Evaluation of argon ages and integrity of fluid-inclusion compositions: Stepwise noble gas heating experiments on 1.87 Ga alunite from Tapajós Province, Brazil
Quantitative analyses are reported for active (N 2 , CH 4 , CO, CO 2 , H 2 , O 2 , HF, HCl, H 2 S, SO 2 ) and noble (He, Ar, Ne) gases released by crushing and step heating of magmatic-hydrothermal alunite from the Tapajós gold province in Brazil. This is the oldest known alunite ( 40 Ar/ 39 Ar age of 1.87 Ga), and because it has undergone minimal postdepositional thermal or tectonic strain, it is excellent material to test the retention of gas species in fluid inclusions and within the crystal structure over geological time. The gas compositions of a single sample, in combination with Ar age-spectrum data derived from stepwise heating of 10 related samples, have been used to constrain the limits of modification of primary gas compositions in fluid inclusions and the possible extent of the loss of radiogenic Ar. The observed variations in the isotopic compositions of He, Ne, and Ar released by stepwise heating have been used to identify the residence sites and determine the diffusion coefficients of the gases in the mineral. The data suggest that the only modification to primary gas compositions after entrapment in fluid inclusions and formation of the mineral is due to radiogenic and nucleogenic processes which affect the noble gas isotopic compositions. Three gas retention sites are recognized in alunite: (1) primary fluid inclusions, (2) crystal structure OH sites, and (3) crystal structure sulfate sites. Alunite undergoes OH loss at <500°C, and K-SO 4 structural decomposition occurs at >600°C. Fluid inclusions generally are ≤1 μm in diameter and have variable but high vapor/liquid ratios. The gases in inclusion fluids are quantitatively released in vacuo by heating at 200°C for ∼1 h. In the inclusion fluids, H 2 O is 32 mol% of total gas, H 2 S/SO 2 ranges from approximately 4 to 2, and N 2 /Ar from 0.3 to 96.3. The presence of large amounts of H 2 and CO indicates disequilibrium among the gas species in the fluids. Helium abundance is 214 ppm. Helium from fluid inclusions ( R /Ra=19.5) makes up about 4% of the total helium, whereas He ( R /Ra=0.2–2.0) from the crystal structure makes up about 65% of the total. Argon from fluid inclusions has 40 Ar/ 36 Ar=584–629 and that from crystal structure sites is >9.6×10 4 . Most gases are released from fluid inclusions at 200 °C, whereas most Ar (≥95%) is released between 525 and 725°C. Argon released from fluid inclusions at 200°C has 38 Ar/ 36 Ar=0.0–0.064. In contrast, Ar released from the matrix of the mineral at high temperature has 38 Ar/ 36 Ar=3.6–14.7. This difference suggests that, since the formation of the alunite at 1.87 Ga, traces of Cl in the mineral structure have undergone Cl ( n , γ ) and 41 K ( n , α ) in situ reactions with neutrons derived from U–Th. The amount of 36 Ar production from Cl nucleogenic reactions used in correcting for atmospheric 40 Ar typically increases the calculated age by 1–5 m.y., which is generally an insignificant component of the determined Ar age. Decay of U–Th over this time contributes 4 He ( α ) buildup in the crystal structure K-SO 4 sites. Atmospheric corrected excess 21 Ne/ 22 Ne=0.028–0.409 indicates that nucleogenic Ne was also produced via ( α , n ) reactions in matrix sites. Diffusion coefficients and activation energies for the diffusion of Ar and He, as determined using Arrhenius plots, indicate two distinct groups definable by their differences in activation energies. Argon log D o =2.45 and 15.33, with activation energies of 225 and 465 kJ mol −1 , respectively; the diffusion of He in alunite is quantified with log D o =−4.33 and E =106.8 kJ mol −1 . Model calculations of simplistic 1/ e -folding times and diffusion distance–time curves indicate that He should remain in alunite for millions of years at ≤100°C, whereas at <200–220°C, the alunite will retain Ar almost indefinitely. The data demonstrate why alunite is suitable for Ar geochronological applications and also show that, unless the alunite is subjected to metamorphic deformation, the inclusion fluids should retain their primary compositions.