Thermal reduction of molybdite and hematite in water and

H2O2-H2O solutions as a tool to determine oxygen fugacity in

HDAC experiments

GAC-MAC 2012 GS-8: Economic Geology

Giulio Solferino & Alan J. Anderson

Solferino      –      GAC-­‐MAC  2012  

Structure  of  the  talk:    A.  An$cipated:  Outcomes  of  the  study  

 1.  Mo2va2on  

2.  Experimental  strategy  and  results  

3.  Discussion  and  conclusions  

4.  Geological  implica2ons:  study  of  hydrothermal  systems    

ANTICIPATED OUTCOMES:

Oxygen fugacity in HDAC experiments imposed BY THE FLUID

Use of metal gasket reduces fO2 BUT DOES NOT act as a buffer

Hydrogen peroxide increases fO2 efficiently

Briefly: Hydrothermal Diamond Anvil Cell

gasket / no gasket

Bassett, W.A., et al., 1993. ‘A new…1200 °C’. Rev. Sci. Instrum., 64, 2340-2345.

Chou, I-M, Anderson, A.J., 2009. ‘Diamond…cells’. Geoc. Cosm. Acta, 73, 6360-6366.

Briefly: HDAC

V

L

S

Temperature of homogenization: Th T at which: L + V = L i.e., the vapor bubble disappears

MOTIVATION:

OXYGEN FUGACITY:

-  Little to no effort (assumed to be controlled by noble metal gasket)

- Attempts to impose it using a solid buffer (Mo rod – Mysen & Yamashita, 2010)

TEMPERATURE:

- 2 thermocouples

- Diamond excellent heat conductor

PRESSURE:

-  Lot of effort

- Raman vibration spectroscopy on: 13C-diamond and quartz (Mysen, 2010, Schmidt & Zieman, 2000)

MOTIVATION: OXYGEN FUGACITY:

It influences:

-  melt/fluid to mineral partitioning of multivalent elements

-  fluid speciation

-  onset of mantle and crustal melting

-  dissolution/precipitation of minerals Indirectly, by imposing redox state of multivalent component:

-  solubility and element speciation in fluid/melt

-  structure of the fluid/melt

STRATEGY + RESULTS:

MOLYBDITE (MoO3 ): Thermal reduction of MoO3 to MoO2 described by Pankratz (1982). His equation defines an oxygen buffer. Easy to visualize: molybdite is highly birefringent (green to pink), whereas tugarinovite (MoO2 ) is non-birefringent and dark-brown to black colored. HEMATITE (Fe2O3 ): Hematite-Magnetite oxygen buffer is well known Immediate to recognize with Raman spectroscopy

STRATEGY + RESULTS:

USE OF HYDROGEN PEROXIDE (H2O2 ): Impose larger fO2 without introducing ‘foreign’ elements to the water medium, like Carbon or transition metals

Raman spectroscopy: After quench. Low laser power critical for magnetite.

STRATEGY + RESULTS:

4 sets of experiments:

Re gasket + MoO3 + water : G series

Without gasket + MoO3 + water : D series

Without gasket + MoO3 + H2O2:

series 0.10M to 0.25M

Without gasket + Fe2O3 + water: Hm series

STRATEGY + RESULTS: First part: Complete dissolution of MoO3. No tugarinovite precipitated. 0.25 M sol. Second part: Tugarinovite precipitates at 370 °C. 0.10 M solution.

Tugarinovite produced in an experiment

Molybdite produced in an experiment

STRATEGY + RESULTS: Raman spectra:

STRATEGY + RESULTS: Raman spectra:

Hematite and magnetite

Hem -rruff

Mag -rruff

Hem SM

Hm-1

Hm-2-b

Hm-2-a

STRATEGY + RESULTS:

Log(fO2) ~ -19.5

Log(fO2) ~ -20.6

Error bars plot inside symbols

DISCUSSION:

1. Oxygen fugacity is HDAC runs seems to be imposed by the fluid medium. In T-P range of about 300-500 °C and 15-140 MPa it does not vary and it’s equal to -19,55 Log units for pure water, independently of the solid phase.

Log(fO2) ~ -19.5

DISCUSSION: 1.2. Gasket has a slightly reducing effect (i.e., 1.1

Log(fO2) unit), but can’t be the actual buffer since at 200-350 °C the value of Log(fO2) imposed by Re-ReO2 is so low (i.e., -32 to -27 Log units) that tugarinovite would have precipitated in all experiments.

Oxygen fugacity is reduced by formation of ReO2 on the gasket wall until completely armored by the oxide. The reaction drag oxygen out from the fluid phase.

DISCUSSION: 1.2. Gasket has a slightly reducing effect (i.e., 1.1

Log(fO2) unit), but can’t be the actual buffer since at 200-350 °C the value of Log(fO2) imposed by Re-ReO2 is so low (i.e., -32 to -27 Log units) that tugarinovite would have precipitated in all experiments.

Oxygen fugacity is reduced by formation of ReO2 on the gasket wall until completely armored by the oxide. The reaction drag oxygen out from the fluid phase.

DISCUSSION: Error bars plot inside symbols

DISCUSSION: 2. Non-linear or Non-log-linear variation of oxygen

fugacity with H2O2 molarity might be due to increase of ‘density of the fluid’ and/or ‘run pressure’ at high temperature. Both parameters reduce the decomposition constant of H2O2. On the contrary hydrogen peroxide breaks off more readily above 350-370 °C. Density of fluid varies due to dissolution of MoO3 into it (remember video).

CONCLUSIONS:

1. Oxygen fugacity in HDAC is imposed by the fluid medium. For pure water a value of -19.55 ± 0.13 Log units over a T-P range of 210-500 °C and 6.5-136.3 MPa was determined.

2. Using a noble metal gasket (Rhenium) as sample

container reduces the fO2 of the system MoO3-H2O. The reduction is estimated to be 1.1 ± 0.12 Log units, due to oxidation of the metal gasket.

3. Hydrogen peroxide can be efficiently used to

increase oxygen fugacity of water-dominated fluid media.

IMPLICATIONS for study of hydrothermal systems:

A. In experimentation with HDAC beware of what the actual fO2 is. It needs to be determined.

B. Introduction of a solid buffer might not be an

appropriate way to operate, without previous determination, e.g., using a thermal reduction reaction.

C. HDAC still remains one of the best tool to

investigate water dominated fluid media, due to possibility of in-situ investigation and VISUALIZATION of the sample.

THANK YOU FOR THE ATTENTION

References:

Mysen, B.O., 2010, ‘Speciation…pressure’. Am.Min., 95, 1807-1816.

Mysen B.O., Yamashita, S., 2010, ‘Speciation…and 800 °C’.

Geoch.Cosmoc., 74, 4577-4588. Pankratz, L.B., 1982, ‘Thermodynamic…oxides’. U.S.

Bureau of Mines, Bulletin 672. Schmidt, C., Zieman, M.A., 2000, ‘In-situ…temperatures’.

Am.Min., 85, 1725-1734.