Carbonyl as a possible carrier of platinum in magmatic fluids
Résumé
CO forms the most stable compounds with bonding (stronger than C2H4, C2H2 and others) with native PGE and some cations including Au +. We find experimentally that pressure of 50-200 MPa stabilizes platinum carbonyls to the magmatic temperatures of 800-950 o C [1]. We performed an experimental study of Pt solubility in CO-bearing fluids at 800-950°C and 0.5-2.0 kbar, using a double capsule technique for fluid generation by the thermal decomposition of MgC2O4, with the fluid trapped in porous welded silica glass and Al2O3 ceramics. Bulk Pt content was measured with electrothermal atomic absorption spectrometry (ETAAS) and LA-ICP-MS. We obtain the first estimates of solubility in CO-CO2 fluid at fO2 near CCO buffer that is no less than 600 ppm at P=200 MPa and T=950 o C. Using of albite glass as a trap allows capturing of the reactive fluid and dissolved carbonyls in the bubbles. Raman spectroscopy of quenching phases in the bubble formed at P=100 MPa and T=950 o C confirms formation of polynuclear carbonyl. Carbonyl of Pt absorbed on carbon was also identified. The pressure dependence of Pt solubility at 950°C between 0.5 and 2 kbar is consistent with the formation of Pt3(CO)6 or Pt3(CO)6 2-(Chini complex) Raman spectroscopy data support this interpretation. LA-ICP-MS analysis demonstrated that iron is transferred by fluid along with Pt. Iron carbonyl is unstable at these PT conditions and neutral complex composed of Fe 2+ and Pt3(CO)6 2-can be responsible for the observed Fe transfer and formation of Pt3Fe. Significant Pt solubility is expected at near solidus temperatures of the upper crustal (P=200-300 MPa) ultramafic-mafic intrusions hosting the largest PGE deposits. Solubility of Pt in the fluid at fO2 near CCO buffer (graphitized rocks) is predicted to be strongly dependent on temperature and at P=500 MPa it will reach hundreds ppm level at the temperatures above 600-700 о C (amphibolite facies metamorphism).