Fluorine-controlled composition of biotite in granulites of Madagascar: the effect of fluorine on thermometry of biotite–garnet gneisses
Because of the strong preference of fluorine (F) for “hydrous” magnesian silicate minerals, the temperature T (0F) given by the reaction (Bi–Gt) of Fe–Mg exchange between biotite and garnet [phlogopite (Phl) + almandine (Alm) = annite (Ann) + pyrope (Py)] can be dramatically underestimated when the...
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| Published in: | Contributions to mineralogy and petrology Vol. 174; no. 9; pp. 1 - 19 |
|---|---|
| Main Authors: | , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
Berlin/Heidelberg
Springer Berlin Heidelberg
01-09-2019
Springer Springer Nature B.V |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | Because of the strong preference of fluorine (F) for “hydrous” magnesian silicate minerals, the temperature
T
(0F) given by the reaction (Bi–Gt) of Fe–Mg exchange between biotite and garnet [phlogopite (Phl) + almandine (Alm) = annite (Ann) + pyrope (Py)] can be dramatically underestimated when the reciprocal effects between octahedral and hydroxyl site occupancies in biotite are ignored. In the granulites of southeastern Madagascar, widespread Mg metasomatism was associated with F-rich fluids. It is mainly manifested by the regional occurrence of diopside skarns hosting F-phlogopite and thorianite mineralisation. At first, biotite- and garnet-bearing granites and leucogneisses appear unaffected by this event. However, their biotites are rich in F [up to 6 wt%;
X
F
=
X
F
/(F + OH) = 0.69) and in Mg (
X
Mg
= Mg/(Mg + Fe) up to 0.8], which shows the tendency of equilibrium with the regional fluids. This equilibration leads to low values of Fe/Mg, which in turn cause strong underestimation of the temperature:
T
(0F) as low as 500 °C, whereas the temperature
T
(Ti) given by the TiO
2
content of biotite is approximately 750 °C. The slightly modified thermodynamic model of Zhu and Sverjensky (Geochim Cosmochim Acta 56:3435–3467, 1992) for reciprocal solid solution is applied to a set of 19 samples to correct this underestimation and to quantify the effect of F on thermometry. The reciprocal activity coefficients for phlogopite (
λ
(Phl)
) and annite (
λ
(Ann)
) calculated by the model are added to the winTWQ software to obtain corrected values
T
(F) of the temperature. Similar values of Δ
T
=
T
(F) −
T
(0F) are obtained by (1) the linear fit of the whole set of the samples and (2) RTln
K
λ
= RTln
λ
(Ann)
− RTln
λ
(Phl)
= 33.417
X
F
− 0.1092 (
r
2
= 0.9994) equation applied to a median sample: Δ
T
= 30–40, 60–70, 95–100, 160, and 200 °C for
X
F
= 0.1, 0.2, 0.3, 0.5, and 0.7, respectively. The same calculations based on the Thermocalc thermodynamic data set lead to values of Δ
T
1.5 times higher than those given by winTWQ. Thus, the results of these calculations confirm (for the first time from a set of natural samples) the validity of the thermodynamic model of reciprocal solid solution and the necessity of accounting for F in thermometry based on Fe–Mg exchanges involving biotite. |
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| ISSN: | 0010-7999 1432-0967 |
| DOI: | 10.1007/s00410-019-1607-1 |