Research ArticleExperimental and thermodynamic constraints on mineral equilibrium inpantelleritic magmas
Introduction
Peralkaline trachyte and rhyolite (peralkalinity index calculated as P.I. = [mol Na + K/Al] ≥ 1.0) frequently represent the felsic end-member in bimodal magmatic suites that characterize both oceanic and continental intra-plate and extensional tectonic settings. The origin of these magmas has been an extensively debated petrological issue, with two principal models: (i) protracted crystal fractionation from mafic (alkali gabbro) parental magma (Civetta et al., 1998; Neave et al., 2012; Romano et al., 2018; White et al., 2009), or (ii) partial melting of mafic (gabbroic) cumulates followed by low pressure crystal fractionation (e.g., Avanzinelli, 2004; Bohrson and Reid, 1997; Macdonald et al., 2008, Macdonald et al., 2011; Marshall et al., 2009). Whatever their origin, the stability of mineral phases crystallising in peralkaline trachyte and peralkaline rhyolite (named as either comenditic if ([Al2O3−4.4]/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 discussion in the petrological community. Carmichael (1962) published the first modern detailed study of the common mineral phases occurring in pantellerite (i.e., anorthoclase, sodian clinopyroxene and amphibole, fayalitic olivine, aenigmatite) and since then there have been many studies focused on the control of temperature, pressure, oxidation and melt peralkalinity on mineral equilibrium and on how these fractionating phases, in particular feldspar and clinopyroxene, affect the transition from metaluminous (P.I. < 1.0) to peralkaline silicic magmas (e.g., Bailey and Schairer, 1966; Carmichael and MacKenzie, 1963; Conrad, 1984; Lindsley, 1971; Macdonald et al., 2011; Marsh, 1975; Nicholls and Carmichael, 1969; Romano et al., 2018; Scaillet and Macdonald, 2001; Scaillet and Macdonald, 2003; Scaillet and Macdonald, 2006; White et al., 2005).
On the island of Pantelleria, the type locality for pantelleritic rocks, metaluminous trachyte and pantellerite comprise about 90% of the outcrop, occurring as ignimbrites, pumice fall deposits, or vitric lava flows and domes (Jordan et al., 2018; Mahood and Hildreth, 1986). Recent petrological modelling and experimental studies have highlighted the low temperatures (T ≤ 750 °C) and reducing conditions (viz., oxygen fugacities an order of magnitude or more lower than the Nickel-Nickel Oxide [NNO] buffer, or log fO2 < ΔNNO-1) that characterize the pre-eruptive conditions in the pantelleritic magma chambers at Pantelleria (Di Carlo et al., 2010; Liszewska et al., 2018; White et al., 2005, White et al., 2009), and other peralkaline systems (Jeffery et al., 2017; Macdonald, 2012; Macdonald et al., 2011, Macdonald et al., 2019; Ren et al., 2006; Scaillet and Macdonald, 2001; Scaillet and Macdonald, 2003; Scaillet and Macdonald, 2006; White et al., 2005). Although considerable progress has been made, previous experimental studies have not been able to fully clarify the relationship between melt composition, intensive parameters, and the observed phenocrysts, particularly the role of fO2 on mineral assemblages that indicate peralkalinity. Recent studies carried out on other (non-peralkaline) felsic systems have shown that small variations in bulk composition may profoundly affect phase relationships (and compositions), most notably the stability fields of non-tectosilicate phases (which are minor phases in evolved silicate magmas) which are generally critical for the determination of pre-eruptive conditions (e.g., Andújar and Scaillet, 2012; Cadoux et al., 2014; Scaillet et al., 2008).
In this study we present the results of phase equilibrium experiments performed on two samples of pantellerites differing slightly in composition and mineral assemblages. Phase relationships were established at 1 kbar, for temperatures ranging between 680 °C–900 °C and redox conditions around the fayalite-magnetite-quartz (FMQ) buffer (equivalent to ΔNNO-0.68 to −0.60). The experimental results are compared with the results of previous experiments carried out on similar material (Di Carlo et al., 2010). We also compare our data with thermobarometric results based on established mineral-mineral equilibria gathered from Pantelleria and other peralkaline localities (Pantelleria: White et al., 2005, White et al., 2009; Liszewska et al., 2018; Eburru, Kenya: Ren et al., 2006; and Menengai, Kenya: Macdonald et al., 2011). We focus our attention on how mineral assemblages are affected by (i) small variations of starting compositions, and (ii) temperature and redox conditions. Our findings shed new light on mineral-liquid and mineral-mineral equilibria governing the occurrence of aenigmatite, olivine, ilmenite and sodian amphibole phases in pantellerites.
Section snippets
Geological and petrological background
The island of Pantelleria is located in the Sicily Channel (Italy), within the transtensional rift system on the northern promontory of the African Plate (Fig. 1). The eruptive history of Pantelleria can be subdivided into three major periods which include the formation of at least two calderas and the eruption of nine ignimbrites, each of which was followed by largely quiescent periods with occasional, volumetrically minor explosive and effusive eruptions (Jordan et al., 2018). The first
Starting materials
We selected two pantellerite samples with similar compositions (Table 1) but slightly different crystal content (about 10% volume), one representing the basal pumice fall of the Green Tuff eruption (GTP; P.I = 1.68) and one representing a younger pumice deposit (‘Fastuca’ eruption, FTP; P.I. = 1.56), which was also used in the experiments of Di Carlo et al. (2010).
General observations
A summary of the results of our experiments is presented in Table 2. Crystallization experiments performed in our study produce results with similar textural features observed in other experiments done with similar compositions (e.g., Di Carlo et al., 2010; Scaillet and Macdonald, 2006). Achievement of near-equilibrium conditions is suggested by (i) the homogeneous distribution of phases within the charges, (ii) the euhedral shape of crystals, (iii) the regular variations of crystal abundances
Comparison with previous experimental works
Previous experimental studies on peralkaline rhyolite include those of Scaillet and Macdonald (2001, 2003, 2006) on Kenyan comendites (Olkaria) and pantellerites (Eburru), and those of Di Carlo et al. (2010) on Pantellerian pantellerites, which used one of the same starting pantelleritic materials (FTP) used in this study. The main differences between the Kenyan comendites and the pantellerites, as described by Di Carlo et al. (2010) and confirmed in this study concern: (i) the crystallization
Conclusions
This study was aimed at exploring the role of composition and fO2 on pantellerite phase relationships by comparing experiments and thermodynamic modelling, along with recent results gathered on peralkaline rhyolites worldwide. Phase relationships and phase compositions show evident differences in relation both to small differences in whole-rock composition and to the imposed experimental parameters, in particular redox conditions. Our main findings can be summarized as follows:
- 1.
The experimental
Declaration of Competing Interest
None.
Acknowledgements
PR is deeply grateful for all the support received from the ‘Magma Team’ of ISTO during the experimental work. BS, JA and IDC acknowledge support from both LabEx VOLTAIRE (ANR-10-LABX-100-01) and EquipEx PLANEX (ANR-11-EQPX-0036) projects.
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