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Mass transfer and element redistribution during chloritization of metamorphic biotite in a metapelite: insights from compositional mapping

Karol Faehnrich
Karol Faehnrich
, 
Karolina Kośmińska
Karolina Kośmińska
 and 
Jarosław Majka
Jarosław Majka
   | Nov 06, 2023
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Article Category: Original Paper
Published Online: Nov 06, 2023
Page range: 43 - 57
Received: Sep 21, 2022
Accepted: Sep 04, 2023
DOI: https://doi.org/10.2478/mipo-2023-0005
Keywords
© 2023 Karol Faehnrich et al., published by Sciendo
This work is licensed under the Creative Commons Attribution 4.0 International License.

Figure 1.

Simplified geologic map of the Svalbard archipelago. Basement Provinces with contrasting pre-Devonian stratigraphy and tectono-metamorphic evolution are separated by N-S trending strike-slip faults. The sampling location is close to the Billefjorden Fault Zone (BFZ), within Ny Friesland, and marked with a star (coordinates are: 79.20248ºN, 16.50917ºE). Modified after Gee and Teben’kov (2004).
Simplified geologic map of the Svalbard archipelago. Basement Provinces with contrasting pre-Devonian stratigraphy and tectono-metamorphic evolution are separated by N-S trending strike-slip faults. The sampling location is close to the Billefjorden Fault Zone (BFZ), within Ny Friesland, and marked with a star (coordinates are: 79.20248ºN, 16.50917ºE). Modified after Gee and Teben’kov (2004).

Figure 2.

Field photo of the outcrop from which sample KK15-4 was collected. The garnet-bearing metapelite is cut by a series of joints that acted as fluid pathways leading to the alteration of biotite into chlorite. Note the chlorite alteration zone.
Field photo of the outcrop from which sample KK15-4 was collected. The garnet-bearing metapelite is cut by a series of joints that acted as fluid pathways leading to the alteration of biotite into chlorite. Note the chlorite alteration zone.

Figure 3.

Photomicrographs of the studied metapelite sample. Photomicrographs show zone next to the joint (KK15-4A), 4 cm (KK15-4B) and 8 cm (KK15-4C) from it. A,B) Biotite crystals unaffected by alteration, occurring 4 and 8 cm away from the joint. C) Pseudomorphic replacement of biotite by chlorite. D) Chlorite replacing biotite and forming intercalated layers within it.
Photomicrographs of the studied metapelite sample. Photomicrographs show zone next to the joint (KK15-4A), 4 cm (KK15-4B) and 8 cm (KK15-4C) from it. A,B) Biotite crystals unaffected by alteration, occurring 4 and 8 cm away from the joint. C) Pseudomorphic replacement of biotite by chlorite. D) Chlorite replacing biotite and forming intercalated layers within it.

Figure 4.

Back-scattered electron (BSE) image of chlorite in KK15-4A. Chlorite, K-feldspar, and titanite all replace biotite. Albite shows various degrees of alteration.
Back-scattered electron (BSE) image of chlorite in KK15-4A. Chlorite, K-feldspar, and titanite all replace biotite. Albite shows various degrees of alteration.

Figure 5.

Distance from the joint plotted against Fe/Zr, K/Zr, and Mg/Zr ratios from whole-rock analyses (see Table 1). Zr was used as a reference due to immobile behavior during fluid-driven reactions (e.g., Ague 2003).
Distance from the joint plotted against Fe/Zr, K/Zr, and Mg/Zr ratios from whole-rock analyses (see Table 1). Zr was used as a reference due to immobile behavior during fluid-driven reactions (e.g., Ague 2003).

Figure 6.

A) Variation of XFe (Fetot/Fetot+Mg) and Ti in biotite in altered (A) and unaltered zone (C). B) Chlorite classification diagram after Hey (1954).
A) Variation of XFe (Fetot/Fetot+Mg) and Ti in biotite in altered (A) and unaltered zone (C). B) Chlorite classification diagram after Hey (1954).

Figure 7.

Concentration ratio diagram (Ague 1994, 2003, 2011) illustrating the gains and losses of oxides and trace elements. Zr is assumed to be immobile. Elements plotting above the yellow line were gained during alteration and points plotting below were lost. The position of the yellow line indicates an overall mass loss of 1.2%.
Concentration ratio diagram (Ague 1994, 2003, 2011) illustrating the gains and losses of oxides and trace elements. Zr is assumed to be immobile. Elements plotting above the yellow line were gained during alteration and points plotting below were lost. The position of the yellow line indicates an overall mass loss of 1.2%.

Figure 8.

Isocon diagram (Grant 1986, 2005) comparing average composition of biotite and chlorite calculated from electron microprobe spot analyses. The assumption is that Al2O3 and Cr2O3 are immobile, oxides above the isocon must have been added to the system to produce chlorite and below the line removed by the fluid or incorporated into new phases.
Isocon diagram (Grant 1986, 2005) comparing average composition of biotite and chlorite calculated from electron microprobe spot analyses. The assumption is that Al2O3 and Cr2O3 are immobile, oxides above the isocon must have been added to the system to produce chlorite and below the line removed by the fluid or incorporated into new phases.

Figure 9.

A) Back-scattered electron (BSE) image of 1.2 by 1.2 mm area of interest within the most altered zone (A). B) X-ray maps showing a variation of Si, Al, K, Mg, and Fe.
A) Back-scattered electron (BSE) image of 1.2 by 1.2 mm area of interest within the most altered zone (A). B) X-ray maps showing a variation of Si, Al, K, Mg, and Fe.

Figure 10.

Distribution of phases and their modal proportions in the first X-ray map produced with XMap Tools (Lanari et al. 2014).
Distribution of phases and their modal proportions in the first X-ray map produced with XMap Tools (Lanari et al. 2014).

Figure 11.

A-I) X-ray maps of the second area showing a variation of Si, Al, K, Fe, Mg, Na, Ca, Ti, and Mn within a single biotite crystal that undergoes chloritization. J) Back-scattered electron (BSE) image of the mapped area. K) Distribution of phases and their modal proportions.
A-I) X-ray maps of the second area showing a variation of Si, Al, K, Fe, Mg, Na, Ca, Ti, and Mn within a single biotite crystal that undergoes chloritization. J) Back-scattered electron (BSE) image of the mapped area. K) Distribution of phases and their modal proportions.

Figure 12.

A-I) X-ray maps of the third area showing a variation of Si, Al, K, Fe, Mg, Na, Ca, Ti, and Mn within chlorite that almost completely replaces biotite. J) Back-scattered electron (BSE) image of the mapped area. K) Distribution of phases and their modal proportions.
A-I) X-ray maps of the third area showing a variation of Si, Al, K, Fe, Mg, Na, Ca, Ti, and Mn within chlorite that almost completely replaces biotite. J) Back-scattered electron (BSE) image of the mapped area. K) Distribution of phases and their modal proportions.

Figure 13.

A) Comparison of moles of Bt, Chl, Kfs, Ttn, and Rt involved in chloritization assuming conservation of molar volume. The equations for the chloritization reaction were calculated using modal proportions and chemical composition obtained from three X-ray maps. B) Gains and losses calculated from the structural formula of substrates (Bt) and product phases (Chl + Kfs ± Rt ± Ttn), assuming conservation of molar volume. Negative values indicate elements that must be added by/from the fluid and positive values indicate elements that must be removed to retain volume.
A) Comparison of moles of Bt, Chl, Kfs, Ttn, and Rt involved in chloritization assuming conservation of molar volume. The equations for the chloritization reaction were calculated using modal proportions and chemical composition obtained from three X-ray maps. B) Gains and losses calculated from the structural formula of substrates (Bt) and product phases (Chl + Kfs ± Rt ± Ttn), assuming conservation of molar volume. Negative values indicate elements that must be added by/from the fluid and positive values indicate elements that must be removed to retain volume.

Whole-rock composition of analyzed zones within sample KK15-4.

KK15-4A KK15-4B KK15-4C
SiO2 61.14 59.09 60.70
Al2O3 16.19 18.01 17.46
Fe2O3 8.06 8.26 7.85
MgO 3.01 2.82 2.65
CaO 1.55 1.88 1.91
Na2O 3.13 2.87 2.92
K2O 3.15 4.06 3.77
TiO2 1.07 1.14 1.02
P2O5 0.14 0.13 0.13
MnO 0.12 0.14 0.13
Cr2O3 0.014 0.015 0.014
Ba 589 703 653
Ni 37 41 39
Sr 132 117 117
Zr 245 258 248
Y 33 38 35
Nb 17 19 16
Sc 18 19 17
LOI 2.2 1.4 1.2
Sum 99.89 99.9 99.91

Comparison of gains (+) and losses (-) of elements during chloritization of biotite.

Whole-rock1 X-Ray Map 12 X-Ray Map 22 X-Ray Map 32
Si + - - -
Ti + - - -
Al - - - -
Fe + + + -
Mn - = = =
Mg + + + +
Ca - + + +
Na + - - -
K - - - -
H2O + n/a n/a n/a
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