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A detailed description of alkali water springs (pH > 10) found within the ultrabasic massif of Mount Soldatskaya in the Kamchatsky Mys Peninsula in Kamchatka is presented for the first time. The chemical composition of the springs and the dependence of the ratios and concentrations of some components on pH are indicative of the fact that these waters were involved in the present-day serpentinization of ultrabasic rocks. The springs with the highest alkali levels (pH 12.3) contain dissolved hydrogen at a concentration of about 0.6 mmol/l. The isotopic composition behavior of carbonate travertines deposited from these springs ([delta].sup.13C and [delta].sup.18O) differs from the known trend of \"meteogenic\" travertines related to serpentinization of ultrabasic rocks in Oman and California. The age of travertines determined by the radiocarbon method is close to modern.

Serpentinites are metamorphic rocks constituted primarily by serpentine-group minerals (antigorite, chrysotile, lizardite) resulting from the transformation and low-temperature hydration of previous olivine-rich ultramafic rocks, such as dunite, lherzolite, wehrlite, and harzburgite. The peculiar features of the serpentinites such as the greenish color and the intricate vein and mesh-like texture, as well as their role in CO[sub.2] sequestration when carbonated, have hugely increased interest in studying these rocks over recent decades. Moreover, since antiquity, serpentinites have long been exploited, traded, and exported worldwide as daily tools, as well as in buildings and decorative stones in both internal and external architectural elements, because of their aesthetic appeal, attractiveness, and durability. In this work, we analyzed and compared petrographic features, geochemical signatures, and physical–mechanical properties of serpentinites from historical quarries from Andalusia (southern Spain), Basilicata, and Calabria (southern Italy) where they have been used as dimension stones in religious and civil buildings and as construction materials. We aim to evaluate and assess differences in petrographic, carbonation, uniaxial compressive strength, and seismic behavior, that could affect the efficiency when these serpentinites are used as either building and construction materials or for preservation/renovation purposes in cultural heritage. Results obtained from petrophysical investigations of serpentinites from these regions highlight that these materials are suitable for use in construction to various extents and are considered a valuable georesource, behind a detailed characterization carried out before their implementation in construction or conservation/restoration of architectural heritage.

To investigate the effect of carbon-bearing phases on the release of fluids in subducted serpentinites, we performed high-pressure multi-anvil experiments on representative ophicarbonate assemblages over a pressure range from 2.5 GPa to 5 GPa and from 450 °C to 900 °C, across the antigorite-out reaction. Parallel experiments were performed on carbonate-free serpentinites. In all experiments, we monitored and/or controlled the oxygen fugacity. The addition of 20 wt. % CaCO.sub.3 to a serpentinite assemblage at 2.5 GPa is found to decrease the onset of the serpentine dehydration by over 100 °C, in comparison to carbonate-free assemblages. Similarly, the final disappearance of serpentine is also affected by the presence of CaCO.sub.3. For a bulk CaCO.sub.3 content of 20 wt. %, this causes a decrease in maximum stability of antigorite by 50 °C. For a bulk CaCO.sub.3 content exceeding 25 wt. %, this difference can be as high as 100 °C in warm and 150 °C in cold subduction zones, causing antigorite to be completely dehydrated at 500 °C. This results from the reaction of CaCO.sub.3 with serpentine to form clinopyroxene and Mg-rich carbonates. This reaction, however, causes no discernible decrease in the proportion of carbonate, indicating that the amount of released carbon is insignificant. Whilst CaCO.sub.3, therefore, influences serpentine stability, there is no significant effect of hydrous phases on the carbonate stability. On the other hand, a MgCO.sub.3-bearing system shows no significant effects on the serpentinite stability field. Further experiments and oxygen fugacity calculations indicate that graphite is not stable in typical magnetite-bearing serpentinites. The reduction of carbonates to graphite would require oxygen fugacities that are 1-2 log units below those of magnetite-bearing serpentinites. This confirms earlier studies and indicates that reduction of carbonates can only occur through the infiltration of external H.sub.2-rich fluids.

Aqueous fluids released by metamorphic dehydration of serpentinites are a key component for seismicity, creep, and geochemical cycling in subduction zones. How these fluids drain and migrate towards the mantle wedge has yet to be fully understood. Here we address the influence of pre-existing structural and mineralogical heterogeneities in serpentinites on dehydration and fluid migration at forearc conditions. We partially dehydrated natural serpentinite containing brucite veins in a piston-cylinder apparatus with a temperature gradient across the conditions of the brucite + antigorite = olivine + fluid reaction (485–520 °C; 1.5 GPa). Micro-tomography, electron microscopy and microstructural analysis of the experimental results, coupled with thermodynamic modelling, show that temperature, mineralogical heterogeneity and variable ingress of external H 2 controlled the dehydration extent. Experimentally formed olivine indicates a topotactic relationship between [100] Ol and [0001] Brc , although the resultant fabric is overall random because brucite was randomly oriented. Olivine forms mono-mineralic aggregates along the walls of brucite veins, displaying very high porosity (up to 32%) and permeability (10 –13 –10 –14 m 2 ). Tracing the pre-existing brucite vein network, these aggregates can form a transient network of interconnected, highly permeable fluid channels that allows drainage and may enhance open-system exchange with neighboring lithologies. Infiltration of reduced external fluids can trigger redox dehydration of magnetite + antigorite to Fe-rich olivine, which renews porosity and propagates focused fluid flow. The distribution of brucite and magnetite, especially as vein networks, therefore has a first-order control on how focused fluid drainage and flow paths develop during subduction of serpentinites.

Serpentinite carbonation contributes to the deep carbon (C) cycle. Recently, geophysical and numerical studies have inferred considerable hydrothermal alteration in plate bending faults, opening the possibility of significant C storage in the slab mantle. However, there is a lack of quantitative determination of C uptake in serpentinized mantle rocks. Here, we experimentally constrain serpentinite carbonation in H 2 O–CO 2 –NaCl fluids to estimate C uptake in hydrated mantle rocks. We find that serpentinite carbonation results in the formation of talc and magnesite along the serpentinite surface. The presence of porous reaction zones (49.2% porosity) promotes the progress of carbonation reactions through a continuous supply of CO 2 -bearing fluids to the reaction front. Added NaCl effectively decreases the serpentinite carbonation efficiency, particularly at low salinities (< 5.0 wt%), which is likely attributed to the reduction in fluid pH and the transport rate of reactants, and the increase in magnesite solubility. Based on previous and our experiments, we fit an empirical equation for the reaction rate of serpentinite carbonation. Extrapolation of this equation to depths of plate bending fault systems suggests that serpentinite carbonation may contribute to an influx of up to 7.3–28.5 Mt C/yr in subduction zones. Our results provide new insights into serpentinite carbonation in environments with high fluid salinities and potentially contribute to the understanding of the C cycle in subduction zones.

New hydrothermal experiments in rapid-quench pressure vessels have been performed to investigate the effect of redox state on the serpentinization reaction. The experimental hydrogen fugacity (fH2) was controlled by monitoring the mobility of H.sub.2 in the reacting system (internal vs. external fH2 control). This was achieved by using either Au (H.sub.2 impermeable) or AgPd (H.sub.2 permeable) capsules and Ar pressurizing gas to control fH2. The experiments were performed with either San Carlos olivine powders or Ãheim dunite chips. Water / rock mass ratios of 1-2, a total pressure of 50 MPa, and temperatures of 300 and 350 °C were investigated. Experimental durations of 30, 45, or â80 d were imposed. Serpentine production is observed in almost all experiments but is favored at 300 °C under external fH2 control. The serpentine-magnetite assemblage is observed in Au capsules (high fH2) at 300 °C, while the serpentine-hematite(-magnetite) is observed in AgPd capsules (low fH2). At 350 °C, less H.sub.2 is produced than at 300 °C and the serpentine-hematite(-magnetite) assemblage is present in both Au and AgPd capsules. Brucite is absent and this is interpreted to reflect both the initially oxidizing conditions and relatively low serpentine production in our experiments. Differences in product phase assemblages found in this study imply that natural serpentinization reaction mechanisms vary with redox conditions, and consequences for H.sub.2 production fluxes and rates can be expected. The high-fH2 (reduced) internally controlled experiments simulate low-permeability \"closed\" oceanic hydrothermal systems. The low-fH2 (oxidized) externally controlled experiments are analogous to \"open\" oceanic hydrothermal systems where serpentinization is driven by tectonically aided infiltration of an external fluid.

Many exposed high‐pressure meta‐serpentinites comprise a channelized network of olivine‐rich veins that formed during dehydration at depth and allowed the fluid to escape from the dehydrating rock. While previous studies have shown that chemical heterogeneities in rocks can control the formation of olivine‐enriched vein‐like interconnected porosity networks on the sub‐millimeter scale, it is still unclear how these networks evolve toward larger scales and develop nearly pure olivine veins. To explore this, we study the effect of reactive fluid flow on a dehydrating serpentinite. We use thermodynamic equilibrium calculations to investigate the effect of variations in the bulk silica content in serpentinites on the dehydration reaction of antigorite + brucite = olivine + fluid and the silica content of this fluid phase. Further, we develop a numerical model that combines the effects of intrinsic chemical heterogeneities with reactive transport with dissolved silica as metasomatic agent. Our model shows how reactive transport can lead to vein widening and olivine enrichment within a vein in an antigorite‐rich matrix, such as observed in the veins of the Erro Tobbio meta‐serpentinites. This is a critical step in the evolution toward larger‐scale vein systems and in the evolution of dynamic porosity, as this step helps account for the chemical feedback between the dehydrating rock and the liberated fluid. Key Points Brucite abundant domains release a fluid with a lower silica content than antigorite‐rich domains Reactive fluid flow can trigger dehydration Preexisting vein‐like structures form pathways for a low silica fluid to generate near pure olivine veins

The iron chemistry of serpentinites and serpentine group minerals is often invoked as a record of the setting and conditions of serpentinization because Fe behaviour is influenced by reaction conditions. Iron can be partitioned into a variety of secondary mineral phases and undergo variable extents of oxidation and/or reduction during serpentinization. This behaviour influences geophysical, geochemical and biological aspects of serpentinizing systems and, more broadly, earth systems. Iron chemistry of serpentinites and serpentines is frequently analysed and reported for single systems. Interpretations of the controls on, and the implications of, Fe behaviour drawn from a single system are often widely extrapolated. There is a wealth of serpentinite/serpentine chemical composition data available in the literature. Consequently, compilation of a database including potential predictors of Fe behaviour and measures of Fe chemistry enables systematic investigation of trends in Fe behaviour across a variety of systems and conditions. The database presented here contains approximately 2000 individual data points including both bulk rock and serpentine mineral geochemical data which are paired whenever possible. Measures of total Fe and Fe oxidation state, which are more limited, are compiled with characteristics of the systems from which they were sampled. Observations of trends in Fe chemistry in serpentinites and serpentines across the variety of geologic systems and parameters will aid in verifying and strengthening interpretations made on the basis of Fe chemistry. This article is part of a discussion meeting issue ‘Serpentinite in the Earth system’.

The De’erni Cu–Zn-Co deposit is a typical altered ultramafic-hosted volcanogenic massive sulfide deposit comprising four lenticular main orebodies (0.57 Mt Cu, 1.27% Cu average ore grade; 0.03 Mt Co, 0.09% Co average ore grade; 0.16 Mt Zn, 1.04% Zn average ore grade) hosted in serpentinite and a 200-m-thick basalt was found below the No. I orebody. Serpentinite spinel Al2O3, TiO2, Cr#, and Mg# indicate a mantle-source. Serpentinite magmatic-hydrothermal genesis is indicated by the following: (i) high Rb/Y and Th/Zr ratios, low Nb/Zr ratios, and low δ65Cu values; (ii) altered magnetite rims on spinel being characterized by high Cr, Ni, and Ti, and low Ga contents; (iii) pyrite appears along the boundary of spinel grains and has a higher Co and Ni content than pyrite in ores. Therefore, the ultramafic host rocks are formed by strong fluid alteration of primary mantle rocks. The compositional zoning of Co, Cu, and Zn in euhedral coarse-grained pyrite from massive sulfide ore suggests that metal enrichment was associated with three fluid phases, with a clear temporal interval between the fluid activity that introduced Co/Cu enrichment and Zn enrichment (Zn-rich veins in magnetite cross-cut early spinel). Serpentinite exhibits a higher Zn content and decoupling of Ni and Co contents compared to Dur’ngoi ophiolite serpentinite distal from the orebody, implying primary ultramafic rocks may have provided Co to the ores. The apparently high Cu content of the Dur’ngoi ophiolite basalt in comparison with ophiolite basalts worldwide indicates basalt may have supplied the Cu.