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John C. White Eastern Kentucky University Minghua Ren University of Texas at El Paso Don F. Parker Baylor University

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.

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John C. White Eastern Kentucky University Minghua Ren University of Texas at El Paso Don F. Parker Baylor University

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  1. 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

  2. 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)

  3. Cuddia Randazzo (CR) 98520, 98521, 98522, 98531 Monte Gelkhamar (MG) 98523, 98526 Green Tuff (GRT) 98527 Mahood and Hildreth (1986) Bull Volc 48: 143-172 Cuddie di Belizzi (CB) 98529

  4. Monte Gelkhamar

  5. Cala Cinque Denti

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

  7. Crystal Fractionation • An assemblage dominated (~86%) by feldspar (Or35-36) 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).

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

  9. Isobaric projection (PH2O = 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)

  10. Rock compositions follow the “Thermal Valley” of Carmichael and MacKenzie (1963), dominated by fractionation of alkali feldspar (Or35-36). • Comparison with experimental data suggests liquidus temperatures decreasing from >825° to 700°C. • Most evolved rocks (98527, 98529) located at the minimum (700-705°C) on the cotectic with quartz.

  11. Magnetite-Ilmenite • Equilibrium two-oxide pairs only found in least evolved samples: 98521-Inc (~63% SiO2, A.I. ~ 1.17) and 98520 (~64% SiO2, A.I. ~ 1.33) • 98521-Inc: 1005°C, -11.5 log fO2 • 98520: 903°C, -13.6 log fO2 • (Andersen and Lindsley, 1988)

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

  13. QUIlF: Ilm + TiMt + Fa + Cpx • 98521-Inc: 990°C, -11.8 log fO2. • 98520: 886°C, -13.9 log fO2. • 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).

  14. QUIlF: Ilm + Fa + Cpx • 98526: 793°C, -15.2 log fO2 • K-spar liquidus temperature: ~780°C • 98531: 764°C, -15.9 log fO2 • K-spar liquidus temperature: ~760°C • 98523: 756°C, -15.8 log fO2 • K-spar liquidus temperature: ~760°C • Putirka et al. (2003) CPX-melt: 745°C • Calculated at P = 1000 bars. • Note: Aenigmatite is present in 98531 and 98523.

  15. T-fO2 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” fO2 given 700°C and cpx + ilm + q assemblage. (-17.0 +/- 0.4) • 98529: cpx + ang + q • 700°C (K-spar) to 701°C (Cpx) • Above FMQ in “no-oxides” field (Nicholls and Carmichael, 1969)

  16. Comparison of methods

  17. Temperature – Oxygen Fugacity

  18. Conclusions • From least-evolved lava (98520) to most-evolved lava (98529): • SiO2 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: -13.8 to <-17.0

  19. Summary • Increasing SiO2, A.I.; Decreasing T-fO2: • Feldspar compositions remain nearly constant (Or35-36) • 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

  20. Summary

  21. Summary

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