Fluid and Multiphase Inclusions
If a mineral inclusion preserves a fragment of mantle rock, a fluid inclusion can preserve a trace of the medium from which the diamond grew or with which it later reacted. This is precisely why micrometer- and nanometer-scale droplets inside diamonds are of exceptional scientific importance.
The problem is that what was a single mobile phase at high pressure and temperature may no longer look like a liquid today. Cooling and decompression can cause minerals, carbonates, salts, and other daughter phases to separate from it. A present-day multiphase inclusion may therefore be the frozen end product of a once more homogeneous deep fluid or melt.
Fluid, Melt, and Supercritical Phase
In geological literature, the boundary between fluid and melt under deep-mantle conditions is not always simple. A mobile phase may be rich in H₂O, CO₂, carbonates, silicate components, salts, and dissolved elements.
For that reason, diamond research often uses the functional term diamond-forming fluid/melt. In this book, the word fluid does not necessarily mean “water trapped in a bubble,” but rather a mobile deep phase that transported matter and could participate in diamond growth or dissolution.
High-Density Fluid Microinclusions
The best-studied group consists of high-density fluid (HDF) microinclusions in fluid-rich diamonds, especially fibrous and coated diamonds.
They are usually submicrometer to micrometer in size. Their abundance can give a diamond a cloudy or fibrous appearance. Under entrapment conditions, the fluid may have been a single phase, whereas cooling produced numerous daughter phases.
Classic studies recognize four principal compositional endmembers:
- saline;
- silicic;
- high-Mg carbonatitic;
- low-Mg carbonatitic.
Actual samples often lie between these endmembers. The classification therefore describes a compositional space, not four completely discrete “types of fluid.”
[VISUAL 42.1: HDF compositional space—saline, silicic, high-Mg carbonatitic, and low-Mg carbonatitic endmembers with transitions]
Fibrous, Coated, Cloudy, and Gem-Quality Diamonds
HDF microinclusions are particularly abundant in fibrous diamonds, where rapid or dendritic growth facilitates the entrapment of large amounts of fluid. Similar inclusion-rich zones may form a fibrous coat around a more transparent core or local cloudy zones.
This does not mean that the fluid record is confined to cloudy diamonds. Analytical studies show that related chemical signals may also occur in gem-quality diamonds, sometimes as optically invisible nanoinclusions or very thin fluid films adjacent to mineral inclusions.
Studies published in 2026 further showed that some gem-quality lithospheric diamonds can preserve micrometer-scale silicic fluid rims around mineral inclusions, whose signal can be separated from that of the mineral inclusion by combining Raman mapping with LA-ICP-MS depth profiling.
This is an important new research direction, but it does not mean that every mineral inclusion in every diamond has the same fluid coat or the same origin.
[VISUAL 42.2: Fibrous, coated, and gem-quality diamonds—different modes of preserving a fluid record]
A Daughter Phase Is Not the Original Fluid
When an HDF inclusion cools and decompresses, silicates, carbonates, and other solid phases may crystallize from the original mobile phase. A multiphase assemblage may therefore be visible today under an optical or electron microscope.
The key rule is:
The composition of an individual daughter mineral is not the same as the bulk composition of the originally trapped fluid.
Reconstructing the bulk composition requires analysis of the entire inclusion population or a method that quantitatively encompasses multiple phases.
Saline Fluids and Subduction
One of the most important models links saline diamond-forming fluids to subducted material. In well-studied diamonds from Canada’s Slave craton, the combination of HDF chemistry, trace elements, and Sr isotopes supported a model in which a highly saline fluid enters from the subducted slab and then reacts chemically with peridotitic and eclogitic mantle.
This is a strong result for that specific population. It is not proof that all saline HDFs in all diamonds have the same source, nor that a single saline microinclusion by itself proves subduction.
Multiple Generations in a Single Diamond
A diamond may grow episodically. Its core, intermediate zone, and outer coat may record different conditions and different fluids.
Consequently, two inclusion-rich zones in the same stone may differ in:
- major-element composition;
- trace-element signature;
- relationship to growth sectors;
- C- and N-isotope context;
- temporal relationship to other zones.
A single bulk result for the entire diamond may erase precisely the evolution that the researcher seeks to reconstruct.
Growth-Related and Secondary Fluid Inclusions
Not every liquid or multiphase feature is a remnant of the primary diamond-forming fluid.
Fluid may also enter later through a fracture that subsequently heals partly or completely. Such healed-fracture inclusions may record a later mantle event. For example, CO₂–N₂ fluid inclusions distributed along healed fractures have been described in some natural diamonds.
The spatial relationship to growth zones and fractures is therefore just as important as chemical composition.
[VISUAL 42.3: Growth-related inclusion versus fluid in a healed-fracture system]
A Natural Fluid Inclusion Is Not Fracture Filling
A natural fluid inclusion formed during a diamond’s geological history is not the same as the artificial filling of a surface-reaching fracture.
Fracture filling is a later treatment in which material is introduced into a fracture to reduce its visibility. It is covered in Chapter 59.
Only the distinction matters here: an optically “fluid-like” appearance must not be identified as a natural mantle inclusion without establishing its spatial and chemical context.
How Composition Is Reconstructed
The analytical strategy depends on the inclusion’s size, depth, and type.
Raman spectroscopy identifies crystalline daughter phases and certain molecular components, while mapping can reveal their spatial distribution.
FTIR is useful for certain molecular and structural information, but signals from the host and the inclusion must be separated carefully.
Micro-XRF and related methods provide elemental maps without necessarily opening the sample.
LA-ICP-MS can provide highly sensitive trace-element data, but it is microdestructive and requires strict control of signals from the host, the inclusion, and any fluid rim.
SIMS enables in situ elemental and isotopic analyses on a very small scale.
FIB-TEM can open the nanometer-scale world of an inclusion and resolve phases far below optical resolution, but it requires invasive preparation.
The method is not an end in itself. The question is always: What exactly do we want to measure, and how much of the sample are we prepared to sacrifice?
[VISUAL 42.4: Reconstructing a fluid inclusion—present-day daughter phases → spatial analysis → bulk chemistry → interpretation of the original mobile phase]
Contamination and Loss of Volatile Components
Opening an inclusion can alter precisely what is to be measured. Volatile components may escape, the surface may become contaminated by polishing agents or the environment, and laser ablation may mix signals from the diamond, fluid rim, and mineral inclusion.
Serious research therefore records:
- whether the inclusion was closed;
- how it was exposed;
- its depth and geometry;
- blanks and standards;
- the sequence of analyses;
- what was removed destructively.
Without that information, an impressive number may have weak evidentiary value.
What Fluid Inclusions Reveal About the Deep Cycle
Fluid inclusions provide more direct insight into mobile mantle components than diamond carbon alone. They can be used to trace H₂O, CO₂, carbonates, Cl, alkalis, and trace elements associated with metasomatism and subduction.
Conclusions, however, must remain proportional to the sample. A single inclusion-rich diamond can demonstrate that a particular fluid existed in a particular geological system. By itself, it cannot determine the global amount of water in the mantle or the universal composition of diamond-forming fluid.
Fluid Inclusions, Cloud, and Transparency
A large number of very small inclusions can scatter light and create a cloud or general cloudiness. This connects the geological story to gemological observation, but the two vocabularies should not be equated.
In the clarity context, cloud describes the appearance of a group of tiny inclusions under standard examination. Geological analysis asks what that population consists of and how it formed. The same observation may therefore serve different functions in Chapter 42 and Chapter 44.
Practical Analytical Sequence
- document the position and 3D relationship of the inclusions;
- distinguish a growth-related zone from a healed-fracture system;
- determine whether the inclusion is fully enclosed;
- map phases nondestructively where possible;
- separate the mineral inclusion from any fluid rim;
- select a chemical method based on the target elements and required spatial resolution;
- before destructive analysis, preserve images, spectra, and geometry;
- do not equate daughter phases with the bulk fluid;
- do not turn a local geochemical interpretation into a global model without additional population data.
Chapter Summary
- A fluid inclusion preserves a trace of a mobile medium, but its present condition may not correspond to the original phase.
- HDF microinclusions are especially well known from fibrous and coated diamonds.
- The four principal HDF endmembers are saline, silicic, high-Mg carbonatitic, and low-Mg carbonatitic.
- Daughter minerals formed during cooling are not the same as the original bulk fluid.
- Related fluid signals may also occur in gem-quality diamonds.
- Studies published in 2026 show silicic fluid rims around mineral inclusions in some gem-quality diamonds.
- Saline HDF may be associated with subducted material in certain populations, but that conclusion is not universal.
- A single diamond may contain multiple generations of fluid.
- Healed-fracture fluids may be younger than the diamond’s principal growth phase.
- A natural fluid inclusion and artificial fracture filling are not the same phenomenon.
- Raman, FTIR, micro-XRF, LA-ICP-MS, SIMS, and FIB-TEM serve different functions and have different degrees of invasiveness.
- Fluid inclusions may be key to the deep cycling of carbon and volatiles, but interpretation must remain proportional to the sample.