OH in zoned amphiboles of eclogite from the western Tianshan,NW-China |
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| Authors: | Wen Su Ming Zhang Simon A T Redfern Jun Gao Reiner Klemd |
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| Institution: | (1) Chinese Academy of Sciences, State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, P.O. Box 9825, 100029 Beijing, China;(2) State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing, China;(3) Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK;(4) Institute of Mineralogy, Wuerzburg University, Wuerzburg, 97074, Germany |
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| Abstract: | Chemically-zoned amphibole porphyroblast grains in an eclogite (sample ws24-7) from the western Tianshan (NW-China) have been
analyzed by electron microprobe (EMP), micro Fourier-transform infrared (micro-FTIR) and micro-Raman spectroscopy in the OH-stretching
region. The EMP data reveal zoned amphibole compositions clustering around two predominant compositions: a glaucophane end-member
(
B
Na2
C
M2+
3 M3+
2
T
Si8(OH)2) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition (
A
0.5
B
Ca1.5Na0.5
C
M
2+
4.5 M
0.53+
T
Si7.5Al0.5(OH)2) (A = Na and/or K; M
2+ = Mg and Fe2+; M
3+ = Fe3+ and/or Al) between winchite (and ferro-winchite) and katophorite (and Mg-katophorite). Furthermore, we observed complicated
FTIR and Raman spectra with OH-stretching absorption bands varying systematically from core to rim. The FTIR/Raman spectra
of the core amphibole show three lower-frequency components (at 3,633, 3,649–3,651 and 3,660–3,663 cm−1) which can be attributed to a local O(3)-H dipole surrounded by
M(1) M(3)Mg3,
M(1) M(3)Mg2Fe2+ and
M(1) M(3) Fe2+
3, respectively, an empty A site and
T
Si8 environments. On the other hand, bands at higher frequencies (3,672–3,673, 3,691–3,697 and 3,708 cm−1) are observable in the rims of the amphiboles, and they indicate the presence of an occupied A site. The FTIR and Raman data from the OH-stretching region allow us to calculate the site occupancy of the A, M(1)–M(3), T sites with confidence when combined with EPM data. By contrast M(2)- and M(4) site occupancies are more difficult to evaluate. We use these samples to highlight on the opportunities and limitations
of FTIR OH-stretching spectroscopy applied to natural high pressure amphibole phases. The much more detailed cation site occupancy
of the zoned amphibole from the western Tianshan have been obtained by comparing data from micro-chemical and FTIR and/or
Raman in the OH-stretching data. We find the following characteristic substitutions Si(T-site) (Mg, Fe)M(1)–M(3)-site] → Al(T-site) AlM(1)–M(3)-site] (tschermakite), Ca(M4-site)□ (A-site) → Na(M4-site) Na + K(A-site) (richterite), and Ca(M4-site) (Mg, Fe) M(1)–M(3)-site] → Na(M4-site) AlM(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism. |
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