# Diamond Formation: A Stable Isotope Perspective

Pierre Cartigny, Médéric Palot, Emilie Thomassot, Jeff Harris

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Pierre Cartigny, Médéric Palot, Emilie Thomassot, Jeff Harris. Diamond Formation: A Stable Isotope Perspective. Annual Review of Earth and Planetary Sciences, 2014, 42, pp.699-732. 10.1146/annurev-earth-042711-105259. hal-01765325

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# Diamond Formation: A Stable Isotope Perspective

Pierre Cartigny,<fnref n="1" /> Médéric Palot,<fnref n="2" /> Emilie Thomassot,<fnref n="3" /> and Jeff W. Harris<fnref n="4" />

Annu. Rev. Earth Planet. Sci. 2014. 42:699-732

First published online as a Review in Advance on
April 3, 2014

The *Annual Review of Earth and Planetary Sciences* is
online at earth.annualreviews.org

This article's doi:
10.1146/annurev-earth-042711-105259

Copyright © 2014 by Annual Reviews.
All rights reserved

## Keywords

diamond, stable isotopes, subduction, metasomatism, chemical geodynamics

## Abstract

Primarily on the basis of C, N, S, and O stable isotope systematics, this article reviews recent achievements in understanding diamond formation and growth in Earth's mantle. Diamond is a metasomatic mineral that results from either the reduction or oxidation of mobile C-bearing liquids (fluids or melts) that intrude preexisting lithologies (eclogites, peridotites, and metamorphic rocks). This process seems ubiquitous, as it occurs over a large range of depths and extends through time. Diamond-forming carbon derives mainly from the convective asthenosphere. Most of its isotopic anomalies reflect fractionation processes in the lithospheric mantle, which are attributed to diamond precipitation itself and/or a mineralogical control occurring prior to diamond precipitation. Evidence for a mineralogical control would be the decoupling of the ¹⁵N/¹⁴N ratios in eclogitic diamond from other tracers of subduction in inclusions in the same diamond. C isotope anomalies related to subduction are rare and are probably best seen in diamonds from the transition zone.

<footnote n="1">¹Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Université Paris Diderot, Centre National de la Recherche Scientifique, UMR 7154, Sorbonne Paris-Cité, 75238 Paris, France; email: cartigny@ipgp.fr</footnote>

<footnote n="2">²Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada</footnote>

<footnote n="3">³Centre de Recherches Pétrographiques et Géochimiques, Centre National de la Recherche Scientifique, Université de Lorraine, 54501 Vandœuvre-lès-Nancy, France</footnote>

<footnote n="4">⁴School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom</footnote>
# INTRODUCTION

## Diamond: A Key Sample to Study Earth's Interior

Although extremely rare (occurring at the parts per billion level even within the most diamond-iferous lamproite; Gurney 1989), diamond is a key mineral that offers insights into a series of open issues in terrestrial and extraterrestrial research. From a terrestrial perspective, diamond is a probe of the deep Earth that allows the evolution and dynamics of both the upper and lower mantle to be studied through time.

Diamond is thermodynamically stable at depths exceeding 150 km for lithospheric P-T conditions (Figure 1). Minerals trapped in diamond indicate that the vast majority of diamonds formed within Earth beneath cratonic zones at depths between 150 and 250 km (Figure 1). A much rarer set of inclusions have been identified that extend diamond formation below the upper mantle-lower mantle boundary (670 km) to about 800 km (see Stachel & Harris 2008, Shirey et al. 2013 for reviews). Other minerals—possibly zircon and carbide (see below)—may originate from such depths, but their potential has not yet been fully explored. In comparison, mantle rocks carried to the surface as xenoliths originate from shallower depths (Creighton et al. 2009; see Sautter et al. 1991 for an exception) and usually are not free of reequilibration and alteration.

Once formed, diamond preserves mineral inclusions because of its general chemical inertness; it also preserves its fine detailed internal growth zonations, which can be revealed by cathodoluminescence. Experimental investigations (Koga et al. 2003, 2005) and detailed C isotope mapping (Craven et al. 2009) show that carbon diffusion in diamond is indeed so slow that any isotope variability is likely to be preserved at mantle temperatures over the entirety of Earth's history. Studying this fine detail of diamond is therefore a powerful method to yield direct constraints on

<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>

P-T phase diagram of elemental carbon. The different geotherms (from 38 to 42 mW/m²) illustrate that diamond is stable at pressures of ~45 kbar (corresponding to depths of 150 km) for continental settings and at ~60 kbar for oceanic settings. In subduction zones, as illustrated here by warm and cold subduction gradients, diamond is stable at ~35 kbar. At depths exceeding 240 km, diamond would be rare; carbon would most likely be dissolved in metal phases or as carbide.
the conditions (pressure, temperature, composition, oxygen fugacity) prevailing in the upper and lower mantle.

Stable and Metastable Growth of Diamond

Graphite is the stable form of carbon on Earth's surface and at crustal depths (Figure 1). Diamond occurs because the mineral has a very high metastability. Following a typical continental geotherm (~38 to 42 mW/m²; Pollack & Chapman 1977, Creighton et al. 2009), the intersection with the graphite-diamond phase transition (Kennedy & Kennedy 1976) means that the minimum conditions for diamond formation occur in the mantle at depths exceeding ~150 km (i.e., 4.5 GPa) and temperatures above 950°C (Figure 1). If colder geotherms are considered, such as those typical of subduction zones, diamond could be stable at shallower depths (>3 GPa) and a temperature of 800°C. Along warmer geothermal gradients, such as those typical of the oceanic mantle, diamond would be stable at depths >200 km, where temperatures exceed 1,600°C. It remains unclear, however, whether diamond is stable in the upper oceanic mantle, as slow ascent rate of mantle plumes would allow graphitization/oxidation of the diamond. From the perspective of experimental petrology, diamond is unlikely to be stable at depths exceeding 240 km, as the reducing character of mantle assemblages favors either carbide or carbon dissolved in metal phases (e.g., Frost et al. 2004, Frost & McCammon 2008, Rohrbach & Schmidt 2011), although diamond coexists with carbide under high C/Fe ratios (Dasgupta & Hirschmann 2010). The rare cases of diamonds from the transition zone (n ≈ 150) and the lower mantle (n ≈ 150) would therefore relate to specific and rare growth environments.

Diamond can also grow metastably because it is only a few electron volts less stable than graphite. Thus, there exists a nonzero probability, when a gaseous carbon compound reduces or oxidizes into elemental carbon—for example, when a CO₂-H₂O-rich bubble cools down in a crystallizing magma—that the diamond may nucleate and grow together with graphite. The reports of nanometer-sized diamonds in detonation products and of amorphous graphite intermixed with diamond in meteorites could represent striking examples of this phenomenon (Badziag et al. 1990, Simakov 2011). There are some limitations to comparing natural diamonds with those formed by chemical vapor deposition (CVD), but the principle that diamond can be kinetically (rather than thermodynamically) formed at the expense of graphite forms the basis of the CVD process, which is increasingly used to synthesize large diamonds (e.g., Butler & Woodin 1993). In other words, ultra-small diamonds formed outside their stability field might actually be more abundant than yet recognized (Badziag et al. 1990). Therefore, the occurrence of nanodiamonds in terrestrial samples such as Hawaii melt inclusions (Wirth & Rocholl 2003) or uranium-rich carbonaceous sediments (the formation of the latter is actually not considered to occur at high P and T; Daulton & Ozima 1996) or in extraterrestrial materials (e.g., diamond/graphite/amorphous carbon intermix in ureilites) is not necessarily symptomatic of high pressures (i.e., >4 GPa).

In the present review, we focus on large terrestrial diamonds (i.e., from >1 µm to several centimeters in size); most mantle and metamorphic diamonds studied so far are a few millimeters and tens of micrometers, respectively. For those, formation in their stability field can safely be assumed (but see Simakov 2011 for an alternative view).

In reviewing the stable isotope compositions of diamonds and their inclusions, we avoid unconfirmed diamond reports. Because diamond is widely used for polishing, grinding, and drilling, every rock and laboratory can be polluted by natural/synthetic microdiamonds. Any diamond report must therefore be independently validated, and in situ observations are best. In consequence, we do not consider the report of microdiamonds from Hadean-aged zircons from Jack Hills, Australia (Menneken et al. 2007), because diamond paste was used to polish the samples

**Graphitization:** the strong hybridized sp³ chemical C-C bonds in diamond require high activation energies to graphitize diamond
and other groups have so far been unable to confirm the original findings (Konishi et al. 2008, Dobrzhinetskaya et al. 2014).

DIAMOND TYPES

The recovery in 1905 of the 3,106-carat Cullinan diamond from the Premier (now the Cullinan) kimberlite mine indicates that Earth's mantle is an appropriate environment to grow large diamonds. However, diamonds of such size and quality are not typical; diamond recoveries can vary widely in shape, size, color, and value. For the purpose of this review, we divide diamond into six groups, which reflect specific depth and age formations within Earth, distinct growth environments, or occurrence in metamorphic rocks. The descriptions are based, in a simplified way, on previous diamond classification schemes (e.g., Harris et al. 1975, Orlov 1977, Jeynes 1978). Additional, complementary information can be found within other reviews (Haggerty 1986, 1999; Gurney 1989; Harris 1992; Navon 1999; Taylor & Anand 2004; Shirey et al. 2013).

Xenocryst Diamonds from the Lithospheric Mantle (150–250 km)

Gem- or near-gem-quality, monocrystalline diamonds represent the largest proportion of dia-
monds studied in classification schemes. Physical characteristics such as size, shape, color, plastic
deformation, surface features, geochemistry (including nonradiogenic isotopes), and syngenetic
inclusion content are available. These diamonds are often referred to as octahedrally shaped to
distinguish them from fibrous or polycrystalline diamond types. The group commonly includes
shapes such as dodecahedra and macles (twinned diamonds) as well as octahedra and their re-
sorbed forms (see Robinson et al. 1989). Radiogenic isotope studies on some of the minerals
trapped within these diamonds clearly indicate that the diamonds are xenocrysts within kimber-
lites/lamproites; their genesis ages range from 1.0 to 3.5 Gyr (e.g., Richardson et al. 1984, Shirey &
Richardson 2011, and references therein), whereas the kimberlites are much younger (typically
a few tens of millions to a hundred million years old; see Janse & Sheahan 1995). A xenocrystic
origin is also compatible with their advanced nitrogen aggregation state (IaAB diamonds) and
the occurrence of plastic deformation (see sidebar, Nitrogen Aggregation States in Diamond).
These diamonds grew in mantle rocks of various lithologies (harzburgite, lherzolite, eclogite, and
websterite), as evidenced by the mineralogy and/or chemistry of their inclusion(s) (rarely larger
than 100 µm; Stachel & Harris 2008), which match the compositions of (usually shallower) man-
tle xenoliths. The terms eclogitic, lherzolitic, and harzburgitic diamond thus describe diamonds
formed in eclogite, lherzolite, and harzburgite, respectively. The term peridotitic (P-type) dia-
mond describes diamonds grown in peridotites (harzburgite or lherzolite) as well as diamonds
for which the inclusion chemistry (e.g., olivine, Ni-rich sulfide) does not allow a precise assign-
ment to one of the two subdivisions. Websteritic (W-type) diamonds contain orthopyroxene,
whereas eclogitic (E-type) diamonds do not. Note that the limited database of websteritic dia-
monds is strongly biased toward diamonds from the Orapa mine in Botswana (Gurney et al. 1984,
Deines et al. 1993, Aulbach et al. 2002) and is not considered further in this review.

Xenocryst Diamonds from the Transition Zone (300–660 km)
and Beyond (>660 km)

Only approximately 150 diamonds have been studied from the transition zone, recognized from the occurrence of majorite garnet inclusions (i.e., garnet with excess Si; Moore & Gurney 1985). Nearly all belong to the eclogitic paragenesis and originate from four localities: São Luís/Juína in
## NITROGEN AGGREGATION STATES IN DIAMOND

Nitrogen is the main substitutional diamond impurity (Kaiser & Bond 1959); it forms the basis of the type I (N-bearing) and type II (N-free) diamond nomenclature (Robertson et al. 1934). In diamond, nitrogen occurs as different N-bearing centers; the most abundant nitrogen defects, designated A, B, and C defects, form the basis of diamond classification into IaA, IaB, and Ib types, respectively. A second-order diffusion process (Chrenko et al. 1977, Evans & Qi 1982) leads C centers (single-substitution N defects) originally present in the diamond matrix to migrate after diamond growth to form A defects (N pairs) and subsequently B defects (clusters of four N atoms around a vacancy). The relative proportions of each defect give insights into the time-temperature history of the diamond; it was originally hoped that these proportions could be used to date the diamond's mantle residence time, but they turned out to be too sensitive to temperature (Evans & Harris 1989); N aggregation is actually a good thermometer (Taylor et al. 1990).

Metamorphic diamonds spent short residence times (typically a few million years; Finnie et al. 1994) at rather low temperatures (<1,000°C) and therefore display low aggregation states (Ib-IaA diamonds). Fibrous diamonds and the coats of coated diamonds, because they are related to kimberlite magmatism, also have short residence times (close to 1 Myr; Boyd et al. 1994, Navon et al. 1988) but at higher mantle temperatures, and they consistently display higher N aggregation states (99% are IaA diamonds, and some rare Ib-IaA diamonds are also present). Finally, having spent billions of years at mantle temperatures, xenocrysts of eclogitic and peridotitic diamonds show higher aggregation states (>99.9% are IaA-IaB diamonds). Transition-zone and lower-mantle diamonds are usually deprived in nitrogen (type II), but the few that are type I are characterized by highly aggregated nitrogen (IaB diamonds), reflecting their high-temperature environment.

Brazil (Harte et al. 1999), Kankan in Guinea (Stachel et al. 2000a), and Jagersfontein and Monastery in South Africa (Moore et al. 1991, Tappert et al. 2005). Genesis age dating of the Juína diamonds from the transition zone only suggests that their formation occurred shortly (<200 Myr) before they were brought to the surface (Tappert et al. 2005, Bulanova et al. 2010). These diamonds therefore appear to be much younger than the eclogitic shallow diamonds.

Approximately the same number of diamonds have been studied from the lower mantle (>660 km), Their deep origin is identified from the inclusion content comprising multiple non-touching inclusions of MgSiO₃, (Mg,Fe)O, and, for the two silicate inclusions, CaSiO₃ recognized to be former perovskite-structured minerals (Scott Smith et al. 1984, Harte & Harris 1994). Most inclusions have chemical composition pointing to a protolith of peridotitic composition and, as inferred from the low Al content in MgSiO₃, originate from the uppermost part of the lower mantle; yet, a few samples formed at depths in excess of 800 km (Harte 2010). More recently, four diamonds with inclusions of eclogitic affinity have been identified (Walter et al. 2011), which suggests that protoliths of basaltic composition might occur in the deepest part of the mantle. Lower-mantle diamonds are found primarily in São Luís/Juína in Brazil (Harte et al. 1999, Kaminsky et al. 2001), Kankan (Stachel et al. 2000b), and pipe DO-27, Slave craton, northern Canada (Davies et al. 1999). As with diamonds from the transition zone, genesis ages suggest very short residence times in the mantle of a few million years (Harte & Richardson 2012).

## Fibrous, Coated, and Cloudy Diamonds

Some diamonds consist wholly of an opaque fibrous structure that usually lends the whole diamond a cuboid shape. In other cases, the opaque fibers grow from a substrate of clear eclogitic or peridotitic diamond, often with a very distinct interface. These specimens are referred to as fibrous
and coated diamonds, respectively (Kamiya & Lang 1964). Near-gem-quality coats have been reported (Moore 1985, Boyd et al. 1994), and sometimes the diamond core is small or even absent. The opaque diamonds range from yellow through shades of green to gray at the darkest. Coated diamonds have cubic or (more often) cuboid shapes, but in some productions—such as those from the Democratic Republic of the Congo—octahedra, dodecahedra, macles, and combined forms such as cubo-octahedra are common. Some diamonds also exhibit white to dark-gray cloud-like impurities, with the cloud occurring in the center of the stone. In some of these so-called cloudy diamonds, a distinct internal shape can be identified that is aligned with the principal directions of the host diamond. In others, the cloud appears to be part of the growth stratigraphy.

The fibrous, radial structures of fibrous diamonds and the clouds of cloudy diamonds correspond to numerous microinclusions, 1 to 5 µm in size, trapped during diamond growth (Kamiya & Lang 1964, Navon et al. 1988). On the basis of the geochemical characteristics, composition, and age of the fluids, a consensus has emerged that these diamonds are related (within ~1 Myr) to the (proto)kimberlite magmatism that carried them to the surface (for early discussions on this topic see Boyd et al. 1987, 1994; Akagi & Masuda 1988; Navon et al. 1988). Thus, fibrous/coated stones sample kimberlite volatiles and are genetically different from the most common, older, eclogitic or peridotitic diamonds. To our knowledge, the suggestion made by Boyd et al. (1994) that fibrous diamonds are restricted to group I kimberlites (see Smith 1983) is still valid, although it must be emphasized that similar diamonds were recently reported in the 2.4-Gyr Wawa lamprophyre of Superior craton, Canada (Smith et al. 2012).

Detailed analytical work led to the identification of several fluid end-members, related by immiscibility process(es), ranging from carbonatitic to saline to hydrous-silicic compositions (Schrauder & Navon 1994; Izraeli et al. 2001, 2004; Klein-BenDavid et al. 2004; Kopylova et al. 2010; Weiss et al. 2011). Subsequent Sr-Nd-Pb isotope geochemistry, however, revealed highly variable isotope compositions. Originally taken as evidence that these fluids are genetically unrelated (Klein-BenDavid et al. 2010), the role of assimilation of lithospheric material with extreme isotope composition was recently favored (Weiss et al. 2013, Klein-BenDavid et al. 2014). In this article, we treat fibrous, coated, and cloudy diamonds as a unique group owing to their C and N isotope homogeneity, which is in agreement with rare gas systematics (Ozima & Zashu 1991, Wada & Matsuda 1998).

## Polycrystalline Aggregates of Diamonds

Polycrystalline diamond consists of very fine-grained to medium-grained diamond aggregated together; it has been reported in nontypical diamond environments such as meteorites and other impact-related materials (Haggerty 1999). In kimberlites and linked alluvial deposits, the abundance of polycrystalline diamond varies, from a few percent (e.g., Koffiefontein, Premier, and Finsch mines; see table 3 in Harris et al. 1975) to tens of percent at Jwaneng and Orapa mines (Robinson et al. 1984). Compared with single crystals, diamond aggregates have not been studied in detail. Terms including boart, ballas, carbonado, stewartite, and framesite are mentioned without any common criteria of classification. Among few others, Orlov (1977) proposed that the textural aspect of diamond aggregation (granular, fibrous, cryptocrystalline) is related to diamond growth conditions (temperature, pressure, and supersaturation conditions). The author determined three polycrystalline diamond families: ballas (spherical fibrous), boart (granular aggregate), and cryptocrystalline carbonado. Because carbonado is well known to represent a specific diamond type (see below), this classification is weakened. Apart from carbonado, other polycrystalline aggregates are of special interest as they often contain mineral inclusions from Earth's mantle. It is commonly assumed that the crystallization of these inclusions is contemporaneous with the formation of the
diamond (Jacob et al. 2000, Heaney et al. 2005), though this assumption was recently questioned (Jacob et al. 2011).

Polycrystalline diamond is commonly interpreted as representing rapid crystallization in localized areas of carbon oversaturation, which allows high nucleation rates (Sunagawa 1990). Although no genesis ages are yet available, some authors (Jacob et al. 2000, Heaney et al. 2005) suggest that polycrystalline diamond aggregates should have relatively young ages, although available C and N geochemical data instead suggest similarities to older octahedrally shaped diamonds (Thomassot et al. 2009a), in particular websteritic diamonds (Mikhail et al. 2013). Occurrence of plastic deformation in these diamonds provides further support for a relatively long residence time in the mantle (DeVries 1973).

## Carbonado

Carbonado is a polycrystalline diamond type with sintered diamond grains; it has never been observed in a volcanic rock. It occurs mostly in Brazil and the Central African Republic metaconglomerates; its crustal inclusions may be primary or, given their high porosity, secondary. The origin of carbonado remains an enigma; models of formation range from impact related (Smith & Dawson 1985) to extraterrestrial (Garai et al. 2006, Haggerty 2014) to crustal (Kaminsky 1991) to mantle derived (Kagi & Fukura 2008, Cartigny 2010). For a detailed description, the reader is referred to Haggerty's most recent review (Haggerty 2014).

## Metamorphic Diamonds

Metamorphic diamonds are those found in situ in crustal rocks subducted at high pressures, and they are increasingly recognized on Earth (see Frezzotti et al. 2011, Brown 2007 for reviews). The largest such diamonds (up to 100 µm in situ and as large as 500 µm in some associated alluvial deposits) that have been described and studied geochemically come from the Kokchetav massif (Sobolev & Shatsky 1990, Finnie et al. 1994). At Kokchetav, the diamonds are typically protected from graphitization/oxidation during retrograde metamorphism by zircon and garnet. The original possibility that these might form metastably is unlikely (but again see Simakov 2011), and variations in diamond abundance and/or morphology from one lithology to another demonstrate that the diamonds were not deposited in sediments before subduction (De Corte et al. 1998). Metamorphic diamonds thus offer a unique window into the geochemistry of deeply subducted materials (e.g., Cartigny et al. 2001a, Hwang et al. 2006, Sobolev et al. 2011), although Sumino et al.'s (2011) recent study emphasized the contribution of mantle volatiles. Due to the typically small (micrometer) size of metamorphic diamonds, most studies remain limited to diamonds from the Kokchetav massif.

# A TERRESTRIAL DIAMOND PARADIGM: DIAMONDS GROW FROM A MOBILE C-O-H-N-S FLUID/MELT PHASE THROUGH OXIDOREDUCTION REACTIONS

## Diamonds Grow from a C-O-H-N-S Fluid/Melt

Within the past 15 years, a broad scientific consensus has emerged in recognition of the fundamental role of a mobile C-O-H-N-S fluid/melt in diamond formation in the mantle, via either reduction of oxidized carbon or oxidation of a reduced phase such as methane. An alternative, but much less favored, mechanism would be diffusion-related solid-state growth. Diamond, therefore, is a metasomatic mineral. The evidence for this conclusion is considerable. Examples include the
**Carbon isotope composition:** because variations are very small, carbon isotope composition is expressed as per mill deviation from the international standard (V-PDB)

**Nitrogen:** the main substitutional diamond impurity

chemical variability of mineral inclusions in a single diamond (Sobolev et al. 1998); sinusoidal rare earth element patterns in mineral inclusions (see Taylor et al. 1996, Stachel et al. 2004 for reviews); metasomatic inclusions within diamonds (Schrauder & Navon 1993, Leost et al. 2003, Sobolev et al. 2009); strongly zoned N aggregation states, requiring several growth episodes separated by protracted time periods (e.g., Swart et al. 1983, Boyd et al. 1994, Fitzsimons et al. 1999); diamonds within a xenolith along veins enriched in other fluid-mobile elements (Schulze et al. 1996, Anand et al. 2004, Liu et al. 2009); polycrystalline aggregates of diamonds, requiring extensive carbon supersaturation (Sunagawa 1990); and marked zonations in C isotope composition (Fitzsimons et al. 1999, Palot et al. 2012, Zedgenizov et al. 2014).

This evidence further indicates that diamond growth occurs in an open-system environment and that diamond and its inclusions record a multistage process or processes associated with significant changes in chemical composition and possibly oxygen fugacity. Note that metamorphic diamond formation (De Corte et al. 1998, Hwang et al. 2006) and unconstrained carbonado (Kagi & Fukura 2008; but see Haggerty 2014) are also considered to result from metasomatic fluids. The only exceptions to metasomatic diamond growth relate to the formation of impact diamonds (Hough et al. 1995) and the formation of nanodiamonds from carbonaceous U-rich sediments (Daulton & Ozima 1996), for which solid-state growth is more likely.

## Oxidoreduction Processes and the Speciation of Carbon in the C-O-H-N-S Fluid/Melt

The growth of diamond through an oxidoreduction process or processes is easy to postulate but less straightforward to demonstrate. The alternative view, outlined above, is that diamond precipitates from elemental carbon dissolved in a C-O-H fluid/melt. As the fluid/melt rises, it supersaturates with respect to elemental carbon, precipitating diamond (or graphite at lower pressures)—a process that occurs without carbon reduction or oxidation. This process forms the basis of synthetic diamond production, commonly from supersaturated elemental carbon systems (typically nickel-iron alloys), some with compositions resembling fibrous diamond growth medium (saline, carbonatitic, ultrapotassic compositions; see Litvin 1998; Pal'yanov et al. 1999, 2002; Bureau et al. 2012). But for reasons detailed below, there is little evidence that this process is widespread in nature.

The role of oxidoreduction in diamond formation is best evidenced by core-to-rim variations of ¹³C/¹²C ratios (i.e., carbon isotope composition) in relationship with other tracers (primarily N content and N isotope composition). At isotope equilibrium, diamond is enriched in ¹³C, by 2–3‰ at T ≈ 1,000°C compared with oxidized carbon (CO₂, carbonate). It is depleted in ¹³C by approximately 1‰ at T ≈ 1,000°C when precipitated from methane. Diamond precipitating from dissolved carbon would be depleted by only a few tenths of a per mill (Figure 2).

Isotope trends in C isotope compositions recorded either within a single diamond or within a diamond population highlight either a decrease or increase in C isotope composition. Diamond formed from oxidized carbon exhibits a core-to-rim increase in ¹³C (Figure 3, case A) and is recognized in both fibrous and octahedrally shaped diamonds (Boyd et al. 1994, Zedgenizov et al. 2006, Smart et al. 2011, Palot 2013); it can also be recognized from the positive skewness of some peridotitic diamond δ¹³C distributions (Deines 1980, Stachel & Harris 2009). In contrast, diamond formed from methane would exhibit a core-to-rim decrease in ¹³C (Figure 3, case B) (Thomassot et al. 2007). In this case, the δ¹³C distribution would exhibit a negative skewness (Deines 1980).

The use of δ¹³C-N relationships is also important but depends heavily on our understanding of the incompatible versus compatible behavior of nitrogen during diamond growth (for discussions
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>

C isotope fractionation among phases. Equilibrium fractionation data among C-bearing phases have been reported for the main phases of geological interest, including carbonates, CO₂, CH₄, diamond/graphite, and carbides. These data are derived from either theoretical calculations or laboratory and empirical measurements. Full agreement between the different estimates is rare, the methods being complementary rather than redundant. The figure illustrates the available data, and the y-axis, showing the scale of the fractionation, is better seen with some inaccuracy (see discussion in Thomassot et al. 2007, Smart et al. 2011). Data are mostly lacking at high temperatures (including for systems involving diamond) and are extrapolated here. The reader is referred to the excellent review by Chacko et al. (2001), and references therein, for more detailed discussion and data sources. Data for iron carbides are from Schauble (2009) and V.B. Polyakov & J. Horita (personal communication). It should be noted that, as for most stable isotope systems, the most reduced species are, at isotope equilibrium, depleted in ¹³C, such as carbide < methane < diamond/graphite < CO₂/carbonate. Furthermore, because stable isotope fractionation primarily depends upon change in redox state, the isotope fractionation between graphite, diamond, and elemental carbon dissolved in a melt (not yet available) will be small compared with other C-bearing phases. Every curve should pass through zero, as the stable isotope fractionation is zero at infinite temperature.

see Cartigny et al. 2001b, Palot et al. 2009, Stachel et al. 2009, Smart et al. 2011). This is an open issue, as the most recent evidence (see also **Figure 3**) suggests that the behavior of nitrogen in diamond may be both compatible and incompatible (Smart et al. 2011, Palot 2013). This possible dual role for nitrogen in diamond probably reflects the diversity of the initial composition(s) of metasomatic C-O-H-N-S fluids and their variability/evolution, in particular in carbon-to-water ratios. In this respect, more experimental data would be useful. For example, diamond formation from carbonate reduction has been achieved experimentally (Arima et al. 2002), but diamond formation from methane oxidation has not.
<figure><img src="image_5.png" /></figure>
Although rarely possible, the speciation of carbon from which eclogitic and peridotitic diamond(s) precipitated can be recognized from the mineralogy of some specific mineral inclusions (Schrauder & Navon 1993, Chinn et al. 1995, Leost et al. 2003, Izraeli et al. 2004). Fluid(s) trapped in fibrous/coated diamonds (e.g., Navon et al. 1988, Zedgenizov et al. 2006) all point to diamond formation from oxidized carbonate/CO₂-bearing systems. The occurrence of syngenetic carbonate inclusions in diamond reflects growth at the CCO oxygen fugacity buffer, although in this case, the reaction may be linked to either a reduced or oxidized C-O-H-N-S fluid/melt in a more oxidized or reduced system (see Pal'yanov et al. 2013, for example). For diamonds studied from the deeper mantle, complex growth patterns do not reveal any obvious isotope trends (Bulanova et al. 2010, Walter et al. 2011, Zedgenizov et al. 2014); this may reflect changing mantle pressure, temperature, and time of diamond formation; lack of spatial resolution for these complex samples (Figure 3, case D); or both. Assuming that the mantle below 240 km is metal saturated (Frost et al. 2004, Rohrbach et al. 2007), rare carbonate inclusions in diamonds deemed to have formed at these depths (Brenker et al. 2005, Walter et al. 2008, Rohrbach & Schmidt 2011) are clearly compatible only with reduction of carbonated melts. The occurrence of carbide and Fe metal (Walter et al. 2011, Kaminsky 2012) would be typical of the reduced conditions prevailing in the deeper mantle.

CCO oxygen fugacity buffer:
CCO denotes the equilibrium redox reaction $C + O_2 = CO_2$

## Implications of Diamond Metasomatic Growth and the Nature and Composition of the C-O-H-N-S Fluid/Melt

The exact relationship or relationships of diamond to its mineral inclusions remain to be addressed, as syngenicity cannot be assumed a priori, even when the morphological evidence provides a strong case for that relationship (e.g., Mitchell & Giardini 1953, Harris & Gurney 1979). For example, a mineral inclusion could be preexisting (i.e., protogenetic) but could then be dissolved/reprecipitated as the fluid penetrated the host rock before being enclosed in the newly formed diamond (the inclusion would be protogenetic in terms of some elements, syngenetic in terms of morphology, and in chemical equilibrium in terms of some trace elements). Alternatively, the inclusion could fully relate to the metasomatic agent (i.e., sygenetic) in that its major and trace elements mostly derive from the C-O-H-N-S fluid/melt.

One implication is that it may or may not be possible to relate the trace element budget of the inclusion to the diamond growth medium (e.g., Stachel et al. 2004), because it may rather reflect

### Figure 3

The upper diagram illustrates a transect through a single growth zone of a hypothetical heterogeneous diamond. Dashed lines illustrate the growth of the diamond, and blue areas illustrate new growth episodes after the diamond was resorbed in the mantle. Within this single growth zone, the lower-left panels illustrate different C isotope core-to-rim variations induced by diamond precipitation from oxidized carbon (case A; modified from Boyd et al. 1994, Smart et al. 2011) and from reduced carbon (case B; modified from Thomassot et al. 2007). Case C illustrates the hypothetical C isotope variability of fluids prior to diamond precipitation, with diamond precipitation playing a negligible role (modified from Cartigny et al. 2001a). In case D, diamond forms from a continuous supply of minute amounts of an isotopically homogeneous fluid of which the resolution of analysis does not allow detection. The lower-right panels illustrate some natural evidence associated with each case. For several of these cases, the evolution in N content was key in deciphering the behavior of nitrogen during diamond growth. ΔC refers to equilibrium isotope fractionation between the fluid and the diamond (see Figure 2). Cases A, B, and D reflect isotope variations induced by diamond precipitation, whereas case C reflects isotope variations primarily induced as a result of fractionations occurring in the mantle. Also note that evidence for case A was inferred from a single diamond, evidence for case B was inferred from a suite of contemporary diamonds, and evidence for case C was inferred from the sector limit toward decreasing C isotope compositions defined for worldwide diamonds by Cartigny et al. (2001a). In the right-hand panel for case C, the upper curve reflects fluid evolution induced by diamond precipitation from oxidized fluids (i.e., similar to case A) as inferred from the sector limit toward increasing C isotope compositions defined for worldwide peridotitic diamonds by Stachel et al. (2009). From Cartigny et al. (2001a), Thomassot et al. (2007), and Smart et al. (2011); adapted with permission from Elsevier.
**Primordial carbon:**
carbon that would
have never been cycled
by plate tectonics and
may or may not be
isotopically
heterogeneous

a previous metasomatic enrichment, as is common within the lithospheric mantle (e.g., Pearson
& Wittig 2008). The lack of relationship between the major elements of a mineral inclusion
and the geochemistry of the diamond host favors a strong decoupling between them (see Deines
et al. 1993 and references therein). A similar conclusion was made on the basis of the contrasting
evidence produced by the S isotope geochemistry of the sulfide inclusions and the C and N isotope
geochemistries of the host diamonds (Deines & Harris 1995, Thomassot et al. 2009b).

Besides fluids trapped in fibrous/coated diamonds, little is known about the exact nature of the diamond growth medium (hence the vague wording, C-O-H-N-S fluid/melt, commonly used in the literature). Some have suggested that the fluid end-members recognized from the study of fibrous/coated diamonds could be involved in the formation of the more abundant and older octa-hedrally shaped diamonds (Navon 1999, Izraeli et al. 2004), but this remains to be demonstrated. At least, the identification of reduced, methane-bearing fluids (Thomassot et al. 2007) illustrates more diverse compositions. The lack of experimental studies involving reduced fluid(s) repre-sents the main difficulty in addressing the nature and behavior of these C-O-H-N-S fluids and determining how diverse the composition of reduced C-O-H fluids might be. In this respect, the analysis of trace elements in diamonds (Rege et al. 2008, 2010; McNeill et al. 2009) will certainly provide new insights.

CARBON ISOTOPES IN ECLOGITIC DIAMONDS AND THEIR
POTENTIAL RELATIONSHIPS TO CHEMICAL GEODYNAMICS

The question is not, is carbon (massively) subducted? Rather, the question is, can we record the isotope heterogeneity introduced in Earth's mantle by subduction and distinguish it from the heterogeneity related to intramantle processes (e.g., metasomatism)? The third possibility that eclogitic and peridotitic diamonds record primordial carbon heterogeneities (e.g., Deines et al. 1991, 1993; Haggerty 1999) now seems unlikely and is not discussed further here (see Palot et al. 2012 for a recent discussion).

Key Arguments in Addressing Carbon Isotopes in Eclogitic Diamonds

As shown by **Figure 4**, C isotopes vary greatly (from −41‰ to +5‰ in δ¹³C) among diamonds occurring worldwide and differ significantly from one paragenesis (or type) to the next in both range and skewness. Most values are contained within a narrow interval of −8‰ to −2‰, centered on a value of approximately −5‰ ± 1‰; this range is similar to that displayed by other mantle products such as mid-ocean ridge basalts, ocean island basalts, carbonatites, and kimberlites (e.g., Javoy et al.

**Figure 4**

Comparative histograms of δ¹³C values of (a) diamonds of known and unknown paragenesis from locations worldwide, (b) eclogitic diamonds with silicate inclusions, (c) peridotitic diamonds with silicate inclusions, (d) eclogitic diamonds with sulfide inclusions, (e) peridotitic diamonds with silicate inclusions, (f) fibrous/coated diamonds, (g) diamonds from the lower mantle, (h) metamorphic diamonds, (i) transition-zone diamonds, and (j) sedimentary carbon represented by carbonates and organic matter. The mantle range, defined by the study of fibrous diamonds, mid-ocean ridge basalts, carbonatites, and kimberlites, is also indicated. Eclogitic (Ni-poor) and peridotitic (Ni-rich) sulfide-bearing diamonds are highlighted, as the most recent studies have noted very distinct isotope distributions compared with those of silicate-bearing diamonds of the same paragenesis, suggesting a different mode of formation, although the details remain to be investigated. As sulfide-bearing diamonds can predominate, available distributions may be biased. Note that with the exception of transition-zone diamonds, every distribution displays a mode within the mantle range. Updated from Cartigny (2005). Data sources and data are available in the **Supplemental Material** (follow the **Supplemental Material link** in the online version of this article or at http://www.annualreviews.org/). Updated from Cartigny (2005, 2010), adapted with permission from the Mineralogical Society of America and Elsevier, respectively.

<figure><img src="image_6.png" /></figure>

**Supplemental Material**
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>
**Mantle carbon:**
carbon that has already been cycled via plate tectonics; considered to be isotopically homogeneous as a result of mantle convection

**Subducted carbon:**
recycled carbon assumed to be isotopically heterogeneous due to the occurrence of two distinct carbon pools in the subducted slab

1986, Cartigny et al. 1998b, and references therein). Any individual mine displays very similar δ¹³C distributions to those illustrated by the worldwide eclogitic and peridotitic distributions. When viewed in detail, however, the mode can differ significantly; some diamond populations, especially those from Central and West Africa, show a mode at -3.5‰. This may be significant or may reflect sample bias introduced during sample selection. Recent evidence that sulfide-bearing diamonds display distinct carbon isotope distributions compared with silicate-bearing diamonds from the same mine and of the same paragenesis (Figure 4) (Thomassot et al. 2009a, Cartigny et al. 2009; see also Deines et al. 1987 for an earlier emphasis) and that microdiamonds and macrodiamonds from the same mine can display significant distributions (with mode and median differing by 0.7‰ and 1‰ respectively; Johnson et al. 2012) leads us to consider these different δ¹³C modes as a second-order aspect.

**Figure 5** illustrates some striking exceptions to the C isotope worldwide distributions outlined in **Figure 4**, such as eclogitic diamond distributions that have a very negative δ¹³C mode and/or are bimodal. These are discussed at the end of this review. The key observation, originally made by Sobolev et al. (1979), is that diamonds belonging to the eclogitic and peridotitic parageneses are distinct, even though they predominantly form over similar P-T ranges (Stachel & Harris 2008). Many authors have concluded that the sources of carbon among peridotitic and eclogitic diamonds must be distinct (e.g., Sobolev et al. 1979, Kesson & Ringwood 1989, Kirkley et al. 1991, Nisbet et al. 1994). Peridotitic—and, by inference, fibrous/coated peridotitic—lower-mantle diamonds are formed from mantle carbon, i.e., carbon that originates from Earth's convective mantle. In contrast, eclogitic diamonds are likely related, at least in part, to subducted carbon. Subducted sediments are composed of a mixture of organic carbon (which becomes anthracite and subsequently graphite as pressure increases) and carbonates, with long time-averaged δ¹³C values of -25‰ and 0‰, respectively (reviewed in Schidlowski 2001 and Veizer & Hoefs 1976, respectively); these sediments show a δ¹³C range somewhat similar to that of eclogitic diamonds.

This model is not free of difficulties (see Cartigny et al. 1998b for a critical view). For example, C isotope heterogeneity in sediments is commonly erased by diffusion at temperatures <700°C (Dunn & Valley 1992); the preservation of δ¹³C values <-25‰ thus requires the virtual absence of subducted carbonates, which represent ~80% of the subducted carbon (see the δ¹³C distribution for metamorphic diamonds in **Figure 4**). Also, δ¹³C values below -40‰ are virtually absent within the sedimentary geological record (De Stefano et al. 2009, Smart et al. 2011), as they are preserved in the mantle only under very specific conditions. However, eclogitic diamond formation in a subducted environment is compatible with independent evidence that some (but not all) eclogite nodules represent Archean recycled oceanic crust (see Jacob 2004 for review), although melting of that crust during subduction and subsequent metasomatic diamond formation might lead to decoupling between the diamond and the eclogite.

If eclogitic and peridotitic diamonds precipitated from exactly the same C-O-H-N-S fluids, they would display the same ¹³C/¹²C distributions, and the range would be narrow (a few per mill). If most eclogitic diamonds precipitated from methane and most peridotitic diamonds crystallized from carbonate (or vice versa; e.g., Stachel et al. 2009, Stachel & Harris 2009), they should have distinct C isotope distributions and, in particular, skewness; however, this possibility is not supported by the study of fluid inclusions and trace elements in diamonds, which has revealed very similar chemical compositions (e.g., Rege et al. 2010; see above). Therefore, the concept emerged that eclogitic diamonds passively record the variable C isotope compositions of C-O-H-N-S fluids in the mantle, rather than producing these isotope compositions during crystallization (Javoy et al. 1986, Galimov 1991). Galimov (1991) postulated that the distinct C isotope distributions of eclogitic versus peridotitic diamonds must result not from distinct carbon sources but instead from specific mechanisms that occur in the mantle, but the nature of these mechanisms has remained
<figure><img src="image_9.png" /><figcaption>**Figure 5**</figcaption></figure>

Comparative histograms of diamond δ¹³C values for unusual mines including (a) Guaniamo, Venezuela; (b) Argyle, Australia; (c) New South Wales, Australia; (d) Jagersfontein, South Africa; (e) Dachine, French Guiana; and (f) Jericho, northern Slave craton, Canada. Note that unlike eclogitic diamonds, peridotitic diamonds still lie in the so-called mantle range. Data sources and data are available in the **Supplemental Material** (follow the **Supplemental Material link** in the online version of this article or at http://www.annualreviews.org/).

<figure><img src="image_10.png" /></figure>
unclear. A central consideration highlighted by Galimov (1991) is that eclogitic and peridotitic
diamonds have the same δ¹³C mode and thus require the same source, with the exceptions outlined
in **Figure 5**. This is a very strong, often neglected, argument that is increasingly verified. For
example, in the most detailed study of diamonds from De Beers Pool mines in South Africa, the
different diamond subpopulations (harzburgitic, lherzolitic, eclogitic, websteritic, or with sulfide
inclusions of eclogitic affinity, etc.) have δ¹³C modes within a 0.6‰ range; this is unlikely to
be a coincidence (**Figure 6**). A similar conclusion was made when comparing lherzolitic and
harzburgitic diamonds from locations worldwide (see figure 6 in Stachel & Harris 2009).

Therefore, instead of implying multiple sources, the distinct eclogitic and peridotitic δ¹³C iso-
tope distributions could result from peculiar evolution of carbonate-bearing (i.e., oxidized) fluids
in each environment (Cartigny et al. 1998a, 2001a), primarily as a consequence of decarbonation
reactions (which are restricted to eclogitic compositions, reflecting the lack of olivine in those
rocks) and the fluid H₂O/CO₂ ratio. Because carbonate decarbonation is associated with ¹³C/¹²C
fractionation (CO₂ is ¹³C enriched; see **Figure 2**), this additional process can account for the in-
creased C isotope variability found in eclogitic diamonds. Although this model is based on previous
experimental work (Luth et al. 1993, Knoche et al. 1999) and is receiving increasing consideration
(e.g., Stachel & Harris 2009), it is important to emphasize that the proposals rely on a series of
limited experiments—in particular, experiments involving methane have not yet been conducted.
So, if the mineralogical control of eclogite/peridotite is a viable explanation to account for the
distinct δ¹³C distributions of these two types of diamond, then further experimental work may
highlight new reactions that better define diamond δ¹³C variability.

Finally, if eclogitic diamonds relate in some way to subduction processes, a relatively small mo-
bility scale (typically tens of meters) for C-O-H-N-S fluids/melts is implied, whereas if peridotitic
and eclogitic diamonds form from mantle-related fluids/melts, a larger mobility scale (typically
tens of kilometers) can be inferred.

Mantle Nitrogen Isotope Signatures in Diamonds

Interest in studying diamond nitrogen isotope compositions arose from the recognition that mantle
nitrogen is deprived in ¹⁵N compared with present surface reservoirs (75‰ in the atmosphere
and 25‰ in crust and sediments with δ¹⁵N = 0‰ and approximately +6‰, respectively) (Javoy
et al. 1986), which makes N isotopes a powerful tracer of recycled material in Earth's mantle
(see sidebar, On the Use of Nitrogen). Subsequent data obtained during the past 20 years (see
Cartigny & Marty 2013 for a review) have strengthened this picture, in particular by extending
the N isotope geological record to the Archean mantle and crust and the present-day altered
oceanic crust and lithosphere (Figure 7). Overall, crustal/sedimentary nitrogen is enriched in
¹⁵N, by approximately +5‰ compared with the atmosphere, whereas mantle nitrogen is depleted
by approximately −5‰ (Figure 7). During sediment subduction, devolatilization (if any) would
preferentially release ¹⁴N, further enriching subducted material in ¹⁵N (Bebout & Fogel 1992,

Figure 6

Comparative histograms of diamond δ¹³C values of individual parageneses from the DeBeers Pool mines (South Africa). Note that all display δ¹³C modes within the same 0.6‰ range. This is common among usual diamond mines, a finding that led Galimov (1991) to suggest that eclogitic and peridotitic diamond likely originate from the same isotope source. In the bottom diagram, lherzolitic and moderately subcalcic diamonds are plotted in blue and gray, respectively. Data sources and data are available in the **Supplemental Material** (follow the **Supplemental Material link** in the online version of this article or at http://www.annualreviews.org/).

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /></figure>
ON THE USE OF NITROGEN

The study of, for example, point defects—substitutional or interstitial impurities trapped within the diamond lattice of octahedrally shaped diamonds—relies on the idea that these would have the same origin as the carbon forming the diamond, which might not be the case for mineral inclusions. Although diamond is almost pure carbon, more than 80 elements have been detected in the crystal structure. Among them, nitrogen, oxygen, and hydrogen (which occur at parts-per-million levels) are the most abundant (see Sellschop 1992). Nitrogen was recognized early on as the main diamond impurity and remains the only element that has been systematically studied.

Busigny et al. 2003) and strengthening the isotope contrast. Data on metamorphic diamonds (Cartigny et al. 2001a, 2004a) also support these statements (Figure 7).

Because nitrogen in sedimentary/crustal rocks occurs as ammonium ions substituting for potassium (Busigny & Bebout 2013), the behavior of nitrogen in subduction zones is expected to follow the fate of potassic minerals, which have been experimentally demonstrated to be stable to mantle depths (Watenphul et al. 2010). Larger N contents in metamorphic diamonds (by more than one order of magnitude compared with peridotitic and eclogitic diamonds) demonstrate that nitrogen survives devolatilization at depth >3 GPa and indicate that significant amounts of nitrogen are presently subducted (Busigny et al. 2011).

Nitrogen isotopes are fractionated by only a few per mill during incorporation into diamond (Thomassot et al. 2009b); diamond is therefore representative of its growth medium, as demonstrated by the striking similarities between fibrous/coated diamonds and mid-ocean ridge basalts (Figure 7). Figure 7 shows that if eclogitic diamonds form from subducted carbon and nitrogen, the nitrogen should display positive δ¹⁵N values, which is not the case: Worldwide, eclogitic diamonds often show negative δ¹⁵N values. This distribution is also grossly similar to that for peridotitic diamonds (Figure 7) and for diamonds from the lower mantle (Figure 7) (Palot et al. 2012), implying that the latter cannot directly form from subducted carbon. The δ¹³C-δ¹⁵N plots in Figure 8 also illustrate how closely eclogitic diamonds resemble peridotitic diamonds. Negative, mantle-like N isotope compositions are typically found in about 50% of diamonds with low δ¹³C values (Figure 8), which is again inconsistent with values found in subducted organic carbon. Modeling of mixing between mantle-related and subduction-related carbon has illustrated the impossibility of matching associated diamond N contents (e.g., Cartigny et al. 2004b and references therein). This is because diamonds outside the main mantle array (δ¹⁵N ≈ -5‰ ± 3‰; δ¹³C ≈ -5‰ ± 2‰) typically have low nitrogen contents; mixing relationships would predict hyperbolic curves that do not fit the available data well (Figure 8) (see also Cartigny et al. 1998a, 2004a).

On the basis of linear relationships of δ¹³C with δ¹⁵N and with the logarithm of N contents observed in several eclogitic suites, it was suggested that the very negative δ¹³C values associated with positive δ¹⁵N values highlight open-system evolution of metasomatic fluids (Figure 8) (Cartigny et al. 2001b). There is, however, some scatter, and this model relies on a limited understanding of processes controlling N uptake in diamond (see above and the opposing views developed in Cartigny et al. 2001b, Palot et al. 2009, Stachel et al. 2009, Smart et al. 2011); this suggestion may therefore need to be reevaluated. Another limitation in fully developing the use of nitrogen isotopes in diamonds is that the speciation of nitrogen in mantle fluids is unknown. For example, the low levels of nitrogen in deep diamonds (i.e., most lower-mantle and transition-zone diamonds are type II; e.g., Deines et al. 1991, Harte et al. 1999, Tappert et al. 2005) may reflect low mantle oxygen fugacities and sequestration of most nitrogen either in nitrides or dissolved in carbides.
<figure><img src="image_13.png" /><figcaption>**Figure 7**</figcaption></figure>

Comparative histograms of δ¹⁵N values of modern and ancient sediments, subducted nitrogen (warm and cold environments), present-day convective mantle, ancient mantle, and lower mantle (from Cartigny & Marty 2013). The few δ¹⁵N data for the oceanic lithosphere (metaperidotites, metagabbros) are not illustrated here, but it is worth noting that available data are all positive, which, in comparison with altered oceanic crust, most likely reflects early enrichment in ¹⁵N during subduction. Data sources and data are available in the **Supplemental Material** (follow the **Supplemental Material link** in the online version of this article or at http://www.annualreviews.org/). Adapted from Cartigny & Marty (2013), with permission from the Mineralogical Society of America.
<figure><img src="image_14.png" /><figcaption>**Figure 8**</figcaption></figure>

δ¹³C-δ¹⁵N distributions. (a) Silicate-bearing peridotitic (contour plot) and sulfide-bearing diamonds (red dots). (b) Silicate-bearing eclogitic diamonds together with two hypothetical mixing curves between a mantle end-member (δ¹⁵N = -5‰ and δ¹³C = -5‰) and a recycled end-member (δ¹⁵N = +9‰ and δ¹³C = -25‰). The mixing curves labeled 1 and 2 illustrate C/N mantle-to-C/N recycled ratios of 4 × 10⁻² and 8, respectively. One value for a silicate-bearing eclogitic diamond from Namibia (δ¹⁵N ≈ -2‰ and δ¹³C ≈ -38.5‰) is not shown. (c) Fibrous/coated diamonds occurring worldwide (contour plot) and metamorphic diamonds from Kokchetav (blue dots). (d) Sulfide-bearing eclogitic diamonds. Note that most diamonds point toward the same δ¹⁵N-δ¹³C field, highlighting striking similarities between peridotitic, eclogitic, and fibrous diamonds. Peridotitic, eclogitic, and fibrous diamonds that fall within the mantle range tend to have the highest N contents, whereas diamonds departing from the mantle window tend to have lower N contents. The mixing curve predicted to account for the low N contents of eclogitic diamonds that have low δ¹³C values is illustrated by curve 2, whereas curve 1 would be required to account for the N-rich character of truly subducted metamorphic diamond. The latter curve would not provide a good fit for any of the observed trends, and the former curve would be able to account only for a few of the available observations. Standard: PDB, Pee Dee Belemnite.

## Subduction Signatures from Inclusions in Diamond

Further insights into diamond formation can be inferred from the stable isotope compositions of oxygen and sulfur in silicate and sulfide inclusions. Oxygen isotopes are a particularly powerful tracer of recycled material in Earth's mantle, as altered oceanic crust and sediments display very variable oxygen isotope compositions (typically δ¹⁸O from +3‰ to more than +12‰; see Ickert et al. 2013 and references therein) compared with the very slight mantle variation (δ¹⁸O ≈ +5‰ to +6‰; see Eiler 2001 for a review). The ¹⁸O/¹⁶O isotope variability of eclogitic inclusions (δ¹⁸O from +2.4‰ to +16.0‰) (Schulze et al. 2003, 2013; Anand et al. 2004; Ickert et al. 2013) contrasts with the mantle homogeneity displayed by both peridotite xenoliths (see Eiler 2001 for a review)

**Oxygen isotope compositions:** expressed as per mill deviation from Standard Mean Ocean Water (SMOW)
and peridotitic inclusions in diamond (Lowry et al. 1999). Although often equilibrated at lower pressure and temperature, eclogite xenoliths have O isotope compositions (see Jacob 2004 for a review) consistent with those measured among eclogitic inclusions in diamond (see **Figure 9**). Thus, oxygen isotopes indicate that eclogitic mantle material is probably derived from altered oceanic crust.

Owing to metasomatic diamond growth and the possible decoupling between the geochemical characteristics of diamond and its inclusion(s) $\delta^{18}\text{O}-\delta^{13}\text{C}$ relationships are of particular interest. Diamond silicate inclusions with high $\delta^{18}\text{O}$ values are generally enclosed in diamond displaying depleted $^{13}\text{C}$ isotope compositions (**Figure 10**) (Ickert et al. 2013, Schulze et al. 2013). However, the data set is presently dominated by data from two unusual diamond mines (Guaniamo in Venezuela and Argyle in Australia; see **Figure 6**) and generally lack diamond with mantle $\delta^{13}\text{C}$. Yet, data from the Orapa kimberlitic field (Ickert et al. 2013) span a range of 15‰ in $\delta^{13}\text{C}$ and illustrate mixing between a mantle ($\delta^{18}\text{O} \approx +5\%$; $\delta^{13}\text{C} \approx -5\%$) and a subducted ($\delta^{18}\text{O} > +7\%$; $\delta^{13}\text{C} < -20\%$) end-member, which supports coupling between C and O systematics.

Despite clear evidence for the occurrence of a subducted component, several aspects remain to be clarified to fully develop the use of oxygen isotopes from inclusions in diamond. In particular, the mode and range of available $\delta^{18}\text{O}$ values from inclusions are distinct, by ~2‰, from those of either altered oceanic crust or eclogite nodules (**Figure 9**). Furthermore, the curvature in the $\delta^{18}\text{O}-\delta^{13}\text{C}$ diagram of **Figure 10** requires the subducted component to have a C/O ratio lower than that of the mantle end-member. Given that the oxygen content of rocks varies little, we can infer that the subducted component has a C content >10 times lower than that of the mantle end-member, which is at odds with the general view of a C-rich subducted component. This is, however, consistent with C-rich mantle-related fluid(s) introduced into eclogite (but not the reverse). An intriguing implication of such a correlation is that mantle fluids buffer the oxygen isotope composition of most diamond inclusions that have mantle $\delta^{13}\text{C}$, in contrast with the common view that mantle assemblages buffer fluids. This could explain the large variability in $\delta^{18}\text{O}$ (up to 6.2‰; Anand et al. 2004). Finally, the anticorrelation of inclusion $\delta^{18}\text{O}$ with host diamond $\delta^{13}\text{C}$ suggests strong coupling between these two elements but is currently difficult to reconcile with $\delta^{13}\text{C}-\delta^{15}\text{N}$ covariations of eclogitic diamonds from the same kimberlite field (Ickert et al. 2013, p. 94). In this respect, new insights would emerge if the oxygen isotope composition of oxygen of mantle fluids, as seen, for example, in fibrous diamonds, could be constrained.

Eclogitic sulfide inclusions also display variable $^{34}\text{S}/^{32}\text{S}$ ratios ($\delta^{34}\text{S}$ from -13‰ to +14‰; Chaussidon et al. 1987, Eldridge et al. 1991) that compare well with those of sediments and altered oceanic crust, but it must be emphasized that the most recent studies did not reproduce such a large range of values ($\delta^{34}\text{S}$ from -5‰ to +6‰; Farquhar et al. 2002, Cartigny et al. 2009, Thomassot et al. 2009b, and references therein). The recognition of mass-independent fractionation of sulfur isotopes (i.e., $\delta^{33}\text{S} \neq 0.5 \times \delta^{34}\text{S}$) is unambiguous evidence for the involvement of subduction-related sulfur in eclogitic inclusions in diamond (**Figure 11**) (Farquhar et al. 2002, Thomassot et al. 2009b). The only known geologically relevant process for sulfur mass-independent fractionation is UV photolysis of sulfur-bearing molecules in anoxic conditions (i.e. no atmospheric $\text{O}_2$); such conditions were met only during the Archean (Farquhar et al. 2000). Therefore, the identification of this type of sulfur isotope fractionation within eclogitic sulfide inclusions (Farquhar et al. 2002, Thomassot et al. 2009a) and its absence from sulfide peridotitic diamonds (i.e., $\Delta^{33}\text{S} \approx 0\%$; Cartigny et al. 2009) demonstrate the recycling of some Archean sedimentary sulfur into the mantle. Coupled C, N, and S isotope systematics has emphasized the distinction between the subduction-related signature of inclusions in diamond and the unambiguous mantle signature of the diamond host (Thomassot et al. 2009a), a relationship that points to metasomatic diamond growth and decoupling between the sulfide inclusion and the diamond

**Sulfur isotopes:**
compositions are expressed as per mill deviation from Canyon Diablo Troilite (CDT); mass-independent fractionation is expressed as deviation from the terrestrial fractionation line
Number of data

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

δ¹⁸O (‰)
(see also Deines & Harris 1995). What remains unclear, however, is whether the small range in $\Delta^{33}\text{S}$ reflects isotope reequilibration within subducting sediments or mixing with mantle sulfur. Isotope heterogeneity in $\Delta^{33}\text{S}$ among sulfide inclusions trapped in the same diamond suggests that metasomatic fluids contain sulfur, hence the term C-O-H-N-S fluids.

## The Specific Case of Unusual Diamond Distributions

Several eclogitic diamond populations (Figure 5) are unique in having a $\delta^{13}\text{C}$ mode that differs from the mantle range, and such values cannot be interpreted as reflecting formation from mantle-related carbon; strikingly, the associated peridotitic diamonds have typical $\delta^{13}\text{C}$ values. At Argyle, for example, the $\delta^{13}\text{C}$ mode at -11‰ is associated with positive $\delta^{15}\text{N}$ values (Van Heerden et al. 1995), and such a relationship would be compatible with a subduction-related heterogeneity—a heterogeneity also supported by the $\delta^{18}\text{O}$ values of inclusions in eclogitic diamonds from Argyle and Venezuela (Schulze et al. 2003, 2013). Diamonds from Jagersfontein (Kaapvaal craton) in South Africa and from alluvial deposits in New South Wales in Australia are further potential examples of diamond formation related to the occurrence of subduction-related carbon, but at present, nitrogen and oxygen isotope information is lacking at both sources.

Eclogitic diamonds from the Jericho kimberlite in the northern Slave craton in Canada and diamonds of unknown paragenesis from the Dachine komatiite in French Guiana have $\delta^{13}\text{C}$ modes of approximately -38‰ and -28‰, respectively (Figure 5), and these values are difficult to interpret in light of the idea that isotope heterogeneity is subduction or mantle related (De Stefano et al. 2009, Cartigny 2010, Smart et al. 2011). Preservation of $\delta^{13}\text{C}$ values less than -25‰ indeed requires the virtual absence of subducted carbonates, and $\delta^{13}\text{C}$ values below -40‰ are exceedingly rare within the sedimentary geological record (De Stefano et al. 2009, Smart et al. 2011). In addition, no mantle reservoir and/or known fractionation mechanism can lead to such unique isotope compositions; these present challenges for future studies. The data sets remain limited to C isotopes (Dachine) and major and trace element contents of inclusions (Jericho).

## The Case for Deep Diamonds from the Transition Zone and the Lower Mantle

Deep diamonds, from both the lower mantle and the transition zone, formed at higher temperatures and might be anticipated to display smaller C isotope variability. This is because the magnitude of isotope fractionation decreases linearly with $1/T^2$ (Figure 2). Yet, in a model wherein the deep mantle is metal saturated (Frost et al. 2004, Rohrbach & Schmidt 2011), the isotope fractionation between iron carbide and other C-bearing phases is significant; diamond in equilibrium with carbide is expected to be enriched in $^{13}\text{C}$ by nearly 5‰ (Satish-Kumar et al. 2011) and to

### Figure 9

Comparative histograms of $\delta^{18}\text{O}$ values of garnets from cratonic eclogite xenoliths worldwide and diamond eclogitic silicate inclusions (from individual locations and in total), and a relative probability distribution of altered oceanic crust. Note that the overall similarities in $\delta^{18}\text{O}$ between altered oceanic crust, eclogite nodules, and eclogitic silicate inclusions demonstrate a link between eclogite and subduction. It is unclear, however, why their mode and range differ. For example, $\delta^{18}\text{O}$ values above +10‰ are rare within the altered oceanic crust but are predominant in the Argyle data set. The range for silicate peridotitic inclusions from the Finsch mine is also indicated (Lowry et al. 1999). Although the $\delta^{18}\text{O}$ range is very restricted, it is significantly larger than the $\delta^{18}\text{O}$ range of peridotite nodules. This either reflects analytical difficulties in analyzing small amounts of material or is a true observation for which no explanation has yet been given. The relative probability distribution of altered oceanic crust is from Ickert et al. (2013). The upper histogram and cumulative curve are adapted from Ickert et al. (2013) with permission from Elsevier.
<figure><img src="image_21.png" /><figcaption>Figure 10</figcaption></figure>

Coupled variations between δ¹⁸O of silicate inclusions and δ¹³C of the host diamond, illustrating coupling between the two parameters (data are from Schulze et al. 2003, 2013; Ickert et al. 2013). The curvature of the hyperbolic curve requires the mantle end-member to be C enriched compared with the subducted end-member, which is at odds with the concept of a C-rich subducted slab but can be reconciled by the infiltration of C-rich fluids derived from the mantle into subducted eclogite lithologies. Standards: PDB, Pee Dee Belemnite; SMOW, Standard Mean Ocean Water.

have δ¹³C up to 0‰ in a carbide-dominated system with a mantle-like δ¹³C value. Also, the iron carbide–CO₂ C isotope fractionation could be as low as –10‰ (Schauble 2009, Satish-Kumar et al. 2011, Mikhail et al. 2014; V.M. Polyakov & J. Horita, personal communication), and even larger isotope variations can be produced if open-system fractionation occurs. These possibilities have been scarcely addressed to date and remain to be carefully evaluated. Deines (2002) suggested that carbide-diamond fractionation might account for mantle variability but requires a C isotope fractionation between diamond and carbide four times larger than theoretically predicted (Schauble 2009; V.M. Polyakov & J. Horita, personal communication) and experimentally determined (Satish-Kumar et al. 2011, Mikhail et al. 2014).

If large C isotope variability is related to fractionation mechanisms at these depths, we would anticipate the range in eclogitic and peridotitic diamonds to be similar, unless there is a mineralogical control in either basaltic or peridotitic composition; no data for such a control currently exist. The available data illustrate very distinct C isotope distributions between the two parageneses (Figure 4), as was found for shallow diamonds. The most likely explanation for this difference is that eclogitic diamonds from both the transition zone and the lower mantle record subduction-related C isotope heterogeneity (Tappert et al. 2005, Walter et al. 2011), a proposal supported by N isotope data (Palot et al. 2012).
<figure><img src="image_22.png" /><figcaption>Figure 11</figcaption></figure>

Decoupling in diamond geochemistry, illustrated by negative (i.e., mantle-like) δ¹⁵N values and a nonzero Δ³³S (subduction-related) signature of sulfide-bearing eclogitic diamonds. Red circles correspond to sulfides from eclogitic diamonds from the Jwaneng and Orapa kimberlites, Botswana (Farquhar et al. 2002, Thomassot et al. 2009b), whereas purple triangles correspond to peridotitic diamonds from the Panda kimberlite of Ekati mine, Canada (Cartigny et al. 2009). The contrasting geochemical signatures can be reconciled by considering metasomatic diamond formation from the infiltration of C-rich fluids derived from the convective mantle to enclose a preexisting sulfide. Modified from Cartigny et al. (2009), with permission from Elsevier.

However, the fact that peridotitic lower diamonds have less variable, mantle-like C isotope compositions coupled with predominantly negative δ¹⁵N values instead supports the view that their formation is not subduction related (Palot et al. 2012). This is in contrast with the common view brought by petrology and/or the identification of tiny Eu anomalies (Stachel et al. 2000b, Harte 2010). On the basis of very negative δ¹⁵N (i.e., less than -25‰), it was suggested that primordial carbon also occurs in some diamonds from Kankan (Palot et al. 2012), but there is at present no additional constraint to support this proposal.

In terms of fluid mobility in the lower mantle and transition zone, diamonds record a large C and N isotope variability, and it is tempting to speculate that fluid mobility is more restricted than in the shallow (150–300 km) mantle. Solid-state growth was suggested for eclogitic diamonds from the transition zone (Tappert et al. 2005), but this view is not supported by the occurrence of fluids/melts in some diamond inclusions (Walter et al. 2008, Bulanova et al. 2010).

## Final Word on Diamond Metasomatic Formation and Some Implications for Diamond Dating

Metasomatic diamond growth is well supported by the stable isotope composition of diamonds and their inclusions: Every study indeed calls for a metasomatic agent and thus for mixing between mobile C-O-H-N-S fluids/melts and preexisting peridotites and eclogites. Metasomatic diamond formation predicts some geochemical decoupling between the inclusion and the host diamond. Studies of sulfide-bearing diamonds support strong decoupling, whereas studies of silicate-bearing diamonds support some relationship. This aspect remains to be investigated through the use of even more integrated approaches.
Strong decoupling might lead one to question diamond genesis ages determined from the analysis of silicate and sulfide inclusions in diamonds (e.g., Richardson et al. 1984, Shirey & Richardson 2011), as some previous publications have done (Pidgeon 1989, Navon 1999, Rice 2003, Thomassot et al. 2009b, Shirey et al. 2013). Diamond isochron genesis ages are still valid if the inclusions were isotopically reset at the time of diamond growth by the penetrating fluid. The isotope variability in Δ³³S and δ¹⁸O among sulfide- and silicate-bearing diamonds does not support such a possibility (Thomassot et al. 2009b, Anand et al. 2004, Ickert et al. 2013) but remains limited to a small number of studies.

## SUMMARY POINTS

1. Diamond formation in the mantle is metasomatic and occurs from mobile C-O-H-N-S fluids/melts of composition(s) that largely remain to be investigated. Fluids trapped in coated/fibrous diamonds provide insights into their chemical and isotopic variability but do not sample the composition of reduced components. How the chemical and isotopic variability of fluids trapped in coated/fibrous diamonds might apply to more common and older octahedrally shaped diamonds and polycrystalline aggregates of diamonds is uncertain.

2. Diamond formation in Earth's mantle is closely related to oxidoreduction processes, from both oxidation of methane and reduction of carbonates/CO₂.

3. Diamond formation occurs from fluids introduced into a series of preexisting rocks, eclogites, and peridotites. Some eclogites represent low-pressure melts that are hydrothermally altered before being buried at depths that correspond to the diamond stability field.

4. Carbon isotope variability in diamonds and among diamonds mostly records the C isotope fractionation that occurred both during and prior to diamond growth. For the latter, identification of the involved mechanisms is heavily limited by available experimental data.

5. Available evidence suggests that diamond forms from primordial carbon only very rarely. Most diamond forms from carbon derived from the convective mantle with rather homogeneous carbon isotope composition.

6. Some carbon isotope variability, such as that recorded in eclogitic diamonds from the transition zone, results from the subduction of sedimentary carbon. It is tempting to suggest that fluid mobility is very restricted compared with that in the shallower mantle.

## FUTURE ISSUES

1. Metasomatic diamond formation has far-reaching implications that remain to be investigated by future studies. In particular, the exact relationship(s) of diamond to its mineral inclusions remain to be fully understood.

2. Experimental studies investigating mineralogical control on mantle fluids are lacking; this is particularly the case for methane-rich fluids.

## DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.
ACKNOWLEDGMENTS

John Valley is greatly thanked for informal discussions on Jack Hills diamonds and Edwing
Schauble, Juske Horita, and Veniamin Polyakov for sharing their unpublished C isotope carbide-
diamond theoretical fractionation data. This article is IPGP contribution 3495 and CRPG con-
tribution 2295.

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Tappert R, Tappert MC. 2011. *Diamonds in Nature: A Guide to Rough Diamonds*. Berlin: Springer-Verlag