Eastern Kentucky University Encompass EKU Faculty and Staff Scholarship Faculty and Staff Scholarship Collection 10-2020 Experimental and thermodynamic constraints on mineral equilibrium in pantelleritic magmas. Pierangelo Romano Universita degli Studi di Palermo Bruno Scaillet Universite d'Orleans John C. White Eastern Kentucky University, [email protected] Joan Andujar Universite d'Orleans Ida Di Carlo Universite d'Orleans See next page for additional authors Follow this and additional works at: https://encompass.eku.edu/fs_research Part of the Geochemistry Commons, Geology Commons, and the Volcanology Commons Recommended Citation Romano, P., Scaillet, B., White, J.C., Andújar, J., Di Carlo, I., Rotolo, S.G., 2020, Experimental and thermodynamic constraints on mineral equilibrium in pantelleritic magmas. Lithos, v. 376-377, 105793, 22 p. (doi: 10.1016/j.lithos.2020.105739) This Article is brought to you for free and open access by the Faculty and Staff Scholarship Collection at Encompass. It has been accepted for inclusion in EKU Faculty and Staff Scholarship by an authorized administrator of Encompass. For more information, please contact [email protected]. Authors Pierangelo Romano, Bruno Scaillet, John C. White, Joan Andujar, Ida Di Carlo, and Silvio G. Rotolo This article is available at Encompass: https://encompass.eku.edu/fs_research/458 Lithos 376–377 (2020) 105793 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Research Article Experimental and thermodynamic constraints on mineral equilibrium in pantelleritic magmas Pierangelo Romano a,⁎,BrunoScailletb, John C. White c, Joan Andújar b, Ida Di Carlo b, Silvio G. Rotolo a,d a Dipartimento di Scienze della Terra e del Mare (DiSTeM), Università degli Studi di Palermo, Via Archirafi 22, 90123 Palermo, Italy b Institut des Sciences de la Terre d'Orléans (ISTO) UMR 7327, Université d'Orléans – CNRS – BRGM, Campus Géosciences, 1A rue de la Férollerie, 45071 Orleans, Cedex 2, France c Department of Geoscience, Eastern Kentucky University, Richmond, KY 40475, USA d Istituto Nazionale di Geofisca e Vulcanologia, sezione di Palermo, Via Ugo la Malfa 153, 90146 Palermo, Italy article info abstract Article history: Crystallization experiments on two pantellerites from Pantelleria, Italy, provide new evidence for the relation- Received 11 April 2020 ships between mineral phases in pantelleritic rocks as well as the influence of temperature and redox conditions Received in revised form 27 August 2020 on mineral assemblages. Experiments were performed at 1 kbar with temperature ranging between 750–900°C, Accepted 17 September 2020 and fluid saturation conditions with XH2O (=H2O/H2O+CO2) between 0 and 1. Redox conditions were fixed at, Available online 21 September 2020 or slightly below, the FMQ buffer. Results show that at temperature of 900 °C pantelleritic magmas are well above the liquidus regardless their water content; we also observed a decrease in liquidus temperature (800°C) with Keywords: Pantellerite increasingly reducing conditions. Mineral assemblages of the natural rocks have been successfully reproduced, Mineral Equilibria particularly the relationship between fayalite and aenigmatite, which appear to be strongly controlled by melt Experimental Petrology peralkalinity, temperature and redox conditions. This is the first time that fayalitic olivine have been synthetized Peralkaline magmas in experimental studies on pantellerites, which was found to be stable only for temperatures ≥750°C while am- phibole can be stable at temperatures as high as 800°C at high fF2. Experimental results have been compared with the composition of mineral phases as well as with the results obtained from mineral equilibrium, geothermometry, and oxygen barometry studies on pantelleritic lava and tuffs from Pantelleria (Italy), Eburru (Kenya) and Menengai (Kenya). Petrological characteristics appear similar at different locations worldwide, with typical assemblages of anorthoclase and sodian clinopyroxene with variable fayalite, aenigmatite, Fe-Ti ox- ides and amphibole. ©2020ElsevierB.V.Allrightsreserved. 1. Introduction FeO* > 1.33 or pantelleritic if [Al2O3−4.4]/FeO* < 1.33, FeO* = total Fe as FeO; Macdonald, 1974) has been also matter of considerable dis- Peralkaline trachyte and rhyolite (peralkalinity index calculated as cussion in the petrological community. Carmichael (1962) published P.I. = [mol Na + K/Al] ≥ 1.0) frequently represent the felsic end- the first modern detailed study of the common mineral phases occurring member in bimodal magmatic suites that characterize both oceanic in pantellerite (i.e., anorthoclase, sodian clinopyroxene and amphibole, and continental intra-plate and extensional tectonic settings. The origin fayalitic olivine, aenigmatite) and since then there have been many of these magmas has been an extensively debated petrological issue, studies focused on the control of temperature, pressure, oxidation and with two principal models: (i) protracted crystal fractionation from melt peralkalinity on mineral equilibrium and on how these fractionat- mafic (alkali gabbro) parental magma (Civetta et al., 1998; Neave ing phases, in particular feldspar and clinopyroxene, affect the transition et al., 2012; Romano et al., 2018; White et al., 2009), or (ii) partial melt- from metaluminous (P.I. < 1.0) to peralkaline silicic magmas (e.g., Bailey ing of mafic (gabbroic) cumulates followed by low pressure crystal frac- and Schairer, 1966; Carmichael and MacKenzie, 1963; Conrad, 1984; tionation (e.g., Avanzinelli, 2004; Bohrson and Reid, 1997; Macdonald Lindsley, 1971; Macdonald et al., 2011; Marsh, 1975; Nicholls and et al., 2008, 2011; Marshall et al., 2009). Whatever their origin, the sta- Carmichael, 1969; Romano et al., 2018; Scaillet and Macdonald, 2001; bility of mineral phases crystallising in peralkaline trachyte and Scaillet and Macdonald, 2003; Scaillet and Macdonald, 2006; White peralkaline rhyolite (named as either comenditic if ([Al2O3−4.4]/ et al., 2005). On the island of Pantelleria, the type locality for pantelleritic rocks, metaluminous trachyte and pantellerite comprise about 90% of the out- ⁎ Correspondig author. fl E-mail addresses: [email protected], [email protected] crop, occurring as ignimbrites, pumice fall deposits, or vitric lava ows (P. Romano). and domes (Jordan et al., 2018; Mahood and Hildreth, 1986). Recent https://doi.org/10.1016/j.lithos.2020.105793 0024-4937/© 2020 Elsevier B.V. All rights reserved. P. Romano, B. Scaillet, J.C. White et al. Lithos 376–377 (2020) 105793 petrological modelling and experimental studies have highlighted the 2001; Scaillet and Macdonald, 2003; Scaillet and Macdonald, 2006; low temperatures (T ≤ 750 °C) and reducing conditions (viz., oxygen fu- White et al., 2005). Although considerable progress has been made, pre- gacities an order of magnitude or more lower than the Nickel-Nickel vious experimental studies have not been able to fully clarify the rela- Oxide [NNO] buffer, or log fO2 < ΔNNO-1) that characterize the pre- tionship between melt composition, intensive parameters, and the eruptive conditions in the pantelleritic magma chambers at Pantelleria observed phenocrysts, particularly the role of fO2 on mineral assem- (Di Carlo et al., 2010; Liszewska et al., 2018; White et al., 2005, 2009), blages that indicate peralkalinity. Recent studies carried out on other and other peralkaline systems (Jeffery et al., 2017; Macdonald, 2012; (non-peralkaline) felsic systems have shown that small variations in Macdonald et al., 2011, 2019; Ren et al., 2006; Scaillet and Macdonald, bulk composition may profoundly affect phase relationships (and Fig. 1. (a) General tectonic sketch of Sicily Channel Rift Zone showing the location of Pantelleria in the rift zone between Sicily and Tunisia. (b) Simplified geological map of Pantelleria Island. 2 P. Romano, B. Scaillet, J.C. White et al. Lithos 376–377 (2020) 105793 compositions), most notably the stability fields of non-tectosilicate Regarding the storage pressure, field evidence suggests that phases (which are minor phases in evolved silicate magmas) which peralkaline silicic magmas are stored at pressures <2 kbar are generally critical for the determination of pre-eruptive conditions (Mahood, 1984), which has been supported at Pantelleria by ther- (e.g., Andújar and Scaillet, 2012; Cadoux et al., 2014; Scaillet et al., 2008). modynamic (White et al., 2009), experimental (Di Carlo et al., In this study we present the results of phase equilibrium experiments 2010), and melt inclusion (Gioncada and Landi, 2010)studies.A performed on two samples of pantellerites differing slightly in composi- low pressure of magma storage is also suggested by a phase equilib- tion and mineral assemblages. Phase relationships were established at rium study of Pantelleria trachyte (Romano et al., 2018), which also 1 kbar, for temperatures ranging between 680 °C–900 °C and redox con- demonstrated that peralkaline silicic derivatives can be produced by ditions around the fayalite-magnetite-quartz (FMQ) buffer (equivalent low pressure fractionation of a metaluminous trachyte. The exten- to ΔNNO-0.68 to −0.60). The experimental results are compared with sive fractionation from a common transitional basaltic magma par- the results of previous experiments carried out on similar material (Di ent that the production of pantellerite liquids requires is in line Carlo et al., 2010). We also compare our data with thermobarometric re- with the