““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

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Ti (

apfu

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Ti (

apfu

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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

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 grid

Garnet/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 (

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Ti (

apfu

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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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Biotite Ti saturation surface - 4 kbar

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Biotite Ti saturation surface - 4 kbar

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Temperature (C)

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Biotite Ti saturation surface - 4 kbar

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Temperature (C)

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Mg/(Mg+Fe)

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Ti

(ap

fu)

Ti

(ap

fu)

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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alc

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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 vector

2VIR2+ = 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