Volatiles in Silicate Melts Francis, 2013

Volatile have an importance beyond that predicted simply by their abundance because: - Volatiles have low molecular weights: H 2 O = 18 CO 2 = 44 SiO 4 = 92 NaAlSi 3 O 8 = 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 NaAlSi 3 O 8 clusters and H 2 O molecules: 0.5 wt. % H 2 0 ~ 45 mole % H 2 O Small amounts of water produce large effects because of its low molecular wt. compared to that of a silicate magma

H 2 O = % CO 2 = % SO 2 = % CO = ≤1 % H 2 = ≤1 % H 2 S = ≤1 % HCL = ≤1 % <2 % Kilauea 98 % Significant Elemental Players: H, C, O, S, lessor Cl, F Dominant molecular species in exolved volatiles at surface CH 4 CO Under more reducing conditions in the mantle < FMQ-3

Water is a Basic Component: H 2 O + O bridging 2 × OH Water depolymerizes silicate melts dymer 2 monomers f H20 ~ P H2O α X H2O. 2 For XH 2 O <

H 2 O 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 % H 2 O and 66 mole % OH.

6 wt.% first boiling xylization second boiling 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 o C 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: Δ o C liquidus = × (H 2 O wt.%) Falloon & Danyushevsky, 2000

Although dissolved water produces a large decrease in the liquidus and solidus temperatures, the Fe-Mg partitioning remains essentially unaffected: Δ o C liquidus = × (H 2 O wt.%) Falloon & Danyushevsky, 2000

H hydrous A anhydrous + water  G (P,T) =  G o (P,T) + RTln(aH 2 O)(aA) = 0 (aH)  G (P,T) =  H o (1bar,T) - T  S o (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, H 2 O compresses, the  V of the reaction decreases, and the slope of the reaction increases and may even become negative because typically V A < V H. If H and A are pure phases, but the fluid phase is diluted by another component such as CO 2, then the maximum thermal stability of H is reduced by an amount given by:  G o (P,T) = - RTln (XH 2 O)

Melting and dehydration of a hydrous phase

Amphibole Melting Damp Solidus

CO 2 is an Acid Component at high pressures: CO 2 + 2O nonbridging CO 3 + O bridging CO 2 polymerizes silicate melts: 2 monomers dymer f CO2 ~ P CO2 α X CO2 At pressures below ~ 25 kbs, CO 2 is dissolved in silicate melts at low levels as the neutral species CO 2. At pressures above 25 kbs, however, the solubility of CO 2 greatly increases with CO 2 dissolved as the carbonate ion species CO 3 =. -6

CO 2 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

CO 2 – Saturated Mantle Solidus At pressures below ~ 25 kbs the solubility of CO 2 in silicate melts is low, and the CO 2 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 CO 2 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 CO 2 – H 2 O

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 CO 2 – H 2 O

Upper Mantle

Oxygen fugacity O 2 + Ni NiO

The Oxidation State of Magmas Korzinski observed long ago that: The ratio of Fe 3+ / Fe 2+ in a silicate melt increases with its basicity 4 FeO Fe O = + O 2 FeO is a basic component: Fe 2 O 3 is a relatively acidic component: K FeO2 = ([ a O = ] 6 ×[ a Fe 2+ ] 4 × [fO 2 ]) / ( a FeO 2 -1 ) 4 FeO Fe 2+ + O = K FeO = ([ a O = ] × [ a Fe 2+ ]) / [ a FeO] K Fe2O3 = [ a FeO 2 - ] 2 / ([ a Fe 2 O 3 ] × [ a O = ]) Fe 2 O 3 + O = 2 × [FeO 2 ] -1

~XFe

Increasing Oxidation State has an effect on a NORM calculation: 3Fe 2 SiO 4 + O 2 2Fe 3 O 4 + 3SiO 2 2SiO 2 + NaAlSiO 4 NaAlSi 3 O 8 SiO 2 + Fe 2 SiO 4 2FeSiO 3 FMQ nepheline albite fayalite to magnetite + quartz Increasing oxidation leads to more SiO 2 which in turn leads to the following transformations in the NORM calculation: Olivine to Orthopyroxene Feldspathoids to Feldspars SiO 2 + Mg 2 SiO 4 2MgSiO 3 SiO 2 + KAlSi 2 O 6 KAlSi 3 O 8 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. Ce 3+ Ce 4+ incompat ible soluble, mobile Eu 2+ Eu 3+ compatible in Feldspar relatively incompatible Cr 2+ Cr 3+ incompatible on Moon compatible in Spinel & Cpx V 2+ V 3+ V 4+ V 5+ compatible in silicates incompatible in silicates compatible in oxides Reducing Oxidizing

Oxidation State of the Cordilleran Mantle 2×Fe 2+ Fe 2 3+ O4 + 6×FeSiO 3 = 6×Fe 2 2+ SiO 4 + O 2 spinel opx oliv Most likely oxygen buffer in the spinel lherzolite field: P

Most likely oxygen buffer in the garnet lherzolite field: 2×Fe 3 2+ Fe 2 3+ Si 3 O 12 + = 4×Fe 2 2+ SiO 4 + 2×FeSiO 3 + O 2 garnet oliv opx Oxidation State of Cratonic Mantle Roots Negative Δ V, means fO 2 decreases with depth

Sulfur The solubility of S in silicate melts is a function of: fO 2, Fe content temperature. At low fO 2, S acts as a basic component: 1/2S 2 + O 2- > 1/2O 2 + S 2- At high fO 2, S acts as a acid component: 1/2S 2 + 3O 2 + O 2- > SO 4 2- S 2- is the dominate species in most natural mafic magmas SO 4 2- is the dominate species in most natural felsic magmas

S 2- appears to be preferentially associated with Fe 2+ 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.