The Fluids from Which Diamonds Grow

When we say that a diamond grows “in the mantle,” it is easy to imagine a solid rock in which a crystal simply appears very slowly. The actual process is far more dynamic.

Evidence from natural diamonds shows that mobile carbon-rich fluids and melts are central to many diamond-forming events. They transport carbon and other components through the mantle, react with rock, and can simultaneously cause diamond to grow, change, or dissolve.

What geologists call a fluid

In the deep Earth, the word fluid does not necessarily mean a dilute aqueous solution like surface water.

At high pressures and temperatures, the mobile medium may be:

• a water-rich solution;
• a carbonate or carbonatitic melt;
• a silicate melt;
• a saline fluid;
• a supercritical medium without a simple boundary between liquid and gas;
• a complex mixture of CO₂, H₂O, carbonates, silicates, chlorides, and other components.

It is therefore more useful to speak of a diamond-forming medium than to imagine one universal fluid.

How we know such media existed

The most direct evidence comes from diamonds that trapped extremely small droplets or pockets of fluid during growth. After cooling and decompression, the original contents may separate into multiple microscopic phases.

Fibrous diamonds and fibrous coats around older cores are especially important. Their more rapid growth often traps large numbers of microinclusions, making them natural reservoirs of diamond-forming fluids.

This does not mean that all nonfibrous diamonds grew without fluids. A clean monocrystalline diamond may simply preserve far less direct material from its growth medium.

[VISUAL 8.1: Monocrystalline core, fibrous coat, and a network of microinclusions of diamond-forming fluid]

High-density fluids

The literature on fibrous diamonds often uses the term high-density fluid (HDF) for microinclusion-bearing media rich in dissolved components.

Here, “high density” does not refer only to high physical density. The emphasis is on a chemically concentrated medium that may contain substantial amounts of carbonate, silicate, saline, and aqueous components.

Measured water content varies among populations; certain carbonate–silicate HDFs may contain water on the order of tens of weight percent. Such values must not be presented as the composition of all diamond-forming fluids.

Principal compositional groups

Analyses of microinclusions have revealed a broad continuum of compositions. The literature often distinguishes end-member groups such as:

• saline—rich in Cl, K, Na, and water;
• silicic—relatively rich in Si, Al, and alkalis;
• carbonatitic—rich in carbonate components and Ca and/or Mg;
• transitional mixtures between these types.

The boundaries are not rigid like those between mineral species. A fluid may change composition as it passes through the mantle, dissolves minerals, precipitates new phases, and reacts with peridotite or eclogite.

Saline end-member compositions are especially important because they show that chlorides and water can be significant parts of a deep mobile medium. Silicic compositions indicate a larger contribution from silicate components, while carbonatitic compositions emphasize the role of carbonates and a very low degree of melting. An individual microinclusion, however, often contains present-day crystals that separated from the original fluid during cooling. Its chemistry must therefore be reconstructed from the phase assemblage, not from a single grain.

This is why it is more useful to examine fluid evolution than to create a catalog of rigid boxes.

Subduction can supply mobile components

One of the most important findings of modern diamond geochemistry is the evidence that some diamond-forming fluids carry traces of material that had previously been near Earth’s surface.

In diamonds from Canada’s Slave craton, for example, chemical and isotopic data support a model in which highly saline fluids were associated with subducted oceanic lithosphere and subsequently evolved by reacting with a mixed peridotitic–eclogitic mantle.

This is a strong example of the deep recycling of water, salts, and carbon.

It is, however, evidence specific to a locality and population. It must not be turned into the claim that all diamond-forming fluids are of subduction origin.

Where the carbon comes from

Carbon in a diamond-forming medium may come from several reservoirs. Stable carbon isotopes in some populations are close to the typical mantle range, while others—especially certain eclogitic populations—show broader values that can be interpreted as involving recycled material.

Such an isotopic signal is not a simple label of “organic” or “inorganic.” Fractionation during reactions, melting, and transport can alter ratios, and multiple sources may mix. The carbon source is therefore assessed together with mineralogy, trace elements, and regional geology.

How a fluid precipitates diamond

Diamond crystallizes when the equilibrium among carbon species in a mobile medium changes. Several mechanisms can cause carbon to precipitate as diamond:

• a change in oxidation state;
• a reaction between oxidized and reduced carbon species;
• a reaction between the fluid and the surrounding rock;
• cooling;
• a change in pressure;
• mixing of two fluids;
• a change in composition or in the activity of water and carbonate.

In nature, these processes may be combined. No single universal mechanism applies to all diamonds.

Important Diamond growth and dissolution are two directions of the same chemical equilibrium. A change in fluid that precipitates diamond at one moment may cause its resorption at another.

Redox reactions

Carbon in the mantle may exist in several oxidation states, from reduced species such as methane to oxidized carbonate or CO₂ components.

When a fluid moves through a rock with a different redox capacity, the chemical equilibrium may change enough for diamond to precipitate.

Such a model is especially important in discussions of cratonic-mantle metasomatism. The redox state, however, cannot be reconstructed from a diamond’s color or from a single inclusion without broader chemical analysis.

A fluid does not remain unchanged during its journey

As it passes through the mantle, a fluid may:

• dissolve minerals;
• change the proportions of Mg, Ca, Si, K, Na, and Cl;
• lose or gain CO₂ and H₂O;
• precipitate carbonates, silicates, and diamond;
• shift from a more saline toward a silicic or carbonatitic composition;
• mix with another medium.

This is a metasomatic front: a chemically mobile system that changes both the fluid and the rock.

[VISUAL 8.2: Evolution of a diamond-forming fluid through peridotite/eclogite—transport, reaction, diamond growth, and resorption]

Primary and secondary inclusions

A microinclusion trapped during growth may preserve direct information about the diamond-forming medium. Material that entered later through a fracture has a different significance.

It is therefore necessary to distinguish:

• growth-related inclusions;
• pseudosecondary inclusions associated with a fracture that developed during growth;
• secondary inclusions formed after the main crystallization phase.

A natural fluid inclusion has nothing to do with artificial fracture filling. Fracture filling is a later treatment of a polished or natural diamond and is discussed in Chapter 59.

What is actually analyzed

After a diamond reaches the surface, the original fluid may no longer exist as a homogeneous droplet. During cooling, it may separate into:

• tiny crystals;
• carbonate or silicate phases;
• a saline component;
• an aqueous or gaseous phase.

Researchers therefore often reconstruct the bulk composition of the original medium from many microinclusions and their present-day products.

Methods include Raman spectroscopy, FTIR, micro-XRF, LA-ICP-MS, SIMS, electron-beam methods, and synchrotron techniques. Each has a different spatial resolution, sensitivity, and degree of destructiveness.

In trace-element work, the object analyzed is often not “one droplet” in an ideal sense, but a large number of microinclusions within a small volume of diamond. The analytical result may therefore be a spatial average of many extremely small phases. Opening inclusions also carries a risk of losing volatile components or introducing external contamination. Detailed methodology belongs to Chapter 42.

Limitations

The greatest interpretive risks are:

• extrapolating from one population to all diamonds;
• confusing the original fluid with minerals that crystallized after entrapment;
• overlooking contamination when an inclusion is opened;
• assuming that the present-day ratio of volatile components perfectly preserves the original ratio;
• using fluid composition to infer a specific mine or unique carbon source directly.

A fluid inclusion is exceptionally powerful evidence, but it is not a simple capsule of unchanged liquid.

Chapter summary

• Many natural diamonds grow from mobile carbon-rich fluids or melts.
• A “fluid” in the deep mantle is not necessarily an ordinary aqueous solution.
• Fibrous diamonds and their microinclusion-bearing coats are important archives of diamond-forming media.
• HDFs are chemically concentrated fluids or melts rich in dissolved components.
• Saline, silicic, and carbonatitic compositions represent principal end-member groups, but transitional compositions occur between them.
• A fluid may evolve through reaction with peridotite and eclogite.
• Subduction is a demonstrated source of some mobile components in certain populations, but it is not a universal explanation.
• Diamond can precipitate through redox reaction, cooling, mixing, or fluid–rock reaction.
• The same system may cause both diamond growth and dissolution over time.
• Present-day microinclusion phases are not necessarily identical to the original fluid; their bulk composition is often reconstructed.
• A natural fluid inclusion is not the same as artificial fracture filling.
• Conclusions about fluids must remain tied to the specific population and analytical methods.

[VISUAL 8.3: From original fluid to laboratory conclusion—entrapment → cooling/phase separation → microanalysis → reconstruction of bulk composition]