Volatiles in Silicate MeltsFrancis, 2013

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

- Volatiles have low molecular weights:

H2O = 18

CO2 = 44

SiO4 = 92

NaAlSi3O8 = 262

- Volatiles are mobile: They can move around as an immiscible fluid phase, entering new regions carrying trace elements and heat, metasomatising their surroundings, lowering melting points and inducing partial melting.

In a melt consisting of NaAlSi3O8 clusters and H2O molecules:

0.5 wt. % H20 ~ 45 mole % H2O

Small amounts of water produce large effects because ofits low molecular wt. compared to that of a silicate magma

H2O = 50 - 80 %CO2 = 5 - 25 %SO2 = 10 - 20 %

CO = ≤1 %H2 = ≤1 %H2S = ≤1 %HCL = ≤1 %

<2 %

Kilauea

98 %

Significant Elemental Players: H, C, O, S, lessor Cl, F

Dominant molecular species in exolved volatiles at surface

CH4

COUnder more reducing conditions in the mantle < FMQ-3

Water is a Basic Component:

H2O + Obridging 2 × OH

Water depolymerizes silicate melts

dymer 2 monomers

fH20 ~ PH2O α XH2O.2

For XH2O < 0.3

-6 -3-3

H2O is an basic component that not only lowers the temperature of the liquidus and solidus of silicate melts, but also shifts the positions of cotectics, eutectics,

etc towards more acidic compositions, and expands the liquidus volumes of minerals rich in basic components with respect to those rich in acidic

components

Effect of Water on the Basalt Tetrahedron

Water acts as a basic component that shifts the positions of cotectics, eutectics, etc towards more acidic compositions, and expands the liquidus volumes of minerals rich in basic components with respect to those rich in acidic components

Some evidence that water preferentially attacks Al-O-Si bridging oxygens, for example the cotectic shift with water pressure in petrogeny’s

residua system.

The Effect of Water

The solubility of water in silicate melts is strongly a function of pressure, and the concentration of dissolved water tends to be highest in felsic silicate melts because water behaves as an incompatible element during crystal fractionation, being concentrated in the residual liquid. The effectiveness of water in lowering the liquidus of silicate melts is a reflection of its low molecular weight and the fact that it appears to dissolve by dissociation into two OH- ions. For example, 6-7 wt.% water corresponds to approximately 50 mole % H2O and 66 mole %

OH.

6 wt.%

first boilingxylization

secondboiling

xylization

Loss of Water:

Water-rich granitic magmas have difficulty reaching the surface because of the loss of dissolved water as pressure decreases, known as first boiling. This leads to solidification because of the consequent rise in the solidus temperature.

A rapid decrease in the solubility of water in granitic melts between 600-700oC can result in the exsolution of enormous volumes of water, known as second boiling. In some cases this leads to the explosive eruption of rhyolitic ash flow deposits (ignimbrites) from ruptured high-level granitic plutons. In other cases, the expulsion of water-rich volatiles may lead to the formation of hydrothermal ore deposits in the surrounding host rocks. By the same account, water-rich

Wet Melting of a Mantle Peridotite

Effect of Water on Plagioclase

Water’s effect is most dramatic on the crystallization of feldspar

Dissolved water produces a large decrease in liquidus and solidus temperatures:

ΔoCliquidus = 74.403 × (H2O wt.

%)0.352

Falloon & Danyushevsky, 2000

Although dissolved water produces a large decrease in the liquidus and solidus temperatures, the Fe-Mg partitioning remains essentially unaffected:

ΔoCliquidus = 74.403 × (H2O wt.%)0.352

Falloon & Danyushevsky, 2000

Hhydrous Aanhydrous + water

G (P,T) = Go(P,T) + RTln(aH2O)(aA) = 0 (aH)

G (P,T) = Ho(1bar,T) - T So(1bar,T) + (P-1) V = 0

Stability of Hydrous Phases

At low pressures, V is positive, and the reaction has a positive slope: (dP/dT = S/V)

With increasing pressure, however, H2O compresses, the V of the reaction decreases, and the slope of the reaction increases and may even become negative because typically VA < VH.

If H and A are pure phases, but the fluid phase is diluted by another component such as CO2, then the maximum thermal stability of H is reduced by an amount given by:

Go(P,T) = - RTln (XH2O)

Melting and dehydration of a hydrous phase

Amphibole Melting

Damp Solidus

CO2 is an Acid Component at high pressures:

CO2 + 2Ononbridging CO3 + Obridging

CO2 polymerizes silicate melts:

2 monomers dymer

fCO2 ~ PCO2 α XCO2

At pressures below ~ 25 kbs, CO2 is dissolved in silicate melts at low levels as the neutral species CO2. At pressures above 25 kbs, however, the solubility of CO2 greatly increases with CO2 dissolved as the carbonate ion species CO3

=.

-6

CO2 is an acid component that shifts the positions of cotectics, eutectics, etc towards more basic compositions, and expands the liquidus volumes of minerals rich in acid components with respect to those rich in basic components

CO2 – Saturated Mantle Solidus

At pressures below ~ 25 kbs the solubility of CO2 in silicate melts is low, and the CO2 saturated solidus of mantle peridotite is only slight depressed with respect to the dry solidus.

At pressures greater than 25 kbs mantle peridotite becomes carbonated in the presence of CO2 and its solidus is greatly depressed with the presence of carbonate-rich initial melt compositions.

Note: amphibole stability field not shown for clarity

After Eggler

DryMixed CO2 – H2O

ZIVC – zone of invariant vapour composition

If insufficient fluid is available to completely amphibolitize or carbonatize mantle peridotite, then the fluid composition will be buffered when amphibole or carbonate is stable.

After Eggler

I1

I2

I3

After Eggler

DryMixed CO2 – H2O

Upper Mantle

Oxygen fugacity

O2 + Ni NiO

The Oxidation State of Magmas

Korzinski observed long ago that:

The ratio of Fe3+ / Fe2+ in a silicate melt increases with its basicity

4 FeO2-1

4 Fe2+ + 6 O= + O2

FeO is a basic component:

Fe2O3 is a relatively acidic component:

KFeO2 = ([aO=]6×[aFe2+]4 × [fO2]) / (aFeO2-1)4

FeO Fe2+ + O= KFeO = ([aO=] × [aFe2+]) / [aFeO]

KFe2O3 = [aFeO2-]2 / ([aFe2O3] × [aO=])Fe2O3 + O= 2 × [FeO2]-1

~XFe3+ 0.05 0.15 0.25 0.40 1.00

Increasing Oxidation State has an effect on

a NORM calculation:

3Fe2SiO4 + O2 2Fe3O4 + 3SiO2

2SiO2 + NaAlSiO4 NaAlSi3O8

SiO2 + Fe2SiO4 2FeSiO3

FMQ

nepheline albite

fayalite to magnetite + quartz

Increasing oxidation leads to more SiO2 which in turn leads to the following transformations in the NORM calculation:

Olivine to Orthopyroxene

Feldspathoids to Feldspars

SiO2 + Mg2SiO4 2MgSiO3

SiO2 + KAlSi2O6 KAlSi3O8

leucite orthoclase

Oxidation State and Trace Element Partitioning

A number of trace elements have variable oxidation states that affect their partitioning between liquid and solid phases.

Ce3+ Ce4+

incompat ible soluble, mobile

Eu 2+ Eu3+

compatible in Feldspar relatively incompatible

Cr2+ Cr3+

incompatible on Moon compatible in Spinel & Cpx

V2+ V3+ V4+ V5+

compatible in silicates incompatible in silicates compatible in oxides

Reducing Oxidizing

Oxidation State of the Cordilleran Mantle

2×Fe2+Fe23+O4 + 6×FeSiO3 = 6×Fe2

2+SiO4 + O2

spinel opx oliv

Most likely oxygen buffer in the spinel lherzolite field:

P

Most likely oxygen buffer in the garnet lherzolite field:

2×Fe32+Fe2

3+Si3O12 + = 4×Fe22+SiO4 + 2×FeSiO3 + O2

garnet oliv opx

Oxidation State of Cratonic Mantle Roots

Negative ΔV, means fO2 decreases with depth

Sulfur

The solubility of S in silicate melts is a function of:

• fO2,

• Fe content• temperature.

At low fO2, S acts as a basic component:

1/2S2 + O2- > 1/2O2 + S2-

At high fO2, S acts as a acid component:

1/2S2 + 3O2 + O2- > SO42-

S2- is the dominate speciesin most natural mafic magmas

SO42- is the dominate species

in most natural felsic magmas

S2- appears to be preferentially associated with Fe2+

in most natural mafic silicate magmas:

Mars

Earth

Martian basalts contain more than 4 times as much S as terrestrial basalts, in part because of their high Fe contents.

Falling temperature, increasing oxidation state and decreasing Fe content lead to saturation in sulfur.

Many terrestrial magmas are saturated in sulfur before they reach the surface and carry immiscible sulfide droplets that are dominantly FeS in composition, but carry most of the chalcophile trace elements, such as Ni, Cu, Zn, Pb, etc.