International Journal of Earth Sciences

, Volume 98, Issue 6, pp 1299–1309 | Cite as

OH in zoned amphiboles of eclogite from the western Tianshan, NW-China

  • Wen Su
  • Ming Zhang
  • Simon A. T. Redfern
  • Jun Gao
  • Reiner Klemd
Original Paper

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 (BNa2CM2+3 M3+2TSi8(OH)2) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition (A0.5BCa1.5Na0.5CM2+4.5 M0.53+TSi7.5Al0.5(OH)2) (A = Na and/or K; M2+ = Mg and Fe2+; M3+ = 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 TSi8 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) Al[M(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) Al[M(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism.

Keywords

Zoned amphibole OH-stretching Infrared spectra Raman EMP Western Tianshan 

Introduction

Amphiboles play a major role in the origin and evolution of metamorphic and igneous rocks. They are indicators of temperature, pressure, volatile content, and oxidation state, and also provide petrogenetic information through their structural phase transitions, cation ordering over multiple crystallographic sites, and coupled substitutions. A long-standing crystal chemical goal is to develop thermodynamic solution models for amphiboles, which relate amphibole compositions to the thermodynamic activities of end-member amphibole components. A fundamental prior requirement in the development of such models is to develop an understanding of the partitioning of major elements over each of the crystallographic sites in amphiboles, so that inter- and intra-mineral partitioning can be accurately modeled, understood, and predicted.

Hydroxyl species are a fundamental feature of amphiboles. Chemical shifts on the principal OH-stretching vibrations in the infrared region reflect both chemical composition and the local charge distribution in the neighborhood of the proton, and the vibrational modes of the hydroxyl species in amphiboles provide a window into their short-range correlated crystal chemistry, including information on site occupancies and local clustering (e.g., Burns and Strens 1966; Ernst and Wai 1970; Gillet et al. 1989; Welch et al. 1994; Hawthorne et al. 1996a, b, 2000; Robert et al. 1989; Skogby and Rossmann 1991; Jenkins et al. 1997; Della Ventura et al. 1999). Understanding the relative importance of each of the main intra-crystalline element partitioning and clustering processes is complicated in natural amphibole-group minerals, which are commonly of mixed (non-end-member) compositions. Therefore, FTIR spectroscopy has previously focused primarily on synthetic amphiboles (e.g., Della Ventura et al. 1998; Gottschalk et al. 1998, 1999; Raudsepp et al. 1987; Robert et al. 1989, 1993; Najorka et al. 2000), for which chemical variations can be relatively reduced and crystal chemical complexities can be decoupled. Such studies have allowed us to gain a better understanding of these naturally complex minerals; from these studies we now have information on cation-anion ordering (e.g., Iezzi et al. 2003a, b, 2004a, 2005a; Della Ventura et al. 1999; Hawthorne et al. 1997, 2000; Ishida et al. 2002; Gottschalk et al. 1998, 1999; Jenkins et al. 2003), symmetry (e.g., Iezzi et al. 2004b), and phase-transition characterization at non-ambient conditions (e.g., Iezzi et al. 2005b).

Amphibole minerals are common in the high-pressure rocks of the western Tianshan metamorphic belt (Gao et al. 1999). Field relations and the textural evidence suggest that these amphiboles were formed though metamorphic reactions at conditions of the blueschist to eclogite transition (Klemd et al. 2002; Gao et al. 1999, 2007). During such a transition a major pulse of fluid release occurs as wet blueschist (4–5 wt. % fluid) transforms to dry eclogite (<1 wt. % fluid), at 50–70 km depths (Peacock 1993). Some authors have concluded that almost all of this fluid, stored in the oceanic crust and the upper part of the hydrated slab mantle, would leave the slab without almost no associated chemical effects (e.g., Hermann et al. 2006), whereas others point to the potential reactivity and catalytic capacity of the released aqueous fluids while traveling through the slab towards the mantle wedge (e.g., Zack et al. 2002; John and Schenk 2003; John et al. 2004, 2008; Zack and John 2007). Amphiboles, being the major hydrous phases in such eclogites, could provide important information about the fluid release processes through their structural phase transitions, cation ordering and partitioning, and coupling to aqueous fluid contents during the subduction (blueschist to eclogite transition) and exhumation of oceanic crustal rocks. This paper reports micro-FTIR and micro-Raman spectroscopic data from zoned amphiboles of eclogite from the western Tianshan. Combined with EMP data, these results show that micro-FTIR and micro-Raman spectroscopy on the OH-stretching region provides a promising methodological approach with which to understand particular features of cation diffusion and partitioning for fine-grained amphiboles during metamorphism.

Sample description

The amphibole crystals analyzed here were extracted from an eclogite sample (ws24-7) of the Akeyazhi area (81°15′41″E, 42°29′47″N) in the western Tianshan high-pressure (HP) and low-temperature (LT) belt. This eclogite occurs as thin layer intercalated with blueschists (Fig. 1a). It is composed of 18–23% garnet, 25–30% omphacite, 8–10% paragonite, 17–19% amphibole, 15–18% epidote, 10% quartz, as well as minor rutile, zircon and apatite (Fig. 1b). The titanite and ilmenite are retrograde phases. Secondary chlorite is sparsely present. Three textural types of amphibole are found: (1) fine-grained amphibole found as inclusions in garnet (Fig. 1b); (2) medium-to-coarse-grained amphibole after garnet and/or omphacite; and (3) relatively coarse-grained matrix amphibole (Fig. 1c). The coarse-grained amphibole crystals range from 0.6 to 2.0 mm long and display compositional zoning. Petrological and mineralogical investigations of the here investigated eclogites from the Western Tianshan region have indicated peak PT conditions for this eclogite to be 490–570°C and 1.8–2.1 GPa corresponding to an assemblage of garnet + omphacite + phengite + quartz ± paragonite ± Na-amphibole (e.g., Klemd et al. 2002; Wei et al. 2003). Post-eclogite facies conditions are characterized by clinozoisite/zoisite and Na–(Ca)–amphibole growth at the expense of garnet and omphacite, supporting the model of a transition from eclogite to epidote–blueschist facies conditions. Metamorphic conditions in this second stage are estimated to be about 570°C and 0.8–1.4 GPa (e.g., Klemd et al. 2002).
Fig. 1

a Eclogite occurs as lenses or layers in association with blueschist from the western Tianshan. b Photomicrograph of an eclogite composed of garnet, omphibole, paragonite, epidote, amphibole and minor rutile and ilmenite (Sample ws24-7, width of view 2 mm). c Photomicrograph amphibole grains with blue cores and colourless or pale green rim of matrix (Sample ws24-7, width of view 1 mm)

Analytical methods

EMP analysis

The major element compositions of the amphiboles of interest were determined using both a Cameca SX-51 and a JXA-8100 electron microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Analyses were undertaken employing a 5–10 mA beam diameter with an accelerating potential of 15 kV. Ten second counting times were used for all elements. The microprobe analytical standards were provided by P&H Developments’s standards, in which the Na standard is albite, the Mg standard is periclase, the Al standard is corundum. The standard for both Si and Ca was wollastonite, the K standard was orthoclase, the Ti standard was rutile, the Mn standard was rhodonite, the Fe standard was specularite, the F standard was fluorite, and the Cl standard was tugtupite.

Spectroscopic analysis

A double-polished thin section with a thickness of 0.2 mm was prepared for the FTIR and Raman spectroscopic analysis.

Fourier-transform infrared spectra

The FTIR data were recorded using a Bruker IFS 113v spectrometer equipped with an IR scope-II infrared microscope at the Department of Earth Sciences of the University of Cambridge. Spectra were obtained using unpolarized infrared spectra because of small grain sizes and difficulties with orientating and cutting the grains. A liquid-nitrogen-cooled MCT detector was used with a Globar source and a KBr beam-splitter. Micro-IR absorption data in the mid-IR region were obtained from the thin section using a 36× objective and a beam size of 20 μm. The IR spectra were obtained from 512 interferogram scans with an instrumental resolution of 4 cm−1.

Raman spectra

Micro-Raman data were collected with a free-sample-space LabRam micro-Raman spectrometer in the Department of Earth Sciences, University of Cambridge. CCD detectors (1,024 pixels), a grating (1,800 or 1,200 grooves/mm), a 50× ultra-long working distance and a 100× objective were adopted for all measurements. The Raman scattering was excited by a Spectra-Physics model 127 He–Ne laser (633 nm) and the data were recorded in the range between 200 and 4,000 cm−1. Typical spot sizes of analysis in this configuration are of the order of 3 μm. The calibration of the Raman shift was conducted using a polished silicon wafer standard.

Results and discussion

Amphibole chemistry

Three chemical zones in the coarse grained, optically zoned amphibole grain in thin section from sample ws24-7 (Fig. 1c) is revealed by electron microprobe profiles (Table 1). In the BSE image mode (Fig. 2) of the metamorphic amphibole range from blue cores to colorless or pale green rims. The most significant features are the absolute and relative (up to ~60%) variations of Si, Fe, Ca and Na (Table 1), while the analyses of Al2O3, FeO and MgO also show relative coupled variations.
Table 1

Representative chemical compositions of amphibole in the eclogite (sample ws24-7)

 

Na

Na

Na

Na–Ca

Na–Ca

Na–Ca

Ca

Ca

Ca

Analysis

i

h

g

f

e

d

c

b

a

Location

Core toward mantle to rim

SiO2

56.92

56.75

55.48

53.03

50.5

50.6

50.34

48.65

48.71

TiO2

0

0.11

0.01

0.1

0.14

0.16

0.19

0.11

0.14

Al2O3

10.33

10.57

9.71

7.39

8.02

9.02

7.14

7.3

8.89

FeO

12.85

11.08

15.66

9.47

11.38

11.52

12.82

14.09

14.87

Cr2O3

0.03

0.16

0.02

0.04

0.05

0.03

0.02

0

0

MnO

0.03

0.08

0.04

0.02

0

0.01

0.12

0.19

0.07

MgO

9.09

9.73

8.8

15.14

13.39

13.39

13.16

12.64

11.4

CaO

0.27

0.92

1.77

8.84

8.69

8.72

10.02

10.12

9.6

Na2O

7.1

7.22

6.47

2.96

3.12

3.1

2.33

2.28

2.9

K2O

0.02

0.06

0.11

0.03

0.13

0.21

0.16

0.19

0.06

F

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Cl

0.003

0.01

0.00

0.00

0.01

0.00

0.00

0.00

0.00

Total

96.61

96.69

98.07

97.02

95.43

96.76

96.30

95.60

96.64

Si

8.00

7.94

7.85

7.52

7.38

7.30

7.37

7.25

7.18

Ti

0.00

0.01

0.00

0.01

0.02

0.02

0.02

0.01

0.01

Al

1.71

1.74

1.62

1.24

1.38

1.53

1.23

1.28

1.55

TAl

0.00

0.05

0.15

0.47

0.60

0.68

0.61

0.74

0.81

T-site

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

CAl

1.71

1.69

1.47

0.77

0.78

0.85

0.62

0.54

0.74

Cr

0.00

0.02

0.00

0.01

0.01

0.00

0.00

0.00

0.00

Mg

1.90

2.03

1.86

3.20

2.92

2.88

2.87

2.81

2.51

Fe2+

1.51

1.30

1.85

1.12

1.39

1.39

1.57

1.76

1.83

Mn2+

0.00

0.01

0.00

0.00

0.00

0.00

0.01

0.02

0.01

C-site

5.12

5.05

5.18

5.10

5.10

5.12

5.07

5.13

5.09

Ca

0.04

0.14

0.27

1.34

1.36

1.35

1.57

1.62

1.52

Na

1.93

1.96

1.77

0.81

0.88

0.87

0.66

0.66

0.83

BNa

1.96

1.86

1.73

0.66

0.64

0.65

0.43

0.38

0.48

B-site

2.00 (M2+0.03)

2.00

2.00

2.00

2.00

2.00

2.00

2.00

2.00

ANa

0.00

0.10

0.04

0.15

0.24

0.22

0.23

0.28

0.35

K

0.00

0.01

0.02

0.00

0.02

0.04

0.03

0.04

0.01

A-site

0.00

0.11

0.06

0.15

0.26

0.26

0.26

0.32

0.36

Fig. 2

Backscattered electron image and element maps image of the amphibole (Sample ws24-7). a Backscattered electron image. b Ca element mapping image. c Na element mapping image. d Fe element mapping image. The numbers in a and b show the position of microprobe, IR analyses and Raman analyses, respectively. These numbers are also used in is Tables 1, 2, Figs. 4 and 5

Crystal chemical formulae were calculated on the basis of 23 oxygens with all iron considered to be ferrous. The site assignments were carried out using general crystal chemical constraints on amphiboles (e.g., Leake et al. 2004). The tetrahedral sites were filled with Si, Al and available Ti; the resulting sums of the cations at the T sites for all the amphibole samples are close to T8 (Table 1). The Al cations can be hosted at both the T and the C sites. The C sites were then filled with Fe2+, Mn, Cr3+ and Mg; again, summing up all these cations resulted in a total amount of cation at the M(1)-, M(2)- and M(3)-sites very close to the ideal value of 5 (Table 1). The quality of the assignments is revealed by the number of cations at T-, C- and B-sites (Table 1). It is not possible, however, to exclude the presence of ferric iron. Taking in account the sum of cations at C-site one may infer that Fe2+/Fe3+ is close to 9-8:1-2. The B-site occupancies were calculated such as to give Na + Ca = 2 apfu (atoms per fomula unit), using all the available Ca. The remainder of Na was hosted at the A-site together with K (Table 1). According to the classification of Leake et al. (1997), the composition of the amphiboles range from sodic amphibole (Na amphibole) in the core to calcic amphibole (Ca amphibole) in the rim and sodic–calcic amphibole (Na–Ca amphibole) in the intermediate mantle between rim and core (Figs.1c, 2).

The amphibole cores are glaucophane (Gln). This glaucophane, with higher Si, Na contents in the B-site and lower Ca contents that ranged from 7.85–8.00, 1.73–1.96 and 0.04–0.27 a.p.f.u., respectively (Table 1), is similar in composition to that of blueschists and/or eclogites from other HP or UHP metamorphic belts (e.g., Ernst 1961; Carswell 1990; Compagnoni et al. 1995). Compared to the glaucophane, the Na–Ca amphibole shows a large variation in the Na content in the A-site with values that range from 0.15–0.35 a.p.f.u., whereas the low Na content in the B-site ranges from 0.48 to 0.66 a.p.f.u (Table 1). The rim Ca amphibole displays the highest Ca contents, from 1.57 to 1.62 a.p.f.u. and Al contents in the T-site from 0.61 to 0.81 a.p.f.u (Table 1). The Ca amphiboles also show the highest Na content in the A-site, from 0.28 to 0.35 a.p.f.u. Figure 4 and Table 1 indicate that the amphibole chemical range is clustered on two main compositions: a glaucophane end-member (BNa2CM2+3 M3+2TSi8(OH)2) (red circle, the core amphibole), whereas the last six analyses are closed to an intermediate amphibole composition (A0.5BCa1.5Na0.5CM2+4.5 M3+0.5TSi7.5Al0.5(OH)2) between winchite to ferro-winchite) and katophorite to Mg-katophorite (blue circles, the amphibole between mantles and rims).

Core-to-rim-profiles of the amphiboles (Fig. 3; Table 1) reveal increasing Ca, Mg, Al (T-site) values and decreasing Na (B-site) and Al (C-site) values (Fig. 3). At the A-site, Na and K increase with decreasing Si from the core towards the rim. The chemical analyses of the zoned amphibole porphyroblasts reveal that the change in chemical composition from the core (Gln) to the intermediate mantle (Na–Ca amphiboles) takes place through the substitution of NaAlCa−1 Mg−1(Gln vector) (Fig. 3). The compositional changes from the mantle (Na–Ca amphiboles) to the rim (Ca amphiboles) indicate that the amphiboles underwent a tschermakitic substitution (MgSiAl−1Al−1) (Fig. 3). Based on the above compositional division of amphiboles, the gap in the (Na + K) content in the A site may indicate a possible solvus in the Na amphibole–Na–Ca amphibole system (Fig. 3), and suggests possible discontinuities in their growth process (Mével and Kiénast 1986). Our chemical analysis does not reveal detectable traces of F and Cl in the samples.
Fig. 3

a (Na + Si) versus (Ca + Al), bBNa versus M2+ (M2+=Mg + Fe2+), cBNa versus A-site, dBNa versus TAl, eBNa versus CAl, f Ca versus M2+ (M2+=Mg + Fe2+), g Ca versus A-site; h Ca versus TAl

Infrared spectroscopy

The FTIR spectra for the amphibole in the hydroxyl-stretching region (3,600–3,710 cm−1) are shown in Fig. 4, and Table 2 where the positions and relative intensities of the component bands are listed. The prominent features of all spectra are the most intense B and C bands, centred at 3,633 and 3,649–3,652 cm−1, respectively. Both bands are roughly symmetric, relatively narrow and have a constant relative intensity for all amphibole grains (Fig. 4). At the lower-frequency side of these bands, bands A (at 3,613 cm−1) and D (at 3,660 cm−1) are evident. Both of these bands are observable in spectra from the core amphibole. These components have variable intensity, in contrast with the B and C bands (Fig. 4). Band A (at 3,613 cm−1) begins to make its appearance in the core to mantle region of the grains (Fig. 4g) and then disappears in the mantle to rim of the amphibole. The broad nature of band A is interpreted to reflect numerous overlapping bands (Fig. 4e), which corresponds to different configurations due to complex chemistry and associated chemical shifts (Table 1). In the spectrum collected at point f, there is also a very small but clearly visible shoulder at 3,565 cm−1 (A* band).
Fig. 4

Micro-FTIR absorption spectra in the region of the hydroxyl stretching frequencies for the amphiboles (Sample ws24-7)

Table 2

Both band position (cm−1) of FTIR and Raman in the hydroxyl-stretching spectra of the amphibole (ws24-7)

Location

Core

Core–mantle

Mantle–rim

Rim

Analysis

FTIR

Raman

FTIR

Raman

FTIR

Raman

FTIR

Raman

Band

i

h

g

D

f

e

C

d

c

B

b

a

A

A

  

3,613

3,612

3,613

3,613

3,610

      

B

3,633

3,633

3,633

3,633

3,633

3,633

3,633

  

3,633

3,633

3,633

3, 633

B*

       

3,641*

3,641*

 

3,639

3,639

 

C

3,649

3,649

3,649

3,650

3,649

  

3,652*

3,649

3,650

3,649

3,649

3,651

D

3,660

3,660

3,660

3,663

         

E

       

3,672

3,672

3,673

3,672

3,672

3,673

F

          

3,691

3,691

3,697

F*

       

3,696*

3,696*

    

G

          

3,708

3,708

 

The mantle to rim of the amphibole contains higher-frequency hydroxyl absorption bands at approximately 3,672, 3,691 and 3,708 cm−1, which are labeled E, F, and G, respectively (Fig. 4a–d). Compared with the infrared spectra of the core, bands E, F, G seen in the rim are higher in frequency and lower in intensity (Fig. 4, Table 2). From the mantle to the rim, the bands B* and F* shift in frequency slightly, suggesting that they overlap with bands B and C, and bands E and F overlap, respectively (Fig. 4d-a). The IR data show a systematic change in the spectra (Fig. 4i-a), which evolve from lower frequencies (Fig. 4g-i) towards higher frequencies (Fig. 4a-d) progressively from the core to the rim.

The bands B–D (Table 2), which occur in the core of amphibole grains, are ascribed to configurations involving a vacant A-site and a Si-occupied T-site (Table 1): [M(1)M(1)M(3)]–OH–A□: T1SiT1Si–[M(4)M(4)] (Fig.4g-i). According to the unit formula of core (Table 1 point g–i), these point analyses correspond to the presence of a glaucophane configurations, i.e., the B-site is close to fully Na (1.96–1.73 apfu), the T-site is close to fully Si (8.00–7.85 pfu) and the M(2)-site is rich in Al (1.71–1.47 apfu), while the A-site is virtually empty (0.00–0.06 apfu). If one considers the possible presence of Fe3+ it would be expected to be hosted at the M(2)-site as well. Since the M(2)-site is a next nearest neighbouring site to the hydroxyl group its effect on the spectra would be to give a small chemical shift on the order of only some cm−1 (Iezzi et al. 2003b, 2004a, 2005a). Such shifts can be used to identify M-site clustering configurations, since they can results in the presence of 3 or 4 relatively narrow bands due to the substitution of Fe2+ and Mg at M(1)- and M(3)-site. Della Ventura et al. (2005) and Reece et al. (2002) reported local cation distributions of Fe2+ and Mg at the M(1)-, M(3)-sites of magnesio-riebeckite from infrared observations. These differ from glaucophane (Gillet et al. 1989) (where the substitution of Al by Fe3+ occurs at the M(2)-site). Substitutions and M-site clustering in riebeckites give an associated OH mode at 3,669 cm−1 for M(1,3) = MgMgMg, at 3,653 cm−1 for (MgMgFe2+), 3,637 cm−1 for (MgFe2+Fe2+) and 3,619 cm−1 for (Fe2+Fe2+Fe2+). The bands of D, C, B and A spectra of Fig. 4g-i seem to be much closer to these frequencies (Table 3). Furthermore these four spectra do not reveal the presence of band indicating a partial occupation of the A-site (which is, characteristically, a broad band at higher frequency) and/or the presence of some Al at the T-site (Table 1), which correspond to local environments where the O(3)-H dipole points towards an empty A-site (Hawthorne 1983; Della Ventura et al. 1999, 2005; Iezzi et al. 2005c). However, a broad band A (Fig. 4e, f; Table 2) occur in the IR spectrum of the core to mantle region of the amphiboles, which associated with an A-site composition with (Na + K) = 0.15 or□ = 0.85 and the presence of partial TAl. This could potentially involve a range of configurations of Mg, Fe2+ and Al at the M(1)- and M(3)-sites. However, the band frequency is the same for all the spectra recorded (3,613 cm−1). This implies that only one cation configuration around the O–H bond is present (possibly a disordered configuration?). The A bands disappear from the intermediate mantle to the rim and/or core (Fig. 4e-f), closely correlated with the apparent discontinuities of mineral chemistry from core to rim.
Table 3

Observed vibration bands and their assigned cation configurations

Band

Position(cm−1)

Assigned configuration

A

3,610–3,613

Fe2+Fe2+Fe2+–OH–A(Na + K)□:T1SiT1Al–NaCa

B

3,633–3,639

MgFe2+Fe2+–OH–A□:T1SiT1Si–NaNa

C

3,646–3,649

MgMgFe2+–OH–A□:T1SiT1Si–NaNa

D

3,658–3,660

MgMgMg–OH–A□:T1SiT1Si–NaNa

E

3,672

MgMgMg–OH–A□(Na + K):T1SiT1Al-CaNa

F–H

3,691–3,708

[M(1)M(1)M(3))–OH–A(Na + K):T1SiT1A–CaNa

All the other spectra (a–d) consist of broader bands, compared to the previous ones. Each of them reveals the presence of higher frequencies absorptions (E: 3,672 cm−1, F: 3,691–3,698 cm−1, G: 3,708 cm−1), that in turn suggest the presence of partial occupation of the A-site. Moreover, the EPMA data (a–d) reveal the presence of significant TAl (0.61–0.81 apfu) coupled with an A-site occupancy ranging between 0.26 and 0.36 apfu. Previous infrared spectroscopic studies of synthetic amphiboles have reported that OH-stretching frequencies are sensitive to the specific type of A-site cation. When the M(1, 3) sites are only occupied by Mg: OH stretching bands, that occur around 3,670–3,675 cm−1 for (MgMgMg)–OH–A□ while they appear around 3,730–3,735 cm−1 for (MgMgMg)–OH–ANa (Robert et al. 1989; Hawthorne et al. 1996a, b, 1997; Welch et al. 1994). However, the Tianshan amphibole has its highest frequency bands at H (3,708 cm−1) and G (3,691 cm−1) (Fig. 4). The presence of bands at 3,691–3,708 cm−1 is similar to observations of the spectra of pargasite (e.g., Della Ventura et al. 1999; Jenkins et al. 2003) which are attributed to full A-site configurations but probably involving Al at the T1 sites. Substitution of Na at the M(4) site will definitely not shift the spectra to higher frequencies but to lower frequencies, as predicted by the theory of Palin et al. (2003) and confirmed experimentally by Jenkins and Corona (2006). On the basis of previous work, the E band centred at ~3,672 cm−1 (Fig. 4d-a; Table 2) can be assigned to a M(1),(3)(MgMgMg)–O(3)–H–A□–TSi8 configuration (e.g., Iezzi et al. 2007), i.e., to the same cation arrangement responsible for the D component. This does not appear to agree with the micro-chemical data, however, (Table 1). Hence we are forced to search for other possible configurations. Three possible choices appear to be consistent with the observed for the frequency position of the E band: (1) the presence of Al replacing Si at the tetrahedral rings; (2) A site occupied by an alkaline cation (Na and/or K); and (3) the substitution of (Na, Ca2+) into M(4) (Table 3).

As discussed above, the substitutions in the zoned amphibole are best described by the charge- and site-balanced exchange components: Si (T-site) (Mg, Fe) [M(1, 3)-site] → Al (T-site) Al [M(1, 3)-site] (tschermakite), Ca [M(4)-site]□ (A-site) → Na [M(4)-site] Na + K (A-site) (richterite), and Ca [M(4)-site] (Mg, Fe) [M(1, 3)-site] → Na [M(4)-site] Al [M(1, 3)-site] (glaucophane) from configurations (Table 3). Although Cl and F substitution are found in amphibole and may complicate the OH spectra due to the chemical substitution, our analyses are not affected by these elements because the samples show no detectable amounts of F and Cl (Table 1).

Raman spectroscopy

To further examine the OH species and any possible local variations of structure, micro-Raman spectra of the same zoned amphibole were also recorded at room temperature in the spectral region of 3,900–3,300 cm−1 (Fig. 5).
Fig. 5

Spectra in the OH-stretching region between 3,300 and 3,900 cm−1 (Sample ws24-7)

The evolution of OH speciation within the zoned amphibole suggested by the FTIR spectra is confirmed by the Raman data in Fig. 5, the OH-stretching vibrations of the zoned amphibole give rise to the Raman bands between 3,600 and 3,700 cm−1 (Table 2). The core contains four relatively narrow hydroxyl absorption bands at 3,612, 3,633, 3,650, and 3,663 cm−1, respectively (Fig. 5d; Table 2). Figure 5b and c show Raman spectra of the mantle amphibole, which reveals that the 3,663 cm−1 mode disappears and the 3,672 cm−1 mode appears in this region (Table 2). The Raman spectra in rim amphibole show relatively higher frequencies absorptions (Fig. 5a; Table 2). These data are in good agreement with results of FTIR spectra (Table 2), and point to further possibilities for understanding amphibole crystal chemistry from convenient and rapid Raman analysis, including the possibility of spatial mapping of spectral features.

Conclusions

  1. 1.

    Natural amphiboles from our high-pressure metamorphic rocks show complex structure and zoning compositional features. These amphiboles occur as inclusions or symplectite of early high pressure minerals,which are typically 0.5 or 1 μm or less in size, and therefore not amenable to EPM analysis. The IR and Raman data reveal different OH features for the core and the rim. The results indicate a correlation between chemical and cations compositions and the frequencies of OH bands for the zoned amphiboles. This study has shown the potential of analyzing high-pressure amphibole phases by FTIR and/or Raman spectroscopy of chemical shifts revealed in the OH-stretching modes. The combination and comparison of data between chemical derived (EPM) and FTIR/Raman (in the OH-stretching region) data allows the identification of cation ordering and partitioning patterns across the interior of a single zoned grain.

     
  2. 2.

    The major chemical evolutions recorded by zoning patterns in amphiboles from western Tianshan, obtained by combination and comparison of data between micro-chemical (EPM) and FTIR/Raman in the OH-stretching region data, are: Si (T-site) (Mg, Fe) [M(1, 3)-site] → Al (T-site) Al [M(1, 3)-site] (tschermakite), Ca [M(4)-site]□ (A-site) → Na [M(4)-site] Na + K (A-site) (richterite), and Ca [M(4)-site] (Mg, Fe) [M(1, 3)-site] → Na [M(4)-site] Al [M(1, 3)-site] (glaucophane).

     
  3. 3.

    A conspicuous compositional gap exists at the distinct compositional boundary between the Na and the Na–Ca amphibole zones. Although the chemical composition of the amphiboles indicates continuous growth, it is possible that the composition of the amphiboles was altered, especially during the Na–Ca amphibole growth.

     

Notes

Acknowledgments

This work was supported by the NNSFC (No.40872059, 40572028), National Basic Research Program of China (No.2007CB411302 and 2009CB825001) and the Key Laboratory of Continental Dynamics in Northwest University. We are especially grateful to Dr. T. Zack, Dr. M. Gottschalk, Dr. G. Della Ventura and Dr. G. Iezzi for their constructive and helpful suggestions of this manuscript.

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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • Wen Su
    • 1
    • 2
    • 3
  • Ming Zhang
    • 3
  • Simon A. T. Redfern
    • 3
  • Jun Gao
    • 1
  • Reiner Klemd
    • 4
  1. 1.Chinese Academy of SciencesState Key Laboratory of Lithospheric Evolution, Institute of Geology and GeophysicsBeijingChina
  2. 2.State Key Laboratory for Mineral Deposits ResearchNanjing UniversityNanjingChina
  3. 3.Department of Earth SciencesUniversity of CambridgeCambridgeUK
  4. 4.Institute of MineralogyWuerzburg UniversityWuerzburgGermany

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