1 / 34

Volatiles in Silicate Melts Francis, 2013

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 = 262.

weston
Download Presentation

Volatiles in Silicate Melts Francis, 2013

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Volatiles in Silicate Melts Francis, 2013

  2. 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 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 of its low molecular wt. compared to that of a silicate magma • 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.

  3. Significant Elemental Players: H, C, O, S, lessor Cl, F Kilauea H2O = 50 - 80 % CO2 = 5 - 25 % SO2 = 10 - 20 % CO = ≤1 % H2 = ≤1 % H2S = ≤1 % HCL = ≤1 % 98 % Dominant molecular species in exolved volatiles at surface <2 % CH4 CO Under more reducing conditions in the mantle < FMQ-3

  4. Water is a Basic Component: fH20 ~ PH2Oα XH2O.2 For XH2O < 0.3 H2O + Obridging 2 × OH -6 -3 -3 dymer 2 monomers Water depolymerizes silicate melts

  5. 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

  6. Effect of Water on the Basalt Tetrahedron

  7. 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

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

  9. 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.

  10. 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. 6 wt.% xylization second boiling 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 first boiling xylization

  11. Wet Melting of a Mantle Peridotite

  12. Effect of Water on Plagioclase Water’s effect is most dramatic on the crystallization of feldspar

  13. Dissolved water produces a large decrease in liquidus and solidus temperatures: ΔoCliquidus = 74.403 × (H2O wt.%)0.352 Falloon & Danyushevsky, 2000

  14. 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

  15. Stability of Hydrous Phases 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 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)

  16. Melting and dehydration of a hydrous phase

  17. Damp Solidus Amphibole Melting

  18. CO2 is an Acid Component at high pressures: fCO2 ~ PCO2α 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=. CO2 + 2Ononbridging CO3 + Obridging -6 2 monomers dymer CO2 polymerizes silicate melts:

  19. 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

  20. CO2 – Saturated Mantle 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. 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. Note: amphibole stability field not shown for clarity

  21. Mixed CO2 – H2O Dry After Eggler

  22. 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. I3 I2 I1 After Eggler

  23. Mixed CO2 – H2O Dry After Eggler

  24. Upper Mantle

  25. Oxygen fugacity O2 + Ni NiO

  26. The Oxidation State of Magmas Korzinski observed long ago that: The ratio of Fe3+ / Fe2+ in a silicate melt increases with its basicity FeO is a basic component: KFeO = ([aO=] × [aFe2+]) / [aFeO] FeO Fe2+ + O= Fe2O3 is a relatively acidic component: KFe2O3 = [aFeO2-]2 / ([aFe2O3] × [aO=]) Fe2O3 + O= 2 × [FeO2]-1 4 FeO2-1 4 Fe2+ + 6 O= + O2 KFeO2 = ([aO=]6×[aFe2+]4× [fO2]) / (aFeO2-1)4

  27. ~XFe3+ 0.05 0.15 0.25 0.40 1.00

  28. Increasing Oxidation State has an effect on a NORM calculation: FMQ 3Fe2SiO4 + O2 2Fe3O4 + 3SiO2 fayalite to magnetite + quartz Increasing oxidation leads to more SiO2 which in turn leads to the following transformations in the NORM calculation: Feldspathoids to Feldspars nepheline albite 2SiO2 +NaAlSiO4 NaAlSi3O8 leucite orthoclase SiO2 +KAlSi2O6 KAlSi3O8 Olivine to Orthopyroxene SiO2 +Fe2SiO4 2FeSiO3 SiO2 +Mg2SiO4 2MgSiO3

  29. Oxidation State and Trace Element Partitioning A number of trace elements have variable oxidation states that affect their partitioning between liquid and solid phases. Reducing Oxidizing Ce3+ Ce4+ incompat iblesoluble, 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

  30. Oxidation State of the Cordilleran Mantle P Most likely oxygen buffer in the spinel lherzolite field: 2×Fe2+Fe23+O4 + 6×FeSiO3 = 6×Fe22+SiO4 + O2 spinel opx oliv

  31. Oxidation State of Cratonic Mantle Roots Most likely oxygen buffer in the garnet lherzolite field: 2×Fe32+Fe23+Si3O12 + = 4×Fe22+SiO4 + 2×FeSiO3 +O2 garnet oliv opx Negative ΔV, means fO2 decreases with depth

  32. 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- S2- is the dominate species in most natural mafic magmas At high fO2,S acts as a acid component: 1/2S2 + 3O2 + O2- > SO42- SO42- is the dominate species in most natural felsic magmas

  33. S2- appears to be preferentially associated with Fe2+ in most natural mafic silicate magmas: 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. Mars Earth Martian basalts contain more than 4 times as much S as terrestrial basalts, in part because of their high Fe contents.

More Related