A pervasive challenge in noble gas geochemistry is to ensure that analytical techniques do not modify the composition of the noble gases in the samples. Noble gases are present in the atmosphere and are used
in a number of manufacturing procedures and by laboratory equipment. Of
particular concern is the introduction of atmospheric or laboratory noble
gases to samples during preparation before samples are placed in a vacuum
chamber for analysis. Recent work has shown the potential for contamination
of crushed samples with air-derived He that is not released by placing the
samples under vacuum at room temperature. Using pure He gas as a tracer, we
show that the act of crushing samples to a fine powder itself can introduce
He contamination but that this is easily avoided by crushing under liquid
or in an inert atmosphere. Because the He is trapped during crushing, the
same concern does not extend to samples that are naturally fine-grained when collected. We also show model results that demonstrate when this effect might significantly impact samples for (U–Th) / He geochronology or 3He cosmogenic nuclide dating. The degree of He contamination from crushing samples to sizes smaller than the > 63 µm range typically used for geochronology is insignificant for samples with a date of at least 1 Ma and 1 ppm U, and the degree of He contamination from crushing samples to sizes smaller than the 100–500 µm range typically used for cosmogenic nuclide dating
is also insignificant for samples with a date of at least 10 ka with typical 3He
production rates.
Introduction
The five noble gases are reservoirs of important geochemical information,
and the two stable isotopes of He have many uses for earth scientists.
Geochemists measure rare 3He (abundance 1.39 × 10-6 of total He) as a tracer of tritium in water (Schlosser et al., 1988), as a cosmogenic nuclide for surface exposure dating (Lal, 1988), as a tracer of mantle fluids (Farley et al., 1995), and as a synthetic tracer of experimental He degassing in minerals (Shuster and Farley, 2005). The most abundant isotope, 4He, is in turn a measure of temperature-dependent solubility in water (Weiss, 1970), a radiogenic daughter product for thermochronology (Zeitler
et al., 1987), and a signature of dust derived from crustal rocks (Winckler
et al., 2005). He gas is also used as an inert environment in various
manufacturing processes and in laboratory cryogenics, sometimes in an
isotopically purified form. There are no radioactive isotopes of He
long-lived enough to be found in measurable quantities in nature, so a
complication of these many uses is that, with only two isotopes, the system
is underdetermined relative to the number of potential components, and one
must use additional contextual information or processing techniques to
isolate the desired signal. Because of this omnipresent burden, we read the
intriguing results of Protin et al. (2016) with some concern.
Protin et al. (2016) found that fine-grained minerals crushed in the
laboratory before measurement appeared to irreversibly adsorb He from the
atmosphere, meaning that the He would be adsorbed at room temperature and
then retained until the sample was heated to high temperature during the
measurement process. The idea that a sample might adsorb He at room
temperature would not itself be cause for concern, as it is well known that
surfaces can adsorb gas. However, this process is usually assumed to be
reversible, meaning that a sample placed in a vacuum chamber will relinquish
the adsorbed air as readily as it was first adsorbed at room temperature or,
to speed up the process, under a very mild heating compared to the
temperatures used for sample gas extraction (for example, gentle heating to
50–150 ∘C prior to extraction at > 1000 ∘C is a common technique for cleaning noble gas samples).
In contrast, Protin et al. (2016) found that the irreversibly trapped He was not
released under vacuum during heating in a temperature range distinct from
that in which the sample gas was released, which would render it impossible to distinguish the two components without isotopic deconvolution.
The problem of trapped air in noble gas measurements is not a new one. In
the Ar isotopic system commonly used for K / Ar and 40Ar /39Ar
dating, an additional isotope can be used to correct for the pervasive air
contamination caused by the high abundance of Ar in the atmosphere
(approximately 1 %, compared to 5 ppm for He), but this is usually not
feasible with He due to the extremely low and relatively uncertain
3He /4He ratio of air and the low and variable 3He /4He of radiogenic He and the potential for mixture with other sources of 3He such as mantle-derived He and cosmogenic He. The 3He /4He ratio of air is usually assumed to be 1.39 × 10-6 and appears to be invariant in
space and time at a level better than the typical uncertainty of He isotope
measurements (Lupton and Evans, 2013; Boucher et al., 2018); however, the
absolute uncertainty of this ratio is worse than 1 % (Blard et al., 2015). Radiogenic He produced directly by the decay of U and Th is 100 %
4He, but neutrons produced primarily by (α,n) reactions as a result of the same radioactive decay processes produce 3He through the reaction 6Li(n,α)3He at a rate dependent on the composition of the mineral, primarily its Li content. Minerals such as apatite (Farley et al., 2001) and zircon (Sliwinski et al., 2018) typically have less than 1 ppm Li, which yields an insignificant quantity of 3He, although these too could be compromised by adjacent Li-rich minerals such as micas and amphiboles due to the relatively long ∼ 30 µm stopping distance of the 3He produced by the 6Li(n,α)3He reaction (Farley et al., 2001). Analytical techniques for
(U–Th) / He dating often do not even allow the possibility of measuring
3He in the sample, as the isotope is frequently used as a spike on
quadrupole mass spectrometers that are not, in any case, capable of
resolving small natural abundances of 3He from the ubiquitous HD (hydrogen deuteride) background. Finally, an isotopic correction for air contamination would have
to assume no isotopic fractionation during the trapping process and a
well-known laboratory air composition, but the extremely small structures
that would be required to irreversibly trap He within the surface layer of
minerals would be likely to fractionate the gas isotopically, and the
pervasive use of He and other compressed gases in laboratories makes the
composition of the laboratory atmosphere variable and difficult to predict
without frequent measurement.
Typical analytical procedures for (U–Th) / He dating, cosmogenic 3He dating, and other He isotope analyses call for approximately sand-sized particles with dimensions in the range of 50–500 µm. There are several reasons for this. In the case of (U–Th) / He analyses, the stopping distance
of α particles in minerals is 15–40 µm, depending on the
mineral and the energy of the particle. Because α particles that
travel outside of a mineral after emission during U and Th decay are lost
even as the radioactive parents remain, one must make an alpha ejection
correction in the course of (U–Th) / He dating, and the magnitude and
uncertainty of this correction becomes very large as the grain size analyzed
approaches the ejection length scale (Farley et al., 1996). In the case of
cosmogenic 3He analyses, crystals larger than 500 µm can suffer from contamination with magmatic He in fluid inclusions, and crystals
smaller than 100 µm can be more contaminated by atmospheric He (Blard,
2021). Practically, powders finer than sand-sized are difficult to handle in
the vacuum system, so one tends to choose coarser material when possible. It
is also simpler to ensure the mineralogical purity of the samples when they
are large enough to manipulate easily and observe under a microscope but
not so large that it is impossible to isolate individual crystals or to see
inclusions. There are two reasons one might choose to analyze minerals that
are finer than sand-sized: in some cases, the grain size of the sample may
simply be smaller; in other cases, one might wish to crush a sample to avoid
pervasive small inclusions or intergrowths, or one might crush samples as
part of the analytical process, for example to separate magmatic and
cosmogenic endmembers in cosmogenic 3He dating (Kurz et al., 1986).
Typically, one can isolate mineralogically pure samples that are no smaller
than 50 µm, which is above the size range of concern based on the
results of Protin et al. (2016), and low-He samples crushed to a powder in
a vacuum before fusion can be handled in a way that minimizes atmospheric He
exposure. However, there are many cases in which the material is naturally
fine-grained. A couple of examples include polycrystalline iron oxides,
which can be dated using the (U–Th) / He and (U–Th)/Ne methods, and which comprise a range of crystallites as small as less than 1 µm (Farley and
Flowers, 2012; Farley and McKeon, 2015), interplanetary dust particles, in
which 3He is measured as a constant flux proxy, and for which the size
range of interest is a few µm to less than 35 µm (Farley et al., 1997), and cosmogenic 3He studies in which finer material is
deliberately targeted in order to avoid the fluid inclusions that are more
prevalent in larger crystals (Puchol et al., 2017; Blard, 2021).
The study of Protin et al. (2016) used isotopic deconvolution and sequential
crushing in an attempt to quantify the amount of He contamination from
laboratory air. Our approach is much simpler: while we sought to confirm in
broad terms the results of Protin et al., we do not attempt to quantify the
surface area effect that they document. Rather, we accept the premise that
contamination from atmospheric He is possible and instead take advantage of
pure He gas (200 000 times more concentrated than He in the atmosphere) to
investigate the mechanism of contamination using a coarser size fraction and
to arrive at a solution that allows us to avoid the problem in practical
situations. While we do not attempt to degas the samples before crushing or
to quantify the isotopic composition of the He components, the use of pure
He gas during crushing makes these tests very sensitive compared to the
concentrations of a few parts per million that one would typically find in a laboratory
atmosphere.
MethodologySample preparation
We prepared samples in a variety of ways to simulate different laboratory
noble gas contamination scenarios. In each experiment, we crushed samples of
San Carlos olivine in the presence of different gas mixtures at atmospheric
pressure, and in some cases under fluids. Initial sample preparation
involved picking inclusion-free San Carlos olivine from working samples in
the Caltech Geological and Planetary Sciences mineral collection. The San
Carlos olivine standard has a variable but moderate He concentration of less
than about 7.5 × 10-12 mol g-1 (Mohapatra and Murty, 2000). We chose the San Carlos standard because inclusion-free crystals are readily available and because 7.5 × 10-12 mol g-1 is small compared to the contamination we expect in a pure He environment based on Protin et al. (2016). The smallest degree of contamination the authors observed in the 50–125 µm size fraction was 8.2 × 10-14 mol g-1 from crushing in atmospheric He, which we scale to 1.6 × 10-8 mol g-1 in the 200 000 times more abundant pure He used in this study. Indeed, the
He concentration in San Carlos olivine is more than 150 times less than the
highest concentrations measured in this study. We treat He concentrations
less than 1.5 × 10-11 mol g-1, or twice the highest concentration
recorded by Mohapatra and Murty (2000), as the background level for this
study because of the use of non-degassed olivine in the experiments (see
background boxes on Figs. 1 and 2).
He trapped by crushing process in initial two-heating-step
experiments. The background rectangle shows the approximate
background level of the experiment based on the variable natural He content
in San Carlos olivine (Mohapatra and Murty, 2000; this work). Analytical
standard errors shown are 2σ and account for quadrupole mass spectrometer (QMS) counting
statistics and are small compared to both the natural background and the He
enrichment measured in these experiments. The larger pale boxes show the
initial low-temperature (800–900 ∘C) extractions of each
sample, while the foreground boxes show the second high-temperature (1500 ∘C) extractions. Compared to the samples that were either
crushed in pure N2 and then soaked in He or simply crushed in N2
and then analyzed, the samples crushed in the presence of He exhibit a
substantial contamination from the He atmosphere during crushing. The
aliquot sieved to less than 50 µm shows an even greater He enrichment, consistent with a surface area dependence as shown by Protin et al. (2016).
He trapped by crushing process in single-step follow-up
experiments. The gray background rectangle shows the approximate background
level of the experiment based on the variable natural He content in San
Carlos olivine (Mohapatra and Murty, 2000; this work). All samples were
sieved to a size range of 37–50 µm to reduce variability from sampling. Analytical standard errors shown are 2σ and are small compared to both the natural background and the He enrichment measured in these experiments. The samples crushed in the presence of pure or enriched He show significant enrichments in He. Acid leaching reduces the amount of He
contamination but not to background levels. The sample crushed under vacuum and then immediately exposed to pure He also shows a He enrichment, but it is
only a few percent of the enrichment seen in the samples exposed to He
during crushing. Samples crushed in a neutral atmosphere and then exposed to
He and the sample crushed under water with a pure He atmosphere are
indistinguishable from background.
After initial picking, we separated aliquots of the sample for the different
crushing treatments. The crushing device we chose was a standard agate
mortar and pestle, which we cleaned extensively between experiments using a
scouring pad, water, and isopropyl alcohol. With the exception of crushing
performed in the presence of air, for which we simply crushed the samples on
the bench in a laboratory in which no compressed gas cylinders are used or
stored, we created the gas atmospheres by placing the mortar and pestle in a
sealed plastic zipper bag during crushing. The gas was introduced through a
small opening in the zipper bag, such that the bag inflated and filled with
the gas but was not sealed sufficiently well to increase the pressure
significantly about ambient atmospheric pressure.
For the initial set of experiments, we simply took the finest portion of the
powder to check for He contamination in the different aliquots. After
finding initial evidence of contamination, we sieved the samples at 50 µm and measured the finer fraction to confirm that the surface area of the crystals was an important control on the degree of He contamination. Finally, we repeated the initial experiments and added some additional tests, in order to determine whether crushing itself or fine grain size was the controlling factor on He contamination. Additional experiments included crushing in vacuum and in a He-free atmosphere and then later exposing to the 100 % He atmosphere, crushing under water in the presence of a 100 %
He atmosphere, and leaching the sample for 20 min in concentrated nitric
acid after crushing in the presence of a 100 % He atmosphere. We sieved
all samples in the second round to a grain size of 37–50 µm after
crushing in order to improve the uniformity of the potentially contaminated
surface area in the measured samples.
Mass spectrometry
We measured the first round of samples using laser degassing followed by
measurement with a Pfeiffer Prisma quadrupole used routinely for (U–Th) / He
dating at Caltech (House et al., 2000). Measurements were interspersed with
background analyses and standardized using a calibrated 4He reference
standard and a 3He spike measured against the samples and the standard.
The 4He chamber background was less than 5 × 10-14 mol, the 3He spike was 1.3 × 10-13 mol, and the 4He reference standard was 3.2 × 10-13 mol. For this set of experiments, we heated each sample to at
least 800 ∘C to verify that reversibly adsorbed He or even
He that could be separated from the sample through step heating was fully
removed. We then heated the samples to about 1500 ∘C to
fully degas them; any additional He in these measurements represents
irreversibly trapped He.
After verifying that the He trapping was irreversible, as reported by Protin
et al. (2016), we conducted the second set of experiments using a single
high-temperature extraction with a vacuum furnace to allow for larger
samples, and we measured the trapped He using an MAP 215-50 mass
spectrometer. Measurements were interspersed with background analyses and
standardized by bracketing with a calibrated 4He-doped air reference
standard (“Caltech Air”). Procedural blanks ranged from 1.4 × 10-14± 4 × 10-16 to 6.6 × 10-14± 3 × 10-16 mol and were
mostly dependent on the cleanliness of the vacuum furnace; blank / ,signal
ratios ranged from 0.9 % for the samples crushed in pure He to 73 % for the sample crushed under water. The 4He concentration of the air
standard was 3.6 × 10-12 mol. Extractions were performed at about 1500 ∘C to fully degas the samples.
Results and discussionHe contamination in crushed samples
As expected based on the results of Protin et al. (2016), we found
significant He contamination in samples crushed in the presence of pure He
(Figs. 1 and 2, Tables S1 and S2 in the Supplement). The contamination was
retained at a surprisingly high temperature, with more than 20 % of the
additional He retained even after pumping the samples under ultra-high
vacuum for several hours and then heating to more than 800 ∘C for 10 min (Fig. 1). Because the crushing was carried out at
laboratory temperatures (∼ 21 ∘C), this
result suggests that the trapping of He is indeed irreversible. He merely
adsorbed onto the surface of the crushed olivine would be released after
placing the samples under vacuum and certainly after even mild heating. We
suggest two possible reasons for this anomalous irreversible He trapping and
retention. First, as suggested by Protin et al. (2016), fine-grained samples
may exhibit a “lobster pot” trap for He. This analogy implies that the He
is trapped by some mechanism that does not allow it to escape (like a
lobster pot, which lobsters can easily enter but then not escape due to the
geometry of the trap). Under this analogy, He exposure at any time after the
sample was crushed would cause contamination, and naturally fine-grained
samples would be subject to contamination from the environment. On the other
hand, the violent crushing action itself may cause the He to be trapped,
perhaps due to tiny fractures or other structures that open during crushing
and then heal or anneal immediately. In this case, the He would only be
trapped during crushing but would not be susceptible to irreversible He
contamination after crushing, and naturally fine-grained samples would not
be subject to the same contamination. Our second set of experiments was
designed to distinguish between these two scenarios.
Irreversible He trapping due to crushing and trapping of other noble gases
The second set of experiments confirms that the irreversible He trapping
occurs during the crushing process rather than as a result of the grain size of
the samples or as a result of the fact that they have been crushed. We show
dramatic reductions in He contamination, to levels similar to the detection
limit of this methodology, in several different scenarios (Fig. 2). First,
crushing in a neutral atmosphere (N2) and then immediately exposing samples to
pure He results in virtually no additional He contamination. This shows that
there is not an anomalous He-specific trapping mechanism that remains
accessible after the crushing event, but it does not rule out an unexpected
irreversible surface trapping mechanism for any gas due to crushing or grain
size. In another experiment, however, we crushed in high vacuum using a
magnetically actuated high-vacuum crusher and then immediately flooded the
vacuum system with pure He. In this case, no other gas was present in the
volume of the chamber during crushing, and pure He was the first gas exposed
to the sample after crushing, but the experiment still showed far less He
trapping (about 5 % of the case in which the sample was crushed in the
presence of pure He). Finally, we also crushed samples in the presence of a
mixture of 25 % each of He, Ne, Ar, and Kr. In this case, the sample
trapped approximately 25 % as much He as the sample crushed in the
presence of pure He. This final experiment, combined with the evidence that
pure He is not trapped after either vacuum crushing or crushing in an
N2 atmosphere, is consistent with a model in which the sample traps
gases present in the atmosphere during crushing according to their volume
ratio rather than trapping He through a mechanism that excludes other gases.
Mechanism of trapping during crushing
The mechanism of trapping during crushing is a matter of speculation, but it
is important in that it is clear from our results and from earlier work by
Niedermann and Eugster (1992) and Protin et al. (2016) that the trapping is
irreversible. The trapping appears to apply to heavier noble gases as well
(Niedermann and Eugster, 1992), meaning that it is not enabled by a
mechanism dependent on the unusual mobility and small size of He. It also
appears to cease shortly after the physical impact that causes it to occur,
as evidenced by the experiments showing minimal contamination of sample
crushed in a He-free atmosphere or under vacuum and then later exposed to
pure He. Therefore, we consider that the most likely mechanism is that
microfractures form during crushing and then close or anneal shortly
thereafter. This mechanism would be consistent with the findings presented
here and would also suggest that naturally fine-grained samples are not
subject to the same phenomenon.
Implications for experimentalists
While potentially troublesome for experiments that require crushing of
low-He samples to a very fine grain size in the laboratory, these results
are not nearly as concerning as the possibility that all fine-grained
samples trap He or even the possibility that crushed samples might trap He
at any point after crushing. Because the problem is largely restricted to
the time of crushing itself, we explored several solutions that exclude He
from this part of the sample preparation. As mentioned above, crushing in a
neutral (N2) atmosphere and then exposing to He did not result in
significant contamination, so using a neutral atmosphere such as N2,
which is relatively cheap as both a compressed gas or a product of liquid
nitrogen boiling, is a solution to the problem (Fig. 2). Indeed, in most
cases of typical He concentrations in samples, crushing in air (which is
only 5 ppm He) and taking care to avoid unusually concentrated or
isotopically fractionated sources of He in the laboratory is adequate, but
extra care should be taken in particular for very young cosmogenic 3He
samples (Blard et al., 2006).
The other solutions we explored were crushing under water and crushing and then
acid leaching the sample. Leaching the sample in concentrated nitric acid
for 20 min removed about 45 % of the mass of the 37–50 µm
fraction and removed about 90 % of the He (Fig. 2). While this is a
dramatic improvement and shows that trapping occurs mostly on the outer
edges of the fragments created during crushing, it is not as effective as
simply avoiding He through crushing in a neutral atmosphere. It is also not
as effective as crushing under water, which may in many cases be the
simplest solution available. He solubility in water is quite low compared to
other gases (Weiss, 1970), so working under water is a good way to exclude
He, and crushing under water is often preferable anyway because it helps
prevent sample loss. For this test, we flooded a bag with pure He in the
same way as the experiments showing dramatic increases in sample He
concentration during crushing but also filled the mortar with a
∼ 1 cm layer of tap water before crushing the sample. The
level of He contamination was below the detection limit of this methodology,
which is limited by the natural background of He in the San Carlos olivine
(Mahopatra and Murty, 2000), and similar to the sample crushed in air
despite the presence of 200 000 times the partial pressure of He in the
atmosphere just above the water layer (Fig. 2).
In addition to the imperative to avoid unwanted sample contamination, the
irreversible trapping mechanism we observe may actually be useful for
applications in which trapping synthetic noble gas mixtures in minerals
would be desirable, such as mineral standard preparation or isotopic spiking
in environments such as on planetary bodies. The scope of the current work
does not explore the degree or variability of isotopic fractionation during
trapping or the extent to which the degree of contamination could be
controlled by different crushing techniques or more careful size separation,
both of which would be critical for such applications.
Modeling of the impact of He contamination for thermochronologyModel description
In order to conceptualize the observed phenomena and to make predictions
about the effect of anomalous irreversible He trapping on mineral samples
used for geochronology, we constructed a simple geometric model to represent
one physical explanation consistent with our observations. In this model,
presented in the Supplement in a Jupyter notebook, the amount of He
contamination due to trapping decreases exponentially from the surface of
the crystal. Crystals are modeled as a sphere, with He contamination
entering from the outer surface. The crystal is approximated as spherical
shells of 1 µm thickness, with a per-unit-volume He contamination
exponential rate constant of 0.29 µm-1, which is based on the
∼ 90 % of contamination removed by leaching ∼ 45 % of the mass of the 37–50 µm samples used here. The choice of an
exponential model for the contamination is a hypothesis designed to be
consistent with the observation that removing about half of the sample by
leaching removed most but not all of the He contamination. If the phenomenon
were merely surface contamination, leaching would have removed all of the
it. Conversely, if it were pervasive in the mineral, leaching would only
remove a similar proportion of the contamination. While a simple surface-area-dependent contamination model would account for the grain size
dependence of He contamination, a model in which contamination decreases
exponentially with depth into the crystal surface makes it possible to
account for the observation that the majority of the contamination occurs
near the surface but some extends deeper into the crystal.
For the purpose of comparison to He concentrations in geochronology and thermochronology,
we simulate the radiogenic ingrowth of He in apatite crystals of several ages
and U concentrations. Alpha ejection is ignored, so there is no difference
between He from U and Th decay, and so only U is considered for the sake of
model simplicity. The same principles can be applied to radiogenic 4He
and cosmogenic 3He in other minerals. In the case of cosmogenic
3He measurements, the greatest concern will be the effect of 4He
contamination on the magmatic 3He correction (Protin et al., 2016;
Blard, 2021). While 3He contamination is also a possibility, this
effect will be scaled down by the 3He /4He ratio of the
contaminant, which is 1.39 × 10-6 if the contaminant is atmospheric He.
This model allows us to scale the degree of contamination to different grain
sizes and to compare the atmospheric He contamination to the radiogenic He
concentration of minerals of different ages and U concentrations. The
reference point used for these scaling calculations is a crystal with a
diameter of 45 µm (radius 22.5 µm), in the middle of the sieve range used for the second round of experiments, and an amount of He
contamination equal to 1.37 × 10-14 mol g-1, which represents 2 times the worst contamination observed in our pure He-soaked experiments (1.31 × 10-9 mol g-1), scaled down to 5.24 ppm (the atmospheric
concentration of He), both to present a conservative model and to account
for the possibility of slightly elevated 4He in laboratory environments
due to the use of concentrated He in laboratory processes.
The concentration of the outer shell of the r= 22.5 µm model is calculated by creating a series of shells assuming spherical geometry,
meaning that the volumes are simply given by Eq. (1). The value Vn is
the volume of the shell beginning at rn, where rn is the outer
radius of the nth shell, counting from the outside of the crystal inward,
and rn+1 is the inner radius of shell n or the outer radius of shell
n+1. The fractional volume of the shell is then given by Eq. (2).
The value Vn,frac is the fractional volume of shell n, Vn is the volume of shell n, r0 is the radius of the crystal, and a is the width of shells used in the model (a=rn-rn-1). Note that a= 1 µm except for crystals < 10 µm in radius, in which case we
split the crystal into 10 shells. We then use this fractional volume to
weight the exponential fit of the He contamination for each shell by volume,
an effect which becomes more significant for smaller crystals in which the
outer shell volumes differ dramatically. The relative per-unit-volume He
contamination is given by Eq. (3).
The value [He]reln is the relative He
contamination of shell n. Note that for the outer shell (n= 0), rn=r0, so the He contamination is defined relative to the outer shell. Based on our measurements, we assume that the total concentration of
contamination He in the crystal will be 1.37 × 10-14 mol g-1; then
we determine the fraction of this that is present in the outer shell. We
used olivine for the experiments as described in Sect. 2.1, but we use
apatite for these calculations in order to demonstrate the relative
significance of the contamination in the mineral most commonly used for He
thermochronology. The volume concentration of He contamination in the
outermost shell is given by Eq. (4).
The value [He]T= 1.37 × 10-14 mol g-1, and ρc is the density of the crystal. We note that while the density of different minerals is small compared to the likely
variability of the cracking effect, we are also assuming that the geometric
model of cracking applies similarly to different crystal systems. The goal
of this work is to show the order of magnitude of the problem and suggest
ways to avoid it when it may be significant, but if this model were to be
used quantitatively, more careful investigation of the behavior of
individual crystal systems might be warranted.
We then take the volume concentration of He in the outer shell [He]outer and apply it to the outer shell of
modeled crystals of different sizes in order to calculate the relative
contamination in each. For each test radius rtest, we break the crystal up into either rtest/ (1 µm) shells of a= 1 µm width or, in the case of crystals < 10 µm in radius, 10 shells of a=rtest/10 width. We then calculate relative He concentrations in each shell in the same manner described above and then calculate the He concentration in each shell n using Eq. (5).
The outer shell concentration [He]outer
will simply be the value calculated for the 45 µm crystal, weighted
according to the actual volume of the test crystal outer shell, and the
inner shells will be weighted by both volume and the exponentially
decreasing cracking frequently defined above. The total amount of He
contamination is then given by simply summing over all the shells n
(Eq. 6).
We then compare the total contamination [He]total for a given crushed crystal of size rtest to predicted He concentrations for crystals of the same volume and a given age and U concentration to show for which (U–Th) / He samples the effect might be
important, or a given age and 3He production rate to show for which
cosmogenic 3He samples the effect might be important. The same logic
can be applied to the direct effect of 3He contamination on cosmogenic
3He by simply considering the He concentrations that result from these
calculations and the dramatically lower 3He concentration (the
3He /4He of atmosphere, Ra= 1.39 × 10-6), but because of
this difference in isotopic abundance, the effect of atmospheric 4He
contamination on the magmatic 3He correction applied to cosmogenic
measurements is larger.
1Vn=43rn3-rn+132Vn,frac=Vn/∑i=0r0/aVi3[He]reln=e0.29rn-r04[He]outer=[He]TρcV0,frac[He]rel0∑i=0r0/a[He]reli5[He]n=[He]outer[He]relnVn6[He]total=∑i=ortest/a[He]i
Modeled effects of He contamination on radiogenic (U–Th) / He
apatite (a, b) and cosmogenic 3He olivine (c, d)
samples. Panels (a) and (b) show contamination as a fraction of total He (radiogenic and contamination) due to cracking in crystal fragments of a certain size, age, and U concentration. Panel (a) shows contamination as a function of crystal radius and U concentration with a fixed age of 1 Ma. Panel (b) shows contamination as a function of age and U concentration with a fixed radius of 31.5 µm (reflecting a 63 µm sieve size). Panels (c) and (d) show the fraction of total cosmogenic He represented by the correction that would be applied to a cosmogenic 3He measurement if the contamination 4He is erroneously assumed to be magmatic 4He with a ratio of 8 Ra (following Blard, 2021). The degree to which this correction is incorrect will be a function of the actual difference between the 3He /4He ratio of the contamination He and the assumed or measured magmatic He. Panel (c) shows the fraction of total 3He as a function of crystal radius and age assuming a fixed 3He production rate of 125 at g-1 yr-1. Panel (d) shows the fraction of total 3He as a function of production rate and age assuming a fixed crystal radius of 50 µm (representing a minimum recommended cosmogenic sample size of 100 µm, following Blard, 2021). The spherical model used here is adequate as an approximation for most crystals, but the model code can be modified to account for extreme geometries. Only very young or low-U radiogenic samples and very young or low-3He cosmogenic samples will exhibit a problematic degree of He contamination compared to radiogenic He ingrowth.
Model results for different crystal sizes and radiogenic He
concentrations
Assuming no loss to diffusion or ejection of alpha particles during
radioactive decay, radiogenic He grows into crystals at a rate proportional
to the concentration of 238U, 235U, 232Th, and other minor
nuclides that decay by alpha emission. The model
allows us to compare the He contamination observed here to radiogenic He
concentrations in minerals of a given U concentration (the model applies to
other alpha emitters, the concentrations of which can simply be scaled
according to alpha activity) and age or to cosmogenic He concentrations in
minerals subjected to a given cosmogenic 3He production rate and age,
and it also allows us to adjust the level of contamination according to the geometric model of
He trapping that assumes exponential decay of the degree of He contamination
with depth into the crystal. This simple model reflects just one hypothesis
for the nature of He trapping that is consistent with our observations, but
it provides an order-of-magnitude tool for determining under which
situations even the relatively simple precautions suggested in Sect. 3.4
need to be considered.
A selection of model results is presented in Fig. 3. The model results for
radiogenic samples (Fig. 3a and b) show that while He contamination
represents a very high fraction (> 20 %) of the radiogenic He
in small crystals (< 45 µm) of moderate age and very low U
concentration (1 Ma and 10 ppb) or moderate U concentration and extremely
young age (10 ka and 1 ppm), the fraction quickly becomes small (< 5 %) for samples in the range of (U–Th) / He applications (1 ppm U, 1 Ma, at least 10 µm radius) and is insignificant for typical samples (for example, < 0.2 % for a 10 ppm U, 1 Ma mineral with a radius of 63 µm). This means that while rock crushing for separation of accessory
minerals like apatite is typically performed using techniques like disk
milling that would be difficult to adapt in a way that keeps the mineral
under water, the He contamination problem is nonetheless insignificant for
typical samples used for (U–Th) / He. Only exceptional samples with very
small, young, and low U concentrations will require special care to avoid He
contamination during processing. Similarly, the model results for cosmogenic
samples (Fig. 3c and d) show that the effect of the contamination He on the
magmatic correction for 3He will be small for most samples > 10 ka with typical cosmogenic 3He production rates, so the possibility
of atmospheric contamination during sample processing must only be accounted
for in samples of exceptionally small size, age, or with unusually low
cosmogenic 3He production rates.
Conclusions
These results confirm the experimental data of Protin et al. (2016).
However, we find overwhelming evidence that the irreversible trapping of He
occurs during the crushing process rather than as a result of the grain size
of the sample. The effect will still scale with grain surface area, as it
appears that the trapping happens in the outer layer of the crushed
fragments and can be mostly removed by acid leaching. However, one must only
avoid He during crushing in order to avoid contamination of geological
samples. Naturally fine-grained samples and samples crushed in the absence
of He do not exhibit the same effect. We hypothesize that the crushing
process opens small damage zones that are quickly healed or annealed and
that can only irreversibly trap gas that is exposed to the sample before the
healing occurs. Our experiments demonstrate that this process is complete
within a few minutes. The simplest means of avoiding contamination is to
crush samples under water or other liquid, so we recommend crushing under
water for all low-He samples that must be crushed to a fine powder in the
laboratory. This solution will work for any mineral and for both isotopes
of He and for other noble gas species. Samples of the grain size commonly
used for most measurements, such as (U–Th) / He dating and cosmogenic
3He dating, and naturally fine-grained samples are not susceptible to
this problem.
Code and data availability
The code and data used in this study are included in the Supplement and as a permanent repository item (Cox, 2022).
The supplement related to this article is available online at: https://doi.org/10.5194/gchron-4-551-2022-supplement.
Author contributions
SEC, HBDM, FH, and KAF contributed to conceptualization of the study. SEC and HBDM designed and executed the experiments. KAF advised on laboratory analyses and experimental design. SEC, HBDM, FH, and KAF participated in discussion about trapping mechanism and designed adjustments to common laboratory procedures. SEC created the software model, prepared figures, and prepared the original draft. HBDM conducted model validation. SEC, HBDM, FH, and KAF contributed to the editing of the paper.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank Greg Balco and the Geochronology staff for editorial handling and Jonathan Tucker and Pierre-Henri Blard for constructive reviews.
Review statement
This paper was edited by Greg Balco and reviewed by Pierre-Henri Blard and Jonathan Tucker.
ReferencesBlard, P.-H.: Cosmogenic 3He in terrestrial rocks: A review, Chem.
Geol., 586, 120543, 10.1016/j.chemgeo.2021.120543, 2021.Blard, P.-H., Pik, R., Lavé, J., Bourlés, D., Burnard, P. G., Yokochi, R., Marty, B., and Trusdell, F.: Cosmogenic 3He production rates revisited from evidences of grain size dependent release of matrix-sited helium, Earth Planet. Sci. Lett., 247, 222–234, 10.1016/j.epsl.2006.05.012, 2006.Blard, P.-H., Balco, G., Burnard, P. G., Farley, K. A., Fenton, C. R.,
Friedrich, R., Jull, A. J. T., Niedermann, S., Pik, R., Schaefer, J. M., Scott, E. M., Shuster, D. L., Stuart, F. M., Stute, M., Tibari, B., Winckler, G., and Zimmerman, L.: An inter-laboratory comparison of cosmogenic 3He and radiogenic 4He in the CRONUS-P pyroxene standard, Quat. Geochron., 26, 11–19, 10.1016/j.quageo.2014.08.004, 2015.Boucher, C., Lan, T., Mabry, J., Bekaert, D. V., Burnard, P. G., and Marty, B.: Spatial analysis of the atmospheric helium isotopic composition: Geochemical and environmental implications, Geochim. Cosmochim. Ac., 237, 120–130, 10.1016/j.gca.2018.06.010, 2018.Cox, S. E.: Dataset for Cox et al. (2022), “Short communication: Mechanism and
prevention of irreversible trapping of atmospheric He during mineral crushing,”
Geochronology 2021-42 [Data set], Zenodo, 10.5281/zenodo.6901795, 2022.Farley, K. A. and Flowers, R. M.: (U-Th)/Ne and multidomain (U-Th) / He
systematics of a hydrothermal hematite from eastern Grand Canyon, Earth
Planet. Sci. Lett., 359–360, 131-140, 10.1016/j.epsl.2012.10.010, 2012.Farley, K. A. and McKeon, R.: Radiometric dating and temperature history of
banded iron formation–associated hematite, Gogebic iron range, Michigan,
USA, Geology, 43, 1083–1086, 10.1130/G37190.1, 2015.Farley, K. A., Maier-Reimer, E., Schlosser, P., and Broecker, W.: Constraints
on mantle 3He fluxes and deep-sea circulation from an oceanic general
circulation model, J. Geophys. Res., 100, 3829–3839, 10.1029/94JB02913, 1995.Farley, K. A., Wolf, R. A., and Silver, L. T.: The effects of long
alpha-stopping distances on (U–Th) / He ages, Geochim. Cosmochim. Ac., 60, 4223–4229, 10.1016/S0016-7037(96)00193-7, 1996.Farley, K. A., Love, S. G., and Patterson, D. B.: Atmospheric entry heating and helium retentivity of interplanetary dust particles, Geochim. Cosmochim.
Ac., 61, 2309–2316, 10.1016/S0016-7037(97)00068-9, 1997.Farley, K. A., Cerling, T. E., and Fitzgerald, P. G.: Cosmogenic 3He in
igneous and fossil tooth enamel fluorapatite, Earth Planet. Sci. Lett., 185,
7–14, 10.1016/S0012-821X(00)00360-5, 2001.House, M. A., Farley, K. A., and Stockli, D.: Helium chronometry of apatite
and titanite using Nd-YAG laser heating, Earth Planet. Sci. Lett., 183,
365–368, 10.1016/S0012-821X(00)00286-7, 2000.Kurz, M. D.: In situ production of terrestrial cosmogenic helium and some applications to geochronology, Geochim. Cosmochim. Ac., 50, 2855–2862, 10.1016/0016-7037(86)90232-2, 1986.Lal, D.: In situ produced cosmogenic isotopes in terrestrial rocks, Annu.
Rev. Earth Planet. Sci., 16, 355–388, 10.1146/annurev.ea.16.050188.002035, 1988.Lupton, J. and Evans, L.: Changes in the atmospheric helium isotope ratio
over the past 40 years, Geophys. Res. Lett., 40, 6271–6275,
10.1002/2013GL057681, 2013.Mohapatra, R. K. and Murty, S. V. S.: Search for the mantle nitrogen in the
ultramafic xenoliths from San Carlos, Arizona, Chem. Geol., 164, 305–320,
10.1016/S0009-2541(99)00148-5, 2000.
Niedermann, S. and Eugster, O.: Noble gases in lunar anorthositic rocks
60018 and 65315: Acquisition of terrestrial krypton and xenon indicating an
irreversible adsorption process, Geochim. Cosmochim. Ac., 56, 493–509, 10.1016/0016-7037(92)90147-B, 1992.Protin, M., Blard, P.-H., Marrocchi, Y., and Mathon, F.: Irreversible
Adsorption of Atmospheric Helium on Olivine: A Lobster Pot Analogy, Geochim.
Cosmochim. Ac., 179, 76–88, 10.1016/j.gca.2016.01.032, 2016.Puchol, N., Blard, P.-H., Pik, R., Tibari, B., and Lavé, J.: Variability
of magmatic and cosmogenic 3He in Ethiopian river sands of detrital
pyroxenes: Impact on denudation rate determinations, Chem. Geol., 448,
13–25, 10.1016/j.chemgeo.2016.10.033, 2017.Schlosser, P., Stute, M., Dörr, H., Sonntag, C., and Münnich, K. O.:
Tritium/3He dating of shallow groundwater, Earth Planet. Sci. Lett., 89,
353–362, 10.1016/0012-821X(88)90122-7, 1988.Shuster, D. L. and Farley, K. A.: Diffusion kinetics of proton-induced
21Ne, 3He, and 4He in quartz, Geochim. Cosmochim. Ac., 69,
2349–2359, 10.1016/j.gca.2004.11.002, 2005.Sliwinski, J. T., Kueter, N., Marxer, F., Ulmer, P., Guillong, M., and Bachmann, O.:
Controls on lithium concentration and diffusion in zircon, Chem. Geol., 501, 1–11, 10.1016/j.chemgeo.2018.09.038, 2018.Weiss, R. F.: Solubility of Helium and Neon in Water and Seawater, J. Chem.
Eng. Data, 16, 235–241, 10.1021/je60049a019, 1970.Winckler, G., Anderson, R. F., and Schlosser, P.: Equatorial Pacific
productivity and dust flux during the mid-Pleistocene climate transition,
Paleoocean. Paleoclim., 20, 2005PA001177, 10.1029/2005PA001177, 2005.Zeitler, P. K., Herczeg, A. L., McDougall, I., and Honda, M.: U-Th-He dating
of apatite: A potential thermochronometer, Geochim. Cosmochim. Ac., 51,
2865–2868, 10.1016/0016-7037(87)90164-5, 1987.