The first billion years of Earth history witnessed the emergence of continental magmatism, oceans and life. Yet, the details of how continents formed remain unknown because of the absence of preserved rocks1-8. Two conflicting Hadean models predominate: early onset of subduction and plate tectonics2-4, compared with early stagnant-lid and plume processes with delayed (post-Hadean) plate tectonics5-7. Here we report trace-element ratios (including Nb-Sc-U-Yb) correlated with age and hafnium and oxygen isotope ratios for Hadean detrital zircons from the Jack Hills (JH), Western Australia, which record unprecedented insights into the timing and setting of early magmatism. More than 70% of Hadean JH detrital zircons have Sc/Yb > 0.1, and 47% have U/Nb > 20, fingerprints for continental-arc and subduction settings. The remainder are ocean-island-like with little evidence for ocean-ridge settings. Hadean JH zircons probably originated from distinct terranes with separate tectonic histories. Subduction-related magmatism in the Hadean, as documented by JH zircons, alternated with periods of magmatic quiescence. This contrasts with dominantly stagnant-lid-like signatures for most Barberton Hadean zircons. The diverse settings for Jack Hills and Barberton detrital zircons imply contemporaneous operation of different tectonic styles during the Hadean, as well as a broader diversity of early crustal origins than previously known.

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.

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).

Isotopic systems and trace elements are ideal proxies to constrain the production and recycling of crust (both mafic and felsic) over time. Within the Rubidium-Strontium (Rb-Sr) system, Rb-87 decays to Sr-87 and due to the preferential partitioning of Rb into the crust (relative to Sr) during partial melting, 87Sr/86Sr ratio of the crust is higher than that of the mantle over time. Trace elements such as Niobium (Nb) and Uranium (U) do not fractionate when mantle melts to form mafic magma (oceanic crust) but they do fractionate when oceanic crust is recycled and undergoes fluid-present melting, i.e., during the production of felsic magmas (continental crust) [Hofmann et al., 1986], thereby resulting in a lower Nb/U of the felsic crust compared to the mantle. In this work, we couple the evolution of the above-mentioned geochemical proxies with the melting processes in global convection models using the code StagYY [Tackley, 2008]. Results from these geodynamic models are then compared with geochemical data obtained from olivine-hosted melt inclusions extracted from komatiites of 3.27 Ga Weltevreden formation (Barberton Greenstone Belt, South Africa).These models self-consistently generate oceanic and continental crust while considering both plutonic and volcanic magmatism [Jain et al., 2019] and incorporate a composite rheology for the upper mantle. Pressure-, temperature-, and composition-dependent water solubility maps calculated with Perple_X [Connolly, 2009] control the ingassing and outgassing of water between the mantle and surface [Jain et al., 2022]. These models show intense production and recycling of continental crust during the Hadean and the early Archean, which is in agreement with new geochemical data [Vezinet et al., in review] and previous geochemical box models [Rosas & Korenaga, 2018; Guo & Korenaga, 2020]. The thermal evolution is also consistent with cooling history of the Earth inferred from petrological observations [Herzberg et al., 2010].As the estimates of total amount of water (at the surface and in the deep interior) vary from 5-15 ocean masses (OMs) based on magma ocean solidification models to 1.2-3.3 OMs based on petrological models [Nakagawa et al., 2018], different initial values of water are also tested, which show a strong influence on the amount of felsic melts produced. Ongoing work includes incorporating the effect of water on the density and viscosity of mantle minerals and adapting the lithospheric strength with surface topography.

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.

Melt inclusions (MIs) in zircon can preserve information on the evolution of magmas. However, MIs in plutonic zircons are typically devitrified, consisting of multiple phases that must be remelted to obtain a homogeneous glass for reconstruction of melt composition and magma processes. We undertake a systematic investigation of melt inclusions in zircons from a similar to 3300 Ma xenolith of tonalite gneiss from the Barberton Greenstone belt, a well-studied section of cratonic lithosphere with components dating back to 3500 Ma. To better understand the influence of experimental heating on zircon and MI chemistry, multiple aliquots of zircons were heated in an internally heated pressure vessel at 0.4 GPa and temperatures ranging from 900 to 1200 degrees C (T-step = 100 degrees C). Homogeneous MIs in domains with low degrees of radiation damage and isolated from cracks in the zircon were found by examination of >5000 zircons by SEM (CL, BSE). Oxygen isotopes (delta O-18), OH/O ratios, U-Pb isotopes, trace and rare earth element (TREE) concentrations in zircon, along with delta O-18, H2O contents, and major element compositions in glassy MIs were measured by SIMS and EPMA. Investigated MIs have granitic compositions with 67 to 81 wt% SiO2. Both heated and unheated host zircon possess statistically identical and uniform delta O-18 values of 6.02 +/- 0.45 parts per thousand (2SD), while OH/O ratios systematically decrease with increasing temperature of laboratory heating. Inclusion textures (BSE contrast homogeneity) and composition (H2O, delta O-18) suggest that experimental heating at 1100 degrees C was the most successful in recovering initial MI compositions. Inclusions in lower temperature experiments either did not homogenize (900 degrees C) or are rarely homogenized (1000 degrees C), while those at higher temperature (1200 degrees C) are systematically dehydrated. Twenty-two hydrous MIs from the 1100 degrees C experiment have 3.1-11.5 wt% H2O and an average delta O-18 of 7.5 +/- 0.9 parts per thousand. Four zircons have a Delta O-18(MI-Zrn) fractionation inconsistent with equilibration at magmatic temperatures. In general, delta O-18 values and TREE concentrations measured in the zircons heated at 1100 degrees C show consistent behavior with unannealed zircons, indicating these systems are not significantly disturbed on the mu m-scale during heating experiments, and support the use of anomalous TREE concentrations/patterns as indicators of alteration. These measurements combined with Delta O-18(MI-Zrn) identify the MI-zircon pairs that are unlikely to represent the melt composition at the time of entrapment. Ti-in-zircon temperatures (alpha SiO2 = 1, alpha TiO2 = 0.3) and rhyoliteMELTS thermometry of the unaltered MI-zircon pairs return similar temperature ranges of 762-833 degrees C and 750-865 degrees C respectively, and combined with the granitic major element compositions, suggest that zircons crystallized relatively late during melt fractionation and entrapped residual evolved melt. More generally, these experiments represent the first direct reconstruction of H2O contents and oxygen isotopes of Archean melts from zircon-hosted MIs, and the approach described here can be used as a model for evaluating potential alteration during experimental heating and the geologic history for MI-zircon pairs from plutonic rocks. As compared with the TREE concentrations of similar-aged detrital zircons in the Barberton terrane, the tonalite xenolith zircons are distinct and attest to the diversity of magma compositions around 3300 Ma that formed the Barberton basement. Given that no comparable rock with zircons of the same age and TREE chemistry is exposed in the Barberton terrane, this tonalite likely represents an unknown component of the Barberton basement.

Upper Paleozoic, probably Permian, komatiites have been found in the Paleo-Tethys suture zone in NE Iran. These rocks are divided into three groups: (i) differentiated and undifferentiated komatiite lava flows, (ii) komatiitic basalts, and (iii) ultramafic-mafic pillow lavas. The rocks have a wide range of textures including random olivine spinifex, layered olivine spinifex, random and string-beef pyroxene spinifex, micrographic intergrowths of plagioclase and clinopyroxene, and cumulate textures. MgO contents range from 7.1 wt % in basalts and gabbros in differentiated flows to 38.0 wt % in cumulates, flow margins and samples with olivine spinifex textures. The MgO content of the parental melt is estimated using the Fo content of olivine (89-91) to be between 20 and 25 wt %, and the higher MgO content in spinifex samples (30 to 36 wt %) is attributed to accumulation of olivine. The rocks have low Al2O3/TiO2 and are relatively depleted in heavy rare earth elements. They, therefore, are classified as Al-depleted komatiite, the first report of this magma type in a Phanerozoic locality. These characteristics are attributed to the presence of garnet in the source during mantle melting and melt extraction. The rocks also have relatively low contents of the more incompatible trace elements indicating derivation from a depleted source. Our study indicates that the parental magma formed by 10% to (about) 20% partial melting in a mantle plume at pressures of about 4 to 5 GPa (depths of 120-150 km). Ascent of the plume into the Late Paleozoic subduction zone at the margin of the Paleo-Tethys Ocean is a possible petrogenetic model for the generation of these komatiites.

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.

We report the discovery of upper Paleozoic komatiites from a location near Mashhad in NE Iran. Like Archean komatiites, they erupted as differentiated lava flows with olivine spinifex−textured upper portions and olivine cumulate lower portions. They are associated with komatiitic basalts and undifferentiated ultramafic units and are intercalated with clastic and carbonate sedimentary rocks. Using the compositions of samples with randomly oriented spinifex and the forsterite contents of olivine, we estimate that the parental magmas contained >20 wt% MgO. As such, they represent only the second known occurrence of post-Archean komatiite. Their geochemical compositions show relatively low Al/Ti and depleted heavy rare earth elements, indicating that they belong to the Al-depleted variety. This is the only reported post-Archean example of this type of komatiite. The parental magmas probably formed by moderate degrees of melting (∼20%) of a hydrous peridotite source at a depth of ∼180 km and erupted into a subduction zone at the margin of the Paleo-Tethys Ocean.

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.

Explosive silicic eruptions pose a significant threat to society, yet the development and destabilization of the underlying silicic magmatic systems are still controversial. Zircons provide simultaneous information on the trace element composition and age of silicic magmatic systems, while melt inclusions in quartz and plagioclase yield important constraints on their volatile content as well as magma storage depth. Melt inclusions in zircons (MIZs) combine these data from a single mineral grain, recording the age, storage depth, temperature, and composition of magmas, and thus provide unique constraints on the structure and evolution of silicic magmatic systems. We studied MIZs from the Laguna del Maule (LdM) volcanic field in the southern Andes that is among the most active Pleistocene-Holocene rhyolitic volcanic centers worldwide and a potentially hazardous system displaying inflation rates in excess of 25 cm/yr. The host zircon ages suggest that the LdM MIZ record extends to 30 kyr before eruption, in contrast to the melt inclusions in LdM plagioclase and quartz crystals that formed only decades to centuries before eruption. The major element compositions of MIZs are minimally affected by post-entrapment crystallization, and agree well with the LdM rhyolitic whole rock data. The MIZs record long-term differences in zircon-saturated melt composition between two eruptive units (rdm: Rhyolite of the Laguna del Maule vs. rle: Rhyolite of Los Espejos). The more evolved major element composition of rle MIZs than rdm MIZs, suggests a long-term deeper connection of the rdm crystal mush to a more primitive magma body than that of the rle. The evidence of slow H diffusion observed in MIZs suggest that their H2O contents are not significantly affected by diffusion of H through the host zircon. The magma storage pressures of 1.1 to 2.8 kbars recorded by the H2O contents of rdm and rle MIZs are consistent with the optimal emplacement window (2.0 ± 0.5 kbar) of silicic magma reservoir growth, storage, and eruptibility based on thermomechanical modeling (Huber et al. 2019).

The Archean–Proterozoic transition in the Earth’s history is marked by significant changes in the mantle dynamics and temperature regimes. A notable consequence is the disappearance of Al-depleted komatiites in the Late Archean and an almost complete absence of Archean-typical peridotitic komatiites since the Proterozoic. This work presents a study of the 2.41 Ga komatiitic basalts from the Vetrenyi Belt, dating back to the early Proterozoic. Unique data on the compositions of olivine and chromite, as well as on the crystallization temperatures based on Al-in-olivine geothermometry for komatiitic basalts from the Vetrenyi Belt are provided. The temperatures of the earliest stages of crystallization were approximately 1240 ± 25°C, which indicates the occurrence of water in the melt and is consistent with measured water contents of 0.4 ± 0.2 wt

Two primary and two subordinate age groups of detrital zircons are distinguished as a result of isotopic–geochronological studies of metasandstones of the Vilenga Formation of the Vetrenyi Belt. A zircon group with an age of 2751 ± 7 Ma was potentially sourced from late granitoids (granodiorite–granite–leucogranites) abundant in the northern part of the Vodlozero domain. No source with an age of 2823 ± 5 Ma has been identified in the adjacent part of the Karelian granite–greenschist area. The zircons with the age of 2874 Ma were sourced from rocks of the BADR (basalt–andesite–dacite–rhyolite) series and adakites of the Sumozero–Kenozero greenschist belt. Previous study of detrital zircon from basal horizons of sections of the Vetrenyi Belt (Toksha Formation), as well as the underlying rocks for the metasandstones (Kozhozero Formation), showed the same main age peaks, which may indicate a stable tectonic setting at the beginning of the Paleoproterozoic and a common provenance of clastic material. Intermediate–felsic rocks with an age of 2940 Ma could also have made a small contribution in the formation of metasandstones.

Isotopic-geochronological studies of meta-sandstones from the Vilenga Suite in the Vetreny Belt have allowed the identification of two primary and two secondary age group of detrital zircons. For the zircon group with a 207Pb/206Pb age of 2751±7 Ma, potential sources could be late granitoids (granodiorite-granite-leucogranites) extensively spread within northern part of Vodlozersky Domain. The source with a 207Pb/206Pb age of 2823±5 Ma has not been identified within the adjacent Karelian granite-greenstone terrain as of today. The source of zircon with an age of 2874 Ma are the rocks of the BADR series and adakites of the Sumozero-Kenozero greenstone belt. Previous studies of detrital zircons from the basal horizons of the Vetreny Belt section (Tokshinskaya Suite) and the underlying meta-sandstones (Kozhozerskaya Suite) revealed exactly the same primary age peaks, suggesting a stable tectonic setting in the early Paleoproterozoic and a limited unified erosion area. A minor contribution to the formation of meta-sandstones could be made by rocks of intermediate-acidic composition with an age of 2940 million years.

Охарактеризовано Мяучанское рудное поле, расположенное в пределах Омсукчанской зоны Охотско-Чукотского вулканогенного пояса. Составляющие рудное поле Ag-Pb-Zn рудопроявления Коренное и Малютка локализованы в центральной части антиклинальной складки северо-западного простирания, сложенной верхнетриасовыми карбонатно-терригенными толщами, прорванными штокообразными телами и дайками позднемеловых андезитов, монцодиоритов, гранит-порфиров и риолитов. Сульфидно-карбонат-флюорит-кварцевые жилы и жильно-прожилковые зоны с арсенопиритом, пиритом, сфалеритом, галенитом, халькопиритом, Ag-тетраэдритом распространены как в интрузиях, так и в терригенных толщах. Геохимический спектр минерализации As-Sb-Ag-Pb-Au-Bi-Cu-Zn-W. Результаты изучения флюидных включений наряду с данными по распределению редкоземельных и рудных элементов указывают на формирование руд в эпитермальной обстановке гидротермально-магматической системы из хлоридных растворов с участием поверхностных вод. Отложение руд происходило в диапазоне температур 106—287 °С из растворов соленостью от 0.5 до 9.0 мас. % NaCl-экв. Руды, содержащие Ag порядка 70 г/т и Pb до 1.2 %, формировались в условиях выкипания из растворов, обогащенных солями Fe; руды с содержанием Ag менее 7 г/т, а Pb до 25 г/т образовались в условиях разбавления из гидротерм, в составе которых преобладали соли Na с незначительной примесью K. Минерализация Мяучанского рудного поля представляет верхний уровень Ag-Pb-Zn рудной системы, аналогичной месторождению Гольцовое. The Myauchan ore field located in the Omsukchan zone of the Okhotsk–Chukotka Volcanic Belt is des­cri­bed. The Korennoe and Malyutka Ag–Pb–Zn ore occurrences forming this field are localized at the center of anticlinal fold of NW strike made up of Upper Triassic carbonate-terrigenous deposits intruded by stock-like bodies and dikes of Upper Cretaceous andesite, monzodiorite, granite-porphyry, and rhyolite-porphyry. Sulfide–carbonate–fluorite–quartz veins and vein–veinlet zones with arsenopyrite, pyrite, sphalerite, galena, chalcopyrite, and Ag-tetrahedrite are localized both in intrusions and in terrigenous strata. The field bears As–Sb–Ag–Pb–Au–Bi–Cu–Zn–W mineralization. Study of fluid inclusions and REE and ore element patterns show that the field ores formed from chloride solutions with the participation of surface waters in hydrothermal-magmatic system under epithermal conditions. The ores were deposited from solutions with TDS = 0.5–9.0 wt.% NaCl equiv. in the temperature range 106–287 ºC. The ores containing ca. 70 ppm Ag and up to 1.2% Pb formed through boiling-off of solutions enriched in iron salts. The ores with Ag < 7 ppm and Pb ≤ 25 ppm were generated from diluted hydrothermal solutions with predominant sodium salts and potassium admixture. Mineralization of the Myauchan ore field marks the upper level of the Ag–Pb–Zn ore system similar to the Gol’tsovoe deposit.

The Au concentrational level of the ores of polymetallic volcanogenic-sedimentary massive sulfide deposits is predetermined by the geodynamic setting of their formation. The ores formed in a mid-ocean ridge setting are characterized by low Au grades. The ores formed under the island arc conditions possess higher Au contents. In the course of the further existence of the ore deposit, its ores experience repeated external influences: underwater halmyrolysis, thermal and dynamic metamorphism, hydrothermal-metasomatic transformations, and exogenous oxidation. These processes lead to the transfer of gold from a micro- or nano-sized form in the sulfides to the free form and to redistribution of Au over the ore field of the deposit. The heterogeneity of the ores in terms of the Au content and existence forms must be taken into account when preparing a feasibility study. The work is intended for geologists conducting mining and geological modeling, designing techniques for the ore extraction, and proposing ore processing technologies.