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High spatial resolution analysis of noble gases using UV lasers
Goldschmidt Conference Abstracts 2006
The solubility of copper in hydrous rhyolitic melts: The effects of oxygen, chlorine and sulfur 1
1,2
N.S. KELLER , J.A. MAVROGENES , B. SCAILLET
High spatial resolution analysis of noble gases using UV lasers S.P. KELLEY
3
1
Research School of Earth Sciences, Australian National University, Australia ([email protected]) 2 Department of Earth and Marine Sciences, Australian National University, Australia ([email protected]) 3 Institut des Sciences de la Terre d’Orle´ans, CNRS-UO, Australia ([email protected]) Porphyry-Cu deposits are unquestionably linked to subduction zone (arc) magmas. Yet the source magmas for these deposits remain unclear, despite recognition that arc-magmas are waterbearing and relatively oxidised. This study focuses on understanding some of the chemical parameters that may affect the solubility of Cu in hydrous melts using experimental petrology. In particular, the effect of oxygen fugacity and Cl and S content of the melt was investigated. The experiments were conducted in an internally heated pressure vessel where the oxygen fugacity is controlled by the addition of H2 to the Ar pressure medium. The sample charges consisted of powdered synthetic glass of rhyolitic composition containing water (7 wt%) and various amounts of NaCl or a S-bearing phase (FeS or CaSO4 depending on fO2). The samples were sealed in welded AuCu capsules and run at 4 kbars and 800 °C and 900 °C for up to 10 days. Twelve samples can be run simultaneously in the same vessel enabling the P, T and fO2 conditions to be identical for each sample. The fO2 was checked using solid Ni–Pd and Co–Pd sensors. The fO2 ranged from NNO1 to approx. NNO+3; the Cl contents varied from O to 9000 ppm at 900 °C, and O to 6500 ppm at 800 °C; the S content in the melt was very low for all runs, typically <80 ppm. Results show that although there is a very slight positive correlation between Cl and Cu, Cl does not appear to play a dominant role in the solubility of Cu. The main control on Cu solubility is oxygen fugacity, which correlate linearly, Cu increasing with fO2. The oxidation state of Cu in these experiments can be determined using the reaction log aCuOglass ¼ n n=2 log f O2 þ C, where 2n is the valence of dissolved Cu. A slope of 0.25–0.30 suggests dissolution as a Cu2O-like compound where Cu is present as Cu+. This explains the strong correlation between oxidised magmas and porphyry-Cu deposits. The solubility of S was too low to be varied significantly within detection limits and excess sulfur was found to form a discrete sulfide phase. At lower oxygen fugacities (approx. NNO1) it consists of a Cu–Fe–S phase, whereas at higher oxygen fugacities (up to NNO+3) a Cu–S phase precipitates. This is contrary to the expectation that at high oxygen fugactities sulfide is no longer stable and suggests that Cu greatly expands the stability field of sulphide liquids. doi:10.1016/j.gca.2006.06.630
A311
Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Open University, Milton Keynes, UK (s.p. [email protected]) The use of UV lasers to analyse noble gases in laboratory experiments has provided new impetus to experimental work attempting to determine the fundamental solubility, diffusion and partition of noble gases. Bulk experiments provided some constraints on diffusion parameters but failed to recover real diffusion parameters where minerals broke down. However, attempts to determine solubility and partition coefficients were largely frustrated by surface adsorption, and effects of damage within powders used in the experiments. UV lasers have been used to analyse, in situ, noble gases introduced during laboratory experiments using depth profile technique, by rastering the laser and collecting gas from thin layers of sample. Spatial resolution achieved by this technique improved from around 2 to 0.2 lm largely resulting from a move to shorter wavelength lasers. While this technique has provided reproducible results, is has also highlighted problems of different diffusion regimes and provoked discussion of the importance of noble gas diffusion through a mineral lattice compared with noble gas diffusion through defects. The ability to control ablation using UV laser has also provided a technique to analyse, in situ, experimental charges where crystals have been grown from a melt. This work highlighted the importance of melt inclusions within such crystals and gas bubbles formed within the melt and occasionally trapped within the crystals. UV laser extraction of gas from crystals of the order of 100 lm in diameter, has provided reproducible partition coefficients for olivine and clinopyroxene in mantle melts. The future of such work lies in yet higher spatial resolution depth profiling to study the interaction of the lattice and defect diffusion regimes, and experiments to study noble gases at very high pressures. doi:10.1016/j.gca.2006.06.631
The solubility of copper in hydrous rhyolitic melts: The effects of oxygen, chlorine and sulfur 1
1,2
N.S. KELLER , J.A. MAVROGENES , B. SCAILLET
High spatial resolution analysis of noble gases using UV lasers S.P. KELLEY
3
1
Research School of Earth Sciences, Australian National University, Australia ([email protected]) 2 Department of Earth and Marine Sciences, Australian National University, Australia ([email protected]) 3 Institut des Sciences de la Terre d’Orle´ans, CNRS-UO, Australia ([email protected]) Porphyry-Cu deposits are unquestionably linked to subduction zone (arc) magmas. Yet the source magmas for these deposits remain unclear, despite recognition that arc-magmas are waterbearing and relatively oxidised. This study focuses on understanding some of the chemical parameters that may affect the solubility of Cu in hydrous melts using experimental petrology. In particular, the effect of oxygen fugacity and Cl and S content of the melt was investigated. The experiments were conducted in an internally heated pressure vessel where the oxygen fugacity is controlled by the addition of H2 to the Ar pressure medium. The sample charges consisted of powdered synthetic glass of rhyolitic composition containing water (7 wt%) and various amounts of NaCl or a S-bearing phase (FeS or CaSO4 depending on fO2). The samples were sealed in welded AuCu capsules and run at 4 kbars and 800 °C and 900 °C for up to 10 days. Twelve samples can be run simultaneously in the same vessel enabling the P, T and fO2 conditions to be identical for each sample. The fO2 was checked using solid Ni–Pd and Co–Pd sensors. The fO2 ranged from NNO1 to approx. NNO+3; the Cl contents varied from O to 9000 ppm at 900 °C, and O to 6500 ppm at 800 °C; the S content in the melt was very low for all runs, typically <80 ppm. Results show that although there is a very slight positive correlation between Cl and Cu, Cl does not appear to play a dominant role in the solubility of Cu. The main control on Cu solubility is oxygen fugacity, which correlate linearly, Cu increasing with fO2. The oxidation state of Cu in these experiments can be determined using the reaction log aCuOglass ¼ n n=2 log f O2 þ C, where 2n is the valence of dissolved Cu. A slope of 0.25–0.30 suggests dissolution as a Cu2O-like compound where Cu is present as Cu+. This explains the strong correlation between oxidised magmas and porphyry-Cu deposits. The solubility of S was too low to be varied significantly within detection limits and excess sulfur was found to form a discrete sulfide phase. At lower oxygen fugacities (approx. NNO1) it consists of a Cu–Fe–S phase, whereas at higher oxygen fugacities (up to NNO+3) a Cu–S phase precipitates. This is contrary to the expectation that at high oxygen fugactities sulfide is no longer stable and suggests that Cu greatly expands the stability field of sulphide liquids. doi:10.1016/j.gca.2006.06.630
A311
Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Open University, Milton Keynes, UK (s.p. [email protected]) The use of UV lasers to analyse noble gases in laboratory experiments has provided new impetus to experimental work attempting to determine the fundamental solubility, diffusion and partition of noble gases. Bulk experiments provided some constraints on diffusion parameters but failed to recover real diffusion parameters where minerals broke down. However, attempts to determine solubility and partition coefficients were largely frustrated by surface adsorption, and effects of damage within powders used in the experiments. UV lasers have been used to analyse, in situ, noble gases introduced during laboratory experiments using depth profile technique, by rastering the laser and collecting gas from thin layers of sample. Spatial resolution achieved by this technique improved from around 2 to 0.2 lm largely resulting from a move to shorter wavelength lasers. While this technique has provided reproducible results, is has also highlighted problems of different diffusion regimes and provoked discussion of the importance of noble gas diffusion through a mineral lattice compared with noble gas diffusion through defects. The ability to control ablation using UV laser has also provided a technique to analyse, in situ, experimental charges where crystals have been grown from a melt. This work highlighted the importance of melt inclusions within such crystals and gas bubbles formed within the melt and occasionally trapped within the crystals. UV laser extraction of gas from crystals of the order of 100 lm in diameter, has provided reproducible partition coefficients for olivine and clinopyroxene in mantle melts. The future of such work lies in yet higher spatial resolution depth profiling to study the interaction of the lattice and defect diffusion regimes, and experiments to study noble gases at very high pressures. doi:10.1016/j.gca.2006.06.631