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Th and U diusion in hydrous rhyolite melt Lingbo Xing([email protected]), Dan Ruscitto, E.Bruce Watson Department of Earth and Environmental Science, Rensselaer Polytechnic Institute We measured diusion coecients of thorium (Th) and uranium (U) in hydrous (up to 6 wt.% H2O) melts of Lake County Obsidian (L.C.O) at 1 GPa and temperatures ranging from 900°C to 1200°C. Diusion couples were assembled from pre-synthesized capsule halves with high (~500 ppm) and low (~100 ppm) concentrations of U and Th. Experiments were run for 4 to 54 hours, depending on temperature, and the resulting concentration proles were characterized by LA-ICP/MS in the case of U and both EPMA and LA-ICP/MS in the case of Th. Th and U have almost identical diusivities, ranging from 10-9 to 10-7 cm2/s over the temperature rangeexamined.We observed Arrhenius behavior for both Th and U, and constrain activation energies to E ~140 and 116 kJ/mole, respectively. Measured diusivities are insensitive to dissolved H2O contents at > 6 wt%. Modest down-temperature extrapolation to conditions relevant to the Earth's crust (700 – 850°C) give Th and U diusivities of ~10-10 cm2/s. Our results are comparable with known values for the major structural constituents of accessory minerals that concentrate U and Th (e.g., zircon, monazite, apatite, xenotime), so the diusive supply of U and Th to growing crystals is adequate to preclude signicant disequilibrium uptake during growth. The results complement and extend previous results on Th and U diusion in hydrous molten granite.. Abstract Experiment method Data Fitting processing High concentration reservior ~ 250 to 550ppm Low concentration reservior ~50-100ppm Diusion direction -3000 -2000 -1000 0 1000 2000 3000 50 100 150 200 250 300 350 400 450 distance (um ) Th Concentration (ppm ) -3000 -2000 -1000 0 1000 2000 3000 0 50 100 150 200 250 300 350 400 450 distance (um ) U Concentration (ppm ) 6.5 7 7.5 8 8.5 9 9.5 x 10 -4 -9.2 -9 -8.8 -8.6 -8.4 -8.2 -8 -7.8 -7.6 -7.4 -7.2 1 / T (K -1 ) log 10 DU(cm 2 /s) 6.5 7 7.5 8 8.5 9 9.5 x 10 -4 -9.2 -9 -8.8 -8.6 -8.4 -8.2 -8 -7.8 -7.6 -7.4 1 / T (K -1 ) log 10 DTh (cm 2 /s) 5 6 7 8 9 x 10 -4 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 1/T (K) logD Th (cm 2 /s) 5 6 7 8 9 x 10 -4 -14 -13 -12 -11 -10 -9 -8 -7 -6 1/T (K) logD U (cm 2 /s) This study @1GPa with 6wt.% H2O in granite melt Harrison and Watson 1987- Zr diusivity @1GPa with 6 wt.% H2O in granite melt Mungall and Dingwell 1994 _4.5 wt.% of H2O @1O kbar, haplogranite melt Mungall and Dingwell 1997 Dry @0.1MPa, haplogranite melt Tourrette 1996 in halpobasltic melt @1Atm and log/O2=-0.7 in air Mungall and Dingwell 1997 ~4.5 wt.% H2O 1GPa, haplogranite melt Tourrette 1996 in halpobasltic melt @1Atm and log/O2~-9 in Fe-FeO -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 0 50 100 150 200 250 300 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 0 100 200 300 400 500 600 700 800 900 U Concentration (ppm Th Concentration (ppm distance (um) distance (um) f ( x ) = ( C H + ( C L C H 2 ) (1 erf ( ( x b ) 2 D t )) Laser beam Interface C2 true prole Analyzed prole -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 50 100 150 200 250 300 350 400 450 500 550 Erf function Erf function convolved with a 160um spot Erf function convolved with a 400um spot Erf function convolved with a 1000um spot Length of diusion zone ~ 2000um Lake county obsidian Na 2 O 4.31 wt.% MgO 0.07 wt.% Al 2 O 3 13.30 wt.% SiO 2 76.40 wt.% K 2 O 4.67 wt.% CaO 0.51 wt.% TiO 2 0.11 wt.% FeO 0.79 wt.% L.O.I 0.17(H2O) wt.% ZrO 2 120ppm P 2 O 5 130ppm ThO 2 20ppm UO 2 15ppm ! Table 1. Composition of Lake County Obsidian Lake County Obsidian (see Table 1) was used the starting material in all experiments 900ºC 1000 1050 1100 1200 P H 2 O T 1 GPa 6 wt.% 9001200ºC A Markov Chain Monte Carlo method (Mosegaard and Tarantola, 1995) was used to invert for the four parameters CH, CL, X0 and D and estimate their uncertainties by tting the concentration curves. Experiment parameters Diusion couple approach has been taken to determine the self-diusivities for Th and U in rhyolit melts under laboratory conditions. Fig.1 shows the schematictly details of the experiments set up. Fig. 1 Diusion couples method has been used for the diusion experiments. pre-synthetic hydrous rhyolite glass cylinders with identical major element composition ( Lake county obsidian), but diering in Th and U concentration, are mated to one another within a Pt capsule. ) error function t Fig. 2. Th and U diusion proles (points) in hydrous rhyolite melt and the best error function ts for experiment carried out @900ºC for 54.5 hrs . The error bar shows 1 sigma error from LA-ICP-MS analysis. 900ºC 1000 1050 1100 1200 900ºC 1100 1200 1400 1600 900ºC 1100 1200 1400 1600 Fig . 4. Summary of all experiment data of Th and U diusivity in various melts. Data sources and conditions are indicated in the gure. Our diusivity data is about 1log unit higher than Mugall and Dingwell’s data,it may due to potential dierence in the initial melt composition, dierent technical method and uncertainties inherent in analysis methods. Fig. 3. Temperature dependence of Th and U diusion in rhyolite melt (6wt.% H2O). Both Th and U show good linear relationships between Log D and 1/T and thus display Arrhenius behavior. DTh and DU shows almost identical diusivity under all experimental conditions. Erro bars indicate 90% condence interval. (Equation 1) Fig . 5 a collection of all possible ttings (green curves) after random walk in the modle space for experiment @ 900ºc for 54.5 hrs. The parameters That give rise to the minimum mist (red curve) are used as the inverted diusivity and the 90% condence intervals are dened using the central 90% Output samples from the montecarlo sampling and are used as error bars in the d vs 1/t plot. 1GPa 6 wt.% H2O 1GPa 6 wt.% H2O Ea_U = 126.07 kJ/mole D0_U = 6.43e04 Ea_Th = 148.77 kJ/mole D0_Th = 0.0051 The convolution eect in the measurement of the diusion proles due to spatial averaging (ganguly et al, 1988) in la-icp-ms analysis is evaluated using numerical simulation.The composition that is Measured by the laser beam represents a weighted spatial average of the composition of the sample around the point of incidence of the beam. It is assumed that the excitation intensity of the sample volume has a homogenous distribution with radial symmetry about the beam axis. Assuming that the ablation volume of the sample is a at bottom cylinder for the spot analysis. Then the apparent concentration is function of real concentration convolved with intensity distribution of the volume. So the convolution equation is : Spot size effect numerical simulation of convolution effect f ( x ) = ( C H + ( C L C H 2 ) (1 erf ( ( x b ) 2 D t ))* g ( x , r ) g ( x , r ) = r 2 x 2 Fig. 7 shows convolution eect on diusion proles with dierent beam size. For our experiment interest, i checked the convolution eect on the observed diusion prole with a beam size of 160 to 1000um in a diusion zone length of 2000um. There is minor (<<5 %) convolution eect for the diusivity. No correction need to be done for our diusivity data. C1 Fig.6. Spatial averaging effect in laser ablation analysis on a concentration profile between two phases (Equation 2) (Equation 3)
