Sample Types and Sample Preparation
The following sample types can be analyzed:
- Grain mounts (minerals embedded in epoxy)
- Rock chips (polished rock chips for in-situ analysis)
- Polished thin sections (cut pieces of polished thin sections in-situ analysis)
- Grains mounted in indium to expose unpolished rims (surface depth profile analysis)
Sample preparation should be done at SUMAC under the supervision of Stanford/USGS staff to ensure samples are of highest quality. Improper sample preparation has resulted in many wasted analytical hours. We strongly recommend that users come to the facility at least two days before the start of analytical work to prepare samples and collect reflect light, cathodoluminescence (CL), and/or backscatter electron (BSE) images. More time will be necessary if your samples require non-typical preparation methods. If it is absolutely impossible to arrive at Stanford two days before the start of analytical work, please make special arrangements with the Stanford/USGS staff at least two weeks in advance of the users visit.
The SHRIMP-RG can only accept epoxy or metal disks that are 25.4 mm in diameter and a maximum thickness of 4.5 mm. There are two styles of mounts: The first is a standard 25.4 mm epoxy or metal mount, which has with an analytical working distance of 15 mm (the inner 60% of the mount). The majority of samples analyzed on the SHRIMP-RG are embedded in 25.4 mm diameter epoxy disks. The second is a “mega-mount” that is slightly larger and has a working distance of 20-24 mm. This is the best approach for larger samples, such as polished thin-sections or polished rock chips where in-situ context is critical (note that thin sections will need to be cut to fit into this geometry).
Because the samples are gold or carbon coated to dissipate charge and each analysis erodes a small area of this conductive surface, a practical limitation to consider when planning your mount(s) is that there is finite number of analyses can be performed on each mount before too much of the gold has been removed. Therefore, a maximum of ~100 U-Pb, Th-Pb, or Pb-Pb analyses per mount is a conservative limitation for every analysis session (including unknown and standard analyses). After ~100 analyses, we will typically observe increased scatter in the standards due to decreased ionization efficiency or other instrument mass fractionation effects. Trace element analyses are less sensitive to these effects, and a larger number of trace element analyses can be performed on a single mount. This is an important consideration when mounts are being prepared. For example, if you have several zircon samples from igneous or metamorphic rocks and your goal is to collect 15 zircon analyses from each sample, the best approach is to have 4 unknown samples plus standards placed on a single mount. If more than 100 analyses need to be performed on a single mount, it will need to be removed from the SHRIMP and re-coated with gold after the first ~100 analyses.
The SHRIMP-RG can hold 5 samples under low-vacuum, but only 2 samples can be in the analysis position at any one time. Logistically, each time we have to switch samples we have to turn off the high voltage to the instrument and it can take 1-2 hours to recover. Therefore, it is impractical to make more than 1 sample change per day.
Sectioned Epoxy Grain Mounts (zircon, monazite, titanite, and other small mineral grains)
Three to six rows of sample and standard grains (40-80 individual grains, each row 10 to 14 mm in length) are carefully placed onto Kapton® double-sided tape, covering approximately a 1.5 cm diameter area in the center of a 25.4 mm circle (see photo of zircon samples below). A 25.4 mm (I.D.) cylindrical teflon mold is positioned so that it surrounds the grains, and a thoroughly blended mixture (25:3 by weight) of Struers® EPOES Resin and EPOAR Hardener is poured over them to a depth of 10 – 15 cm. The epoxy is left to cure for 12-24 hours at room temperature or in a slow (60 °C) oven. The mount is cooled and the mold is removed. The epoxy plug is trimmed on a lathe (pictured below) to form a disc about ~4.5 mm thick. The epoxy plug should be no larger than 25.5 mm and no smaller than 24 mm in diameter. The sample thickness cannot exceed 4.5 mm.
The specimen side is polished to expose the individual grains (1500 grit wet/dry sandpaper, followed by 6 µm, then 1 µm diamond powder slurries on a Struers LabPol5 rotary polisher. The back (non-specimen) side is also polished so the specimens can be viewed through the back while in the instrument. Prior to placing the mount in the instrument, it is cleaned with soap, 1M HCl, and/or an EDTA solution, thoroughly rinsed in de-ionized water, and dried in a vacuum oven for approximately one hour. The sample is then covered with roughly 10 nm of gold in a Denton sputter coater. The samples can also be easily carbon coated with a carbon evaporator if gold “contamination” is problem.
Unpolished Surface Grain Mounts – depth profiling of mineral rims (Zircon, titanite, and other small grains)
There are many situations where it is desirable to focus on the outermost 2-5 microns of mineral growth. These types of analyses are most commonly performed on relatively young zircons (<300 ka) where the youngest age of zircon growth is targeted. However, in practice, depth profiling can be done on almost any material for U-Th or U-Pb or Th-Pb ages and/or trace element analyses. Mount preparation requires pressing flat, unpolished, mineral surfaces into soft indium metal. If the grains have adhering glass or other particles, the samples may need to be rinsed in dilute HF to clean the sample surfaces. Three to six rows of sample and standard grains (40-80 individual grains) are carefully arranged on a glass slide covered in a thin layer of vacuum grease. The grease allows grains to be easily rotated to align flat grain surfaces.
The grains are pressed into polished indium metal, which has been pushed into 4 mm x 12 mm troughs milled into 25.4 mm diameter polished epoxy or aluminum disk. The samples have to be carefully cleaned with soap and 1M HCl with great care not to pluck out grains, thoroughly rinsed in de-ionized water, and dried in a vacuum oven for approximately half hour.
Unpolished indium mounted grains can also be carefully polished to expose the grain interiors and re-analyzed for depth-profiling sections or to obtain ages of mineral “cores.”
Sample preparation for the larger samples (e.g., shards of glass, corals, large phenocrysts, rock fragments) is very similar to standard epoxy grain mounts. Care must always be taken to place the specimens in the center of the disc, as secondary ion extraction is affected by the edge of the sample holder. This places a 15 mm x 15 mm limit (roughly speaking) on sample size. Larger samples (~24 x 24 mm) can be accommodated using the larger “mega mounts” pictured above. Because the Kapton® double-sided tape has very little surface texture, it is most convenient to have pre-polished specimens so that the amount of polishing is minimized after the epoxy is hardened. If precise U-Pb or U-Th geochronology is a prerequisite for your analyses, polished age standards must also be located on the mount. The best approach is to use pre-polished standards imbedded epoxy that have been carefully cut into a thin block and can be placed alongside the thin-section chips.
Large specimen mounts require very thorough documentation and un-distorted reflect light, secondary electron (SE), cathodoluminescent (CL), and/or backscatter electron (BSE) images of the entire specimen to facilitate easy navigation on the SHRIMP-RG software. “Hunting” for grains on a polished, gold-coated surface has proven to be challenging and an inefficient use of instrument time.
Polished Thin Sections
Where petrologic context is critical to understanding the compositional or geochronologic variation in a sample, it is possible to embed trimmed pieces of thin sections into epoxy and analyze them on the SHRIMP-RG. The approach requires samples that are abundant in the mineral of interest. For example, many thin sections of granitoids or metamorphic rocks may contain several dozen titanite grains, but it is rare to find a thin section with abundant zircon phenocrysts that are large enough (>18-25mm). Therefore, careful inspection of thin sections or polished rock chips should be undertaken before in-situ analyses are proposed. Please contact the Stanford/USGS staff at least 2-3 weeks in advance to discuss sample preparation strategies. Samples are generally prepared in the same manner as large specimens using either a standard or “mega” mount geometry.
1-inch round (25.4 mm diameter) polished thin sections are preferred. Alternatively, a standard polished thin section (27mm x 46mm) will need to be cut and trimmed on a wire saw or band saw to accommodate the size limitations of the SHRIMP-RG sample holders (either 15 mm or 24 mm). If precise U-Pb or U-Th geochronology is a prerequisite for your analyses, polished age standards must also be located on the mount. The best approach is to use pre-polished standards imbedded epoxy that have been carefully cut into a thin block and can be placed alongside the thin-section chips.
Like large specimen mounts, thin sections require very thorough documentation and un-distorted reflect light, secondary electron (SE), cathodoluminescent (CL), and/or backscatter electron (BSE) images of the entire thin section to facilitate easy navigation on the SHRIMP-RG software.