Crustal Imaging and Characterization Team
Task Contact: Lawrence Snee
Argon geochronology provides fundamental isotopic age constraints for a wide range of earth-science studies. The method is used to date material ranging from the age of the Earth to material in the historic age range. The Denver argon geochronology laboratory is currently participating in studies that span ages from the over 3.0 billion years ago to less than 30,000 years. Because of this robustness and versatility, argon geochronologic results are in much demand and as a result, the method is continually being pushed into uncharted ground. In the Denver argon laboratory we are focused on the application of argon geochronology to difficult problems in two general categories. First, we are developing equipment and methodology to date very young and very small samples. Second, we intend to develop methods to date difficult-to-date "fringe" material. Finally, to release valuable staff from manually operating the "workhorse" argon system, we will automate our system that has been manually operated since 1987.
High resolution argon geochronology in the Denver Central Region geochronology lab will be reconfigured to take account of the many changes including retirement of L.W. Snee, and relocation of Kunk lab operations to Reston center. Based upon direction from program committee directives and decisions, the laboratory and staffing will be organized potentially with a new staff position to direct and operate the laboratory with a focus on high age resolution of processes and igneous-mineralizing events and on difficult analytical geochronologic problems. IF, the decision is not to restaff the geochron lab, strategies to minimally support the most urgent project geochron needs will be developed with existing staff and facilities, while preparing for closure of the lab.
The new MAP215-50 argon mass spectrometer with unique automated sample chamber, extraction line, and CO2-laser system is nearing testing stage. The CO2-laser system has recently been assembled at the University of Lehigh and is operational. The automated sample chamber is designed to rotate any of 60 samples to the loading port over a low-blank double vacuum furnace; this unique self-designed system is ready to install. This system will allow bulk samples or single crystals to be dropped into the furnace. The Labview-based software that has already been put into operation in the noble-gas laboratory has been shown to be far more effective than any other automation language and has been modified to meet the special requirements of a two-electron multiplier collector system. A new upright extraction table has been built and is ready for assembly of furnace and laser extraction systems.
A second highlight is the confirmation of the 1.87-Ga age of Brazilian alunite from a high-sulfidation gold prospect on the Amazon Craton. The type and extent of this deposit was deduced from sediment sampling, geologic mapping, and satellite imagery. Exploratory drilling revealed the high-sulfidation nature of the deposit and discovered alunite. The USGS obtained alunite samples for sulfur isotope analysis and argon geochronology. A test sample for argon analysis confirmed the Paleoproterozoic age assumption. Nine additional samples were dated and from these and the noble-gas work of Gary Landis, isotopic systematics, including diffusion characteristics were defined. One sample is 51 Ma and has a sulfur isotopic signature typical of supergene processes. These results are of major importance not only for the isotope community but also open the questions on extent of volcanic-hosted gold deposits on the Amazon craton and on the nature of climate and weathering conditions in the Tertiary of this area. The results of these experiments are the subject of 2 papers currently in press in Chemical Geology. A four-member USGS team including Snee was hosted by the Brazilians on a tour of these deposits in the Amazon.
References to non-USGS products and services are provided for information only and do not constitute endorsement or warranty, express or implied, by the USGS, USDOI, or U.S. Government, as to their suitability, content, usefulness, functions, completeness, or accuracy.
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