GEOCHEMISTRY
Primordial and recycled helium
isotope signatures in the mantle
transition zone
S. Timmerman^1 *†, M. Honda^1 , A. D. Burnham^1 , Y. Amelin^1 , S. Woodland^2 ,
D. G. Pearson^2 , A. L. Jaques^1 , C. Le Losq^1 , V. C. Bennett^1 , G. P. Bulanova^3 , C. B. Smith^3 ,
J. W. Harris^4 , E. Tohver^5
Isotope compositions of basalts provide information about the chemical reservoirs in
Earth’s interior and play a critical role in defining models of Earth’s structure. However,
the helium isotope signature of the mantle below depths of a few hundred kilometers
has been difficult to measure directly. This information is a vital baseline for understanding
helium isotopes in erupted basalts. We measured He-Sr-Pb isotope ratios in superdeep
diamond fluid inclusions from the transition zone (depth of 410 to 660 kilometers)
unaffected by degassing and shallow crustal contamination. We found extreme He-C-Pb-Sr
isotope variability, with high^3 He/^4 He ratios related to higher helium concentrations.
This indicates that a less degassed, high-^3 He/^4 He deep mantle source infiltrates the
transition zone, where it interacts with recycled material, creating the diverse
compositions recorded in ocean island basalts.
W
ide ranges of Sr-Nd-Hf-Pb isotopic com-
positions for ocean island basalts (OIBs)
and continental plume–related basalts
indicate the existence of different geo-
chemical reservoirs in the mantle ( 1 , 2 ).
This is also evident from their widely variable
helium (He) isotope composition of 4.3 to 50
( 1 , 3 , 4 ), with^3 He/^4 He ratios expressed as R/Ra,
the ratio of^3 He/^4 Hesampleto the^3 He/^4 Heairstan-
dard (1.4 × 10−^6 ). The chemical evolution, nature,
and scale of these different reservoirs remain
problematic. By contrast, mid–ocean ridge basalts
(MORBs) are formed by shallow melting of the
upper mantle and show much more uniform
helium isotope compositions [8 ± 1 R/Ra (SD)]
( 5 ) and more homogeneous Sr-Nd-Hf-Pb isotope
compositions. The higher R/Ra values of plume-
related basalts relative to MORBs are taken as
key evidence of a primordial undegassed reser-
voir with high^3 He/^4 He ratios present in Earth’s
lower mantle ( 6 – 10 ). However, the preservation of
such a reservoir over the Earth’shistoryhasbeen
questioned on the basis of geophysical evidence of
slab subduction into the lower mantle and mantle
convection models ( 11 ). An upper mantle loca-
tion for the high-^3 He/^4 He reservoir has also been
suggested on the basis of seismic anomalies, het-
erogeneities sampled by small degrees of melt,
and modeled low–U-Th/^3 He domains formed
through melt depletion ( 1 , 12 – 17 ). Resolving the
existence and location of a long-term preserved,
primordial, undegassed, high-^3 He/^4 He reservoir,
and where it interacts with other reservoirs in
the mantle, is key to understanding the evolution
of Earth and deep mantle convection.
Diamonds are physically and chemically robust,
allowing retention of He isotope signatures thatreflect their formation environment ( 18 – 20 ). Here,
we present helium isotope data of fluid inclusions
from sublithospheric diamonds; these data pro-
vide an unparalleled perspective on He isotopes
from transition zone depths (410 to 660 km).
We studied 24 diamonds (1.3 to 6 mm in size)
from the Juina-5 and Collier-4 kimberlites and
São Luiz River (Juina, Brazil). The Juina area is
renowned for the unusually predominantly sub-
lithospheric origin of its diamonds ( 21 – 23 ). Our
diamonds have physical features and properties
that are consistent with other diamonds from
Earth’s transition zone (410 to 660 km depth),
including dislocations or diffuse growth zones
(database S1) and no detectable nitrogen (N) or
fully aggregated N defects (database S2) ( 24 ). The
diamonds contain mineral inclusions seen in other
transition zone diamonds ( 21 , 25 , 26 ), including
breyite (database S3 and fig. S1, A and B), coesite,
carbonates, sulfides, and amorphous silicate (fig.
S1, C and D). Although only diamond C4-106 has
a confirmed superdeep origin, on the basis of its
breyite mineral inclusions, all diamonds show
typical sublithospheric features. After establishing
the sublithospheric origin for our diamonds, we
measured helium isotopes of the fluid inclusions.
We combined these data with picogram analyses
of Pb-Sr isotopes of fluid inclusions, trace-element
patterns of fluid inclusions, and carbon isotope
values of the diamond hosts ( 24 ).
Helium is trapped in submicrometer fluid in-
clusions (database S1), as the Juina diamonds are
relatively young, likely formed less than 500 million
years ago ( 23 , 27 ), and He diffusion through dia-
mond is extremely slow ( 19 ). Preservation of He
and its isotope signatures in diamond is supportedRESEARCH
Timmermanet al.,Science 365 , 692–694 (2019) 16 August 2019 1of3
(^1) Research School of Earth Sciences, Australian National
University, 142 Mills Road, Acton, ACT 2601, Australia.^2 Earth
and Atmospheric Sciences, University of Alberta, 116 Street
and 85 Avenue, Edmonton, Alberta T6G 2R3, Canada.
(^3) School of Earth Sciences, University of Bristol, Queens
Road, Bristol BS8 1QU, UK.^4 School of Geographical and
Earth Sciences, Gregory Building, University of Glasgow,
Glasgow G12 8QQ, UK.^5 University of Sao Paolo, Sao Paolo,
Brazil.
*Corresponding author. Email: [email protected]
†Present address: Institut für Mineralogie, University of Münster,
Corrensstrasse 24, 48149 Münster, Germany.
Fig. 1. Helium isotope compositions (R/Ra) of fluid inclusions and carbon isotope composi-
tions (d^13 C) of their diamond hosts.Two trends (A and B) are observed in the studied
sublithospheric Brazilian diamonds. Values for São Luiz cloudy diamonds ( 31 ), OIBs from Hawaii
( 30 ), and MORBs ( 5 ) are shown for comparison. R/Ra values are shown with 1 SD uncertainty, and
thed^13 C values are averages ± 1 SD variations of multiple analyses across the diamond.