Komatiitic Lavas Research Articles

Petrogenesis and Geochemistry of Upper Paleozoic Komatiites (Mashhad, NE Iran)

Abstract 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- (about) 20% partial melting in a mantle plume at pressures of about 4-5 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.

Nickel isotope fractionation in komatiites and associated sulfides in the hart deposit, Late Archean Abitibi Greenstone Belt, Canada

Extremely light and highly variable δ60Ni values have been observed in komatiite-associated magmatic sulfides in recent studies. In this study, we examine the mechanisms of Ni isotope fractionation between silicate and sulfide liquids in the Hart komatiite-associated Fe-Ni-Cu-sulfide system. We assess the petrogenetic significance of these mechanisms using Ni isotope and concentration data. The concentration of Ni in bulk rock varies from 774 to 2690 ppm in komatiite samples with no sulfide minerals to 8380–39,300 ppm in samples almost entirely consisting of sulfide minerals. The δ60Ni values vary from +0.14‰ in komatiite samples with no sulfide minerals to −1.06‰ in samples dominantly consisting of sulfide minerals. A theoretical model of fractionation between the komatiitic lava and sulfide xenomelt with nickel isotope exchange followed by fractional crystallization during crystallization of the sulfide melt can produce a range of δ60Ni values from +0.17‰ to −1.02‰ in sulfide-rich rocks depending on the extent of fractional crystallization and the amount of trapped melt between the sulfide mineral grains, which corresponds well with the range of values observed in these rocks. This proposed model requires fractionation of Ni isotopes between sulfide liquid and the earliest formed sulfide crystals during crystallization. Effects of later crystallization during peritectic reactions and subsolidus exsolution could be tested by in situ measurements of Ni isotopes in different textural varieties of pentlandite that formed over a large range of temperatures during cooling.

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.

Spontaneous reheating of crystallizing lava

Abstract We show that recalescence, or spontaneous reheating of a cooling material due to rapid release of latent heat, can occur during disequilibrium crystallization of depolymerized Mg-rich melts. This can only happen at fast cooling rates, where the melt becomes undercooled by tens to hundreds of degrees before crystallization begins. Using a forward-looking infrared (FLIR) camera, we documented recalescence in pyroxene (Fe, Mg)SiO3 and komatiite lavas that initially cooled at 25–50 °C s–1. Local heating at the crystallization front exceeds 150 °C for the pyroxene and 10 °C for komatiite and lasts for several seconds as the crystallization front migrates through. We determined the latent heat release by differential scanning calorimetry to be 440 J g–1 for pyroxene and 275 J g–1 for komatiite with a brief power output of ∼100 W g–1 or ∼300 MW m–3. Recalescence may be a widespread process in the solar system, particularly in lava fountains, and cooling histories of mafic pyroclasts should not be assumed a priori to be monotonic.

Molybdenum isotope systematics in cumulate rock of the 2.8 Windimurra layered intrusion: A test for igneous differentiation and the composition of the Archean mantle

Molybdenum isotopes (reported as δ98Mo relative to NIST-3134) show resolvable isotope differences in igneous rocks with the continental crust being markedly heavier in isotope composition than mid-ocean ridge lavas, lunar basalts or the Earth’s mantle. The tholeiitic differentiation series at the intra-plate Hekla volcano (Iceland) shows no resolvable Mo isotope differences from basaltic to rhyolitic compositions. In contrast, convergent margin lavas show a transition from isotopically lighter mantle to heavy continental crust, suggesting that subduction processes drive continental crust towards heavier values. Archean komatiitic lavas, presumed probes of the Archean mantle, have Mo isotope values identical to modern depleted mantle, raising the questions if and how the Mo isotope crust-mantle disparity developed so early in Earth’s history. Here we present new Mo isotope data for a set of cumulate rocks from the Upper Zone of late Archean (2.8 Ga) Windimurra Igneous Complex, a mafic/ultramafic layered intrusion. The intrusion is not subduction related and contains no apparent primary hydrous minerals. We tested the effect of crystal fractionation on Mo isotopes in relatively dry melt along a tholeiitic liquid line of descent by using the cumulate effect of normally anhydrous minerals in the layered intrusion. Near mono-mineralic olivine-pyroxene-rich, feldspar-rich and Fe-Ti-rich oxides show small variations (~0.15‰) in Mo isotope signatures. This is consequently to predominantly isotopically light Fe-Ti-oxide-rich and isotopically heavier feldspar-rich rocks, respectively. This is suggesting minor Mo isotope fractionation, even in dry, tholeiitic systems, which however, counterbalance each other and thus potentially remain undetected. On average, the Windimurra mantle source is indistinguishable, or slightly isotopically lighter than the Mo isotope signature of komatiites. This is reinforcing an isotopically light Mo isotope signature of Archean mantle sources of high-degree mantle melts and is extending these signatures to predominantly mafic Archean crust. It remains to be tested if Archean felsic crust resembles modern continental crust in its heavy isotope values and to which extend the mantle was already isotopically depleted in Mo isotopes at Mesoarchean time.

Multi-stage alteration, rheological switches and high-grade gold mineralization at Sheba Mine, Barberton Greenstone Belt, South Africa

Structural, petrographic and geochemical data from Sheba Gold Mine in the Barberton Greenstone Belt of South Africa indicate that gold mineralization formed at the end, and as the result of a multi-stage structural, fluid flow and alteration history. This multi-stage evolution is a prerequisite for the formation of high-grade (>40 g/t Au) ore bodies. The Sheba Mine is situated in well-bedded metaturbidites of the Mesoarchaean Fig Tree Group and structurally infolded ultramafic lavas in the first-order, refolded Ulundi syncline in the northern parts of the belt. Original komatiitic lava flows are pervasively altered to talc-carbonate greyschist. This regionally widespread alteration relates to an early phase of deformation and associated fluid flow. The reaction softening associated with this alteration accentuates rheological anisotropies within the heterogeneous wall-rock sequence. This results in the localization of strain along the contact between talc-carbonate greyschist and overlying, massive greywacke and chert units during later-stage refolding and associated flexural slip. Pervasive quartz veining, silicification, K-enrichment and the progressive replacement of the earlier talc-carbonate assemblages by a high-variance quartz-fuchsite green schist alteration testify to the localized fluid flow along this contact. The newly formed, finely intergrown green schist assemblage leads, in turn, to a reaction hardening and embrittlement of the former greyschist-chert contact. High-grade gold mineralization is hosted by brittle quartz-carbonate vein networks and breccias developed along brittle-ductile structures that cross-cut the competent green schist at high angles. The siting of gold and predominance of free gold suggest that gold precipitation is related to extreme fluid pressure cycling and phase separation of the mineralizing fluid during the formation of the brittle fracture networks. The thrust kinematics of the late-stage controlling structure studied here indicate that the mineralized structure accommodated regional NW-SE subhorizontal shortening when further tightening of the Ulundi syncline passed the lock-up stage could no longer be accommodated by flexural slip. The results highlight that the formation of favourable conditions for high grade mineralization, such as those at Sheba Mine, may be the result of the interaction of temporally distinct fluid flow and alteration events and lithological heterogeneities during the progressive structural evolution of a region.

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.

Physical volcanology and petrogenesis of the Archean Quebra Osso komatiite flow field, Rio das Velhas greenstone belt, Quadrilátero Ferrífero (Brazil)

The Quebra Osso komatiite flow field is part of the 2.9–2.7 Ga Rio das Velhas greenstone belt, in the Quadrilátero Ferrífero province, located in the Southern São Francisco craton (Brazil). It is mostly composed of metamorphosed komatiites with a minor association of pyroclastic and metasedimentary rocks. Through a detailed and systematic study, the komatiite flows were subdivided into coherent facies (massive, layered, and pillowed facies) and autoclastic facies (autobreccias and hyaloclastites), formed by effusive flows; and minor pyroclastic facies (tuffs and lapilli-tuffs) that are related to explosive flows. The igneous textures are still preserved, although their primary minerals (olivine and pyroxene) were completely pseudomorphosed by serpentine and/or chlorite. The major proportion of the komatiites of Quebra Osso Group is massive and cumulate-textured, which suggests channelized flow systems. The layered flows are subordinate and encompass an upper zone of random spinifex that grades to a lower cumulate zone. They are restricted to the marginal zones and interpreted as breakouts of lava from the main pathway. The occurrence of pillow lavas, hyaloclastites, and chemical sediments suggests that lavas erupted in a submarine environment. The abundance of autobreccias and local hybrid rocks point out to turbulent percolation of komatiites and/or multiple influxes of melts that caused the fragmentation of the solidified lava and magma mixing/mingling. The studied komatiites were variably affected by hydration due to seafloor alteration, regional metamorphism, and hydrothermal alteration. The mineral assemblage is secondary and comprises serpentine (mostly antigorite), chlorite, tremolite, talc, carbonate, and opaque minerals as accessory phases. The chemical composition of chromites suggests metamorphic overprint up to lower amphibolite facies conditions. Some komatiites are enriched in LREE (La, Sm, Nd), Rb, Ba, Th, Zr, Ti, and Y, and exhibit low [Nb/Th]MN, suggesting contamination by continental crustal. The bulk-rock and the mineral chemistry of amphibole, serpentine/chlorite, and chromite show enrichment of Al and decrease in Si contents in the spinifex-textured komatiites, as a result of crustal material assimilation and/or hybridization processes. The partial melting model for the HREE composition refers to a 50% batch melting of a garnet-free source with a composition similar to the pyrolytic mantle. The formation of Quebra Osso komatiite lavas is associated with the ascension of a mantle plume that was subsequently erupted onto the Archean sialic crust of the Santa Bárbara Complex and was later reworked by subsequent metamorphic and deformational events.

Volcanic succession, petrology, and geochemistry of the Sujiagou komatiite from the North China Craton

The ~2.7‐Ga Taishan greenstone belt from the Eastern Block of the North China Craton has well‐exposed komatiite sections. The komatiite outcrops at Sujiagou have well‐preserved primary volcanic texture although they have experienced amphibolite‐facies metamorphism. Two komatiite lava flows are identified in the Sujiagou outcrops: “Flow Ι,” with a thickness of 55 m, is characterized by poly‐sutured crust at the top, spinifex texture in the middle, and olivine cumulates at the flow bottom. “Flow ΙΙ” consists of abundant pahoehoe lava lobes with no interstitial materials. These volcanic characteristics, as well as the interbedding with terrestrial sediments, suggest that the Sujiagou komatiite erupted in a subaerial continental environment. The olivine–chromite geothermometer gives peak metamorphic temperatures from 684°C to 764°C. Phase equilibria modelling suggests that the mineral proportions and mineral compositions are consistent with a retrograde temperature of ~488°C, at which the komatiite would have low olivine Fo (79–89) and high bulk‐rock Mg# (83–90). Linear mixing of the in situ trace element compositions of the metamorphic olivine, augite, tremolite, serpentine, and chlorite from Sujiagou komatiite samples can reproduce the trace elements of representative komatiite compositions, suggesting that later metamorphism and fluid–rock interactions played key roles in trace element variations in the bulk‐rock komatiite samples. These results also imply that the “arc signature” of the Sujiagou komatiite may be an overprint from posteruption fluid alteration rather than a primary subduction geochemical feature. Therefore, the Sujiagou komatiites may belong to a part of the global plume‐related greenstone belts at ~2.7 Ga.

Ancient high Pt/Os crustal contaminants can explain radiogenic 186Os in some intraplate magmas

The origin of variations in 186Os/188Os ratios amongst mantle-derived basaltic and komatiitic lavas remains controversial, with opposing models arguing for deep core-mantle versus shallow mantle sources. Crustal contamination has generally not been favored due to the low Os contents of such sources, meaning that variations in 186Os/188Os would require involvement of extremely high proportions of crustal material. Here we re-examine crustal contamination as an effective means for generating significant 186Os/188Os variations in Earth materials. Using chromitites and peridotites from the Stillwater, Muskox and Rum layered intrusions, we show that radiogenic 186Os/188Os ratios are correlated with 187Os/188Os ratios and can only be explained by shallow-level mixing processes and crustal contamination. The samples have δ186Os ([{(186Os/188Ossample[t]/186Os/188OsPM(t)) − 1} × 1000], where the modern primitive mantle [PM] 186Os/188Os is 0.1198388) values ranging between 0.04 to 0.15 for the ∼2.7 Ga Stillwater Igneous Complex, −0.05 to 0.17 for the ∼1.27 Ga Muskox Intrusion, and 0.02 to 0.13 for the ∼0.06 Ga Rum Layered Suite. The highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents of the chromitites and peridotites can be modeled through high sulfide-melt partitioning (typically >8000) and emphasize the role of S-saturation and HSE scavenging. Considering the high sulfide-melt partitioning and accounting for high silicate melt to sulfide melt ratios (R-factors), it is possible to explain the variations in 186Os–187Os in layered intrusions using calculated Os isotope crustal evolution growth models. These calculations indicate that <4% of ancient high Pt/Os crustal contributions can explain the composition of the chromitites and peridotites that were examined. Our observations are consistent with published models for chromitite genesis that invoke either crustal melt-primitive melt mixing, or cumulate assimilation. A crustal origin for radiogenic 186Os is a possible cause for 186Os/188Os ratio variations observed in some komatiites. It is more difficult to explain radiogenic 186Os/188Os measured in Hawaiian lavas by crustal contamination processes. Instead, ancient high Pt/Os oceanic crust, shallow mantle sources such as metasomatic sulfide, or metal-rich large low-shear wave velocity provinces at the core-mantle boundary, all remain valid explanations.

Mineralogical and petrographic features of metakomatiites of the Kostomuksha greenstone structure (Karelia)

Object of study. The paper presents the mineralogical and petrographic study results of metamorphosed and metasomatized komatiites and komatiitic peridotites from the Ozerki soapstone deposit and Pentinsuo prospect, located in the Kostomuksha greenstone structure of the Karelian Craton, Fennoscandian Shield. Material and methods. Surface and drill core samples of various mineral and structural-textural varieties of altered komatiites were studied by optical microscopy, electron microscopy with an attachment for microanalysis, XRD phase, thermogravimetric and chemical analysis. Results. Soapstone formation in the investigated localities displays a multistage alteration and associates with the superimposed alteration of Mg-rich metakomatiite flows and olivine cumulates due to influx of carbon dioxide bearing fluids enriched in calcium and potassium. The chemical composition of initial komatiite (MgO content of the rock) and the degree of its fracture intensity are the main factors controlling soapstone formation. In differentiated lava flows soapstone is formed mainly in high-magnesium cumulate zones characterized by a high content of serpentine. In the flow top and spinifex zones an early amphibole-chlorite-magnetite mineral association is preserved in varying degrees. Talc and carbonate are formed by the decomposition of serpentine and amphibole. The chlorite content in soapstone is controlled by the Al2O3 concentration in the respective flow zones. Conclusion. The carbonate-chlorite-talc is the general natural type of soapstone associated with thin komatiite lava flows of distal volcanic facies. In more thick proximal lava flows the formation of soapstone of chloritecarbonate-talc and talc-carbonate composition is possible. The most prefered prospecting areas for soapstone are fields proximal to the eruptive vent, main lava conduits and subvolcanic analogues of komatiites.

Droplets and Bubbles: Solidification of Sulphide-rich Vapour-saturated Orthocumulates in the Norilsk-Talnakh Ni–Cu–PGE Ore-bearing Intrusions

A common feature of the mineralised chonolith intrusions of the Norilsk-Talnakh camp is the presence of globular disseminated Ni–Cu–Fe sulphide ores, particularly well developed within the olivine orthocumulate layers of the intrusions, locally referred to as picritic gabbrodolerites. In many cases these globules partially fill subspherical intercumulus spaces within a framework of crystals of olivine and/or plagioclase. The sulphide-free component of these intercumulus spaces is partially or completely infilled with crystallization products of highly fractionated residual trapped liquid derived from the interstitial liquid of the original crystal mush; these intergrowths typically develop as ‘caps’ above the globules. Similar partially-filled cavities are well known within basaltic and komatiitic lava flows and are referred to as ‘segregation vesicles’; they have also been observed as caps above sulfide globules in quenched dolerite dyke margins in other localities. In the Norilsk-Talnakh intrusions, where the caps are incompletely filled by magmatic material the remaining space is occupied by amygdaloidal linings, which in some cases include anhydrite. These relationships provide evidence that the caps represent original gas bubbles and the close association with the sulphide globules arises from the strong tendency for sulphide droplets to attach to vapour bubbles in magmas. The original outlines of the vapour–bubble pairs are preserved by the rigidity of the surrounding crystal framework, which is interpreted to have developed largely by simultaneous in situ nucleation and growth of olivine and clinopyroxene on the basis of detailed textural observations. Pressure increase during solidification coupled with gas filter pressing forced residual silicate melt into the bubbles, while gas filter pressing and vapour–liquid capillary attraction drove differentiation of the droplets themselves. Whether the globules were deposited already attached to bubbles, or whether the bubbles were generated by volatile saturation within the crystal mush after deposition, cannot be determined definitively, but the geometry of flattened bubble–droplet pairs tends to favour the former option. The distinctive nature of the Norilsk globular ores, along with other unusual aspects of these deposits, is interpreted to be the result of low confining pressures and syn-crystallization degassing, an unusual condition in intrusion-hosted magmatic sulfide ores.

Mashhad komatiitic rocks in NE Iran: Origin and implications for the evolution of the Paleo‐Tethyan Ocean

AbstractMashhad ultramafic volcanic rocks, located within the Paleo‐Tethys suture zone in the north‐eastern Iran, are composed mainly of komatiitic rocks with interbedded picrobasalts. The Mashhad komatiitic rocks are characterized by low Al2O3/TiO2 ratios (9.85–11.44) and depleted HREE patterns with slightly high (Gd/Yb)N ratios (1.48–1.74), similar to those of the Al‐depleted Barberton‐type komatiites. These rocks have ƐNd (t) values ranging from +9.0 to +10.4 and initial 87Sr/86Sr ratios ranging from 0.7039 to 0.7072, indicating they were derived from a depleted mantle source. They have a total 187Re/188Os and 187Os/188Os ratios ranging from 0.278 to 72.254 and from 0.12741 to 0.14012, respectively, and display consistent PGE patterns similar to that of the primitive mantle. The picrobasalts have high contents of SiO2 (45.08–48.87 wt.%), MgO (7.28–11.51 wt.%), nearly flat REE pattern, and ƐNd (t) values ranging from +6.5 to +7.9 and initial 87Sr/86Sr ratios focused on 0.7040 and 0.7070, illustrating that they were derived from a relatively enriched source compared with the komatiitic rocks. The thermobarometric calculations suggest that the Mashhad komatiitic rocks were generated by high degree partial melting between 20% and 30% at 1,604–1,681°C and 3.20–4.65 Gpa, essentially consistent with those of the Hawaiian mantle plume (1,500–1,600°C). These new data demonstrate that the Mashhad komatiitic rocks are of a mantle plume origin, which might have caused the opening of the Paleo‐Tethyan Ocean, and the mantle plume was likely located beneath or nearby the mid‐ridge of the Paleo‐Tethyan Ocean when the Mashhad komatiitic lava erupted, a scenario resembling the present Iceland and Ontong Java.

DIFFERENTIATED MESOARCHEAN KOMATIITE LAVA FLOWS: MINERALOGICAL AND GEOCHEMICAL CHARACTERISTICS, OUTFLOW AND CRYSTALLIZATION CONDITIONS

The paper presents the results of the study of thin differentiated Mesoarchean komatiite lava flows from the Koikary and Sovdozero greenstone domains. Several zones were recognized in the section of differentiated lava flows: autobreccia, spinifex texture of olivine and pyroxene types, massive, cumulate (ortho- and mesocumulates), as well as the lower chilled contact at the basement of the flow. The stratification of the lava flows is due to magmatic differentiation in situ coupled with the continuing injection of more molten rock until the stage of complete crystallization.

Evidence of magmatic degassing in Archean komatiites: Insights from the Wannaway nickel-sulfide deposit, Western Australia

Magmatic degassing from komatiite lava flows potentially influenced the geochemical evolution of the Archean atmosphere and hydrosphere. We argue that the escape of SO2-rich volatiles from komatiites impacted on the mineralogical, geochemical and isotopic composition of associated nickel-sulfide mineralization leaving behind detectable and measurable footprints that can be best observed where the polarity of the magmatic sequence is clearly recognizable. Here we focus on the ore-bearing sequence of the Archean komatiite-hosted N01 nickel-sulfide orebody at Wannaway, Yilgarn Craton, Western Australia. This deposit displays a volcanic sequence with a well-defined succession of stratigraphically-correlated facies comprising a massive sulfide horizon at the base of the channelized komatiite flow, overlain by matrix and disseminated sulfide mineralization. Pyrrhotite is the dominant sulfide phase in the lower part of the ore profile. The amount of troilite gradually increases from the base of the matrix ore over several meters up-sequence, eventually becoming dominant at the expense of pyrrhotite. In the upper portion of the mineralized sequence troilite is associated with accessory Mn sulfide alabandite (MnS), which is usually reported in reduced terrestrial and extra-terrestrial environments. Such mineralogical and volcanological features are consistent with upwards decreasing in fS2 and fO2 away from the basal contact of the komatiite flow. After evaluating the possible role of metamorphism, the pyrrhotite-troilite-alabandite assemblage and the progressive up-sequence decrease of the pyrrhotite/troilite ratio across the upper part of the mineralized sequence are interpreted as magmatic and indicative of progressive loss of sulfur with concomitant establishment of reducing conditions within the sulfide melt ponding at the base of the komatiite lava. In this context, the investigation of spatially constrained sulfur isotopic signatures allows to isolate the multiple sulfur fractionation processes that impacted on sulfide mineralization and ultimately permits the identification of the isotopic shift associated with magmatic degassing. Following this approach we recognize two distinct sulfur isotope exchanges processes triggered by 1) assimilation of sulfidic shales during emplacement of the komatiite flow, and 2) equilibration between the sulfide melt and the sulfur dissolved in the silicate melt. We finally correlate the remaining δ34S depletion up-stratigraphy with the loss of heavy sulfur isotopes through magmatic degassing of SO2-rich volatiles from the ultramafic flow. The emission of SO2 upon emplacement and cooling of the magma flow would also explain the progressive reducing fO2 and fS2 conditions indicated by variations in mineral assemblages from the base of the komatiite upwards.

Ultramafic lavas and high-Mg basaltic dykes from the Othris ophiolite complex, Greece

We evaluate the petrography and geochemistry of an unusual suite of subduction-related Phanerozoic high-MgO rocks from the Othris ophiolite complex in Greece, some of which have previously been described as komatiitic lavas. In particular, we study ultramafic, olivine-phyric lavas from the Agrilia area and high-Mg basaltic dykes from the Pournari area. We seek to define primary magmatic MgO contents and initial liquidus temperatures as well as the differentiation sequence and cooling rates experienced by the lavas and dykes. One of our goals is to relate the Othris case to known komatiite and boninite occurrences and to address whether Othris documents an important new constraint on the temporal evolution of ambient mantle temperature, plume-related magmatism, and subduction of oceanic lithosphere. We conclude that, despite whole-rock MgO contents of 31–33wt%, the olivine-phyric lavas at Agrilia had an upper limit liquid MgO content of 17wt% and are therefore picrites, not komatiites. The Agrilia lavas contain the unusual Ti-rich pyroxenoid rhönite; we discuss the significance of this occurrence. In the case of the Pournari high-Mg dykes, the distinctive dendritic or plumose clinopyroxene texture, though it resembles in some ways the classic spinifex texture of komatiites, is simply evidence of rapid cooling at the dyke margin and not evidence of extraordinarily high liquidus temperatures. We correlate the dendritic texture with disequilibrium mineral chemistry in clinopyroxene to constrain the cooling rate of the dyke margins.

Numerical modelling of erosion and assimilation of sulfur-rich substrate by martian lava flows: Implications for the genesis of massive sulfide mineralization on Mars

Mantle-derived volcanic rocks on Mars display physical and chemical commonalities with mafic-ultramafic ferropicrite and komatiite volcanism on the Earth. Terrestrial komatiites are common hosts of massive sulfide mineralization enriched in siderophile-chalcophile precious metals (i.e., Ni, Cu, and the platinum-group elements). These deposits correspond to the batch segregation and accumulation of immiscible sulfide liquids as a consequence of mechanical/thermo-mechanical erosion and assimilation of sulfur-rich bedrock during the turbulent flow of high-temperature and low-viscosity komatiite lava flows. This study adopts this mineralization model and presents numerical simulations of erosion and assimilation of sulfide- and sulfate-rich sedimentary substrates during the dynamic emplacement of (channelled) mafic-ultramafic lava flows on Mars. For sedimentary substrates containing adequate sulfide proportions (e.g., 1wt% S), our simulations suggest that sulfide supersaturation in low-temperature (< 1350°C) flows could be attained at < 200km distance, but may be postponed in high-temperature lavas flows (> 1400°C). The precious-metals tenor in the derived immiscible sulfide liquids may be significantly upgraded as a result of their prolonged equilibration with large volumes of silicate melts along flow conduits. The influence of sulfate assimilation on sulfide supersaturation in martian lava flows is addressed by simulations of melt-gas equilibration in the CHOS fluid system. However, prolonged sulfide segregation and deposit genesis by means of sulfate assimilation appears to be limited by lava oxidation and the release of sulfur-rich gas. The identification of massive sulfide endowments on Mars is not possible from remote sensing data. Yet the results of this study aid to define regions for the potential occurrence of such mineral systems, which may be the large canyon systems Noctis Labyrinthus and Valles Marineris, or the Hesperian channel systems of Mars’ highlands (e.g., Kasei Valles), most of which have been periodically draped by mafic-ultramafic lavas.

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.

Earth’s thermal evolution, mantle convection, and Hadean onset of plate tectonics

During Solar System condensation, the early Earth formed through planetesimal accretion, including collision of a Mars-sized asteroid. These processes rapidly increased the overall thermal budget and partial fusion of the planet. Aided by heat supplied by radioactivity and infall of the Fe-Ni core, devolatilization and chemical-density stratification attended planetary growth. After the thermal maximum at ∼4.4Ga, terrestrial temperatures gradually declined as an early Hadean magma ocean solidified. By ∼4.3–4.2Ga, H2O oceans+a dense CO2-rich atmosphere blanketed the terrestrial surface. Near-surface temperatures had fallen well below the low-P solidi of dry peridotite, basalt, and granite, ∼1300, ∼1120, and ∼950°C, respectively. At less than half their melting T, rocky materials existed as thin lithospheric platelets in the surficial Hadean Earth. Upper mantle stagnant-lid convection may have operated locally, but was rapidly overwhelmed by heat build-up-induced asthenospheric circulation, rifting and subduction, because massive heat transfer required vigorous mantle overturn in the early, hot planet. Bottom-up mantle overturn, involving abundant plume ascent, brought deep-seated heat to the surface. It decreased over time as cooling, plate enlargement, and top-down plate descent increased. Thickening, lateral extension, and contraction typified the post-Hadean lithosphere. Geologic evolutionary stages included: (a) ∼4.5–4.4Ga, the magma ocean solidified, generating ephemeral, ductile platelets; (b) ∼4.4–2.7Ga, small oceanic and continental plates were produced, then were destroyed by mantle return flow before ∼4.0Ga; eventually, continental material began to accumulate as largely subsea, sialic crust-capped lithospheric collages; (c) ∼2.7–1.0Ga, progressive suturing of old shields and younger orogenic belts led to cratonal plates typified by emerging continental freeboard, intense sedimentary differentiation, and episodic glaciation during transpolar plate drift; temporally limited stagnant-lid mantle convection occurred beneath growing supercontinents; (d) ∼1.0Ga-present, laminar-flowing mantle cells are capped by giant, stately moving plates. Near-restriction of komatiitic lavas to the Archean, and formation of multicycle sediments, ophiolite complexes±alkaline igneous rocks, and high-pressure/ultrahigh-pressure (HP/UHP) metamorphic belts in youngest Proterozoic and Phanerozoic orogens reflect increasing density of cool oceanic plates, but decreasing subductability of enlarging, more buoyant continental plates. Attending assembly of supercontinents, negative buoyancy of thickening oceanic lithosphere began to control the overturn of suboceanic mantle as cold, top-down convection. The scales and dynamics of hot asthenospheric upwelling versus plate foundering and mantle return flow (bottom-up plume ascent versus top-down plate subduction) evolved gradually, due to planetary cooling. After accretion of the Earth, heat transfer through mantle convection has resulted in the existence of surficial rocky plates or platelets, and vigorous, lithosphere-coupled mantle overturn since ∼4.4Ga. Thus plate-tectonic processes have typified the Earth’s thermal history since Hadean time.

Primary stratigraphic controls on ore mineral assemblages in the Wannaway komatiite-hosted nickel-sulfide deposit, Kambalda camp, Western Australia

The 18m-long UWA-04-02 drillcore from the Fe-Ni-Cu-PGE Wannaway deposit in the Widiemooltha Dome district (Eastern Goldfields, Western Australia) intersects the whole sequence of a komatiite-hosted layer of metal-rich sulfide magma. In spite of regional deformation and amphibolite facies metamorphism the sequence in the drillcore still preserves some of the original, magmatic textures and assemblages and these were examined in a great detail. The magmatic orebody typically consists of basal massive sulfides grading to net-textured (matrix) and disseminated sulfide mineralization upward into the komatiite host. The ore zone is underlined by sulfide-rich black shale passing to basalts. Country rock xenoliths are locally enclosed in the massive sulfides. Portions of the drillcore untouched by penetrative deformation and with minimal imprint by late-stage serpentinization allow the construction of a fairly complex framework where mineral assemblages and mineral chemistry of sulfides, spinels and silicates vary systematically with stratigraphy and may reflect original conditions of ore deposition. The general ore assemblage is dominated by Fe-sulfide and pentlandite, with minor sphalerite and chalcopyrite, spinels (Zn-rich chromite, Ti-magnetite), alabandite (MnS), accessory PGE-rich sulfarsenides and tellurides and rare molybdenite. Monoclinic and high-S hexagonal pyrrhotite and fresh Zn-Mn-rich chromite characterize the basal massive facies, whereas the matrix ore facies is marked by magnetite, sphalerite and a gradually S-depleted and reduced assemblage now represented by troilite exsolving low-S hexagonal pyrrhotite and alabandite. Compositional modifications of the Fe-sulfides across the whole orebody and occurrence of alabandite testify to progressive sulfur loss concomitant with the establishment of low fO2 conditions over several meters upsequence in the matrix ore facies. PGE-rich sulfarsenides disseminated across the whole mineralized sequence display igneous textures and PGE fractionation trends. The composition of olivine intergrown with matrix sulfides and in the serpentinized hangingwall komatiite deviates from the typical unmetamorphosed komatiite-related, highly-forsteritic type. However the Fe, Mn and Zn contents of olivine crystals decrease systematically and gradually with distance from mineralization towards the hangingwall komatiite. Contamination may be an alternative to metamorphic recrystallization of olivine as the cause of these trends. The role of contamination is also shown by the trends of whole-rock data from the mineralized sequence across the entire drillcore. Textures and mineral chemistry of minerals from the different rock facies in the drillcore are evaluated in terms of metamorphic effects, although the remarkable relationship observed between stratigraphy and several major and accessory phases over metric distances is suggestive of alternative options including primary processes involving the komatiitic lava flow in its interaction both with the black shale substrate and with the sulfide melt ponding at its base.