DIAMONDS FROM THE DEEP
WINDOWS INTO SCIENTIFIC RESEARCH
Karen V. Smit and Steven B. Shirey

How Do Diamonds Form in the Deep Earth?
Carbon is one of the most important elements on ourplanet, which led the Geological Society of London toname 2019 the Year of Carbon. Diamonds are a main hostfor carbon in the deep earth and also have a deeper origin
than all other gemstones. Whereas ruby, sapphire, andemerald form in the earth’s crust, diamonds form many
hundreds of kilometers deep in the earth’s mantle. Colored
gemstones tell scientists about the crust; gem diamonds
tell scientists about the mantle. This makes diamonds
unique among gemstones: Not only do they have great
beauty, but they can also help scientists understand carbon
processes deep in the earth. Indeed, diamonds are some ofthe only direct samples we have of the earth’s mantle.
But how do diamonds grow in the mantle? While Hollywood’s depiction of Superman squeezing coal captured
the public’s imagination, in reality this does not work. Coalis a crustal compound and is not found at mantle pressures.
Also, we now know that diamond does not prefer to form
through direct conversion of solid carbon, even though thepressure and temperature conditions under which diamond
forms have traditionally been studied experimentally as
the reaction of graphite to diamond.
Generally, two conditions are needed for diamond formation: Carbon must be present in a mantle fluid or meltin sufficient quantity, and the melt or fluid must become
reduced enough so that oxygen does not combine with carbon (see below). But do diamonds all grow by the samemechanism? What does their origin reveal about their
growth medium and their mantle host rock? Surprisingly,
diamonds do not all form in the same way, but rather they
form in various environments and through varying mechanisms. Through decades of study, we now understand that
diamonds such as the rare blue Hope, the large colorlessCullinan, and the more common yellow “cape” diamonds
all have very different origins within the deep earth.
Diamonds Form from Fluids in the Mantle That Migrate
Due to Plate Tectonics
Diamond is a metasomatic mineral that forms during migration of carbon-bearing fluids, which means that it forms
from fluids and melts that move through the mantle. Dia-

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DIAMONDS FROM THE DEEPmonds can form in both peridotite and eclogite (box A) inthe cratonic lithospheric mantle, as well as their higherpressure equivalents in the much deeper transition zone
and lower mantle (box B). Regardless of a diamond’s formation depth, many diamond fluids and melts appear to be related to the recycling of surficial material into the deep
earth or to deep melting processes when tectonic plates
split apart, or rift, to form new oceans. Both processes occur
as part of the geologic cycles that accompany plate tectonics
or, in ancient times, some type of pre-plate tectonics.
Since diamonds come from deep and otherwise inaccessible regions, they can be used to study many largerscale tectonic processes in earth that cannot be studied anyother way. Diamonds reveal processes such as early craton
development, craton growth and stabilization processes,
tectonic processes that can modify and destroy the cratonic
lithosphere, and deep subduction (box B) into the lower
mantle that may reintroduce volatile elements into thedeep earth. This makes the study of diamond source fluids,
as well as inclusions in diamond, a powerful way (if not
the only way) to study the deep earth cycles of many elements such as carbon, nitrogen, boron, sulfur, and oxygen.
How Do Lithospheric Diamonds Form?
Lithospheric diamonds (box B) often contain detectable nitrogen, implying that they crystallize from carbon- and nitrogen-bearing (C-N-bearing) fluids. Through the study of
diamonds from many different localities, we now know that
there are subtle differences in the compositions of these CN-bearing fluids and melts. These differences manifest aschanges in the type of carbon and nitrogen compounds contained in these fluids. “Oxidized” hydrous fluids and melts
can contain CO3, CO2, and N2, whereas more “reduced” hydrous fluids contain CH4, NH3, and minor H2.
Traditional models for diamond formation from fluids
in the mantle invoke either carbonate (CO3) reduction or
methane (CH4) oxidation to remove the elements bonded
to carbon. Both these mechanisms require some oxygen
exchange with the surrounding rocks—peridotite or eclogite (box A)—at the site of diamond precipitation so that elemental carbon can be produced to crystallize diamond.
Sometimes peridotite has a limited capacity to exchange
oxygen, and we now know that cooling of hydrous fluids
containing methane (CH4) and carbon dioxide (CO2) is analternative way to precipitate diamonds in these rocks (figure 1; Luth and Stachel, 2014; Smit et al., 2016; Stachel etal., 2017).

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BOX A: ROCK TYPES IN THE MANTLE
Figure A-1. Red-purple
garnet (with Cr-pyrope
compositions) and brightgreen clinopyroxene (with
Cr-diopside compositions)separated from a crushed
peridotite rock. Photos by
Karen Smit/University ofAlberta.

Peridotite. Peridotite is the predominant rock type in the
mantle. It contains the minerals orthopyroxene, clinopyroxene, and olivine (figure A-1). Peridotite also contains
an aluminous phase that can be either spinel or garnet,
depending on depth. At the higher pressure conditionsin the diamond stability field, the aluminous phase is always garnet (figure A-1). Peridotite melts to form basalt
at mid-ocean ridges and ocean islands. This melting removes the basaltic melt from peridotite, leaving theresidual peridotite depleted in elements such as Ca, Al,
and Fe. This is because the melt moves upward to dikes
that eventually feed shallow magma chambers. Lherzolite, the most fertile peridotite, has not undergone significant melt depletion and will contain somecombination of the minerals listed above. With high proportions of melt depletion, clinopyroxene is eventually
exhausted in the residual peridotite, resulting in the
clinopyroxene-free rock known as harzburgite. Witharound 40–50% melting, orthopyroxene is also exhausted and olivine dominates the peridotitic assemblage. After these high degrees of melting where most of
the Ca, Al, and Fe have been lost, the residual peridotite
becomes the most depleted dunite. Importantly, both depleted harzburgite and dunite can be re-enriched by passing melts that could reintroduce many of these minerals
and convert depleted peridotite back to fertile lherzolite.

tle through subduction (Jacob, 2004). Despite theirscarcity in the lithospheric mantle (estimated to be lessthan 1%), eclogites are often sampled by kimberlites and,due to their frequent recycled origin, can provide important insights into the role of subduction in the assemblyof cratonic keels and recycling of surficial components
into the deep earth.

Figure A-2. Bimineralic eclogite containing pale green
clinopyroxene and orange-red garnet (Victor mine,
Canada). This eclogite rock was sawed into a thin section(30 µm thick) and polished so that it could transmit lightand be studied under a microscope. Photo by Karen
Smit/University of Alberta.

Cpx

Grt

Eclogite. Eclogite is another rock type in the lithosphericmantle, a bimineralic rock consisting of a sodium-richclinopyroxene (known as omphacite) and garnet with pyrope (Mg-rich), grossular (Ca-rich), and almandine (Ferich) components (figures 3 and A-2). There are two mainmodels for the origin of eclogites in the lithosphericmantle: as high-pressure mantle melts or as former
oceanic crust that has been recycled back into the man-

1 cm

Unfortunately, gem-quality diamonds almost nevercontain any trapped source fluids that can directly indicate
how they formed. However, there are so-called fibrous diamonds (figure 2) that grow much more rapidly than gem-

quality diamonds, and even coat earlier gem diamondcores, allowing them to trap abundant fluids. Fibrous diamonds from all over the world, including Canada, West
Africa, Siberia, and South Africa, have compositions that

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BOX B: WHERE DO DIAMONDS FORM IN THE EARTH?
Natural diamonds typically form 150–200 km below thesurface of the earth. Diamond formation does not occur
everywhere at these depths, but only below the oldest continents that have been stable for billions of years; theseareas are known as cratons (see figure 2 in Summer 2018
Diamonds from the Deep). This is because these old cratons all have thick continental roots with cool temperature
profiles conducive to diamond formation that geologists
term “continental lithospheric mantle.” Diamonds thatform within these continental roots are known as lithospheric diamonds and are carried up to the surface of theearth by rare volcanic eruptions known as kimberlites.
Other diamonds form much deeper in the earth, in
sublithospheric regions of the mantle, below these continental roots. So-called superdeep diamonds form at

depths much greater than 200 km, in areas of the mantle
known as the transition zone (410 to 660 km below theearth’s surface) and lower mantle (> 660 km). After formation they are transported to shallower depths in the mantle, likely through mantle convection cells, and then alsobrought to the surface by kimberlite eruptions. Earth’s
mantle convection associated with plate tectonics is responsible for both the availability of fluids necessary toform diamonds and the return of diamonds to the surface.
Ocean floor is thrust deep into the mantle by the process
known as subduction and carries diamond-forming fluids
in its seawater-altered minerals and rocks. Mantle convection return flow also causes hot mantle to rise to shallower levels and melt, generating the kimberlites thatdeliver diamonds to the surface.

are different mixtures of three end-member fluid compositions: carbonate-like, silica-rich, or salty (e.g., Navon et al.,
1988; Izraeli et al., 2001, 2004; Schrauder and Navon, 1994;
Smith et al., 2012; Weiss et al., 2013; Timmerman et al.,2018).
The origins of fibrous diamonds and their fluids are still
debated. Some researchers argue that fibrous diamonds
may be related to the kimberlite magmas that transport diamonds to the surface. If so, fibrous diamonds could formright before the kimberlite eruption to the earth’s surface,which would make them much younger than many gemquality diamonds that may have resided in the mantle formillions to billions of years. One study, however, linked
the full range of fluids in some fibrous diamonds from
northern Canada to subducting oceanic lithosphere, concluding that the original subducting fluid was related to>200-million-year-old recycled sea water (Weiss et al.,2015). This implies that some fibrous diamonds could have
longer mantle residence times and are not necessarily always related to kimberlite eruption.

Figure 1. We now have evidence for a wider range of diamond source fluid compositions in both the lithosphereand sublithosphere that do not include any carbonate components. Both peridotitic and eclogitic diamonds fromthe lithosphere are now known to also precipitate from non-carbonate-bearing fluids. The Marange cuboctahedral
diamonds formed from water-rich fluids that contain both methane and carbon dioxide (Smit et al., 2016), andthe Zimmi eclogitic diamonds crystallized from water-rich fluids that are more reduced and contain significant
methane (Smit et al., 2019), similar to fluids indicated for peridotitic diamonds from the Cullinan mine (Thomassot et al., 2007). Photos by Joshua Balduf and Karen Smit/GIA.

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Figure 2. A fibrous cubic diamond (1.3 × 1.3 mm) from Wawa (Superior craton, Canada) that has been cut throughthe middle and polished into a double-sided plate (right). Fibrous diamonds such as this example typically contain a darker rim of fluid inclusions that have compositions ranging between carbonate-rich, saline-rich, and silica-rich. Fluids similar to those in fibrous diamonds have been found along twin planes in gem-quality diamonds
from Voorspoed and Venetia (Jablon and Navon, 2016). Photos by Evan Smith/University of British Columbia.

Fluids similar to those in fibrous diamonds have beenfound along twin planes in rare gem-quality diamonds fromSouth Africa, indicating that some gem-quality diamonds
may have similar formation conditions to fibrous diamonds(Jablon and Navon, 2016). However, fluid inclusions in gemquality diamonds are actually very rare, and source compositions and the nature of reactants to make diamond must
be inferred from the covariation of carbon isotope composition and nitrogen content, either in individual diamonds
or diamond suites (Deines, 1980; Stachel et al., 2009). These
carbon isotope studies suggest that gem-quality diamonds
may form through varying mechanisms, though they typically involve hydrous carbon-bearing fluids that also contain oxygen and hydrogen (either as CO3, CO2, or CH4).
Carbon isotope studies can infer whether a diamondsource fluid is “oxidized” or “reduced” (see above) but do
not reveal the fluid’s full elemental composition. Diamonds are comprised mostly of carbon, and any other elements present in the diamond are normally at incredibly
low abundances that are difficult to measure. For this reason, trace element data for gem-quality diamonds are only
now being realized with improvements in analytical sensitivity. But the available trace element data for gem-quality diamonds do appear to show many compositional
similarities to fluid-rich fibrous diamonds (Krebs et al.,2019), further suggesting that fibrous and gem-quality diamonds crystallize from similar types of mantle fluids.
Sublithospheric Diamonds:
The Product of Very Deep Subduction

stellation diamond from Karowe mine in Botswana; Smithet al., 2016). Some superdeep diamonds are plastically deformed, making them pink or brown (e.g., diamonds fromthe Juina area in Brazil). Rarely, they can contain someboron imparting a blue color (e.g., the Hope diamond;
Smith et al., 2018). Based on studies of inclusion-bearing
superdeep diamonds, we now know that they cannot allform in the same environment and that they must crystallize from a wide variety of source fluids and melts that all
relate to different aspects of very deep subduction (box B).
Many superdeep diamonds, such as those from the Juina
and Machado River areas in Brazil, preserve inclusion evidence for their crystallization from carbonate-rich fluids (figure 3; Walter et al., 2008; Bulanova et al., 2010; Burnham etal., 2016; Thomson et al., 2016). These CO3-rich source fluids likely derive from oceanic lithosphere that was deeply
subducted into the mantle transition zone (box B). CLIPPIR
diamonds (figure 4) are now also known to crystallize frommetallic melts in the deep earth that are associated insteadwith reduced carbon (CH4) and hydrogen (Smith et al., 2016).
These diamonds have carbon isotopic compositions that are
unlike those expected for the mantle, but rather indicate anultimate source for the carbon in the oceanic lithosphere, as
do other superdeep diamonds. The third kind of superdeep
diamond, boron-containing blue diamonds, are also related
to deeply subducted oceanic lithosphere (figure 4; Smith etal., 2018). In this case, however, the source for the boroncontaining fluids is likely to be altered peridotite in the
deeper portions of the subducted lithosphere.

So-called superdeep diamonds that form in the sublithospheric mantle (box B) contain very little nitrogen and are
relatively pure, making them nearly colorless (e.g., CLIPPIR diamonds such as the Cullinan diamond and the Con-

Our Fluid Understanding of How Diamonds Form:
Future Studies of Diamond-Forming Fluids

DIAMONDS FROM THE DEEPGEMS & GEMOLOGYOur understanding of the fluids from which diamonds form
is improving with advances in the sensitivity of analytical

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Figure 3. Before 2016, the majority of diamonds were thought to crystallize from fluids and melts that containedcarbonate. For example, the diamonds contained in Jericho eclogites (Slave craton, Canada; left) have covariations
in carbon isotopic composition and nitrogen content that indicate carbonate-rich source fluids/melts (Smart et al.,2011). Similarly, the trace element compositions of inclusions in Juina superdeep diamonds (right) also indicate
their formation from carbonate-rich fluids or melts in the deep mantle (Walter et al., 2008; Bulanova et al., 2010;
Thomson et al., 2016). Photos by Katie Smart/University of Alberta and Galina Bulanova/University of Bristol.

techniques. Most diamonds are only comprised of carbon,
with less than a few parts per million of nitrogen, boron, andother trace elements. Many elements that scientists are interested in detecting in diamond are currently not able to bemeasured accurately. With improved analytical techniquesthat can measure the very low concentrations of trace ele-ments in diamonds, we may be able to better understandthe different origins for diamond source fluids.
Future work will also reveal how the different diamondforming environments relate to each other. Our understanding of the origin of lithospheric diamond source fluids isbetter developed than it is for sublithospheric diamonds.

Figure 4. Although some superdeep diamonds have been shown to crystallize from carbonate-rich melts (figure3), there are also some rare superdeep diamonds that crystallize from boron-rich fluids and reduced metallic
melts, both variably associated with subducted oceanic lithosphere (Smith et al., 2016, 2018). On the left is a
29.62 ct boron-containing rough blue diamond, and on the right is the 812.77 ct rough Constellation diamond,
which contained metallic inclusions. Photos by Jian Xin (Jae) Liao/GIA.

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This is because they contain much more nitrogen that canbe studied along with carbon, and sometimes they are foundstill trapped within their peridotite or eclogite host rocks(figure 3). Since superdeep diamonds contain little or no nitrogen, and are never found within their original superdeep
mantle host rocks, they provide less information about thegeological conditions by which they formed. We do know,
however, that the carbonate-rich fluids, metallic melts, andboron-rich fluids in superdeep diamonds all relate in some
way to deeply subducted material from the earth’s oceanic
lithosphere. But the challenge remains for geologists to predict where these domains, all with different oxidation states,
occur in the deep earth. Current diamond exploration models have a near-exclusive focus on lithospheric diamonds, so
geologists still need to do some work to figure out how to
explore for potentially high-value superdeep diamonds.

REFERENCES
Bulanova G.P., Walter M.J., Smith C.B., Kohn S.C., Armstrong L.S.,
Blundy J., Gobbo L. (2010) Mineral inclusions in sublithospheric diamonds from Collier 4 kimberlite pipe, Juina, Brazil:
subducted protoliths, carbonated melts and primary kimberlite
magmatism. Contributions to Mineralogy and Petrology, Vol.
160, No. 4, pp. 489–510.
Burnham A.D., Bulanova G.P., Smith C.B., Whitehead S.C., Kohn
S.C., Gobbo L., Walter M.J. (2016) Diamonds from the MachadoRiver alluvial deposit, Rondonia, Brazil, derived from bothlithospheric and sublithospheric mantle. Lithos, Vol. 265, pp.
199–213.
Deines P. (1980) The carbon isotopic composition of diamonds: relationship to diamond shape, colour, occurrence and vapour
deposition. Geochimica et Cosmochimica Acta, Vol. 44, pp.
943–961.
Izraeli E.S., Harris J.W., Navon O. (2001) Brine inclusions in diamonds: a new upper mantle fluid. Earth and Planetary Science
Letters, Vol. 187, No. 3-4, pp. 323– 332.
Izraeli E.S., Harris J.W., Navon O. (2004) Fluid and mineral inclusions in cloudy diamonds from Koffiefontein, South Africa.
Geochimica et Cosmochimica Acta, Vol. 68, No. 11, pp. 2561–
2575.
Jablon B.M., Navon O. (2016) Most diamonds were created equal.
Earth and Planetary Science Letters, Vol. 443, pp. 41–47.
Jacob D.E. (2004) Nature and origin of eclogite xenoliths from kimberlites. Lithos, Vol. 77, No. 1-4, pp. 295–316.
Krebs M.Y., Pearson D.G., Stachel T., Laiginhas F., Woodland S.,
Chinn I.L., Kong J. (2019) A common parentage – low abundance trace element data of gem diamonds reveals similar fluids to fibrous diamonds. Lithos, Vol. 324-325, pp. 356–370.
Luth R.W., Stachel T. (2014) The buffering capacity of lithosphericmantle: implications for diamond formation. Contributions toMineralogy and Petrology, Vol. 168, No. 5, p. 1083.
Navon O., Hutcheon I.D., Rossman G.R., Wasserburg G.J. (1988)
Mantle-derived fluids in diamond micro-inclusions. Nature,
Vol. 335, No. 6193, pp. 784–789.
Schrauder M., Navon O. (1994) Hydrous and carbonatitic mantle
fluids in fibrous diamonds from Jwaneng, Botswana. Geochimica et Cosmochimica Acta, Vol. 58, No. 2.
Smart K.A., Heaman L.M., Chacko T., Simonetti A., Kopylova M.,
Mah D., Daniels D. (2009) The origin of high-MgO diamondeclogites from the Jericho Kimberlite, Canada. Earth and Planetary Science Letters, Vol. 284, No. 3-4.
Smart K.A., Chacko T., Stachel T., Muehlenbachs K., Stern R.A.,
Heaman L.M. (2011) Diamond growth from oxidized carbon
sources beneath the Northern Slave Craton, Canada: A d13C-N
study of eclogite-hosted diamonds from the Jericho kimberlite.
Geochimica et Cosmochimica Acta, Vol. 75, No. 20, pp. 6027–
6047.
Smit K.V., Shirey S.B., Stern R.A., Steele A., Wang W. (2016) Dia-

mond growth from C-H-N-O recycled fluids in the lithosphere:
evidence from CH4 micro-inclusions and d13C-d15N-N content
in Zimbabwe mixed-habit diamonds. Lithos, Vol. 265, pp. 68–
81.
Smit K.V., Stachel T., Luth R.W., Stern R.A. (2019) Evaluating
mechanisms for eclogitic diamond growth from reduced fluids:
an example from Neoproterozoic Zimmi diamonds (WestAfrican craton). Submitted to Chemical Geology.
Smith E.M., Kopylova M.G., Nowell G.M., Pearson D.G., Ryder J.
(2012) Archean mantle fluids preserved in fibrous diamonds
from Wawa, Superior craton. Geology, Vol. 40, No. 12, pp.
1071–1074.
Smith E.M., Shirey S.B., Nestola F., Bullock E.S., Wang J., Richardson S.H., Wang W. (2016) Large gem diamonds from metallic
liquid in Earth’s deep mantle. Science, Vol. 354, No. 6318, pp.
1403–1405.
Smith E.M., Shirey S.B., Richardson S.H., Nestola F., Bullock E.S.,
Wang J., Wang W. (2018) Blue boron-bearing diamonds from
Earth’s lower mantle. Nature, Vol. 560, pp. 84–87.
Stachel T., Harris J.W., Muehlenbachs K. (2009) Sources of carbon
in inclusion-bearing diamonds. Lithos, Vol. 112S, pp. 625–637.
Stachel T., Chacko T., Luth R.W. (2017) Carbon isotopic fractionation during diamond growth in depleted peridotite: Counterintuitive insights from modelling water-maximum CHO fluids
as multi-component systems. Earth and Planetary Science Letters, Vol. 473, pp. 44–51.
Thomassot E., Cartigny P., Harris J.W., Viljoen K.S. (2007)
Methane-related diamond crystallisation in the Earth’s mantle:
Stable isotope evidences from a single diamond-bearing xenolith. Earth and Planetary Science Letters, Vol. 257, pp. 362–
371.
Thomson A.R., Kohn S.C., Bulanova G.P., Smith C.B., Araujo D.,
Walter M.J. (2016) Trace element composition of silicate inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite:
Evidence for diamond growth from slab melts. Lithos, Vol. 265,
pp. 108–124.
Timmerman S., Jaques A.L., Weiss Y., Harris J.W. (2018) N-d13Cinclusion profiles of cloudy diamonds from Koffiefontein: Evidence for formation by continuous Rayleigh fractionation andmultiple fluids. Chemical Geology, Vol. 483, pp. 31–46.
Walter M.J., Bulanova G.P., Armstrong L.S., Keshav S., Blundy J.D.,
Gudfinnsson G., Lord O.T., Lennie A.R., Clark S.M., Smith
C.B., Gobbo L. (2008) Primary carbonatite melt from deeply
subducted oceanic crust. Nature, Vol. 454, pp. 622–625.
Weiss Y., Griffin W.L., Navon O. (2013) Diamond-forming fluids
in fibrous diamonds: The trace-element perspective. Earth andPlanetary Science Letters, Vol. 376, pp. 110–125.
Weiss Y., McNeill J., Pearson D.G., Nowell G.M., Ottley C.J.
(2015) Highly saline fluids from a subducting slab as the source
for fluid-rich diamonds. Nature, Vol. 524, p. 339.

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