Metapelites Francis, 2014 garnet muscovite qtz muscovite qtz

Solid - Solid Reactions: One Component andalusite sillimanite Al 2 Si0 5 Al 2 Si0 5 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) =  H o (1bar,T) - T  S o (1bar,T) + (P-1)  V = 0 d  G (P,T) = 0 = -  S o 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 = bar / K  V = Joules / bar mol  S = 4.72 Joules / mol K

Solid - Solid Reactions: Two Component albite jadeite + quartz NaAlSi 3 O 8 NaAlSi 2 O 6 + SiO 2 dP/dT =  S/  V = bar / oK  V = Joules / bar mol  S = 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 – F = 1, if C = 3 univariant Ab Jd Qtz Ab Jd Qtz

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

Temperature A + B + C D Pressure 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

Metapelites: 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: K 2 O, Al 2 O 3, SiO 2, FeO, MgO, H 2 O With excess quartz & water: C = 4 With excess water and quartz ~ 4 component system

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

Al 2 O 3 * Al 2 O 3 * + MgO +FeO Al 2 O 3 * = Al 2 O 3 - 3×K 2 O MgO / (MgO + FeO)

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

Lower Greenschist Facies green mica - in montmorillonite chlorite + water (Al 1-X (Mg,Fe) X ) 2 Si 3 O 10 (OH) 2 + XNa+.nH 2 O(Mg,Fe) 3 (Al,Si) 4 O 10 (OH) 2  (Mg,Fe) 3 (OH) 6 white mica - in kaolinite+ qtz muscovite + water Al 2 Si 2 O 5 (OH) 4 KAl 2 (Al,Si 3 )O 10 (OH) 2

Lower Greenschist Facies biotite - in K-spar + chlorite biotite + muscovite + qtz + water KAlSi 3 O 8 + (Mg,Fe) 3 (Al,Si) 4 O 10 (OH) 2  (Mg,Fe) 3 (OH) 6 K(Mg,Fe) 3 (Al,Si 3 )O 10 (OH) 2 + KAl 2 (Al,Si 3 )O 10 (OH) 2 kyanite or andalusite - in pyrophyllite Al-silicate + qtz + water Al 2 Si 4 O 10 (OH) 4 Al 2 Si SiO 2

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

Upper Greenschist Facies garnet – in (Fe-rich systems) Fe-chlorite + muscovite + qtz garnet + biotite + water (Mg,Fe) 3 (Al,Si) 4 O 10 (OH) 2  (Mg,Fe) 3 (OH) 6 (Fe,Mg) 3 Al 2 (SiO 4 ) 3 + (Mg,Fe) 3 (Al,Si 3 )O 10 (OH) 2 + KAl 2 (Al,Si 3 )O 10 (OH) 2 + SiO 2

staurolite - in garnet + chlorite staurolite + biotite + water (Fe,Mg) 3 Al 2 (SiO 4 ) 3 + K(Mg,Fe) 3 (Al,Si 3 )O 10 (OH) 2 Fe 2 Al 9 O 6 (SiO 4 ) 4 (O,OH) 2 + K(Mg,Fe) 3 (Al,Si 3 )O 10 (OH) 2 Upper Greenschist Facies – con’t

(Mg,Fe) 7 Al 4 Si 4 O 15 (OH) 12 + KAl 2 AlSi 3 O 10 (OH) 2 + Fe 2 Al 9 O 6 (SiO 4 ) 4 (O,OH) 2 chlorite muscovite staurolite K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 + Al 2 SiO 5 + water biotite kyanite Lower Amphibolite Facies The transition from greenschist facies to the amphibolite facies in metapelites corresponds to the discontinuous reaction:

(Mg,Fe) 7 Al 4 Si 4 O 15 (OH) 12 + KAl 2 AlSi 3 O 10 (OH) 2 + Fe 2 Al 9 O 6 (SiO 4 ) 4 (O,OH) 2 chlorite muscovite staurolite K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 + Al 2 SiO 5 + water biotite kyanite Staurolite Stability Field

chlorite-out chlorite biotite + Al-silicate + cordierite + water (Mg,Fe) 3 (Al,Si) 4 O 10 (OH) 2  (Mg,Fe) 3 (OH) 6 K(Mg,Fe) 3 (Al,Si 3 )O 10 (OH) 2 + Al 2 Si0 5 1 st cordierite-in + (Fe,Mg) 2 Al 3 (Al,Si 5 )O 18.nH 2 O + H 2 O Lower Amphibolite Facies

staurolite garnet + biotite + Al-silicate + water Fe 2 Al 9 O 6 (SiO 4 ) 4 (O,OH) 2 (Fe,Mg) 3 Al 2 (SiO 4 ) 3 + K(Mg,Fe) 3 (Al,Si 3 )O 10 (OH) 2 + Al 2 Si0 5 Upper Amphibolite Facies muscovite + qtz muscovite - out K-spar + Al-silicate + water KAl 2 (Al, Si 3 )O 10 (OH) 2 + SiO 2 KAlSi 3 O 8 + Al 2 Si0 5 garnet sillimante schist staurolite-out garnet-in

Granulite Facies muscovite muscovite - out K-spar + corundum + water KAl 2 (Al, Si 3 )O 10 (OH) 2 KAlSi 3 O 8 + Al 2 O 3 biotite + Al-silicate K-spar + cordierite + garnet K(Mg,Fe) 3 (Al,Si 3 O 10 (OH) 2 + 3Al 2 SiO 5 KAlSi 3 O 8 + (Fe,Mg) 2 Al 3 (Al,Si 5 )O 18.nH 2 O + Fe 3 Al 2 (SiO 4 ) 3 2 nd cordierite-in

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. Musc + Qtz Al-Sil + K-spar + H 2 O H 2 O + MetapeliteGranite melt muscovite breakdown: wet melting:  G o = - RTlnX H2O (+)  G o = - RTln(1/X H2O ) (-) migmatite XH 2 O=0.7