Volatile metal mobility and fluid melt partitioning: Experimental constraints and applications to degassing magmas.

摘要

Volatile trace metals are variably enriched in volcanic gases. Metal concentrations in sub-aerially erupted magmas are also depleted in many of these metals. The causes of variable metal enrichment in volcanic gasses, however, remain enigmatic. The objective of this work is to place experimental constraints on kinetic and thermodynamic factors that influence the concentrations of trace metals in volcanic gases. To measure metal mobility in silicate melts, Pt crucibles packed with metal doped glasses of broadly basaltic composition were equilibrated with air and mixed gases at atmospheric pressure. The metals in the melt diffused to the gas/melt interface where they were released as a volatile species. The experiments produced concentration-distance profiles from which diffusivity was derived. Experiments were also conducted in a piston-cylinder apparatus at 1 GPa pressure. In these experiments, melts were equilibrated with Cl-bearing fluids at high temperature and pressure. At equilibrium, trace metals partitioned between the melt and fluid phase as a function of temperature and fluid composition. The diffusivity of Re in melts of natural basalt, andesite and a synthetic composition in the CaO-MgO-Al2O3-SiO 2 (CMAS) system has been investigated at 0.1 MPa and 1250--1350°C over a range of fO2 conditions from log fO2 = -10 to -0.68. Re diffusivity in natural basalt at 1300°C in air is logDRe = -7.2 +/- 0.3 cm2/sec and increases to logDRe = -6.6 +/- 0.3 cm2/sec when trace amounts of Cl were added to the starting material. At fO2 conditions below the nickel-nickel oxide (NNO) buffer Re diffusivity decreases to logDRereducing = -7.6 +/- 0.2 cm2/sec and to logDRe andesite = -8.4 +/- 0.2 cm2/sec in andesite melt. Cd, Re, Tl, Pb, Sb and Te diffusivity in CMAS and Na2O-MgO-Al 2O3-SiO2 (NMAS) melts were also determined at 0.1 MPa and 1200--1350°C. In the CMAS composition at 1300°C, the fastest diffusing element was Cd having a logDCd = -6.5 +/- 0.2. The slowest element was Re with logDRe = -7.5 +/- 0.3. Diffusivities of Sb, Te, Pb and Tl have intermediate values where logD Sb = -7.1 +/- 0.1, logDTe = -7.2 +/- 0.3, logD Pb = -7.1 +/- 0.2, logDTl = -7.0 +/- 0.2 cm 2/sec. In the NMAS composition, logDRe = -6.5 +/- 0.2, logDSb = -6.0 +/- 0.2, logDPb = -6.1 +/- 0.1, logDTl = -5.8 +/- 0.2 cm2/sec. Fluid/melt partition coefficients ( Kdf/mx ) of Re, Mo, W, Tl and Pb between fluid (H2O + Cl) and a haplobasaltic melt in the CMAS system were measured between 1200 and 1400°C at 1 GPa and fluid chlorine molarities from 7.7 to 27 mol/L. At 1300°C and fluid molarity of 7.7 mol/L, Kdf/mRe = 9.8 +/- 1.8, Kdf/mMo = 11.8 +/- 1.6, Kdf/mW = 3.7 +/- 1.6, Kdf/mTl = 4.5 +/- 1.4 and Kdf/mPb = 2.4 +/- 1.8. Both Mo and Re were shown to partition most strongly into the fluid at all temperatures and fluid chlorinities. Differences in diffusivity of volatile heavy metal ions to a lead to significant fractionation between these metals in magmas during degassing. Given the observed differences in Cd and Re diffusivities, an increase in the normalized Cd/Re ratio in the gas phase with increasing bubble growth rate is predicted. Monitoring of the Cd/Re ratios in aerosols from degassing volcanoes may provide a tool for predicting volcanic eruption. Modeling of Re using the values measured here support the contention that subaerial degassing is the cause of lower Re concentrations in arc-type and ocean island basalts compared to mid-ocean ridge basalts. The model results were also compared with emanation coefficients for trace metals from natural volcanoes. The magnitudes of the modeled Re/Tl and Re/Pb in fluids at 1300°C and the lowest fluid chlorinities were less than that observed from their emanation coefficients. Re and Pb are more sensitive to fluid chlorinity than Tl. The ratios of Re/Tl and Re/Pb expected from emanation coefficients are closely matched if partitioning values for experiments having fluid chlorinities of &sim16--20 MCl at 1300°C are used.