SCIENCE science.orgAs of today, davemaoite is the first and
only high-pressure silicate recovered from
the lower mantle. Together with the 2014
discovery of another lower-mantle mineral
“bridgmanite,” found inside a meteorite and
named after physicist Percy Bridgman ( 7 ),
the two form an exclusive club as the only
lower-mantle silicate minerals confirmed in
nature. The discovery of these high-pressure
phases in nature reflects the technologi-
cal advancement in the characterizations
of minerals at the submicrometer scale.
Recovery of davemaoite and bridgmanite
provides two distinct pathways for obtain-
ing high-pressure phases in nature: through
rising mantle rocks from deep interior and
through falling meteorites that were highly
shocked because of collisions of planetary
bodies (see the figure).
The ability of a material to preserve the
high-pressure phase in materials depends
heavily on what happens once the pressure
is removed. Synthetic CaSiO 3 -perovskite
has been observed to revert to glass upon
release from high pressure ( 3 , 4 ); thus, it
was considered impossible to retrieve the
high-pressure phase from the lower mantle.
This expectation was further strengthened
by bridgmanite—which has a more stable
high-pressure phase than davemaoite—hav-
ing never been recovered from the lower
mantle, only having been found serendipi-
tously in a highly shocked meteorite ( 7 ).
The high residual pressure of the entrapped
davemaoite in the host diamond is likely to
have helped preserve the structure by slow-
ing down the retrogression of davemaoite
back to its low-pressure phase. Residual pres-
sure in diamond is often observed because of
the relative volume difference between the
entrapped phase and diamond in responding
to their formation condition.
The pressure-temperature-time path-
ways also play an important role in the
survival of the high-pressure phase as the
davemaoite made its journey to the sur-
face. Not all davemaoites embedded in
deep diamonds can stay as davemaoites,
and there have been multiple studies of
CaSiO 3 inclusions in the diamonds where
the high-pressure cubic perovskite phase
was nowhere to be found (8 –13). For ex-
ample, the CaSiO 3 sample from a mine in
Mato Grasso of Brazil should have formed
in its high-pressure phase because it was
found alongside hydrous ringwoodite that
had remained in the high-pressure phase
inside the same super-deep diamond ( 8 ).
However, spectroscopic measurements re-
vealed that the CaSiO 3 had reverted to an
intermediate-pressure polymorph known
as breyite. Another CaSiO 3 inclusion, found
in a diamond from the Cullinan mine in
South Africa ( 9 ), also did not have suffi-
cient evidence to link it to the synthetic
cubic CaSiO 3 -perovskite phase. In recent
years, it has been recognized that breyite is
one of most abundant inclusions found in
these super-deep diamonds, and its abun-
dance has fueled an ongoing debate on the
formation depth of the CaSiO 3 inclusions
and the survivability of the cubic CaSiO 3 -
perovskite phase ( 10 – 13 ). The discovery
of davemaoite by Tschauner et al. dem-
onstrates a viable pathway to recover the
cubic CaSiO 3 -perovskite phase from the
lower mantle and opens the door for the
recovery of other seemingly “unquench-
able” high-pressure phases.
The davemaoite sample recovered by
Ts c h a u n e r et al. might have been special
because of its very high potassium content.
According to chemical composition analy-
sis, the davemaoite sample has more than
half of calcium atoms replaced by other
elements, such as potassium, iron, and
aluminum. In comparison, the unquench-
able synthetic CaSiO 3 -perovskite is usually
relatively pure in composition, even when
it is synthesized by using natural basalts or
peridotites. It is possible for the davemaoite
sample to have retained its high-pressure
phase because of the substituted calcium
atoms, which can be the focus of future ex-
periments. The high potassium content of
the davemaoite sample also points toward
some chemical heterogeneity in the lower
mantle, such as the regional differences in
potassium concentration.
The work by Tschauner et al. inspires
hope in the discovery of other difficult high-
pressure phases in nature, either through
careful search in deep-origin diamonds or
in highly shocked meteorites. The success-
ful retrieval of davemaoite from the lower
mantle encourages the ongoing search for
bridgmanite from the Earth’s lower mantle.
Such direct sampling of the inaccessible
lower mantle would fill our knowledge gap
in chemical composition and heterogeneity
of the entire mantle of our planet. jREFERENCES AND NOTES- O. Tschauner et al., Science 374 , 891 (2021).
- A. E. Ringwood, A. Major, Earth Planet. Sci. Lett. 2 , 106
(1967). - L. G. Liu, A. E. Ringwood, Earth Planet. Sci. Lett. 28 , 209
(1975). - H. K. Mao et al., J. Geophys. Res. 94 (B12), 17889 (1989).
- S.-H. Shim, R. Jeanloz, T. S. Duffy, Geophys. Res. Lett. 29 ,
2166 (2002). - H. W. Chen et al., Am. Mineral. 103 , 462 (2018).
- O. Tschauner et al., Science 346 , 1100 (2014).
- D. G. Pearson et al., Nature 507 , 221 (2014).
- F. Nestola et al., Nature 555 , 237 (2018).
- C. Anzolini et al., Am. Mineral. 103 , 69 (2018).
- E. M. Smith et al., Nature 560 , 84 (2018).
- A. B. Woodland, A. V. Girnis, V. K. Bulatov, G. P. Brey, H. E.
H ö fe r, Eur. J. Mineral. 32 , 171 (2020). - F. E. Brenker, F. Nestola, L. Brenker, L. Peruzzo, J. W.
Harris, Am. Mineral. 106 , 38 (2021).
10.1126/science.abm4742GEOCHEMISTRYA surprise
from the deep
Isotope patterns of methoxyl
groups reveal the origin
an d production potential of
methane from coal
By Frank Keppler1,2I
n 1885, Simon Zeisel established a
method to quantify the content of me-
thoxyl groups in plant tissue ( 1 ). He cer-
tainly could not have envisaged how his
work would advance environmental re-
search and our understanding of carbon
cycling, particularly with regard to the for-
mation of volatile compounds containing
a one-carbon unit—C1 compounds—such
as methane, methanol, or chloromethane.
These volatile compounds play an impor-
tant role in atmospheric chemistry and
physics. On page 894 of this issue, Lloyd et
al. ( 2 ) report the concentration and isoto-
pic patterns of methoxyl groups for humic
samples at different stages of coalifica-
tion—the process by which vegetative mat-
ter is converted into coal. They show the
involvement of microbes in transforming
plant matter in deep-Earth subsurface sedi-
ments over geological time scales and es-
tablish their contribution to the occurrence
of coal-bed methane (CBM) and its limits
for exploitation.
Chemically, methoxyl or methoxy de-
fines a methyl group bound to an oxygen
atom in an organic molecule (R–OCH).
These functional groups, which contribute
to biochemical reactions under specific
conditions, are ubiquitous in terrestrial
plant matter and can constitute up to 7 % of
wood. Most methoxyl groups in wood are
found in lignin as a component of coniferyl
or synapyl alcohol ( 3 ). During humifica-
tion of plant matter and further matura-
tion processes in sediments, lignite meth-
oxyl content increases up to 10% on a total
weight basis because lignin is more stable
than other plant components, such as cel-
lulose or starch, which are lost during the
initial phase of humification. Further coali-
fication processes under increased temper-(^1) Biogeochemistry Group, Institute of Earth Sciences,
Heidelberg University, Heidelberg, Germany.
(^2) Heidelberg Center for the Environment (HCE),
Heidelberg University, Heidelberg, Germany.
Email: [email protected]
12 NOVEMBER 2021 • VOL 374 ISSUE 6569 821