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“Ground truthing” biotite in peraluminous and metaluminous metamorphic rocks

“Ground truthing” biotite in peraluminous and metaluminous metamorphic rocks. Darrell J. Henry Louisiana State University Charles V. Guidotti University of Maine. Interplay among mineral physics, crystallography and petrologic “ground truth” i.e. petrologic mineralogy.

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“Ground truthing” biotite in peraluminous and metaluminous metamorphic rocks

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  1. “Ground truthing” biotite in peraluminous and metaluminous metamorphic rocks Darrell J. HenryLouisiana State University Charles V. Guidotti University of Maine

  2. Interplayamong mineral physics, crystallography and petrologic “ground truth” i.e. petrologic mineralogy

  3. Phyllosilicate - Biotite T O T K T O T K T O T Phlogopite: K Mg3 [Si3AlO10] (OH)2 T-layer - triocathedral (Mg2+) layer - T-layer - K

  4. Ti in Biotite – General Results from Experiments 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

  5. Calibration 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

  6. Primary 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

  7. W Maine

  8. Temperature 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

  9. Supplementary 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)

  10. SC Massachusetts

  11. Temperature 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

  12. Surface-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 • Near-isobaric (~4-6 kbar) Ti saturation surface for biotite in aluminous, graphitic metapelites

  13. Distinct changes in slope of Ti-saturation surface • 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

  14. Single mineral Ti-in-biotite thermometer T(°C) = ([ln(Ti) – a – c(XMg)3]/b)0.333[+/- 25°C] 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

  15. Monitor of local chemical equilibrium involving 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

  16. Distinct changes in slope of Ti-saturation surface • 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

  17. Most commonly-cited Ti substitution schemes and exchange vectors * VIR2+ represents the sum of the divalent cations in the octahedral sites and VIrepresents the octahedral site vacancies.

  18. Crystal 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

  19. Compositional change of metamorphic fluids 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

  20. Direct evidence of biotite deprotonation as 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

  21. Biotite 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

  22. Conclusions – “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

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