VOLCANIC HELIUM
J. E. Lupton, Hatfield Marine Science Center,
Newport, OR, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Volcanic activity along the global mid-ocean ridge
system and at active seamounts introduces a heliumrich signal into the ocean basins that can be used to
trace patterns of ocean circulation and mixing. Helium is extracted from oceanic volcanic rocks by
circulating sea water and then injected into the ocean
as helium dissolved in submarine hydrothermal vent
fluids. Hydrothermal venting produces plumes in the
ocean that are highly enriched in a variety of tracers,
including heat, helium, manganese, iron, methane,
and hydrogen. Among these, volcanic helium is a
particularly useful tracer because it has such a high
concentration in hydrothermal fluids relative to the
background values ofhelium in sea water, and because it is stable and conservative, i.e., helium does
not decay radioactively and is not affected by any
chemical or biological processes. By making careful
measurements of the relative abundance of heliumisotopes, it is possible to trace hydrothermal helium
plumes for thousands of kilometers from the source
regions.
There are two stable isotopes of helium,
3
He and
4 He, which vary in their ratio by over three orders of
magnitude in terrestrial samples. The Earth’s atmosphere is well mixed with respect to helium and
contains helium with a uniform isotopic composition
of
3
He/
4
He ¼ 1.39 Â 10
À6
. Atmospheric helium is a
convenient standard for helium isotope determinations, and terrestrial
3 He/
4
He ratios are usually
normalized to the air ratio and expressed in units of
R=R A , where R ¼
3
He/
4 He and R A ¼ ð
3 He
=4 HeÞ air .
In contrast to atmospheric helium (R=R A ¼ 1), the
radiogenic helium produced by a-decay of U and Th
series isotopes has a much lower ratioof R=R A 0:1,
while the volcanic helium that is derived from
the Earth’s mantle is highly enriched in
3
He
(R=R A ¼ 5230). Thus volcanic helium has an isotopic composition distinct from other sources such as
atmospheric helium or the helium produced by
radioactive decay. This
3
He-rich mantle helium is
sometimes called ‘primordial’ helium, since it is
thought to be the remnant of a primitive component
trapped in the Earth’s interior since the time of its
formation. This trapped component probably had
3 He/
4
He ¼ 1 Â 10
À4
or 100 R A , similar to the helium
found trapped in meteorites or in the solar wind, but
has been modified to R ¼ 30R A by dilution with
radiogenic helium since the time the Earth was
formed. Although there is a wide variety of volcanic
sources in the oceans, including subduction zone
volcanoes and hot spot volcanoes, most of the
oceanic volcanic helium is derived from activity
along the global mid-oceanridge system. While the
3 He/
4
He ratio of mantle helium shows a wide range
of variation, the helium from mid-ocean ridgesfalls
in a much narrower range of R=R A ¼ 729.
In order of decreasing importance, the most
abundant forms of helium in sea water are dissolved
atmospheric helium, volcanic helium, and to a lesser
degree radiogenic helium from sediments. Thereis
also an input of pure
3 He into the oceans from tritium(
3 H), the radioactive isotope of hydrogen, which
decays to
3 He with a half-life of 12.4 years. Because
tritium isgenerally found only in the upper ocean,
3 He from tritium decay(tritiogenic helium) is only
significant at depths less than about 1000 m.
Although there are only two isotopes of helium, it
is still possible to clearly distinguish submarine volcanic helium from the other components because of
its high
3 He/
4
He ratio and because volcanic helium is
introduced at mid-depth rather thanat the ocean
surface or on the abyssal plain.
Units
For samples highly enriched in helium such as volcanic rocks and hydrothermal vent fluids, the helium
isotope ratio is usually expressed in the R=R A notation described above. However, for the relatively
small variations observed in sea water samples, the
3 He/
4
He variations are usually expressed as dð
3 HeÞ,
which is the percentage deviation from the ratio in
air, defined as in eqn [1].
dð
3 HeÞ ¼ 100½ðR=R A Þ À 1
½ 1
Here again R ¼
3 He=
4 He and R A ¼ ð
3 He
=4 HeÞ air .
Thus R=R A ¼ 1:50 is equivalent to dð
3 HeÞ ¼ 50%.
History and Background
The first attempt to detect nonatmospheric helium in
the oceans was made by Suess and Wa ¨ nke in 1965,
who predicted that the deep oceans should contain
147
J. E. Lupton, Hatfield Marine Science Center,
Newport, OR, USA
Copyright & 2001 Elsevier Ltd.
Introduction
Volcanic activity along the global mid-ocean ridge
system and at active seamounts introduces a heliumrich signal into the ocean basins that can be used to
trace patterns of ocean circulation and mixing. Helium is extracted from oceanic volcanic rocks by
circulating sea water and then injected into the ocean
as helium dissolved in submarine hydrothermal vent
fluids. Hydrothermal venting produces plumes in the
ocean that are highly enriched in a variety of tracers,
including heat, helium, manganese, iron, methane,
and hydrogen. Among these, volcanic helium is a
particularly useful tracer because it has such a high
concentration in hydrothermal fluids relative to the
background values ofhelium in sea water, and because it is stable and conservative, i.e., helium does
not decay radioactively and is not affected by any
chemical or biological processes. By making careful
measurements of the relative abundance of heliumisotopes, it is possible to trace hydrothermal helium
plumes for thousands of kilometers from the source
regions.
There are two stable isotopes of helium,
3
He and
4 He, which vary in their ratio by over three orders of
magnitude in terrestrial samples. The Earth’s atmosphere is well mixed with respect to helium and
contains helium with a uniform isotopic composition
of
3
He/
4
He ¼ 1.39 Â 10
À6
. Atmospheric helium is a
convenient standard for helium isotope determinations, and terrestrial
3 He/
4
He ratios are usually
normalized to the air ratio and expressed in units of
R=R A , where R ¼
3
He/
4 He and R A ¼ ð
3 He
=4 HeÞ air .
In contrast to atmospheric helium (R=R A ¼ 1), the
radiogenic helium produced by a-decay of U and Th
series isotopes has a much lower ratioof R=R A 0:1,
while the volcanic helium that is derived from
the Earth’s mantle is highly enriched in
3
He
(R=R A ¼ 5230). Thus volcanic helium has an isotopic composition distinct from other sources such as
atmospheric helium or the helium produced by
radioactive decay. This
3
He-rich mantle helium is
sometimes called ‘primordial’ helium, since it is
thought to be the remnant of a primitive component
trapped in the Earth’s interior since the time of its
formation. This trapped component probably had
3 He/
4
He ¼ 1 Â 10
À4
or 100 R A , similar to the helium
found trapped in meteorites or in the solar wind, but
has been modified to R ¼ 30R A by dilution with
radiogenic helium since the time the Earth was
formed. Although there is a wide variety of volcanic
sources in the oceans, including subduction zone
volcanoes and hot spot volcanoes, most of the
oceanic volcanic helium is derived from activity
along the global mid-oceanridge system. While the
3 He/
4
He ratio of mantle helium shows a wide range
of variation, the helium from mid-ocean ridgesfalls
in a much narrower range of R=R A ¼ 729.
In order of decreasing importance, the most
abundant forms of helium in sea water are dissolved
atmospheric helium, volcanic helium, and to a lesser
degree radiogenic helium from sediments. Thereis
also an input of pure
3 He into the oceans from tritium(
3 H), the radioactive isotope of hydrogen, which
decays to
3 He with a half-life of 12.4 years. Because
tritium isgenerally found only in the upper ocean,
3 He from tritium decay(tritiogenic helium) is only
significant at depths less than about 1000 m.
Although there are only two isotopes of helium, it
is still possible to clearly distinguish submarine volcanic helium from the other components because of
its high
3 He/
4
He ratio and because volcanic helium is
introduced at mid-depth rather thanat the ocean
surface or on the abyssal plain.
Units
For samples highly enriched in helium such as volcanic rocks and hydrothermal vent fluids, the helium
isotope ratio is usually expressed in the R=R A notation described above. However, for the relatively
small variations observed in sea water samples, the
3 He/
4
He variations are usually expressed as dð
3 HeÞ,
which is the percentage deviation from the ratio in
air, defined as in eqn [1].
dð
3 HeÞ ¼ 100½ðR=R A Þ À 1
½ 1
Here again R ¼
3 He=
4 He and R A ¼ ð
3 He
=4 HeÞ air .
Thus R=R A ¼ 1:50 is equivalent to dð
3 HeÞ ¼ 50%.
History and Background
The first attempt to detect nonatmospheric helium in
the oceans was made by Suess and Wa ¨ nke in 1965,
who predicted that the deep oceans should contain
147