Advances in 3D imaging and volumetric reconstruction of fluid and melt inclusions by high resolution X-ray computed tomography

Fluid and melt inclusions are tiny pockets of fluid and melt trapped in natural and synthetic minerals. Characterizing the 3D distribution of fluid and melt inclusions within minerals, their shape and the volume fraction of their different phases is crucial for determining the conditions of crystal growth and paleostress analysis. However, their relatively small size (typically 5 to 100 μm), complex shape, heterogeneous content, the opaque nature of some host minerals and projection bias frequently hamper accurate imaging and volumetric reconstruction using conventional microscopic techniques. High resolution X-ray computed tomography (HRXCT) is a non-destructive method which uses contrasts of X-ray attenuation in a series of contiguous radiographs with different view angles to reconstruct the 3D distribution of areas of different densities within a large variety of materials. In this work, we show the capabilities of HRXCT for: (i) imaging the 3D distribution of aqueous and hydrocarbon-bearing fluid inclusions and silicate melt inclusions in a crystal; (ii) characterizing the shape of fluid and melt inclusions and (iii) reconstructing the total volume and the volume of the different phases (liquid, glass, crystal, vapor) of fluid and melt inclusions. We have used a variety of hand specimens and chips of transparent and opaque minerals (olivine, quartz, feldspar, garnet, emerald, wolframite), that we analyzed using three different HRXCT setups. When a resolution of ~1 μm3/voxel is achieved, HRXCT allows identifying >5 μm fluid inclusions, and the identification and volumetric reconstruction of the different phases can be carried out with reasonable confidence for relatively large (>25 μm) inclusions. Density contrasts are high enough to properly identify: (i) a silicate melt inclusion, and its different phases (glass, vapor and crystals such as clinopyroxene and spinel) in an olivine crystal; (ii) aqueous monophase (liquid) and two-phase (liquid + vapor) fluid inclusions in transparent and opaque minerals (quartz, garnet, emerald, wolframite). In the case of hydrocarbon-bearing fluid inclusions containing a vapor phase and two liquid phases (oil and aqueous solution), the two liquid phases could not be distinguished from each other. Volumetric reconstruction of liquid and vapor phases of aqueous and hydrocarbon-bearing fluid inclusions show compatible results with independent calculations using known pressure, temperature, molar volume and composition (P-T-V-x) conditions of trapping or imaging using confocal laser scanning microscopy respectively. Collectively, our results show that HRXCT is a promising tool for non-destructive characterization of fluid and melt inclusions.