and body wave studies suggesting that a low-velocity zone may not exist beneath some continents (2, 12), although this absence is potentially
an artifact of the broad depth resolution of the
Our results correspond well to lithospheric thickness that we calculated from diamond-bearing
xenolith pressure-temperature (PT) estimates (Figs.
1 and 3E). Our SS Moho results provided a crustal
thickness, and we calculated lithospheric thickness
from diamond-bearing xenoliths assuming a crustal density of 2600 kg/m3 and a mantle density of
3330 kg/m3. We used the highest reported pres-
sure in continental interiors where there are
multiple analyses of diamond-bearing xenoliths.
We assumed that continental lithospheric dia-
monds are not stable at greater depths than the
highest reported pressure. Our estimates of litho-
spheric thicknesses from diamond-bearing xen-
oliths range from 130 to 190 km (Fig. 3E and
table S3); these values correlate with the depth
extent of the lithosphere from diamond thermo-
barometry (correlation coefficient of 0.93) (19)
(Fig. 3E). Our SS results are shallower than those
from diamonds by ~15 km (Fig. 3E). This is much
smaller than the previously large discrepancy
in which the geophysical lithosphere was up to
hundreds of kilometers deeper than the geo-
chemical lithosphere. If this small difference
between our seismic discontinuity depths and
diamond origin depths is robust, it could reflect a
subtle difference in sensitivity between the seis-
mic wave speeds and diamond stability to the
underlying mechanism (Fig. 3E). The good overall
correlation between the seismic discontinuity and
the depth extent of the diamonds suggests that
our result beneath the continents corresponds
to the chemical depth extent of the tectonic plate.
The discontinuity at 130 to 190 km depth is not
defined by temperature or anisotropy alone. The
sharpness of the discontinuity (<30 km depth) is
inconsistent with thermal gradients between the
lithosphere and asthenosphere in geodynamic
models, which generally occur over >70 km depth
(31). Anisotropy does not likely define our observation on its own. A change in azimuthal anisotropy would be accompanied by a change in polarity
with back-azimuth (32), which was not observed.
An increase in radial anisotropy with depth is also
not a likely explanation, because recent surface
wave models beneath the continents suggest radial
anisotropy (VSH > VSV) decreases in magnitude
with depth at 130 to 190 km beneath the continents (10).
Nor is the discontinuity at 130 to 190 km defined
by composition alone. Xenolith compositions suggest that the continents are depleted at shallow
depths and more fertile at greater depths. However, our velocity drop is also too large to be
explained by typical continental bulk compositional depletion, which can only explain up to
~1% velocity contrast (33). The deep continental
mantle may be enriched in volatiles, and it has
been suggested that this could lower seismic velocity substantially by enhancing the effect of
elastically accommodated grain boundary sliding.
However, the predicted depths for this effect (~60
to 150 km) are shallower than our discontinuity at
up to 190 km depth (34).
Another possible mechanism to explain strong,
sharp seismic velocity gradients is a transition
from a melt-free shallow layer to a deeper layer
containing a small degree of partial melt. To investigate the melt hypothesis, we compared the
carbonate-silicate solidus (35) to collated PT
estimates for xenoliths with textural information
(table S3). We examined three regions: the Slave
province, South Africa, and Siberia. Textural information provides an indication of mantle deformation. Coarse-grained samples are interpreted
as being within the continental lithosphere, whereas
the deformed xenoliths are generally believed to
have been affected by eruptions that brought them
to the surface and related processes (36). These
deeper samples indicate high adiabatic eruption
temperatures and show evidence of multiple events
of metasomatism, which produced enrichment
in incompatible trace elements, volatiles, and SiO2
(6, 37). Geothermobarometry of coarse-grained
samples indicates a geotherm consistent with
steady-state conductive cooling down to depths
582 11 AUGUST 2017 • VOL 357 ISSUE 6351 sciencemag.org SCIENCE
Fig. 3. Comparison of SS precursor results to previous studies. (A) SS Moho depths compared to
average crustal thickness from CRUST1.0 (18). (B) SS MLD depths compared to results of receiver
function studies (table S1). (C to E) SS LAB depths compared to the depth of the maximum negative
gradient in Voigt average shear velocity in SEMum2 (10) (C), receiver function studies (table S1) (D), and
depth of the deepest diamond-bearing xenoliths (table S3) (E). Estimates for lithospheric thickness
from diamond-bearing xenoliths: NA, 181 km; EUR, 204 km; SIB, 150 km; SA, 178 km; WAF, 170 km; SAF,
190 km; AUS, 180 km. The gray bars in (B) and (D) show the depth range of discontinuities reported
using receiver functions.
Fig. 4. SS LAB depths lie at the intersection of the conductive geotherm and the carbonate-silicate solidus. Depth-temperature relations for xenoliths from NA, SAF, and SIB (table S3).
Continental conductive geotherms (dashed gray lines) were calculated after (4). Geotherms terminate
in a mantle adiabat from 1200° to 1300°C (shaded gray region). The graphite (G)–diamond (D)
equilibrium (solid black line) is from (41). The carbonate-silicate solidus (solid red line) is from (35).
SS LAB results are indicated by dashed green lines, with the deeper SIB estimate shown as a dashed
magenta line (16).