HR: 11:40h
AN: V32C-06    [Abstracts]
TI: Identifying Source Heterogeneity of Italian K-rich Volcanism With Melt Inclusions: Roccamonfina Revisited
AU: * van Bergen, M J
EM: vbergen@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, Budapestlaan 4, Utrecht, 3484 CD, Netherlands
AU: Nikogosian, I K
EM: iniki@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, Budapestlaan 4, Utrecht, 3484 CD, Netherlands
AB: Volcanism along the Tyrrhenian border of peninsular Italy is characterized by a wide compositional variety of potassic and ultrapotassic rocks, inferred to be related to metasomatic enrichment of mantle sources. Assessing the diversity of their source components and processes from erupted products remains challenging, since the bulk lavas rarely represent primary melts. Roccamonfina stratovolcano has erupted lavas and pyroclastics (e.g., Luhr and Giannetti, 1987; Giannetti and Luhr, 1983, 1990) that have traditionally been distinguished into a qtz- normative K-series (KS), and a Si-undersaturated high-K series (HKS). We have studied olivine-hosted (Fo=93- 87) melt inclusions of five representative mafic lavas (46-52 wt.% SiO₂, 5-8 wt.% MgO, 1.4-6.3 wt.% K₂O) from both series, aiming to assess the compositional diversity of primary melts. Relatively high CaO contents of HKS olivines (0.4-0.6 wt.%) are clearly distinct from those of KS olivines (0.03- 0.35 wt.%), irrespective of Fo contents, and appear to correlate with the degree of alkali enrichment. Heating- stage-homogenized MI (n=150) show strong compositional variations in each series. No systematic variations exist between the CaO contents of MI and host olivine, but highest K₂O, P₂O₅, F concentrations were found in MI of HKS, and highest CaO (up to 19 wt.%), CaO/Al₂O₃, (up to 1.5), S and Cl in those of KS. Nevertheless, potassium contents of the primitive melts (MgO>7 wt.%) show a strong diversity in samples from both series (0.5-5 wt.% K₂O in KS; 0.5-9 wt.% in HKS). Collectively, the MI data show that most of the bulk lava compositions reported so far do not represent primary melts, tending to overestimate SiO₂, K₂O, Na₂O, and Al₂O₃, and underestimate MgO, CaO and TiO₂. Importantly, bulk samples appear to be composed of multiple melts, suggesting contributions from heterogeneous mantle domains. MI in HKS olivines have the highest incompatible trace element contents. Both series show subduction-type LILE/HFSE ratios, but there is considerable variation in the degree of overall enrichment, particularly in the KS, which includes some anomalous depleted melts. Pb isotopic signatures obtained on MI show an extreme diversity (e.g., ²⁰⁷Pb/²⁰⁶Pb=0.8-0.86, ²⁰⁶Pb/²⁰⁴Pb=18.0-19.6) encompassing much of the range seen in the entire spectrum of Italian magmas. Despite some scatter, KS and HKS melts trend towards isotopically different crustal/sedimentary source components. Our findings are consistent with the hypothesis that distinct mantle domains below Roccamonfina were metasomatized by multiple agents with different properties. Similarities with signatures of the Roman Province suggests that the source of the HKS melts was affected by input from the Adria slab, whereas that of the KS melts was modified by the Ionian subduction in view of a greater affinity with Campanian magmas. We speculate that virtually simultaneous melting of a heterogeneous mantle column below Roccamonfina was promoted by upwelling hot asthenosphere in response to slab detachment, inferred from seismic tomography. References Luhr and Giannetti, 1987, CMP 95, 420-436; Giannetti and Luhr, 1983, CMP 84, 235-252; Giannetti and Luhr 1990, EPSL 101, 404-424.
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