B08_Attachment_-_SOW_Mass_Spectrometer.docx

Noble gas mass spectrometer for 40Ar/39Ar geochronology and cosmogenic neon geochronology

Bascom Argon Dating Laboratory Performance Work Statement (PWS)

1.0 INTRODUCTION AND BACKGROUND

The Bascom Argon Dating (BARD) Laboratory aims to procure a multi-collector noble gas mass spectrometer. The dual purpose of this mass spectrometer will be to measure, in multi-collection, the ionized gas species required to produce isotopic ages for both 40Ar/39Ar geochronology and cosmogenic neon geochronology.

The determination of absolute rock and landform ages is a critical component of the U.S. Geological Survey’s mission. Constraining the formation ages of rocks is essential for accurately correlating map units in seamless geologic map databases, modeling the genesis of ore materials hosting rare earth elements (REEs) and other critical minerals, and understanding volcanic patterns and hazards. Determining the ages of landforms is similarly essential for dating the accumulation of economic ores (e.g., heavy mineral-rich sand deposits along continental rivers and coastal plains), earthquake recurrence intervals, and slope failures that produce massive rockslides. While these two geochronologic tasks – dating rocks and dating landforms – are typically done in separate labs using distinctly different isotopic systems, a noble gas mass spectrometer capable of measuring both argon and neon isotopes provides an opportunity to achieve both goals on a single machine.

Constraining the ages of rocks themselves can be achieved using 40Ar/39Ar geochronology, which leverages the decay of radioactive potassium-40 into argon-40 to estimate when a rock cooled and began retaining argon. The Bascom 40Ar/39Ar Dating (BARD) Lab has exploited this technique to date ore genesis, rock exhumation and deformation, and volcanism in service of USGS mapping and scientific research. However, the mass spectrometry equipment at BARD Lab is >30 years old, difficult to service, and incapable of analyzing with the speed, precision and flexibility of modern noble gas instruments. These issues pose a significant risk to the lab’s ability to address high-priority scientific goals in a timely manner.

Dating landforms, on the other hand, is typically accomplished using cosmogenic nuclides. Cosmogenic nuclides (including 26Al, 10Be, and 21Ne) are produced in quartz near the Earth’s surface from interactions with cosmic radiation, and their measurement can provide insight into how long geologic materials have been buried or exposed near the surface. The BARD Lab shares mineral separation facilities with the Reston Cosmogenic Nuclide (RECON) Lab, which specializes in processing quartz for cosmogenic nuclide geochronology. The RECON Lab has historically focused on 26Al and 10Be, but these isotopes decay over geologically short time periods and limit the reliable dating of landforms to the last 3-4 million years. Furthermore, measuring these isotopes requires the use of accelerator mass spectrometry, a time-consuming and costly endeavor that must be outsourced. Cosmogenic 21Ne provides an attractive alternative to 26Al and 10Be, because it does not decay over time and can be readily measured using an appropriately equipped noble gas mass spectrometer. In this way, 21Ne extends the ability to date landforms to before the Pliocene while also providing a simpler, faster, and more cost-effective alternative to the more frequently used 26Al and 10Be.

Acquiring a mass spectrometer capable of measuring argon and neon isotopes would revolutionize the argon and cosmogenic nuclide labs at USGS in several ways. By adding state-of-the-art technology to the BARD Lab’s argon dating facilities, it would increase the reliability and speed with which the lab could operate to date rocks for critical science, and make tractable research that require the greater analytical precision of a modern instrument (e.g. dating of young volcanics). Uniquely, it would also introduce a new capability to the RECON lab, and the USGS in general, in the form of stable isotope cosmogenic neon dating. By providing an in-house facility for the measurement of cosmogenic neon, this acquisition would both (1) provide the new capability to date geologic deposits and landforms over unbounded timescales and (2) significantly reduce the cost and processing time needed to perform cosmogenic nuclide studies. These benefits would carry through to the BARD and RECON Labs’ colleagues in the USGS mapping and energy programs, as well as collaborators at state institutions and universities. In doing so, this upgrade would have broader impacts on USGS science overall, particularly in the eastern United States.

2.0 SCOPE

The required end product is a noble gas mass spectrometer that can be used to perform both 40Ar/39Ar geochronology and cosmogenic neon geochronology in multi-collection mode; that is, simultaneous collection of 36Ar, 37Ar, 38Ar, 39Ar, and 40Ar for 40Ar/39Ar and 20Ne, 21Ne, and 22Ne for cosmogenic neon geochronology. More details regarding this overarching requirement are given in section 4.

3.0 APPLICABLE DOCUMENTS

See technical requires for performance terms of what is required.

4.0 TECHNICAL REQUIREMENTS

4.1 Noble gas mass spectrometer capable of multicollection for both 40Ar/39Ar and Ne Comsogenic Nuclide Geochronology General Description The mass spectrometer shall be constructed of stainless steel, have a minimum radius of 20cm with a high-sensitivity ion source and collectors capable of simultaneous measurement of 40Ar, 39Ar, 38Ar, 37Ar and 36Ar; and separately, simultaneous measurement of 22Ne, 21Ne, and 20Ne. Although the instrument will be used for argon and neon analysis, it must be also capable of operation over the mass range 2-100, to permit investigation of other gas species. Some of these gas species are required for corrections for isobaric interferences on the isotopes of interest (e.g. mass 18, 19, 40 and 44). The entire analytical volume must be bakeable to 300°C and removable bakeout ovens and heaters must be provided. The mass spectrometer table must be constructed so its electromagnet can be manipulated easily by one person before and after instrument bakeout.

Beam Separation We seek a system where the mass separation is accomplished solely by the electromagnet, without “zoom optics” or electrostatic energy analyzers (ESA) following the magnetic separation.

Transition Between Argon and Neon Analytical Setups We seek an instrument that can routinely be used for multicollection of isotopes of both Argon and Neon. Ideally, as in the beam separation, the transition between the two analytical setups will require only changes to the electromagnet and source settings; which can be controlled by software and are reproducable. If additional changes to the instrument setup (e.g. physical movement of the magnet) are required those must be fully described—and the method of reproducing any manual movements described, proven, and documented in detail—such that routine transitions between Argon and Neon are achievable. Routine here is defined as transitioning the mass spectrometer from the Argon to the Neon setup such that a multi-collection measurement of Neon isotopes in air can be made the following day.

Mass Resolution We require a resolution of 180 or higher. The resolution is defined as the ratio of the m/e 40 peak (M) to its width (∆M in amu) at 10% peak height. A mass resolution of 600 or greater preferred to allow for partial resolution of 40Ar++ from 20Ne, H19F+ and H218O+ (e.g. Ma et al. 2015).

Abundance Sensitivity It is critical that we are able to measure small peaks adjacent to large peaks, as well as the valley or baseline between them. We require the instrument to be delivered with <5 ppm abundance sensitivity from m/e 40 at m/e 39 and from m/e 37 at m/e 36 at a minimum operating resolution of 180 and normal operating pressures.

Emission Regulator/Source Control Total emission current must be stable to 100ppm. Additionally, the hardware must permit switching ionization off during gas inlet and equilibration to avoid fractionation prior to measurement. Ionization shutoff is a critical component we require to allow for extended gettering without gas consumption, this will help mitigate CO2 interferences on Neon, among other issues.

High V power supplies Power supplies for source, lenses and electron multiplier must be fully adjustable over their useful range with a metered display and 5% accuracy. Stability shall be better than 20ppm over a 20-minute interval. The ability to deflect the beam programmatically during peakhopping or to protect detectors is required.

Magnet We require a stable electromagnet with temperature-controlled Hall Probe. Magnetic field must be stable to better than 100ppm in mass over a 20-minute interval. With 50 percent of the 40Ar beam in the collector, the change in beam intensity must be less than 50ppm over a 20-minute interval. Stepping and sweeping of the ion beam must be accomplished by variation of the magnetic field at a constant accelerating voltage. Manufacturers must specify magnet settling time for a 4-mass unit jump in a single detector—best visualized by jumping from m/e 36 to the 50% height of m/e 40. Manufacturers must specify a magnet settling time for a 20-mass unit jump in a single detector, best visualized by jumping from m/e 20 to 50% height on either m/e 40 or m/e 44. These large jumps are required for corrections of isobaric interferences in Neon analyses (e.g. Sarda et al., 1988, Marrochi et al., 2009).

Detectors Argon: We require detectors for simultaneous acquisition of 5 argon peaks (40, 39, 38, 37, 36). The collectors for M/e of 40, 39, 38, and 37 should be Faraday detectors with amplifiers capable of electronic intercalibration. For faraday collectors we require noise @ 10 second integration time of < 1E-17 amps, and a decay time (tau) to 10 ppm of < 0.3 seconds. The collector(s) for M/e 36 must include a multiplier of the discrete dynode type, and the specifications of the multiplier must be included in the proposal. M/e 40, 39, 38 and 37 signals must be measurable from 1e-17 to 1e-11 moles, m/e 36 must be measurable from 1e-19 to 1e-16 moles (see below). There must be designed capabilities to keep radioactive 37Ar from hitting the electron multiplier, and to intercalibrate the detectors.

The instrument needs to be able to measure radiogenic argon from a variety minerals of a variety of ages (0.05-1000Ma). Typical isotopic abundances to be measured for six types of irradiated materials are listed in the table below. Please provide baseline-corrected time-zero intercept intensities (in amps, volts or counts per second) and 1 uncertainties the instrument would deliver for each molar quantity of isotope in the table, allowing up to five minutes for the isotopic analysis. If more than one detector geometry is specified, please include estimates for each geometry. If manufacturer recommends baseline measurement before or after aliquant measurement, please also report results for measurements when baselines are measured during the five-minute analysis, as well.

Argon Isotopic abundances to be measured

Neon: We require detectors for simultaneous acquisition of 3 neon peaks (22, 21, 20). The collectors for M/e of 22 and 20 should be Faraday detectors with amplifiers capable of electronic intercalibration. For faraday collectors we require noise @ 10 second integration time of < 1E-17 amps, and a decay time (tau) to 10 ppm of < 0.3 seconds. The collector(s) for M/e 21 must include a multiplier of the discrete dynode type, and the specifications of the multiplier must be included in the proposal. Capability for multi-collection of mass 40 and 44 (isobaric interferences, 40Ar+ and 44CO2) is desired. M/e 22 and 20 signals must be measurable from 1e-12 to 2e-17 moles, m/e 21 must be measurable from 1e-13 to 1e-19 moles (see below). There must be designed capabilities to intercalibrate the detectors. There must be designed capabilities to precondition the electron multiplier to reduce potential CO2 release from this component (e.g. Gyore et al. 2021).

The instrument needs to be able to measure neon at a variety of abundances. Expected isotopic abundances to be measured for several materials are listed in the table below. Please provide baseline-corrected time-zero intercept intensities (in amps, volts or counts per second) and 1 uncertainties the instrument would deliver for each molar quantity of isotope in the table, allowing up to five minutes for the isotopic analysis. If more than one detector geometry is specified (e.g. multi-collection, single collector CDD peak-hopping), please include estimates for each geometry. If manufacturer recommends baseline measurement before or after aliquant measurement, please also report results for measurements when baselines are measured during the five-minute analysis, as well.

Neon Isotopic abundances to be measured

Peak Flat Vendors must specify beam intensity variation over 50% of the peak top for a large (~1e-13 mol) beam of 40Ar.

Stability of air measurements Measurement of 100 sequential air aliquants ranging from 1e-13 to 1e-12 moles of 40Ar must yield weighted mean 40Ar/36Ar measurements with acceptable dispersion (MSWD < 1.3; Mahon, 1996). Measurement of 100 sequential air aliquants ranging from 1e-13 to 1e-12 moles of 20Ne must yield weighted mean 22Ne/20Ne measurements with acceptable dispersion (MSWD < 1.3; Mahon, 1996).

Linearity 40Ar/36Ar air measurements with at least five different signal sizes ranging from 1e-14 to 1e-12 moles 40Ar must be within analytical error.

Valves Valves must be high-conductance, VAT Series 57 or equivalent, fabricated of all metal, have mating surfaces to accommodate standard ultra-high vacuum gaskets and bakeable to >350°C in the open or closed position. The mass spectrometer pump valve(s) must be pneumatically actuated with electronic position indicators and have demonstrated helium leak rates for valve body and seat <1x10-10 cc STP/sec. They must be serviceable, but service-free for at least 10,000 cycles. Additional valves on instrument must have identical specifications, but may be manually actuated. The sample inlet valve must have a standard mini-Conflat (1.33 inch) flange to facilitate mating with extraction line. Recommend gold-plated stainless-steel seals (rather than standard VAT silver-plated stainless) to minimize hydrocarbon gettering.

Pumping System Roughing system shall be oil-free to prevent hydrocarbon contamination. An ion pump designed to pump noble gas shall be installed as the ultra-high vacuum pump. All pumps must be pneumatically isolatable from the analyzer to allow automated, static measurement. An ultimate pressure of less than 1x10-9 torr must be attained. If pumping is required on the collector housing, the pump must be vibrationally isolated.

Data acquisition The instrument must be delivered with computer-controlled source, magnet and collector electronics as well as data acquisition software. The program must use industry-standard communication protocols with electronic components. The program must provide an accurate time-base for all data acquisition operations, control all necessary peak-switching, perform peak centering, acquire background diagnostics, linearly regress peak intensities to inlet time with appropriate background and intercept statistics. The program must allow for integration with LabView and Python softwares that may operate the argon extraction line valves and laser systems.

Documentation Two sets of manuals, parts list, and other documents sufficient to permit operation and routine maintenance shall be supplied.

Spares kit A spares kit containing spare filaments (5), spare HV supply module (1), and a spare multiplier (1) will be supplied.

Power All components shall be operable with 60 Hz power, with either 240 or 120 VAC supply.

Delivery An instrument meeting or exceeding these specifications must be delivered, assembled and demonstrated at the USGS in Reston, VA within 1 year of purchase. The BARD Lab will provide an extraction line capable of delivering argon and neon to the mass spectrometer upon delivery.

Warrantee The vendor shall provide a minimum 1-year warrantee for the mass spectrometer, including parts, labor, shipping, and any necessary travel by support engineers, beginning on acceptance of the instrument by the Government. (2 year warrantee at thermos?)

5.0 DELIVERABLES

An instrument meeting or exceeding the above specifications must be delivered, assembled and demonstrated at the USGS in Reston, VA within 1 year of purchase. The BARD Lab will provide an extraction line capable of delivering argon and neon to the mass spectrometer upon delivery.

6.0 Performance Standards and Monitoring Methods

Machine acceptance tests include the following:

Stability of Air Measurements test: Measurement of 100 sequential air aliquants ranging from 1e-13 to 1e-12 moles of 40Ar must yield weighted mean 40Ar/36Ar measurements with acceptable dispersion (MSWD < 1.3; Mahon, 1996). Measurement of 100 sequential air aliquants ranging from 1e-13 to 1e-12 moles of 20Ne must yield weighted mean 22Ne/20Ne measurements with acceptable dispersion (MSWD < 1.3; Mahon, 1996).

Linearity test: 40Ar/36Ar air measurements with at least five different signal sizes ranging from 1e-14 to 1e-12 moles 40Ar must be within analytical error. 22Ne/20Ne air measurements with at least five different signal sizes ranging from 1e-14 to 1e-13 moles 20Ne must be within analytical error.