After ~100 analyses, we will typically observe increased scatter in the standards due to decreased ionization efficiency or other instrument mass fractionation effects. 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). 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).īecause 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. The second is a “mega-mount” that is slightly larger and has a working distance of 20-24 mm. The majority of samples analyzed on the SHRIMP-RG are embedded in 25.4 mm diameter epoxy disks. 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 SHRIMP-RG can only accept epoxy or metal disks that are 25.4 mm in diameter and a maximum thickness of 4.5 mm. 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. More time will be necessary if your samples require non-typical preparation methods. 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. Improper sample preparation has resulted in many wasted analytical hours. Sample preparation should be done at SUMAC under the supervision of Stanford/USGS staff to ensure samples are of highest quality. Grains mounted in indium to expose unpolished rims (surface depth profile analysis).Polished thin sections (cut pieces of polished thin sections in-situ analysis).Rock chips (polished rock chips for in-situ analysis).Grain mounts (minerals embedded in epoxy).The following sample types can be analyzed:
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