Evaluation of argon ages and integrity of fluid-inclusion compositions: stepwise noble gas heating experiments on 1.87 Ga alunite from Tapajo´ s Province, Brazil

Authors

G.P. Landis, U.S. Geological Survey
L.W. Snee, U.S. Geological Survey
C. Juliani, Instituto de Geoscieˆncias-Universidade de Sa˜o Paulo

Date of this Version

2005

Comments

Published in Chemical Geology 215 (2005)

Abstract

Quantitative analyses are reported for active (N₂, CH₄, CO, CO₂, H₂, O₂, HF, HCl, H₂S, SO₂) and noble (He, Ar, Ne) gases released by crushing and step heating of magmatic-hydrothermal alunite from the Tapajo´ s gold province in Brazil. This is the oldest known alunite (⁴⁰Ar/³⁹Ar 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-SO₄ 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, H₂O is 32 mol% of total gas, H₂S/ SO₂ ranges from approximately 4 to 2, and N₂/Ar from 0.3 to 96.3. The presence of large amounts of H₂ 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 ⁴⁰Ar/³⁶Ar = 584–629 and that from crystal structure sites is >9.6x10⁴. Most gases are released from fluid inclusions at 200 °C, whereas most Ar (≥95%) is released between 525 and 725 °C.