Aenigmatite-ilmenite-clinopyroxene equilibria and applications to geothermometry and oxygen barometry in pantelleritic melts:

Report
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 (PChem), 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 postcaldera 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(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
2. Cpx + Ol temperature
En allowed to “float”.
1. Cpx + Ol temperature
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).
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(O2) from
these assemblages? (Answer:
Theoretically)
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).
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 + 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; Marsh’s (1975) estimate
for Aenigmatite.
• Ilm-Hem solution model: Andersen and
Lindlsey (1988)
1
NNO
FMQ
FMQ
0
X-Hem = 0.07
0.06
0.05
WM
0.04
0.03
-1
-2
0.02
Ilmenite + Quartz
-3
Aenigmatite
700
0.01
800
900
1000
T (°C)
FMQ = log f(O2) – FMQ(T)
Ilmenite-Hematite activities calculated with Andersen and Lindsley (1988)
1
NNO
FMQ
FMQ
0
0.07
0.06
0.05
0.04
0.03
-1
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
FMQ
0
ilm
aen
Ilm93
-1
Ilm97
-2
mgt
Usp74
aen
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
900
Usp78
P = 1500 bar, XIlm = 0.95
FMQ-0.5
fa + ilm
mgt
(Usp 0.7)
1.0
aSiO2(Qtz)
0.9
aen
fa + ilm
mgt
(Usp 0.8)
ilm
0.8
aen
fa + ilm
fs
0.7
fa
0.6
700
750
800
850
900
T (°C)
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 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?
Limitations / Work Needed
• Aegerine in clinopyroxene (Nicholls and
Carmichael, 1969; Marsh, 1975; Conrad, 1984).
• Free-energy data: inconsistent from source-tosource 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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