Noble Gas Geochemistry

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Task Contact: Gary Landis

Task Objectives

Task Statement of Work

Task Highlights & Key Findings

Task Products

Task Objectives

This task handles operations of a modern and highly versatile noble gas mass spectrometry lab with the following capabilities:

cosmogenic surface exposure age determinations from olivine, pyroxene, clinopyroxine, garnet, quartz, and possibly other mineral targets using 3He and 21Ne and (through collaboration) 10Be and 26Al and 36Cl. With detection of as few as 10^∧4 atoms, age range is from approximately 600 years to as old as geomorphic evidence can demonstrate a stable exposure rock surface (800,000 yr to > 1 myr), e.g. dating young lavas, pediment surfaces, fault and landslide scarp faces, debris flow surfaces, stream terraces and beach strand lines, determination of erosion rates;
in vacuo crushing and separation of gases for isotopic and chemical analysis (hi-res sector and quadrupole mass spectrometers) followed by additional analysis of gases thermally released by step heating or bulk heating for fluid inclusion gas analysis (both active and noble gases) and determination of "inherited" non-cosmogenic nuclides, samples are maintained under vacuum between crush and heating steps;
extraction, cryogenic separation, and purification of gases dissolved in water samples for tritium-3He age calculations, noble gas solubilities (recharge temperatures-infiltration rates for groundwater), in-growth nuclide geochronology using 4He, 39Ar, possibly 81Kr, and isotopic tracer studies including isotopically enriched xenon injection, to determine flow rates and dispersion, age of groundwaters, recharge regions, infiltration processes, interconnection of aquifers;
U-Th-4He cooling ages from apatite and sphene to determine uplift rates and cooling histories and general tectonics of a region.

This task has three objectives:

maintenance of the instrumentation and support infrastructure,
evolving development of new methods and techniques and calibrations appropriate for general operation of the lab, and with importance to a broad spectrum of applications originating in any Program; and
continued development of software enabling instrument control, data acquisition / reduction, and report generation.

Routine maintenance and repair or replacement of electronics, computers, vacuum pumps and valves, hardware development including welding and machining of parts necessary to implement new or modified procedures, and software are routine expenses in operation of this type of analyical facility that is separate from a project-specific cost. Pumps, valves, and electronics fail and must be repaired/replaced. On-going software development is improving the level of automation and ease of operation while performing very complex multi-step procedures. In addition to unforseen but predictable maintenance costs, continued development of improved automation, and of general procedures for sampling and analysis of gases/noble gases in groundwater samples will be performed this year, with creation of calibration standards and methods. Additionally, development of more efficient methods for analysis of gases extracted from fluid inclusions will be completed this year. Software for routine data reduction and reporting will be written.

Statement of Work

In preparation for mineral assessments strategies, and in response to increased demands cosmogenic surface exposure age dating in mapping and surficial processes research, this project task continues to develop and to expand the analytical capabilities for active and noble gas quantitative analysis and their element isotopes. Both very low detection limits at high precision and analysis from wide spectrum of sample types are high priority. Integration of capabilities into GD program priorities is crucial, and expansion of network of scientific colleagues with collaboration in both BRD and WRD continues in hydrologic and ecosystem dynamics studies.

Highlights & Key Findings

Major development highlight has been the implementation of complex full computer automation of feasable portions and steps of analyses. Developed new control thermocouple placement apparatus for high temperature low blank tantalum furnace. Successful design and fabrication of low blank furnace multi-sample drop loader with gate valve interlock and drop funnel-furnace blast "shutter" plate, easy sample loading while maintaining quick pumpdown and vacuum recovery. Developed inexpensive water sampling device and reliable method for attaching to gas extraction manifold, with inexpensive custom copper tube clamps. Developed new crushing device for opening fluid inclusions for active and noble gas analyses, and modified/improved design for extraction and purification of gas dissolved in water samples. Designed and fabricated super cold He cryotrap for improved selective separation of Ar, Kr, Xe for analysis and calibration of each noble gas. Concentrations and isotopes of these noble gases are now routine. In cooperation with Auslog Ltd. (Australia) we have developed fully integrated sampling and logging suite of tools for discrete depth sampling and parameter measurement in water wells to depth of >3000 ft. Highlight is a sensor using a semi-permeable membrane to measure dissolved gas pressures at depth in well with fast real-time response. The total dissolved gas sensor employs a high tech space shuttle pressure transducer in a USGS designed chamber that is deployed in a conventional Auslog sonde. These tools are critical to obtaining appropriate samples and Temp. conductivity, hydrostatic pressure, and dissolved gas pressure profiles to characterize aquifer waters with depth in open well boreholes. Our recent studies illustrate that minimal water flow occurs vertically along the annulus of open water wells, minimizing the need for strategically placed packed intervals within the well. These new USGS-developed tools are applicable to hydrologic studies, investigations of coal bed methane well gases (and water geochemistry), methane hydrate studies, and 3D modeling of aquifer flow models.

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