-0821    [Abstracts]
TI: Multiple Sources of K-rich Melts in Central Italy: Evidence From Trace-element and Pb- isotopic Signatures of Melt Inclusions in a Single Lava Flow From Latera Volcano
AU: Nikogosian, I K
EM: iniki@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, Budapestlaan 4, Utrecht, 3584 CD, Netherlands
AU: * van Bergen, M J
EM: vbergen@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, Budapestlaan 4, Utrecht, 3584 CD, Netherlands
AU: de Hoog, C
EM: cees-jan@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Whitehouse, M J
EM: martin.whitehouse@nrm.se
AF: Lab. for Isotope Geology, Swedish Museum of Nat. History, Frescativaegen 4, Stockholm, SE-104 05, Sweden
AB: Latera stratovolcano is part of Vulsini, the northernmost volcanic complex of the Roman Province, Central Italy. Lava compositions range between strongly silica-undersaturated leucite-bearing High-K (HKS) and near silica- saturated Low-K (KS) products. We analyzed homogenized melt inclusions trapped in primitive olivines (Fo=91- 87) from a single KS lava for major and trace-element compositions and Pb-isotope ratios. A wide range in CaO contents of high-Mg# olivine and clinopyroxene phenocrysts points to crystallization from a diversity of primary melts, as observed elsewhere in Roman Province HKS (>0.3 wt.%) and KS (<0.3 wt.%). Some olivines have low CaO contents (<0.15 wt.%), similar to those of Tuscan lamproites (LMP). Based on the combination of major and trace element contents of MI and mineral chemistry, the lava appears to host a collection of different alkali-rich melts: KS-type compositionally similar to the host rock, HKS-type close to lavas of the surrounding Roman Province, a high-SiO₂, low-CaO, low-Na₂O melt type close to the lamproite compositions, and high-Na₂O, low-CaO melt which have no equivalent in nearby erupted lavas. Melt inclusions were further analyzed for Pb isotope ratios, using the Nordsim Cameca-1270 ion microprobe (Swedish Museum of Natural History, Stockholm). A clear relation was found between Pb concentrations of MI and analytical uncertainty in Pb-isotope ratios, with a strong increase in error below 10 ppm. For MI with >10 ppm, 2-sigma errors were <0.003 for ²⁰⁷Pb/²⁰⁶Pb, <0.007 for ²⁰⁸Pb/²⁰⁶Pb, <0.25 for ²⁰⁶Pb/²⁰⁴Pb, <0.2 for ²⁰⁷Pb/²⁰⁴Pb and <0.5 for ²⁰⁸Pb/²⁰⁴Pb. The 25 MI of Latera analyzed contain up to 130 ppm Pb with 75% higher than 10 ppm. The MI show an extreme Pb-isotopic diversity (e.g., ²⁰⁷Pb/²⁰⁶Pb=0.815-0.86, ²⁰⁸Pb/²⁰⁶Pb=2.03-2.11, ²⁰⁶Pb/²⁰⁴Pb=18-19.2, ²⁰⁷Pb/²⁰⁴Pb=14.6-16, ²⁰⁸Pb/²⁰⁴Pb=36.5-40.5), with each group being characterized by its own signature. The Pb-isotope composition of the host lava appears to be a mixture of (at least) three isotopically distinct end- members. The melts form arrays between MORB-like mantle and different crustal/sedimentary end-members. Subduction of the Adriatic plate was probably responsible for the introduction of the one that affected other Roman Province sources as well. Different geodynamic scenarios can be invoked for the introduction of additional heterogeneity in the Latera mantle. The Pb isotopes of the high-Na₂O, low-CaO group are consistent with either a contribution of deeply subducted crustal components from the Southern Alps, as has been proposed for the LMP source, or with involvement of local lower-crust lithologies. A recent thermal event must have been responsible for the simultaneous melting of the strongly heterogeneous mantle column below the volcano.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting