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

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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

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• My favorite reference since 1994.
• MS, PhD, teaching Mineralogy, Petrology,
  Geochemistry
• Mineral formula calculations, conversions,
  thermodynamics (P-Chem), modelling

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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.

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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.

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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)

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Eburru, Kenya

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

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T-P- a(SiO2)-f(O2) 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 SiO2
• Fs Fa SiO2
• Oxygen fugacity (ol ilm)
• 2 Fa O2 2 Hem 2 SiO2 (AHQ)
• En 2 Ilm Fs 2 Gk

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2. Cpx Ol temperature En allowed to float.
1. Cpx Ol temperature
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4. Add Hem (let Gk float), Calcualte log fO2.
3. Add Q (set SiO2 1.0), Calculate P (or set P
and cacluate silica activity).
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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)

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New Problem

• Many strongly peralkaline rocks (i.e.,
  Pantellerite with A.I. gt 1.6) lack olivine.
• Common assemblages in highly evolved
  pantellerites include aenigmatite clinopyroxene
  /- ilmenite /- ferrorichterite
• Is it possible to constrain T-f(O2) from these
  assemblages? (Answer Theoretically)

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Clinopyroxene-Ilmenite equilibria

• AHQ (from QUIlF)
• 2 Fs O2 2 Hem 4 SiO2
• Temperature (gu)es(s)timated from other methods,
  fO2 calculated from AHQ (or vice versa).

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Estimate of temperatures from Fa-free
assemblages 98521, 98522 740C 98257
700C 98529 lt 700C (Fa- and Ilm-free
assemblage!)
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Aenigmatite-Ilmenite equilibria

• Aen O2 Ilm 2 Hem 2 SiO2 Na2Si2O5
• a(Nds) 1.0, a(SiO2) 1.0 relative to qtz
• Free energy data Robie and Hemmingway (1995) for
  Ilm, Hem, SiO2 JANAF for Nds Marshs (1975)
  estimate for Aenigmatite.
• Ilm-Hem solution model Andersen and Lindlsey
  (1988)

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DFMQ log f(O2) FMQ(T) Ilmenite-Hematite
activities calculated with Andersen and Lindsley
(1988)
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DFMQ log f(O2) FMQ(T) T-fO2 data from
Pantelleria and Eburru calculated with QUILF95
(White et al., 2005 Ren et al., 2006).
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What else?

• Aen 2 Q Ilm 2 Fs Nds
• AHQ combined with Aen-Ilm equilibria
• 2 SiO2 2 Na2Si2O5 2 Fa O2 4 Aeg
• Line that defines the Fa-out reaction.
• 2 Aeg Hem 2 SiO2 Na2Si2O5
• Line that defines the Ilm-out reaction (minimum T
  for oxide-free rocks)
• Aen Na2Si2O5 O2 4 Aeg Ilm
• Line that defines Aen-Ilm-Cpx equilibrium
• Nicholls and Carmichael (1969)
• Aen Hem Wilk Ilm
• Aenigmatite-Ilmenite geothermometer?

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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!

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