From Outcrop to Ions: development and application of in-situ isotope ratio measurements to solve geologic problems

Science Center Objects

Project objectives are to (1) develop innovative analytical techniques for isotope geochemistry and U-Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and (2) apply these techniques to collaborative research projects of high priority to the Mineral Resources Program, including studies related to the formation of "critical mineral" deposits, and studies related to Alaska and U.S. midcontinent regions.

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 houses a new double-focusing multiple-collector plasma ionization mass spectrometer (MC-ICPMS), a Nu Plasma II, and a new 193 nm laser ablation system (RESOlution-SE). This LA-MC-ICPMS is designed to perform in-situ high-precision measurements of radiogenic and stable isotopes of trace elements. The other instrumentation in the Plasma Laboratory includes double-focusing single-collector lasma ionization mass spectrometer (SC-ICPMS), a Nu AttoM ES, and a new Agilent 7900 quadruple (Q) ICPMS. The latter two instruments are primarily 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 element abundances.

This project will allow us to (1) develop innovative in-situ 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 (MRP), including studies related to the U.S. midcontinent region and Alaska, and/or processes related to the formation of critical mineral deposits.

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.)

Methods to Address Issue

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. 

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 are following 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, Hf isotopes in zircon; Nd isotopes in monazite) to those with higher risk and greater impact (e.g., St isotopes in apatite, feldspars, carbonates, and barite; Pb isotopes in feldspar and tourmaline; Li isotopes in tourmaline; Nd isotopes in bastnaesite or scheelite; Sn isotopes in cassiterite, or in situ U-Pb dating of ore-related minerals, such as bastnaesite, 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 through collaboration with other scientists on current and future MRP projects. We will involve USGS scientists in research with isotopes through a series of case studies related to high-priority Mineral Resources Program projects.

Project tasks are focused on the following activities:

  • In-situe Isotope analysis of critical mineral isotope systems for geologic applications
  • In-situ U-Pb geochronology of ore-forming and ore-related minerals applied to geologic processes
  • In-situ U-Pb geochronology and trace element geochemistry of ore minerals and ore-forming processes
fume hoods

Metal-free fume hoods in the Clean Lab.

(Credit: Aaron Pietruszka, USGS. 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, USGS. 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, USGS. Public domain.)

 

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