New mineral activity–composition relations for thermodynamic calculations in metapelitic systems

New activity–composition (a–x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell ( , Journal of Metamorphic Geology, 29, 333–383). The a–x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order–disorder in all ferromagnesian minerals where Fe–Mg mixing occurs on multiple sites. The a–x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the first time, the entire set of a–x relations for the common ferromagnesian minerals in metapelitic rocks is parameterized simultaneously, with attention paid to ensuring that they can be used together to calculate phase diagrams of geologically appropriate topology. The a–x relations developed are for use in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system for both subsolidus and suprasolidus conditions. Petrogenetic grids in KFMASH and KFMASHTO are similar in topology to those produced with earlier end‐member datasets and a–x relations, but with some notable differences. In particular, in subsolidus equilibria, the FeO/(FeO + MgO) of garnet is now greater than in coexisting staurolite, bringing a number of key staurolite‐bearing equilibria into better agreement with inferences from field and petrographic observations. Furthermore, the addition of Fe3+ and Ti to a number of silicate phases allows more plausible equilibria to be calculated in relevant systems. Pseudosections calculated with the new a–x relations are also topologically similar to equivalent diagrams using earlier a–x relations, although with many low variance fields shifting in P–T space to somewhat lower pressure conditions.

[1]  R. Powell,et al.  On parameterizing thermodynamic descriptions of minerals for petrological calculations , 2014 .

[2]  R. Powell,et al.  A new thermodynamic model for sapphirine: calculated phase equilibria in K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 , 2014 .

[3]  S. Boger,et al.  The importance of iron speciation (Fe+2/Fe+3) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar , 2012 .

[4]  R. Powell,et al.  Garnet and spinel lherzolite assemblages in MgO–Al2O3–SiO2 and CaO–MgO–Al2O3–SiO2: thermodynamic models and an experimental conflict , 2012 .

[5]  R. Powell,et al.  Revised activity–composition models for clinopyroxene and amphibole , 2012 .

[6]  J. H. Stout,et al.  Stability of sapphirine + quartz in the oxidized rocks of the Wilson Lake terrane, Labrador: calculated equilibria in NCKFMASHTO , 2012 .

[7]  Roger Powell,et al.  An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids , 2011 .

[8]  T. Holland,et al.  Garnet–chloritoid–kyanite assemblages: eclogite facies indicators of subduction constraints in orogenic belts , 2010 .

[9]  R. Powell,et al.  Influence of ferric iron on the stability of mineral assemblages , 2010 .

[10]  F. Ferri,et al.  An Experimental Determination of the Effect of Bulk Composition on Phase Relationships in Metasediments at Near-solidus Conditions , 2009 .

[11]  B. Cesare,et al.  A thermodynamic model for titanium and ferric iron solution in biotite , 2009 .

[12]  R. Powell,et al.  Phase equilibria modelling of kyanite‐bearing anatectic paragneisses from the central Grenville Province , 2008 .

[13]  R. Powell,et al.  Partial melting of metagreywacke: a calculated mineral equilibria study , 2008 .

[14]  R. Powell,et al.  Granulite facies metamorphism and subsolidus fluid‐absent reworking, Strangways Range, Arunta Block, central Australia , 2008 .

[15]  R. Powell,et al.  Progress relating to calculation of partial melting equilibria for metapelites , 2007 .

[16]  R. Powell,et al.  Mineral activity–composition relations and petrological calculations involving cation equipartition in multisite minerals: a logical inconsistency , 2006 .

[17]  D. Pattison,et al.  CONTRASTING SEQUENCES OF METAPELITIC MINERAL-ASSEMBLAGES IN THE AUREOLE OF THE TILTED NELSON BATHOLITH, BRITISH COLUMBIA: IMPLICATIONS FOR PHASE EQUILIBRIA AND PRESSURE DETERMINATION IN ANDALUSITE–SILLIMANITE-TYPE SETTINGS , 2005 .

[18]  A. Garcia‐Casco,et al.  Synthesis of Staurolite in Melting Experiments of a Natural Metapelite: Consequences for the Phase Relations in Low-Temperature Pelitic Migmatites , 2003 .

[19]  R. Powell,et al.  Activity–composition relations for phases in petrological calculations: an asymmetric multicomponent formulation , 2003 .

[20]  T. Holland,et al.  Mixing properties of phengitic micas and revised garnet‐phengite thermobarometers , 2002 .

[21]  M. Dyar,et al.  Fe3+ and Fe2+ partitioning among silicates in metapelites: A synchrotron micro-XANES study , 2002 .

[22]  R. Powell,et al.  The interpretation of reaction textures in Fe‐rich metapelitic granulites of the Musgrave Block, central Australia: constraints from mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 , 2002 .

[23]  R. Powell,et al.  Calculation of Phase Relations Involving Haplogranitic Melts Using an Internally Consistent Thermodynamic Dataset , 2001 .

[24]  R. Powell,et al.  Calculation of partial melting equilibria in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) , 2001 .

[25]  Worley,et al.  The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 , 2000 .

[26]  R. Powell,et al.  Relating formulations of the thermodynamics of mineral solid solutions: Activity modeling of pyroxenes, amphiboles, and micas , 1999 .

[27]  Roger Powell,et al.  An internally consistent thermodynamic data set for phases of petrological interest , 1998 .

[28]  R. Powell,et al.  Thermodynamics of order-disorder in minerals: II. Symmetric formalism applied to solid solutions , 1996 .

[29]  B. Harte,et al.  The Effect of Mn on the Phase Relations of Medium-Grade Pelites: Constraints from Natural Assemblages on Petrogenetic Grid Topology , 1995 .

[30]  J. Connolly,et al.  Melting of the continental crust: Some thermal and petrological constraints on anatexis in continental collision zones and other tectonic settings , 1995 .

[31]  R. Powell,et al.  The applicability of least squares in the extraction of thermodynamic data from experimentally bracketed mineral equilibria , 1993 .

[32]  A. Navrotsky,et al.  Substitution of (super [6,4]) Al in phlogopite; high-temperature solution calorimetry, heat capacities, and thermodynamic properties of the phlogopite-eastonite join , 1992 .

[33]  J. Ague.  Evidence for major mass transfer and volume strain during regional metamorphism of pelites , 1991 .

[34]  E. Ghent,et al.  PELITES FROM THE WESTERN ROCKY MOUNTAINS, NORTHWEST OF GOLDEN, BRITISH COLUMBIA , 1991 .

[35]  R. Powell,et al.  Calculated mineral equilibria in the pelite system, KFMASH (K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O) , 1990 .

[36]  F. Spear,et al.  Geothermometry, geobarometry and the Al2SiO5 triple point at Mt. Moosilauke, New Hampshire , 1982 .

[37]  S. Guggenheim,et al.  Distribution of titanium between coexisting muscovite and biotite in pelitic schists from northwestern Maine , 1977 .

[38]  James B. Thompson.  The Graphical Analysis of Mineral Assemblages in Pelitic Schists , 1957 .

[39]  T. Hor.  On the formulation of simple mixing models for complex phases , 1993 .

[40]  C. Guidotti.  Micas in metamorphic rocks , 1984 .

[41]  B. Harte,et al.  Pelite facies series and the temperatures and pressures of Dalradian metamorphism in E Scotland , 1979, Geological Society, London, Special Publications.

[42]  CHenI-rs V. Gulnorrt,et al.  Margarite pseudomorphs after chiastolite in the Rangeley area' Maine , 1976 .

[43]  H. Winkler,et al.  Petrogenesis of metamorphic rocks , 1965 .