Variability in mineral assemblage, temperature, and oxygen fugacity in a suite of strongly peralkaline lavas and tuffs from Pantelleria, Italy John C. White Eastern Kentucky University Minghua Ren University of Texas at El Paso Don F. Parker Baylor University

Pantelleria, Italy Strait of Sicily transtensional rift Type locality for pantellerite (strongly peralkaline quartz trachyte and rhyolite) Cinque Denti caldera and related post- caldera vents (45 to 5.5 ka)

Mahood and Hildreth (1986) Bull Volc 48: Cuddia Randazzo (CR) 98520, 98521, 98522, Monte Gelkhamar (MG) 98523, Green Tuff (GRT) Cuddie di Belizzi (CB) 98529

Monte Gelkhamar

Cala Cinque Denti

Classification of samples studied, following Macdonald (1974). Key to rock types: C, Comendite; CT, Comenditic Trachyte; P, Pantellerite; PT, Pantelleritic Trachyte.

Crystal Fractionation An assemblage dominated (~86%) by feldspar (Or ) with Na-Cpx, Aenigmatite, Fayalite, Ilmenite, and Apatite. F = 76-79% Civetta et al. (1998), Lowenstern and Mahood (1991), White and Parker (2000), White et al. (2003).

Alkali feldspar analyses. Sanidine (San) / Anorthoclase (Anr) boundary following Deer et al. (1966)

Isobaric projection (P H2O = 980 bars) of the liquidus surface of the 8.3% ae + 8.3% ns plane in the system Ab-Or-Q. (Carmichael and MacKenzie, 1963)

Rock compositions follow the “Thermal Valley” of Carmichael and MacKenzie (1963), dominated by fractionation of alkali feldspar (Or ). Comparison with experimental data suggests liquidus temperatures decreasing from >825° to 700°C. Most evolved rocks (98527, 98529) located at the minimum ( °C) on the cotectic with quartz.

Magnetite-Ilmenite Equilibrium two-oxide pairs only found in least evolved samples: Inc (~63% SiO 2, A.I. ~ 1.17) and (~64% SiO 2, A.I. ~ 1.33) Inc: 1005°C, log fO : 903°C, log fO 2 (Andersen and Lindsley, 1988)

Clinopyroxene and Olivine analyses. Classification of clinopyroxene following Morimoto et al. (1988)

QUIlF: Ilm + TiMt + Fa + Cpx Inc: 990°C, log fO : 886°C, log fO 2. –K-spar liquidus temperature: ~820°C (Carmichael and MacKenzie 1963) to ~860°C (Tuttle and Bowen, 1958). Calculated at P = 1000 bars (Lindsley and Frost, 1992; Frost and Lindsley, 1992; Andersen et al., 1993).

QUIlF: Ilm + Fa + Cpx 98526: 793°C, log fO 2 –K-spar liquidus temperature: ~780°C 98531: 764°C, log fO 2 –K-spar liquidus temperature: ~760°C 98523: 756°C, log fO 2 –K-spar liquidus temperature: ~760°C –Putirka et al. (2003) CPX-melt: 745°C Calculated at P = 1000 bars. Note: Aenigmatite is present in and

T-fO 2 Constraints on other (fa-free) assemblages? 98521, 98522: cpx + ilm +ang –740°C (K-spar) to 700°C (Cpx) –Pyrrhotite microphenocrysts = closer to 740°? 98527: cpx + ilm + ang + q + Fe-richterite –705°C (K-spar) to 678°C (Cpx) –Use QUIlF to try to “recover” fO 2 given 700°C and cpx + ilm + q assemblage. ( /- 0.4) 98529: cpx + ang + q –700°C (K-spar) to 701°C (Cpx) –Above FMQ in “no-oxides” field (Nicholls and Carmichael, 1969)

Comparison of methods

Temperature – Oxygen Fugacity

Conclusions From least-evolved lava (98520) to most- evolved lava (98529): –SiO 2 increases from ~64 to ~70 wt%. –A.I. increases from 1.33 to 1.97 –Th: 17 to 42, Nb: 253 to 407, Zr: 822 to 2010 –Temperature decreases: 886° to 700°C –Oxygen fugacity decreases: to <-17.0

Summary Increasing SiO 2, A.I.; Decreasing T-fO 2 : Feldspar compositions remain nearly constant (Or ) Continuous Reaction Series: –Augite to Sodic Augite / Aegirine-Augite Discontinuous Reaction Series: –(to ~880°C) Fayalite + Ilmenite + Ti-Magnetite –(to ~780°C) Fayalite + Ilmenite –(to ~760°C) Fayalite + Ilmenite + Aenigmatite –(to ~700°C) Ilmenite + Aenigmatite –(to <700°C) Aenigmatite

Summary