Phase Equilibria in Silicate Systems

Intro. Petrol. EPSC-212, Francis-14

Fundamentals: Units

Types of units commonly used:

Weight units: = gms species/ 100gms (X-Ray analytical techniques determine weight fraction element)

• Molar Units: = gms species / (molecular wt. of species), normalized to 100 (most chemical phenomena are proportional to molecular proportions)

• Atomic units: = (no. moles species) x (no. atoms per species), normalized to 100 atoms

• Cation units: = (no. moles species) x (no. cations per species), typically normalized to some total number of cations (or anions).

• Oxygen units: = (no. moles species) x (no. oxygens per species), commonly normalized to some total number of anions (closest to volume proportions)

Example:

Coordinates of Enstatite (MgSiO3) in: Mg2SiO4 - SiO2 space

Weight units: 70 30Molecular units: 50 50Cations units: 75 25Oxygen units 66.7 33.3

Winter, J.D.; 2001: An Introduction to Igneous and Metamorphic Petrology. Prentice Hall, Chapter 4, 2001.

Volatile have an importance beyond that predicted simply by their abundance because:

- Volatiles have low molecular weights:

H2O = 18

CO2 = 44

SiO4 = 92

NaAlSi3O8 = 262In a melt consisting of NaAlSi3O8 clusters and H2O molecules:

0.5 wt. % H20 ~ 45 mole % H2O

Small amounts of water produce large effects because of its low molecular wt. compared to that of a silicate magma. This effect is enhanced by the fact that at XH2O < 0.3, molecular water dissociates into 2 OH’s

H2O + Obridging 2 × OHfH20 ~ PH2O α XH2O.2

The Rock Forming Minerals:

A mineral is defined as a naturally occurring crystalline phase or compound that is made up of a 3-D ordered atomic arrangement or structure of different atoms.

The dominant rock forming minerals are silicates, compounds of Si and O, because Si is the most abundant metal and oxygen the most abundant anion in the Earth's crust.

Minerals can be thought of as close packings of oxygens to a first approximation, because of the large size of O, with the smaller Si and other metals occupying the interstices, or sites, between the oxygens.

Olivine

Y2TO4

(Mg,Fe)2SiO4

Feldspar

WT4O8

(K,Na)AlSi3O8

Pyroxene

XYT2O6

Ca(Mg,Fe)Si2O6

Mineral Name:Structural Formula:Chemical Composition:

Rock Forming Minerals:The ratio of Si to O determines the type and abundance of the silicate mineral(s) present, and the other metals distribute themselves in sites between the oxygens according to the size and charge of their cations. The amounts of different elements in any given mineral may vary somewhat, but the type and proportion of occupied sites is fixed by the structure of the silicate mineral and can be expressed by a formula of the type:

Ww Xx Yy Tt Aa

Where the capital letters stand for occupied sites of different co-ordination number and the small letters are small whole numbers.

A = anion site

W = 12 co-ordinated site

X = 8 co-ordinated site

Y = 6 co-ordinated site

T = 4 co-ordinated site

K, Rb, Ba, Na

Ca, Mn, Na

Mg, Fe, Mn, Al, Ti

Si, Al

Silicates (lithophile elements) Olivine: Y2TO4 Mg2SiO4 - Fe2SiO4

Garnet: X3Y2(TO4)3 Mg3Al2(SiO4)3 - Fe3Al2(SiO4)3

Clinopyroxene: XYT2O6 CaMgSi2O6 - CaFeSi2O6

Orthopyroxene: Y2T2O6 Mg2Si2O6 - Fe2Si2O6

Amphibole: W01X2Y5T8O22(OH)2 Ca2Mg5Si8O22(OH)2 - NaCa2Mg4Al(Al2Si6O22(OH)2

Feldspar: WT4O8 CaAl2Si2O8 - NaAlSi3O8 - KAlSi3O8

Quartz: TO2 SiO2

Micas: W Y2-3T4O10(OH)2 KAl2(AlSi3)O10(OH)2 - KMg3(AlSi3)O10(OH)2

Clays & Serpentine: Y3T2O5(OH)4 Mg3Si2O5(OH)4

Carbonates: (lithophile elements)

Calcite - Dolomite: YCO3 CaCO3 - Ca(Mg,Fe)(CO3)2

Other Oxides: (lithophile elements)

Spinel – Magnetite: Y3O4 MgAl2O4 - FeCr2O4 - Fe3O4

Hematite – Ilmenite: Y2O3 Fe2O3 - FeTiO3

Sulfides: (chalcophile elements) Pyrite - Pyrrhotite FeS2 - Fe1-xS

Rock Forming Minerals

FDegrees of Freedom = Components - Phases + 2

The Mineralogical Phase Rule

In any chemical system at equilibrium, the following relationship holds:

F equals the minimum number of variables that must be specified in order to completely define the state of a system. F is thus the variance of the system, the number of unknowns.

C equals the number of independent chemical components needed to define the composition of the system.

P equals the number of physical phases present in the system, which include the number of solid minerals, plus liquid and gas phases, if present.

2 represents the variables pressure and temperature.

Single Component Systems: SiO2

When a solid consist of 2 coexisting minerals (phases):

F = C – P + 2 = 1 - 2 + 2 = 1

Such a system is invariant at any given pressure, and thus a single component solid phase will melt at 1 unique temperature at any specified pressure. The boundary between the 2 phases in P - T space will be a univariant line with a slope approximated by:

dG = - SdT + VdP = 0

dP/dT = S/ V

This is also true for solid - liquid phase boundaries because, to a first approximation, Ho and So are constant for small changes in temperature (true for all reactions not involving a relatively compressible vapour phase).

Two Component Systems:

Mg2SiO4 – SiO2 Pure forsterite melts at 2163oC at 1 atm. If extra SiO2 is added to thesystem, SiO2 will be present only in the melt phase, while forsterite will remain a pure phase. The temperature at whichforsterite crystallizes is now an inverse function of SiO2 content:

at equilibrium:

GFoOl = GFoLiq and GFo = 0

GoFo + R × T × Ln(aFoOl) = Go

FoLiq + R × T × Ln(aFoLiq) Go

Fo = - R × T × Ln(aFoLiq/aFoOl) or

GoFo = - R × T × Ln(1-XSiO2),

aFoOl = 1.0, assume activity (a) = mole fraction X)

and Go

Fo = HoFo – T × So

Fo = HoFo – T × Ho

Fo / TFo

If Ho and So are insensitive to small changes in T & P, then: Ho

Fo – T × HoFo / TFo = - R × T × Ln(1-XSiO2)

or

Ln(1-XSiO2) = HoFo / R × (T - TFo) / (T×TFo)

van't Hoff equation for melting point depression

Binary Phase Diagrams

No solid-solutionBetween end-members

If a system is comprised of 2 components, then where a solid and liquid phase coexist:

F = 2 - 2 + 2 = 2

Such a system is univariant at any given pressure, and thus the melting point of a solid will depend on the proportions of the two components. In the absence of extensive solid solution, the presence of an additional component will reduce the melting temperature of single component solid phases because the additional component typically dissolves preferentially in the liquid phase.

The Eutectic point “e” of a two component system is invariant (F = 0, if pressure fixed) and is defined by the intersection of two univariant (F = 1) liquidus curves, originating from the melting temperatures of the two pure end-member phases.

e

Two Component Systems:

Peritectic versus EutecticInvariant points

p - oliv + liq opx

e - liq opx + qtz e2 - liq neph + albite

e1 - liq albite + qtz

e1

e2

The Lever Rule

T1 : Forst / Enst : d / a; Forst / whole = d /(a+d)

T2 : Forst / liquid : c / a; Forst / whole = c /(a+c)

T3 : Forst / liquid : b / a; Forst / whole = b /(a+b)

X

For bulkComposition X

Liquids

vs

Cumulates

Fractional Crystallization vs Partial Melting

Upon cooling to 1557oC, early crystallized olivine exhibits a reaction relationship with the residual liquid of composition “p” to form orthopyroxene. Either olivine or melt must disappear before cooling can continue. During partial melting, orthopyroxene begins to melt incongruently at 1557 oC to form olivine plus a liquid of composition “p”. Orthopyroxene must be consumed before the temperature can increase.

Equilibrium

vs

Fractional

Cumulate Rocks versus Rocks that represent liquids

1557

The presence of other components in solid solution at levels that are insufficient to stabilize a separate phase destroys the invariant nature of melting.

The temperature and composition of the first melt are determined by the amount of the additional component. During partial melting, these additional components are typically the first to be refined out into the melt.

Bianary Systems with extensive Solid Solution:Olivine exhibits complete solid solution between the forsterite (Mg2SiO4) and fayalite (Fe2SiO4) end-members. In Fe and Mg bearing systems, neither the olivine solid nor the olivine liquid are pure end-member components:

We now have two van't Hoff equations:

Ln(XFoLiq / XFoOl) = HoFo / R × (T - TFo) / (T × TFo)

Ln(XFaLiq / XFaOl) = HoFa /R × (T - TFa) /(T × TFa)

Because: XFaLig = 1-XFoLiq and XFaOl = 1-XFoOl

Then:

Ln(XFoLiq / XFoOl) = HoFo / R × (T - TFo) / (T × TFo)

Ln(1-XFoLiq / (1-XFoOl)) = HoFa /R × (T - TFa) / (T × TFa)

The choice of any T between TFo and TFa will enable the calculation of the compositions of the coexisting olivine and liquid for that T, and thus the solidus and liquidus at any T. Exactly analogous solid solution relationships can be developed for the plagioclase series feldspars:

anorthite CaAl2Si2O8 - albite NaAlSi3O8

P = 1 atm

Ternary Systems: Forsterite – Diopside – Anorthite Liquidus Projection

In order to portray the magmatic phase relations of systems with more than two chemical components, we need to develop specialized projection schemes.

Three component systems can be represented on a two dimensional sheet of paper, if we project only those phase relationships for which a magmatic liquid is present.

X

P = 1 atmA Liquid of bulk composition X cools to the olivine liquidus surface at 1600°C, at which point Forsterite begins to crystalize

The liquid composition moves directly away from Fo, producing a dunite cumulate, until it reaches the cotectic, at which point Diopside begins to crystallize with Forsterite.

Ternary Systems: Forsterite – Diopside - Anorthite

X

P = 1 atm

Co-precipitation of Forsterite + Diopside causes liquid composition to move down cotectic curve, producing a wehrlite cumulate.

Forsterite – Diopside - Anorthite

X

P = 1 atmThe liquid composition reaches the ternary eutectic at 1270°C, at which point Anorthite begins to crystallize with Diopside and Forsterite, producing a gabbroic cumulate.

The composition of the liquid remains at the eutectic point until all the liquid is consumed.

Forsterite – Diopside - Anorthite

The composition of the first melt of an assemblage ABD is that of invariant eutectic point eABD, while the composition of the first melt of assemblage DBC is that of invariant eutectic point eDBC.

The intersection of a univariant curve with the Alkemade line joining the compositions of the coexisting solid phases defines a thermal maximum along the univariant curve.

Ternary Systems

Ternary Systems with

Solid Solution:

The invariant point “p”, at which olivine, clinopyroxene and orthopyroxene coexist with a liquid, is a peritectic point because it lies outside of the compositional volume of the solid phases. It represents the first melt of any assemblage consisting of olivine, opx, and cpx (mantle peridotite) at 1 atm., and is analogous in composition to a quartz-normative basalt.

Similarly, the univariant curve along which olivine and orthopyroxene coexist with a liquid is a reaction curve because the tangent to the curve at any point cuts the olivine - orthopyroxene Alkemade line with a negative olivine intercept.

The invariant point “e” is a eutectic and represents the composition of the first melt of an assemblage of quartz-diopside-orthopyroxene. The composition of “e” approximates that of the Earth’s continental crust.

Oliv - Cpx - Qtz Liquidus Projection:

SiO2 45.2 49.4 60.3 TiO2 0.7 1.4 1.0 Al2O3 3.5 15.4 15.6 MgO 37.5 7.6 3.9 FeO 8.5 10.1 7.2 CaO 3.1 12.5 5.8 Na2O 0.6 2.6 3.2 K2O 0.1 0.3 2.5 Total 99.2 99.3 99.5

Cations normalized to 100 cations Si 38.5 46.1 56.4 Ti 0.5 1.0 0.7 Al 3.6 16.9 17.2 Mg 47.6 10.6 5.4 Fe 6.0 7.9 5.6 Ca 2.8 12.5 5.8 Na 0.9 4.7 5.8 K 0.1 0.5 3.0 O 140.2 153.0 161.3

Mineralogy (oxygen units, XFe3+ = 0.10) Quartz 0.0 0.0 13.0 Feldspar 13.2 57.3 64.3 Clinopyroxene 6.7 25.7 5.9 Orthopyroxene 18.3 4.1 14.7 Olivine 59.9 9.9 0.0 Oxides 1.8 3.0 2.0

Mantle Ocean Continent crust crust

Oceanic crust - MORB basalt p

Continental crust - granite e

Liquidus Projections for haplo-basaltsThe Basalt Tetrahedron at 1 atm:

The olivine - clinopyroxene - plagioclase plane is a thermal divide in the haplo-basalt system at low pressures and separates natural magmas into two fundamentally different magmatic series. Sub-alkaline basaltic magmas with compositions to the Qtz-rich side of the plane fractionate towards Qtz-saturated residual liquids, such as rhyolite. Alkaline basaltic magmas with compositions to the Qtz-poor side of the plane fractionate towards residual liquids saturated in a feldspathoid, such as nepheline phonolite.

Since the dominant mineral in the mantle source of basaltic magmas is olivine, we can achieve a further simplification by projecting the liquidus of basaltic systems from the perspective of olivine:

Alkaline basaltic lavas are volumetrically insignificant (~1%), but strongly enriched in highly incompatible trace elements profiles compared to sub-alkaline lavas, and low in HREE, Y, & Sc. These characteristics are generally ascribed to small degrees of partial melting at elevated pressures, leaving garnet as a phase in the refractory residue.

Alkaline basalts fall to the Foid-side of the olivine-clinopyroxene-plagioclase plane (1 atm thermal divide) and fractionate to foid-saturated residual liquids. Sub-alkaline basalts fall to the Quartz-side and fractionate towards quartz-saturated residual liquids.

The Effect of Pressure

Increasing pressure shifts the oliv-cpx-opx peritectic point towards less Si-rich compositions. At approximately 10 kbs this invariant point moves into the oliv - cpx- opx compositional volume, and the first melt of the mantle has an olivine basalt composition. The invariant point is still a peritectic point, however, because of the extensive solid solution of cpx towards opx. At pressures exceeding 15-20 kbs, this invariant point moves outside the simple olivine - cpx - qtz system, into the Neph-normative volume of the basalt tetrahedron. The first melt of mantle peridotite is an alkaline olivine basalt at these high pressures.

1 atm

Since the dominant mineral in the mantle source of basaltic magmas is olivine, we can achieve a further simplification by projecting the liquidus of basaltic systems from the perspective of olivine:

Movement of the invariant point determining the composition of the first melt with increasing pressure.