Antoine Bénard

• PhD
• ClerVolc Research Fellow at University of Clermont Auvergne

Chen et al. (2023) reported serpentinized, SiO2-rich ophiolitic peridotites from Central Asia and argued that these were formed by reworking of residual mantle by boninite melts in arc settings. While SiO2- rich ophiolitic peridotites were reported and linked to subduction zone processes earlier (e.g., Ionov et al., 2017), Chen et al. claimed that...

Ultramafic rocks of the Yarlung-Tsangpo ophiolites (lherzolites, harzburgites, dunites and pyroxenites) have been interpreted as lithospheric mantle relics of the Cretaceous Neo-Tethys Ocean (NTO). Studying these rocks could provide key constraints on the NTO’s evolution and invaluable information on sub-ridge mantle heterogeneity. The Xiugugabu op...

In the first article, we have reported petrological data for a new, glass-bearing orthopyroxenite vein cutting a sub-arc mantle xenolith from Kamchatka. As similar veins from the West Bismarck arc, this orthopyroxenite is sulfide-rich and formed by cooling of parental melts derived by partial melting of spinel harzburgite sources. Here, I report ne...

We report abundances of major trace and volatile elements in an orthopyroxenite vein cutting a sub-arc, mantle-derived, spinel harzburgite xenolith from Kamchatka. The orthopyroxenite contains abundant sulfides and is characterized by the presence of glass (formerly melt) both interstitially and as inclusions in minerals, comparable with similar ve...

Recent studies of serpentine-free, spinel peridotite xenoliths from the mantle lithosphere beneath the active Kamchatka and West Bismarck arcs have shown that these rocks are enriched in silica and highly depleted in incompatible elements in comparison with melting residues of either primitive or mid-ocean ridge mantle. It has been suggested that t...

Layered mafic-ultramafic intrusions (LMI) are among the largest igneous bodies on Earth, and represent aggregations of large volumes of mantle- and some crustal-derived melts. Melts are emplaced over time-intervals of less than 1 million years, predominantly through multiple pulses of injections into pre-existing melt-crystal slurries. The dynamic...

Hybridization-melting is a concept that emerged from highpressure experimental studies by P.J. Wyllie’s group in the 80’s [1, 2]. The experiments have highlighted that liquidus phase relationships of tonalite-peridotite mixtures from 1.5 to 3 GPa are characterised by (orthopyroxene ± garnet)-rich mineral assemblages. Recent data from the lithospher...

Many island arc lavas (IAL) in the western Pacific carry a distinct DupAl-or Indian-type mantle signature in radiogenic Pb isotopes. This isotope anomaly, consequent to elevated, time-integrated Th/U in the mantle, is considered a result of eastward mantle flow from the mantle domain underlying and feeding Indian Ocean ridges. Boninites, a distinct...

Magmatism on Earth mainly occurs at mid-ocean ridges and subduction zones, where it is generated by mantle melting at different redox conditions. This study examines the role of sulfur in redox reactions within subduction zones by determining the sulfur valence state in glass inclusions in mantle minerals.

We report the abundances of major and lithophile trace elements and volatiles (H2O, Cl and S) in orthopyroxenite veins cutting mantle-derived, spinel harzburgite xenoliths from the active Ritter volcano in the West Bismarck Arc (Papua New Guinea). The veins preserve sulfide-bearing glass coexisting with crystals. The glass formed by the quench of r...

Iron isotopes in ocean floor basalts (OFB) away from convergent margins comprising mid-ocean-ridge and ocean island lavas show significant variation of >0.4‰ (expressed in the delta notation δ⁵⁷Fe relative to IRMM-014), but processes responsible for this variation remain elusive. Bond-valence theory predicts that valence states (Fe³⁺ vs. Fe²⁺) cont...

Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising s...

Supplementary Material

It is debatable whether oxygen fugacity (fO2), the usual measure of the oxidation state of a geological system, can vary during partial melting in the Earth's mantle or not. Notably, samples of mantle peridotite recovered from lavas and pyroclastic deposits in island arcs are mostly more oxidized than those from other tectonic settings. However, th...

Petrological models [1] suggest that orthopyroxene-rich peridotites from the sub-arc lithospheric mantle can originate from melting of hybrid, silica-rich sources. However, several processes can account for the silica-rich nature of these sources, such as modal modifications of lherzolite before melting (e.g. pyroxenite formation) or continuous enr...

Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes...

We report chlorine (Cl) and fluorine (F) abundances in minerals, interstitial glasses, and melt inclusions in 12 andesitehosted, spinel harzburgite xenoliths and crosscutting pyroxenite veins exhumed from the sub-arc lithospheric mantle beneath Avacha volcano in the Kamchatka Arc (NE Russia). The data are used to calculate equilibrium mineral-melt...

Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc l...

___________________________________________________________________________ It has long been recognised that arc-related peridotites are more oxidised than those in other tectonic settings [1]. However, the samples studied to date were frequently small, heterogeneous and altered, while it remains unclear whether these are all melting residues forme...

Tholeiitic and calcalkaline rock suites respectively predominate at mid-ocean ridges and subduction zones. The low-Fe character of calcalkaline suites has mainly been attributed to the earlier phase saturation with Fe-Ti oxides relative to ferromagnesian silicates therein. However, experiments have shown that this requires either a high oxidation s...

An ongoing controversy in mantle geochemistry concerns the oxidation state of the sources of island arc lavas (IAL). Three key factors control oxidation-reduction (redox) of IAL sources: (i) metasomatism of the mantle wedge by fluids and/or melts, liberated from the underlying subducted slab; (ii) the oxidation state of the wedge prior to melting a...

Various models can explain the formation of subalkaline andesitic arc magmas with compositions that most closely resemble that of the bulk continental crust. Backed up by large but sometimes conflicting sets of high-pressure experiments, andesite genesis models can be classified into four main types: (i) fractionation of basalt; (ii) partial meltin...

Arc lavas are more oxidised compared to mid ocean ridge lavas [1]. This is commonly explained by the addition of hydrous slab-derived agents to the mantle source of arc lavas The resulting fO 2 of the sub-arc mantle is also in the range where the S valence state in silicate melts change from S 2-to S 6+. From the observation that the S 6+ /S 2-equi...

Kamchatka and West Bismarck peridotites from the sub-arc lithospheric mantle preserve percolation of low-Ca boninite melts as orthopyroxene and sulfide-rich veins. Noble metals (platinum group elements [PGE], Au and Ag), Re and base metals in bulk peridotites and veins, combined with in-situ analyses of sulfides, show these low-Ca boninites have ex...

Iron is a major element in arc lavas and occurs in two valence states in natural igneous systems (Fe 3+ and Fe 2+ , commonly expressed as Fe 3+ /ΣFe). It is clear that fractionation in mafic igneous system preferentially removes minerals with Fe 2+ , i.e., in olivine and pyroxene, inevitably forcing a shift of residual liquids towards a higher Fe 3...

We report petrographic, major and trace element data for xenoliths from the andesitic Avacha volcano (Kamchatka), which host orthopyroxene (opx)-rich veins of mantle origin formed either by rapid crystallization of intruded melts or by their interaction with the host harzburgite. Studies of such veins may give better insights into subarc mantle pro...

Rare xenoliths carried by recent andesites or arc tholeiites are the only direct witnesses of active sub-arc processes. We present a detailed study on such xenoliths from Avacha volcano in southern Kamchatka (Russia) and Ritter volcano, West Bismarck Arc (Papua New-Guinea Region). Avacha is located on a ‘mature’ subduction zone (~120 km Pacific sla...

We investigated alkali and alkaline earth silicate glasses and sodium borate and borosilicate glasses, containing chromium and nickel (0.1 to 1%) mostly as Cr 3+ and Ni 2+ . The local and medium range order was investigated with UV-VIS-NIR absorption spectroscopy and X-ray absorption spectroscopy. EXAFS shows Cr-O distances of 2Å for Cr 3+ , a conf...

Magma-sediment interactions provide important information to reconstruct paleo-eruption dynamics in hydro-magmatic environments. The lithofacies resulting from these interactions are usually named "peperite" as first described on rocks from the Limagne area (Massif Central, France). Nevertheless, it is nowadays admitted that both fluidal and blocky...

Low-Ca boninites (LCB) are arc-related magmatic rocks enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), Zr and Hf relative to medium to heavy REE (MREE-HREE). These signatures are commonly attributed to a unique slab-derived agent that metasomatized a depleted mantle source but their origin in such an agent remains...

In subduction zones the f O 2 increases from the subducted slab to lavas erupted at the Earth's surface from reducing (log f O 2 = QFM-1) to oxidising conditions (log f O 2 = QFM+3). This increase of f O 2 is generally manifested and determined by an increase of Fe 3+ /Fe 2+ of mantle wedge derived xenoliths [1] or arc lavas [2]. The mantle wedge,...

Spinel harzburgite xenoliths from the andesitic Avacha volcano (Kamchatka, Russia) record a two-stages history: their protolith was (a) initially formed by high degrees (28-35%) of flux-melting in the mantle wedge asthenosphere and (b) slowly re-equilibrated at 900-1000˚C in the mantle wedge lithosphere [1]. Avacha harzburgites contain two types of...

Nous avons étudié des harzburgites à spinelles échantillonnées au volcan andésitique Avachinsky (Sud du Kamchatka, Russie). Ces roches contiennent des inclusions vitreuses dans des spinelles et sont recoupées par des veines pyroxénitiques. L’étude des inclusions vitreuses dans les spinelles révèle deux générations de liquides: (1) un liquide formé...

We report the first application of multiphoton microscopy (MPM) to generate three-dimensional (3D) images of natural minerals (micron-sized sulfides) in thick (∼120 μm) rock sections. First, reflection mode (RM) using confocal laser scanning microscopy (CLSM), combined with differential interference contrast (DIC), was tested on polished sections....

Laser-induced photo-luminescence spectroscopies: probes for sulfide crystal-chemistry

Harzburgite xenoliths from the andesitic Avacha volcano (Kamchatka, Russia) contain two types of spinel-hosted melt inclusions: (a) high-T inclusions (homogenized at 1200°C) containing opx±cpx+glass and (b) low-T inclusions (homogenized at 900°C) containing amph±sulf+glass. Homogeneous glass in the high-T inclusions is similar in major element comp...

We combine Two-Photon Fluorescence (TPF), Confocal Laser Scanning Microscopy (CLSM) and Confocal Laser- Induced Luminescence (LIL) to acquire 2-D and 3-D-resolved luminescence emission spectra from transition metal- and (Ca, REE)-bearing mantle-derived and meteoritic (enstatite chondrite) sulfides. The latter include primary condensates and high-de...

Glass-bearing inclusions hosted by Cr-spinel in harzburgite xenoliths from Avacha are grouped based on homogenization temperatures and daughter minerals into high-T (1200°C; opx+cpx), intermediate (900-1100°C; cpx±amph) and low-T (900°C; amph) and are commonly accompanied by larger “melt pockets”. Unlike previous work on unheated inclusions and int...

Petrographic and geochemical data were obtained on melt inclusions in spinels from fresh mantle harzburgite xenoliths in andesitic tephras from the Avacha volcano in Kamchatka, Russia. Three types of melt inclusions (10-50µm) are identified: Type 1 are the more numerous and are crystal free or, depending on their size, contain exclusively fine, euh...

Peridotite xenoliths in calc-alkaline volcanic rocks are direct samples of the sub-arc mantle. They are generally thought to have reacted with slab-derived fluids, yet many show no or minor whole-rock enrichments in incompatible elements [1]. Metasomatic veins that may cut these samples are crystallisation products of fluids and melts and give then...

Petrogenetic relations in igneous rocks are usually studied in natural samples using classical optical microscopy and subsequent geochemical data acquisition. Multiphotonic Laser Scanning Confocal Microscopy (MLSCM) can be a powerful tool to section geological materials optically with sub-micrometric resolution and then generate a three-dimensional...

Nickel speciation in multi-oxydes components silicate glasses: new UV-VIS-NIR spectroscopic results and geochemical implications

PGE and lithophile trace elements were determined by LA-ICPMS in minerals of pyroxene-rich veins in spinel harzburgite xenoliths from the andesitic Avacha volcano in Kamchatka, Russia. Type 1 veins are quenched magmatic liquids with arc-related trace element signatures. Type 2 veins are produced by fractionation of the initial liquids and their rea...

Silicate glasses are usually considered as good analogues for natural melts and thus frequently synthesized to study their structural properties. The optical spectra of Nickel are well constrained in crystal phases [1]. Only recent studies permit real advances of Ni speciation in glasses [2, 3]. In alkali and alkaline-earth silicate glasses, Ni 2+...

Peridotite xenoliths in calc-alkaline volcanic rocks are direct samples of the sub-arc mantle. They are generally thought to have reacted with slab-derived fluids, yet many show no or minor whole-rock enrichments in incompatible elements [1]. Metasomatic veins that may cut these samples are crystallisation products of fluids and melts and give then...

The mafic lavas and the diabases of Crozon (Armorican Massif, France), belong to an anorogenic Ordovician volcanic complex, emplaced on a rifted passive margin in North Gondwana. Magma passed through syn-volcanic soft sedimentary substrate, which is today mostly composed of alternating sandstones and mudstones, from Llanvirn to Ashgill in age. Fiel...