Titanium in Muscovite, Biotite, and Hornblende: Modeling, Thermometry, and Rutile Activities of Metapelites and Amphibolites

Jennifer A. Chambers; Matthew J. Kohn

Titanium in Muscovite, Biotite, and Hornblende: Modeling, Thermometry, and Rutile Activities of Metapelites and Amphibolites

Jennifer A. Chambers, Matthew J. Kohn

Geosciences Department

Boise State University

Research output: Contribution to journal › Article › peer-review

Abstract

Reactions involving the VI Ti IV Al- VI Al IV Si exchange in muscovite, biotite, and hornblende were calibrated thermodynamically using a set of experimental and natural data in rutile-plus quartz/coesite-bearing assemblages. The specific respective reactions are K(Al 2 )(AlSi 3 )O 10 (OH) 2 + TiO 2 = K(AlTi)(Al 2 Si 2 )O10(OH) 2 + SiO 2 (R1) K(&square;MgAl)Si 4 O 10 (OH) 2 + TiO 2 = K(&square;MgTi)AlSi 3 O 10 (OH) 2 + SiO 2 (R2) Ca 2 Mg 3 Al 2 Al 2 Si 6 O 22 (OH) 2 + 2TiO 2 = Ca 2 Mg 3 Ti 2 Al 4 Si 4 O 22 (OH) 2 + 2SiO 2 . (R3) Ideal mixing on octahedral or octahedral plus tetrahedral sites and a non-ideal van Laar solution model yield the best regression results for thermodynamic fit parameters, with R 2 values of 0.98–1.00. Isopleths of the equilibrium constant ( K eq ) show minimal pressure dependencies of <1 >°C/kbar, implying that the equilibria are poor barometers. Model reproducibility of the ideal portion of the equilibrium constant ( K id ) is excellent (ca. ±0.1 to 0.3, 2σ), but the absolute value of the combined term Δ S + K id is quite small (absolute values from 0 to 4), so calibration residuals propagate to temperature errors >±50–100 °C, 1σ. Whereas the consistency of a mica or hornblende composition with a known T can be evaluated precisely, Ti chemistry in these reactions is sensitive to composition and does not resolve T (or P ) well. The activity of TiO 2 in rutile [ a (rt)] was also evaluated using both the garnet-rutile-ilmenite-plagioclase-quartz (GRIPS) equilibrium and our new calibrations in rutile-absent, ilmenite-bearing rocks whose peak P-T conditions are otherwise known. Metapelites have average a (rt) of 0.9 (GRIPS) and 0.8 (R1), whereas amphibolites have a (rt) of 0.95 (GRIPS and R3). A value for a (rt) of 0.80 ± 0.20 (metapelites) and 0.95 +0.05/−0.25 (amphibolites) is recommended for trace-element thermomobarometers in ilmenite-bearing, rutile-absent rocks. The dependence of Ti contents of minerals on a (rt) and the reequilibration of Ti during metamorphic reactions both deserve further exploration, and may affect application of trace-element thermobarometers.

Original language	American English
Journal	American Mineralogist
State	Published - 1 Apr 2012

Keywords

biotite
hornblende
muscovite
rutile
titanium

EGS Disciplines

Earth Sciences
Geophysics and Seismology

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https://doi.org/10.2138/am.2012.3890

Cite this

@article{e9a87ad7e6ff4c32bb3cd2315226221d,

title = "Titanium in Muscovite, Biotite, and Hornblende: Modeling, Thermometry, and Rutile Activities of Metapelites and Amphibolites",

abstract = " Reactions involving the VI Ti IV Al- VI Al IV Si exchange in muscovite, biotite, and hornblende were calibrated thermodynamically using a set of experimental and natural data in rutile-plus quartz/coesite-bearing assemblages. The specific respective reactions are K(Al 2 )(AlSi 3 )O 10 (OH) 2 + TiO 2 = K(AlTi)(Al 2 Si 2 )O10(OH) 2 + SiO 2 (R1) K(&square;MgAl)Si 4 O 10 (OH) 2 + TiO 2 = K(&square;MgTi)AlSi 3 O 10 (OH) 2 + SiO 2 (R2) Ca 2 Mg 3 Al 2 Al 2 Si 6 O 22 (OH) 2 + 2TiO 2 = Ca 2 Mg 3 Ti 2 Al 4 Si 4 O 22 (OH) 2 + 2SiO 2 . (R3) Ideal mixing on octahedral or octahedral plus tetrahedral sites and a non-ideal van Laar solution model yield the best regression results for thermodynamic fit parameters, with R 2 values of 0.98–1.00. Isopleths of the equilibrium constant ( K eq ) show minimal pressure dependencies of <1 >°C/kbar, implying that the equilibria are poor barometers. Model reproducibility of the ideal portion of the equilibrium constant ( K id ) is excellent (ca. ±0.1 to 0.3, 2σ), but the absolute value of the combined term Δ S + K id is quite small (absolute values from 0 to 4), so calibration residuals propagate to temperature errors >±50–100 °C, 1σ. Whereas the consistency of a mica or hornblende composition with a known T can be evaluated precisely, Ti chemistry in these reactions is sensitive to composition and does not resolve T (or P ) well. The activity of TiO 2 in rutile [ a (rt)] was also evaluated using both the garnet-rutile-ilmenite-plagioclase-quartz (GRIPS) equilibrium and our new calibrations in rutile-absent, ilmenite-bearing rocks whose peak P-T conditions are otherwise known. Metapelites have average a (rt) of 0.9 (GRIPS) and 0.8 (R1), whereas amphibolites have a (rt) of 0.95 (GRIPS and R3). A value for a (rt) of 0.80 ± 0.20 (metapelites) and 0.95 +0.05/−0.25 (amphibolites) is recommended for trace-element thermomobarometers in ilmenite-bearing, rutile-absent rocks. The dependence of Ti contents of minerals on a (rt) and the reequilibration of Ti during metamorphic reactions both deserve further exploration, and may affect application of trace-element thermobarometers.",

keywords = "biotite, hornblende, muscovite, rutile, titanium",

author = "Chambers, {Jennifer A.} and Kohn, {Matthew J.}",

year = "2012",

month = apr,

day = "1",

language = "American English",

}

TY - JOUR

T1 - Titanium in Muscovite, Biotite, and Hornblende: Modeling, Thermometry, and Rutile Activities of Metapelites and Amphibolites

AU - Chambers, Jennifer A.

AU - Kohn, Matthew J.

PY - 2012/4/1

Y1 - 2012/4/1

N2 - Reactions involving the VI Ti IV Al- VI Al IV Si exchange in muscovite, biotite, and hornblende were calibrated thermodynamically using a set of experimental and natural data in rutile-plus quartz/coesite-bearing assemblages. The specific respective reactions are K(Al 2 )(AlSi 3 )O 10 (OH) 2 + TiO 2 = K(AlTi)(Al 2 Si 2 )O10(OH) 2 + SiO 2 (R1) K(&square;MgAl)Si 4 O 10 (OH) 2 + TiO 2 = K(&square;MgTi)AlSi 3 O 10 (OH) 2 + SiO 2 (R2) Ca 2 Mg 3 Al 2 Al 2 Si 6 O 22 (OH) 2 + 2TiO 2 = Ca 2 Mg 3 Ti 2 Al 4 Si 4 O 22 (OH) 2 + 2SiO 2 . (R3) Ideal mixing on octahedral or octahedral plus tetrahedral sites and a non-ideal van Laar solution model yield the best regression results for thermodynamic fit parameters, with R 2 values of 0.98–1.00. Isopleths of the equilibrium constant ( K eq ) show minimal pressure dependencies of <1 >°C/kbar, implying that the equilibria are poor barometers. Model reproducibility of the ideal portion of the equilibrium constant ( K id ) is excellent (ca. ±0.1 to 0.3, 2σ), but the absolute value of the combined term Δ S + K id is quite small (absolute values from 0 to 4), so calibration residuals propagate to temperature errors >±50–100 °C, 1σ. Whereas the consistency of a mica or hornblende composition with a known T can be evaluated precisely, Ti chemistry in these reactions is sensitive to composition and does not resolve T (or P ) well. The activity of TiO 2 in rutile [ a (rt)] was also evaluated using both the garnet-rutile-ilmenite-plagioclase-quartz (GRIPS) equilibrium and our new calibrations in rutile-absent, ilmenite-bearing rocks whose peak P-T conditions are otherwise known. Metapelites have average a (rt) of 0.9 (GRIPS) and 0.8 (R1), whereas amphibolites have a (rt) of 0.95 (GRIPS and R3). A value for a (rt) of 0.80 ± 0.20 (metapelites) and 0.95 +0.05/−0.25 (amphibolites) is recommended for trace-element thermomobarometers in ilmenite-bearing, rutile-absent rocks. The dependence of Ti contents of minerals on a (rt) and the reequilibration of Ti during metamorphic reactions both deserve further exploration, and may affect application of trace-element thermobarometers.

AB - Reactions involving the VI Ti IV Al- VI Al IV Si exchange in muscovite, biotite, and hornblende were calibrated thermodynamically using a set of experimental and natural data in rutile-plus quartz/coesite-bearing assemblages. The specific respective reactions are K(Al 2 )(AlSi 3 )O 10 (OH) 2 + TiO 2 = K(AlTi)(Al 2 Si 2 )O10(OH) 2 + SiO 2 (R1) K(&square;MgAl)Si 4 O 10 (OH) 2 + TiO 2 = K(&square;MgTi)AlSi 3 O 10 (OH) 2 + SiO 2 (R2) Ca 2 Mg 3 Al 2 Al 2 Si 6 O 22 (OH) 2 + 2TiO 2 = Ca 2 Mg 3 Ti 2 Al 4 Si 4 O 22 (OH) 2 + 2SiO 2 . (R3) Ideal mixing on octahedral or octahedral plus tetrahedral sites and a non-ideal van Laar solution model yield the best regression results for thermodynamic fit parameters, with R 2 values of 0.98–1.00. Isopleths of the equilibrium constant ( K eq ) show minimal pressure dependencies of <1 >°C/kbar, implying that the equilibria are poor barometers. Model reproducibility of the ideal portion of the equilibrium constant ( K id ) is excellent (ca. ±0.1 to 0.3, 2σ), but the absolute value of the combined term Δ S + K id is quite small (absolute values from 0 to 4), so calibration residuals propagate to temperature errors >±50–100 °C, 1σ. Whereas the consistency of a mica or hornblende composition with a known T can be evaluated precisely, Ti chemistry in these reactions is sensitive to composition and does not resolve T (or P ) well. The activity of TiO 2 in rutile [ a (rt)] was also evaluated using both the garnet-rutile-ilmenite-plagioclase-quartz (GRIPS) equilibrium and our new calibrations in rutile-absent, ilmenite-bearing rocks whose peak P-T conditions are otherwise known. Metapelites have average a (rt) of 0.9 (GRIPS) and 0.8 (R1), whereas amphibolites have a (rt) of 0.95 (GRIPS and R3). A value for a (rt) of 0.80 ± 0.20 (metapelites) and 0.95 +0.05/−0.25 (amphibolites) is recommended for trace-element thermomobarometers in ilmenite-bearing, rutile-absent rocks. The dependence of Ti contents of minerals on a (rt) and the reequilibration of Ti during metamorphic reactions both deserve further exploration, and may affect application of trace-element thermobarometers.

KW - biotite

KW - hornblende

KW - muscovite

KW - rutile

KW - titanium

UR - https://scholarworks.boisestate.edu/geo_facpubs/120

UR - https://doi.org/10.2138/am.2012.3890

M3 - Article

SN - 0003-004X

JO - American Mineralogist

JF - American Mineralogist

ER -

Titanium in Muscovite, Biotite, and Hornblende: Modeling, Thermometry, and Rutile Activities of Metapelites and Amphibolites

Abstract

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