Post on 16-Mar-2016
description
““Ground truthing” biotite in Ground truthing” biotite in peraluminous and metaluminous peraluminous and metaluminous
metamorphic rocksmetamorphic rocks
Darrell J. HenryDarrell J. HenryLouisiana State UniversityLouisiana State University
Charles V. GuidottiCharles V. GuidottiUniversity of MaineUniversity of Maine0 0
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Ti (a
pfu)
Ti (a
pfu)
Biotite Ti saturation surface - 4 kbar
InterplayInterplay among mineral physics, crystallography and among mineral physics, crystallography and petrologic “ground truth” i.e. petrologic “ground truth” i.e. petrologic mineralogypetrologic mineralogy
Phyllosilicate - BiotitePhyllosilicate - Biotite
Phlogopite: K MgPhlogopite: K Mg33 [Si [Si33AlOAlO1010] (OH)] (OH)22
T-layer - triocathedral (MgT-layer - triocathedral (Mg2+2+) layer - T-layer - K) layer - T-layer - K
T T O O T T K K T T O O T T K K T T O O TT
Ti in Biotite Ti in Biotite – General Results from – General Results from Experiments and Natural SettingsExperiments and Natural Settings
• Ti solubility in phlogopite and intermediate biotite– Ti increases with Temperature – Ti decreases with Pressure
• Ti solubility in Fe-Mg biotite– Ti increases with Fe
Calibration of Biotite Ti-Saturation SurfaceCalibration of Biotite Ti-Saturation Surface• Metapelites from NW Maine,
USA (>530 analyses)
• Near-isobaric (~ 4 kbar) regional-contact metamorphism (380 Ma) with well-equilibrated assemblages
• Range of biotite Mg/(Mg+Fe) due to sulfide-silicate interactions
Mooselookmeguntic Lake NW Maine
Primary Biotite Calibration Data SetPrimary Biotite Calibration Data Set• Biotite data derived from
metapelitic rocks with assemblages that restrict bulk compositional effects– Graphite present - restriction to low and
relatively constant fO2 and Fe3+ (~12% of Fetotal)
– Quartz present - Si at maximal levels– Ilmenite or rutile present - Ti at maximal
levels– Aluminous minerals present - Al at
maximal levels
W MaineW Maine
Temperature ConstraintsTemperature Constraints• Isogradic reactions at 4 kbar calibrated against Spear et
al. (1999) petrogenetic gridGarnet/Staurolite Zone (545°C)
Grt + Chl = St + Bt + H2O
Staurolite/Lower Sillimanite Zone (580°C) St + Chl = Bt + Sil + H2O
Lower /Upper Sillimanite Zone (620°C) St = Grt + Bt + Sil + H2O
Upper Sillimanite/Sillimanite Kfs Zone (660°C) Ms = Kfs + Sil + H2O
Supplementary Data SetsSupplementary Data Sets• Biotite/garnet zone data
for Mg-rich biotite from sulfidic schists of WC Maine (Ferry, 1981)
• Sillimanite-Kfs zone and garnet-cordierite zone data from sulfidic and non-sulfidic schists, slightly higher pressures (5-6 kbar) (Tracy, 1978; Tracy and Robinson, 1988; Thomson, 2001)
SC MassachusettsSC Massachusetts
Temperature ConstraintsTemperature Constraints• Dehydration melting reactions at 6 kbar calibrated
against Spear et al. (1999) petrogenetic grid
• Muscovite melt Zone (650°C) Ms + Ab = Sil + Kfs + melt
• Biotite-Garnet-Cordierite melt Zone (745°C) Bt + Sil = Grt + Crd + melt
• Biotite-Opx-Cordierite melt Zone (815°C) Bt + Grt = Opx + Crd + melt
Surface-fit of near-isobaric biotite dataSurface-fit of near-isobaric biotite data• ln(z) = a + bx3 + cy3
• where x is T°C, y is X(Mg) and z is Ti
• data range: T=490-800°C, X(Mg)=0.3-1.0 and Ti=0.04-0.6
• Coefficients: a = -2.3594, b = 4.6482e-9 and c = -1.7283
• r2=0.924 • Uncertainty of fit +/- 25°C
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Ti (a
pfu)
Ti (a
pfu)
Biotite Ti saturation surface - 4 kbar
• Near-isobaric (~4-6 kbar) Ti saturation surface for biotite in aluminous, graphitic metapelites
Distinct changes in slope of Ti-saturation Distinct changes in slope of Ti-saturation surfacesurface
• Region 1 – steeper-sloped Mg-rich portion of the surface
• Region 2 – shallow-sloped portion at XMg < 0.65 and T < 600oC
• Region 3 – higher T region (T > 600oC) with nonlinear change
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Temperature (C)
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Mg/(Mg+Fe)
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Ti (a
pfu)
Ti (a
pfu)
Biotite Ti saturation surface - 4 kbar
450500
550600
650700
750
Temperature (C)
0.20.30.40.50.60.70.80.9
Mg/(Mg+Fe)
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0.60.6
Ti (a
pfu)
Ti (a
pfu)
Biotite Ti saturation surface - 4 kbar
450500
550600
650700
750
Temperature (C)
0.20.30.40.50.60.70.80.9
Mg/(Mg+Fe)
0 00.1
0.10.2 0.20.3 0.30.4 0.40.5 0.50.6 0.6
Ti (a
pfu)
Ti (a
pfu)
Biotite Ti saturation surface - 4 kbar
450500
550600
650700
750
Temperature (C)
0.20.30.40.50.60.70.80.9
Mg/(Mg+Fe)
0 0
0.1 0.1
0.2 0.2
0.3 0.3
0.4 0.4
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Ti (a
pfu)
Ti (a
pfu)
Biotite Ti saturation surface - 4 kbar
Single mineral Ti-in-biotite thermometerSingle mineral Ti-in-biotite thermometer
For strict application:• should have quartz,
graphite, ilmenite or rutile, and aluminous phase
• P is 4 - 6 kbar • T limitation = 480-800°C)Can provide limiting T in
other cases
T(°C) = ([ln(Ti) – a – c(XMg)3]/b)0.333 [+/- 25°C]
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T (reference)
Pattison Symmes Alias Florence Dymek Shearer Ikeda Ibarguchi Thomson Peterson
graphitic, peraluminous with Ti-saturation
Monitor of local chemical equilibrium Monitor of local chemical equilibrium involving biotiteinvolving biotite
• Local equilibrium is a particular problem at upper amphibolite and granulite facies conditions due to local retrograde re-equilibration
• Ti concentrations provide an insight into likely peak thermal biotite composition
Distinct changes in slope of Ti-saturation Distinct changes in slope of Ti-saturation surfacesurface
• Region 1 – steeper-sloped Mg-rich portion of the surface
• Region 2 – shallow-sloped portion at XMg < 0.65 and T < 600oC
• Region 3 – higher T region (T > 600oC) with nonlinear change
Most commonly-cited Ti substitution Most commonly-cited Ti substitution schemes and exchange vectorsschemes and exchange vectors
* VIR2+ represents the sum of the divalent cations in the octahedral sites and VI represents the octahedral site vacancies.
Site substitution Exchange vector2VIR2+ = VITi + VI* Ti R-2
2VIAl = VITi + VIR2+ TiRAl-2
VIR2+ + 2IVSi = VITi + 2IVAl TiAl2R-1Si-2
VIR2+ + 2(OH)1- = VITi + 2O2- TiO2R-1(OH)-2
Crystal Chemical ConsiderationsCrystal Chemical Considerations
• Within Region 1 biotite (Mg-rich region), TiAl2R-1Si-2 is the dominant exchange vector and is a likely response to crystallographic constraints – Decrease in Ti alleviates the size disparity
between the octahedral and tetrahedral sheets– Increase in amounts of Si helps reduce the size
disparity between sheets and maintain overall charge balance
Compositional change of metamorphic Compositional change of metamorphic fluids associated with graphitefluids associated with graphite
• Graphite-saturated COH fluids during dehydration tend to approach H/O ratio of 2:1 (Connelly and Cesare, 1993)
• Isobaric heating at ~ 4 kbar produces fluids with lower X(H2O), esp. > 600oC
Direct evidence of biotite deprotonation Direct evidence of biotite deprotonation as a function of metamorphic gradeas a function of metamorphic grade
• Graphite-bearing metapelites with biotite (intermediate XMg) that is part of the original calibration data set (Dyar et al, 1991)
• Significant reduction in H content in biotite above transition zone
Biotite from metaluminous rocksBiotite from metaluminous rocks• Biotite-bearing metatonalite at ~800 ºC and 8 kbar
under locally variable a(H2O) conditions [Seward, Alaska] (Harlov and Forster, 2002) – Dehydration zone (80 cm) of a hornblende metatonalite proximal to
a marble unit– host rock metatonalite has a uniform assemblage of hbl + bt + pl +
qtz + gr, but no Ti-saturating mineral (ilmenite or rutile) – within 50 cm of the marble unit and has an assemblage of opx + cpx
+ Ti-rich bt + pl + qtz + Kfs
– 470 biotite analyses from 12 samples with multiple biotite analyses from the same grain and from several grains from the same sample
Conclusions – “Ground truthing” biotiteConclusions – “Ground truthing” biotite• Established useful Ti-in biotite geothermometer and
monitor of chemical equilibrium• Can constrain the interplay of crystallochemical controls
with the petrologic environment– For peraluminous, Mg-rich biotite TiAl2R-1Si-2 is the dominant exchange
vector– For peraluminous biotite in graphitic rocks above staurolite zone, TiO2R-
1(OH)-2 becomes the dominant exchange as the activity of H2O is reduced in metamorphic fluids
– For metaluminous biotite TiO2R-1(OH)-2 becomes the dominant exchange as the activity of H2O is reduced in metamorphic fluids, but TiAlR-2 is also significant