Magma emplacement at anomalous spreading ridge: Constraints due to plagioclase crystals from basalts of Marsili seamount (Southern Tyrrhenian back-arc) D. Barca a, ⁎, T. Trua b a Dipartimento di Scienze della Terra, Università della Calabria, Ponte P. Bucci 12b, I-87036 Arcavacata-Rende (CS), Italy b Dipartimento di Scienze della Terra, Universita' degli Studi di Parma, Via G.P. Usberti 157A, I-43100 Parma, Italy abstract article info Article history: Received 26 March 2012 Accepted 15 June 2012 Available online 28 June 2012 Keywords: Plagioclase Basalt LA–ICP-MS Marsili seamount Southern Tyrrhenian Plagioclase crystals from four basalt samples dredged from different portions of the Marsili seamount (Southern Tyrrhenian, Italy) provide a detailed record of the changes occurring in the surrounding magma during its ascent in the crust. The textural features and chemical zoning (i.e., An content and Fe, Mg, Sr and Ba concentrations) of the plagioclases from each sample show that aggregation of various generations of crystals occurred in the host melt prior to eruption. Plagioclases resulting from crystallisation in small sill intrusions, where local flotation of plagioclase may have occurred, are ubiquitous. Instead, large (up to 1 mm) homogeneous An-rich plagioclases are rare and have only been found in two of the studied basalts, recovered from the northern and southern portions of the volcano, respectively. These crystals record crystallisation events occurring long before eruption, and probably derive from deep-seated crystalline material. At the highest An values, the Sr and Ba concentrations of the studied plagioclases also indicate the existence of two distinct mafic magmas within the lower crustal plumbing system of the Marsili volcano, one with lower Sr and Ba contents than the other. Three of the four stud- ied basalts came from magma with low Sr and Ba, whereas the fourth, which erupted from the southern portion of the volcano, sampled magma richer in Sr and Ba. The ascent of magma caused resorption of the previously formed plagioclase crystals and regrowth of normally or inversely zoned portions, depending on the water-saturated or water-undersaturated conditions of the surrounding magma, respectively. Small shallow magmatic reservoirs may be located under the southern part of the Marsili volcano, as testified by the resorbed sodic cores (An 52–57 ) found in the two basalt samples from this portion. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Many studies combine compositional data and textural character- istics of plagioclase crystals from basaltic rocks in order to investigate the open- or closed-system processes occurring within volcanic plumbing systems throughout the history of a volcano (e.g., Ginibre and Wörner, 2007; Hellevang and Pedersen, 2008; Viccaro et al., 2010; and references therein). This is because the compositional and textural zoning patterns which develop during primary growth are preserved in plagioclase, due to slow CaAl–NaSi diffusion (Morse, 1984; Smith et al., 2009), and the composition of plagioclase is sensi- tive to melt composition (Housh and Luhr, 1991; Putirka, 2005) and physical conditions, such as temperature, pressure and H 2 O(Blundy and Wood, 1991; Bindeman et al., 1998). More recently, studies carried out on plagioclase crystals from mid-ocean ridge basalts (MORB) have demonstrated that open-system processes are also com- mon in these magmatic systems and reveal that plagioclases hosted in MORB basalts can form in a mixture of environments, including deep-seated or shallow magma reservoirs and even in conduits (Costa et al., 2010; Zellmer et al., 2011). Here we investigate the textural and compositional characteristics of plagioclase crystals in four basalt samples dredged from the Marsili seamount (Fig. 1) in the Tyrrhenian Sea, which represents the active spreading centre of the Southern Tyrrhenian back-arc basin (Marani and Trua, 2002). The Marsili volcano is an ideal case for this type of study, both because of its distinctive morphology, resembling that found in spreading mid-ocean ridges (Marani and Trua, 2002), and the geochemical and isotope characteristics of the erupted basaltic lavas, which reveal that a compositional variation occurred during the growth of the volcano, from dominant island-arc basalt (IAB) magmas to younger ocean-island basalt (OIB) lavas (Trua et al., 2010, 2011). Here we use the compositional record preserved in plagioclase crystals from selected Marsili basalt samples. The data obtained allowed us to study the plumbing system of the southern and northern sectors of the volcano, revealing, in a single basalt sam- ple, plagioclases of different origins which crystallised over variable time-spans. They also record the input of two distinct mafic magmas under the ridge axis of the volcano, consistent with conclusions based on previous petrological studies of Marsili basic lavas (Trua et al., 2010, 2011). Journal of Volcanology and Geothermal Research 241–242 (2012) 61–77 ⁎ Corresponding author. Tel.: +39 0984493571. E-mail addresses: d.barca@unical.it (D. Barca), teresa.trua@unipr.it (T. Trua). 0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2012.06.021 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores