One of the largest salt deposits in the world, a salt giant, is hidden in the subsurface of the Red Sea area, including deep-sea zones. Besides, brine and salt layers blanket different parts of the Red Sea floor. In general, mass-balance calculations and geological considerations indicate that climate-driven evaporitic processes cannot form salt giants, especially those in deep marine basins. Conversely, hydration of anhydrous mafic minerals can produce effective salinization of wide sectors of the oceanic lithosphere, if the newly formed minerals, such as serpentine, assume only water in the form of OH groups and reject seawater salts. Therefore, concentrated brines and salts can be temporarily stored in the oceanic serpentinites and released during a later time by various mechanisms, giving rise to saline deposits even gigantic in size. Moreover, some of the released brines can rise to the seafloor as hydrothermal solutions or as buoyant saline diapirs. On these premises, the Red Sea salt giant and the related saline vents may have a serpentinite origin, consistent with geophysical indications of diapiric structures in the sub-surface and occurrences of ultramafic rocks in the region, particularly at Zabargad Island.

As shown in previous chapters, nanodiamonds can form in the stability field of graphite in the presence of organic matter and water. Such conditions characterize different shallow-seated geological contexts, including oceanic serpentinite systems. In fact, nanodiamond clusters were found in some serpentinite xenoliths from Sicily. The discovery was made by HRTEM electron microscopy observations coupled with Raman spectroscopy analyzes. NDs formed in the temperature interval of 150–350 °C and at pressures lower than 2 kbar from an organic-water system. As indicated by an increasing set of literature reports, microdiamonds occur in various ophiolite complexes around the world. Although the origin in deep oceanic mantle is the most popular hypothesis for ophiolitic microdiamonds, it is also possible that they were formed in the stability field of graphite on the seeds of nanodiamonds carried out in supercritical hydrothermal fluids circulating in oceanic serpentinite systems, particularly during hydrodynamic cavitation episodes. It is also highlighted that nanodiamonds in oceanic serpentinite systems possibly stimulated the assemblage of complex bioorganic molecules relevant to the emergence of life on early Earth. Nano- and micron-sized diamonds were also found in garnet peridotite xenoliths from the Salt Lake Crater (Hawaii). Although a deep mantle origin for the Hawaiian diamonds is generally indicated, here, it is suggested that they precipitated metastably under low-pressure conditions from reduced C-O-H fluids during the interaction between the upwelling basaltic magma and a serpentinized section of the oceanic crust. Accordingly, the enigmatic microdiamond aggregates worldwide known as “carbonados” possibly formed during the final stage of serpentinization processes in the primary oceans of Earth from an organic-water system. Although the origin of nonkimberlitic continental microdiamonds may be related to buried serpentinite bodies cross-cut by upwelling magma, this hypothesis cannot yet be proven due to the lack of relevant data.

The model of nanosized diamond particle formation at metastable P‒T parameters from a C–H–O fluid system is presented. This explains the hydrothermal formation and growth of diamond and the specifics of CVD diamond synthesis gas mixtures at low P‒T parameters. The present model makes the simplest possible assumptions about the key processes and is then able to account for various tendencies seen in experimental and natural data. The role of carbon isotopes and nitrogen in diamond nucleus formation is also discussed. The determined relations may help to develop new models needed for diamond formation and deposition.

The results of experiments at low P‒T parameters on nanosized diamond, diamond-like phases, and fullerene-like structure formation from organics are presented. The products of experiments at 500 °C and 1 kbar include diamond, carbine, cubic carbon, and lonsdaleite particles 70–80 nm in size, as indicated by Raman observations and TEM analyzes. Fullerene and fullerene-like phases were synthesized at 700–750 °C and 5 kbar, resembling onion-like structures in wood charcoals carbonatized at 700 °C. The experimental data prove that nano- and microdiamonds can originate from fluids without crystal seeds at P‒T parameters corresponding to graphite stability, according to the theoretical assumption provided in Chap. 2 . Experiments also show that fullerenes, carbon nanotubes, and onion-like carbon structures can be formed from reduced hydrocarbon fluids at temperatures lower than 1000 °C.

Samples of anthraxolites, graphite coals from Taimyr, shungites from Karelia, and anthracites from Donbass and Kuzbass were studied using Raman spectroscopy. In summary, peaks at 1311–1326 cm–1 corresponding to nanodiamonds were recorded in all studied samples. This shows that nanocrystalline diamonds can form both in the late stages of lithogenesis (catagenesis and metagenesis) and during contact metamorphism. The experimental results reported in Chap. 3 show that rare onion-like carbon structures from Karelian shungites could be formed from hydrocarbon gases under pyrometamorphic conditions. Mineralogical, geological, and geochemical data on the Kokchetav massive allow us to conclude that Kokchetav microdiamonds were formed during crustal metamorphic processes from hydrothermal fluids bearing hydrocarbon and nitrogen at P‒T parameters corresponding to graphite stability and that their UHP origin is questionable.

This book explains the properties and formation mechanisms of various types of diamonds, especially nanodiamond

The large body of published mineralogical and experimental data and P‒T- $$f_{{O_{2} }}$$ estimates were used to establish the processes of mantle and kimberlitic diamond formation. As a result, three main stages of diamond formation should be highlighted: 1. Asthenospheric melt-driven; 2. Lithospheric fluid-driven; 3. Shallow mantle or crustal hydrothermal-driven. Diamond formation in the upper mantle is generally related to aqueous fluids at pressures and temperatures corresponding to “cold” geotherms. Macrodiamonds can have multiple origins. Extralarge Type IIa diamonds can originate in a P‒T range from lower mantle to crustal depths, and their main build-up takes place from fluids during the solidification of kimberlite magma at crustal depths.

This paper reports the first multidisciplinary petrologic, mineralogical, and geochemical studies of the near-crater tephra discharged by the 1669 catastrophic eruption of Etna stratovolcano, Sicily. We studied the grain-size distribution, chemical and mineral-phase composition of the tephra. We determined the composition of trace elements and the composition of encapsulated lithogenic gases. Etna is classified as an intraplate volcano with a deep-seated magma chamber. Of special importance is the fact that the Etnean products were found to contain volcanogenic organoids that have phase, elemental, and isotope compositions similar to the organoids encountered in diamond-bearing products discharged by some Kamchatka volcanoes. This corroborates out earlier inference that carbonaceous abiogenesis is ubiquitous in the conditions of onshore volcanism.

Rock fragments from the deepest parts of a buried hydrothermal system belonging to the Mesozoic Tethys Ocean entered as xenoliths in a Miocenic diatreme, hence brought to the surface, in the Hyblean Plateau (Sicily). Some xenoliths consist of strongly serpentinized ultramafic rocks bearing blebs of abiotic organic matter, where clusters of amorphous carbon nanoparticles, including nanodiamonds, are immersed. Such an occurrence conjures up established hypotheses that diamond surfaces are suitable catalytic platforms stimulating the assemblage of complex bio-organic molecules relevant to the emergence of life on Earth. The appearance of bio-organic molecules under primitive Earth conditions is one of the major unsolved questions on the origin of life. Here we report new micro-Raman spectra on blebs of abiotic organic matter from a selected xenolith. Diamond bands were related to hydrogenated nanocrystalline diamonds, with size of nearly 1–1.6 ​nm, formed from organics at low pressures and temperatures. In particular, diamond surfaces can give rise to crystalline interfacial water layers that may have played a fundamental role in the early biosphere evolution as a good medium for rapidly transporting positive charges in the form of hydrated protons. Nowadays, proton gradients in alkaline hydrothermal vents along oceanic ridges are generally viewed as key pre-biotic factors. In general, serpentinites span the entire geological record, including prebiotic times. These hydrous ultramafic rocks often display evidence of abiotic carbon species, both organic and inorganic, including nanodiamonds, being also capable to give rise to chemiosmotic processes and proton gradients necessary to the organisms, such as the “Last Universal Common Ancestor” (LUCA), in the prebiotic Earth.

Geochemical characteristics of middle ocean ridge basalts (MORBs) testify partial melting of spinel-peridotite mixed with a few amounts of garnet-pyroxenite. The latter can be considered either autochthonous products of the crystallization of partial melts in the sub-oceanic mantle or allocthonous recycled crustal materials originated in subduction contexts. Here we suggest the “autocthnous recycled” origin for garnet-pyroxenites. Such a hypothesis derives from the study of garnet-bearing pyroxenite xenoliths from the Hyblean Plateau (Sicily). These consist of Al-diopside, pyralspite-series garnet, Al-spinel and Al-rich orthopyroxene. Trace element distribution resembles an enriched MORB but lower chromium. Major-element abundances closely fit in a tschermakitic-horneblende composition. Assuming that a high-Al amphibolite was formed by hydrothermal metasomatism of a troctolitic gabbro in a slow-spreading ridge segment, a transient temperature increasing induced dehydroxilization reaction in amphiboles, giving Al-spinel-pyroxenite and vapor as products. Garnet partially replaced spinel during an isobaric cooling stage. Density measurements at room conditions on representative samples gave values in the range 3290–3380 kg m−3. In general, a density contrast ≥300 kg m−3 can give rise to convective instability, provided a sufficient large size of the heavy masses and adequate rheological conditions of the system. Garnet-pyroxenite lumps can therefore sink in the underlying mantle, imparting the “garnet geochemical signature” to newly forming basaltic magma.

Ultramafic magmas (MgO ≥ 18 wt%) are generally thought to be primary mantle melts formed at temperatures in excess of 1600 °C. Volatile contents are expected to be low, and accordingly, high-Mg magmas generally do not yield large explosive eruptions. However, there are important exceptions to low explosivity that require an explanation. Here we show that hydrous (hence, potentially explosive) ultramafic magmas can also form at crustal depths at temperatures even lower than 1000 °C. Such a conclusion arose from the study of a silicate glass vein, ~1 mm in thickness, cross-cutting a mantle-derived harzburgite xenolith from the Valle Guffari nephelinite diatreme (Hyblean area, Sicily). The glass vein postdates a number of serpentine veins already existing in the host harzburgite, thus reasonably excluding that the melt infiltrated in the rock at mantle depths. The glass is highly porous at the sub-micron scale, it also bears vesicles filled by secondary minerals. The distribution of some major elements corresponds to a meimechite composition (MgO = 20.35 wt%; Na2O + K2O < 1 wt%; and TiO2 > 1 wt%). On the other hand, trace element distribution in the vein glass nearly matches the nephelinite juvenile clasts in the xenolith-bearing tuff-breccia. These data strongly support the hypothesis that an upwelling nephelinite melt (MgO = 7–9 wt%; 1100 ≤ T ≤ 1250 °C) intersected fractured serpentinites (T ≤ 500 °C) buried in the aged oceanic crust. The consequent dehydroxilization of the serpentine minerals gave rise to a supercritical aqueous fluid, bearing finely dispersed, hydrated cationic complexes such as [Mg2+(H2O)n]. The high-Mg, hydrothermal solution "flushed" into the nephelinite magma producing an ultramafic, hydrous (hence, potentially explosive), hybrid magma. This hypothesis explains the volcanological paradox of large explosive eruptions produced by ultramafic magmas.

Pyrite and organic matter closely coexist in some hydrothermally-altered gabbroic xenoliths from the Hyblean Plateau, Sicily. The representative sample consists of plagioclase, Fe-oxides, clinopyroxene, pyrite and minor amounts of many other minerals. Plagioclase displays incipient albitization, clinopyroxene is deeply corroded. Pyrite grains are widely replaced by spongy-textured magnetite, which locally hosts Ca-(and Fe-)sulfate micrograins and blebs of condensed organic matter. Whole-rock trace element distribution evidences that incompatible elements, particularly the fluid-mobile Ba, U and Pb, are significantly enriched with respect to N-MORB values. The mineralogical and geochemical characteristics of the sample, and its U-Pb zircon age of 216.9 ± 6.7 MA, conform to the xenolith-based viewpoint that the unexposed Hyblean basement is a relict of the Ionian Tethys lithospheric domain, mostly consisting of abyssal-type serpentinized peridotites with small gabbroic intrusions. Circulating hydrothermal fluids there favored the formation of hydrocarbons trough Fischer-Tropsch-type organic synthesis, giving also rise to sulfidization episodes. Subsequent variations in temperature and redox conditions of the system induced partial de-sulfidization, Fe-oxides precipitation and sulfate-forming reactions, also promoting poly-condensation and aromatization of the already-formed hydrocarbons. Here we show organic matter adhering to a crystal face of a microscopic pyrite grain. Pyrite surfaces, as abiotic analogues of enzymes, can adsorb and concentrate organic molecules, also acting as catalysts for a broad range of proto-biochemical reactions. The present data therefore may support established abiogenesis models suggesting that pyrite surfaces carried out primitive metabolic cycles in suitable environments of the early Earth, such as endolithic recesses in mafic rocks permeated by hydrothermal fluids.

Serpentinites occur worldwide in various geological settings, such as in magma-starved oceanic basins. Abyssal serpentinites are long-term storage for chlorine and carbon, the latter forming dominantly reduced and sub-ordinately oxidized compounds. Hence, serpentinites, salts, and reduced carbon phases coexist in an "abyssal association". Although huge reservoirs of salts and reduced carbonaceous matter in abyssal serpentinites are thought to be lacking, this may be more a reflection of intrinsic limits in research methods than an indication of their absence. On the other hand, shallow-seated hydrocarbon-rich mud volcanoes and diapirs, consisting of salts, hydrocarbons (oils and gases), and clayey muds (including mudded serpentines) can provide evidence for such an oceanic association. Here, we search for scientific data in support of the "oceanic association", and hence we propose a reappraisal of exploitable worldwide salt-hydrocarbon reservoirs, even gigantic in size. Abyssal serpentinites are well-known source rocks of abiotic gaseous hydrocarbons via Fischer-Tropsch-type reaction (FTT), whereas macromolecular carbonaceous matter (mostly bitumen) found In serpentinites is postulated to be biogenic. This inference is based mainly on three assumptions: (i) FTT in nature does not yield oils; (ii) hydrocarbons derive essentially from the thermal alteration of microbial molecules; (iii) the presence of bio-markers, traditionally considered evidence of the biological origin of oils. Nevertheless, some lines of strong evidence disprove these accepted conventions: (i) Lab experiments demonstrated that FTT can synthesize significant quantities of liquid hydrocarbons (oils) under conditions compatible with the deeper and more reduced level of serpentinite-hosted hydrothermal systems; (ii) macromolecules, generally thought to derive from microbial activity, can be abiotically generated as documented in extraterrestrial bodies; (iii) biomarkers are evidence for the proliferation of microbial communities, which subsist on serpentinization and FTT products, thus they are biological pollutants. Moreover, seawater-driven serpentinization consumes water producing huge amounts (similar to 11 kg halite.m(-3) of peridotite) of salts that can be stored in the deeper zone of abyssal-type hydrothermal systems. Thus, the origin of shallow-sited salt deposits, even gigantic in size, can be related to either the advective upwelling of hot hydrothermal brines, due to the dehydration of abyssal serpentinites, or to the passive upwelling of buoyant saline geobodies. Piercement structures are indeed seismically imaged in several localities, such as South Atlantic, the Gulf of Mexico, the Serpentinite belt extending from Cuba to Hispaniola, the North Sea trench, and the Zagros orogenic belt. Similar lines of evidence at the microscopic scale have been found in some Hyblean serpentinite xenoliths (south-eastern Sicily), being representative of the unmetamorfosed in-situ relic of the Palaeo-Tethys Ocean, upon which Sicily and its off-shore areas lie. In spite of the accepted stereotype of hydrocarbons as fossil fuels, and hence finite resources, reduced carbon compounds abiotically produced in abyssal serpentinites may be copious, protracted over time, and thus renewable.

Interpretation of seismic profiles and results of scientific drillings in the Mediterranean subseafloor provided indication of gigantic salt deposits which rarely crop out on land, such as in Sicily. The salt giants were ascribed to the desiccation, driven by the solar energy, of the entire basin. Nevertheless, the evaporite model hardly explains deep-sea salt deposits. This paper considers a different hypothesis suggesting that seawater reached NaCl saturation during serpentinization of ultramafic rocks. Solid salts and brine pockets were buried within the serpentinite bodies being later (e.g., in the Messinian) released, due to serpentinite breakdown, and discharged at seafloor as hydrothermal heavy brines. Therefore, sea-bottom layers of brine at gypsum and halite saturation were formed. The model is applicable to the Mediterranean area since geophysical data revealed relicts of an aged (hence serpentinized) oceanic lithosphere, of Tethyan affinity, both in its western “Atlantic” extension (Gulf of Cádiz) and in eastern basins, and xenoliths from Hyblean diatremes (Sicily) provided evidence of buried serpentinites in the central area. In addition, the buoyant behavior of muddled serpentinite and salts (and hydrocarbons) gave rise to many composite diapirs throughout the Mediterranean area. Thus, the Mediterranean “salt giant” consists of several independent geobodies of serpentinite and salts.

Eight samples of Pliocenic/Pleistocenic volcanic rocks from the Hyblean area (south-eastern Sicily, Italy) were investigated for noble gases, trace elements, and Sr-Nd isotopes. The samples consist of tholeiitic basalts, basanites, and nephelinites and display a variable Sr-Nd isotopic composition (Sr-87/Sr-86 = 0.70275-0.70284 and Nd-143/Na-144, = 0.51312-0.51316 in the tholeiites; Sr-87/Sr-86 = 0.70294-0.70331 and Nd-143/Nd-144 = 0.51293-0.51308 in the basanites/nephelinites). Most of the investigated lavas show almost constant He-3/He-4 ratios of about 7.0 Ra, except one basanite and one tholeiite-basalt which exhibit He-3/He-4 ratios of 3.5 Ra and 8.3 Ra, respectively. The two samples, both characterized by low abundance of gases, are affected by secondary inputs of He-4 and He-3 that led to a wide deviation from the leading value of 7.0 Ra. We suggest that a heterogeneous lithosphere fed the Quaternary volcanism beneath the Hyblean Plateau. This mantle source is featured by a variable Sr-Nd isotopic composition as a consequence of patchy partial melting or cryptic metasomatism events. Its almost constant isotopic helium is due to the homogenizing effect of deep fluids periodically infiltrating the lithosphere. Although the samples show the same He isotope signatures as mantle harzburgite xenoliths from Hyblean Miocenic diatremes, the distribution of some trace element pairs argues against a close geochemical parentage between the Pliocenic-Pleistocenic volcanic rocks and mantle xenoliths. This can be reconciled if the xenoliths come from mantle portions whose trace element distribution was modified by carbonatitic metasomatic melts responsible for the enrichment of incompatible trace elements but not able to carry significant He amounts. (C) 2018 Published by Elsevier B.V.

In this study, we derived the first 3-D P-wave seismic attenuation images (Q(P)) as well as new 3-D V-P and V-P/V-S models for the crust in southeastern Sicily. We used a large data set of local seismic events occurring in the time span 1994-2013. The results of this tomographic study have important implications on the seismic behaviour of the region. Based on velocity and attenuation images, we identified distinct volumes characterized by different fluid content, which correlate well with seismicity distribution. Moreover, the obtained velocity and attenuation tomographies help us to provide a more complete picture of the crustal structure of the area. High V-P, high Q(P) and high V-P/V-S values have been obtained in the crustal basement, below a depth of 8 km, and may be interpreted as due to the presence of Sserpentinized peridotites. Accordingly, the new model for the degree of serpentinization, retrieved from V-P values, shows that the basement has an average serpentinization value of 96 +/- 3 vol.% at 8 km, decreasing to 44 +/- 5 vol.% at about 18-20 km. Our joint interpretation of geophysical and petrophysical evidence suggests that the nature and composition of the Hyblean upper lithosphere may differ from accepted and long-established geological models, which consider this lithospheric block a continuation of the Africa continental plate.