The dehydration of altered oceanic lithosphere is a source of aqueous fluids in subduction zones. Serpentine minerals, hosting ~ 13 wt.% H2O, are one of the main water carriers of the hydrated oceanic mantle. Antigorite, the stable serpentine mineral in deep subduction conditions, breaks down at temperature above 600 °C (Atg-out reaction), releasing free aqueous fluid. Compilation of bulk compositions of oceanic and exhumed subduction-collision zones serpentinites from the literature indicates that brucite (Brc) should also be an important hydrous (30 wt.% H2O) component of the oceanic lithosphere. Thermodynamic modeling with an updated thermochemical database shows that the Brc + Atg = Ol + H2O reaction (Atg-Brc reaction) occurs at lower temperature and can even produce more fluid than the Atg-out reaction. Moreover, the Atg-Brc reaction occurs in a narrow temperature range (< 10 °C), implying relatively high dehydration rates in the slab. Furthermore, the released aqueous fluid is calculated to be highly magnesian (> 1 mol/kg) with MgOaq as the dominant aqueous species. We studied the products of the Atg-Brc reaction in Zermatt-Saas (Swiss Alps) and Mont Avic (Italian Alps) meta-ophiolites, involved in the Alpine subduction. The development of metamorphic olivine and Ti-clinohumite veins within metamorphic serpentinites crosscut by pure magnesian brucite (Mg# > 99) indicates strong magnesian segregation, in agreement with thermodynamic modeling. From the size of the segregation, it is estimated that a Mg-rich fluid interacted with the host rock for around a hundred years before being drained. Finally, based on the idea that dehydration reactions can trigger seismicity in subduction zones, we located in a PT diagram the Low-Frequency Earthquakes (LFE) recorded in present-day subduction zones (Mexican, Nankai and Cascadia). The conditions under which these LFE are generated coincide with the PT conditions of the Atg-Brc dehydration reaction, supporting its central role as a main source of aqueous fluid in subduction zones.

Magmas from mantle plumes are potentially the best monitors of Earth's compositional and thermal evolution over time. However, their erupted products are commonly modified by syn- and post-magmatic processes and thus do not fully retain original information about their mantle sources. Such data can be recovered from melt inclusions in olivine phenocrysts in the most primitive magmas from mantle plumes. Such inclusions, shielded by host olivine, retain original isotopic and critical trace element signatures of deep mantle sources even for Archean and Hadean Eons.We will present the results of a study of chemical and Rb-Sr isotope composition (EPMA, LA-ICP-MS and RAMAN) of melt inclusions and chemical (EPMA, LA-ICP-MS) compositions of host olivines for komatiites and plume-related picrites with eruption age from 3.3 Ga to 1 Ka.Recent advances in in-situ split stream LA-ICP-MS measurements of 87Sr/86Sr ratios and trace element contents of olivine-hosted melt inclusions revealed significant mantle source heterogeneities of magmas from individual plumes. The results are confirmed by geodynamic modelling (Jain et al., this meeting).We show that the melt inclusions of most studied mantle plumes display heterogeneous populations in age-corrected 87Sr/86Sr ratios and include groups with model ages more than 1 Ga older than the emplacement age. The oldest inclusion groups found in Archean komatiites correspond to Hadean (4.3±0.2Ga, Vezinet et al., in review) and Eo-Paleoarchean (3.6±0.2 Ga) model ages. These and most inclusions from studied komatiites and picrites display Nb/U, Nb/Th and Ce/Pb significantly higher than in BSE.Evolution over time of canonical proxies of continental crust generation (Nb/U, Th/U and Ce/Pb, Hofmann et al., 1986) in mantle plumes, combined with geodynamic modelling, suggests:Most of the continental crust was generated in several Hadean and Archean pulses by plume-induced subduction and melting of the hydrated mafic/ultramafic crust or mantle. Hadean continental crust was subducted or/and reworked. Restites left after extraction of continental crust were continuously subducted to the core-mantle boundary from the mid-Hadean and later recycled in Archean mantle plumes. Active formation of both continental and oceanic crust in Hadean was governed by plume-induced subduction, which ceased after cold subducted material hindered the propagation of large plumes at the core-mantle boundary. After heating the recycled lithosphere at the core-mantle boundary, the process repeats, producing oscillating subduction and crustal formation in Hadean-Archean.

Oxygen is the most abundant element in the terrestrial mantle and crust. We have recently reported on a 0.2‰ δ18O decrease of continental mantle peridotites from the original primary Bulk Silicate Earth-Moon value of 5.57‰ [1] in the mid-Archean to the Phanerozoic explained by the initiation of surface recycling (linked to intensity and style of plate tectonics) sometime in the Archean. Even small variations in the volatile mass balance are critical in explaining phenomena such as the Great Oxidation Event at ~2.4 Ga that may have mantle origin. As low-δ18O subduction fluids are derived by the dehydration (and potentially oxidation) of low-δ18O interiors of subducted slabs, this work further explores this process to observe temporal changes related to the progressive input of volatile elements and potential lithospheric mantle oxidation. This study presents a record of trace elements measured in same olivines (Li, Na, Al, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Ga, Y, Zr) including oxidation-sensitive elemental ratios V/Sc and Zn/Fe for this collection. Prior melt-depletion of mantle peridotites, estimated using bulk Al2O3 content of the xenoliths, increases with age from ~25 to 35%, leading to depletion of Yb, Y, Co, Mn, Ca, P, with smaller effects on the elemental ratios. We observe significant ranges of V/Sc (0.2-14), Li/Y and other ratios, not related to prior melt depletion that may be linked to subduction-related re-distribution of incompatible elements by subduction [2], and scattered correlation with age and δ18O values. Further trends will be analyzed during the talk after considering craton-specific domains and global trends. This work can potentially contribute to constraining a global mass balance of crustal growth and recycling based on co-variations of isotopes of a major element oxygen and trace elements in the predominant lithospheric reservoir of subcontinental mantle.[1]Bindeman ea, (2022) Nat Comm 13, 3779; [2] Doucet ea, (2020) NatGeosci 13, 511.

Ultramafic magmas are the ideal trac & iecy;rs of mantle composition, as they are derived from high degrees of partial melting and their compositions approach that of mantle peridotite. The Permian Song Da ultramafic volcanic rocks in northern Vietnam represent a rare example of well-preserved Paleozoic picrites of the Emeishan Large Igneous Province (ELIP), with the most Mg-rich olivine phenocrysts of up to Fo(93.5). As such, they are invaluable for constraining temperature, pressure, and composition of the ELIP mantle source. Here, we report the results of a study of olivine-hosted melt inclusions from ultramafic lavas of both low-Ti and high-Ti types in the Song-Da zone of the ELIP, providing new information on the concentrations of mobile trace elements, volatile components, and Sr isotope compositions of parental melts. Our data suggest a significant difference between the mantle source compositions of low-Ti and high-Ti primary melts of the Song Da zone. The latter melt likely originally contained 2.7 +/- 1.2 wt% CO2 and 0.6 +/- 0.1 wt% H2O, but lost most of its volatile inventory during degassing. Its enrichment in trace elements is attributed to a low degree of partial melting (less than 9 %) of a PREMA-type peridotitic source, as evidenced by the bulk rocks Nd and melt inclusions Sr isotopic data, contaminated by metasomatic fluids or recycled components bringing Pb, Sr, and Na. The low-Ti komatiite-type Song Da melts contain an excess of H2O (H2O/Ce up to 5500), similarly to some Archean komatiites, show a decoupling of Nd and Sr isotopes, and reveal mixing of at least two unusual components: (1) one with low initial Sr-87/Sr-86 (down to 0.7040), low Nb/U and Ce/Pb ratios and relatively enriched in La, Th, U, Pb, and Sr, and (2) the other with high initial Sr-87/Sr-86 (up to 0.7080), high Nb/U and Ce/Pb ratios, and strongly depleted in highly incompatible lithophile elements such as Ba, Rb, Nb, K, U, Th, La, and Sr. We suggest that component (1) reflects contamination with lower continental crust or other subduction-related constituents in the mantle source, while component (2) represents partial melt derived from dehydrated seawater-altered recycled harzburgite that was part of a subducted oceanic slab 300 to 500 Ma in age. The latter component can be recognized only in the melts that are exceptionally depleted in highly incompatible elements. The data from this study and Kazzy et al. (2024) suggest that the ELIP was produced by a mantle plume with a maximum potential temperature (T-p) of similar to 1600 degrees C at similar to 260 Ma. Because of its high temperature, this plume passed through the hydrated mantle transition zone (MTZ) in a partially molten state, which allowed it to entrain in its hottest part H2O and components of the subducted 300-500 Ma Paleo-Tethyan oceanic plate stagnated in the MTZ for similar to 200 Ma. An alternative source of dehydrated serpentinites of recycled Paleo-Tethyan slab in the Emeishan mantle plume could be a core-mantle boundary if this slab had been subducted shortly (less than 240 Ma) before the initiation of this plume. This option requires a fast Tethyan crust recycling rate of >2.3 cm/year. Low-Ti komatiite-type melts of the Song Da area came from the hottest part of the Emeishan mantle plume and contain traces of these components, whereas the high-Ti Song Da melts came from the coldest part of the Emeishan mantle plume (Tp similar to 1450 degrees C) and reflect a PREMA-type mantle source metasomatized by addition of a component enriched in Na, Sr, and Pb.

In subduction zones, serpentinized oceanic mantle is expected to dehydrate above 600 degrees C due to antigorite breakdown (Atg-out reaction). Analysis of compilatory bulk composition of serpentinites shows that brucite (Brc) should also be an important hydrous component, capable of carrying water to depth. The Atg + Brc = Ol + H2O reaction (R1) which occurs at lower temperature than Atg-out, is thus highly relevant for fluid release in subduction zones. Depending on the initial composition of the serpentinized mantle, R1 can produce more water than the Atg-out reaction. The consideration of most recent thermochemical data for brucite and serpentine solid-solutions shows that the R1 reaction proceeds in a narrow temperature range (< 10 degrees C), implying relatively high dehydration rates. Thermochemical modeling also shows that the fluid released during R1 is highly magnesian ([Mg]/[Si] > 100), i.e., likely to promote Mg metasomatism. In parallel, metamorphic olivine veins formed according to R1 in the Zermatt-Saas meta-ophiolite were examined. They are interpreted as magnesium-rich segregations resulting from the interaction between an Mg-rich fluid and the host serpentinite for around a hundred years. Furthermore, they are crosscut by brucite veinlets formed at R1 conditions or higher, which confirms, in agreement with thermodynamical modeling, that R1 is not a brucite-out reaction. Finally, P-T conditions of R1 were found to coincide with the location of Low Frequency Earthquakes recorded in the Mexican, Nankai and Cascadian subductions; this indirectly supports the role played by R1 as a significant source of fluid in subduction zones.

Melt inclusions hosted in highly magnesian olivine crystals have proven invaluable for probing the composition of the mantle through time since their geochemical signature is reflecting that of parental melt. Additionally, the geochemical study of melt inclusions has shown to be more suited to identify the heterogeneity in the magma from which they crystallized, particularly the chemically depleted domains [1, 2]. Here, we will present new major, minor & trace elements, H2O contents and Sr-isotope signature of more than 300 olivine-hosted naturally quenched melt inclusions from Pu’u Wahi (910 yr-old) and Puʻu Mahana (ca. 50 kyr-old), two ash cones associated with Mauna Loa, the largest shield volcano of the Hawai’ian seamount chain. In order to have a high degree of confidence in the geochemical proxies, Sr-isotope and trace elements analyses were conducted through laser ablation split stream (LASS) protocol on top of EPMA and Raman (for H2O contents) analytical spots. Preliminary results in our new set of inclusions show the presence of high (Sr/Ce)N inclusions, previously interpreted as indicating either gabbro influence in the source of the plume [3] or interactions between plagioclase-rich cumulates and percolating mantle-derived melts [4]. Further, “ultra-depleted melts”, UDM, indicated by K2O contents < 0.1 wt.% identified in [1], have also been re-identified in this new set of inclusions (not analyzed for Sr-isotope yet). 87Sr/86Sr of non-UDM inclusions ranges from 0.70361±0.00025 to 0.70427±0.00025, i.e. analogous to the most recent TIMS values [4, 5]. Additional LASS analyses will be conducted before the meeting. The full set of analyses will be confronted to published results on the same volcano [1, 3-6] and integrated in a larger framework of interactions between mantle plume and consequences for plate tectonic. References: Sobolev, A.V., et al., Nature, 2011. 476(7361). Stracke, A., et al., Nature Geoscience, 2019. 12(10). Sobolev, A.V., et al., Nature, 2000. 404(6781). Anderson, O.E., et al., Geochemistry, Geophysics, Geosystems, 2021. 22(4). Reinhard, A., et al., Chemical Geology, 2018. 495. Sobolev, A.V., et al., Nature, 2005. 434(7033).

Meimechite (i.e., rare high MgO and TiO2 ultramafic rocks) concluded the Permo-Triassic Trap magmatism ca. 250 Ma-ago, known as a Siberian Large Igneous Province (SLIP) in the Meimecha-Kotui region, northern Siberia (e.g. [1]). In addition to their elevated MgO contents, meimechite’s melts display almost no crustal contamination, making them ideally suited to investigate the mantle source of the SLIP. Formerly, two opposing models were evoked for the origination of the meimechite: i) the hottest phanerozoic mantle plume [1] or ii) water fluxing of the asthenospheric mantle in a long-lived subduction zone [2]. Based on an extended analytical workflow we will shed new light on the source of these unusual rocks. Here we present new results for more than 300 olivine-hosted homogenized melt inclusions from Siberian meimechite including major, minor and trace elements, water and Sr-isotopes contents (EPMA, LA-ICP-MS and Raman spectrometry) along with the chemical composition of their host olivine (EPMA, LA-ICP-MS). When encountered, spinel inclusions were analysed by EPMA for major element abundances. We show that the Siberian meimechite crystallised from a highly magnesian (MgO > 22 wt%) parental melt deficient in H2O compared to Ce and K concentrations, which was degassed of most of its CO2 and likely part of its H2O while rising to shallower depths. Three independent geothermometers (Mg-Fe and Sc-Y olivine melt and Al olivine-spinel) confirm the high crystallisation temperature of the Siberian meimechite, ca. 1400oC. Furthermore, the calculated potential temperatures (over 1500oC) imply a mantle plume origin of the Siberian meimechite and, consequently, of the SLIP. Initial 87Sr/86Sr values of melt inclusions reveal heterogeneous populations ranging from 0.7022±0.0002 to 0.7039±0.0004 suggesting mixing between at least two depleted mantle components. The less depleted group has an average Bulk Silicate Earth (BSE) model age of 876±88 Ma, whereas the more depleted group is significantly older with an average model age of 1716±76 Ma. All source components display significantly fractionated proxies of continental crust extraction (Nb/U, Th/U and Ce/Pb [3]), indicating major events of continental crustal formation and deep recycling of residual lithosphere before the Proterozoic Eon. References: [1] – Sobolev, A.V., et al., Russ. Geol. Geophys., 2009 and references therein. [2] – Ivanov, A.V., et al., Chem. Geol., 2018. [3]- Hofmann, A.W. et al. EPSL, 1986.

The rates of continental crust growth and recycling on early Earth remain unclear due to the lack of information resulting from the extensive alteration of ancient rocks. Melt inclusions trapped and shielded from alteration in Archean high-Mg olivine crystals offer a solution to this problem. We report an unprecedented unradiogenic Sr mantle source component (87Sr/86Sr = 0.69932 +/- 0.00024, 95% confidence interval) of melts included in olivine from 3.27 Ga komatiitic lava flows in the Barberton Greenstone Belt, South Africa. This component indicates a model age of 4.31 +/- 0.19 Ga and significant chemical fractionation (Nb/U = 36.9 +/- 1.5, Ce/Pb=16.7 +/- 1.1), suggesting up to 80% +/- 16% of the present-day continental crust's mass was extracted by the late Hadean from the whole mantle. Geodynamic models support this finding, explaining geochemical data by producing 40% to 70% of the present-day continental crust mass during the Hadean in a variable tectonic regime with tens of millions of years-long periods of massive impulsive subduction induced by mantle plumes.

The discovery of systematic differences in the trace element composition of forsteritic olivines in primitive magmas from within-plate, arc and mid-ocean ridge volcanoes engendered much debate about a causal link to the recycling of oceanic crust into the mantle sources of within-plate and arc magmas. Here we address this problem using Cr-spinel bearing, forsteritic (similar to Fo(80-91)) olivines from high-Mg# = 50 = 73 [Mg# = molar ratio of Mg/(Mg + Fe2+)*100] arc magmas from the Trans-Mexican Volcanic Belt (TMVB). The TMVB arc front olivines have similar high Ni, low MnO, and low Mn/Fe as forsteritic olivines from within-plate basalts erupting through thick lithosphere (= WPB-thick). However, the olivines in TMVB arc front primary melts crystallize at much lower temperatures of T-cryst(oliv) similar to 1119 +/- 38 degrees C (calculated with olivine-spinel aluminum exchange thermometry) in hydrous (similar to 4-9 wt % H2O), silicic, less magnesian (<= 10 wt % MgO) mantle melts from mostly garnet-free mantle sources. Model calculations suggest that the primary arc front melts last equilibrated in the mantle at pressures of similar to 1.4 to similar to 1.9 GPa (similar to 51-69 km depth) and low temperatures (T-source = 1150 +/- 45 degrees C) that are only slightly higher than the olivine crystallization temperatures. While the Kd(oliv/melt)(Ni) increases in the cooler and silicic melts, such modulation cannot account for the full range of Ni concentration in TMVB magmatic olivines. A small population of very high-Ni olivines (>4000-5500 mu g/g Ni) is best explained by crystallization in Ni-rich components melt that formed by melt rock reaction processes in the mantle wedge. Unlike Ni, olivine MnO is not sensitive to melt temperature and only moderately to melt composition, and thus retains mantle source characteristics. In the TMVB, olivine Fo-MnO-Mn/Fe systematics record an ambient mantle wedge (= mantle without slab component) that is similar to WPB sources and that is variably depleted by slab flux-driven melt extraction. Overall, the olivine Fo-Ni-MnO systematics confirm with greater detail than possible by bulk rock studies that the TMVB primary melts are hydrous and silicic and originate from a mantle wedge that is strongly and variably modified by the slab flux. These results reaffirm a strong genetic link between slab recycling and the genesis of silicic arc magmas.

The Permian (similar to 260 Ma) Song Da volcanic suite in Vietnam is one of very few known occurrences of Phanerozoic ultramafic volcanic rocks that are similar in composition to komatiites. Despite continuous efforts to determine the primary melt composition of Song Da ultramafic lavas, the concentrations of the volatile and fluid-mobile elements are still poorly constrained due to widespread alteration and low-grade metamorphism of bulk rocks. This study reports high-precision in-situ major- and trace element abundances in host olivine and inclusions of melt and Cr-spinel from the Song Da ultramafic lavas. Two different types were identified: low-Ti lavas, previously described as komatiites, and newly discovered Ti- and Na-rich picrites. The application of olivine-melt Sc/Y, olivine-spinel Al, and olivine-melt Fe/Mg geothermometers indicates crystallization temperatures of up to 1450 degrees C for the Song Da low-Ti suite, which are within the range of komatiite crystallization temperatures, and up to 1330 degrees C for the high-Ti picrites. These conditions correspond to mantle potential temperatures of 1590 degrees C and 1450 degrees C, respectively. The estimation of oxygen fugacity, based on V partitioning between olivine and melt and Fe2+/Fe3+ between spinel and melt, indicates that low-Ti melts crystallized in a closed system under reducing conditions starting from one to half an order of magnitude below the QFM buffer. The high-Ti melt crystallized at higher oxygen fugacity (triangle QFM +0.5) in a buffered open system. The primary melt of the Song Da komatiites contained 0.7 wt% H2O, which was likely entrained from the hydrated Mantle Transition Zone (MTZ) by a partially molten plume. Our results indicate that the Song Da low-Ti ultramafic volcanics were likely derived from an ultramafic komatiite-like parental melt with an MgO content between 21 and 23 wt%. It was produced by a high degree (>26%) of partial melting of a depleted mantle source. The high-Ti picrite melt had 17-18 wt% MgO and was produced by a lower degree of partial melting (<9%) in a colder part of the same plume.

The structure of serpentinite melange of the North Balkhash ophiolite zone (Central Kazakhstan; west Central Asian Orogenic Belt) is ascertained to be high-grade formations assigned to epidote-glaucophane eclogites and garnet blueschists, and their varieties. The rocks follow a clockwise 'subduction-type' evolution with the estimated near-peak metamorphic conditions of 1.6-1.9 GPa and 500-560 degrees C. 40Ar-39Ar phengite ages of -491 Ma and - 465 Ma obtained for the eclogites and garnet blueschists, respectively, are interpreted to reflect the nearpeak to shortly retrograde stages of rock evolution and to record the late Cambrian and Middle Ordovician episodes of high-pressure re-equilibration in the North Balkhash zone. Protoliths of the eclogites were N-MORBlike mafic rocks, which comprised structurally different (from the lower gabbroic to the upper dolerite/basalt) parts of pre-late Cambrian oceanic crust and were formed in the spreading centre setting at the expense of depleted mantle source. Protoliths of the garnet blueschists and associated rocks were represented by volcanogenic-sedimentary, predominantly mafic, complexes (tuffaceous sandstones, greywackes), an accumulation of which and subsequent involvement into subduction occurred at -478-465 Ma in the intra-oceanic (fore-arc) setting. Zircon core ages indicate formation of the source of the protoliths of the garnet blueschists occurred in the 509-478 Ma range. A comprehensive correlation of the metamorphic and igneous formations of the North Balkhash zone with those assigned to the adjacent complexes of the southern part of the West Junggar area (NW China) suggested their mutual Early Palaeozoic tectonic evolution within the Junggar-Balkhash Ocean.

AbstractIn this study, we investigated the high-pressure (HP) metamorphism of the Precambrian continental crust exposed in the Zheltau terrane in South Kazakhstan (Koyandy complex) and the Chu-Kendyktas terrane in the North Tien Shan of Kyrgyzstan (Aktyuz, Kemin and Kokdzhon complexes) within the SW part of the Central Asian Orogenic Belt. HP quartz–feldspar lithologies of the Koyandy complex consist of migmatized kyanite-bearing garnet–mica paragneisses, garnet–kyanite paragneisses and their derivatives associated with eclogites. Paragneisses demonstrate prograde evolution involving mica dehydration melting and producing magnesium-rich garnet, kyanite and K-feldspar at the near-peak to retrograde stages at pressures of 15–18.5 kbar and temperatures of 800–870°C. The widespread growth of micas in these rocks reflects lower stages of retrogression at P = 10–12 kbar and T = 720–770°C. The age distributions of the cores of detrital zircon grains from the paragneisses indicate a predominance of Neoproterozoic and minor occurrence of Mesoproterozoic and Palaeoproterozoic sources of their protoliths. The ages of ∼487–485 Ma obtained from the zircon rims of the paragneisses reflect the timing of their HP metamorphic re-equilibration. These age clusters are consistent with the age estimates obtained from the rims of zircons in the eclogite-bearing garnet gneisses of the adjacent Aktyuz complex in the North Tien Shan. The P–T paths and zircon ages obtained from the high-grade quartz–feldspar gneisses of the Zheltau and Chu-Kendyktas terranes are thus interpreted to indicate involvement of the crustal material derived from the Precambrian basement (magmatic zircons aged ca. 844 Ma) and its Ediacaran–Cambrian sedimentary cover (detrital zircons with maxima at 1 Ga and 800–600 Ma) in the latest Cambrian subduction processes induced by the closure of the oceanic basins assigned to the Palaeo-Asian Ocean.

In this study we report on the petrography, major and trace element and mineral chemistry, platinum-group elements and Re–Os isotope systematics of depleted ultramafic rock suites from the Itmurundy Block in the North Balkhash ophiolite zone in Kazakhstan. Represented mainly by variably serpentinized harzburgites and dunites, our samples are characterized by low whole-rock Al 2 O 3 (0.33–0.86 wt%), CaO (0.51–0.86 wt%) and Na 2 O (0.07–0.25 wt%) concentrations, and high-Mg olivine (Fo = 91–92) and orthopyroxene (Mg# = 92–93) contents, together with moderately high spinel Cr-numbers (Cr# = 63–68). They are depleted in incompatible elements ( Σ REE, Nb, Sc) and enriched in compatible elements, such as Cr (up to 2817 ppm) and Ni (up to 2327 ppm), representing highly refractory mantle residues derived from a forearc mantle wedge. They underwent 19–23% hydrous partial melting to produce boninitic melts. 187 Os/ 188 Os values vary from 0.1202 to 0.12599, and 187 Re/ 188 Os ratios from 0.230 to 0.316. The Re–Os model ages (T MA ) and maximum Re depletion model age (T RD ) were calculated based on the obtained data. Re–Os isotope systematics suggests that the analysed peridotites formed in two stages: a first stage around 1.5 Ga and a later stage around 668–589 Ma. Supplementary material: A supplementary figure is available at https://doi.org/10.6084/m9.figshare.c.6845683 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

An Erratum to this paper has been published: https://doi.org/10.1134/S0016702923210024

Secondary fluorescence (SF) is known to be a potential source of error in electron probe microanalysis (EPMA) when analyzing for a trace or minor element near a phase boundary. This often overlooked effect leads to a concentration enhancement whenever the neighboring phase contains a high concentration of the analyzed element. Here we show that SF may also lead to a concentration decrease, which can be mistakenly interpreted as a depletion. To examine this issue, we compare Ni profiles measured on well-characterized, homogeneous olivine [(Mg,Fe)2SiO4] grains embedded in basaltic glass, with semi-analytical calculations and numerical simulations of SF across phase boundaries. We find that the Ni content consistently decreases with decreasing distance to the interface or grain radius, deviating from the expected concentration by ∼2-5% at 10 μm from the interface. This decrease is explained by the lower bremsstrahlung fluorescence emitted from the sample as compared to that emitted from the standard. The analytical error due to boundary fluorescence affecting other elements of petrologic importance in olivine is discussed.