Aenigmatite-ilmenite-clinopyroxene equilibria and applications to geothermometry and oxygen barometry in pantelleritic melts: Examples from Pantelleria (Italy) and Eburru (Kenya).

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

• My favorite reference since 1994.

• MS, PhD, teaching Mineralogy, Petrology, Geochemistry • Mineral formula calculations, conversions, thermodynamics (P Chem), modelling…

Original Problem

• Temperature data would help us evaluate petrogenetic models: partial melting vs. FC.

• Geothermobarometry and Oxygen barometry for igneous rocks requires: • Two feldspars, two pyroxenes, and/or two oxides; Hornblende for P (given T).

• Assemblages in peralkaline rocks typically consist of one feldspar (kspar), one pyroxene (cpx), and one oxide (ilm), maybe fayalite and/or aenigmatite; if an amphibole is present, ferrorichterite or arfvedsonite instead of hbl.

So far…

• White, J.C., Ren, M., and Parker, D.F. (2005): Variation in mineralogy, temperature, and oxygen fugacity in a suite of strongly peralkaline lavas and tuffs, Pantelleria, Italy.

Canadian Mineralogist

, 43: 1331-1347.

• Ren, M., Omenda, P.A., Anthony, E.Y., White, J.C., Macdonald, R., and Bailey, D.K. (2006): Application of the QUIlF thermobarometer to the peralkalne trachytes and pantellerites of the Eburru Volcanic Complex, East African Rift, Kenya.

Lithos

, in press.

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)

Eburru, Kenya

• East African Rift • Trachyte, Pantellerite, High-silica Comendite • 1.2 to 0.4 Ma (“Older”) • 400 y B.P. (“Younger”)

T-P-

a

(SiO

2

)-

f(

O

2

) from QUIlF

(Frost and Lindsley, 1988; Lindsley and Frost, 1988; Andersen et al., 1993) • Temperature (ol + cpx): – Fo (ol) + Fs (cpx) = Fa (ol) + En (cpx) – Fa (ol) + Hd (cpx) = CaFeOl (ol) + Fs (cpx) – Fo (ol) + Di (cpx) = CaMgOl (ol) + En (cpx) • Pressure and Silica activity (ol + cpx): – En = Fo + SiO 2 – Fs = Fa + SiO 2 • Oxygen fugacity (ol + ilm): – 2 Fa + O 2 = 2 Hem + 2 SiO 2 (AHQ) – En + 2 Ilm = Fs + 2 Gk

1. Cpx + Ol temperature 2. Cpx + Ol temperature En allowed to “float”.

3. Add Q (set SiO2 = 1.0), Calculate P (or set P and cacluate silica activity).

4. Add Hem (let Gk float), Calcualte log fO2.

QUIlF results are…

• Consistent with experimental data from both synthetic pantellerite (Carmichael and MacKenzie, 1963) and Eburru (Scaillet and Macdonald, 2001) • Consistent with clinopyroxene-melt geothermometer (Putirka et al., 2001) • Also proven useful with silica undersaturated rocks with similar mafic assemblages (Gadar, Greenland: Markl et al., 2001a, b; Marks and Markl, 2001; Marks et al., 2003)

New Problem:

• Many strongly peralkaline rocks (i.e., Pantellerite with A.I. > 1.6) lack olivine.

• Common assemblages in highly evolved pantellerites include aenigmatite + clinopyroxene +/- ilmenite +/ ferrorichterite • Is it possible to constrain T-

f

(O 2 ) from these assemblages? (Answer: Theoretically)

Clinopyroxene-Ilmenite equilibria

• AHQ (from QUIlF) • 2 Fs + O 2 = 2 Hem + 4 SiO 2 • Temperature (gu)es(s)timated from other methods,

f

O 2 calculated from AHQ (or vice versa).

Estimate of temperatures from Fa-free assemblages: 98521, 98522 = ~740 °C 98257 = ~700 °C 98529 < 700 °C (Fa- and Ilm-free assemblage!)

Aenigmatite-Ilmenite equilibria

• Aen + O 2 = Ilm + 2 Hem + 2 SiO 2 + Na 2 Si 2 O 5 • a(Nds) = 1.0, a(SiO 2 ) = 1.0 relative to qtz • Free energy data: Robie and Hemmingway (1995) for Ilm, Hem, SiO 2 ; JANAF for Nds; Marsh’s (1975) estimate for Aenigmatite.

• Ilm-Hem solution model: Andersen and Lindlsey (1988)

1 NNO FMQ 0 -1 X-Hem = 0.07

0.06

0.05

0.04

0.03

WM -2 Ilmenite + Quartz 0.02

Aenigmatite 0.01

-3 700 800 900 1000 T (°C)  FMQ = log f(O2) – FMQ(T) Ilmenite-Hematite activities calculated with Andersen and Lindsley (1988)

1 NNO FMQ 0 -1 0.07

0.06

0.05

0.04

0.03

WM -2 0.02

Eburru Pantelleria -3 700 800 900 1000 T (°C)  FMQ = log f(O2) – FMQ(T) T-fO2 data from Pantelleria and Eburru calculated with QUILF95 (White et al., 2005; Ren et al., 2006).

1 0 -1 -2 aen ilm Ilm93 700 750 800 T (°C) 2Aen+O2 = 2Usp+2Mgt +2Nds+8Q Aen+O2 = Ilm+2Hem+Nds+4Q Pantelleria (ilm+mgt) Pantelleria (ilm +/- aen) Pantelleria (mgt) Menengai (mgt) 850 Ilm97 mgt Usp74 aen Usp78 900

1.0

P = 1500 bar, X Ilm = 0.95

FMQ-0.5

0.9

0.8

aen ilm aen fa + ilm 0.7

0.6

fa + ilm mgt (Usp 0.7) fa + ilm mgt (Usp 0.8) fa fs 700 750 800 T (°C) 850 900 fa + ilm + mgt fa + ilm fa + mgt fa + ilm + aen fa + mgt + aen ilm +aen mgt + aen Menengai (fa + mgt) fa + ilm + aen ilm + aen (FMQ-0.5) QUIlF (Frost et al., 1988) Col 41 vs Col 43

What else?

• Aen + 2 Q = Ilm + 2 Fs + Nds – AHQ combined with Aen-Ilm equilibria • 2 SiO 2 + 2 Na 2 Si 2 O 5 + 2 Fa + O 2 – Line that defines the Fa-out reaction.

= 4 Aeg • 2 Aeg = Hem + 2 SiO 2 + Na 2 Si 2 O 5 – Line that defines the Ilm-out reaction (minimum T for oxide-free rocks) • Aen + Na 2 Si 2 O 5 + O 2 = 4 Aeg + Ilm – Line that defines Aen-Ilm-Cpx equilibrium – Nicholls and Carmichael (1969) • Aen + Hem = Wilk + Ilm – Aenigmatite-Ilmenite geothermometer?

Limitations / Work Needed

• Aegerine in clinopyroxene (Nicholls and Carmichael, 1969; Marsh, 1975; Conrad, 1984).

• Free-energy data: inconsistent from source-to source for aegerine (Marsh, 1975; Robie and Hemingway, 1995), estimated for aenigmatite, non-existent for wilkinsonite!

• Solution models unavailable for Aegirine in Cpx, Aenigmatite-Wilkinsonite, etc.

• More samples from Pantelleria and Kenya!