Application of Plasma Ionization Isotope Ratio Mass Spectrometry

Science Center Objects

This project will allow us to (1) develop innovative analytical techniques for isotope geochemistry and U-Pb geochronology using plasma ionization mass spectrometry and (2) apply our new analytical techniques to collaborative research projects of high priority to the Mineral Resources Program, including studies related to the U.S. midcontinent region and Alaska, and/or processes related to the formation of critical mineral deposits. Our laboratory will be used to conduct innovative studies directly related to the USGS Mission Areas.

plasma lab instruments

The major instrumentation in the Plasma Lab that is used for U-Pb dating of accessory and ore minerals, and trace element analyses of geological materials by laser ablation ICPMS. (Credit: Kate Souders, USGS. Public domain.)

Scientific Issue and Relevance

The Plasma Laboratory itself is now complete and a new double-focusing multiple-collector plasma ionization mass spectrometer (MC-ICPMS) has been installed. This MC-ICPMS is designed to perform high-precision measurements of radiogenic and heavy metal stable isotopes of elements that will be purified from geological materials in the adjacent Clean Laboratory. The other instrumentation in the Plasma Laboratory will primarily be used to perform rapid in situ measurements of Pb isotope ratios (i.e., common Pb for tracer studies or radiogenic Pb for U-Pb dating) and trace elementabundances. Our next major tasks are to refine our existing techniques and establish a series of new methods for isotopic and geochronological research using a philosophy of innovation through collaboration.

laser ablation craters

Scanning electron microscope (SEM) image of typical laser ablation craters in cassiterite after the LA-ICPMS runs. (Credit: Denver Microbeam Laboratory, USGS. Public domain.)

Methodology to Address Issue

Innovation: Our goal is to improve the precision and accuracy of each technique to reach or surpass the cutting edge, while increasing efficiency (i.e., reducing the time and cost per analysis). We will use a phased approach of establishing or refining the analytical techniques from the basics (e.g., in situ U-Pb dating of zircon, apatite, titanite, and rutile, or high-precision Pb isotopes) to those with higher risk and greater impact (e.g., stable Mo isotopes, or in situ U-Pb dating of ore-related minerals, such as bastnäsite, columbite, cassiterite, or scheelite).

Collaboration: The closely related fields of isotope geochemistry and geochronology are the power tools in the Geologist's tool chest. A major goal of this project is to increase the usage of these tools within the USGS with collaboration on a series of case studies related to high-priority Mineral Resources Program projects. Project tasks are focused on the following activities:

  • Isotope geochemistry of ore-forming processes
  • In situ U-Pb geochronology and Pb isotope geochemistry of ore minerals and ore-forming processes
  • In situ U-Pb geochronology and trace element geochemistry of ore minerals and ore-forming processes in simultaneous split stream mode

 

fume hoods

Metal-free fume hoods in the Clean Lab.

(Credit: Aaron Pietruszka, U.S. Geological Survey. Public domain.)

Tera-Wasserburg Concordia diagram

Tera-Wasserburg Concordia diagram for combined cassiterite samples Llallagua and Siglo XX, Llallagua tin deposit, Bolivia. A cathodoluminescence (CL) image of an analyzed crystal fragment is shown as an insert here and in the following figures. Laser spot diameter is 135 μm. (Public domain.)

drying stations

Class-10 drying stations in the Clean Lab, Denver, CO.

(Credit: Aaron Pietruszka, U.S. Geological Survey. Public domain.)

lab column boxes

Modular workstations for ion-exchange chromatographic separations of single elements from geological materials in the Clean Lab, Denver, CO.

(Credit: Aaron Pietruszka, U.S. Geological Survey. Public domain.)