HR: 11:20h
AN: V22B-05 INVITED    [Abstracts]
TI: Comparison of synthetic fluid inclusion and quartz-trap methods for determining platinum- group element (PGE) solubility in hydrous salt melts at magmatic conditions
AU: * Hanley, J J
EM: jake.hanley@gmail.com
AF: Department of Geology, Saint Mary's University, 923 Robie Street, Halifax, NS B3H 3C3, Canada
AB: Layered intrusions preserve magmatic inclusions containing hydrous halide melt phases composed of transition metal chlorides with less than 5 wt% H₂O. Previous attempts at measuring platinum-group element (PGE) solubility in such high salinity volatiles through the analysis of synthetic fluid inclusions have shown that, although the PGE appear to be highly soluble at geologically-realistic conditions, it is difficult to determine if trapped fluids represent equilibrium fluid compositions. Wide ranges in trapped metal content may result from (i) premature formation of fluid inclusions owing to rapid rates of silicate mineral healing (i.e., hours at magmatic conditions in a saline fluid), (ii) premature entrapment of fluids in which the approach to equilibrium metal solubility requires the generation of the metal-complexing ligands (e.g., HCl) from a slow fluid-mineral or fluid- melt buffer reaction, (iii) the dissolution and reprecipitation of PGE in opened inclusions due to subtle temperature or chemical gradients in run capsules, or (iv) heterogeneous fluid compositions due to the presence of PGE micronuggets or colloids that are more abundant near metal-fluid or metal-buffer interfaces. To circumvent some of the problems associated with synthetic inclusion methods, the solubility of platinum in a hydrous salt melt (S-free; 75 wt% CaCl₂+MgCl₂, 25 wt% H₂O) was investigated at elevated T (700^oC) and low crustal pressure. The salt melts were reacted with PtAs2 (natural sperrylite) in Pt capsules buffered at an oxygen fugacity of FMQ-1 using a mixed gas buffer and by the solid mineral assemblage tremolite- diopside-enstatite-quartz which fixes the concentration of relevant metal-complexing ligands at run conditions. Salt melts were trapped (simultaneously) in the matrix of a quartz trap (granulated natural quartz partially isolated in a smaller gold capsule) and in synthetic melt inclusions trapped in pre-fractured quartz. After quenching, the melt inclusions and quartz trap material (in a frozen state) were analyzed by laser ablation ICP-MS (ETH Zürich). Platinum solubility in the presence of the mineral sperrylite is in the low ppm range. Analysis of different portions of the quartz traps using a 90 micron pit size yielded relatively consistent platinum concentrations of 4.9 ppm +/- 1.4 ppm (2 sigma, n=28) with a routine detection limit of 80-90 ppb for Pt. Analyses of traps at different experiments run durations indicate that Pt equilibrium is reached after 150 hours and is approached from over- saturation. Based on these observations, it is suggested that the measured concentrations are representative of Pt solubility at run conditions and that the laser sampling size sufficiently overcomes local heterogeneity in the distribution of Pt quench products. By contrast, Pt concentrations in hydrosaline melt inclusions from different areas of a 3 mm x 10 mm quartz cylinder range from below detection limits (0.2 ppm) to 4.7 ppm (n=102). The range in observed metal concentrations in the fluid inclusions varies between different areas in the quartz cylinder. The data demonstrates that measurements of PGE solubility in hydrous salt melt phases using a frozen quartz trap eliminates some uncertainties concerning the true equilibrum metal solubility that are associated with synthetic inclusion methods. Additionally, the detection limits achievable using the quartz trap method are up to an order of magnitude lower than for synthetic inclusions, owing to the much higher mass of halide melt that may be analyzed in a single ablation.
DE: 1000 GEOCHEMISTRY
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1036 Magma chamber processes (3618)
DE: 1043 Fluid and melt inclusion geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting