Disequilibrium thermal breakdown of staurolite: natural example

Disequilibrium thermal breakdown of staurolite: � natural example

RODNEY GRAPES1,* and XU-PING LI2

1 Department of Earth and Environmental Sciences, Korea University, Seoul, 136-713, Korea*Corresponding author, e-mail: [email protected]

2 College of Geological Science & Engineering, Shandong University of Science and Technology, Qingdao,266510, China

Abstract:A partially melted pelitic xenolith, Wehr volcano, East Eifel, Germany, contains staurolite porphyroblasts pseudomorphedby a hercynite-rich assemblage that includes ferrogedrite, sillimanite-mullitess, quartz and siliceous, peraluminous glass, which can beschematically represented by the reaction, staurolite! hercynite þ gedrite þ Al-silicate þ quartz þ melt. Minerals and glass of thepseudomorph are a disequilibrium breakdown assemblage that represents a time-temperature-transformation of staurolite duringshort-term heating and cooling. Microstructural evidence indicates the hercynite formed first together with melt, followedby ferrogedrite, and crystallization of Al-silicate and quartz from the melt. The absence of cordierite, the presence of ferrogedrite(a possible metastable intermediate phase to formation of almandine) in the staurolite breakdown assemblage and evidence formelting of muscoviteþ quartz and plagioclase (An20)þ quartz in the xenolith indicates minimum pressure conditions of 0.35 GPa atca. 665–700 �C with probable overstepping of staurolite stability by 40–95 �C. Such conditions were kinetically favourable for theformation of the metastable breakdown assemblage that was preserved by quenching on eruption.

Key-words: xenolith,Wehr volcano, East Eifel, disequilibrium staurolite breakdown, hercynite, gedrite, sillimanite-mullitess, quartz,glass, time-temperature-transformation, reaction overstepping.

1. Introduction

In rocks that have undergone time-temperature histories ofrapid heating and cooling, i.e. contact aureoles surroundingsmall igneous intrusions and fragments of country rockincorporated as xenoliths in mafic magmas, minerals typi-cally undergo breakdown reactions by melting and crystal-lization of metastable phases (e.g. Grapes, 2006, andreferences therein). Disequilibrium reactions are preservedin the form of partial mineral transformations, microstruc-tures, metastable phases and glass (¼melt). Such transfor-mations typically follow the Ostwald Step Rule whichpredicts that in any reaction the kineticallymost favourablesequence of phases (including melt) will form, rather thanthose with the lowest free energy. The formation of inter-mediate metastable phases is well known from experimen-tal kinetic studies of mineral breakdown reactions, e.g.enstatite (Greenwood, 1963); chlorite (Cho & Fawcett,1986), muscovite (Rubie & Brearley, 1987; Brearley &Rubie, 1990), biotite (Brearley, 1987a,b), and the presenceof water enhances the rate of metastable melting relative tothat of a predicted equilibrium assemblage (e.g. Rubie &Brearley, 1987).

Staurolite occurs as porphyroblasts within aluminousschist xenoliths of regional metamorphic basement originin a trachytic tephra unit (Wehr III) of the Wehr volcano,

East Eifel area, Germany. These and other basement-derived xenoliths have been subjected to high-tempera-ture/low-pressure metamorphism as described by Worneret al. (1982), Worner & Fricke (1984) and Grapes (1986,2003, 2006). In some extensively melted xenoliths(buchites), staurolite is partially replaced by hercynite; inothers that show only a low degree of melting (10–15 %),Worner et al. (1982) noted a breakdown assemblageof spinel-cordierite-sillimanite on the basis of X-raydiffraction analysis. Apart from the spinel, it is difficultto optically resolve the other phases that replace stauroliteand in this paper we use EPMA backscattered imaging(BEI) and quantitative analysis to describe another staur-olite replacement assemblage associated with glass inorder to establish the nature, reaction pathway and condi-tions of staurolite breakdown.

2. Petrography

The focus of this study is a xenolith of garnet-stauroliteschist with alternating quartz- and mica-rich layers derivedfrom regionally metamorphosed high-T/low-P amphibo-lite-facies basement rocks underlying the East Eifel area(Worner et al., 1982). Porphyroblasts consist of garnet

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Eur. J. Mineral.

2010, 22, 147–157

Published online October 2009

(zoned from Alm68.1Py3.9Sp22.4Gr5.6 core to Alm75.4

Py5.2Sp6.4Gr13.0 rim, with narrow overgrowths of Alm82.3

Py15.7Sp1.8Gr0.2) and a hercynite-rich pseudomorphdescribed below. The primary non-porphyroblastic assem-blage in the schist is quartz, minor oligoclase (An19–21),muscovite, biotite, chloritoid, tourmaline, zircon, apatite,spinel, ilmenite, rutile, and graphite. A colourless glassoccurs between quartz grains, between quartz and plagio-clase and along quartz-muscovite contacts indicating thebeginning of fusion of these minerals (Grapes, 1986).Glass along cleavage planes and margins of muscovitecontains spinel and needles of Al-silicate. Optically, biotiteoften appears black to dark brown due to the presence offine magnetite and possibly pleonaste spinel indicative ofoxidation during dehydroxylation (e.g. Grapes, 1986;Brearley, 1987b). Chloritoid also appears black to verydark green due to the presence of abundant magnetite.Partial melting of muscovite, quartz and plagioclase,oxidation of biotite and chloritoid, and thin overgrowthson garnet are evidence of a contact metamorphic overprinton the regional amphibolite-facies assemblage in thexenolith.

One of the totally replaced (pseudomorphed) porphyro-blasts in the xenolith is shown in Fig. 1a. About 70% of theporphyroblast consists of dimensionally-orientated darkgreen spinel that also occurs as crosscutting veins (inter-preted as growth of hercynite along cracks) (Fig. 1b). Grainsize is highly variable and growth habits extremely irregu-lar (Fig. 2a). Because of this the spinel forms complexintergrowths with orthoamphibole (Fig. 1b, 2b) that oftenencloses smaller spinel grains or narrow extensions oflarger grains (Fig. 2b). Interstitial areas to spinel andorthoamphibole are occupied by a pale yellow glass thatcontains euhedral crystals of aluminium silicate with glassinclusions, and newly formed subhedral-anhedral quartz,as distinct from quartz inclusions, that invariably partlysurround Al-silicate (Fig. 2b). Unmelted, oxidised biotiteoccurs around the margins of the porphyroblast, and ilme-nite and quartz occur as inclusions (Fig. 1a). The size,habit, textural relations of the porphyroblast and mineralinclusions are the same as porphyroblasts of unreactedstaurolite in other Wehr schist xenoliths and we concludethat the example shown in Fig. 1a is completely replacedstaurolite.

Fig. 1. (a) Back-scattered electron image (BEI) of staurolite porphyroblast extensively replaced by hercynitic spinel in a partly melted peliticxenolith, Wehr volcano, East Eifel, Germany. Fine-grained phases in dark interstitial areas to hercynite are not resolved at these magnifica-tions but are shown in Fig. 2a. Qtz ¼ quartz; Ms ¼ muscovite (partially melted); Bt ¼ biotite; Grt ¼ garnet. Elongate bright grains inporphyroblast are ilmenite. Light grey areas within biotite are spinel-rich. Muscovite is characterised by abundant thin black striations that areglass-lined cleavage planes associated with tiny bright grains of spinel. (b). High-contrast BEI enlargement of part of (1a) showingdimensional orientation of hercynite (white), probably parallel to the {010} cleavage of staurolite, and cross-cutting veins of hercynitethat presumably represent original cracks in the staurolite porphyroblast. The hercynite is intergrown with gedrite (light grey).

148 R. Grapes, X.-P. Li

3. Mineralogy

3.1. Analytical methods

Electronmicroprobe analyses (WDS) of minerals and glassof the porphyroblast were carried out using a JEOL JXA-733 SuperProbe, at Victoria University of Wellington,New Zealand, using Moran Scientific software that incor-porates modified Bence-Albee and ZAF matrix correc-tions. Accelerating voltage was 15 kV using an emissioncurrent of 10 nA. The analysing spot was 1–2 micrometresfor mineral and 10 (sometimes 20) micrometres for glass.All analysis positions were located using BEI. Natural and

synthetic mineral standards were used. Mineral and glassanalyses are listed in Tables 1 and 2, respectively.

3.2. Staurolite

No staurolite is preserved in the studied sample, butporphyroblastic staurolite analyzed from another Wehrschist xenolith is compositionally homogeneous, Fe-rich[XFe � 0.79], and contains �0.5 wt.% ZnO. The formulacalculated on the basis of 48(O), 4 hydroxyls and all ironas FeO is H4(Fe,Mn,Mg,Zn)4.2Al17.5Si7.8O48. The stauro-lite is associated with graphite and ilmenite (as in the

Fig. 2. (a) High-contrast BEI showing hercynite growth habit and compositional zoning related to Si-enrichment (darker grey areas)(Table 1). Black ¼ glass (in hercynite) and gedrite þ Al-silicate þ quartz þ glass (interstitial to hercynite). (b) BEI showing texturebetween hercynite (Hc), gedrite (Gd), Al-silicate (Als) (sillimanite-mullitess), quartz (Qtz) and glass (Gl) in the pseudomorphed porphyr-oblast shown in Fig. 1a. Note that Al-silicate and quartz are confined to interstitial areas of glass and that Al-silicate contains glass inclusions.Quartz surrounds Al-silicate.

Disequilibrium thermal breakdown of staurolite 149

Table1.EPMAanalysesofstauroliteandpseudomorphmineralsin

Wehrxenoliths.

Staurolite

Spinel

Gedrite

‘‘Quartz’’

Al-silicate

Analyses(n)

71s

SiO2

27.94

0.15

0.70

1.08

1.16

1.38

1.52

2.69

37.92

38.90

40.11

41.50

94.22

96.00

95.40

97.27

96.87

36.60

35.84

34.14

31.18

30.69

TiO

20.57

0.02

0.02

0.02

0.05

0.02

0.02

Al 2O3

53.35

0.26

60.55

60.21

59.08

59.45

59.61

60.99

26.02

22.42

21.00

20.75

3.82

2.79

2.33

1.86

1.50

58.13

60.10

62.31

65.62

64.86

Fe 2O3*

3.90

3.34

2.74

2.55

3.56

FeO

**

12.85

0.10

35.74

36.19

36.22

35.73

34.92

33.65

29.00

30.98

29.28

30.53

1.54

1.20

1.39

0.97

0.90

MnO

0.48

0.03

0.20

0.22

0.17

0.41

0.30

0.41

0.40

0.65

0.77

0.78

MgO

2.07

0.06

2.70

2.64

2.51

3.10

2.98

2.60

4.05

3.95

4.98

4.71

0.18

0.12

0.11

0.08

0.04

0.83

0.81

0.58

0.67

0.73

ZnO

0.54

0.04

0.08

0.04

0.06

0.04

0.07

0.05

Na 2O

0.23

0.29

0.38

0.27

H2O

2.16

Total

99.96

99.97

100.38

99.20

100.11

99.40

100.39

97.62

97.19

96.52

98.54

99.76

100.11

99.23

100.18

99.31

99.48

100.11

99.82

100.04

99.86

48(O

)4(O

)23(O

)2(O

)5(O

)

Si

7.757

0.020

0.030

0.033

0.039

0.043

0.074

5.710

5.962

6.138

6.232

0.958

0.969

0.972

0.979

0.982

1.008

0.980

0.936

0.855

0.847

Al

17.456

2.002

1.985

1.976

1.963

1.976

1.975

4.619

4.051

3.789

3.673

0.046

0.033

0.028

0.022

0.018

1.886

1.936

2.012

2.121

2.110

Fe3

þ0.081

0.069

0.056

0.053

0.074

Fe2

þ2.983

0.838

0.846

0.859

0.837

0.820

0.773

3.652

3.971

3.748

3.834

0.013

0.010

0.012

0.008

0.008

Mn

0.113

0.005

0.005

0.004

0.010

0.007

0.010

0.051

0.084

0.100

0.099

Mg

0.857

0.113

0.110

0.106

0.130

0.125

0.106

0.909

0.902

1.136

1.054

0.003

0.002

0.002

0.001

0.001

0.034

0.033

0.024

0.027

0.030

Ti

0.119

0.001

Zn

0.111

0.002

0.001

0.001

0.001

0.001

0.001

Na

0.076

0.086

0.113

0.079

OH

4.000

P2.980

2.977

2.979

2.980

2.970

2.939

15.018

15.056

15.024

14.971

1.020

1.014

1.014

1.010

1.009

3.009

3.018

3.029

3.056

3.061

XFe

0.78

0.88

0.88

0.89

0.86

0.87

0.88

0.80

0.81

0.77

0.78

Si/R3þ

0.51

0.49

0.45

0.39

0.39

*Allironas

Fe 2O3;**Allironas

FeO

Spinelendmem

bers

Fe 2SiO

42.0

3.0

3.3

3.9

4.3

7.3

FeA

l 2O4

81.4

80.9

81.9

79.0

69.6

68.6

MgAl 2O4

11.2

10.9

10.5

12.9

12.6

10.4

MnAl 2O4

0.5

0.5

0.4

1.0

0.7

1.0

ZnAl 2O4

0.2

0.1

0.1

0.1

0.1

0.1

Al 2O3

3.7

3.5

2.7

2.2

11.2

9.5

Vacancy

1.0

1.2

1.1

0.8

1.5

3.1


xenolith containing pseudomorphed staurolite) and isinferred to contain negligible Fe3þ. The recalculated com-position lies within the range of Fe3þ-poor staurolitesgiven by Holdaway et al. (1991).

3.3. Spinel

Spinel is hercynite-rich (XFe ¼ 0.86–89; 69–82 mol%FeAl2O4) with smaller amounts of MgAl2O4 (10–13 mol%) and minor MnAl2O4 (0.4–1.0 mol%). The ZnOcontent is very low at 0.04–0.08 wt.%. The spinel is unu-sual in having 0.70–2.70 wt.% SiO2 and is zoned to silica-rich rims (Fig. 2a). Silica in magnetites has been recordedas a Fe2SiO4 component (e.g. Shcheka et al., 1977;Newberry et al., 1982), but the presence of silica (inamounts . 0.6 wt.%) in the hercynite poses a problemfor estimation of Fe2O3 as recalculation on the basis of3 cations results in negative Fe3þ values. With all iron asFeO, oxide totals of the hercynite vary between 99.2 and100.4 wt.% and cation totals per 4(O) range between 2.94(highest Si) and 2.98 (lowest Si) which suggests the pre-sence of some Fe2O3 and/or the possibility of vacancies inthe octahedral site. Recasting the analyses into spinel end-member compositions, and assuming silica is present as ag-Fe2SiO4 component (2.0–7.3 mol%), yields excessAl2O3 of 2.2 to 9.5 mol% implying a limited solid solutiontowards corundum and vacancies of between 0.8 and3.1 suggesting operation of the exchange vectors

(Fe.Mg)�3Al2&vacancy and Fe�6Si�3Al8&vacancy. Non-stoichiometric magnesian spinels with excess (Al,Cr)2O3

up to 20.5 mol% and vacancies up to 2.6 have beenrecorded by Pedersen (1978) in a graphitic shale xenolith.

3.4. Orthoamphibole

The orthoamphibole intergrown with hercynitic spinel isferrogedrite (Fig. 2b) with the average compositionNa0.1(Fe3.8Mn0.1Mg1.0Al2.0)6.9(Si6.0Al2.0)8.0O22(OH)2 andXFe ¼ 0.78–0.81. Individual grains are zoned with higherAl2O3 near contacts with spinel (up to 26 wt.%) comparedwith central parts of the grains or areas in contact with glassthat have �20 wt.% Al2O3.

3.5. Aluminium silicates

There is considerable replacement of Al by Fe3þ inthe Al-silicate with Fe2O3 contents ranging from 2.6 to3.9 wt.%. The ratio of Si/R3þ (where R3þ ¼ Al þ Fe)varies from 0.51–0.39, i.e. between ideal cation ratios of0.5 (sillimanite) and 0.333 (mullite), and on the basis of5(O) cation totals deviate from near the ideal 3.000 (silli-manite) to 3.061. Thus, some Al-silicate grains havecompositions intermediate between sillimanite(Al2SiO5) and ideal mullite (3Al2O3�2SiO2) (Cameron,1976, 1977a and b). Significant is the high amounts of

Table 2. EPMA analyses and norms of glass.

S S S Q/M Q/P

SiO2 75.93 75.33 76.99 76.05 78.36TiO2 0.03 0.05 0.02 0.16 0.04Al2O3 11.76 12.32 11.87 15.29 11.58FeO** 4.28 3.02 3.90 1.16 0.62MnO 0.16 0.16 0.17 0.15 0.00MgO 0.40 0.35 0.49 0.28 0.12CaO 0.08 0.61 0.17 0.50 0.26Na2O 0.59 1.19 1.23 1.29 2.15K2O 1.55 3.45 1.86 2.12 3.36

Total 94.78 96.48 96.70 97.00 96.49

NormQ 62.74 51.20 58.81 58.14 51.86C 8.97 5.52 7.53 9.97 3.94Or 9.16 20.39 10.99 12.53 19.86Ab 4.99 10.07 10.41 10.92 18.19An 0.40 3.03 0.84 2.48 1.29Hy 7.71 5.64 7.38 2.46 1.17Mt 0.82 0.58 0.75 0.22 0.12Ilm 0.06 0.09 0.04 0.20 0.08A/(CNK) 4.25 1.83 2.70 2.83 1.50

Norms calculated on basis of Fe2O3/FeO ¼ 0.15.** All iron as FeO.S ¼ in pseudomorphed staurolite porphyroblast.Q/M ¼ along quartz-muscovite contact.Q/P ¼ along quartz-plagioclase contact.


MgO (0.51–0.91 wt.%) in the Al-silicate, i.e. muchgreater than a maximum of 0.21 wt.% analysed byOkrusch & Evans (1970) in hornfels sillimanite and anda-lusite (see also Deer et al., 2006). TiO2 is below0.05 wt.%.

3.6. Quartz

The newly formed quartz contains significant amounts(wt.%) of Al2O3 (1.50–3.82) together with FeO (0.90–1.54) and MgO (0.04–0.18). No alkalis were detected inthe BEI-positioned analysis spots indicating that Al, Fe andMg contents of the quartz are not the result of glass con-tamination. In contrast, inclusions of quartz within theporphyroblast (Fig. 1a) are pure SiO2. The quartz reactionproduct is probably metastable quartz in which Si4þ issubstituted by Al3þþ 0.5(Fe,Mg)2þ (e.g. Schreyer &Schairer, 1961a, b).

3.7. Glass

Glass within the pseudomorph is siliceous (75.3–77.0 wt.%SiO2), with 2.1–4.6 wt.% alkalis, 3.0–4.3 wt.% FeO,normative corundum between 5.5 and 9.0 %, and Al2O3/(CaO þ Na2O þ K2O) (A/(CNK) of 1.8–4.3 (Table 2).Glass along quartz-muscovite contacts adjacent to theporphyroblast (Fig. 1) has A/(CNK) of �2.8, �10 % nor-mative corundum, 3.4 wt.% alkalis. Glass along quartz-plagioclase (An20) contacts has high SiO2 (78.4 wt.%),A/(CNK) of 1.50, 3.9 % normative corundum and5.5 wt.% alkalis.

4. Discussion

4.1. Staurolite breakdown reaction: textural evidence

The earliest stage of staurolite breakdown is shown in abuchitic staurolite schist xenolith with the formation ofhercynite and melt (glass) along cracks, grain bound-aries and some cleavage planes and strongly embayedcontacts between staurolite and glass (Fig. 3) indicatingmelting. As a discrete (mass balance) process, thisshould be strongly degenerate in the FMASHO system(where ‘‘O’’ accounts for redox reactions involvingFe2þ and Fe3þ) with colinearity between hercynite-staurolite -melt that requires the melt to be extremelyAl-rich. As shown in Fig. 4, this is not the case. Mass isnot conserved and the melt is silica-rich and Al-poorindicating open system behaviour as implied by theabundance of glass shown in Fig. 3.

In the schist xenolith containing pseudomorphedporphyroblasts of staurolite, hercynite is the dominantreaction product (ca. 70 %) (Fig. 1a). Dimensionalorientation of most hercynite in the porphyroblast(Fig. 1b) may have been controlled by expansion along{010} cleavage planes during dehydroxylation of thestaurolite with heating (e.g. Gibbons et al., 1981). This

would have provided optimal low activation energy sitesfor the nucleation and growth of hercynite, the kineticallymost favourable phase to form with increasing tempera-ture. Reaction along cleavage planes is also likely to havebeen relatively fast because nucleation and growth werepromoted by rapid diffusion (e.g. Brearley, 1987a), par-ticularly if melt is present as in this case. Substitutionof Si in hercynite of the porphyroblast, particularly inrims that are in contact with glass and inclusions ofglass (Fig. 2a; see also Fig. 3), imply that it grew incontact with melt.

Textural relations between gedrite and hercynite(Fig. 2b) suggest that gedrite formed later, probablywhen temperature reached an appropriate level (possi-bly slight cooling) allowing it to nucleate along inter-faces between hercynite and remaining staurolite. Thegedrite intergrowths therefore have a crystallographicorientation that is determined by hercynite. Delayedformation of Fe-Mg silicate with respect to an oxidephase is also observed during an experimental andkinetic study of biotite melting at 800 �C where ortho-pyroxene forms after spinel (Brearley, 1987a). Possiblereasons for this include requirement of an incubationperiod before a critical nucleus can form and thatnucleation only occurs when spinel-reactant silicateinterfaces have formed to provide high-energy nuclea-tion sites (Brearley, 1987a). The presence of Na ingedrite implies that it grew in contact with melt asalso indicated by euhedral growth extensions intoglass (Fig. 2b). There are no textures showing

Fig. 3. BEI showing staurolite (St) breakdown to hercynitic spinel(Hc; white grains) and melt (glass) along cracks in a buchitic peliticxenolith, Wehr volcano, East Eifel, Germany. Grt ¼ garnet; Bt ¼biotite; Ilm ¼ ilmenite; Gl ¼ glass. The staurolite exhibits stronglyembayed contacts with glass indicating melting. Note that most ofthe hercynite grains contain inclusions of glass (darker grey spots).Biotite shows the initial stage of partial melting with the develop-ment of melt-lined cracks that sometimes contain small granules ofpleonaste spinel (Bt, lower right side of photo). Although disruptedby glass, almandine-rich garnet (Grt) is unaltered. The web-likepattern of glass around vesicles (black) is the result of H2O exsolu-tion from the melt when the xenolith was erupted.


intergrowths between gedrite and Al-silicate to indicatethe relative timing of their formation after hercynite,but Al-silicate crystallized from the melt as evidencedby its euhedral habit and glass inclusions (Fig. 2b).Newly formed quartz is always associated withAl-silicate (Fig. 2b) suggesting that it formed as aresult of local Si-saturation caused by crystallizationof Al-silicate.

Zoned hercynite and ferrogedrite, Al-silicate composi-tions intermediate between sillimanite and mullite,‘‘impure’’ quartz and glass that pseudomorph stauroliterepresent a quenched breakdown assemblage that can beschematically described as

Stþ O2 ! Hcþ Gdþ Silþ Qtzþ L (1)

Fig. 4. Compositions of hercynite, ferrogedrite, sillimanite-mullitess, ‘‘quartz’’ and glass (Tables 1,2) in pseudomorphed porphyroblast(Fig. 1a) together with staurolite composition from another schist xenolith, Wehr volcano, plotted in terms of mol% (Fe,Mg)O-(Al,Fe)2O3-SiO2. Dashed tie-lines suggest a possible stable (melt-absent) breakdown assemblage of staurolite in a degenerate FMASH system. Upperdiagram: mineral compositions in the system FeO-MgO-Al2O3-SiO2-H2O (FMASH) projected on to the mol% (Fe,Mg)O-Al2O3-SiO2 plane.


Despite evidence of metastable melting, the pseudo-morph mineral assemblage could mimic that of a possiblestable melt-absent staurolite breakdown reaction

St ¼ 3Hcþ 0:2Gdþ 5:6Silþ 1:2Qtzþ 1:8V (2)

as represented by the analyzed mineral compositionsplotted in Fig. 4. However, the stoichiometric coefficientsin the reaction are clearly at variance with the modal com-position of the staurolite pseudomorph. The discrepancymay be explained by the presence of siliceous peraluminousmelt from which Al-silicate and quartz crystallized.

Staurolite breakdown proceeded together with partialmelting of muscovite, and plagioclase and the melt com-position of reaction (1) has been modified by elementdiffusion from these phases. The high alkali content ofglass within the porphyroblast (2.1–4.6 wt.%; Table 2)implies K from muscovite, that also contains between1.47–1.72 wt.% Na2O, with additional Na and smallamounts of Ca (0.08–0.81 wt.% CaO) from plagioclase(�An20) dissolution in the presence of quartz via anintergranular film of melt. High FeO (3.0–4.3 wt.%) andlower MgO (0.35–0.49 wt.%) of glass in the porphyro-blast are inferred to be mainly derived from staurolite,as pleonaste spinel is a reaction product of muscovitemelting. Because the melt contains K, Na and Ca frommuscovite and plagioclase melting, the staurolite break-down reaction could also be described in terms of theKNCFMASHO system, e.g.,

StþMsþ Plþ O2 ¼ Hcþ Gdþ Alsþ Qtzþ L (3)

The textural evidence described above leads to theinterpretation that the staurolite breakdown assemblagein the Wehr xenoliths is an example of a time-tempera-ture-transformation (TTT) (Putnis & McConnell, 1980)with the sequence hercynite ! gedrite/Al-silicate !quartz in the presence of melt. We suggest that thissequence reflects formation of hercynite during an initialperiod of rapid temperature increase and maximum over-stepping of staurolite stability followed by an interval ofslower cooling when gedrite, Al-silicate and quartzformed. Such metastable time-temperature mineral trans-formations often involve melting and are preserved inrocks that have undergone rapid heating and cooling asin this case (e.g. Brearley, 1986, 1987a; Grapes, 1986,2003; Worden et al., 1987; Cesare, 2000). An alternativeinterpretation that staurolite breakdown took place wellabove equilibrium temperature as a single, rather than astepped, reaction process is a possibility but seems to beprecluded from the textural evidence.

4.2. Comparison with staurolite-breakdown reactions

Subsolidus breakdown of staurolite to hercynite-bearingassemblages has been recorded in thermal aureoles(Pattison & Tracy, 1991, and references therein). In pelitichornfels, Atkin (1978) describes hercynite in cordierite

that occasionally encloses small irregular staurolite grains,and complex intergrowths of staurolite, hercynite andsillimanite in cordierite, that suggest the reaction

St ¼ Hcþ Crdþ Sil (4)

In another contact aureole, Cesare (1994) proposedthe reaction

Fe-St ¼ Hcþ Silþ Qtzþ V (5)

to explain clusters of hercynite grains surrounding relics ofstaurolite in sillimanite. In this example the absence ofquartz suggests removal of silica from the reaction sitetogether with H2O released by staurolite dehydration.Unlike the Wehr xenolith, there is no evidence of meltingin these examples and hercynite does not pseudomorphstaurolite.

The Wehr xenolith appears to be the only known nat-ural example where orthoamphibole is a replacementphase of staurolite, although its formation has beendemonstrated experimentally. Richardson (1968) notedthe presence of small amounts of orthoamphibole (analuminous member of the ferro-anthophyllite–gedriteseries) in experimental runs at 650 and 700 �C/0.5 and0.45 GPa, respectively, to produce Alm þ Sil and Crd þSill from St þ Qtz. He suggested that, although this mayindicate a stability field for an amphibole-bearing assem-blage, it is probably metastable. This raises the questionas to whether Fe-gedrite in the staurolite pseudomorphmight be a metastable intermediate phase in the formationof almandine, and that the stable staurolite breakdownreaction (which excludes quartz) is,

Fe-St ¼ Hcþ Almþ Silþ V (6)

or cordierite (reaction 4)(Fig. 4), depending on pressure.However, a stability field of aluminous ferro-orthoamphi-bole (approximating the gedrite compositionFe5Al2Si6Al2O22(OH)2) coexisting with sillimanite andquartz has been delineated by Grieve & Fawcett (1974) atpressures . 0.34 GPa between maximum temperatures of�660–710 �C (QFM buffer conditions), bounded on its lowtemperature side by the reaction, St þ Qtz ¼ OAm þ Sil(Fig. 5). At the same pressure, but at considerably lowertemperatures, aluminous ferro-orthoamphibole, togetherwith staurolite, first appears as a reaction product of Cldþ Qtz (Fig. 5), and stable St þ Gd assemblages have beenrecorded from regional metamorphosed pelitic and maficrocks (e.g. Hudson & Harte, 1985; Moen, 1991). An upperstability of St þ Gd is indicated by the formation of smallgranules of spinel, magnetite and locally cordierite alongmutual grain boundaries suggesting the reaction StþGd¼Crd þ Sp � Mt (Sharma & MacRae, 1981). In the Wehrxenolith, textural evidence that ferrogedrite (metastable ornot) formed together with hercynite precludes the possibi-lity that it is a relict phase within staurolite that reacted tohercynite. For rocks rich in K2O (e.g. 4.23 wt.% K2O for a


staurolite schist xenolith from the Wehr volcano; unpub-lished data), orthoamphibole-bearing assemblages inthe KFMASH system do not become stable until near orabove the metapelite melting curve below 0.4 GPa (Xuet al., 1994).

As stated above, although the staurolite breakdownproceeded under strongly disequilibrium conditions, theHc þ Gd þ Sil þ Qtz reaction products may haveapproached an equilibrium assemblage under the P-T con-ditions of transformation. Comparison with appropriateexperimentally determined breakdown reactions and melt-ing curves in the KNCFMASH system can therefore beused to help provide minimum P-T conditions of thequenched staurolite breakdown assemblage in the Wehrxenoliths.

Inclusions of unreacted quartz within, and adjacent tothe pseudomorphed staurolite (Figs. 1a,b) suggest that themelting reaction occurred at a lower temperature than thebreakdown of St þ Qtz, although siliceous melt could beregarded as a substitute for quartz. Modelling quartz-absent staurolite-breakdown reactions in the simplifiedFASH system assuming a C–O–H fluid composition(H:O¼ 2 coexisting with graphite as in the Wehr xenolith)by Cesare (1994) indicates that reaction (5) occurs at�570–660 �C at pressures between 0.2–0.5 GPa, and thatan almandine-bearing assemblage is produced according toreaction (6) some 20 �C degrees lower over the samepressure range (Fig. 5). Compared with experimentallydetermined St þ Qtz-out curves of Richardson (1968)

and Dutrow & Holdaway (1989), reaction (5) is 55� and35 �C lower, respectively, at 0.35 GPa (Fig. 5).

Cordierite and almandine were not products of staurolitebreakdown in theWehr xenolith. The absence of cordieritemay reflect pressure conditions . ca. 0.34 GPa, whileferrogedrite (instead of almandine) implies a minimumtemperature of ca. 665 �C at this pressure based on thereaction St þ Qtz ¼ OAm þ Sil (Fig. 5). Partial meltingof muscovite, sodic plagioclase and quartz in the Wehrxenolith indicates temperature conditions . 645 �C fromthe melting reaction

Msþ Qtzþ Abss þ V ¼ Alsþ L (7)

From these data, minimum T-P conditions for the pseu-domorph assemblage after staurolite in schist wall-rocks ofthe Wehr magma chamber were 665 �C and 0.35 GPa, i.e.about 20 �C above the pelitic wet solidus, 20 �C below theSt þ Qtz stability curve of Richardson (1968), and coin-cident with that of Dutrow & Holdaway (1989) (location1 in Fig. 5). If the temperature reached that of the meltingreaction

Msþ Qtzþ V ¼ Alsþ L (8)

(�705 �C at 0.35 GPa; location 2 in Fig. 5), as seems likely,then staurolite reactions (5), (6), and the OAm þ Sil-form-ing reaction would have been significantly overstepped byca. 75, 95 and 40 �C, respectively, and the peliticwet solidus temperature exceeded by ca. 55 �C (Fig. 5).A C-O-H fluid phase with H:O ¼ 2, would lower thetemperatures of reactions (7) and (8) by about 10 �C(Connolly & Cesare, 1993).

To our knowledge, evidence of staurolite melting innature has not been reported. However, Garcıa-Castroet al. (2003) have experimentally determined the existenceof a narrow stability field of St þ Lþ Qtz þMsþ Pl þ Vin the NKMASH system, the high-temperature boundaryof which is bounded by a fluid-absent St-breakdown reac-tion that produces melt (PlþMsþQtzþ St¼ Grtþ BtþAls þ L) between ca. 660–685 �C/0.68–0.73 GPa in thesillimanite stability field, i.e., at a lower temperature thanfluid-absent melting of Ms þ Qtz þ Pl. Metastable extra-polation of the reaction curve to 0.35 GPa would lower thereaction temperature to �625 �C.

4.3. Reaction overstepping and heating conditions

Temperature overstepping of mineral equilibrium asproposed for staurolite breakdown might be expected tooccur in thermal aureoles where the duration of heating andcooling prior to quenching on eruption were sufficientlyshort to prevent complete reaction from occurring, result-ing in the preservation of a metastable or disequilibriummineralþ glass assemblage. In this respect, it is interestingto compare the extensively melted staurolite-bearingbuchite xenolith (Fig. 3) where staurolite is only partlyreplaced by hercynite, with the low degree of melting

Fig. 5. P-T diagram related to staurolite (St) stability. Fe-staurolitebreakdown curves to hercynite (Hc), aluminium silicate (AS), alman-dine (Alm) and to Hc, Als and quartz (Qtz) assuming a C-O-H, H:O¼2, graphite-saturated fluid from Cesare (1994). Experimentally deter-mined Stþ Qtz¼ Almþ Sil þ H2O stability curves from Dutrow &Holdaway (1989) (DH) and Richardson (1968) (R); chloritoid (Cld),cordierite (Crd), alm þ sillimanite (Sil) þ Qtz, St þ orthoamphibole(OAm) stabilities and OAm stability field (grey-shaded area) afterGrieve & Fawcett (1974) where OAm is aluminous ferro-anthophyl-lite (i.e., ferrogedrite). Dashed curves represent metastable exten-sions of the reactions muscovite (Ms) þ Qtz þ albite (Ab) þvapour (V) ¼ Als þ liquid (L) (extrapolated from Thompson &Algor, 1977), Ms þ Qtz þ V ¼ Als þ L and Ms þ V ¼Co (corundum) þ L (after Rubie & Brearley, 1987). The An20 þQtz ¼ L curve is after Johannes & Holtz (1996). PWS ¼ peliticwet solidus curve (from Thompson, 1982) that is approximatelyequal to the Qtz þ K-feldspar (Kfs) þ Ab þ V ¼ L and Ms þQtz þ Kfs þ Ab þ V ¼ L reaction curves at P , 0.5 GPa.Andalusite (And)-Sil stability fields from Holdaway &Mukhopadhyay (1993). Location points 1, 2 and 3 are referred to inthe text.


(�10%) in the xenolith containing pseudomorphed staurolite(Fig. 1a). This may be explained by different heating timesand temperature. According to Worner et al. (1982), wallrock within 200 m of the Wehr magma chamber could haveattained temperatures of ca. 630 �C shortly after phonolitemagma emplacement at �900 �C (two feldspar thermome-try). If the magma temperature was 1000 �C (phonoliteMELTS liquidus temperature) the maximum wall-rock tem-peratures may have initially risen to ca. 700 �C. The thinovergrowths on regionalmetamorphic garnet in the stauroliteschist have Fe/(FeþMg)¼ 0.84 and at 0.35 GPa indicate atemperature of ca. 660 �C from XFe isopleths in theKFMASH system (Spear & Cheney, 1989).

Partial melting of wall rock (spotted schist) was in theorder of 10–15 % compared with buchitic xenoliths thathave undergone 40–80 % melting at higher temperature(800–850 �C; Grapes, 1986) by their incorporation into themagma. Thus, schist with pseudomorphed staurolite hadundergone only a small amount of melting at lower tempera-ture and over a longer time that may have involved a smallamount of cooling before being fragmented and entrained inevolved trachytic magma at the time of phreatic eruption.The minimum pressure estimate of 0.34 GPa from the staur-olite pseudomorph assemblage indicates that the xenolithwas derived from a depth of ca.12–13 km. On the otherhand, buchite xenoliths were incorporated into the magmawhere they underwent rapid heating to higher temperatures.An experimental and kinetic study of staurolite breakdownover a period of 1 to 8 weeks at 800 �C, 0.1 GPa, H2Osaturated and undersaturated, and fO2¼ NNO buffer condi-tions (Grapes, in preparation), indicates the development ofabundant hercynite with melt along fractures and expansioncracks parallel to the {010} cleavage of staurolite in two-week experiments. Textures are very similar to that shown inFig. 3. The partly reacted staurolite in the buchitic xenolith isassociated with biotite that shows only the initial stages ofbreakdown to spinelþ melt, and strongly resorbed relics ofquartz (Fig. 3). No muscovite or plagioclase remain indicat-ing that temperatures of reactions (7) and (8) were exceeded.Although not shown in Fig. 3, clusters of sillimanite needlestogether with spinel, newly formed biotite, corundum andsanidine in glass that mimic the habit and orientation oforiginal muscovite are common. This suggests minimumtemperature overstepping of the staurolite and staurolite þquartz breakdown reactions shown in Fig. 5 of 75–90 �C and25–50 �C, respectively,with reference to reaction (8) and theAn20 þ quartz solidus, and possibly higher temperatures of150–165 �C if theMsþV¼CoþL curvewas reached (e.g.775 �C at 0.35 GPa; location 3 in Fig. 5). Such conditionsindicate temperatures of 60–125 �C above the pelitic wetsolidus, and in the case of Msþ Vmelting, 55 �C above theAn20 þ Qtz solidus. Comparison with experimental evi-dence of hercynite replacement of staurolite implies a shortheating time of perhaps 200–300 hours before eruption andquenching.

Acknowledgements: We are indebted to Antonio Garcia-Castro (University ofGranada), BernardoCesare (University

of Padova), and Max Schmidt (ETW, Zurich) for their per-ceptive and constructive comments. Editorial comments andsuggestions from Fernando Nieto Garcia and ChristianChopin are appreciated.

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Received 5 February 2009

Modified version received 1 June 2009

Accepted 16 July 2009


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Disequilibrium thermal breakdown of staurolite: natural example