ZIRCON SOLUBILITY IN ALKALINE AQUEOUS FLUIDS AT UPPER CRUSTAL CONDITIONS

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1 ZIRCON SOLUBILITY IN ALKALINE AQUEOUS FLUIDS AT UPPER CRUSTAL CONDITIONS Ayers J.C. 1, Zhang L. 1,2, Luo Y. 1,3, Peters T Dept. of Earth & Environmental Sciences, Vanderbilt University 2. Division of Geological and Planetary Sciences, California Institute of Technology 3. Department of Earth Sciences, The University of New Brunswick, Canada

2 Zircon Solubility Diagram In alkaline solution: ZrSiO 4 (xt) + 2H 2 O + OH - = SiO 2 (aq) + Zr(OH) log Zr concentration (molality) Baddeleyite Zr(OH)5 - a Zircon b Quartz saturation We measured the solubilities of the assemblages a and b log Si concentration (molality) Solubility diagram for the system ZrO 2 -SiO 2 -H 2 O at 200 C and 1m NaOH calc. using LLNL.dat of GWB (Bethke, 1996) and the stability constants of Adair et al. (1997).

3 ph Dependence of Solubility of Zircon + Baddeleyite The solubility of the assemblage baddeleyite + zircon at 200 C as a function of ph, calculated using LLNL.dat of GWB (Bethke, 1996) and stability constants of Adair et al. (1997).

4 Zircon Solubility In Quartz-saturated Fluid 8 log a Zr(OH) Zr(OH) +++ Zr(OH) 2 ++ Zircon Zr(OH) 3 + Zr(OH) 4 (aq) ph Zr(OH) 5-25 C ayersj Thu Apr C and 1 bar pressure, thermodynamic data from Adair et al. (1997)

5 Zircon Solubility In Quartz-saturated Fluid 0 2 log a Zr(OH) Zircon Zr(OH) ZrOH Zr(OH) 4 (aq) C ph berrios Mon Apr Diagram Zr(OH) 2 ++, T = 25 C, P = bars, a [H2 O] = 1, a [Quartz] = 1 25 C and 1 bar pressure, default LLNL thermodynamic data from Naumov et al. (1974)

6 Methods Quench Zr and Si leached from outer capsule; leachate analyzed by ICP-MS.

7 Zir-003 (Z + B) Run Product Zircons

8 Zir-001 (Z + Q) Run Product Zircon

9 SM BE SM CL Zircon grains microns in diameter. RP BE RP CL

10 Baddeleyite Crystals on Surface of Run Product Zircon Grain

11 Baddeleyite Crystals Filling a Pore Within a Run Product Zircon Grain

12 Zircon Unstable in 1m NaOH at 450 C? Vlasovite (Na 2 ZrSi 4 O 11 ) Zircon

13 Summary Of Zircon Experiments At 0.2 GPa Sample SM a T ( C) NaOH (m) ph t (days) RP SEM RP XRD moles Zr/kg H 2 O moles Si/kg H 2 O c Zir-001 Z Z+B 3.62(0.03) Zir-002 Z Z+B Z 2.66(0.09) Zir-003 Z+Q Z+Q 1.30(0.03) Zir-004 Z+Q Z+Q+V Z+Q+V 3.32(0.03) Zir-005 Z Z+B+V Z+B+V 1.46(0.003) Zir-006 Z Z+B+V 2.0(0.3) Zir-007 Z Z+B+V 4.0(2.4) Zir-008 Z Z+B+V 8.2(2.9) Zir-009 Z (0.3) Zir-010 Z Z+B+V 2.5(0.4) Zir-011 Z Z+B 1.7(0.3) Zir-012 Z Z+B Z 2.1(0.3) Zir-013 Z Z+B Z 1.8(0.8) Zir-014 Z Z+B 1.2(0.3) Zir-015 Z Z+B 1.2(0.3) Zir-016 Z Z+B Z 4.5(2.3) Zir-017 Z Z 4.4(3.3) Zir-018 Z+Q Z+Q+V 1.1(0.3) Zir-019 Z Z 3.6(0.3) Zir-020 Z Z 2.3(0.3) Zir-021 Z (0.6)

14 Time Dependence of Z+B Solubility in 1m NaOH at 600 C 1e-3 Zr concentration (molality) 1e-4 1e-5 1e-6 1e-7 1e-8 PB MDL IDL 1e Duration (days)

15 ph Dependence of Zircon Solubility 1e-4 Zr concentration (molality) 1e-5 1e-6 1e C 750 C 1e ph

16 Zircon Solubility at 1.0 GPa and 1000 C (Single Crystal Weight Loss in Piston Cylinder) y = 0.50x R 2 = m ZrSiO DC y = 0.35x R 2 = Zircon in NaF DC Zircon in NaOH m Na

17 Solubility Equation Derivation In alkaline solution: ZrSiO 4 (xt) + 2H 2 O + OH - = SiO 2 (aq) + Zr(OH) 5 - ln K = ln [Zr(OH) 5- (aq)] + ln [SiO 2 (aq)] ln [OH - ] = -ΔG/RT ln [Zr(OH) 5- (aq)] = -ΔH/RT + ΔS/R + ln [OH - ] ln [SiO 2 (aq)] - ln γ SiO2(aq) ln γ Zr(OH)5-(aq) Performed MLR with dependent variable ln (Zr) and independent variables 1/T, [OH-], and ln (Si); the terms ΔS/R, ln γ Zr(OH)5-(aq), and ln γ SiO2(aq) are lumped together as the intercept b: ln (Zr) = -ΔH/R(1/T) + ln [OH - ] ln (Si) + b The term ln (Si) has a (-) coefficient, implying that as (Si) (Zr) should. We made measurements at 3 different values of 1/T and [OH-], but the coefficients for these parameters were not statistically different from zero at the 95% level (P > 0.05). The term ln (Si) accounted for 85% of the observed variation in ln (Zr): ln (Zr) = 2.9* ln (Si) The positive coefficient for ln (Si) gives the zircon solubility curve a (+) slope, implying that Zr complexes with Si.

18 Evidence of Zr-Si Complexing -6 ln Zr concentration (molality) y=2.9x+0.25 r= ln Si concentration (molality)

19 Revised Zircon Solubility Diagram GPa, 600 C ln Zr concentration (molality) Baddeleyite Zr(OH)5 - Zircon Quartz ln Si concentration (molality)

20 from Manning (2007) Evidence for Al-Si Complexing

21 Conclusions Zircon dissolved incongruently to form baddeleyite at all temperatures, + vlasovite at 450 C. In quartz-saturated fluids zircon dissolved congruently at 600 and 750 C, while vlasovite formed at 450 C. Zircon solubility increases with increasing molality of NaOH (ph), possibly due to formation of Zr(OH) 5- at low silica activities. Zircon solubility is enhanced by aqueous complexing of Zr with Si according to: ln (Zr) = * ln (Si)

22 This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. References Adair, J. H., Krarup, H. G., Venigalla, S., and Tsukada, T., 1997, A review of the aqueous chemistry of the zirconium-water system to 200C: Materials Research Society Symposium Proceedings, v. 432, p Bethke, C., 1996, Geochemical Reaction Modeling: Concepts and Applications: New York, NY, Oxford University Press, Inc., 397 p. Manning, C. E., 2007, Solubility of corundum + kyanite in H2O at 700 C and 10 kbar: evidence for Al-Si complexing at high pressure and temperature: Geofluids, v. 7, p Naumov G. B. et al.. (1974) Handhook of Thermodynamic Data. U.S. Dept. Comm. Natl. Techn. Info. Serv., Reproduction No. Pb , 328 pp.