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Article

Methane in Fluid Inclusions in Ophiolitic Chromitites Revealed by Raman Spectroscopy: Preliminary Results

by
Federica Zaccarini
1,*,
Gabriella B. Kiss
2,
Giorgio Garuti
1,
Daniela Mauro
3,4,
Maria Economou-Eliopoulos
5,
Máté Hegedűs
2,6 and
Cristian Biagioni
3,7
1
Geosciences Programme, Faculty of Science, University Brunei Darussalam, Jalan Tungku Link, Bandar Seri Begawan BE1410, Brunei
2
Department of Mineralogy, Institute of Geography and Earth Sciences, Faculty of Sciences, ELTE Eötvös Loránd University, 1117 Budapest, Hungary
3
Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy
4
Museo di Storia Naturale, Università di Pisa, Via Roma 79, I-56011 Calci, Italy
5
Department of Geology and Geoenvironment, University of Athens, 15784 Athens, Greece
6
Departement of Materials Physics, Institute of Physics and Astronomy, Faculty of Sciences, ELTE Eötvös Loránd University, 1117 Budapest, Hungary
7
Centro per l’Integrazione della Strumentazione dell’Università di Pisa, I-56126 Pisa, Italy
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(4), 335; https://doi.org/10.3390/min15040335
Submission received: 29 January 2025 / Revised: 25 February 2025 / Accepted: 21 March 2025 / Published: 23 March 2025
(This article belongs to the Section Mineral Deposits)
Browse Figures
Versions Notes

Abstract

:
This contribution provides a petrographic and Raman investigation of fluid inclusions found in chromitites collected in the ophiolites of Santa Elena (Costa Rica), Bracco (Italy), Otrhys and Vourinos (Greece), and Troodos (Cyprus). Most of the analyzed chromites are classified as high-Cr, with the exception of those from Bracco and some of the Othrys complexes that are high-Al. Although the investigation of fluid inclusions in chromitites is very challenging due to the poor transparency of the host chromite, the studied samples contain numerous fluid inclusions. The fluid inclusions look to be more abundant in the high-Cr chromitites, related to a subduction zone environment, compared to the high-Al chromitites generated in a mid-ocean ridge. This is in agreement with the petrogenetic model for the formation of podiform chromitites that implies the presence of a metasomatic event caused by hydrous fluids that reacted pervasively with variable depleted mantle tectonites, especially in the subduction zone setting. The fluid inclusions, between 1 and 15 µm in size, show negative crystal or irregular angular shapes. They occur when enclosed in chromite crystals that have not been affected by low-temperature processes. The fluid inclusions consist of liquid (L), vapour(V~30–50 area%) and L + V (V~40–60 area% rarely 10–80 area%). The fluid inclusions may contain only vapour and a vapour and a solid phase, too. The Raman spectra reveal the presence of CH4 in certain fluid inclusions. Considering the high number of fluid inclusions that potentially contain CH4, we suggest that the fluid inclusions in the chromite crystals and their leaching can be a possible source in order to explain the high amount of CH4 detected in some podiform chromitites, previously attributed to the Sabatier reaction. The mode of the occurrences of the studied CH4 bearing fluid inclusions, i.e., entrapped in unaltered chromite crystals formed at a magmatic temperature, suggest their abiotic origin from mantle-derived fluids, rather than those related to the low-temperature serpentinization processes. The investigation of fluid inclusions, although it is difficult and challenging or even impossible when the chromite is too opaque, can be applicable to other chromitites worldwide to verify the presence of H2O, CH4 or other gases. This information will greatly improve our understanding of the nature of the fluid phases during the formation of podiform chromitites.

1. Introduction

Ophiolites may host podiform chromitite in residual mantle peridotites, especially in harzburgite, and layered chromitite that occurs close to the Moho or in the crustal sequence [1]. In addition to their economic relevance because they represent one of the most important sources for the chromium recovery, the podiform chromitites have proven to be an efficient guide to understand the tectono-magmatic evolution of their host ophiolites.
Based on the chemical composition of chromite, the podiform chromitites have been classified as high-Cr, with Cr/(Cr + Al) > 0.6, and high-Al with Cr/(Cr + Al) < 0.6. Since the composition of chromite is sensitive to the parent melt composition which is generated in different geotectonic settings, there is a general consensus to consider the formation of high-Cr and high-Al chromitites related to boninitic and mid-ocean ridge basalt (MORB) magmas, respectively, and, as a consequence, those formed in subduction-related or unrelated settings [1,2,3,4,5,6,7,8].
The mechanisms proposed in order to understand the origin of the podiform chromitites can be summarized as follows: (1) fractional crystallization from a mafic magma [2], (2) partial melting of the mantle source [3], (3) melt–rock interaction [4,5], and (4) hydrothermal processes [6,7]. The predominant petrogenetic model for the formation of podiform chromitites implies the presence of a metasomatic event caused by hydrous fluids that reacted pervasively with variable depleted mantle tectonites [4,5,8,9].
The importance of hydrous fluids in the formation of podiform chromitites was proven by experimental work [9], and the presence of abundant inclusions of hydrous minerals, such as amphibole and phlogopite, hosted in chromite crystals [6,8,10].
Recently, considerable amounts of abiotic CH4 have been detected in podiform chromitites associated with the Othrys ophiolite of Greece [11] and in the Cedrolina chromitites occurring in an Archean-Paleoproterozoic Greenstone Belt of Brazil [12]. Chromitites and associated serpentinites in ophiolites have been considered as the rocks able to generate abiotic CH4 at low temperatures via the Sabatier reaction, 4H2 + CO2 = CH4 + 2H2O, also known as carbon dioxide hydrogenation [11,12]. According to Etiope and Ionescu [13], Etiope et al. [11] and Portella et al. [12], chromium, Fe-Ni alloys and ruthenium may play an important role as metal catalysts for abiotic CH4 production via the Sabatier reaction.
Fluids and gases can be trapped in minerals as fluid inclusions that consist of tiny droplets of fluid formed either during the initial growth from solution, i.e., primary inclusions, or at some later stage when minerals are affected by the process of healing fractures, i.e., secondary or pseudosecondary inclusions [14,15].
Despite their importance to determine ore-forming, only a few studies describe the presence of fluid inclusions in ophiolitic chromitites [6,8,16,17,18,19,20,21] due to the fact that the fluid inclusions are difficult to observe in thin sections because of the opacity of the host mineral, a chromium-rich spinel.
In order to provide more information about the presence of CH4 in ophiolitic chromitites, in this contribution, we report the preliminary results of a fluid inclusions investigation based on petrographic observation and Raman data obtained in selected podiforms and thin layers of chromitites from different ophiolites. The main target of this work is to discover where CH4 is hidden in ophiolitic chromitites, suggesting an alternative source to explain the enrichment in this gas detected in some chromitites [11,12], and to briefly discuss its possible origin. However, due to the opaqueness of the chromite, the traditional methods to study fluid inclusions, such as microthermometry, cannot be applied. Therefore, the analytical limitations and uncertainties of the investigation of fluid inclusions in chromitites are also discussed.

2. Sample Set

More than 300 thin sections representing massive chromitites sampled in the field were studied using a transmitted-light microscope. However, only 50 samples were selected because the host chromite was transparent enough to allow for the finding of tiny fluid inclusions.
The analyzed samples were collected in the following ophiolites: Santa Elena (Costa Rica), Bracco (Italy), Otrhys and Vourinos (Greece), and Troodos (Cyprus) (Table 1).
Simplified geological maps of the studied ophiolites showing the sample location are reported in Figure 1A–E.
The podiform chromitites from Costa Rica were collected from the Santa Elena Peninsula on the northern Pacific coast of Costa Rica (Figure 1A). They occur in the Santa Elena ultramafic nappe, and are Cretaceous in age (Table 1), close to the Cretaceous igneous-sedimentary sequence of the Santa Rosa Accretionary Complex (Figure 1A). The studied chromitites consist of small mineralized bodies (Table 1, Figure 2A) associated with strongly altered harzburgite. According to Zaccarini et al. [22], they represent podiform chromitites formed in the mantle section of a supra-subduction zone ophiolite. The following samples have been studied: JAG2, JAG1, JAG3, JAG4, JAG5 and JAG6.
The Bracco chromitites are hosted in the Jurassic Ligurian ophiolites (Table 1) located in the northern Italian Apennines of the Liguria region, north of the Levanto village (Figure 1B). The Ligurian ophiolites are classified as subduction-unrelated, continental-margin types [23]. The Bracco chromitites occur in the homonymous complex composed of gabbroic bodies that intrude mantle lherzolite, are partially to totally serpentinized, and are in contact with pillow lavas and pelagic sediments (Figure 1B). The studied chromitites form small rhythmic layers (Table 1), up to a maximum 5 cm in thickness, in contact with troctolite, serpentinized peridotite and anorthosite, hosted in a layered peridotite (Figure 1B and Figure 2B) [24]. They were sampled in the localities of Canegreca (CN53), Cima Stronzi (SR40) and Mattarana (MT30) (Figure 1B).
The Jurassic-Cretaceous Othrys ophiolite (Table 1) is located in eastern central Greece and mainly consists of serpentinized upper mantle harzburgite and lherzolite, overlain by gabbroic cumulates (Figure 1C). Several mafic dikes, cutting across both the mantle peridotites and the gabbros, have also been described [25]. The Othrys ophiolite is characterized by a complex geotectonic system, varying from an oceanic spreading centre to a supra-subduction zone [26]. Several chromitite deposits have been mined in the past, and occur in two tectonically separated ultramafic bodies, in the areas of the Domokos village (west Othrys ophiolite) and the Eretria village (east Othrys ophiolite) (Figure 1C). The studied chromitites were collected from the Agios Stefanos (DK2 AS) and Metallion (DK12 MET) mines in the area of Domokos and in the deposit of Tsangli (TS2) located in the Eretria area (Figure 1C). They consist of massive chromitite with a coarse-grain texture (Table 1, Figure 2C).
The podiform chromitites from the supra-subduction Vourinos ophiolite, which are Jurassic-Cretaceous in age (Table 1), were collected from the Voidolakkos mine (VS11 VOI, VS8 VOI) (Figure 1D). The Vourinos complex consists of harzburgite–dunite mantle tectonite, partially to totally serpentinized, that hosts the studied chromitites (Figure 1D), with subordinate mafic–ultramafic cumulates overlain by volcanic rocks [27,28]. The studied chromitites show the typical schlieren texture (Figure 2D).
The Late Cretaceous Troodos supra-subduction zone ophiolite (Table 1) crops out in the central part of Cyprus Island and represents a complete ophiolitic sequence from a basal tectonite harzburgite to pillow lavas. Several chromitite deposits in the Troodos ophiolite have been mined in the past [29]. The mineralized bodies consist, in most cases, of podiform chromitites associated with dunite and harzburgitic tectonite (Figure 1E), that are variably serpentinized, and are in contact with a cumulate sequence made up of wehrlite, pyroxenite and gabbro (Figure 1E). The chromitites investigated in this contribution were collected from the mines of Kokkinorotsos (TD1 KOK) and Kannoures (TD8 KAN) located close to the Monte Olympus area (Figure 1E). They represent large pods of massive chromite as the example reported in Figure 2E.

3. Methods

Generally, fluid inclusion investigation is carried out by making doubly polished sections of 100–300 µm in thickness [14]. However, due to the optical properties of chromite, i.e., it being poorly transparent and not colourless, only thin sections with a maximum thickness of 30 µm were suitable for fluid inclusion study. The obtained thin sections, representing massive chromitites, were previously investigated by a transmitted-light microscope with the aim to find and to classify the fluid inclusions using the approach provided by Roedder [15] and van den Kerkhof and Hein [14]. Fluid inclusion petrography was performed with a Zeiss Axioplan research microscope (Oberkochen, Germany), and photo documentation was prepared with a Zeiss Axiocam 208 camera and Zeiss Zen software (version 3.11).
Oxyspinel-group minerals (hereafter spinels) were quantitatively analyzed by an electron microprobe using a Superprobe JEOL JXA 8200 installed in the Eugen F. Stumpfl Laboratory at the University of Leoben, Austria, using WD systems. The analysis of spinels was performed using an accelerating voltage of 15 kV, an electron beam current of 10 nA, and a beam diameter of about 1 µm. The analyses of Mg, Al, Si, Ti, V, Cr, Zn, Mn, Fe and Ni were obtained using the Kα lines and were calibrated on natural chromite, rhodonite, ilmenite, albite, pentlandite, sphalerite and metallic vanadium. The following diffracting crystals were used: TAP for Mg and Al; PETJ for Si; and LIFH for Ti, V, Cr, Zn, Mn, Fe and Ni. The peak and background counting times were 20 s and 10 s, respectively, for the major elements. The detection limits were automatically calculated by the microprobe software, and they are listed in the following as ppm: Mg, Mn, and Fe (100); V and Ni (150); Cr and Zn (200); and Al, Si, and Ti (250). The amount of Fe3+ in chromite was calculated assuming the ideal spinel stoichiometry. The analyses are available in Supplementary Materials Table S1.
Raman data were previously collected at the Department of Earth Sciences, University of Pisa, Italy, using a Horiba Jobin-Yvon XploRA Plus apparatus (Kyoto, Japan), with a 50× objective lens and the 532 nm line of a solid-state laser attenuated to 25% (i.e., 6.25 mW). The spectra were collected in the range between 100 and 4000 cm−1 through multiple acquisitions, with counting times ranging between 60 s and 120 s. Backscattered radiation was analyzed with a 1200 grooves/mm grating monochromator.
Subsequently, in order to check the reproducibility of the obtained data, systematic Raman spectroscopy analyses was carried out at the ELTE Faculty of Sciences Research and Industrial Relations Center, Eötvös Loránd University, Budapest, Hungary, using a HORIBA JobinYvon LabRAM HR 800 Raman micro-spectrometer. A frequency-doubled Nd-YAG green laser with a 532 nm excitation wavelength was used to illuminate the samples, displaying 130 mW at the source and ~25 mW at the sample surface. OLYMPUS 100 × (N.A.  =  0.9) objectives were used to focus the laser. A 50 μm confocal hole, 1800 grooves/mm optical grating, and 60 s cumulated exposition time were used with 3 accumulations. The spectral resolution of measurements was 0.7 cm−1. The spectrometer was calibrated to the Rayleigh-line at 0 cm−1.

4. Results

4.1. Texture of the Chromitites and the Chromite Composition

Microscopically, the studied chromitite consists of a massive aggregate of spinel grains (0.3–5.0 mm) with small amounts of silicate gangue (<20 vol.%), mainly composed of low-temperature secondary minerals such as chlorite and serpentine. The chromite crystals are brecciated and strongly fractured and, as a consequence, the magmatic grain boundary relationship is obliterated (Figure 3A,B). Evidence of gravitational settling and a cumulitic texture was observed only in the chromitite of the Bracco ophiolite. Chromite is usually fresh, showing the typical ferrian–chromite alteration only along grain boundaries and cracks. Therefore, the magmatic composition of spinel was deduced exclusively from results of the electron-microprobe analysis of the unaltered grain cores.
Based on the value of Cr/(Cr + Al), most of the analyzed chromites are classified as high-Cr, with the exception of all the chromitites from the Bracco ophiolite, from the Tsangli deposit, and some of the Aghio Stefanos and Metallion of the Othrys ophiolite (Figure 4).
Following the mineralogical classification of the spinel supergroup minerals provided by Bosi et al. [30], based on the relationships between Cr/(Cr + Al) and Mg/(Mg + Fe2+), the great majority of the analyzed samples are plotted in the field of chromite (Figure 4).
The samples from Canegreca and Cima Stronzi from the Bracco ophiolite can be classified as Cr-rich hercynite, whereas oxyspinels in the chromitite samples from Mattarana (Bracco ophiolite) and the Santa Elena ophiolite (sample JAG6) can be classified as magnesiochromite, although with low and high Cr/(Cr + Al) values, respectively (Figure 4).
The calculated Fe2O3 (wt%) content is between 0 and 7.48, with the highest value detected in the Canegreca and Mattarana chromitites and the lowest detected in Santa Elena chromitites (Supplementary Materials, Table S1).
The values of TiO2 vary from 0.08 wt% to 0.34 wt% in the chromitites formed in a subduction-related setting, such as those of Santa Elena, Othrys, Vourinos and Troodos ophiolites (Supplementary Materials, Table S2). A relatively high TiO2 amount, from 0.49 wt% to 0.78 wt%, has been detected in the Bracco chromitites that are believed to have formed at a spreading centre [24].
The SiO2, ZnO, MnO, V2O3 and NiO amounts are typically below 1 wt% (Supplementary Materials, Table S2).

4.2. Fluid Inclusion Description and Raman Data

All the investigated chromitites contain numerous fluid inclusions (Supplementary Materials, Table S2), generally with a size comprised between 1 µm and 8 µm, more rarely up to 15 µm. Following the recommendations provided by Chi et al. [31] dealing with the importance of the paragenetic position of the host mineral in order to conduct a meaningful study of fluid inclusions, we observed that in our samples, all the fluid inclusions that occur are enclosed in unaltered chromite crystals crystallized at magmatic temperatures that have not been affected by alteration processes. The inclusions that occur are either isolated, widely scattered in chromite crystals or are arranged along trails (Figure 5A–H and Figure 6A–D).
The fluid inclusion trails, essentially, represent healed microcracks and can be described following the terminology proposed by Simmons and Richter [32] and Kranz [33]. The trails observed in the studied chromitites can be classified as intergranular inclusions occurring within single chromite grains as well as transgranular inclusions cross-cutting grain boundaries and cracks (Figure 6A–D).
Some of the fluid inclusions display a negative crystal shape, but irregular angular shapes also occur. Few inclusions display implosion–decrepitation textures. The identification of the phases is often impossible due to the bad transparency of chromite.
The inclusions consist of liquid (L) and vapour (V) (V~30–50 area%), although only vapour and solid phase inclusions have also been observed, and two-phase inclusions of L + V (V~40–60 area%, more rarely 10–80 area%), as well as those that are only vapour or only a solid phase.
Based on the petrographic observation, the fluid inclusions are more abundant in the Santa Elena, Othrys, Vourinos and Troodos high-Cr chromitites, related with a subduction zone environment, compared to the high-Al chromitites of the Bracco complex generated in a mid-ocean ridge geodynamic setting.
Despite the presence of abundant fluid inclusions, only a few of them were analyzed by Raman, because their small size and the opacity of the host spinels prevent the obtainment of reliable data as well as carrying out microthermometric investigations. Therefore, Raman spectroscopy was only applied to selected fluid inclusions that were big enough and found in enough transparent host spinel (sample JAG5). The host spinels were also analyzed to check the possible overlapping of the Raman bands. They show two well-defined bands between 530 cm−1 and 600 cm−1 and between 680 cm−1 and 730 cm−1, respectively. Several tiny fluid inclusions, characterized by a negative crystal shape (Figure 5E–H) and decrepitation texture, show some Raman bands comprised between 4100 cm−1 and 4400 cm−1. These bands correspond to those of the epoxy used during the samples’ preparation. The Raman bands of the host spinels and the epoxy are different from those reported by Frezzotti et al. [34] for the most common gases in fluid inclusions, and thus do not interfere with the identification of the phases that can be present in the selected fluid inclusions analyzed in this work. The presence of several gases, including CO2, H2, H2S, and N2, were tested; however, only CH4 was detected. It occurs in the secondary and indeterminable L + V inclusions of sample JAG5 in its V phase (Figure 7A–H). The peak of CH4 at 2917 cm−1 was found to be punctual and was observed in several fluid inclusions.
On the contrary, it was not possible to obtain reproducible data for the composition of the L phase. Only one fluid inclusion containing CH4 as a gaseous phase showed a weak peak at around 1630 cm−1, compatible with those of H2O liquid [34]. Based on these observations, we can suggest that the L phase of the inclusions is less Raman-active than the CH4 and the bad transparency of chromite prevents the collection of reliable signals.

5. Discussion

Fluid inclusions consist of small droplets that contain liquid, gases and/or solid phases, that may occur in almost all natural minerals. They provide an important tool to evaluate the nature of the various fluids responsible for the formation and evolution of rocks and minerals in a wide range of geological processes. Despite their ubiquity, more than 90% of the fluid inclusion studies are carried out on transparent minerals, usually quartz and, to a lesser extent, calcite [14]. In some cases, fluid inclusions have also been described in mafic minerals that may occur associated with chromitites, such as olivine and pyroxene [14,35]. Most of the published data on fluid inclusions in chromitites were obtained using microthermometry and mass spectrometer techniques that revealed the presence of low to moderate salinity aqueous inclusions with complex gas contents, including CH4, H2 and CO2 in the samples from Massif du Sud (New Caledonia) [18], from Troodos (Cyprus) [19], and from Kempirsai (Kazakhstan) [8]. The isotopic study of hydrogen in fluid inclusions analyzed in the Oman chromites revealed δD values that wer typical of mantle-derived fluids [17]. Raman spectroscopy is a versatile, non-destructive technique that is widely applied for fluid inclusion analysis to obtain the qualitative detection of the solid, liquid and gaseous components [34]. However, it has never been systematically used to investigate the nature of the fluid inclusions hosted in chromitites. The Raman data presented in this contribution, although they are preliminary and challenging, revealed that this methodology can be considered as a potential efficient tool to decipher the nature of the fluids available during the formation and later evolution of ophiolitic chromitite.
Our systematic finding of CH4-rich fluid inclusions provides direct natural evidence for the presence and importance of CH4 during the formation and evolution of podiform chromitites, as inferred by Etiope et al. [11] and Portella et al. [12].
However, these authors provided only whole-rock analyses and, in the absence of petrographic and mineralogical observations, they explained the anomalous enrichment in abiotic CH4 detected in certain podiform chromitites as solely due to the Sabatier reaction. An alternative model proposed by Klein et al. [35] suggests that the consumption of CH4-rich secondary fluid inclusions hosted in olivine during low-temperature processes might lead to widespread and significant contribution of abiotic CH4 content in submarine and subaerial vent systems on Earth. This option was dismissed by Etiope and Whiticar [36] in the case of the serpentinized peridotite of Chimeria, Turkey. Therefore, open questions still exist regarding the abiotic CH4 content of the mafic-ultramafic rocks of the oceanic lithosphere. The systematic presence of CH4-rich fluid inclusions in the investigated chromitites, however, can contribute to the understanding of those systems.
The occurrence of CH4-rich secondary fluid inclusions in the chromite points to an annealing process of fluid-filled fractures in the host mineral. This could take place at any time after the mineral formation, though there is an upper limit of temperature given by the brittle/ductile boundary of the host mineral/rock. In the case of the oceanic lithosphere, depending on the pressure, a maximum of 600–800 °C [37] can be taken into consideration as a favourable condition for fluid inclusion formation. These temperatures are consistent with primary temperatures, re-equilibration and the closure of the chromite crystallization system reported for several ophiolitic chromitites [24,38,39]. A simple explanation for the presence of the secondary inclusions in the studied chromitites could be their formation during low-temperature serpentinization, as that process includes the formation of CH4 via the Sabatier reaction [11]. However, all the fluid inclusions analyzed in this contribution occur in magmatic chromite grains that have not experienced low-temperature alteration processes. Therefore, similarly to the case of olivine described by Klein et al. [35], the fluid inclusions enriched in CH4 described in this work could have been trapped at high temperatures during the crystallization and annealing of the host chromite. During the evolution from magmatic to the low temperature of the host chromitites, several reactions, including CH4 formation, took place between the trapped fluids and the host chromite in a closed system. As a result, abundant CH4 content was detected in the secondary inclusions of the studied chromitite. Later, the CH4 hosted in both primary and secondary fluid inclusions can be released from the chromitite via fracturing and decrepitation processes. Once liberated from the fluid inclusions, CH4 cannot bind to any mineral present in the system. Therefore, it can be hosted only in pores and microfractures, as described in the work of Pappalardo et al. [40]. Hence, in addition to the already described Sabatier reaction [11], fluid inclusions in chromite grains should also be taken into consideration in order to explain the enrichment in CH4 reported for some ophiolitic chromitites. The CH4 reported in ophiolitic chromitites and those analyzed in fluid inclusions enclosed in olivine is considered to be abiotic in origin, i.e., formed by chemical reactions which do not involve organic matter [11,12,35]. On the contrary, Arai et al. [41] proposed that CH4 detected in micro-inclusions of olivine is associated with the meta-dunite from the Fujiwara, Sanbagawa metamorphic belt of Japan, which was supplied from the subducted and maturing organic matter. Our findings, although they are preliminary, suggest an abiotic origin for the CH4 analyzed in fluid inclusions enclosed in magmatic-formed host chromite. It is very likely that they were trapped at high temperatures from mantle fluids formed in the asthenosphere and originally enriched in CH4 [42] and not at low temperatures during alteration processes.

6. Summary and Conclusions

Examination of podiform chromitites from several ophiolite complexes located in Costa Rica, Greece and Cyprus, as well as layered chromitites associated with the Bracco ophiolite of Italy, has revealed the presence of abundant fluid inclusions in chromite. The major conclusions are as follows:
  • The research and Raman investigation of fluid inclusions in podiform chromitites is very challenging due to their lack of or poor transparency, although they may host millions of μm-sized fluid inclusions per cubic centimetre. A maximum thickness of 30 μm in thin, polished sections is considered to be optimal, because in some cases, enough light can pass through. This thickness prevents the application of the traditional methods for fluid inclusion studies in the analyzed samples. Furthermore, during the sample preparation, only very tiny fluid inclusions may preserve their original content of fluid, vapour, gas and solid phases.
  • Despite the analytical challenges, the Raman spectra reveal the systematic presence of CH4 in certain fluid inclusions. Considering the exceptionally high number of fluid inclusions that potentially contain CH4 found in the investigated chromites as well as the CH4-rich fluid inclusions previously reported in the New Caledonia chromitites [6,18], we suggest that fluid inclusions in chromite crystals and their leaching can be a possible alternative source of the high amount of CH4 detected in some podiform chromitites and not only through the Sabatier reaction as previously proposed by Etiope et al. [11] and Portella et al. [12] based on whole-rock analyses. Therefore, a careful mineralogical and petrographic study is strongly recommended in order to support the whole-rock geochemical data.
  • The mode of the occurrences of the studied CH4 bearing fluid inclusions, i.e., entrapped in unaltered chromite crystals formed at a magmatic temperature, suggest their origin from mantle-derived fluids, rather than those related to low-temperature serpentinization processes. Fluids with a mantle origin responsible for the presence of fluid inclusions were also described in the Oman chromitites [17].
  • The fluid inclusions in high-Cr chromitites are exceedingly numerous compared to the high-Al ones from the Bracco complex, Italy, suggesting more abundant fluids in chromitites related to a subduction zone rather than those in the mid-ocean ridge. This is in agreement with the model proposed for the formation of podiform chromitites from basaltic melts with primary H2O contents high enough to exsolve a H2O-rich fluid phase during decompressing in the mantle, in a supra subduction geodynamic setting, in contrast to basaltic magmas in mid-ocean ridges, which are too dry to exsolve a H2O-rich fluid [9].
  • Taking into consideration the wide distribution of podiform chromitites, the investigation of fluid inclusions, although difficult and challenging or even impossible when the chromite is opaque, can be applicable to other chromitites worldwide in order to verify the presence of H2O, CH4 and other volatile species. This information will greatly improve our understanding of the nature of the fluid phases during the formation of podiform chromitites.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15040335/s1, Table S1:Composition of spinel (wt%) in the studied chromitites; Table S2: Summary of the fluid inclusions observed in the studied samples.

Author Contributions

Field work and sample collections, F.Z., G.G. and M.E.-E.; conceptualization and data acquisition, F.Z., G.B.K., G.G., D.M., M.E.-E., M.H. and C.B.; writing—original draft preparation, F.Z. and G.B.K.; writing—review and editing, F.Z., G.B.K., G.G., D.M., M.E.-E., M.H. and C.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We would like to thank the ELTE Eötvös Loránd University Research and Industrial Relations Center for the access to the Raman spectroscopy laboratory. The work of G. B. Kiss is supported by the János Bolyai Research Scholarship awarded by the Hungarian Academy of Sciences.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Simplified geological maps of the studied ophiolites showing the locations of the studied chromitites: the Santa Elena ophiolite, Costa Rica [22], (A) the Bracco complex, Italy [24] (B), the Othrys [25] (C) and Vourinos [27] (D) ophiolites of Greece and the Troodos complex, Cyprus [16,29] (E).
Figure 1. Simplified geological maps of the studied ophiolites showing the locations of the studied chromitites: the Santa Elena ophiolite, Costa Rica [22], (A) the Bracco complex, Italy [24] (B), the Othrys [25] (C) and Vourinos [27] (D) ophiolites of Greece and the Troodos complex, Cyprus [16,29] (E).
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Figure 2. Examples of the studied chromitites. A field image of the small chromitites associated with a strongly altered peridotite, from Santa Elena ophiolite, Costa Rica (A); a hand sample of the layered chromitite at the transition between anorthosite and serpentinized peridotite of the Bracco complex, Italy (B); hand sample of massive coarse-grains of the Othrys ophiolite, Greece (C); a field image of a schlieren chromitite from the Vourinos ophiolite, Greece (D); and a field image of a massive pod in contact with serpentinite, from the Troodos complex, Cyprus (E).
Figure 2. Examples of the studied chromitites. A field image of the small chromitites associated with a strongly altered peridotite, from Santa Elena ophiolite, Costa Rica (A); a hand sample of the layered chromitite at the transition between anorthosite and serpentinized peridotite of the Bracco complex, Italy (B); hand sample of massive coarse-grains of the Othrys ophiolite, Greece (C); a field image of a schlieren chromitite from the Vourinos ophiolite, Greece (D); and a field image of a massive pod in contact with serpentinite, from the Troodos complex, Cyprus (E).
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Figure 3. Transmitted light microphotographs showing the texture of the studied samples. (A) Chromitite from Costa Rica (sample JAG5) and (B) chromitite from the Kannoures mine of the Troodos ophiolite, Cyprus. The white and greenish minerals are serpentine and chlorite.
Figure 3. Transmitted light microphotographs showing the texture of the studied samples. (A) Chromitite from Costa Rica (sample JAG5) and (B) chromitite from the Kannoures mine of the Troodos ophiolite, Cyprus. The white and greenish minerals are serpentine and chlorite.
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Figure 4. Classification of the oxyspinel-group minerals from the studied chromitites (Supplementary Materials, Table S2) in the diagram of Cr/(Cr + Al) versus Mg/(Mg + Fe2+).
Figure 4. Classification of the oxyspinel-group minerals from the studied chromitites (Supplementary Materials, Table S2) in the diagram of Cr/(Cr + Al) versus Mg/(Mg + Fe2+).
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Figure 5. Examples of fluid inclusions found in the studied chromitites. Abundant fluid inclusions (A) in the chromitite from Costa Rica (sample JAG6), (B) in the sample TD8 KAN from the Kannoures mine of the Troodos ophiolite, Cyprus, and (C) fluid inclusions containing liquid + gases, liquid + solid phases, and only liquid in the Tsangli deposit (sample TS2) of the Othrys ophiolite (Greece). Isolated liquid fluid inclusions (D) in the chromitite of the Kokkinorotsos mine (sample TD1 KOK), Troodos ophiolite (Cyprus), and (E) in the Cima Stronzi occurrence (sample SR40), Bracco ophiolite (Italy), the (F) liquid + solid phase fluid inclusion in the Canegreca chromitite (sample CN53), Bracco ophiolite (Italy), and fluid inclusions showing a ‘negative’ crystal shape (G) in the Mattarana chromitites of the Italian Bracco ophiolite (sample MT30) and (H) from the Santa Elena chromitite (sample JAG2) from Costa Rica.
Figure 5. Examples of fluid inclusions found in the studied chromitites. Abundant fluid inclusions (A) in the chromitite from Costa Rica (sample JAG6), (B) in the sample TD8 KAN from the Kannoures mine of the Troodos ophiolite, Cyprus, and (C) fluid inclusions containing liquid + gases, liquid + solid phases, and only liquid in the Tsangli deposit (sample TS2) of the Othrys ophiolite (Greece). Isolated liquid fluid inclusions (D) in the chromitite of the Kokkinorotsos mine (sample TD1 KOK), Troodos ophiolite (Cyprus), and (E) in the Cima Stronzi occurrence (sample SR40), Bracco ophiolite (Italy), the (F) liquid + solid phase fluid inclusion in the Canegreca chromitite (sample CN53), Bracco ophiolite (Italy), and fluid inclusions showing a ‘negative’ crystal shape (G) in the Mattarana chromitites of the Italian Bracco ophiolite (sample MT30) and (H) from the Santa Elena chromitite (sample JAG2) from Costa Rica.
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Figure 6. Examples of intergranular and transgranular fluid inclusions arranged along trails (A) in the chromitite from Voidolakkos mine (sample VS8 VOI), Vourinos ophiolite, Greece, (B) in the sample TS2 collected from the Tsangli chromium mine of the Othrys ophiolite, Greece, and (C,D) from the Santa Elena chromitites of Costa Rica (samples JAG2 and JAG6).
Figure 6. Examples of intergranular and transgranular fluid inclusions arranged along trails (A) in the chromitite from Voidolakkos mine (sample VS8 VOI), Vourinos ophiolite, Greece, (B) in the sample TS2 collected from the Tsangli chromium mine of the Othrys ophiolite, Greece, and (C,D) from the Santa Elena chromitites of Costa Rica (samples JAG2 and JAG6).
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Figure 7. (A) Two-phase (L + V, V~40 area%) fluid inclusions in the JAG 5 sample. The Raman spectra shown in (B) were obtained from the inclusion marked with an arrow. (B) Raman spectra of the inclusion of (A), showing the characteristic peak of CH4 at 2914 cm−1. (C) Two-phase (L + V, V~40 area%) fluid inclusions in the JAG 5 sample. The Raman spectra shown in (D) were obtained from the inclusion marked with an arrow. (D) Raman spectra of the inclusion of (C), showing the characteristic peak of CH4 at 2914 cm−1. (E) Two-phase (L + V, V~75 area%) fluid inclusions in the JAG 5 sample. The Raman spectra shown in (F) were obtained from the inclusion marked with an arrow. (F) Raman spectra of the inclusion of (C), showing the characteristic peak of CH4 at 2914 cm−1. (G) Two-phase (L + V, V~50 area%) fluid inclusions in the JAG 5 sample. The Raman spectra shown in (H) was obtained from the inclusion marked with an arrow. (H) Raman spectra of the inclusion of (G), showing the characteristic peak of CH4 at 2914 cm−1.
Figure 7. (A) Two-phase (L + V, V~40 area%) fluid inclusions in the JAG 5 sample. The Raman spectra shown in (B) were obtained from the inclusion marked with an arrow. (B) Raman spectra of the inclusion of (A), showing the characteristic peak of CH4 at 2914 cm−1. (C) Two-phase (L + V, V~40 area%) fluid inclusions in the JAG 5 sample. The Raman spectra shown in (D) were obtained from the inclusion marked with an arrow. (D) Raman spectra of the inclusion of (C), showing the characteristic peak of CH4 at 2914 cm−1. (E) Two-phase (L + V, V~75 area%) fluid inclusions in the JAG 5 sample. The Raman spectra shown in (F) were obtained from the inclusion marked with an arrow. (F) Raman spectra of the inclusion of (C), showing the characteristic peak of CH4 at 2914 cm−1. (G) Two-phase (L + V, V~50 area%) fluid inclusions in the JAG 5 sample. The Raman spectra shown in (H) was obtained from the inclusion marked with an arrow. (H) Raman spectra of the inclusion of (G), showing the characteristic peak of CH4 at 2914 cm−1.
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Table 1. Ophiolitic chromitites containing fluid inclusions investigated in this contribution.
Table 1. Ophiolitic chromitites containing fluid inclusions investigated in this contribution.
CountryOphioliteAgeGeodynamic SettingName of the DepositStratigraphyHost PeridotiteChromitite TypeChromite CompositionReferences
COSTA RICASanta ElenaCretaceousSSZ MantleHarzburgiteSmall podHigh-Cr[22] p.w.
ITALYBraccoJurassicMORCanegreca,Mantle-cumulateLherzoliteLayeredHigh-Al[23,24] p.w.
Cima Stronzi,Mantle-cumulateLherzoliteLayeredHigh-Al
MattaranaMantle-cumulateLherzoliteLayeredHigh-Al
GREECEOthrysJurassic-Cretaceous MOR-SSZAgios Stefanos,MantleHarzburgite-LherzoliteMassive podHigh-Al, High-Cr[25,26] p.w.
Metallion,MantleHarzburgite-LherzoliteMassive podHigh-Al, High-Cr
TsangliMantleHarzburgite-LherzoliteMassive podHigh-Al
VourinosJurassic-Cretaceous SSZVoidolakkosMantleHarzburgiteSchlierenHigh-Cr[27,28] p.w.
CYPRUSTroodosCretaceousSSZKokkinorotsos,MantleHarzburgite-DuniteMassive podHigh-Cr[16,29] p.w.
KannouresMantleHarzburgiteMassive podHigh-Cr
SSZ = supra-subduction zone; MOR = mid-ocean ridge; High-Cr = Cr/(Cr + Al) > 0.6; High-Al = Cr/(Cr + Al) < 0.6; p.w. = present work.
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Zaccarini, F.; Kiss, G.B.; Garuti, G.; Mauro, D.; Economou-Eliopoulos, M.; Hegedűs, M.; Biagioni, C. Methane in Fluid Inclusions in Ophiolitic Chromitites Revealed by Raman Spectroscopy: Preliminary Results. Minerals 2025, 15, 335. https://doi.org/10.3390/min15040335

AMA Style

Zaccarini F, Kiss GB, Garuti G, Mauro D, Economou-Eliopoulos M, Hegedűs M, Biagioni C. Methane in Fluid Inclusions in Ophiolitic Chromitites Revealed by Raman Spectroscopy: Preliminary Results. Minerals. 2025; 15(4):335. https://doi.org/10.3390/min15040335

Chicago/Turabian Style

Zaccarini, Federica, Gabriella B. Kiss, Giorgio Garuti, Daniela Mauro, Maria Economou-Eliopoulos, Máté Hegedűs, and Cristian Biagioni. 2025. "Methane in Fluid Inclusions in Ophiolitic Chromitites Revealed by Raman Spectroscopy: Preliminary Results" Minerals 15, no. 4: 335. https://doi.org/10.3390/min15040335

APA Style

Zaccarini, F., Kiss, G. B., Garuti, G., Mauro, D., Economou-Eliopoulos, M., Hegedűs, M., & Biagioni, C. (2025). Methane in Fluid Inclusions in Ophiolitic Chromitites Revealed by Raman Spectroscopy: Preliminary Results. Minerals, 15(4), 335. https://doi.org/10.3390/min15040335

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