FAQ about preparing good mineral separates:
1) I plan to send my rocks to a commercial lab for mineral separates. Are all commercial labs the same?
Unfortunately, not all commercial facilities deliver high quality mineral separates. Please contact us in advance before sending samples to a commercial lab for separation.
2) What is the main problem with commercially prepared mineral separates?
Surprisingly, the main problem with commercially prepared separates is often simply that the sample’s owner never looks at the separates before arriving here at the lab. We often hear this familiar apology for a poor separate: “I didn’t realize these would be so problematic… I only looked at them today for the first time”. The lesson here is that good familiarity with one’s samples before starting to make a mount is invariably the best way to reduce the chances of making a poor one.
FAQ about sample mounts and strategies for making good ones:
1) What are the dimensions of the finished mount?
Sample mounts are 2.5cm diameter (1”) round epoxy disks, not exceeding 4.5mm in thickness.
2) My mount is slightly thicker than 4.5mm. Is that still OK?
No, mounts that are too thick will not fit in the sample holder. Mounts may be less than 4.5mm thick, however. Mounts that are too thick must be sawed or ground down to the required thickness.
3) Can I put a thin section into the SHRIMP?
Yes and no. A regular thin section will not fit in the sample holder, but a 1” round thin section will. However, for most applications a thin section would not be ideal. The three main considerations are that navigating a thin section is much more difficult than a grain mount, one’s “grains of interest” invariably seem to be outside of the 60% central area (see next FAQ), and typically a thin section may afford too few suitable spots for analysis before a mount change is necessary (mount changes are time and labor intensive and therefore should be kept to a minimum). In some instances, the spatial information available with a thin section is important to a project, or perhaps delicate overgrowths might not survive the mineral separation procedure. In these cases, we recommend drilling out small cores of interest from one or more thin sections and assembling an epoxy mount with the set of recovered cores.
4) How many samples can I put on one mount?
Mounts can generally accommodate up to 6-8 rows of samples, occasionally more on a very carefully made mount. Due to the geometry of the extraction field, samples are limited to the central 60% area of the mount (roughly a circular area 1 cm in diameter). To ensure convenient mount change times, it is not always advantageous to have too many sample rows on one mount. Ideally, a mount should be able to be completed in one or more discrete 24 hour periods. This means an even number of detrital sample rows, or roughly 6 igneous or metamorphic sample rows, or an appropriate combination (see FAQ #7 below).
5) One of my sample rows extends outside of the central 60% area. Will I still be able to analyze those outer zircons?
Possibly, although differential extraction fields near the mount edges may yield unreliable results; hence, the ages may be virtually meaningless.
6) How closely spaced can I place my sample rows?
They can be placed as closely as possible, although for ease of analysis a visible space between rows is desirable. We generally recommend one row’s width (0.5-0.7mm) between rows. There are several advantages to tightly spaced rows: you can fit more samples on one mount, shorter traverses across the mount means it is easier to navigate from sample to sample, and the extraction field in the center is less likely to be variable.
7) How many grains should I put into each sample row?
The nature of samples differs according to the type of work to be done, and is based on the number of grains necessary for a reliable data set, and the type of imaging best suited for the material. A zircon sample for precise geochronology (igneous or metamorphic ages) consists of about 30-50 grains arranged in a row about 0.7mm wide by 6-7mm in length. A zircon sample for a detrital study consists of about 100 or more grains in a row 0.7mm wide by up to perhaps 8-9mm in length. Titanite and monazite samples for geochronology should conform to the zircon geochronology guidelines, but the row widths should ideally not exceed 0.5mm (see samples and mounts FAQ #10). It is OK to make a mount with diverse samples of zircon, monazite and/or titanite, but individual samples should be pure separates and not mixed phases. It is also OK to have detrital and igneous/metamorphic samples on the same mount, although if possible there should always be an even number of detrital samples (2, 4, etc.).
8) Why are samples of detrital zircons treated differently from those used for precise igneous or metamorphic ages?
For detrital zircons, enough grains need to be analyzed to be statistically confident that all populations present are represented by at least one grain. Depending on the reference, this number varies from 60 to 100 grains. Because some grains will be unsuitable for analysis, more than 100 grains should be available for analysis. In contrast, roughly 8-15 analyses of one age, for an igneous or metamorphic zircon, are adequate to statistically delineate it. When cores or overgrowths are present and are of interest, another one to two sets of 8-15 spots are necessary to define each of those ages. Hence, depending on the complexity of the sample, anywhere from 8 to 40 analyses will be sufficient.
9) Other than just the number of grains, are detrital zircons mounted differently than igneous/metamorphic samples?
Actually, there is a slight difference. Igneous and metamorphic zircons are hand-picked with tweezers or a needle and are carefully aligned on the tape. This is simple for only 20-40 grains. Grains are typically placed about one grain width’s spacing apart. In contrast, detrital grains, if the separate is clean, can just be “poured” onto the tape through a mask and gently spread throughout the row with a clean fingertip. By not hand-picking detrital zircons, no picking bias is introduced to the sample and the laborious task of hand-picking 100+ grains is also avoided. Using this technique, detrital grains are often almost touching in a sample row.
10) Why should sample rows of monazite or titanite be thinner than zircon rows?
Zircons are typically imaged with cathodoluminescence (CL), which gives good detail at even fairly low magnification (100X). Monazite and titanite, however, require back-scattered electron (BSE) imaging which is usually more subtle and requires higher magnification to bring out the detail. At even moderate magnifications (~200X), a thick row will not fit into the field of view, so imaging becomes inefficient and laborious. For these minerals, we recommend a single row, where sequential grains are slightly offset, as in a saw-tooth pattern. Incidentally, very old, U-rich zircons (often purplish in color) are oftentimes also best imaged with BSE and thus also may benefit from this sample arrangement.
11) What limitation is there to the size of the grains?
Really just the size of the mount. It’s possible with care to mount a single cm-sized zircon in the central 60% of a mount and do a traverse across the grain. However, for more typical samples, a key limitation is that all of the grains within one mount should be roughly the same size, although in practice this is not always possible. Certainly, samples with fairly big grains should not be put on the same mount with very small grains. For a mount of detrital zircons, it may be possible to mix big and small samples, but there are some important caveats (see next two FAQs).
12 Why is mixing large and small samples a problem?
Ideally, zircons in the sample rows are polished down to their equatorial plane, exposing any cores that may be present. Hence, the smallest zircons in a mount ultimately limit the cutting depth; once small zircons are sufficiently exposed, polishing must stop, even if larger zircons are not exposed. Additional polishing invariably causes the loss of small zircons.
13) But I have a mix of large and small zircons. What can I do?
If the large and small zircons represent different samples, the best approach is to mount them on separate mounts… one mount for only larger grains and one for smaller grains. On a mount of detrital samples, it is also possible to work with two rows of small zircons (~150 total spots; 24 hr total analysis time), and then polish away the small grains to expose the larger ones. Of course, this approach means the small grains are eventually lost forever, so this might not be acceptable if the grains are valuable and you perhaps want to re-examine them in the future. Also, due to the time it takes to complete a row and the timing of mount changes, this approach only works efficiently for detrital samples when there is an even number of small samples. A mix of large and small grains in igneous or metamorphic samples is never desirable. When both large and small grains occur in one sample (more typical in detrital samples), it may be difficult or impossible to expose the equatorial plane of the larger crystals without sacrificing smaller grains.
14) Do I have to analyze my samples in any particular order?
Yes and no. Due to extraction field geometries, it is necessary to always analyze sample rows from left to right to avoid passing the primary beam over old craters. However, because the sample and standard rows are arranged all horizontal and parallel, there is no limitation on which sample is started, or the subsequent order of samples. Indeed, it is even possible to do some analyses in one sample row, move to another sample row, and then later return to the first row (of course, always moving from left to right along the row). However, in most cases it is usually preferable to complete a row before moving on to another. This question brings up another important topic: sample priority. If you make only one mount, this is not an issue. But if you make two or more mounts, see next FAQ.
15) I have more than seven samples so I’ll probably need to make two mounts. How do I decide which samples to put on which mount in order to be the most efficient with my analytical time?
There are several issues here. If you have to deal with samples with large grains and samples with small grains, then segregating the mounts by grain size is perhaps the most important criterion. If grain size is not an issue and you are dealing with detrital samples, you should segregate them by priority: #1 and #2 go on one mount; #3 and #4 go on the other, and so on. For mounts of igneous or metamorphic samples, the top priority samples (#1, #2, #3, etc.) can go on one mount and lower priority samples (#7, #8, #9, etc.) can go on another (again, if grain size is not an issue). Some users like to make separate mounts for zircon, monazite and titanite; this is not necessary, but it does reduce the number of standard rows required for a particular mount. Similarly, it may be desirable, but not absolutely necessary, to segregate older samples (>~1 Ga) from younger ones (see next FAQ).
16) Is it helpful to have an idea of the age of my sample before I make the mount?
It may seem paradoxical to know the age of the sample before you determine its age on the SHRIMP, but even a general idea of its age (from a knowledge of the local geology of the sample) can be very helpful in selecting the most appropriate standard and the most appropriate imaging technique. We try to use age-appropriate standards whenever possible. Detrital samples with diverse ages can be the most problematic in this respect.
17) What do I need to know about standards when preparing my mount?
Based on the anticipated age ranges of your samples, we select appropriate standards, which we add to the mount as one or more additional rows. A standard row usually consists of 30 to 40 grains, and may be placed wherever there is available space within the central analytical area of the mount.