Metapelites Francis, 2013 qtz muscovite muscovite qtz qtz qtz garnet

Solid - Solid Reactions: One Component andalusite sillimanite Al2Si05 Al2Si05 From the phase rule, we know that this is reaction is univariant and thus can be represented by a line in P-T space: F = C - P + 2 = 1 At equilibrium: G (P,T) = Ho(1bar,T) - T So(1bar,T) + (P-1) V = 0 dG (P,T) = 0 = - So dT + VdP S, V, and H vary little with T and P for solid - solid reactions because the changes in the reactants with T and P tend to parallel those in the products. Thus the above equation approximates that of a straight line in P - T space with a slope of: dP/dT = S/ V = - 3.3 bar / K V = - 1.44 Joules / bar mol S = 4.72 Joules / mol K

F = C – P + 2 F = 2 – 3 + 2 F = 1, if C = 3 univariant Solid - Solid Reactions: Two Component albite jadeite + quartz NaAlSi3O8 NaAlSi2O6 + SiO2 dP/dT = S/ V = 25.02 bar / oK V = - 1.734 Joules / bar mol S = - 43.39 Joules / mol K Most solid - solid reactions have positive slopes in P - T space because the higher temperature side of the reaction typically has both higher entropy and volume. F = C – P + 2 F = 2 – 3 + 2 F = 1, if C = 3 univariant Jd Ab Qtz Jd Qtz Jd Ab Qtz

F = 3 - P + 2 F = 2, if P =3 F = 1, if P = 4, univariant Temperature Three Component Systems: F = 3 - P + 2 F = 2, if P =3 F = 1, if P = 4, univariant Tie-line switching (2D) or piercing plane (nD) reactions Temperature B + D A + E Pressure

F = 3 - P + 2 F = 2, if P =3 F = 1, if P = 4, univariant Temperature Terminal reactions at which phases appear or disappear B + C = A appearance D = A + B + C disappearance F = 3 - P + 2 F = 2, if P =3 F = 1, if P = 4, univariant Temperature A + B + C D Pressure

Metapelites: ~ 4 component system Shales are typically depleted in Ca and Na because they were lost to solution during the breakdown of tecto-, ino-, and orthosilicates to clay minerals during weathering. Their bulk compositions can be approximated in a 6 component system Components C = 6: K2O, Al2O3, SiO2, FeO, MgO, H2O With excess quartz & water: C = 4 ~ 4 component system With excess water and quartz

Metapelites C = 3 and F = 3 - P + 2 AFM Projections J.B. Thompson, 1957 If muscovite is present: we can project the mineral assemblages onto the Al2O3* – FeO – MgO plane, where: C = 3 and F = 3 - P + 2 Muscovite is typically a ubiquitous phase in metapelites.

MgO / (MgO + FeO) 1.0 0.5 Al2O3* Al2O3* + MgO +FeO 0.0 -0.5 Al2O3* = Al2O3 - 3×K2O MgO / (MgO + FeO)

Phase Rule: if P and T are held Constant F = C - P F = 3 - P

F = 3 - P

Lower Greenschist Facies montmorillonite chlorite + water green mica - in montmorillonite chlorite + water (Al1-X(Mg,Fe)X)2Si3O10(OH)2 + XNa+.nH2O (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 white mica - in kaolinite + qtz muscovite + water Al2Si2O5(OH)4 KAl2(Al,Si3)O10(OH)2

Lower Greenschist Facies biotite - in K-spar + chlorite biotite + muscovite + qtz + water KAlSi3O8 + (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 K(Mg,Fe)3(Al,Si3)O10(OH)2 + KAl2(Al,Si3)O10(OH)2 kyanite or andalusite - in pyrophyllite Al-silicate + qtz + water Al2Si4O10(OH)4 Al2Si05 + 3SiO2

Biotite - in The temperature of the first appearance of biotite depends on the Mg/Fe ratio of the whole rock composition.

Upper Greenschist Facies garnet – in (Fe-rich systems) Fe-chlorite + muscovite + qtz garnet + biotite + water (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 (Fe,Mg)3Al2(SiO4)3 + (Mg,Fe)3(Al,Si3)O10(OH)2 + KAl2(Al,Si3)O10(OH)2 + SiO2

Upper Greenschist Facies – con’t staurolite - in garnet + chlorite staurolite + biotite + water (Fe,Mg)3Al2(SiO4)3 + K(Mg,Fe)3(Al,Si3)O10(OH)2 Fe2Al9O6(SiO4)4(O,OH)2 + K(Mg,Fe)3(Al,Si3)O10(OH)2

Lower Amphibolite Facies The transition from greenschist facies to the amphibolite facies in metapelites corresponds to the discontinuous reaction: (Mg,Fe)7Al4Si4O15(OH)12 + KAl2AlSi3O10(OH)2 + Fe2Al9O6(SiO4)4(O,OH)2 chlorite muscovite staurolite K(Mg,Fe)3AlSi3O10(OH)2 + Al2SiO5 + water biotite kyanite Temp

Lower Amphibolite Facies chlorite-out chlorite biotite + Al-silicate + cordierite + water (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 K(Mg,Fe)3(Al,Si3)O10(OH)2 + Al2Si05 1st cordierite-in + (Fe,Mg)2Al3(Al,Si5)O18.nH2O + H2O

Upper Amphibolite Facies staurolite-out garnet-in staurolite garnet + biotite + Al-silicate + water Fe2Al9O6(SiO4)4(O,OH)2 (Fe,Mg)3Al2(SiO4)3 + K(Mg,Fe)3(Al,Si3)O10(OH)2 + Al2Si05 garnet sillimante schist muscovite + qtz muscovite - out K-spar + Al-silicate + water KAl2(Al,Si3)O10(OH)2 + SiO2 KAlSi3O8 + Al2Si05

Granulite Facies 2nd cordierite-in muscovite muscovite - out K-spar + corundum + water KAl2(Al,Si3)O10(OH)2 KAlSi3O8 + Al2O3 2nd cordierite-in biotite + Al-silicate K-spar + cordierite + garnet K(Mg,Fe)3(Al,Si3O10(OH)2 + 3Al2SiO5 KAlSi3O8 + (Fe,Mg)2Al3(Al,Si5)O18.nH2O + Fe3Al2(SiO4)3

Partial Melting of Metapelites: In the presence of a vapor phase, the partial melting of metapelite lithologies is controlled by the intersection of the breakdown curve for muscovite with the wet solidus curve for granite. In more mafic systems, the breakdown curve for biotite may be the controlling factor. XH2O=0.7 muscovite breakdown: musc + qtz Al-sil + K-spar + H2O Go = - RTlnXH2O (+) wet melting: H2O + metapelite granite melt Go = - RTln(1/XH2O) (-) migmatite