Multistage growth of Fe-Mg-carpholite and chloritoid, from field evidence to
thermodynamic modelling
Contributions to Mineralogy and Petrology
Amaury Pourteau*, Romain Bousquet, Olivier Vidal, Alexis Plunder, Erik Duesterhoeft, Osman Candan, Roland
Oberhänsli
*
Corresponding author:
Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl-Liebknecht-Straße 24–25, 14476
Potsdam–Golm, Germany
E-mail: pourteau@geo.uni-potsdam.de
Electronic Appendix 3: Phase diagrams calculated with the alternative datasets.
EVALUATION OF THE AVAILABLE DATASETS
In the literature, two self-consistent thermodynamic datasets are available for calculation of phase diagrams for
carpholite-bearing assemblages: that of Holland & Powell (1998)(hereafter “HP02”) with recent complements,
and the dataset of Le Bayon et al. (2006) developed from the JUN92.bs update of Berman’s (1988) database. In
the following, either dataset is evaluated in the light of the phase diagrams that were computed for theoretical
FMASH compositions, as in Figure 7.
Dataset HP02
During the past decade, the HP02 dataset has been used by most of the studies investigating HP–LT metapelites,
as well for phase relation systematics in the systems KFMASH and NaKFMASH (Wei and Powell 2003; Wei
and Powell 2004), as for thermobarometric purposes (e.g., Song et al. 2007; Agard et al. 2010; Plunder et al.
2012). Recently, Lanari et al. (2014) introduced a new chlorite solid-solution model, Chl(LWV), that follows
site distribution after Vidal et al. (2001) and accounts for the di/tri-octahedral [VI☐,2VIAl=3VI(Mg,Fe2+)]
substitution that is crucial in LT metamorphic rocks. The Chl(LWV) model especially places the chlorite–
carpholite transition to reasonably high pressure, whereas the original Chl(HP) model (Holland et al. 1998)
predicts Mg-carpholite stability until LP conditions (< 0.3 GPa at 300°C), which contradicts natural and
experimental evidence (e.g. Goffé et al. 1973; Chopin and Schreyer 1983; Vidal et al. 1992). The HP02 dataset
was evaluated using PERPLE_X software package (version 6.6.9; Connolly 2005) and the hp02ver.dat file with
the following solid-solution models: Chl(LWV) for chlorite (Lanari et al. 2014), Carp for carpholite, Ctd(HP)
for chloritoid, Gt(HP) for garnet and St(HP) for staurolite (Holland and Powell 1998). Phase diagrams obtained
for theoretical FMASH quartz vein compositions (as in Fig. 7) are shown in Figure A3-1.
Mg-rich carpholite is stable solely above 0.8 GPa at 300–400°C in agreement with experimental
constraints by Vidal et al. (1992) and Chopin and Schreyer (1983). On the other hand, the predicted stability of
Fe-rich carpholite (XMg ≤ 0.3) is restricted to very high pressures, along very low thermal gradients (< 5°C/km),
long considered to be a “forbidden zone” not realised in the Earth (Schreyer 1988). Such extreme conditions
seem unlikely as Fe-rich carpholite compositions were reported from various low-grade HP units (e.g., Ligurian
Alps, Goffé 1984; Oman, Goffé et al. 1988; Western–Central Anatolia, this study). Using this dataset for
samples containing Fe-rich carpholite shall therefore yield erroneously high pressures. The HP02 dataset implies
strong Mg/Fe partitioning between carpholite and chloritoid: KP(Car/Cld) values, that depend mostly on
temperature, spread between <3 at 550°C (>1.6 GPa) to >25 below 300°C, and thus include the range of values
obtained from natural data (Theye et al. 1992; Azañon and Goffé 1997; this study) although systematic
variations with metamorphic conditions were never evidenced (see Vidal and Theye 1996).
Therefore, although the new Chl(LWV) model allows Mg-rich carpholite to be stable only above
upper–greenschist-facies conditions, and predicted KP(Car/Cld) values generally satisfy natural constraints,
critical issues with the thermodynamic data for Fe-carpholite still prevent calculating reliable P–T phase
diagrams with this database.
Dataset LB06
The second available dataset is that of Le Bayon et al. (2006) (hereafter “LB06”), which consists in the
thermodynamic database JUN92.bs (updated after Berman 1988), complemented with new data calculated for
Mg-carpholite, Mg-chloritoid and Fe-chloritoid. Dataset LB06, compatible with THERIAK–DOMINO, was
developed with the aim to calculate phase diagrams in KFMASH chemical systems, for carpholite-free,
eclogite-facies magnesian metapelites of the Monte Rosa nappe (Western Alps). New standard enthalpies of
formation (fH°), and standard entropies (S°) were calculated by mathematical optimisation using, as
calculation constraints, P–T positions of reactions as bracketed experimentally by Ganguly (1969), Chopin and
Schreyer (1983) and Vidal et al. (1994). fH° and S° of Fe-carpholite and daphnite (ferrous end-member of trioctahedral chlorites) were estimated based on the approximate P–T positions of reactions by Chopin & Schreyer
(1983) and Fe–Mg exchange between chlorite and biotite (after Laird 1988).
The topologies obtained for FMASH compositions using the LB06 dataset (Fig. A3-2) are very similar
to the published petrogenetic grids of Theye et al. (1992), Vidal et al. (1992), Oberhänsli et al. (1995), and
Bousquet et al. (2008). Fe-rich carpholite (XMg ≤ 0.3) is stable from <0.3 GPa and up to 350–400°C (depending
on XMg), i.e. not only at blueschist- but also at greenschist-facies conditions, in agreement with natural
constraints from Goffé (1984), whereas Mg-rich carpholite is stable from upper greenschist-facies conditions, in
agreement with experimental constraints from Vidal et al. (1992). Yet, Vidal et al. (1992) investigated the
stability of carpholite vs. sudoite + quartz, whereas the chlorite model used in the LB06 dataset consists of an
ideal-mixing daphnite–clinochlore solid solution. Thermodynamic data for Mg-carpholite, which differ from
values estimated by previous studies (Vidal et al. 1992; Bertoldi et al. 2006), might therefore be questionable.
Carpholite–chloritoid di-variant fields are restricted to narrow temperature ranges (<50°C), and shift towards
higher temperatures with increasing XMg values. Calculated KP(Car/Cld) values (<3.5) are in clear contradiction
with those determined in natural samples (7–11; Goffé 1982; Theye et al. 1992; Azañon and Goffé 1997; this
study).
Therefore, although the LB06 dataset yields topologies very similar to well-accepted petrogenetic grids
for HP/LT metapelites, erroneous predictions of Mg–Fe partitioning between carpholite and chloritoid hamper
its use for carpholite-bearing rocks.
REFERENCES
Agard P, Searle MP, Alsop GI, Dubacq B (2010) Crustal stacking and expulsion tectonics during continental
subduction: P-T deformation constraints from Oman. Tectonics 29 doi:10.1029/2010tc002669
Azañon JM, Goffé B (1997) Ferro- and magnesiocarpholite assemblages as record of high-P, low T
metamorphism in the Central Alpujarrides, Betic cordillera (SE Spain). European Journal of Mineralogy
9:1035-1051
Berman RG (1988) Internally-Consistent Thermodynamic Data for Minerals in the system Na2O-K2O-Ca-MgOFeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2. Journal of Petrology 29(2):445-522
Bertoldi C, Dachs E, Theye T (2006) Calorimetric data for naturally occurring magnesiocarpholite and
ferrocarpholite. American Mineralogist 91(2-3):441-445 doi:10.2138/am.2006.1958
Bousquet R, Oberhänsli R, Goffé B, Wiederkehr M, Koller F, Schmid SM, Schuster R, Engi M, Berger A,
Martinotti G (2008) Metamorphism of metasediments in the scale of an orogen: A key to the Tertiary
geodynamic evolution of the Alps. In: Siegesmund S, Froitzheim N (eds) Tectonic aspect of the AlpineDinaride-Carpathian System, vol 298. Geological Society Special Publication of London, London, pp 393-411
Chopin C, Schreyer W (1983) Magnesiocarpholite and magnesiochloritoid: two index minerals of pelitic
blueschits and their preliminary phase relations in tne system MgO-Al2O3-SiO2-H2O. American Journal of
Science 283-A:72-96
Connolly JAD (2005) Computation of phase equilibria by linear programming: A tool for geodynamic modeling
and its application to subduction zone decarbonation. Earth and Planetary Science Letters 236(1-2):524-541
Ganguly J (1969) Chloritoid stability and related parageneses - Theory, experiments, and applications.
American Journal of Science 267(8):910-944
Goffé B (1982) Définition du faciès à Fe,Mg-carpholite-chloritoide, un marqueur du métamorphisme de HP-BT
dans les métasédiments alumineux. Paris
Goffé B (1984) Le facies à carpholite-chloritoide dans la couverture briançonnaise des Alpes Ligures: un témoin
de l'histoire tectono-metamorphique régionale. Memorie della Societa Geologica Italiana 28:461-479
Goffé B, Goffé-Urbano G, Saliot P (1973) Sur la présence d'une variété magnésienne de la ferrocarpholite en
Vanoise (Alpes françaises) : sa signification probable dans le métamorphisme alpin. Comptes Rendus de
l'Académie des Sciences Paris 277:1965-1968
Goffé B, Michard A, Kienast J-R, Le Mer O (1988) A case study of obduction-related high pressure, low
temperature metamorphism in the upper crustal nappes, Arabian continental margin, Oman: P-T paths and
kinematic interpretation. Tectonophysics 151:363-386
Holland T, Baker J, Powell R (1998) Mixing properties and activity-composition relationships of chlorites in the
system MgO-FeO-Al2O3-SiO2-H2O. European Journal of Mineralogy 10(3):395-406
Holland TJB, Powell R (1998) An internally consistent thermodynamic dataset for phases of petrological
interest. Journal of Metamorphic Geology 16(3):309-343
Laird J (1988) Chlorites: Metamorphic Petrology. In: Bailey SW (ed) Hydrous Phyllosilicates, vol 19.
Mineralogical Society of America, Washinton, D.C., pp 405-453
Lanari P, Wagner T, Vidal O (2014) A thermodynamic model for di-trioctahedral chlorite from experimental
and natural data in the system MgO-FeO-Al2O3- SiO2-H2O: applications to P-T sections and geothermometry.
Contributions to Mineralogy and Petrology 167(2) doi:10.1007/s00410-014-0968-8
Le Bayon R, De Capitani C, Frey M (2006) Modelling phase-assemblage diagrams for magnesian metapelites in
the system K2O-FeO-MgO-Al2O3-SiO2-H2O: geodynamic consequences for the Monte Rosa nappe, Western
Alps. Contributions To Mineralogy And Petrology 151:395-412
Oberhänsli R, Goffé B, Bousquet R (1995) Record of a HP-LT metamorphic evolution in the Valais zone:
Geodynamic implications. In: Lombardo B (ed) Studies on metamorphic rocks and minerals of the western Alps
A Volume in Memory of Ugo Pognante, vol 13, n°2. Bollettino del Museo Regionale di Scienze Naturali
(suppl.), Torino, pp 221-239
Plunder A, Agard P, Dubacq B, Chopin C, Bellanger M (2012) How continuous and precise is the record of P-T
paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics
(Schistes Lustres, W. Alps). Journal of Metamorphic Geology 30(3):323-346 doi:10.1111/j.15251314.2011.00969.x
Schreyer W (1988) Experimental studies on metamorphic of crustal rocks under mantle pressures. Mineralogy
Magazine 52:1-26
Song SG, Zhang LF, Niu Y, Wie CJ, Liou JG, Shu GM (2007) Eclogite and carpholite-bearing metasedimentary
rocks in the North Qilian suture zone, NW China: implications for early palaeozoic cold oceanic subduction and
water transport into mantle. Journal of Metamorphic Geology 25(5):547-563
Theye T, Seidel E, Vidal O (1992) Carpholite, sudoite and chloritoid in low-grade high-pressure metapelites
from Crete and the Peloponnese, Greece. European Journal of Mineralogy 4:487-507
Vidal O, Goffé B, Theye T (1992) Experimental study of the stability of sudoite and magnesiocarpholite and
calculation of a new petrogenetic grid for the system FeO-MgO-Al2O3-SiO2-H2O. Journal of Metamorphic
Geology 10:603-614
Vidal O, Parra T, Trotet F (2001) A thermodynamic model for Fe-Mg aluminous chlorite using data from phase
equilibrium experiments and natural pelitic assemblages in the 100-600°C, 1-25 kbar range. American Journal
of Science 301:557-592
Vidal O, Theye T (1996) Comment on "Petrology of Fe-Mg-carpholite-bearing metasediments from NE Oman".
Journal of Metamorphic Geology 14:381-386
Vidal O, Theye T, Chopin C (1994) Experimental study of chloritoid stability at high pressure and various fO 2
conditions. Contributions To Mineralogy And Petrology 118:256-270
Wei C, Powell R (2004) Calculated Phase Relations in High-Pressure Metapelites in the System NKFMASH
(Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O). Journal of Petrology 45(1):183-202
Wei CJ, Powell R (2003) Phase relations in high-pressure metapelites in the system KFMASH (K2O–FeO–
MgO–Al2O3–SiO2–H2O) with application to natural rocks. Contributions To Mineralogy And Petrology
301:301–315