Michele Menna

Al-Si order and spinodal decomposition texture of a sanidine from igneous clasts of Stromboli (southern Italy): insights into the timing between the emplacement of a shallow basic sheet intrusion and the eruption of related ejecta

A sanidine from subvolcanic cognate monzonite clasts within the debris flow deposits belonging to the 5 ka Secche di Lazzaro pyroclastics of Stromboli volcano (Neostromboli period) was studied by X-ray diffraction (single crystal and powder) and transmission electron microscopy (TEM). The sanidine is poikilitic, Ab 31 Or 65 An 4 , coexisting with plagioclase, iron-rich diopside and olivine, phlogopite, Ti-magnetite, ilmenite and apatite. The subvolcanic clasts are the slowly cooled equivalent of the potassic magmas erupted as lavas during the Neostromboli period. They are representative of basic magmas arrested at shallow levels as sheet intrusions (sills or dykes). Single-crystal X-ray diffraction of the sanidine ( R 4σ = 4.4 % for 1317 unique reflections) enabled us to obtain Al-Si site ordering [X A1 T1 = 0.301(2)] from average tetrahedral bond lengths. TEM microstructural analysis showed spinodal decomposition modulations, but no cryptoperthitic exsolution lamellae were found. Although quantitative application of the above microstructures and single-crystal results could have been hindered by the uncertainty due to the effect of anorthitic component in the sanidine, Al-Si ordering data suggest that, at 700 °C, the sheet intrusion was cooling on the order of 1–10 °C/day. The spinodal decomposition texture seems to be the result of the disruption of the shallow subvolcanic rocks at time of the 5 ka paroxystic explosive eruption which allowed a syn-eruptive very rapid cooling and prevented a long, in situ, subsolidus cooling history of the ejecta. In fact, the lack of cryptoperthitic exsolution lamellae in the sanidine suggests the monzonite fragments were rapidly ejected to the surface before the temperature of the albite-sanidine solvus was reached. At time of disruption, the basic sheet intrusion had to be characterized by a temperature just below its solidus and therefore it should have been emplaced shortly before the 5 ka eruption of the Secche di Lazzaro pyroclastics.

Mineralogical, Geochemical, and Isotopic Characteristics of the Ejecta from the 5 April 2003 Paroxysm at Stromboli, Italy: Inferences on the Preeruptive Magma Dynamics

The 5 April 2003 explosive eruption at Stromboli emplaced typical basaltic scoria, pumice, and lithic blocks. This paper reports a detailed set of mineralogical, geochemical, and isotopic data on the juvenile ejecta and fresh subvolcanic blocks, including micro-Sr isotope analyses and major and dissolved volatile element contents in olivine-hosted melt inclusions. The juvenile ejecta have compositions similar to those of their analogs from previous paroxysms; the 2003 pumice, however, does not contain stable high-MgO olivine, usually typical of large-scale paroxysms and has lower compatible element contents. Texture, composition, and Sr isotope disequilibrium of crystals in pumice indicate that most of them are inherited from the shallow crystal-rich magma and/or crystal mush. The most primitive magma is recorded as rare melt inclusion in olivine Fo 85―86 . It has a typical S/Cl (1.1) and a total volatile content of 3.1 wt % from which the total fluid pressure was evaluated >240 MPa. Hence, moderate pressure conditions can be envisaged for the mechanism triggering the April 2003 paroxysm. The subvolcanic blocks are shoshonitic basalts with 45―50 vol % of phenocrysts (plagioclase + clinopyroxene + olivine). The late-stage crystallization of the crystal-rich magma lead to the formation ofNa-sanidine with plagioclase An 60―25 + olivine Fo 68―49 + Ti-magnetite ± apatite ± phlogopite ± ilmenite assemblage. Mineralogy, chemistry, and Sr―Nd isotopic signatures of the subvolcanic blocks indicate they represent the slowly cooled equivalents of batches of crystal-rich basaltic magma stored in the uppermost subvolcanic feeding system during the last few years. Cooling might be facilitated by short breaks in the summit crater activity.