Geoscience Reference
In-Depth Information
The helium carrier
The carrier of the high
R
/
R
a signature is
unknown. An ancient gas-rich bubble in the inte-
rior of an olivine grain represents one extreme,
but plausible, carrier. Newly formed bubbles
derived from ascending MORB magmas, will have
present-day MORB isotopic signatures.
Locations with the highest
3
He/
4
He materi-
als -- Hawaii, Iceland, Galapagos -- also have
the highest variance and are associated with
basalts with MORB-like ratios. On the basis of
Pb, Sr and Nd isotopes, and LIL ratios, the high-
est
3
He/
4
He magmas have depleted MORB-like
signatures. They also exhibit large variance in
3
He/
4
He and have low
3
He abundances. The Os
isotopes suggest a peridotitic protolith, such as a
depleted restite or cumulate. This would explain
why the high
3
He/
4
He component is the C- or
'common component' and why it is low in
3
He.
Peridotites can freeze-in ancient He isotopic sig-
natures because of their low U/He ratios.
If the mantle is composed of peridotite with
blobs of recycled eclogitized oceanic or lower
continental crust (HIMU), then metasomatism or
mixing could occur during ascent of diapers or in
ponded melts. In such a mixture, concentrations
of noble gases and compatible elements (Ni, Cr,
Os and heavy rare-earth elements) would come
almost completely from the peridotitic mantle,
whereas the highly incompatible elements are
supplied by melts. Ascending magmas interact
with the surrounding material -- wall-rock reac-
tions -- exchanging heat, fluids and gases, and
causing melting and crystallization. Such pro-
cesses may explain the complexities of isotopic
and trace-element arrays.
Ta relative to MORB and has high
206
Pb/
204
Pb and
207
Pb/
204
Pb ratios. Type-islands include Tubuaii,
Mangaia and St. Helena.
EM1 has extremely low
143
Nd/
144
Nd, moder-
ately high
87
Sr/
86
Sr and very low Pb-isotope ratios.
EM1 includes either recycled oceanic crust plus a
few percent pelagic sediment or metasomatized
continental lithosphere. Examples are Pitcairn
and the Walvis Ridge.
EM2 is an intermediate Pb-isotope compo-
nent with enriched Nd- and Sr-isotope signatures.
Samoa and Taha (Societies) are examples, and
Kerguelen trends towards EM2. EM2 may be recy-
cled oceanic crust containing a few percent of
continent-derived sediment.
LOMU is distinguished from EM1 by its unusu-
ally high
87
Sr/
86
Sr and
207
Pb/
204
Pb ratios, and very
low
143
Nd/
144
Nd and
177
Hf/
176
Hf ratios, suggesting
an ancient continental source. EM1, EM2, LOMU
and HIMU and mixtures of these are referred
to as EM components; all may include recycled
materials of continental derivation.
LONU is low
238
U/
3
He (
ν
), an ancient compo-
3
He/
4
He ratios over time
nent that retains high
compared with high
components such as MORB
and other melts. This component is most likely
U- and Th-depleted peridotites or cumulates with
trapped CO
2
-rich inclusions. Gases that escape
from ascending magmas or crystallizing cumu-
lates are partially trapped at shallow depths in
LIL-poor surroundings, such as the lithosphere
and olivine-rich cumulates and the
3
He/
4
He ratio
is frozen in. Meanwhile, the U and Th in fertile
and LIL-rich mantle causes the
4
He content to
increase with time in these components. High
3
He/
4
He components do not need to represent
undegassed mantle; they can be
3
He-poor.
The end-members constrain the composi-
tional, or at least the isotopic, extremes in mag-
mas. Some isotopic data (Sr, Nd, Pb) from ocean
islands form arrays that trend toward a lim-
ited region of isotopic space that has been given
various names, including C ('common' man-
tle component), FOZO ('focus zone') and PHEM
('primary helium mantle'). The fact that there
appears to be a common component suggests
that melt percolation through a shallow buoyant
peridotite, such as depleted lithosphere, may be
involved.
ν
Mixing arrays
The chemical and isotopic properties of many
mantle magmas can be approximated by binary
mixing between two end-members [see
mantle
mixing trends HIMU EM-1 EM-2 DMM
]. How-
ever, the mantle contains many different com-
ponents. Some of the 'end-members' themselves
appear to be mixtures. Magmas are likely to be
blends of melts that represent different degrees
of partial melting and crystal fractionation from
