Komatiitic Melt Research Articles

The multiple sulfur isotope architecture of the Kambalda nickel camp, Western Australia

New data on the multiple sulfur isotope signature of Archean sulfides from country rocks and magmatic mineralization at the Moran deposit (Kambalda, Western Australia) were combined with previously published geochemical data to constrain the various stages of the dynamic evolution of this magmatic system, unveiling new insights into the transport mechanisms of sulfide liquids in komatiite magmas. Sulfides in the Archean magmatic and sedimentary host rocks of the komatiites display a unique mass-independent sulfur isotope signature (Δ33S), which records a photochemical reaction of sulfur in an oxygen-poor atmosphere prior to the Great Oxidation Event.Sedimentary rocks that are thought to be assimilated by komatiite show a distinctly positive Δ33S signature (+ 0.9 to + 2.4‰). Early ore sulfides situated above these sedimentary rocks contain relatively few valuable metals and display an overlapping Δ33S range (+ 0.6 to + 1.0‰). Subsequent but still early ore sulfides are situated above basalt, as the sedimentary rocks were thermo-mechanically eroded by the sulfide melt, displaying more mantle-like signatures (+ 0.2 to + 0.3‰) and valuable metal content - indistinguishable from the main ore deposit. This reflects a progressive dilution of the contaminant signature by the magmatic isotope signature of the komatiite liquid. Calculated volumes of the interaction of silicate melt and sulfide melt to explain the metal tenor of the ore and its Δ33S signature indicate a decoupling between chemical and isotopic signatures. This can be explained by upgrading the sulfide melt with valuable metals simultaneously with the dissolution of sulfur in the komatiite melt.

Decoupling of Sulfur Isotope Signatures from Platinum Group Elements in Komatiite-Hosted Ore Systems: Evidence from the Mount Keith MKD5 Ni-(Co-Cu) Deposit, Western Australia

Abstract Komatiites require external sulfur from country rocks to generate immiscible sulfide liquid, which concentrates metals to form economic nickel sulfide deposits. Although signatures related to mass-independent fractionation of S isotopes (MIF-S, denoted as Δ33S) may identify external S sources, their values may not be directly indicative of the S reservoirs that were tapped during the ore-forming process, because of dilution by S exchange between assimilated sulfide xenomelt and komatiite silicate melt. To quantify this process and be confident that MIF-S can be effectively used to track S sources in magmatic systems, we investigated the effect of silicate melt-sulfide liquid batch equilibration, using the proxy of silicate/sulfide mass ratio, or R factor, on the resulting MIF-S signatures of pentlandite-rich ore from the Mount Keith MKD5 nickel sulfide deposit, Agnew-Wiluna greenstone belt, Western Australia. We carried out in situ multiple S isotope and platinum group element (PGE) analyses on pentlandite from a well-characterized drill core through the deposit. The variability in Pd tenor and MIF-S signature suggests that these are decoupled during batch equilibration and that the latter is not controlled by metal-derived R factor. Rather, the observed spread of MIF-S signatures implies that the sulfide xenomelt was initially heterogeneous and that chemical equilibration of S isotopes is incomplete as opposed to that of PGEs in a komatiite melt. Consequently, magmatic sulfides, which formed in the hottest, most dynamic, and likely fastest equilibrating magmatic systems on Earth, may still preserve their initial MIF-S isotope compositions, reflecting the range of crustal S reservoirs that were available upon komatiite emplacement.

Mineral–Geochemical and Geotectonic Features of the Lotmvara-II Ultrabasic Sill, Serpentinite Belt (Kola Peninsula)

Abstract —In this paper, we present a description of the characteristics of the Lotmvara-II sill, which is a representative of the Serpentinite Belt (SB) composed of a series of shallowly emplaced ultrabasic intrusive bodies. The Paleoproterozoic SB complexes were derived from a large-scale mantle plume of komatiitic melt. The sill consists predominantly of fine-grained (locally nearly micrograined) harzburgites with subordinate zones of dunites and orthopyroxenites, located in the central and marginal parts, respectively. It formed from an Al-undepleted komatiitic magma of extremely high Mg content and may represent a near-surface laccolithic “ridge.” In general, the sill is comparatively homogeneous and does not have distinct zoning in the distribution of Mg# values in rock compositions (Mg# = 84.2–88.9, average 86.7). Detailed studies show that olivine, chromian spinel, and ilmenite are the most strongly magnesian in the central part of the body. The maximum values of Mg# equal to 90.7–91.4 in the compositions of olivine at the center of the sill are interpreted as “centers of initial crystallization”. The low values of Mg# equal to 73.4–76.4 are attributed to manifestations of the recurrent generation of olivine. The values of Mg# of orthopyroxene in the sill are within the range 84.6 to 92.3. Orthopyroxene grains in a porphyritic texture are surrounded by a rim of calcic amphibole (autometasomatic in origin); they do not differ compositionally from normal grains. The Zn content of the chromian spinel generally decreases toward the marginal parts of the sill. There is an insignificant degree of magmatic differentiation in the sill with respect to the principal components, but incompatible elements (REE and HFSE) locally show increased levels of their relative enrichment, which is reflected in the nature of the mineral associations described. Thus, the sill has a cryptic zonal structure, which is consistent with its overall crystallization from the center to the edges. The data gathered suggest the presence and significant development of volatile components, halogens, CO2, and especially magmatic H2O, which are capable of strongly lowering the liquidus and reducing the density and viscosity of the high-magnesium melt, thereby improving its mobility during ascent from the mantle to the near-surface level of the crust. An increase in fO2 is observed during in situ subvolcanic crystallization of the sill, as noted earlier in the related complexes of the belt. The relatively small volume of the komatiitic magma in the sill crystallized fairly quickly, resulting in unusual mineral intergrowths. Thus, the Lotmvara-II sill is a novel member in the Serpentinite Belt–Tulppio Belt (SB–TB) in the Paleoproterozoic SB–TB megastructure of the Fennoscandian Shield.

ПАЛЕОПРОТЕРОЗОЙСКИЕ УЛЬТРАМАФИТОВЫЕ ДАЙКИ КУН-МАНЬЁНСКОГО АРЕАЛА (ЮГО-ВОСТОК СИБИРСКОЙ ПЛАТФОРМЫ): СТРУКТУРНОЕ ПОЛОЖЕНИЕ, СОСТАВ И ОБСТАНОВКА ФОРМИРОВАНИЯ

Dykes of Paleoproterozoic ultramafic rocks from the Kun-Manie area were studied within the southeastern fringe of the Siberian Platform. At the end of the Paleoproterozoic, the processes of overthrust faulting and folding were widely manifested here, the latest of which were accompanied by the emplacement of mafic-ultramafic dykes and sills that control the location of Cu-Ni-PGE sulfide mineralization. The petrographic structure of ultramafic dykes suggests their emplacement in tectonic settings. The elevated concentrations of LREE relative to HREE, coupled with other data, allow them to be referred to high-Mg ultramafic rocks of the picrite-tholeiite series, the parent magmas of which could be not ultramafic, but picrobasaltic magmas. The level of REE concentrations in ultramafic rocks and the nature of the normalized spectra indicate the involvement of sufficiently enriched sources in the generation of primary melts from which rocks crystallized that radically differed from peridotites of ophiolite associations and typical komatiite melts. In terms of spectral shapes and contents of indicator elements (Nb, Yb and Th), they are close to the «enriched» mantle source of the E-MORB type differing in lower REE concentrations. The Zr/Y – Nb/Y ratio in ultramafic rocks suggests a non-plume source of their primary melts in the upper depleted mantle. Rocks derived from such a melt are characteristically enriched in fluid components (S, Cl, F, As, Te, Se, and H2O), which is evident in the presence of mineral phases rich in hydroxyl, that is, amphiboles and phlogopite of igneous origin. The ore-forming system evolved from Ni-bearing at the early magmatic stage during the formation of disseminated mineralization to Cu-enriched at the postmagmatic stage. The geological data indicate that they formed during the collision between the Paleoproterozoic island arc and the microcontinent. The combination of subduction and intraplate geochemical characteristics in magmatites is characteristic of the transform margin as a result of the oceanic (Verkhnemaya) and continental (Uchur) microplates sliding horizontally past each other.

Komatiitic Subvolcanic Rocks in the Mount Khanlauta Massif, Serpentinite Belt (Kola Peninsula)

Abstract —For the first time, exposures of ultramafic rocks of subvolcanic origin have been investigated at the foot of Mount Khanlauta (Kola Peninsula) located at a distance of ~2 km from the southwestern margin of the Pados-Tundra layered complex of dunite–harzburgite–orthopyroxenite composition, which hosts zones of chromitite and unconventional PGE mineralization. The ultramafic body is composed of micro- to fine-grained harzburgite and subordinate orthopyroxenite and has a cryptically zoned structure. The body is of E–W strike and has a small size with an apparent thickness of ~0.1 km; the exposed outcrops exhibit a blocky surface as a result of extensive cracking caused by degassing and rapid cooling of a parental komatiitic melt. Elements of columnar parting are recognized, which have a hexagonal shape in cross section and are consistent with the inferred subvolcanic origin. Two zones are identified. Zone I is formed by a more magnesian olivine with Mg# = 86.0–87.9 (0.15–0.21 wt.% MnO). In Zone II, olivine grains are notably less magnesian (Mg# = 81.8–84.1) and invariably have higher contents of manganese (0.19–0.30 wt.% MnO). The presence of this zoning is corroborated by the lateral distribution of accessory chromian spinel grains with maximum Mg# values (>20) in Zone I. The Khanlauta rocks also contain subordinate orthopyroxene (Mg# = 86.3–87.2), amphiboles of the tremolite–actinolite series, and anthophyllite of deuteric (autometasomatic) origin along with accessory minerals: ilmenite, hematite (~15 mol.% escolaite, Cr2O3, in solid solution), and mono- and diphase grains of sulfides in the form of intergrowths of Co-bearing pentlandite (Ni/Fe = 0.9–1.3; 1.00–16.74 wt.% Co; up to 1.7–6.8 wt.% Cu) and heazlewoodite (locally in intergrowth with hematite). The inferred front of crystallization moved in the western direction, causing the formation of Zone II from a more fractionated melt with a notably lower Mg# value. A sharp increase in oxygen fugacity locally caused the formation of anomalous parageneses of chromite and ilmenite, in which the observed Mg# values of ilmenite are considerably greater than those of the coexisting chromite. The geochemical whole-rock characteristics based on major, minor, and trace elements (including LILE, REE, and HFSE), as well as the compositions and trends of chromian spinels, are similar to those in differentiated (zoned) sills of the Chapesvara complex, which are closely associated with the Pados-Tundra layered complex. The obtained data indicate comagmatic relationships among the Khanlauta massif, zoned sills of the Chapesvara complex, and the Pados-Tundra layered complex. All of them crystallized from a primitive highly magnesian Fe- and Cr-enriched komatiitic magma (Al-undepleted). Thus, they belong to a single subvolcano–plutonic association being part of the Serpentinite belt–Tulppio belt (SB–TB) megastructure of presumably Paleoproterozoic age.

Fractionation of trace and platinum-group elements during metamorphism of komatiitic chromites from the early Archean Gorumahishani greenstone belt, Singhbhum Craton (eastern India)

It is well established that the major and minor element contents of chromites are subject to change during greenschist to amphibolite facies metamorphism. During upper amphibolite facies metamorphism, chromite can be completely converted to chrome magnetite. However, not all elements are affected to the same degree, the concentrations of +2 ions (e.g. Zn, Co, Mn) being particularly vulnerable to modification. The degree to which trace elements, particularly the platinum-group elements (PGE), are affected has not been closely examined. The compositions and textures of chromites from komatiites of the Gorumahishani greenstone belt of the Singhbhum Craton (India) have experienced a range of metamorphic conditions from greenschist to amphibolite facies, providing the opportunity to study the changes of trace and platinum-group element composition with metamorphic grade. Five types of altered chromites are identified from the komatiitic suite of rocks in the ~120-km-long greenstone belt. The type-I chromites are non-porous and characterized by the least modified cores. These chromites are mostly present in the northern Maharajgunj-Tua Dungri section where rocks show metamorphism from greenschist to greenschist-amphibolite transition facies. The type-II and type-III chromites are porous and mostly found in the southern Kapili section of the greenstone belt where rocks show metamorphism up to the mid-amphibolite facies. Type-IV and type-V chromites are completely modified to ferritchromit and chrome magnetite, respectively, and are present in the komatiitic rocks from the entire greenstone belt. The central cores of the type-I and type-II grains have relatively higher concentrations of mobile trace elements (e.g. Zn, Co, and Mn) with higher Mg# [Mg/(Mg + Fe2+)], lower Cr# [Cr/(Cr + Al)], and lower Fe3+/R3+ (R3+ = Fe3+ + Cr3+ + Al3+) ratios than their respective rims. Significantly higher concentrations of the immobile trace elements (e.g. Ti and V) in the cores of the type-II grains relative to their chrome magnetite rims from the Kapili section and to the type-I varieties from other sections might be due to the metamorphism of the komatiitic rocks under higher-grade conditions (amphibolite facies). In situ LA-ICPMS analysis for PGE reveals a relatively higher concentration of Ru and Rh in the rims of the type-I chromites than in the cores which is due to the diffusion of these elements from the normal spinel structure of the cores towards the bivalent octahedral sites of the inverse spinel structure of the chrome magnetite rims during metamorphic processes. The lower concentrations of Os, Ir, Ru, and Rh in the cores of the type-II chromites from the Kapili section might be related to the metamorphism of the rocks under higher-grade conditions that facilitated the diffusion of these elements to associated sulphide or platinum-group mineral or alloy phases. The calculated partition coefficients of Sc, Ti, V, Mn, Ni, Ga, Os, Ir, Ru, and Rh from the least altered chromite cores assuming equilibrium with the parental komatiitic melt also suggest the variable effects of metamorphism when compared with global experimental and empirical values of the natural samples.

Chalcophile Element Heterogeneity in Ni-Cu-(Platinum Group Element) Orebodies of Raglan Horizon in Cape Smith Belt: Implications for Ore-Forming Processes

Abstract The komatiitic Raglan Horizon in northern Quebec hosts one of the largest komatiite-associated magmatic sulfide deposits in the world. Several studies have suggested that the sulfide ores were formed by melting and transporting sedimentary sulfides from upstream after the komatiite melt had assimilated a large amount of S-rich country rocks. In this study, we report new compositional data of the sulfide ores in Zones 5-8 and 13-14 along with the legacy assay database to investigate variability of the ore-forming processes at Raglan at the camp scale. More than 90,000 drill core samples were selected, and the rocks are categorized into disseminated, net-textured, and massive sulfide based on their sulfide abundance. We demonstrate that the majority of the compositional variability of the sulfide-bearing rocks can be attributed to the difference in the mass ratio between silicate and sulfide melts (R factor), by applying thermodynamic modeling of silicate and sulfide magma evolution. The massive sulfides display depletions in Cu, Pd, and Pt that are best explained by sulfide liquid fractionation, which differentiates the residual Cu-, Pd-, and Pt-rich sulfide liquid from the Ni-rich monosulfide solid solution (MSS) cumulate now preserved within the massive sulfide ores. A small population of sulfide-bearing samples that is strongly depleted in all the economically important metals has been interpreted to represent externally derived xenomelts that were poorly equilibrated with the metal-rich host magma due to low residence time. A portion of massive sulfides has been found to be depleted in Pd and Cu but not Pt. This decoupling of these metals can be well explained by a postmagmatic process that persisted to temperatures as low as 300°–350°C. Similar depletions of Pd have also been observed in the Expo Intrusive Suite immediately to the south. The regional variations in the compositions of individual orebodies suggest that they occurred in highly localized environments that record only local magmatic conditions and that the sulfide mineralization did not reach equilibrium with a single regionally continuous silicate magma system. Each zone preserves a unique signature of assimilation, sulfide segregation, and sulfide crystallization; hence, they likely represent separate magmatic pulses within the larger Raglan system. Lastly, based on the spatial locations of the low tenor sulfides, we tentatively suggest that Zone 2 and the Katinniq zone are more proximal to the as-yet-unidentified eruption site of the Raglan komatiite lava flow field.

The Palaeoproterozoic Gallujavri Ultramafic Intrusion, Karasjok Greenstone Belt; Petrogenesis of a Trans-Crustal Magma System

Abstract The modern concepts of magmatic plumbing systems have evolved from simple models of melt-dominated crustal magma chambers to magmatic systems that span the continental crust. The geochemistry of these systems is challenging to model numerically because of a complex polybaric and multiphase evolution, which is not well described by mathematic end-member formulas. We propose that a recent modelling tool, the Magma Chamber Simulator (MCS), can reveal part of the petrological complexities of such systems. The MCS numerically models phase equilibria, mineral chemistry and major and trace elements in multicomponent–multiphase systems by finding the extremes of the appropriate state functions. This study presents new petrographical and geochemical data from the Palaeoproterozoic Gallujavri ultramafic intrusion in the Karasjok Greenstone Belt, Arctic Norway. Our data indicate that (1) the intrusion is affected by crustal assimilation, (2) phase equilibria require polybaric crystallisation and (3) spinel compositions are similar to sub-volcanic intrusions of continental flood basalt provinces. We utilise the MCS to simulate the petrogenesis of the intrusion and test the model by comparing it against our reported petrological and geochemical data. Our modelling shows that the parental magma of the intrusion is consistent with coeval Karasjok-type komatiites observed at various places throughout the greenstone belt. First, the komatiitic primary magma intruded Archaean lower crust (c. 700 MPa), where fractionation and assimilation resulted in a hybrid melt. Second, the hybrid melt migrated and accumulated in a mid-crustal magma chamber (c. 400 MPa), where it continued to fractionate and assimilate while remaining open to recharge of mantle-derived komatiitic melts, forming the Gallujavri intrusion. We interpret the Gallujavri intrusion as part of a Palaeoproterozoic trans-crustal magma plumbing system, with many similarities to other Fennoscandian ultramafic-mafic layered intrusions.

The komatiite-mantle platinum-group element paradox

The platinum-group element (PGE) contents of komatiites have been used to estimate the PGE contents of their mantle sources and investigate temporal variations in the PGE systematics of the mantle throughout Earth history. However, the use of mantle melts to investigate source characteristics is critically dependent on understanding the fractionation of the elements in question between mantle sources and their derived magmas. We present new PGE and Re abundance data, together with Re-Os isotope data for the pristine 1.9 Ga Winnipegosis komatiites, Canada. Whole rock Re-Os isotope data define a 1865 ± 40 Ma isochron, consistent with previous age estimates, with initial γOs of +0.9 ± 1.4. The PGE and Re variations in these komatiites were controlled by fractionation of olivine, chromite, Os-Ir alloys, a Ru-Os rich phase (possibly laurite) from the komatiite melt. The calculated parental melt composition at 23.6 ± 1.6 wt% MgO has 1.1 ± 0.1 ppb Os, 0.91 ± 0.08 ppb Ir, 4.0 ± 0.5 ppb Ru, 7.0 ± 0.8 ppb Pt, 7.2 ± 0.7 ppb Pd, and 0.53 ± 0.06 ppb Re. The calculated PGE contents are low relative to many komatiites; using existing methods for calculating the PGE contents of mantle sources suggests the Winnipegosis source had PGE contents from 25 ± 19 wt% to 57 ± 7 wt% of primitive mantle values. This surprising finding substantiates a growing ‘PGE paradox’, where almost all komatiites appear to have formed from PGE-depleted mantle sources. We suggest the simplest resolution to this paradox is that current methods systematically underestimate mantle source PGE contents. We test the assumptions implicit in these methods by comparing detailed models of PGE behaviour during mantle melting to a large database of parental melt compositions of high degree melts. Ruthenium behaviour in high degree melts can be adequately modelled using our current understanding of Ru partitioning. However, the Ru contents of high degree mantle melts are strongly dependent on the degree of melting, as Ru-rich melts formed after sulphide exhaustion are increasingly mixed into early-formed Ru-poor melts. Platinum and Pd behaviour during mantle melting cannot be successfully modelled under the widespread assumption that they are strongly incompatible following sulphide exhaustion, suggesting that these elements cannot be used to estimate PGE contents of the mantle sources of high degree melts. Parental melt Pt and Pd in many high degree lavas are correlated with Al2O3/TiO2, suggesting a pressure control on their partitioning. We attribute the relatively low PGE contents of the Winnipegosis komatiites to their moderate degrees and depth of melting compared to other komatiites, rather than invoking a PGE-depleted mantle source. More broadly, the complex controls (T, P, fO2, S content, prior depletion) on the PGE contents of komatiites mean that most komatiites can be reconciled with being derived from sources with primitive mantle-like PGE contents, and few komatiites can be confidently asserted to come from PGE-depleted mantle sources. Pending a better understanding of PGE behaviour during mantle melting, we recommend caution when using komatiite PGE systematics to infer the highly siderophile element evolution and accretionary history of Earth’s mantle.

The δ53Cr isotope composition of komatiite flows and implications for the composition of the bulk silicate Earth

Data on the chromium stable isotope composition of planetary reservoirs have the potential to provide information about core formation, partial melting and conditions of the Moon formation. In order to detect the small Cr isotopic differences between various reservoirs in the solar system, their compositions need to be precisely constrained. The current BSE value of δ53Cr = −0.11 ± 0.06‰ (Sossi et al., 2018) cannot resolve differences between achondrites, (Vesta δ53Cr = −0.17 ± 0.05‰) and chondrites (carbonaceous δ53Cr = −0.12 ± 0.05‰; ordinary δ53Cr = −0.11 ± 0.04‰). The composition of the bulk silicate Earth (BSE) is often used as a reference point for comparisons to other planetary reservoirs. However, past attempts to estimate the Cr isotopic composition of the BSE have been unable to provide a well-constrained BSE value. Traditional methods, using mantle peridotites, are affected by the susceptibility of Cr isotopes to fractionation during metasomatism. More recently, the Cr isotope composition of the BSE has been calculated using komatiites, in addition to mantle peridotites, to produce a more precise value (Sossi et al., 2018). In order to constrain the BSE composition to a higher precision, the δ53Cr of remarkably fresh komatiite lava flows from three localities, ranging in age from 2.7 Ga to 89 Ma, have been investigated in detail. These included the Tony's Flow in the Belingwe Greenstone Belt, Zimbabwe, the Victoria's Lava Lake in Fennoscandia, and komatiites from Gorgona Island in Colombia.In the komatiites studied, a range in Cr isotopic compositions was found, from δ53Cr = −0.16 ± 0.02 to −0.01 ± 0.02‰. We show that the high degrees of partial melting that produced the komatiites, did not result in Cr isotopic fractionation between the komatiitic melt and mantle residue. However, limited Cr isotopic fractionation is found to be a consequence of komatiite lava differentiation. For the lava flows with high Mg content and high Cr2+/ƩCrTOT (the molar ratio of Cr2+/(Cr2+ + Cr3+)), such as Tony's Flow and Gorgona, δ53Cr increases in the evolved portion of the magma during olivine fractionation due to the preferential inclusion of light Cr into olivine. Other flows with lower MgO content do not show this behaviour because a smaller fraction of the Cr is contained in olivine. The effects of fractional crystallisation must, therefore, be taken into account when calculating the Cr isotopic composition of the source of komatiite lavas.The weighted average of δ53Cr for the komatiite lavas analysed is −0.12 ± 0.04‰ (n = 5) and represents our best estimate for the Cr isotopic composition of the BSE. It agrees with the previous estimates, while providing an improvement to the uncertainty. There is no resolvable difference between this value and that of chondritic meteorites. Our data also indicate that the δ53Cr of the mantle has been constant since at least the Archean.

A Fractional Crystallization Link between Komatiites, Basalts, and Dunites of the Palaeoproterozoic Winnipegosis Komatiite Belt, Manitoba, Canada

AbstractThe rock type most commonly associated with komatiite throughout Earth’s history is tholeiitic basalt. Despite this well-known association, the link between komatiite and basalt formation is still debated. Two models have been suggested: that tholeiitic basalts represent the products of extensive fractional crystallization of komatiite, or that basalts are formed by lower degrees of mantle melting than komatiites in the cooler portions of a zoned plume. We present major and trace element data for tholeiitic basalts (∼7·5 wt% MgO) and dunites (46–48 wt% MgO) from the Palaeoproterozoic Winnipegosis Komatiite Belt (WKB), which, along with previous data for WKB komatiites (17–26 wt% MgO), are utilized to explore the potential links between komatiite and basalt via crystallization processes. The dunites are interpreted as olivine + chromite cumulates that were pervasively serpentinized during metamorphism. Their major and immobile trace element relationships indicate that the accumulating olivine was highly magnesian (Mg# = 0·91–0·92), and that small amounts (4–7 wt% on average) of intercumulus melt were trapped during their formation. These high inferred olivine Mg# values suggest that the dunites are derived from crystallization of komatiite. The tholeiitic basalts have undergone greenschist-facies metamorphism and have typical geochemical characteristics for Palaeoproterozoic basalts, with the exception of high FeO contents. Their REE patterns are similar to Winnipegosis komatiites, although absolute concentrations are higher by a factor of ∼2·5. The ability of thermodynamic modelling with MELTS software to reproduce komatiite liquid lines of descent (LLD) is evaluated by comparison with the crystallization sequence and mineral compositions observed for Winnipegosis komatiites. With minor caveats, MELTS is able to successfully reproduce the LLD. This modelling is extended to higher pressures to simulate crystallization of komatiitic melt in an upper crustal magma chamber. All major and rare earth element characteristics of the tholeiitic basalts can be reproduced by ∼60 % crystallization of a Winnipegosis komatiite-like parental melt at pressures of ∼1·5–2·5 kbar at oxygen fugacities between QFM − 1 and QFM + 1, where QFM is the quartz–fayalite–magnetite buffer. Winnipegosis basalts have low Mg# values that are not in equilibrium with mantle peridotite. They therefore cannot represent primary mantle melts derived from cooler mantle than the komatiites, and require fractional crystallization processes in their formation. Furthermore, their trace element characteristics indicate a depth of melting indistinguishable from that of the Winnipegosis komatiites, and derivation from an identical depleted mantle source. All geochemical and geological evidence is therefore consistent with their derivation from a komatiitic melt, and the presence of a large komatiite-derived dunite body in the WKB provides evidence of extensive fractionation of komatiite in the upper crust. The observed uniform basalt compositions are interpreted as the result of a density minimum in the evolving komatiitic melt at temperatures between clinopyroxene and plagioclase saturation, with efficient extraction of melt from a mixture containing ∼60 % crystals. We conclude that the WKB basalts formed by fractional crystallization of a komatiitic parental melt, and suggest that this model may be more broadly applicable to other localities where komatiites and associated basalts show similar geochemical or isotopic characteristics.

Interspinifex Ni sulfide ore from Victor South-McLeay, Kambalda, Western Australia

Spinifex-textured olivine plates hosted in sulfides are usually named “interspinifex ore” in komatiite-hosted sulfide deposits. This ore type is rare but provides important genetic information on sulfide deposits, komatiite volcanology and thermomechanical erosion processes. Occurrences in Victor South-McLeay and Moran South (Kambalda, Western Australia) differ significantly from previously reported occurrences in their stratigraphic location, position within the ore profile and textural appearance. Thus, their formation process has to be reconsidered. Interspinifex ore reported here is situated in the lower portion of the basal lava flow between massive and net-textured sulfides in the centre of the embayment and between massive sulfides and older basalt in a “pinchout” where the sulfides melted sideways into older basalt on the embayment edge. Interspinifex ore is composed of up to 10-cm-long aggregates of parallel plates in the upper portion of massive sulfides and is overlain by barren komatiite. The texture does not allow for a classic single explanation. Thus, two possible formation mechanisms are envisaged: (1) A younger komatiite melt intrudes into its own olivine and sulfide liquid cumulate pile, while the sulfides are still liquid. The injection on top of the sulfides causes the formation of an emulsion, from which the spinifex forms due to the temperature gradient between the melts. (2) Interspinifex ore is a relic of an early komatiite flow formed in a series of successive pulses of komatiite and sulfide liquid. The spinifex of the komatiite is invaded by a younger batch of sulfide liquid replacing interstitial silicate melt.

1860-Ma I-Shaped Mafic Sills in the Murmansk Craton, Eastern Fennoscandia: Petrology and Tectonic Setting of Within-plate Mafic Events during Nuna Assembly

1860-Ma within-plate mafic rocks of the Eastern Murman Sill Province were studied in the northern part of the Murmansk Craton, where they compose sheet bodies (sills) up to 100 m thick in Archean granitoids and gneisses. The sills have a simple inner structure, show no macroscopic signs of in situ differentiation even in large bodies, and consist of homogenous medium-grained poikilophitic dolerites weakly varying in composition. Minor variations in mineral and chemical composition of the rocks allow us to regard the mafic sills as I-shaped sills. Their homogeneity is likely caused by the rapid crystallization of aphyric magmas. The studied sills are close to N-MORB in major-element composition, but significantly differ in trace element signatures. The low Cr and Ni concentrations in the dolerites indicate the high degrees of fractional crystallization, while the low HREE and HFSE contents suggest that primary komatiite melts were formed through the high-degree melting of a depleted mantle source. The absence of phenocrysts in chilled dolerites prevented the use mineralogic and petrographic criteria for estimating the crystallization conditions in intermediate magma chambers. AlphaMELTS 1.9 and PRIMELT3 MEGA modelling results indicate that the primary melts could be formed through a high-degree melting of ascending high-temperature mantle plume, with subsequent contamination and fractional crystallization in mid-crustal intermediate magma chambers. The Nd and Sr isotopic composition of the dolerites and constituent minerals suggests that the crustal contamination included several stages. The first, most significant crustal contribution (up to 10%) was related to the contamination by Archean tonalites in the intermediate mid-crustal chambers. This provided negative eNd values and radiogenic Sr isotopic composition (Sri = 0.702–0.704) in the dolerites. The second stage included the upper-crustal contamination during lateral propagation of melts along gentle weakened zones, which resulted in the wide variations of Sr-Nd isotopic composition in rocks and minerals. The third stage was related to the contamination by 87Sr-enriched fluid that is supported by the sharp increase of Sri (up to 0.708) at constant Nd isotopic composition in gabbro-pegmatites. Such fluid could be formed via biotite dehydration caused by heating of host granites by large volume of emplaced mafic melts. The age, volume, and composition of the studied dolerites and similarity with coeval mafic rocks in the Superior Craton indicate that the I-shaped mafic sills in the Murmansk Craton could be formed on the periphery of the Circum Superior large igneous province. It suggests that the Murmansk Craton of the Fennoscandian Shield and the Superior Craton of the Canadian Shield at 1860 Ma were joined into a single consolidated lithospheric block that host within-plate mafic rocks and likely represented an early core of the Paleoproterozoic Nuna supercontinent.

An appraisal of geochemical signatures of komatiites from the greenstone belts of Dharwar Craton, India: Implications for temporal transition and Archean upper mantle hydration

Archean ultramafic–mafic magmatism associated with dynamic mantle melting processes represents an integral part of crustal evolution and continental growth. Archean mantle plumes and their high temperature derivatives in komatiitic magmas provide a cornerstone in understanding the thermal and chemical evolution of the early Earth and its mantle. In this study, we evaluate the geochemical characteristics of the temporally distinct Sargur Group and Dharwar Supergroup greenstone belts of Dharwar Craton in southern India. The komatiites associated with these greenstone belts are characterized by Al‐depleted and Al‐undepleted compositions. Their MgO, Ni, Co, and Cr concentrations reflect primitive, undifferentiated nature of precursor melts, whereas the HFSE and REE chemistry corroborates high‐degree (30–50%) partial melting of mantle with prominent role of majorite garnet and perovskite in the generation of komatiitic melts. The Al‐depleted komatiites were formed at a greater depth (~13 GPa) by equilibrium melting leaving a garnet‐bearing residue. The resultant komatiitic melt retained buoyancy under extreme pressure and high temperature conditions, segregated and accumulated in the ambient peridotite mantle source and subsequently escaped with progressive decrease in pressure in the ascending plume. The Al‐undepleted and relatively rare Al‐enriched komatiites were derived by fractional melting at intermediate to shallow depths after the discharge of a large proportion of melt ensued by the collapse and entrance of residual garnet into the melt phase. The Mesoarchean–Neoarchean Dharwar komatiites provide evidence for heterogeneous, hydrated Archean upper mantle trapped by ascending mantle plumes. The non‐plume, arc‐related signatures invoke contributions from (a) sub‐cratonic lithospheric mantle roots influenced by ancient subduction events and (b) hydrated Archean upper mantle. The Sargur Group and Dharwar Supergroup komatiites record a distinct temporal transition and geochemical heterogeneity in the Archean mantle beneath the Dharwar Craton and can be attributed to Archean upper mantle hydration by flat slab subduction of >3.3 Ga oceanic crust at shallow level with vestiges of primordial oceanic crust forming deep cratonic mantle roots beneath the Dharwar Craton. The temporal transition towards mantle hydration beneath Dharwar Craton during the Mesoarchean can be correlated with the onset of subduction‐driven plate tectonics and ocean–crust–mantle interactions.

Remnant lenses of komatiitic dykes in Kambalda (Western Australia): Occurrences, textural variations, emplacement model, and implications for other komatiite provinces

Remnants of komatiitic dykes have been intersected for the first time at Kambalda (Western Australia), a classical locality for komatiite-hosted Ni deposits. The dykes consist of metamorphosed lenses, aligned on three planar and parallel surfaces. They are composed of brecciated komatiitic rocks, differentiated lenses with spinifex- and hopper-textured olivine and clinopyroxene cumulates (all now composed of variable amounts of tremolite, chlorite and biotite), and chloritite.Igneous textures (spinifex and cumulate, skeletal chromite, amygdales) are well preserved and their combination with a low carbon content (carbonation is common in more altered komatiites) implies an isochemical metamorphic alteration. The composition of the komatiite dyke near the contact is similar to the Kambalda Komatiite (but lower in MgO) and changes to a clinopyroxene composition in the dyke centre. This is interpreted to be a clinopyroxene cumulate crystallized from a komatiitic basalt. Furthermore, trace elements in whole rock samples from the centre of the dyke record contamination by felsic crust. This means one drill hole intersecting the dyke has a record of contaminated to uncontaminated mantle melts within a few centimetres.The rare occurrence of komatiite dykes compared to sills in all komatiite occurrences worldwide is probably a consequence of three factors – the komatiite melt erupting above its liquidus, the melt's very low viscosity and its laminar flow. In areas of obstacles (e.g., change in lithology), local eddies can cause a local temperature decrease which leads to crystallization of the liquidus phase. A decrease in temperature might be accelerated by locally induced convection cells of pore fluid in the host rock. In the laminar flowing part of the dykes, however, the overall liquidus of the melt is only reached when the eruption ceases. Due to decreasing melt pressure, the fracture closes to adjust to the changing pressure, squeezing the melt out before the liquidus is reached. Such dykes leave no trace of their former existence. These areas of changes in flow direction therefore form chains of komatiite lenses.

Deep komatiite signature in cratonic mantle pyroxenite

AbstractWe present new and compiled whole‐rock modal mineral, major and trace element data from extremely melt depleted but pyroxenite and garnet(‐ite)‐bearing Palaeoarchean East Greenland cratonic mantle, exposed as three isolated, tectonically strained orogenic peridotite bodies (Ugelvik, Raudhaugene and Midsundvatnet) in western Norway. The studied lithologies comprise besides spinel‐ and/or garnet‐bearing peridotite (dunite, harzburgite, lherzolite) garnet‐clinopyroxenite and partially olivine‐bearing garnet‐orthopyroxenite and ‐websterite. Chemical and modal data and spatial relationships between different rock types suggest deformation to have triggered mechanical mixing of garnet‐free dunite with garnet‐bearing enclosures that formed garnet‐peridotite. Inclusions of olivine in porphyroclastic minerals of pyroxenite show a primary origin of olivine in olivine‐bearing variants. Major element oxide abundances and ratios of websterite differ to those in rocks expected to form by reaction of peridotite with basaltic melts or silica‐rich fluids, but resemble those of Archean Al‐enriched komatiite (AEK) flows from Barberton and Commondale greenstone belts, South Africa. Websterite GdN/YbN, 0.49–0.65 (olivine‐free) and 0.73–0.85 (olivine‐bearing), overlaps that of two subgroups of AEK, GdN/YbN 0.25–0.55 and 0.77–0.90, with each of them being nearly indistinguishable from one another in not only rare earth element fractionation but also concentration. Websterite MgO content is high, 22.7–29.0 wt%, and Zr/Y is very low, 0.1–1.0. The other, non‐websteritic pyroxenites overlap—when mechanically mixed together with garnetite—in chemistry with that of AEK. It follows an origin of websterite and likely all pyroxenite that involves melting of a garnet‐bearing depleted mantle source. Pyroxene exsolution lamellae in the inferred solidus garnet in all lithological varieties require the pyroxenites to have crystallized in the majorite garnet stability field, at 3–4 GPa (90–120 km depth) at minimum 1,600°C. Consequently, we interpret the websterites to represent the first recognized deep plutonic crystallization products that formed from komatiite melts. The other pyroxenitic rocks are likely fragments of such crystallization products. An implication is that a mantle plume environment contributed to the formation of (one of) the worldwide oldest lithospheric mantle underneath the eastern Rae craton.

Redox state of the Archean mantle: Evidence from V partitioning in 3.5–2.4 Ga komatiites

Oxygen fugacity of the mantle is a crucial thermodynamic parameter that controls such fundamental processes as planetary differentiation, mantle melting, and possible core-mantle exchange. Constraining the evolution of the redox state of the mantle is of paramount importance for understanding the chemical evolution of major terrestrial reservoirs, including the core, mantle, and atmosphere. In order to evaluate the secular evolution of the redox state of the mantle, oxygen fugacities of six komatiite systems, ranging in age from 3.48 to 2.41 Ga, were determined using high-precision partitioning data of the redox-sensitive element vanadium between liquidus olivine, chromite and komatiitic melt. The calculated oxygen fugacities range from −0.11 ± 0.30 ΔFMQ log units in the 3.48 Ga Komati system to +0.43 ± 0.26 ΔFMQ log units in the 2.41 Ga Vetreny system. Although there is a slight hint in the data for an increase in the oxygen fugacity of the mantle between 3.48 and 2.41 Ga, these values generally overlap within their respective uncertainties; they are also largely within the range of oxygen fugacity estimates for modern MORB lavas of +0.60 ± 0.30 ΔFMQ log units that we obtained using the same technique. Our results are consistent with the previous findings that argued for little change in the mantle oxygen fugacity since the early Archean and indicate that the mantle had reached its nearly-present day redox state by at least 3.48 Ga.

Geochemistry and Sm-Nd isotopic characteristics of the Paleoarchean Komatiites from Singhbhum Craton, Eastern India and their implications

Komatiites near Kapili village, located in Palaeo-Mesoachean Gorumahishani-Badampahar greenstone belt of Singhbhum Craton, Eastern India preserve excellent igneous textures and exhibit co-association of Al-depleted (ADK) and Al-undepleted komatiites (AUK). The Kapili komatiites exhibit repeated spinifex textured flows (cooling units) overlying massive cumulate zone. The basal cumulate part of Kapili komatiite is chemically similar to ADK which is characterized by subchondritic Al2O3/TiO2 (9.38–10.5) and HREE depleted nature (average (Gd/Yb)PM∼1.35) elucidating presence of majorite in residuum whereas compound upper spinifex lava typically display superchondritic Al2O3/TiO2 (∼22–28.5) and HREE enriched nature (average (Gd/Yb)PM∼0.81) demonstrating majorite free residuum.Calculated primary magma compositions of Kapili ADK and AUK reveal that generation of primary liquid of Kapili ADK or basal cumulate took place at greater depth (>300km) or at higher pressure (∼9GPa) at around 30% melting of source in contrast with primary liquid of AUK (spinifex lava), which was generated at shallower depth (∼150km) or at lower pressure (∼6GPa) beyond majorite stability and at higher degree of melting (∼30–40%). ‘Hump-shaped’ pattern of primitive mantle (PM) normalized REE spectra of Kapili ADKs and their primary liquid is caused by depletion of both LREE and HREE with respect to MREEs. Depletion in LREE and HREE is explained by depletion of source prior melting and sequestration of HREE in residual majorite respectively. However, Kapili AUK or spinifex lava samples show flat, near primitive mantle (REE/PM∼1) pattern exclusive of any significant depletion and implying higher degree of melting (between 30 and 40%) of a PM like source.Sm-Nd isotopic studies of Kapili ADK and AUK reveal that they both were derived from a mantle source with long term depletion (εNd∼+2 to +4). Except differences in LREE/HREE ratio featured due to differences in melting conditions, similarity in overall immobile HREE and high field strength element (HFSE) concentrations of both the types and their corresponding primary magmas suggest a single plume source rather than multiple sources, whose composition was closer to primitive mantle and eventually segregated chemically different komatiitic melts (Al-depleted and undepleted) at different depths. Whole rock Th/Yb, Th/Nb, Zr/Nb, Nb/Y, (La/Nb)PM ratios of both Kapili AUK and AUKs are comparable to oceanic island basalt (OIB) and mid oceanic ridge basalt (MORB) similar to typical greenstone belt volcanics from oceanic provenance. In addition, presence of copious pillow basalt and oceanic chert associated with komatiites, absence of negative δNb values recommend emplacement of komatiitic lava in oceanic rather than continental or arc setting.

Modelling komatiitic melt accumulation and segregation in the transition zone

Komatiites are probably produced in very hot mantle upwellings or plumes. Under such conditions, melting will take place deep within the upper mantle or even within the mantle transition zone. Due to its compressibility at such pressures, melt might be denser than olivine, but would remain buoyant with respect to a peridotitic mantle both above and below the olivine–wadsleyite phase boundary because of the presence of its higher temperature and denser garnet. We studied the physics of melting and melt segregation within hot upwelling mantle passing through the transition zone, with particular emphasis on the effect of depth-dependent density contrasts between melt and ambient mantle. Assuming a 1D plume, we solved the two-phase flow equations of the melt-matrix system accounting for matrix compaction and porosity-dependent shear and bulk viscosity. We assumed a constant ascent velocity and melt generation rate. In a first model series, the level of neutral buoyancy zneutr is assumed to lie above the depth of onset of melting, i.e. there exists a region where dense melt may lag behind the solid phases within the rising plume. Depending on two non-dimensional numbers (accumulation number Ac, compaction resistance number Cr) we find four regimes: 1) time-dependent melt accumulation in standing porosity waves that scale with the compaction length. The lowermost of these waves broadens with time until a high melt accumulation zone is formed in steady state. During this transient solitary porosity waves may cross the depth of neutral density and escape. 2) steady-state weak melt accumulation near zneutr, 3) no melt accumulation due to small density contrast or, 4) high matrix viscosity. In regime 4 the high mantle viscosity prevents the opening of pore space necessary to accumulate melt. In a second series, the rising mantle crosses the olivine–wadsleyite phase boundary, which imposes a jump in density contrast between melt and ambient mantle. A sharp melt porosity contrast develops and a large melt porosity accumulates immediately above the phase boundary. Both model series show 1) that not only melt density, but also porosity-dependent matrix viscosity controls the melt ascent or accumulation, 2) that there are parameter ranges and physical conditions which may lead to the accumulation of very large melt porosities (> degree of melting), 3) that in spite of melt being denser than olivine at some depths, in general these melts escape these regions and continue to percolate upward faster than the rising mantle. Melting and melt transport under the conditions predicted by the numerical modelling is able to reproduce the compositions of the main types of komatiite. Thus, the accumulation of large melt fractions, and sequential escape of melt from porosity waves, explains several puzzling features of the geochemical compositions of komatiites.

Liquidus temperatures of komatiites and the effect of cooling rate on element partitioning between olivine and komatiitic melt

Archean komatiites are the hottest magmas preserved on Earth and are thus unique probes of its thermal evolution. Estimating their eruption temperatures remains problematic, however, because the uppermost (A1, A2) zones of komatiite flows contain randomly oriented spinifex-textured olivines, indicative of rapid cooling and growth. Fe–Mg partitioning between olivine and assumed komatiitic liquid typically shows departures from equilibrium, extending towards higher \(K_{\text{D}}^{{{\text{Fe}}^{2 + } - {\text{Mg}}}}\). If these higher values are a disequilibrium effect, using them to calculate parental magma composition would lead to errors in estimated liquidus temperatures. In order to investigate this possibility, we have performed experiments on two komatiite compositions, the classic Barberton Aluminium Undepleted Komatiite (AUK) sample 49J (32.2 % MgO) and Munro AUK sample 422/95 (23 % MgO). Isothermal experiments to constrain phase equilibria on 49J at atmospheric pressure, between 1360 and 1600 °C at 1.7 log units below and 1.1 above the fayalite–magnetite–quartz (FMQ) buffer reveal a liquidus temperature (T liq) of 1616 °C, ~40 °C lower than a previous estimate. The \(K_{\text{D}}^{{\varSigma {\text{Fe}}{-}{\text{Mg}}}}\) ranges between 0.320 and 0.295 at FMQ − 1.7, with a slight negative dependence on temperature. To replicate the conditions that prevailed during the quenching of komatiites in their upper chill zones, experiments with a constant cooling rate at FMQ − 1.7 were performed on 422/95 (T liq = 1450 °C) at 0.5, 1.5, 2.5, 6.5 and 16 °C/min. Olivine morphology changes from euhedral to tabular at low cooling rates, hopper at intermediate, and skeletal and chain structures at high rates. Concurrently, the \(K_{\text{D}}^{{\Sigma {\text{Fe}}{-}{\text{Mg}}}}\) increases monotonically from an equilibrium value of 0.305 to 0.376 at 16 °C/min, reflecting the inability of unwanted cations to diffuse away from the growing olivine. The high \(K_{\text{D}}^{{\varSigma {\text{Fe}}{-}{\text{Mg}}}}\) between olivine and komatiitic liquid caused by rapid cooling accounts for the systematically lower Mg#s in spinifex olivines with respect to their euhedral counterparts in natural komatiites. The maximum MgO content of komatiite liquids in the Archean is revised upwards to 32 wt%, implying temperature excesses in Archean plumes relative to ambient mantle were greater than in their contemporary equivalents.