Noble Gas Mass Spectrometry Lab

OSU Marine Geology

Geochronology

     Geochronology is continually evolving and growing more precise, with studies focusing on the ages of MORB, OIB, lunar and planetary rocks, hydrothermal minerals, and clays.  Employed in this lab is the ⁴⁰Ar/³⁹Ar dating method using an MAP 215-50 rare gas mass spectrometer for analysis and a resistance furnace or laser for gas extraction.  This lab supports both basic and applied research projects in earth and marine science.

Geochronology Group Members

Robert Duncan

COAS

John Huard

COAS

Facilities

Noble Gas Mass Spectrometer

     Our laboratory is centered on a Mass Analyser Products model 215-50 rare gas mass spectrometer with all-metal extraction system for ⁴⁰Ar/³⁹Ar age determinations.  The mass spectrometer is a 15 cm radius, 90° sector instrument with a Nier-type source.  It has an electron multiplier for high sensitivity and an electrostatic analyzer with adjustable collector slit for high resolution of masses in the Argon isotope range.  For incremental heating the system is equipped with a Heine low-blank, double-vacuum resistance furnace and a Merchantek integrated CO₂ continuous fire laser with infrared pyrometer gas extraction system.  Each is connected to an ultra-clean, low volume (~1000 cc) gas cleanup line with Zr-Al and Zr-V-Fe getters.  For gas extractions using the resistance furnace, irradiated samples, either whole rock mini-cores or Cu-wrapped mineral separates, are loaded into a sample manifold that feeds into a Ta/Nb-crucible with a Mo-liner.  Temperatures are precisely controlled at the bottom of the crucible with a programmable power supply thermocouple system.  For experiments using the laser system, irradiated samples, either crushed whole-rock, groundmass, mineral separates, single-grain, or in-situ mineral vein are loaded into a Cu-planchette designed with a variety of pits/pans that hold <1 to 50 mg of material, which is then pumped within a sample chamber fitted with a ZnS window that is transparent to the CO₂ laser wavelength.  Temperatures are monitored with an infrared pyrometer and controlled with laser power by computer. 

CO2 LASER

Ion beam currents are measured with the electron multiplier at m/z = 35, 36, 37, 38, 39, and 40, and intervening baselines.  Measurement times, peak/baseline voltages, data acquisition and storage are computer controlled using an 8.5 digit integrating multimeter.  Peak heights decay during an analysis and the regressed peak heights vs. time generally follow first-order polynomial fits.  The background for the mass spectrometer (subsequent to 20 minutes isolation from pumps) is 5x10^-18 mol at m/z = 36, 4x10^-18 mol at m/z = 39 and 1x10^-15 mol at m/z = 40.  The extraction line blank for the laser system is 1x10^-18 mol at m/z = 36, 3x10^-18 mol at m/z = 39 and 1x10^-16 mol at m/z = 40.  Mass discrimination is monitored using zero age basalts.  All resulting ages are calculated using the ArArCALC v2.2 software package (Koppers, 2002), http://earthref.org/tools/ararcalc.htm

Laser Chamber with ZnS Window

Sample Preparation

    We employ a variety of methods to prepare samples for age determinations.  Fine grained, unaltered whole-rocks (basalts, andesites) are cored with a 5 mm diameter diamond-tipped drill bit, then sectioned into disks of 100-300 mg.  Unaltered phenocrysts in otherwise altered rocks are separated by crushing, sieving, washing and drying, then passing through a Frantz magnetic separator, followed by mild acid cleaning (HNO₃, HF), ultrasonic wash, drying, and a final hand-picking under binocular microscope.  Clays are separated by size fraction (2-20µm, 20-63µm) then treated with acetic acid to remove carbonate, washed and dried. 

Laser Chamber Sample Holders (Scale: cm)

Irradiations  

     Samples are irradiated at the Oregon State University TRIGA experimental reactor, typically at 1 MW power for periods appropriate for the age and composition of sample unknowns.  The neutron flux is monitored with a variety of standard samples (Mmhb-1 hornblende, FCT-3 biotite, TCR sanidine). There is also a Cd-shielded irradiation location which is designed to block slow (thermal) neutrons in preference to fast neutrons.  Irradiated materials are disposed through the OSU Radiation Safety Office. All workers in our laboratory are trained in the safe handling of radioactive materials.

Sample Requirements

Resistance Furnace Sample Manifold

Sample Amounts

     -  Typically,  furnace whole rock, groundmass, or plagioclase analysis requires 100mg of prepared, separated, clean, and hand-picked sample.  Young(<500ka), low K whole rock analysis may require up to 300mg.  Analysis of most other K-bearing mineral separates require 30-50mg.

     -  Laser extraction analysis range from <1mg to ~50mg of sample.  Low K-bearing samples of <5mg may not produce usable data and may be best analyzed as total fusion extractions.

     -  If sending already separated and prepared samples the above amounts are minimums.  Ideally,  double the above amount to account for final sample cleaning, inspection and hand-picking conducted by lab personnel, and to decrease heterogeneity effects of the sample.

     -  For unprepared raw sample send 1-2kg of a whole rock hand-sample for whole rock core or groundmass analysis; or 1-2kg of fragments for crushed whole rock or groundmass analysis.  Raw material required for mineral separations is dependant on mineral abundance, however, most are successful using ~2kg.

     NOTE:  When considering type of sample preparation (whole rock, groundmass, mineral separate), sample amount, irradiation duration, and subsequent analysis, the use of thin-sections and elemental analysis are highly recommended prerequisites for Argon dating, and if available should be provided.

Sample Preparation

     The following are recommended guidelines if preparing samples:

     -  All whole rock core preparation will be performed in lab to ensure proper size for irradiation packaging.

     -  Groundmass or mineral separates are crushed and sieved according to grain size of the desired separate.  Usually, a maximum size sieved fraction of 1/2 the original grain size or less is most successful.  Sample is then rinsed with distilled/de-ionized water several times and placed in ultrasonic wash for 5 minutes.  After rinsing several times, repeat ultrasonic wash and rinse until water is clear.  Decant as much water as possible and dry sample at ~80º C for several hours.  Separation by primarily hand-picking, magnetic separation, heavy-liquids, or other means is then performed.  After a concentration of groundmass or mineral separate is obtained, acid treat the sample as follows:

          *  ~5% HF, ultrasonic 1 minute, rinse with DI water. (plagioclase only)

          *  ~5% HNO₃ ultrasonic 20 minute, rinse three times with DI water.

          *  DI water ultrasonic 20 minute, rinse three times, dry in oven.

     -  Continue groundmass or mineral separation techniques, end with final hand-picking under binocular microscope.  Re-sieve sample, wash with acetone followed by DI water, acetone, then oven dry.

     -  Pre-separated sample should be shipped in a leak proof vial.  Highly recommended is a ~14x45mm size vial capped with a tightly fitting plastic push closure.  For further security tape cap to vial using a clear tape.  Screw capped vials often leak sample during shipping, even when taped.

     NOTE:  Do not use HCl for acid treating samples.  Use acetic acid to remove carbonates.

     NOTE:  Please inform if heavy-liquids were used for sample preparation.

Analysis Fee

     Standard analysis fee per sample for an incremental heating experiment with 8-10 steps is $600 U.S.;  sample preparation, irradiation, final disposal of irradiated material, and data reduction included.  Samples are typically analyzed in the order received.  For other than standard fee schedule (total fusion experiments, priority (expedited) research, visiting domestic or international researchers) contact lab personnel.

Current Research Projects

Geochronology of volcanic rocks from the ocean basins. These include: mid-ocean ridge basalts and small seamounts near ridges, fracture zone ridges, and mid-plate volcanic lineaments related to hotspots. Current projects involve radiometric dating of:

     - dredged rocks from the Rano Rahi seamounts near the super-fast spreading East Pacific Rise 

     - Ocean Drilling Program cores from the Emperor Seamounts, Leg 197 

     - dredged rocks from the Line Islands and Easter-Salas y Gomez seamount chains

     - submersible-collected rocks from the Gorda Escarpment and Mendocino Fracture Zone  

Geochronology of Large Igneous Provinces (LIPs), both on land and in the ocean basins. Current projects include:

     - North Atlantic Igneous Province (East and West Greenland, Faeroe Islands)

     - Kerguelen Plateau (ODP Leg 183)

     - Caribbean Plateau (Costa Rica, Haiti, Curacao, Colombia, Beata Ridge) 

     - Karoo Igneous Province (South Africa, Lesotho, Namibia, Zimbabwe, Antarctica)

Chronology of basin formation on the Moon, through radiometric dating of lunar impact glasses returned by the Apollo 16 mission.

Geochronology of ash layers in marine sediments to revise biostratigraphic timescales. Current projects are:

     - Eocene-Oligocene boundary and Pacific NW flora assemblages

     - Pliocene-Miocene sections from ODP Legs 185 and 191

Geochronology of terrestrial sediments as fingerprints for source provenance. Current projects include:

     - total fusion ages for feldspar grains from glacial tills

     - clay formation ages to identify river drainages contributing to Pacific NW margin sediments on glacial-interglacial timescales

Geochronology of mineralization in economic ore deposits. Current projects include:

     - Vein adularia in the Hishikari gold deposit, Japan

     - Timing and duration of mineralization at the Yanacocha district, Peru

Contact

For more information:  Prof. Robert Duncan or John Huard

                                  -email: geochronology@coas.oregonstate.edu

                                  -tel:  (541) 737-5226

Send samples to:   

Oregon State University
COAS - Burt 130

Corvallis, OR 97331-5503

Attn: John Huard
                        

Please contact via telephone or email before shipping samples.