1

The Noble Gases as Geochemical Tracers: history and background

Pete Burnard1, Laurent Zimmermann1 and Yuji Sano2

1. Centre de Recherches Pétrographiques et Géochimiques, BP20, Vandoeuvre-lès-Nancy Cedex, France 2. Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo, Nakano, Tokyo 164–8639, Japan

Abstract

This chapter describes the discovery of the noble gases and the development of the first

instrumentation used for noble gas isotopic analysis before outlining in very general terms how

noble gases are analysed in most modern laboratories. Most modern mass spectrometers use

electron impact sources and magnetic sector mass filters with detection by faraday cups and

electron multipliers. Some of the performance characteristics typical of these instruments are

described (sensitivity, mass discrimination). Extraction of noble gases from geological samples is

for the most part achieved by phase separation, by thermal extraction (furnace) or by crushing in

vacuo. The extracted gases need to be purified and separated by a combination of chemical and

physical methods. The principles behind different approaches to calibrating the mass

spectrometers are discussed.

2

Introduction

Occupying the last column of the Periodic Table and therefore characterized by a

filled outer valence shell, the noble gases do not form chemical compounds at

conditions relevant to natural processes on Earth: while Kr and Xe do have an

extensive chemistry (Grochala 2007), this is typically only with highly

electronegative elements such as F and O, which in natural terrestrial systems are

already bound to more reactive elements than the noble gases. The noble gases

and their isotopes are able to provide unique constraints on certain geological

processes owing to their inert behavior and because numerous nuclear reactions

are recorded in the noble gas isotopic compositions. While many of these

applications are related to constraining the duration of geologic events

(thermochronology, absolute dating, surface exposure dating etc), this volume

aims to describe sampling strategies, analytical methodologies and data

interpretation for the use of noble gases as tracers in the Earth Sciences: tracers of

geological fluids, of mantle circulation, of seawater circulation, of dust falling

from space, of hydrocarbon reserves, of past climate change and of potential

future CO2 storage in crustal reservoirs.

History

The discovery of the noble gases was initially by inference rather than isolation.

Helium was first identified as an element simply from its absorption spectrum in

the solar chromosphere (by Lockyer in 1868; it would be another 27 years before

it was actually isolated as an element). Similarly, the presence of an unknown gas

of low density (which later turned out to be Ar) in the atmosphere was proposed

by Rayleigh in 1895 because nitrogen from distilled air had a different density to

chemically produced nitrogen. This observation lead to more refined distillation

experiments by Ramsey (1898) which positively identified Ar as an element and

also led to the isolation of Kr, Ne and Xe (for which Ramsey and Rayleigh would

receive Noble Prizes).

The discovery that an alpha particle produced by decay of U was in fact a helium

nucleus (by accumulating α particles in an evacuated tube by recoil through a thin

window) allowed Rutherford and Strutt to make the first radiometric age

measurements in 1905: by heating various U-bearing minerals and weighing the

3

He that was produced, they were able to constrain the age of the earth to be

greater than 400 Ma. Rutherford and Strutt were well aware of the problem of

potential He loss from minerals on geological timescales and so correctly

considered the 400 Ma estimate to be a minimum estimate; but even so, this was a

major advance as previous estimates considered that the maximum age of the

Earth was of the order 20 Ma. However, as a result of the He loss problem the

U+Th/He radiochronometer was largely ignored until the beginning of the 1990s

when a better understanding of the kinetics of He loss in certain U bearing

minerals ushered in the possibility of quantitative U+Th/He thermochronometry

(Farley 2002).

Noble gases were central to understanding the structure of the atom, and the first

isotopic separation – using the first mass spectrometer – was made by JJ Thomson

and F.W. Aston in 1913 to separate 20Ne from 22Ne. This development opened the

floodgates for research into isotopes, and ten years later multiple isotopes of all

the noble gases except for 3He had been identified (despite a hiatus forced by the

First World War), for the most part by Dempster at the University of Chicago:

Dempster's 1918 mass spectrometer (Dempster 1918) laid down the blueprint for

the magnetic field filters used in mass spectrometers to this day. Helium-3 was

only identified in 1939 by cyclotron mass spectrometry at Lawrence Berkely Nat.

Labs, CA, by Alvarez and Cornog (Alvarez and Cornog 1939). who used the 60-

inch cyclotron as a mass spectrograph to show that 3He is a stable isotopic

constituent of natural helium. The 3He2+ ions were detected using the nuclear

reaction 28Si(3He,p)30P(β)30Si and the radio-activity of 30P quantified by a Geiger

counter. The activity of pure atmospheric helium was twelve times higher than

that of tank helium derived from a natural gas well. They concluded that the 3He/4He ratio of air helium is about ten times greater than that of natural gas

helium.

The advances in mass spectrometry resulting from the Manhattan Project (1942-

1946) opened up opportunities in geo- and cosmochemistry in the post war years.

At the forefront of mass spec development during the Manhattan project, Nier,

now in Minnesota, returned to the geochronology and geochemistry he started in

the pre-war years as a grad student at Harvard. Seminal papers on K-Ar dating

(Aldrich and Nier 1948a) and on the He isotopic composition of the atmosphere

(Daunt, et al. 1947) resulted from the impressive 15cm radius, 60° mass

4

spectrometers and electron impact ion sources that Nier built at that time. These

machines were capable of separating 3He+ from HD+ and HHH+ interferences.

Nier and his co-workers discovered large variations in 3He/4He ratios of natural

materials: Aldrich and Nier presciently stated in their 1948 paper (Aldrich and

Nier 1948b) that “The present study can hardly be regarded as more than a

preliminary exploration of a new and fascinating field of investigation. It is

apparent that a far more comprehensive and systematic study will be required to

definitely establish the natural source of 3He and 4He.”

Noble gas mass spectrometers owe significantly to Nier’s legacy; indeed, many

modern mass specs still employ a Nier-type ion source (see Section Noble Gas

Mass Spectroscopy, below). Further advances in noble gas mass spectroscopy in

the 65 years or so since Nier’s inventions have been gradual rather than leaps in

performance. Two exceptions are Reynold’s (at Berkeley) development of static

(i.e. isolated from pumps) mass spectrometry in 1956 (Reynolds 1956) thereby

significantly increasing sensitivity, and the development of a new electron impact

source by Baur and Signer in the late 1970s (Baur 1980) which is characterized by

better pressure linearity.

Clarke et al. (Clarke, et al. 1969) measured the 3He/4He ratios of deep Pacific

water by using a 10-inch radius, low volume, and static vacuum mass

spectrometer, with an electron multiplier for ion detection, reducing samples sizes

by three orders of magnitude compared to Aldrich and Nier (Aldrich and Nier

1948a). Two examples of Clarke mass spectrometers remain operational to this

day (Scripps Institution of Oceanography and Woods Hole Institution of

Oceanography (e.g., Jenkins (Jenkins 1987) , Lupton (Lupton, et al. 1990)). In

1976, Takaoka and co-workers developed a noble gas mass spectrometer to

measure all noble gas isotopes, which was commercialized in 1982 as the VG

MM-3000 (Hooker, et al. 1985). At the same time Sano et al. (Sano, et al. 1982)

used a 6-inch static mass spectrometer (6-60-SGA, Nuclide Co.) for terrestrial He

isotope measurements, reporting an atmospheric 3He/4He ratio of (1.43±0.03)x10-

6, which agreed well with the critical value of (1.399±0.013)x10-6 determined by

Mamyrin et al. (Mamyrin, et al. 1970) (see below).

5

In the ex-Soviet Union, Mamyrin et al. (Mamyrin, et al. 1969)

independently developed the “Reflectron” time of flight mass spectrometer with

m/ΔM ~ 2000 and even more sensitive than the latest magnetic sector mass

spectrometers. Until the mid- 1980s, there were only seven laboratories in the

world routinely measuring terrestrial He isotopes: Ontario, St. Petersburg, Boston,

San Diego, Tokyo, Osaka and Cambridge.

In 1986, the VG-5400, a successor of the MM-3000 was introduced and

soon after the MAP 215-50 became available. These machines have since

produced the majority of terrestrial noble gas analyses in many laboratories

around the world (e.g., Sano and Wakita (Sano and Wakita 1985)). However,

further development in noble gas mass spectrometry ceased for almost twenty

years following the success of the VG5400 and MAP215 instruments.

In 2004 two brand-new multicollector noble gas mass spectrometers

(HELIX-SFT, GV instruments and Noblesse, Nu Instruments) were announced,

with the possibility of precisely measuring the atmospheric 3He/4He ratio on

samples size of 1x10-7 ccSTP, three order of magnitude smaller than the original

Clarke-type instrument. Figure 1 shows the technical progress of helium isotope

measurements with time. Taking into account only mass spectrometry

(MS1~MS6) developments, the sample size required for helium isotope analysis

as well as the precision of the analyses have improved on an approximately

logarithmic scale with time since their invention. Recently, high sensitivity

(sample sizes of 10-7 ccSTP) and high precision (estimated error of 0.1% at 1σ)

helium isotope measurements have been reported by Sano et al. (Sano, et al.

2008).

With the advent of high resolution, multicollector noble gas mass

spectrometry, the precision and sensitivity for the remaining noble gases – which

all have more than two isotopes – have also increased. It is important to note that

all noble gases have at least one (and usually more) non-abundant isotopes

requiring multicollection using secondary electron multipliers (SEM) and

6

sometimes requiring multiple SEMs. Multiple SEM machines are considerably

more complex to construct and operate than faraday-only multicollector mass

spectrometers.

One of the major issues for Ne isotope analyses are the isobaric

interferences on 20Ne+ by 40Ar+ and H218O+ and on 22Ne+ by CO2

++ (see Table 1),

requiring high resolving power mass filters in order to cleanly separate the

signals. This is now possible for 20Ne+ on both Nu and GV (now Thermofisher)

mass spectrometers, producing data with 20Ne/22Ne and 21Ne/22Ne uncertainties that

have decreased by factor 2 – 5 compared to single collector data (Colin, et al. 2011;

Marrocchi, et al. 2009; Parai, et al. 2009).

Multicollection also provides major advantages for the analysis of Kr and

Xe isotopes, as both noble gases have numerous isotopes (6 and 9 stable isotopes,

respectively) and their atoms have short half lives in the mass spectrometer due to

their high ionization cross-sections compared to the lighter noble gases. These

multicollector (>2 collector) machines have been routinely producing data for two

or three years at the time of writing (Holland and Ballentine 2006; Holland, et al.

2009); from this early work it is clear that the precision of noble gas analyses that

is now possible will open up new opportunities and directions in the isotope

geochemistry of the noble gases.

Figure 1. Technical advances in helium isotope measurements with time. Top

and bottom show the decrease in sample size required to measure He isotopes and

the improvement in analytical precision, respectively. MS1: Aldrich and Nier

(1948); MS2: Clarke et al. (1969); MS3: Mamyrin et al. (1969); MS4: Nagao et

al. (1981); MS5: Sano and Wakita (1988); MS6: Lupton (1990); MS7: Sano et al.

(2008); NAA: Coon (1949); LR: Wang et al. (2003).

Obviously, improvements in materials, vacuum techniques, multiple

collectors and ion optics have contributed to the high standards expected of

commercial mass spectrometers today, but the for the most part modern mass

spectrometers closely resemble those of Nier and Reynolds (indeed, some are still

operational with excellent data still produced by glass ‘Reynolds’ machines).

Some individual labs have made technical advances that have very specific

applications. Wang et al. (Wang, et al. 2003) reported that a sensitive laser

spectroscopic method could be been applied to the quantitative determination of 3He/4He for ratios in the range 10-7 ~ 10-5. Resonant absorption of 1083 nm laser

light photons by metastable 3He atoms in a discharge cell was measured by

frequency modulation saturation spectroscopy while the abundance of 4He was

7

measured by a direct absorption technique. Even though the results on three

different samples extracted from the atmosphere and commercial helium gas were

in good agreement with values obtained by mass spectrometry, a large amount of

sample helium, about 0.2 ccSTP was required for analysis. This is far beyond the

terrestrial helium sample size for practical use. Resonance Ionization Mass

Spectrometry has also been developed in some noble gas laboratories which

increases precision and reduces detection limits for Xe and Kr isotope

determination (Crowther, et al. 2008; Gilmour, et al. 1994; Iwata, et al. 2010;

Mamyrin, Tolstikh.In, Anufriye.Gs and Kamenski.Il 1969; Strashnov, et al. 2011;

Thonnard 1995). The turbo compressor source developed by H. Baur of ETH

increases the partial pressure of (noble) gases in the source by using a molecular

drag pump within the mass spectrometer volume, thereby increasing sensitivity

for He and Ne by a factor ~100 (Baur 1999).

Conventions

Quantities of noble gases are reported either in moles of gas or as cm3 STP

(Standard Temperature and Pressure); it should be noted that although STP is

usually defined as 273.15 K and 101.325 kPa (= 1 bar), some definitions use

100.000 kPa (McNaught and Wilkinson 1997). Use of moles should be

encouraged. As a consequence, noble gas concentrations are usually expressed as

moles (or cm3 STP) per gram of rock or fluid. Sometimes, although rarely, these

are converted to percent or ppm of the noble gas by weight, as is common for

other elements in many branches of geochemistry. However, performing the

simple conversion demonstrates how ‘rare’ the noble gases are: a typical olivine

crystal might contain ~ 10-13 moles 4He g-1, or about 0.5 ppt 4He. The abundances

of 3He or the rare isotopes of Xe are typically >106 times lower.

There is no single convention for reporting isotope ratios. Most commonly, the

actual ratio is reported; this is practical where isotopic variations are relatively

large. Where small isotopic variations are involved, the ‘delta’ notation is

commonly used where:

( )( ) 10001

tan

•⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡−=

dardsr

isample

ri

X

XX

XX

iδ

8

Where iX is the ‘anomalous’ isotope and rX is the reference isotope. For all

systems with the exception of He, it is usual to place a non-radiogenic isotope as

the denominator. Thus δ40Ar would refer to the permil variations in the 40Ar/36Ar

ratio.

Because the non-radiogenic He isotope (3He) is the least abundant isotope,

historically He isotope ratios have been reported as 3He/4He ratios (i.e. radiogenic

isotope as the denominator). Furthermore, it is common to report 3He/4He ratios

normalized to the atmospheric 3He/4He ratio, Ra. Thus a sample with a 3He/4He

ratio of 1 Ra would have real ratio of 1.39 x10-6 (see chapter 2, this volume).

However, when plotting He isotopes against other isotope ratios (which are

ubiquitously quoted with the more traditional non-radiogenic/radiogenic notation),

this often leads to unnecessarily complex, inverse relationships. In these instances,

it is preferable to use 4He/3He (not normalized to Ra).

Relative noble gas abundances are frequently expressed normalized to 36Ar and

the ratio normalized to that of air, commonly known as the F-value notation:

( ) ( )( )air

sample

AriAri

iF 36

36

//

=

Thus F(80Kr) = 2 corresponds to a 80Kr/36Ar ratio twice the ratio of air.

Noble Gas Mass spectrometry

Principles

As with all mass spectrometers, noble gas mass spectrometers consist of an ion

source, a mass filter and collector(s). The “standard” configuration which

accounts for >95% of operational mass spectrometers at the time of writing

consists of an electron impact source, a magnetic sector filter and detection using

a combination of Faraday cups and secondary electron multipliers (SEMs). The

magnetic field is measured by one or more Hall probes. Magnet control and data

collection are usually done via proprietary software; on the modern generation of

mass spectrometers (post 2002), control of source and multiplier voltages are also

performed via a software interface.

The most common “non-standard” mass spectrometer solution involves ionisation

using resonant lasers and a time of flight mass filter with detection by

microchannel plates (Gilmour, Lyon, Johnston and Turner 1994) (Lavielle, et al.

9

2006). Alternatively, atom trap or penning trap noble gas mass spectrometers also

exist (Lu and Mueller 2010), (Neidherr, et al. 2009). These configurations are

adapted to specific analytical problems and are not “off the shelf” solutions,

therefore will not be dealt with here: the reader is instead referred to the

publications above.

Most, but not all, noble gas mass spectrometers are operated in “static” mode, that

is to say all the gas available is admitted into the mass spectrometer with the

pumps isolated (Reynolds 1956) (by contrast, “dynamic” mode is where the

sample gas is slowly bled into the mass spectrometer while simultaneously

pumping the spectrometer). “Static” mode considerably increases mass

spectrometer sensitivity with the disadvantage that the signal changes over time as

the gas is gradually consumed by ionisation. Usually this is accounted for by

performing an extrapolation to the instant the gas was diluted into the mass

spectrometer (Figure 2). An additional consequence of operating in “static” mode

is that very low mass spectrometer backgrounds are required because, as soon as

the pumps are isolated, background levels in the spectrometer envelope will begin

to rise. This is countered by including one or more getters in the isolated volume

and by rigorous vacuum techniques: noble gas mass spectrometers are exposed to

air very infrequently (usually several years between ventings) and are thoroughly

baked at temperatures up to 350°C after being at atmospheric pressure.

FIGURE 2: Signal of 40Ar vs. time in a VG5400 mass spectrometer (200 µA trap current). ≈ 200

seconds are necessary to dilute the gas into the mass spectrometer and to determine field positions

for each argon isotope. The signals were extrapolated to the instant the gas was diluted into the

mass spectrometer (t=0) in order to account for consumption by ionisation of the gas during

measurement.

10

Electron bombardment sources

While magnet and collector configurations are relatively straightforward

and have much in common with non-noble gas mass spectrometers, good mass

spectroscopic analyses require a thorough understanding of how a noble gas

source functions. All commercial noble gas mass spectrometers past and present

use either modified Nier-type sources (Figure 3) or Baur-Signer sources, both of

which depend on a filament to create an electron beam which then ionize gas

molecules in an ionization chamber by electron bombardment (where an energetic

electron colliding with a neutral gas atom or molecule dislodges an electron from

the neutral species, thereby creating a positive ion). An ion repeller behind the

electron beam and a series of lenses extract the ions from the source and focus

these into a well-collimated ion beam.

FIGURE 3: Schematic of a Nier-type source. A tungsten filament operating at a current of around

2.5 A emits a beam of electrons which passes through the ionisation volume and is measured by

the trap. The electron current received by the trap, H, is usually of the order 10 – 800 μA; the

filament current is regulated in order that H is kept constant. A magnetic field (B) in the source

increases the electron path length and thus increases sensitivity. The ions are extracted by

adjusting the voltages on the repeller and drawout plates.

The ionisation chamber of Nier-type sources are placed within a magnetic

field (usually created by small external permanent magnets) which increases the

path length of the electrons in the ionisation region, thereby increasing the 11

sensitivity of the source but with the disadvantage that this induces mass

discrimination effects in the source itself. Baur-Signer sources have no magnetic

field within the source relying instead on a circular filament to provide a focussed,

conical electron beam. These sources therefore have considerably less mass

discrimination but also typically have lower sensitivity than Nier-type sources.

The efficiency of the source increases with mass (Figure 4) for a given source

setting.

FIGURE 4: Sensitivity vs. mass of gas analysed (4He, 20Ne, 36Ar, 84Kr & 130Xe). For the same

source settings, sensitivity increases with the mass of the gas analysed and depends only on the

effective ionisation cross section of the target atom (Botter and Bouchoux 1995)

The intensity of the electron beam is monitored within the source (the

“trap current”, usually between 10 and 800 μA) and the filament current

controlled so as to keep electron beam intensity constant. Intensity of the electron

beam has a strong effect on both sensitivity and mass discrimination (see section

Pressure dependent effects below) of the mass spectrometer (Figure 5), therefore

it is important to ensure the electron beam intensity does not fluctuate.

12

FIGURE 5: 40Ar signal as a function of trap current, VG5400. A constant pressure of argon was

introduced in the mass spectrometer for each measurement. For all other source settings kept

constant, the signal increases approximately linearly with the trap current.

Pressure dependent effects

The response of a noble gas ion source depends on the pressure within the source

itself (Burnard and Farley 2000) {Mabry, 2012 #1738}; as a result, the sensitivity

of the mass spectrometer is not linear with pressure, and mass discrimination also

depends on the quantity of gas being analyzed (i.e. the pressure in the source).

These effects are considerably less important for Bauer-Signer type sources than

for modified Nier-type sources (see Figures 6 and 7). As a result, it is important

that pure noble gases are admitted into the mass spectrometer preferably one

noble gas at a time, and that samples and standards are similar in purity, pressure

and composition (see section Purification). It is thought that these pressure

dependent effects result from the distribution of space charges within the

ionization volume (Burnard and Farley 2000), (Baur 1980). Space charges result

from passing an electron beam through the insulated ionization volume. At small

ion currents (low source pressures), the ion chamber is essentially well isolated

and large space charge potentials can exist. However, increasing the density of

ions in the source (high source pressures), these space charges can be redistributed

by the ions themselves. Hence the pressure-dependent effects are more significant

at high electron beams (high trap currents; Figure 6). Pressure dependent effects

are most significant for He and can be > 5% for typical sample sizes for Nier-type

13

14

sources (Sano, Tokutake and Takahata 2008): mass discrimination will be greater

for He than for the remaining noble gases due to the large mass difference

between the two He isotopes, and, in addition, He requires higher electron beam

currents (higher trap currents) than the heavier noble gases in order to compensate

for the lower He ionization efficiency (Figure 4), and increasing trap current is a

convenient way of increasing He sensitivity.

FIGURE 6: Non-linearity of a Nier-type source. a) He isotope discrimination and He sensitivity as

a function of pressure (≡4He mol) for constant source settings (trap current = 400 μA). b) 40Ar/36Ar

discrimination as a function of trap current and pressure (≡40Ar signal). Coulié (Coulie 2001)

similarly observes an increase in mass discrimination for Ar, up to 8%, as a function of the trap

current.

15

FIGURE 7: Non-linearity of Ar sensitivity of a Baur-Signer source (from (Baur 1980)). Measured

(shaded areas) and simulated (data points) relative Ar-sensitivity of a GSVII (GS98) Baur-Signer

ion source versus equivalent Ar pressure (equivalent Ar pressures were obtained by measuring the

ion current of 40Ar during stepwise admission of known amounts of He to the static mass

spectrometer; the measured 4He ion current was multiplied by √(4He/40Ar) to get an equivalent 40Ar ion current and a corresponding equivalent Ar pressure that would cause the same space

charge). The measurements were performed with two different α-slit settings, 0.063 rad for the

upper curve and circular symbols, 0.038 rad for the lower curve and square symbols. The

maximum sensitivities (=100%) for the two slits are 1.1 and 1.0 mA torr-1 respectively. The

measurements were made at 2.5 x10-4 A total filament emission and 100eV electron energy.

Isobaric interferences

Despite thorough baking and the presence of getters in the mass spectrometer,

there are frequently background atoms or molecules in the region of the noble gas

peaks. Common interferences that are known in noble gas mass spectrometers are

given in Table 1 (an exhaustive list is not possible, particularly at high masses

where many different molecules may exist).

Species Mass/Charge Noble gas

Resolution required1

comment

1H2D+ 3.0215 3He 509 Present in all mass spectrometers

3H+ 3.0161 3He 150 000 1H1H1H+ 3.0238 3He 384 Present in all mass

16

17

spectrometers 12C+++ 4.0000 4He 1538 1H2

18O+ 20.015 20Ne 894 1H19F+ 20.006 20Ne 1406 40Ar++ 19.981 20Ne 1778 Liquid N2 trap during

Ne analysis 20NeH+ 21.000 21Ne 3276 12C3

1H6++ 21.023 21Ne 709

63Cu+++ 20.976 21Ne 1202

CO2++ 21.995 22Ne 6232 Liquid N2 + getters

during Ne analysis 66Zn+++ 21.975 22Ne 1405 12C3

+ 36.000 36Ar 1109 1H35Cl+ 35.977 36Ar 3947 1H37Cl+ 37.974 38Ar 3458 12C3

1H2+ 38.016 38Ar 699

12C31H4

+ 40.031 40Ar 580 12C6

1H6-12+ 78 - 86 Kr

isotopes ~600

12C101H4-16

+ 124 - 136 Xe isotopes

~550 Common pump oil or grease fragments

Table 1: common interferences found in the region of the noble gases. 1 Mass resolution is

expressed as mass/peak width; the quoted resolutions are the resolutions necessary to separate the

interference from the neighboring noble gas isotope.

Extraction and purification

In general, the noble gases are present in low to very low concentrations in

geological materials (fluids and rocks); in certain circumstances, radiogenic noble

gases such as 4He or 40Ar may comprise up to a few percent of a fluid or mineral

(e.g. air contains nearly 1% 40Ar by volume) but more usually these are present at

ppm concentrations or lower. The less abundant noble gases may be present at

only a few thousand atoms per gram. There are two obvious consequences of this

observation: 1. relatively large bulk samples are required compared to many other

geochemical analyses and 2. it is necessary to extract and separate the noble gases

from their host phase(s) for analysis. Extraction of noble gases depends on the

nature of the major phase (fluid, solid) and on the scientific objectives (bulk or

microsampling). Analysis by mass spectrometry typically requires samples of a

18

single, pure noble gas, therefore purification and separation of the noble gases is

required following extraction. Most modern extraction-purification systems are

made of steel as most types of glass are permeable to He.

Extraction: Fluid samples

The noble gases are in general poorly soluble in both aqueous and hydrocarbon

liquids, hence, under vacuum, they will preferentially partition into the head space

above a liquid. Extraction of noble gases from liquids makes use of this property:

the sample is equilibrated with a volume under static vacuum (i.e. a volume

isolated from all pumps), and the noble gases (along with any other insoluble

gases such as CH4 or CO2) exsolve from the liquid into the headspace (Figure 8).

As the equilibration time may be long (>30 minutes for the heavy noble gases),

samples may be heated and/or stirred to accelerate degassing. Further details on

preparation of petroleum samples are available in (Pinti and Marty 1995) and

Chapter 9 of this volume, and of water samples in (Beyerle, et al. 2000) and

Chapters 5 and 6 of this volume.

Fluid samples are commonly collected in copper pinch-off tubes where a length of

annealed, refrigeration grade Cu tubing is placed between two clamps (Figure 9).

The tube is connected to the fluid source (gas, water or petroleum) and once

satisfactorily filled with the fluid (i.e. no air remains in the tube, sometimes

requiring extensive flushing of the Cu tube), the clamps are tightened creating a

cold-weld seal trapping the fluid between the clamps. In order to extract the fluid

from the sample, the end of the Cu tube is connected to the vacuum system

(usually via the expansion chamber), the clamp closest to the extraction system is

removed and the tube gently opened using pliers or a small vice.

FIGURE 8: Schematic diagram of an expansion device for extracting noble gases from liquid

samples. The tube is connected to the expansion vessel via a viton gasket; once a good vacuum has

been obtained, the lower clamp is opened, transferring the water from the tube into the vacuum

vessel. Being insoluble, the noble gases preferentially partition into the headspace above the

liquid, ready for purification and analysis.

FIGURE 9: A copper pinch-off tube in place for analysis.

19

20

Extraction: solid samples

Noble gases may be sited either within a mineral’s lattice (for example, in situ

production of the radiogenic noble gases or dissolved components) or within a

liquid or a melt inclusion trapped in the mineral in question. Solutions exist which

favour preferential extraction of noble gases from either site. Heating the sample

using a furnace or a long-wavelength laser (CO2 or Nd-YAG) will release gases

from the mineral lattice either by diffusion or by melting the sample, but this will

also release any inclusion-hosted gases within the sample. Fluids can be released

from fluid inclusions by crushing the sample under vacuum; some targeted

applications also use short wavelength lasers to open vesicles or fluid inclusions

(Burnard, et al. 1998; Raquin, et al. 2008), although the quantities of the less

abundant noble gases in individual inclusions are generally insufficient for

analysis.

Most furnaces for noble gas extraction are double vacuum systems, where the

sample is placed in an interior tube which is heated by an external resistance

element, itself located in a second, evacuated tube (otherwise the resistance

element would be rapidly consumed by atmospheric oxygen) (Figure 10).

Temperatures of 2150 °C have been reported using this type of furnace (Turner, et

al. 1990), although more commonly maximum temperatures are of the order 1600

- 1800°C.

Figure 10: Schematic diagram of a classical double-vacuum high temperature furnace

It is also possible to place the resistance element within the same vacuum as the

sample (i.e. no double vacuum system) with the advantage that less refractory

metal is required (reducing fabrication and running costs). However, having the

heating element in the same volume as the sample considerably degrades vacuum

quality, requiring more rigorous purification of the noble gases after extraction.

21

22

Induction furnaces can also be used to heat samples for noble gas analyses; a

radiofrequency alternating current in a solenoid coil external to the vacuum

system induces a current in a metal crucible (usually of Ta or Mo) within the

vacuum chamber, thereby heating the crucible to temperatures up to 1800 °C.

While induction furnaces are efficient and reliable, the vacuum envelope cannot

be made of a conducting material: usually this is made of glass which, in turn,

results in high He blanks.

Although largely used for U+Th/He thermochronology, diode (808 nm) or Nd-

YAG (1064 nm) lasers can be used instead of resistance elements in order to heat

small (10’s of mg) samples. In these systems, the sample is placed within a laser

cell with a suitable window that transmits the laser wavelength and yet withstands

the UHV conditions required for noble gas analyses (sapphire is commonly used)

(Foeken, et al. 2006; House, et al. 2000). Addition of a flux which couples well

with the laser wavelength may permit larger sample volumes to be melted (e.g.

upto 150mg quartz according to Foeken (Foeken, Stuart, Dobson, Persano and

Vilbert 2006)).

Crusher devices aim to reduce the sample grainsize under vacuum thus rupturing

vesicle and (fluid) inclusions in the samples (Scarsi 2000). It is important

therefore to start with grains that are considerably larger than the

inclusions/vesicles in the sample. All crushers are “home-made” and as a result

there are many different designs. Two broad types of crusher can be identified,

those that crush by repeated “hammering” under vacuum, and those that crush the

sample principally by application of pressure to the sample. The former (Figure

11) typically place a soft-iron piston in a vertical tube above the sample and then

use external electromagnets to raise and drop the piston on the sample. Repeated

(up to several hundred) strokes can be used to completely pulverise the sample.

Pressure-type crushers have a mortar connected via a bellows to an external

device which applies pressure to the samples (Burnard, et al. 2003). The pressure

is created either by a hydraulic press or by screw threads (common examples use a

modified valve assembly to transfer the movement onto the sample (Stuart, et al.

1994)).

Figure 11: Schematic view of a Piston type crusher from (Scarsi 2000). The sample is placed in the

holder at the top of the tube. When ready (i.e. when blanks are acceptable), the holder is rotated

and the sample falls to the base of the tube (slots cut in the piston or “sledge” permit the sample to

pass to the bottom of the tube) and the piston actuated by means of the external electromagnet

induced in the coils around the tube.

Purification

It is necessary to purify the extracted gases in order to admit pure (or as pure as

possible) noble gases into the mass spectrometer. Purification usually takes

advantage of the non-reactivity of the noble gases, chemically removing active

species from the extracted gases. “Getters” are active metal surfaces which

combine with the reactive gases either by adsorption or more commonly by

chemi-sorption. Common metals are Ti or Zr based alloys (Ag is also known),

either in the form of a sponge or deposited onto a high surface area substrate.

Usually the getter is “activated” at high temperature (up to 900 °C), which creates

the active metal surface. “Flash getters” evaporate a layer of the metal onto the

interior of the vessel. Getters are usually more efficient for most gases when held

at moderate temperature (e.g. c. 500 °C for Ti sponge) but they will also consume

active gases at room temperature. The precise purification cycles vary

considerably from laboratory to laboratory and/or depending on the nature of the

sample (e.g. the quantity of active gases to be removed) but will commonly use

both hot and cold getters. Although in theory the noble gases are not affected by

getters, there is some evidence that in fact Xe can be irreversibly adsorbed on

some getter materials (D Pinti pers comm.). 23

24

Cryoseparation

In order to separate mixtures of (purified) noble gases released from the sample

into single noble gas aliquots suitable for analysis, the noble gas mixture is

commonly reversibly adsorbed onto a substrate at low temperature (Reynolds, et

al. 1978). The substrate is usually activated charcoal or sintered metal; plain metal

stainless tube can also be used (Lott 2001). Raising the temperature will

sequentially release the noble gases in order of mass; the exact temperatures

depend on the installation and substrate but common values (temperatures in

Kelvin) for activated charcoal are He: 20; Ne: 50; Ar: 140; Kr 180; Xe 280. For

condensing Ar, Kr and Xe on activated charcoal, the trap can be cooled with an

external liquid nitrogen bath (T= 77 K) but in order to condense He and Ne

temperatures lower than liquid nitrogen are required (c. 15K for He and c. 40 K

for Ne (Reynolds, Jeffery, McCrory and Varga 1978)) a refrigerator that cools to

below c. 20 K is required. The most common solutions use a Gifford McMahon

cooler where a compressed He expansion cycle cools a cryogenic trap containing

activated charcoal or similar adsorbent. Numerous commercial propositions exist.

Calibrating noble gas mass spectrometers

The response of the mass spectrometer has to be calibrated against standards of

known isotopic composition and/or known quantity of noble gas. While some

mineral standards are beginning to appear, most calibrations are performed

relative to a gas standard where a bottle contains a pre-determined pressure of gas

of known isotopic composition. An aliquot of this gas (trapped between two

valves) is expanded into the mass spectrometer (after purification and/or

cryogenic separation as necessary). For Ne, Ar, Kr and Xe, the gas used is almost

always air (at reduced pressure); the composition of the atmosphere is assumed to

be known and constant (see Chapter 2 of this voloume). Some laboratories also

use air for He calibration; however, given the low abundance of He in the

atmosphere, relatively large volumes of air are required with attendant difficulties

purifying the He. Various laboratories have fabricated their own He standards for

example by extracting the He from He-rich rocks (MORB glasses), by purifying

He-rich volcanic gases (most commonly from “Murdering Mudpots”,

Yellowstone National Park, USA, or Reunion Island) or by mixing known

quantities of 3He and 4He. The best known of the latter is the HeSJ standard

25

(Matsuda, et al. 2002) which has been distributed to several noble gas labs

worldwide. However, it should be noted that all the “home-made” He standards

are nevertheless ultimately calibrated against atmospheric He which is also

assumed to have a constant concentration and composition.

26

References

Aldrich LT, Nier AO (1948a) Argon 40 in potassium minerals. Physical Review 74(8):876-877

Aldrich LT, Nier AO ( 1948b) The occurence of He3 in natural sources of helium. Physical

Review 74( 11): 1590-1594

Alvarez LW, Cornog R (1939) He3 in helium[4]. Physical Review 56(4):379

Baur H (1980) Numerische simulation und praktische erprobung einer rotationssymmetrischen

ionenquelle für gasmassenspektrometer. No. 6596. In, vol. ETH, Zürich

Baur H (1999) A noble-gas mass spectrometer compressor source with two orders of magnitude

improvement in sensitivity. EOS 80:F1118

Beyerle U, Aeschbach-Hertig W, Imboden DM, Baur H, Graf T, Kipfer R (2000) A mass,

spectrometric system for the analysis of noble gases and tritium from water samples.

Environmental Science and Technology 34(10):2042-2050

Botter R, Bouchoux G (1995) Spectrométrie de masse. Techniques de l'ingenieur, Analyse et

caracterisation 4:2615

Burnard PG, Farley KA (2000) Calibration of pressure-dependent sensitivity and discrimination in

Nier-type noble gas ion sources. Geochemistry, Geophysics, Geosystems vol.1:paper number

2000GC000038

Burnard PG, Farley KA, Turner G (1998) Multiple fluid pulses in a Samoan harzburgite. Chemical

Geology 148:99 - 114

Burnard PG, Harrison DW, Turner G, Nesbitt R (2003) Degassing and contamination of noble

gases in Mid-Atlantic Ridge basalts. Geochem. Geophys. Geosysts.

4(1):doi:10.1029/2002GC000326

Clarke WB, Beg MA, Craig H (1969) Excess 3He in the sea: evidence for terrestrial primordial

He. E.P.S.L. 6:213 - 220

Colin A, Burnard P, Graham DW, Marrocchi Y (2011) Plume-ridge interaction along the

Galapagos Spreading Center: discerning between gas loss and source effects using neon isotopic

compositions and 4He-40Ar-CO2 relative abundances. G.C.A. doi:

10.1016/j.gca.2010.11.018(75):1145-1160

Colin A, Burnard P, Marty B (2009) A study of degassing at mid-oceanic ridges by bubble-by-

bubble analyses. G.C.A. 73:A237

Coulie E (2001) Chronologie Ar/Ar et K/Ar de la dislocation du plateau éthiopien et de la

déchirure continentale dans la corne d'afrique depuis 30 Ma. In, vol. Université de Paris XI, Orsay

Crowther SA, Mohapatra RK, Turner G, Blagburn DJ, Kehm K, Gilmour JD (2008)

Characteristics and applications of RELAX, an ultrasensitive resonance ionization mass

spectrometer for xenon. Journal of Analytical Atomic Spectrometry 23(7):938-947

Daunt JG, Probst RE, Johnston HL, Aldrich LT, Nier AO (1947) A new method of separation of

the isotopes He3 and He4. Physical Review 72(6):502-503

Dempster AJ (1918) A new method of positive ray analysis. Physical Review 11(4):316-325

27

Farley KA (2002) (U-Th)/He dating: Techniques, calibrations, and applications. In: Reviews in

Mineralogy and Geochemistry, vol 47.

Foeken JPT, Stuart FM, Dobson KJ, Persano C, Vilbert D (2006) A diode laser system for heating

minerals for (U-Th)/He chronometry. Geochemistry, Geophysics, Geosystems 7(4)

Gilmour JD, Lyon IC, Johnston WA, Turner G (1994) RELAX: An ultrasensitive, resonance

ionization mass spectrometer for xenon. Review of Scientific Instruments 65(3):617-625

Grochala W (2007) Atypical compounds of gases, which have been called 'noble'. Chemical

Society Reviews 36(10):1632-1655

Holland G, Ballentine CJ (2006) Seawater subduction controls the heavy noble gas composition of

the mantle. Nature 441(7090):186-191

Holland G, Cassidy M, Ballentine CJ (2009) Meteorite Kr in earth's mantle suggests a late

accretionary source for the atmosphere. Science 326(5959):1522-1525

Hooker PJ, Bertrami R, Lombardi S, O'Nions RK, Oxburgh ER (1985) Helium-3 anomalies and

crust-mantle interaction in Italy. Geochimica et Cosmochimica Acta 49(12):2505-2513

House MA, Farley KA, Stockli D (2000) Helium chronometry of apatite and titanite using Nd-

YAG laser heating. E.P.S.L. 183(3-4):365-368

Iwata Y, Ito C, Harano H, Aoyama T (2010) Improvement of the resonance ionization mass

spectrometer performance for precise isotope analysis of krypton and xenon at the ppt level in

argon. International Journal of Mass Spectrometry 296(1-3):15-20

Jenkins WJ (1987) H-3 and HE-3 in the Beta Triangle - observations of gyre ventilation and

Oxygen Utilization rates. Journal of Physical Oceanography 17(6):763-783

Lavielle B, Gilabert E, Thomas B, Lavastre V (2006) New RIS-TOF facility for measuring very

small Kr and Xe gas samples. G.C.A. 70(18):A344-A344

Lott DE (2001) Improvements in noble gas separation methodology: a nude cryogenic trap.

Geochemistry, Geophysics, Geosystems 2:2001GC000202

Lu ZT, Mueller P (2010) Atom Trap Trace Analysis of Rare Noble Gas Isotopes. In: Advances in

Atomic, Molecular and Optical Physics, vol 58. pp 173-205

Lupton JE, Johnson HPe, Embley RWe (1990) Water column hydrothermal plumes on the Juan de

Fuca Ridge

Special section; Axial Seamount; an active ridge axis volcano on the central Juan de Fuca Ridge.

Journal of Geophysical Research, B, Solid Earth and Planets 95(8):12,829-812,842

Mamyrin BA, Anufriyev GS, Kamenskiy IL, Tolstikhin IN (1970) Determination of the isotopic

composition of atmospheric helium. Geochemistry International 7: 498-505

Mamyrin BA, Tolstikh.In, Anufriye.Gs, Kamenski.Il (1969) ISOTOPIC ANALYSIS OF

TERRESTRIAL HELIUM ON A MAGNETIC RESONANCE MASS SPECTROMETER.

Geochemistry International Ussr 6(3):517-&

Marrocchi Y, Burnard PG, Hamilton D, Colin A, Pujol M, Zimmermann L, Marty B (2009) Neon

isotopic measurements using high-resolution, multicollector noble gas mass spectrometer: HELIX-

MC. Geochem. Geophys. Geosysts. 10:art. no. Q04015

28

Matsuda J, Matsumoto T, Sumino H, Nagao K, Yamamoto J, Miura Y, Kaneoka I, Takahata N,

Sano Y (2002) The 3He/4He ratio of new internal He Standard of Japan (HESJ). Geochemical

Journal 36(2):191-195

McNaught AD, Wilkinson A (1997) IUPAC Compendium of Chemical Terminology (2nd ed),

vol. Blackwell Science, Cambridge

Neidherr D, Cakirli RB, Audi G, Beck D, Blaum K, Böhm C, Breitenfeldt M, Casten RF, George

S, Herfurth F, Herlert A, Kellerbauer A, Kowalska M, Lunney D, Minaya-Ramirez E, Naimi S,

Rosenbusch M, Schwarz S, Schweikhard L (2009) High-precision Penning-trap mass

measurements of heavy xenon isotopes for nuclear structure studies. Physical Review C - Nuclear

Physics 80(4)

Parai R, Mukhopadhyay S, Lassiter JC (2009) New constraints on the HIMU mantle from neon

and helium isotopic compositions of basalts from the Cook-Austral Islands. E.P.S.L. 277(1-2):253-

261

Pinti DL, Marty B (1995) Noble gases in crude oils from the Paris Basin, France: Implications for

the origin of fluids and constraints on oil-water-gas interactions. Geochimica et Cosmochimica

Acta 59(16):3389-3404

Raquin A, Moreira MA, Guillon F (2008) He, Ne and Ar systematics in single vesicles: Mantle

isotopic ratios and origin of the air component in basaltic glasses. E.P.S.L. 274(1-2):142-150

Reynolds JH (1956) High sensitivity mass spectrometer for noble gas analysis. Review of

Scientific Instruments 27(11):928-934

Reynolds JH, Jeffery PM, McCrory GA, Varga PM (1978) Improved charcoal trap for rare gas

mass spectrometry. Review of Scientific Instruments 49(4):547-548

Sano Y, Tokutake T, Takahata N (2008) Accurate measurement of atmospheric helium isotopes.

Analytical Sciences 24(4):521-525

Sano Y, Tominaga T, Nakamura Y, Wakita H (1982) HE-3/HE-4 RATIOS OF METHANE-RICH

NATURAL GASES IN JAPAN. Geochemical Journal 16(5):237-245

Sano Y, Wakita H (1985) Geographical distribution of (super 3) He/ (super 4) He ratios in Japan;

implications for arc tectonics and incipient magmatism. JGR. Journal of Geophysical Research. B

90(10):8729-8741

Scarsi P (2000) Fractional extraction of helium by crushing of olivine and clinopyroxene

phenocrysts: Effects on the 3He/4He measured ratio. Geochimica et Cosmochimica Acta

64(21):3751-3762

Strashnov I, Blagburn DJ, Gilmour JD (2011) A resonance ionization time of flight mass

spectrometer with a cryogenic sample concentrator for isotopic analysis of krypton from

extraterrestrial samples. Journal of Analytical Atomic Spectrometry 26(9):1763-1772

Stuart FM, Turner G, Taylor R (1994) He-Ar isotope systematics of fluid inclusions: resolving

mantle and crustal contributions to hydrothermal fluids. In: Matsuda J (ed) Noble Gas

Geochemistry and Cosmochemistry, vol. Terra Scientific, Tokyo, pp 261-277

Thonnard N (1995) RESONANCE IONIZATION OF HEAVY NOBLE-GASES - THE

POTENTIAL OF KRYPTON AND XENON MEASUREMENTS FROM SINGLE PRESOLAR

GRAINS. Meteoritics 30(5):588-588

29

Turner G, Burgess R, Bannon M (1990) Volatile-rich mantle fluids inferred from inclusions in

diamond and mantle xenoliths. Nature 344:653-655

Wang LB, Mueller P, Holt RJ, Lu ZT, O'Connor TP, Sano Y, Sturchio NC (2003) Laser

spectroscopic measurement of helium isotope ratios. Geophysical Research Letters 30(11)