Tourmaline as a Geochemical Tape Recorder in Metamorphic Rocks

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Tourmaline as a Geochemical Tape Recorder in Metamorphic Rocks Powered By Docstoc
					              “Ground truthing” biotite in
            peraluminous and metaluminous
                  metamorphic rocks

                  Biotite Ti saturation surface - 4 kbar

                                                                                                                     Darrell J. Henry
                                                                                                                    Louisiana State University
                                                                                                  0.6
            0.6
                                                                                                  0.5
            0.5
                                                                                                  0.4
            0.4
                                                                                                        Ti (apfu)
Ti (apfu)




                                                                                                  0.3
            0.3

            0.2
            0.1
                                                                                                  0.2
                                                                                                  0.1
                                                                                                                    Charles V. Guidotti
              0
                  0 .9 .8                                                             700
                                                                                            750
                                                                                                  0
                                                                                                                       University of Maine
                      0                                                         650
                          0 .7 .6                                         600                e   (C)
                    Mg/       0
                                  0 .5 .4                           550                  tur
                        (Mg                                   500                    era
                           + Fe        0
                                          0 .3                                  mp
                               )                        450                Te
                                                 0 .2
Interplay among mineral physics, crystallography and
petrologic “ground truth” i.e. petrologic mineralogy
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
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
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
  Mooselookmeguntic Lake
        NW Maine
                             sulfide-silicate interactions
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
W Maine
         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
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)
SC Massachusetts
          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
            Surface-fit of near-isobaric biotite data
                                                                                                                  • ln(z) = a + bx3 + cy3
                    Biotite Ti saturation surface - 4 kbar
                                                                                                                    • where x is T°C, y is X(Mg)
                                                                                                                      and z is Ti
                                                                                                0.6
                                                                                                                    • data range: T=490-800°C,
              0.6

              0.5
                                                                                                0.5                   X(Mg)=0.3-1.0 and Ti=0.04-
                                                                                                0.4
              0.4
                                                                                                                      0.6

                                                                                                      Ti (apfu)
Ti (apfu)




                                                                                                0.3
              0.3
                                                                                                0.2
              0.2
              0.1
                                                                                                0.1                 • Coefficients: a = -2.3594,
                0
                    0 .9 .8                                                   650
                                                                                    700
                                                                                          750
                                                                                                0
                                                                                                                      b = 4.6482e-9 and c = -1.7283
                        0
                            0 .7 .6                                     600                    (C)
                                                                                                                    • r2=0.924
                                                                                           e
                      Mg/       0
                                    0 .5 .4                       550                  tur
                          (Mg                               500                    era
                             + Fe        0
                                            0 .3                              mp
                                 )                     4                 Te
                                                   0.2 50

                                                                                                                    • Uncertainty of fit +/- 25°C

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




                                                                                 0.6
            0.6                                                                  0.5

            0.5                                                                 0.4




                                                                                         Ti (apfu)
            0.4                                                                 0.3
Ti (apfu)




            0.3                                                                 0.2

            0.2                                                                 0.1

            0.1                                                                 0
                                                                           750
                                                                        700          )
              0                                                     650            (C
                   0 .9 0 .8                                      600           re
                                                                              tu
                             0 .7 .6                            550        ra
                                                                        pe
                                  0   0 .5 .4                500       m
                       Mg/(M               0                        Te
                              g+ Fe           0 .3        450
                                    )                0 .2
                  Biotite Ti saturation surface - 4 kbar




                                                                               0.6
            0.6
                                                                               0.5
            0.5
                                                                               0.4




                                                                                      Ti (apfu)
            0.4
                                                                               0.3
Ti (apfu)




            0.3                                                                0.2
            0.2                                                                0.1
            0.1                                                               0
                                                                         75
              0                                                      700 0
                  0 .9 .8                                        650              )
                       0                                      600             e (C
                            0 .7 .6                        550            tur
                                 0                                     ra
                      Mg/(          0 .5 .4            500         mpe
                           Mg+           0                      Te
                                Fe)         0.3 .2 450
                                                0
                  Biotite Ti saturation surface - 4 kbar




                                                                                             0.6
            0.6
                                                                                             0.5
            0.5
                                                                                             0.4
            0.4




                                                                                                   Ti (apfu)
Ti (apfu)




                                                                                             0.3
            0.3
                                                                                             0.2
            0.2
                                                                                             0.1
            0.1
                                                                                             0
              0                                                                        750
                  0 .9 .8                                                        700
                      0                                                    650            )
                         0 .7 .6                                     600              e (C
                             0                                 550                 ur
                   Mg/
                       (Mg       0 .5 .4                 500                  era
                                                                                  t
                          + Fe        0                                     p
                              )          0 .3       4                 T   em
                                                0.2 50
                  Biotite Ti saturation surface - 4 kbar




            0.6                                                                               0.6
            0.5                                                                               0.5
            0.4                                                                             0.4
Ti (apfu)




                                                                                                    Ti (apfu)
            0.3                                                                             0.3
            0.2                                                                             0.2
            0.1                                                                             0.1
              0                                                                             0
                  0 .9                                                               750
                     0 .8 .7                                                   700
                          0                                              650
                  Mg          0 .6 5                               600                  (C)
                    / (M          0.                         550                   ur e
                         g+ F        0 .4 3            500                     rat
                              e)         0.                             pe
                                                  4
                                              0.2 50                T em
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
                                        graphitic, peraluminous with Ti-saturation
                          Pattison
                 850
                          Symmes
                          Alias
                 800
                          Florence
                          Dymek
                 750      Shearer
                          Ikeda
T (calculated)




                 700      Ibarguchi
                          Thomson
                 650      Peterson


                 600

                 550

                 500

                 450
                    450   500     550      600    650     700    750    800     850

                                           T (reference)
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
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
    Most commonly-cited Ti substitution
      schemes and exchange vectors
   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


* VIR2+ represents the sum of the divalent cations in the octahedral sites and VI represents the octahedral site vacancies.
  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
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
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
    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
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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