Enikő Bali

Geologist, Current residence: Reykjavík, Iceland (Home: XV. district (Rákospalota), Budapest, Hungary)

What are fluid inclusions?


Fluid inclusions are small droplets of fluid that are trapped in minerals. We might be able to recognize them by naked eyes, if we have a big crystal which is not transparent for example. The “haziness” of the crystal is caused by high number of microscopic inclusions, in many cases these inclusions contain some fluids. 


Why fluid inclusions are interesting?


They serve us with information on the formation or evolution of the crystal and thus the rock in which we find them. These inclusions can be trapped in the minerals during their growth, in that case they are primary (or pseudosecondary) or after crystal growth along healed fractures.  In later case we call them secondary. Primary inclusions represent the hot fluid from which the mineral was growing. Secondary inclusions record fluid processes after mineral growth.


Primary (left), secondary and pseudosecondary (right) fluid inclusions. Primary inclusions are trapped during crystal growth. In the left hand side picture the earliest inclusions are in the middle of the mineral (in mineral core as we call), the ones along the edge are trapped latter. In the right hand side picture secondary and pseudosecondary inclusions are shown. Pseudosecondary inclusions (black ones) are found along healed cracks, but these cracks formed during crystal growth.  Real secondary inclusions are the ones that crosscut grain boundaries (like the black and white inclusion row).



A very systematic microscopic study without any chemical analysis can reveal lots of important information about these inclusions. The first figure above shows how we distinguish inclusions trapped at different times, the figure below shows what special characteristics of a single inclusion generation (assemblage of inclusions trapped at the same time) can have.

First of all, most fluid inclusions are built up by at least one liquid and one gas phase (here liquid is represented by white, gas with black). Gas phase forms a bubble in the inclusions. Small crystals (grey square, here) can also occur in these inclusions, many cases these crystals were growing from the fluid inside the inclusions. If the proportion of liquid-gas-crystal phases are the same in all inclusions (figures a and c), we can say that the inclusions were trapped from a homogeneous high temperature (and maybe pressure) fluid and all the phases developed inside the inclusions during the cooling of the rock. If the proportion of liquid-gas-crystal phases are different (figures b and d) in these inclusions, than the fluid was inhomogeneous during the growth of the crystal and the trapped proportions are completely accidental.



Drawing of fluid inclusions composed of a liquid (white) gas (black bubble) and a crystal (grey) phase. Inclusions in figures a and c were trapped from a homogeneous fluid (phase proportions are the same in them).  Inclusions in figure b and c were trapped from an inhomogeneous fluid (phase proportions are variable in them).



As these inclusions are closed in the mineral we can reheat and freeze them in a heating stage. We can determine the temperature where they were trapped (homogenisation temperature) and their composition (freezing point gives us information on their major components). Some components of the gas and liquid phase can qualitatively analysed by Raman spectroscopy, others by further chemical analyses like nuclear microprobe  or laser ablation inductively coupled mass spectrometry (commonly abbreviated as LA-ICPMS). As the latter analysis destroys our inclusions it should always be the last. 


Fluid inclusion studies have special importance in:

the study of ore deposits (determining the composition of ore-   forming fluid);

in oil exploration (tracing the formation and migration of hydrocarbons);

and geothermal research (temperature and composition of the geothermal fluid used for energy generation and space heating).


Some nice videos on heating stage experiments and the behaviour of fluid inclusions during these experiments can be seen on the web page of the fluid inclusion lab of the University of Lille.  


Synthetic fluid inclusions (SFI):

Fluid inclusions can be produced artificially. In industrial products of course the aim is to avoid inclusion production, because these inclusions can change the mechanical properties of the material. They always produce a “defect” in crystal lattice or in the structure of a glass. Through these defects the mineral or glass brakes easily.

In high pressure and temperature experiments however they can be very useful. The fluid that equilibrated in experimental conditions is trapped and when we finish our experiments by quenching (dropping the temperature, than decrease the pressure) its composition will be unmodified. Some small crystals can crystallize inside the inclusions, but we know that all those were part of the fluid at high temperature if the solid/fluid/bubble ratio is constant in them.

These kind of experiments help us to understand how economic grade ore deposits form or how aqueous fluid behaves in the deeper Earth.




Synthetic fluid inclusions in quartz, from one of my experiments at 15 kbas pressure and 800°C. The fluid inclusions have a very nice negative crystal shape. At room presssure and temperature conditions they contain a liquid phase, a vapor bubble and some tiny minerals (in this case U-oxide).  These tiny minerals precipitated while I was cooling my experiment. At 800 °C and 15 kbar pressure, they were dissolved in the fluid.


Silicate melt inclusions


At very high temperatures, where molten silicate is present in the system (like in a magma chamber) small droplets of silicate melt can also be trapped in growing minerals. These are called silicate melt inclusions.  One example can be seen below, a pseudosecondary silicate melt inclusion row in an olivine crystal from a basalt from Iceland.  These melt inclusions are composed of silicate glass and a fluid bubble phase.





Silicate melt inclusions in orthopyroxene of a mantle xenolith.  The rounded inclusions contain brownish silicate glass, and a small CO2 fluid bubble. The size of the inclusions is approximately 30 micrometers.  



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