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Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean.
The high degree of partial melting (F) and pressure (P) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate
compositional variations in Archean komatiites,...
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Context 1
... this adjustment is not correct for re-addition of fractionated olivine because it assumes C M is a linear function of a single olivine and melt composition, but it is a good approximation, considering that most non- melt compositions are cumulates and not evolved li- quids. The preferred major and trace element compos- ition of the parental liquid for each of the seven suites is listed in Table 2. ...
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... Table 2, the standard deviation associated with the mean value for the magma of a suite is given for each element. This is an expression of the spread of values for each corrected whole-rock composition, which is summarised in Fig. 4, where the elements are arranged by concentration (Fig. 4a) and atomic number (Fig. 4b). ...
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... Sc to Ni, the RSD on the corrected value varies between about 5 and 10%, where the variability on the XRF-determined values is comparable with (Sc, Ti and V) or better than (Co, Cr, Ni) those determined by ICP- MS (Fig. 4b). A departure from this behaviour is observed for Cu, and to a lesser extent Zn, which dis- play an order of magnitude greater variability than other first-row elements, even in the Reliance Formation (Table 2). This would suggest post-magmatic mobilization of Cu by metamorphism, a process that can also affect Zn, though only in the more altered suites (Coonterunah, Regal-Ruth Well). ...
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... mobil- ity of Eu 2þ is well documented, whereas the tendency for other REE to remain in the trivalent state ensures their reliability. In detail, Eu behaves coherently in the less-altered suites (Belingwe, Barberton, Yilgarn, Munro), whereas the uncertainty increases to 26% for the Coonterunah suite (Table 2). Thorium and U have rather large uncertainties of about 20%, although this is probably a function of their very low abundances, par- ticularly for U, rather than post-emplacement disturb- ance. ...
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... Fe/Mg ratio of the Earth's mantle is well constrained (Mg# ¼ 89) and cannot have changed appre- ciably since the cessation of core formation (O'Neill & Palme, 1998;Delano, 2001). Additionally, the Fe/Mg of the parental komatiite melts is also known to 65% rela- tive (Table 2), making the application of K D Fe-Mg ex- change between olivine and liquid possible (Rajamani et al., 1993). The thermodynamic underpinning of this exchange reaction is determined up to 2073 K and7 GPa (Toplis, 2005), covering the eruptive conditions and the majority of P and T values previously posited for the . ...
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... intermediate, best-fit value was taken, ameliorating any bias between the two determinations. Using these temperatures, taking the pressure estimates from the 'Pressure of melting' sec- tion, and compositions from Table 2, the K D Fe-Mg can be determined at the P and T conditions of magma formation using the parameterisation of Toplis (2005). The calcu- lated K D Fe-Mg ¼ 0Á35 6 0Á02 (1SD) depends on compos- ition; higher temperature, higher pressure liquids (Barberton AUK) have higher K D Fe-Mg , whereas the oppos- ite is true for the Munro komatiites (Table 3). ...
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... assume that dif- ference between the Fe 3þ content of the mantle and that of komatiite liquids is within uncertainty of their known concentration (e.g. Table 2), such that no correction for Fe 3þ is made. Regardless of whether pure batch or pure fractional melting is modelled, estimates for P and F increase to- gether, such that the hottest magmas also segregate at the highest pressures. ...
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Citations
... Because of their large degree of melting, komatiites (particularly those that have not experienced extensive crustal contamination) are considered representative mantle archives simply because most enrichment in incompatible trace elements, including variations in element ratios, during low-degree melting are erased. Depending on the source mineralogy, low-degree melting can strongly influence the trace elements and, to a lesser degree, the major element composition of a rock (Sossi et al., 2016). Because of the low viscosity of these melts, coupled with their assigned extreme melting temperatures (to facilitate large degree melting in the first place), they are also considered melts with nearprimitive mantle composition with rapid mantle extraction. ...
Subduction is a key geodynamic feature on modern Earth that drives crustal chemical diversity, bridging the atmo-, hydro-, and lithosphere, but remains an enigmatic, unique planetary feature. Indisputable is the critical role of subduction in shaping Earth's geomorphology and crustal dichotomy (ocean vs continental crust) and its impacts on long-term climate, making it arguably the most important process on present-day Earth across all geosciences. It is thus important to understand to what degree, or if at all, subduction was operational during the billions of years that led to our geological status quo. Here, we assess the feasibility of Archean subduction with a focus on early Earth geodynamics. We argue that convection-driven rifting, but not spreading, formed the first keels under the primordial crust, providing the necessary stability for crustal survival. These sections of crustal rejuvenation would counterintuitively forge the first stable proto-cratonic terranes, which later evolved into cratons. Hydrated upper crustal rocks were vital in generating early fluxed mantle melting and related volcanism, but also for partial melting in hydrated lower crustal sections within proto-cratons, giving rise to tonalite-trondhjemite granodiorites (TTGs). Both processes operated off-and on-craton, respectively, and required melting of hydrated crust and crustal convergence but are unrelated. Away from proto-cratonic regions of minor episodic divergence and rifting, relative motions were accommodated by convergence and shuffle tectonics, leading to Archean-style subduction in localised regions that were prone to destruction. This primitive form of subduction and crustal maturation has operated from the earliest Archean time in a plate-and-lid regime. Crucially, this 'Archean subduction' represents short-lived crustal shuffle-tectonics outside areas of today's cratons with fluxed melting in upper mantle regions but does not resemble present-day Benioff-style subduction. The development of subduction akin to present-day processes towards the end of the Archean could plausibly have driven atmospheric oxygenation over a few hundred million years between ca. 2.8-2.3 Ga, with H-loss to space accompanied by atmospheric oxidation through subduction-related global volcanic SO 2 emissions.
... The origin of komatiites and the water content in their mantle source have long been debated. The most widely accepted hypothesis is that these rocks formed in high-temperature mantle plumes (Campbell et al., 1989;Campbell and Griffiths, 1992;Herzberg, 1992Herzberg, , 1995McDonough and Ireland, 1993;Nisbet et al., 1993;Arndt et al., 1998;Sobolev et al., 2016Sobolev et al., , 2019Sossi et al., 2016;Asafov et al., 2018;Waterton and Arndt, 2023;Puchtel and Arndt, 2024). Recently, it has been shown by direct measurements of olivine-and spinel-hosted melt inclusions that the parental melts of komatiites from the Gorgona island, Abitibi, Belingwe, and Barberton greenstone belts contain between 0.2 and 0.9 wt% H 2 O and show high (>1000) H 2 O/Ce ratios (Shimizu et al., 2001;Kamenetsky et al., 2010;Gurenko et al., 2016;Sobolev et al., 2016Sobolev et al., , 2019Asafov et al., 2018Asafov et al., , 2020. ...
The Permian (~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 ◦C for the Song Da low-Ti suite, which are within the range of komatiite crystallization temperatures, and up to 1330 ◦C for the high-Ti picrites. These conditions correspond to mantle potential temperatures of 1590 ◦C and 1450 ◦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 (ΔQFM +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- ike 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 older part of the same plume.
... The MgO content of the parental melt was computed to be 25 wt% (Figure 10 Figure S2), which is found to be slightly higher than the estimated MgO of Munro komatiites, but lower than the most magnesian Eastern Yilgarn komatiites, both of which represents AUKs (e.g. Sossi et al. 2016; Supplementary Table S3). This result was further used to compute the eruption temperature for the komatiite liquid, which corresponded to nearly 1500°C (T liquidus ¼ MgO liquid �20 þ 1000; Nisbet 1982). ...
... The model calculated trace-element partitioning between melt and the residue during batch melting of a given source (garnet lherzolite). The modelling suggests that the mantle source of the Banasandra samples was derived from a PM or a 1% depleted PM-like source, which seems to be less depleted than the Yilgarn (1%) and Munro (4%) sources (Sossi et al. 2016; Figure 10(b)). While taking these results into consideration, we need to also take into account that the Banasandra komatiites from this study have slightly higher La concentration, which may increase the La/Sm N and La/Yb N ratios and Data for banasandra greenstone belt is from this study, while data for Nuggihalli is taken from Mukherjee et al (2012Mukherjee et al ( , 2014. ...
... Thus, the degree of mantle melting for the Banasandra komatiites has been modelled using PETROMODELER for a source that has undergone 1% as well as 2% depletion with respect to the PM (Figure 11). When a 1% depleted PM is melted to 40%, a melt composition similar to Yilgarn komatiites was produced (Figure 11(a)) as predicted by Sossi et al. (2016). However, when the same source was melted to 45% it is noted that even though the HREE patterns are in correspondence, the LREE abundances are much higher, except for La (Figure 11(a)). ...
... These greenstone belts contain metamorphosed clastic and chemical sedimentary rocks, volcanic rocks, tholeiitic basalts, komatiitic basalts and komatiites, many of which were deposited or erupted underwater [16][17][18][19][20] . Of particular interest are the extensively serpentinized komatiitic rocks, which are highly magnesian (MgO > 18 wt%) lavas and therefore contained high proportions of olivine [21][22][23] . The serpentinization of komatiites via reactions such as equations (2) and (3) has therefore been identified as a probable source of H 2 in the Archaean 3,24,25 . ...
The oxidation of iron from rocks during subaqueous alteration is a key source of the molecular hydrogen (H2) used as an energy source by chemosynthetic organisms, which may represent some of the earliest forms of life on Earth. In the Archaean, a potential source of ultramafic material available for serpentinization reactions that release H2 are komatiites. Komatiites are highly magnesian lavas, which contain evidence of extensive serpentinization and magnetite (Fe²⁺Fe³⁺2O4) production close to the Archaean seafloor. H2 production in komatiitic compositions has been modelled and experimentally investigated; however, the natural rock record has remained unexplored. Here we examine the geological evidence of H2 production from the basaltic to komatiitic rock record held in Archaean cratons. From the petrological investigation of 38 samples of komatiitic basalt to komatiite, we identify the unique serpentinization reaction responsible for H2 production from these lithologies. With support from over 1,100 bulk rock geochemical analyses, we directly quantify Fe³⁺ and therefore H2 production of komatiites in the Archaean. The chemical (high Mg) and physical (low viscosity flow) characteristics of komatiite flows allowed for extensive hydration and serpentinization in oceanic plateaus and therefore high H2 production available to chemosynthetic early life.
... These Al-depleted Buffalo River komatiites geochemically resemble archetypal komatiites from the Komati Formation in the , which implies similar magma origins. The Al-depleted komatiites from the Komati Formation have been explained by > 30 % of high-pressure partial melting of an incompatible element depleted mantle source in the garnet/majorite stability field (Fig. 19a;Jahn et al., 1982;Cattell et al., 1984;Gruau et al., 1990;Xie et al., 1993;Arndt, 2003;Sossi et al., 2016). The presence of residual garnet/majorite accounts for the high (Gd/Yb) N ratios in these Aldepleted komatiites because this high-pressure Al-phase preferentially retains the HREE during mantle melting. ...
... The 'komatiitic' actinolite schists from the Buffalo River Greenstone Belt correspond geochemically to the Al-undepleted komatiites of the Weltevreden Formation in the Barberton Greenstone Belt (Fig. 11b). The geochemical character of Al-undepleted komatiites has been attributed to melt generation by very high degrees of partial melting at relatively shallow mantle depths without garnet involvement (Herzberg, 1992;Arndt, 2008;Robin-Popieul et al., 2012;Sossi et al., 2016). The flat REE patterns, low (Gd/Yb) N values and high Al 2 O 3 /TiO 2 compositions can be explained by re-melting of an upwelling mantle source that had previously produced Al-depleted komatiites at greater depths (i.e., a sequential melting model linked to the same rising mantle plume; Sossi et al., 2016) (Fig. 19b). ...
... The geochemical character of Al-undepleted komatiites has been attributed to melt generation by very high degrees of partial melting at relatively shallow mantle depths without garnet involvement (Herzberg, 1992;Arndt, 2008;Robin-Popieul et al., 2012;Sossi et al., 2016). The flat REE patterns, low (Gd/Yb) N values and high Al 2 O 3 /TiO 2 compositions can be explained by re-melting of an upwelling mantle source that had previously produced Al-depleted komatiites at greater depths (i.e., a sequential melting model linked to the same rising mantle plume; Sossi et al., 2016) (Fig. 19b). The third geochemical type is represented by Alenriched ultramafic rocks (some of which in the form of cumulates) that display suprachondritic Al 2 O 3 /TiO 2 ratios of > 25 (Fig. 9f). ...
... Even though komatiite-hosted Ni sulfides sit in an extrusive rock, they are considered to reflect a significant proportion of mantle input (Herzberg, 1992;Barnes and Fiorentini, 2012;Mole et al., 2014;Sossi et al., 2016;Fig. 2). ...
... 2). The rapid ascent of komatiitic melts during decompression melting of the mantle could render associated Ni sulfides a Pb source proxy for the upper mantle and/or the lower crust (Barnes and Fiorentini, 2012;Sossi et al., 2016). Plume tail-related volcanism and rifting along the eastern margin of the Youanmi Terrane and the western margin of the EGST has been proposed to have caused two major komatiite pulses at ~2900 and 2700 Ma (Hoatson et al., 2006;Mole et al., 2014). ...
Various geological processes that affect Earth's crust may be encoded into isotopic tracers preserved in rocks and minerals. The enhanced sensitivity of U, Th, and Pb to crustal fractionation processes allows Pb isotopes to complement information from the Nd and Hf isotope systems. However, melt fractionation, crustal contamination and recycling, hydrothermal fluid flow and fluid-rock interaction, and other processes in the continental crust can lead to mixing of Pb isotopic signatures. Here, we report new Pb isotopic data from granite-K-feldspar and integrate these data with published Pb isotope ratios from granite K-feldspar and Pb-rich ores across the Yilgarn Craton in Western Australia. The aim of this study is to explore how the variability of Pb isotope ratios and derivative parameters can be used to gain information on specific geological processes occurring throughout the crustal column. We develop a model that subdivides different sampling media into chemical process groups and links their initial Pb signatures to Pb source regions and fractionation processes at various locations within the crust and upper mantle. Equilibration of Pb signatures with a primary mantle source reservoir (in part represented by volcanic-hosted massive sulfides) is contrasted with granite formation in the mid to lower crust (granite K-feldspar), and mineralization of ore deposits in the mid to upper crust (Pb-rich ores). Spatial trends similar to those in Nd and Hf isotopic data are recorded by Pb isotopic derivative parameters (μ = source 238U/204Pb, ω = 232Th/204Pb, and Δt - the difference between true sample age and Pb model age) calculated for komatiite-hosted Ni sulfide ores, granite K-feldspar, and volcanic-hosted massive sulfide (VHMS) ores. The significance of subtle differences in absolute values of derivative parameters is supported by the diversity of Pb isotope ratios, Pb model ages, and ∆t as tracked by a statistical metric, quantifying the variability of Pb sources involved in the formation of different chemical process groups. Generally greater variety in an older terrane (Youanmi) documents more ancient and recycled continental crust as compared with more homogeneous Pb isotopic signatures in a younger terrane (Eastern Goldfields Superterrane). The Pb signatures are interpreted, in part, to relate to the timing of source fractionation in the upper mantle with a legacy of this source signal preserved through various depths in the lithospheric section. The least radiogenic VHMS ore samples appear to provide a good approximation of mantle Pb signatures, indicated for example by a deficit in 206Pb and 208Pb relative to the other process groups. A significant heterogeneity recorded in Pb isotopic data from Pb-rich gold ores is explained by the interplay of hydrothermal fluids with diverse sources leading to the mineralization of gold deposits (e.g., leaching of Pb from surrounding rocks or fluid mixing). Such gold ore Pb signatures are distinct from other process groups, which together track sources less heterogeneous in age and/or U and Th.
... Быстрая закалка исходных коматиитовых расплавов в объеме подушки подразумевает термостатированность и сохранность первичных изотопно-геохимических характеристик расплава так, как это происходит в зонах спинифекс и прикровельных брекчиях лавовых потоков коматиитов (Sossi et al., 2016). О закрытости изотопно-геохимических систем в подушках можно судить по особенностям их строения. ...
The article presents new data on structure and composition of the concentrically zoned pillow of komatiite from the Ruvinvaara Formation of the Neoarchean Kostomuksha Greenstone Belt, Fennoscandian Shield. The zoning of the komatiite pillow is characterized by narrow variations of rare earth element concentrations and similar REE spectra for the dacite and andesite in the core and for komatiite and komatiite basalt in the outer part of the pillow, as well as higher concentrations of lanthanides in andesite compared to dacites. The REE distribution is a distinctive feature of the immiscibility differentiation in basic melts.
The Sm-Nd isotope systematics of komatiite matrix rocks and liquate dacite made it possible to establish their isochron age (2874 ± 35 Ma, εNd = +1.5). These data are evidence in favour of older age of the Kontok Series, Kostomuksha Greenstone Belt than considered before.
Keywords: komatiite, andesite, dacite, liquid immiscibiliy, REE geochemistry, isotopic systematics and age, Neoarchean Kostomuksha Greenstone Belt, Fennoscandian Shield
... Most komatiites are Archean-Paleoproterozoic in age, and are the signature of a higher ambient mantle temperature on the early Earth (Herzberg et al., 2007). All Archean komatiites are hydrated and metamorphosed, comprising serpentine, chlorite, and amphibole-bearing assemblages (Arndt, 2003;Sossi et al., 2016), as many were likely erupted onto the seafloor and hydrated through interaction with seawater, and later metamorphosed to greenschist-amphibolite facies. While this technically marks all Archean komatiitic rocks as metakomatiites, we use the term komatiites for simplicity. ...
... 3.5 Ga Komati formation of the Barberton Greenstone Belt (BGB), Kaapval Craton, South Africa (Fig. 1), was selected, based on its mineralogy, bulk rock composition and mineral compositions. This sample, B331-783, is an Al-depleted komatiite from Sossi et al. (2016) and Nesbitt et al. (1979). It exhibits spinifex textures, presumably once olivine, now completely pseudomorphed by antigorite, chlorite and chromian magnetite (Fig. 2a). ...
... 2.7 Ga Abitibi Belt, Superior Craton, Canada (Fig. 1). It is sample A442-84 from Sossi et al. (2016). It contains oriented spinifex-textures which are up to 1 cm in length, now completely pseudomorphed by antigorite, chlorite and magnetite with chromian-magnetite rims, in a matrix of finer-grained amphibole, chlorite, magnetite and ilmenite (Fig. 2f). ...
The preserved Archean continental crust is dominantly comprised of tonalite-trondhjemite-granodiorite (TTG) suites associated with less abundant low-grade greenstone belts. The exact processes that form TTGs, as well as the source rock they are derived from, are difficult to constrain from the sparse Archean geological record. However, studies show that the water-present partial melting of metamorphosed basalt at temperatures of 750–950 °C is required to produce large volumes of partial melt with TTG compositions. In this contribution, we investigate if hydrated komatiites — a constituent of greenstone belts — played a vital role in TTG genesis. Using petrology, mineral chemistry and phase equilibria modelling of representative komatiite samples, combined with analysis of a global geochemical dataset of komatiites and basaltic komatiites, we show that during metamorphism hydrated komatiites can release at least 6 weight % mineral-bound water. Up to 5 weight % of this water is released by breakdown of chlorite and tremolite at temperatures between 680 and 800 °C, regardless of the P–T path (i.e., tectonic scenario) experienced by the komatiitic rocks. As the temperatures of komatiite dehydration are above the water-saturated basalt solidus, the released water can trigger voluminous partial melting of basalt to ultimately create TTG batholiths. This considerable hydration potential of komatiites is due to their high XMg (XMg = molar Mg/[Mg+Fe]), which stabilises water-rich minerals during oceanic alteration on the seafloor, but also extends the stability of Mg-rich chlorite to high temperatures. During prograde metamorphism, the XMg, CaO and Al2O3 content of the reactive rock composition determines the proportion of chlorite vs amphibole, and therefore the volume of water which can be transported to temperatures of >750 °C. Despite the low abundance of komatiites in greenstone belts, they potentially played a vital role in crustal formation and the Earth's early water cycle.
... Our results strengthen this hypothesis by highlighting new reaction routes that encompass the progressive time-line of geologic events in such rock systems. Unexplored prebiotic reaction pathways based on similar processes may have occurred in the primitive Earth and on Mars where hydrothermal environments rooted on olivine-rich magmatic rocks (e.g., komatiites on Earth) are thought to be widespread [82][83][84][85] . The more reduced state of the mantle on early planets should have favored reduced species 12,86,87 in the percolating magmatic fluids. ...
How simple abiotic organic compounds evolve toward more complex molecules of potentially prebiotic importance remains a missing key to establish where life possibly emerged. The limited variety of abiotic organics, their low concentrations and the possible pathways identified so far in hydrothermal fluids have long hampered a unifying theory of a hydrothermal origin for the emergence of life on Earth. Here we present an alternative road to abiotic organic synthesis and diversification in hydrothermal environments, which involves magmatic degassing and water-consuming mineral reactions occurring in mineral microcavities. This combination gathers key gases (N 2 , H 2 , CH 4 , CH 3 SH) and various polyaromatic materials associated with nanodiamonds and mineral products of olivine hydration (serpentinization). This endogenous assemblage results from re-speciation and drying of cooling C–O–S–H–N fluids entrapped below 600 °C–2 kbars in rocks forming the present-day oceanic lithosphere. Serpentinization dries out the system toward macromolecular carbon condensation, while olivine pods keep ingredients trapped until they are remobilized for further reactions at shallower levels. Results greatly extend our understanding of the forms of abiotic organic carbon available in hydrothermal environments and open new pathways for organic synthesis encompassing the role of minerals and drying. Such processes are expected in other planetary bodies wherever olivine-rich magmatic systems get cooled down and hydrated.
... Rapid chilling of primary komatiitic melts within the pillow means thermal stability and preservation of primary isotopic-geochemical characteristics of melts similarly to that occurs in spinifex zones and top breccias of komatiitic lava flows (Sossi et al., 2016). The closed isotopic-geochemical systems of pillows are evident from their structural features. ...
This paper presents the data on the structure and composition of a concentric-zonal komatiite pillow from the Ruvinvaar Formation of the Neo-Archean Kostomuksha greenstone structure of the Fennos-candian Shield. The features of the zonal pillow structure include a narrow range of variations in concentrations of rare-earth elements (REEs), similar REE patterns of dacite and andesite from the pillow core and komatiite and komatiitic basalt from the outer part, and higher REE contents of andesite relative to dacite. These features of REE distribution indicate liquation differentiation of basic melts. According to the Sm-Nd isotopic systematics of rocks of the komatiite matrix and dacite, their isochron age is 2874 ± 35 Ma (εNd = +1.5). Our data indicate an older (than was previously considered) age of the Kontok Group of the Kosto-muksha greenschist structure.
