Serpentine minerals in natural samples are dominated by lizardite and antigorite. In spite of numerous petrological experiments, the stability fields of these species remain poorly constrained. This paper presents the petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P < 4 kbar, T ~ 200-300°C) to eclogite facies conditions (P > 20 kbar, T > 460°C) along a subduction geothermal gradient. We used the observed mineral assemblage in natural serpentinite along with the T max estimated by Raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures. We show that below 300°C, lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, lizardite is progressively replaced by antigorite at the grain boundaries through dissolutionprecipitation processes in the presence of SiO 2 enriched fluids and through a solid-state transition in the cores of the lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.

. (2016) 'Titanium stable isotope investigation of magmatic processes on the Earth and Moon.', Earth and planetary science letters., 449 . pp. 197-205. Further information on publisher's website: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.

Subduction zones are one of the most important sites of chemical interchange between the Earth's surface and interior. One means of explaining the high Fe 3+ /ƩFe ratios and oxidized nature of primary arc magmas is the transfer of sulfate (SO X), carbonate (CO 3-), and/or iron (Fe 3+) bearing fluids from the slab to the overlying mantle. Iron mobility and Fe stable isotope fractionation in fluids are influenced by Fe redox state and the presence of chlorine and/or sulfur anions. Here we use Fe stable isotopes (d 56 Fe) as a tracer of iron mobility in serpentinites from Western Alps metaophiolites, which represent remnants of oceanic lithosphere that have undergone subduction-related metamorphism and devolatilization. A negative correlation (R 2 = 0.72) is observed between serpentinite bulk d 56 Fe and Fe 3+ /ƩFe that provides the first direct evidence for the release of Fe-bearing fluids during serpentinite devolatilization in subduction zones. The progressive loss of isotopically light Fe from the slab with increasing degree of prograde metamorphism is consistent with the release of sulfate-rich and/or hypersaline fluids, which preferentially complex isotopically light Fe in the form of Fe(II)-SO X or Fe(II)-Cl 2 species. Fe isotopes can therefore be used as a tracer of the nature of slab-derived fluids.

No abstract

No abstract

Subduction zones facilitate chemical exchanges between Earth’s deep interior and volcanism that affects habitability of the surface environment. Lavas erupted at subduction zones are oxidized and release volatile species. These features may reflect a modification of the oxidation state of the sub-arc mantle by hydrous, oxidizing sulfate and/or carbonate-bearing fluids derived from subducting slabs. But the reason that the fluids are oxidizing has been unclear. Here we use theoretical chemical mass transfer calculations to predict the redox state of fluids generated during serpentinite dehydration. Specifically, the breakdown of antigorite to olivine, enstatite, and chlorite generates fluids with high oxygen fugacities, close to the hematite-magnetite buffer, that can contain significant amounts of sulfate. The migration of these fluids from the slab to the mantle wedge could therefore provide the oxidized source for the genesis of primary arc magmas that release gases to the atmosphere during volcanism. Our results also show that the evolution of oxygen fugacity in serpentinite during subduction is sensitive to the amount of sulfides and potentially metal alloys in bulk rock, possibly producing redox heterogeneities in subducting slabs.

International audienceThe Cerro del Almirez massif (Spain) represents a unique fragment of serpentinized oceanic lithosphere that has been first equilibrated in the antigorite stability field (Atg-serpentinites) and then dehydrated into chlorite–olivine–orthopyroxene (Chl-harzburgites) at eclogite facies conditions during subduction. The massif preserves a dehydration front between Atg-serpentinites and Chl-harzburgites. It constitutes a suitable place to study redox changes in serpentinites and the nature of the released fluids during their dehydration. Relative to abyssal serpentinites, Atg-serpentinites display a low Fe3+/FeTotal(BR) (=0.55) and magnetite modal content (=2.8–4.3 wt%). Micro-X-ray absorption near-edge structure (μ-XANES) spectroscopy measurements of serpentines at the Fe–K edge show that antigorite has a lower Fe3+/FeTotal ratio (=0.48) than oceanic lizardite/chrysotile assemblages. The onset of Atg-serpentinites dehydration is marked by the crystallization of a Fe3+-rich antigorite (Fe3+/FeTotal = 0.6–0.75) in equilibrium with secondary olivine and by a decrease in magnetite amount (=1.6–2.2 wt%). This suggests a preferential partitioning of Fe3+ into serpentine rather than into olivine. The Atg-breakdown is marked by a decrease in Fe3+/FeTotal(BR) (=0.34–0.41), the crystallization of Fe2+-rich phases and the quasi-disappearance of magnetite (=0.6–1.4 wt.%). The observation of Fe3+-rich hematite and ilmenite intergrowths suggests that the O2 released by the crystallization of Fe2+-rich phases could promote hematite crystallization and a subsequent increase in fo2 inside the portion of the subducted mantle. Serpentinite dehydration could thus produce highly oxidized fluids in subduction zones and contribute to the oxidization of the sub-arc mantle wedge

International audienceWe compare magnetic properties of 58 variably serpentinized peridotites from three ophiolite complexes (Pindos, Greece; Oman; Chenaillet, France) and the mid-Atlantic Ridge near the Kane fracture zone (MARK). The Pindos and Oman sites show low susceptibility and remanence (K < 0.02 SI; M s < 0.4 Am 2 / kg), while the Chenaillet and MARK sites show instead high susceptibility and remanence (K up to 0.15 SI; M s up to 6 Am 2 /kg), regardless of serpentinization degree. Petrographic observations confirm that Pindos and Oman samples contain serpentine with very little magnetite, while Chenaillet and MARK samples display abundant magnetite in serpentine mesh cells. Bulk rock analyses show similar amounts of ferric iron at a given serpentinization degree, suggesting that iron is oxidized during the serpentinization reaction in both cases, but that its distribution among phases differs. Microprobe analyses show iron-rich serpentine minerals (5–7 wt % FeO) in low-susceptibility samples, while iron-poor serpentine minerals (2–4 wt % FeO) occur in high susceptibility samples. The contrasted magnetic properties between the two groups of sites thus reflect different iron partitioning during serpentinization, that must be related to distinct conditions at which the serpentinization reaction takes place. We propose that magnetic properties of ophiolitic serpen-tinites can be used as a proxy to differentiate between high temperature serpentinization (>250–3008C) occurring at the axis (i.e., Chenaillet, similar to serpentinites from magmatically poor mid-ocean ridges), from lower temperature serpentinization (<200–2508C), likely occurring off axis and possibly during obduction (i.e., Pindos and Oman). At both settings, serpentinization can result in significant hydrogen release