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

2. Interplay among mineral physics, crystallography and
The mica group minerals have been a common theme in the RIMG series with two volumes devoted specifically to the micas.
One of the hallmarks of the RIMG volumes over the last 30 years has been an interplay ranging over mineral physics, crystallography, and petrologic “ground truth” for a wide variety of mineral groups and rock systems, an interplay which, for rock-forming minerals, can be termed "petrologic mineralogy".
Interplay among mineral physics, crystallography and
petrologic “ground truth” i.e. petrologic mineralogy

3. 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
A good example of “petrologic mineralogy” is from the recent investigations on the systematics of Ti in biotite in natural settings.
Based on experiments and observations we know that Ti increases as a function of T and Ti decreases as a function of P. Further, there seems to be some crystallochemical control on Ti such that Ti generally increases with Fe content in biotite.

4. 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
So how do we take advantage of the “petrologic mineralogy” approach? --- We go to a natural laboratory such as NW Maine.
This well-studied metamorphic terrane has the following characteristics…
In this case we want to isolate the temperature and crystallochemical influences on Ti in biotite.
Mooselookmeguntic Lake
NW Maine

5. 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
We also restrict bulk compositional effects by choosing samples with mineral assemblages that minimize this effect. For instance, …

6. How do we calibrate the T effect in this kinds of rocks
How do we calibrate the T effect in this kinds of rocks? --- with a temperature template.
The well-studied and mapped metamorphic zones provide a basis for our temperature grid. If we can assign temperatures to the isograds we can extrapolate temperatures for individual samples with the zones. This is in lieu of using other independent geothermometers.

7. We can then assign temperatures based on intersection of the isogradic reactions at 4 kbar using the calibrated petrogenetic grid for metapelites developed by Spear et al. (1999).
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
So we can now assign specific temperatures for the isograds.

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)
We can also extend temperature range with biotite data from other W Maine samples and higher-grade samples from central Massachusetts

10. In the case the SC Massachusetts we made the assumption of roughly isobaric heating at 6 kbar.

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
The higher T isograds are essentially dehydration melting reactions of different types.

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= °C, X(Mg)= and Ti=
Coefficients: a = , b = e-9 and c =
r2=0.924
Uncertainty of fit +/- 25°C
So, taking all 530 data points we plot the Ti data as a function of temperature and X(Mg) and fit the data to the optimal surface fitting expression.
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

18. Single mineral Ti-in-biotite thermometer
T(°C) = ([ln(Ti) – a – c(XMg)3]/b) [+/- 25°C]
For strict application:
should have quartz, graphite, ilmenite or rutile, and aluminous phase
P is kbar
T limitation = °C)
Can provide limiting T in other cases
In turn we can solve the surface expression for T, either as a simple expression or graphically. However, there are limitations in its application…

19. As is common practice with geothermometers, this can be tested against independent geothermometers or estimates based on literature data.
The TIB geothermometer does quite well, typically within 25 C. However, we do see departures that can become quite significant, especially at the higher T realm. This may imply departures from chemical equilibrium and may also serve as an effective monitor of equilibrium. For example…

20. 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
In upper amphibolite to granulite facies metapelites it is fairly common to find a range of Ti contents within a given sample represent local re-equilibration.
This can be carried to extremes in some instances in which coexisting red (high Ti) and green (low Ti) biotites are developed in the sample. However, you should note the smaller effect on the Mg-Fe partitioning geothermometer.
Ti concentrations provide an insight into likely peak thermal biotite composition

21. 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
Can we learn more about biotite? Is there crystallochemical information on the manner in which Ti substitutes in biotite?

22. 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
Substitutions 1 and 2 involve octahedral site substitutions only
Substitution 3 is a coupled substitution involving octahedral and tetrahedral sites.
Substitution 4 involves multiple sites and is essentially a deprotonation reaction.
These different types of substitutions may be influenced by a variety of factors: crystallochemical, T, P and fluid composition.
* VIR2+ represents the sum of the divalent cations in the octahedral sites and VI represents the octahedral site vacancies.

23. Ti vs. other compositional parameters plots are used to test substitutions in roughly isothermal staurolite zone samples (regions 1 and 2) and for transition and lower sillimanite samples which grade into regions 3 and 1.
Taken as a whole, the plots are most consistent with TiAl2(R-1Si-2) at X(Mg)>0.65 (Region 1).
However, in the higher grade samples the dispersion of the data appears to be mostly controlled by TiO2(R-1(OH)-2).

24. 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
How do we explain the Region 1 substitution? … However, how do we explain the substitutions on region 3?

25. 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
In graphite-bearing systems undergoing isobaric heating, the maximum H2O diminishes, particularly above 600 C – i.e. moving into region 3.
This would be consistent with a tendency for the biotites to experience deprotonation with increase in temperature.

26. 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
There is direct evidence of loss of H in higher grade biotites from NW Maine in this study by Darby et al., 1991)

27. 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
What about biotites from metaluminous rocks? Again, using the “petrologic mineralogy” approach we examine a natural data set to get insights.
In this case we can examine a natural isothermal, isobaric data set in which a(H2O) clearly changes.

28. The biotite from the samples with anhydrous mineral assemblages (red) clearly have elevated Ti contents (generally with some dispersion of data – local equilibrium).
The most clear cut substitution is TiO2(R-1(OH)-2) compatible with conditions of reduced a(H2O).
However, there is also an indication in (d) that TiR(Al-2) is also significant in metaluminous biotites in such a way that suggests that Ti will be enhanced in these biotites more so than biotites from peraluminous rocks.

29. 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