by He heterogeneities within individual diamonds
( 18 , 20 ). We removed the outer rim of the diamond
to eliminate any^4 He implantation [up to 30mm
fromthediamondsurface( 19 )] from the surround-
ing matrix, either from the mantle or the kimberlite.
The potential impact of production of radiogenic
(^4) He within the diamond generally results in an
R/Ra shift of <0.8, based on low U-Th-Sm concen-
trations measured on 10 of the studied diamonds
(database S2) and assuming a diamond forma-
tion age of 500 million years ago (fig. S2). Cos-
mogenic production of^3 He at Earth’ssurfacemay
also modify He isotope compositions, which is
a concern for diamonds from alluvial sources
(São Luiz River). The highest R/Ra value for our
diamonds was from the Juina-5 kimberlite, and
it coincides with the R/Ra of 49.8 from Baffin
Island picrites ( 4 ), the highest observed in basalts.
Diamonds recovered from kimberlite pipes are
less likely to have been exposed to cosmic rays,
but we cannot exclude exposure entirely, as the
diamonds from Collier-4and Juina-5 kimberlite
pipes were likely recovered from the near-surface
oxidized yellow ground kimberlite material. Thus,
our studied diamonds provide the most direct
and undegassed evidence of the variation in helium
isotope compositions in Earth’s transition zone.
Previous studies ofd^13 C-d^18 O diamond-mineral
inclusion correlations ( 25 ) and major and trace-
element abundances of mineral inclusions ( 23 , 28 )
have established the presence of subducted mate-
rial in the transition zone under the Juina region.
The He isotopic signatures released from the
sparse fluid inclusions vary from 0.7 to 49.9 R/Ra
(Figs. 1 and 2). This range is larger than the varia-
tion found in plume-related basalts. Lead isotope
ratios also bear similarities to plume-related basalts
(Fig. 3, fig. S3, and database S2) and, along with
trace-element patterns (Fig. 4 and fig. S4), provide
evidence for the involvement of subducted mate-
rial.Furthermore,thelargerangein^87 Sr/^86 Sr ratios
(0.7051 to 0.7260) (Fig. 3) suggests the involvement
of old continental crust [enriched mantle (EMII)
component]. Low Rb/Sr ratios that do not sup-
port the observed radiogenic Sr can be caused by
subduction-related loss of Rb relative to Sr. A
negative Nb anomaly characterizing the involve-
ment of subducted material is present in all the
trace-element patterns of our studied samples.
This anomaly implies that a recycled crustal com-
ponent, and not the ambient mantle, dominates
the trace-element budget in the fluid inclusions.
Furthermore, the negative Y/Ho, negative Sr, and
positive Eu anomalies and the Zr-Hf depletion
measured in our samples are all best explained
by shallow (crustal) processes ( 24 ). Eu anomalies
range from 0.01 to 3.2 Eu/Eu* and correlate pos-
itively with La/Nd ratios. We found no correla-
tion of trace-element ratios with diamond host C
or fluid He isotope compositions, indicating de-
coupling of volatile from lithophile elements.
The carbon isotope compositions of the dia-
monds form two arrays with the R/Ra values
(Fig.1).ThelargerangeinR/Ravalues(trendA,
Fig. 1) could be the result of mixing of various
mantle reservoirs and oceanic crust–lithosphere
with different R/Ra ratios ( 29 ) but with typical
mantle carbon isotope values centered around
− 5 ‰. Similar ranges in He isotope composi-
tion were observed in a compilation of MORBs
( 5 ), OIBs ( 30 ), and two cloudy São Luiz diamonds
( 31 ). We also found diamonds that presented a
negative correlation betweend^13 C and R/Ra
(trend B, Fig. 1). Although unexpected, a high-
(^3) He/ (^4) He source dominating the isotope ratio
could explain why the R/Ra values are higher
than those found for MORBs. This would be most
visible in rocks that have low helium concentra-
tions and low U-Th-Sm contents, such as recycled
pelagic sediments strongly depleted in almost
all their helium and U-Th during subduction;
indeed, the lowd^13 C values of some Juina dia-
monds have previously been related to subducted
pelagic sediments ( 23 ). The diamonds that showed
mantle-like carbon and low R/Ra values may be
related to oceanic lithosphere, which would likely
be more retentive of U-Th than sediments would,
given their lower water content and diminished
propensity to form melt. Mixing between fluids
Timmermanet al.,Science 365 , 692–694 (2019) 16 August 2019 2of3
m m
Fig. 2. Helium isotopic composition of the fluid inclusions.(AandB)R/RaversusHe
concentrations of the fluid inclusions in alluvial and Juina-5 sublithospheric diamonds from array
A. (CandD) R/Ra versus He concentrations of fluid inclusions in Collier-4 and Juina-5
sublithospheric diamonds from array B. Helium data from lithospheric fibrous diamonds ( 34 )
with a plume component are given for comparison. As most of the noble gases are contained in
fluid inclusions in diamond, some of the variability in He concentrations can be attributed to
differences in fluid inclusion population densities between diamonds. Errors are 1 SD. Helium
concentrations are in cubic centimeters per gram (cm^3 /g).
Fig. 3. Strontium^87 Sr/^86 Sr versus
(^206) Pb/ (^204) Pb isotopic compositions of
fluid inclusions.The data support the
presence of enriched mantle (EMII
component) in three of the diamonds and
a HIMU component in one diamond.
Strontium isotope compositions shown
are for present day and with a correction
for 500 million years of^87 Sr ingrowth
from^87 Rb decay. Data for OIB and MORB
fields are from ( 38 ). Errors are 2 SD.
RESEARCH | REPORT