Archean Komatiites Research Articles

Calcium isotopic composition of the bulk silicate Earth: A komatiite perspective

Calcium (Ca) isotopic composition of the mantle has been previously estimated as δ44/40Ca. However, δ44/40Ca of the mantle records both its stable isotopic composition and excess/deficit in radiogenic 40Ca that can be produced by the decay of 40K. Here, we present the high-precision Ca isotopic compositions of 16 Archean komatiites from five representative localities (Barberton, Munro, Abitibi, Yilgarn, and Taishan). Due to the high degrees of partial melting that led to their formation, these komatiites are used here to simultaneously estimate ε40/44Ca, δ44/40Ca, and δ44/42Ca of the bulk silicate Earth. The komatiites exhibit a resolvable negative ε40/44Ca relative to SRM915a, with an average of −0.6 ± 0.1 (2SE, 95 c.i.% if not specified). Given the lack of significant differences among the samples and the non-zero value of ε40/44Ca, we suggest using δ44/42Ca to report the stable Ca isotopic composition in this study. Excluding one sample (SD6/400), which may have been affected by alteration, the remaining komatiites display relatively homogeneous δ44/42Ca, ranging from 0.39 ± 0.02‰ (2SE, N = 16) to 0.49 ± 0.02‰ (2SE, N = 10), defining a mean of 0.44 ± 0.02‰ (2SE, N = 15). The absence of correlations between δ44/42Ca and LOI, Sr/Nd, (Dy/Yb)N, and (La/Sm)N among the komatiites indicates that post-eruption alteration, the influence of residual garnet, as well as prior melt extraction from the mantle source have had a negligible influence on their stable Ca isotopic compositions. Based on available experimental petrological data and isotope fractionation factors, it is demonstrated that komatiites were insignificantly fractionated from their mantle sources (i.e., <0.02% in δ44/42Ca). Their δ44/42Ca represents the ambient mantle at the time of segregation. Accordingly, we suggest that the average δ44/42Ca and ε40/44Ca of the komatiites measured here are representative of the bulk silicate Earth. Our new estimate can serve as a reference point for Ca isotopes as a two-dimensional tracer for sources and geological processes.

Concordance of V-in-olivine and Fe-XANES oxybarometry methods in mid-ocean ridge basalts

Oxygen fugacity (fO2) is a fundamental parameter controlling the behavior of redox-sensitive elements, as well as the speciation of degassing volatiles in igneous systems. Therefore, the average fO2 of typical mid-ocean ridge basalts (MORB) is important to constrain for models of atmospheric and mantle chemical evolution. Although there are several independent oxybarometry methods, in recent years, the Fe X-ray absorption near edge spectroscopy (Fe-XANES) oxybarometry of volcanic glasses has become the gold standard for evaluating the fO2 of modern lavas. Other oxybarometry methods, such as the V-in-olivine method, are useful for samples in which fresh glass is not preserved, but it is not clear whether the two methods give comparable results when applied to the same samples due to the lack of inter-calibration studies. Presented here are V-in-olivine oxybarometry data for eleven MORB lavas, eight of which have been previously characterized by Fe-XANES oxybarometry. The new data show that the V-in-olivine method is not applicable to MORB with glass containing <8.3 wt. % MgO, and that MORB with glass having MgO >8.3 wt. % have an fO2 averaging -0.28 ± 0.28 ΔFMQ, overlapping with the global Fe-XANES MORB average of -0.18 ± 0.16 ΔFMQ. It is therefore recommended that this oxybarometry technique only be applied to MORB with primitive (>8.3 wt. % MgO) glass. The lack of any systematic offset between the data from the two oxybarometers supports our previous conclusion that ocean island basalt sources (OIB) are more oxidized than average MORB, and that the majority of Archean komatiite sources record significantly lower fO2 than modern MORB.

Modern ocean island basalt–like 182W signature in Paleoarchean mafic rocks: Implications for the generation, preservation, and destruction of early mantle heterogeneities

Komatiites and picrites generated by high degrees of mantle partial melting serve as potential probes of Earth’s deep mantle. Tungsten (W) isotopes in these rocks offer a rare chance to better understand early differentiation, late accretion, core-mantle interaction, and subsequent evolution of Earth’s mantle. We present new W isotope data for Archean komatiites and basalts from the Barberton (South Africa) and Suomussalmi (Finland) Greenstone Belts and Permian picrites from the Emeishan large igneous province (China). The Paleoarchean samples from the Barberton Greenstone Belt have modern ocean island basalt (OIB)−like μ182W values ranging from −20.4 to +5.6, whereas the Mesoarchean komatiites from the Suomussalmi Greenstone Belt show μ182W values of −2.2 to +11.3. The Permian Emeishan picrites give μ182W values of −7.1 to +3.1. Our data, combined with the published global data set, show that W isotope heterogeneity in the mantle has existed throughout Earth’s history, with positive μ182W values transitioning to near-zero in the upper mantle by the end of the Archean. The negative μ182W values of Paleoarchean samples in the Barberton Greenstone Belt and modern OIBs likely result from either early differentiation or core-mantle interaction. The incorporation of a plume-delivered negative μ182W component and enhanced mantle mixing is a viable mechanism to explain the transition of μ182W values in the upper mantle from positive to near-zero, while recycling of crustal materials into the mantle would result in a shift of negative μ182W values of the lower mantle closer to zero since the onset of plate tectonics. The latter process could possibly explain the slightly negative to near-zero μ182W values of the Emeishan picrites and some kimberlites. The well-resolved negative μ182W anomalies observed in this study provide important insights into the generation, preservation, and obliteration of W isotope heterogeneities in the lower mantle.

KOMATIITE LAVAS FROM THE QUEBRA OSSO GROUP (RIO DAS VELHAS GREENSTONE BELT, SOUTHEAST BRAZIL): A FIELD GUIDE TO AN ARCHEAN FLOW FIELD

The Quebra Osso Group encompasses one of the best preserved Archean komatiite flows in the Quadrilátero Ferrífero region, southern São Francisco craton (Brazil). It is commonly associated with the base of the Rio das Velhas greenstone belt within the Santa Bárbara tectonostratigraphic block. The Quebra Osso rocks are well exposed along the valley of the homonymous river, near the Catas Altas city (Minas Gerais state), and in inactive serpentine quarries. In this field guide, we provide the descriptions of the key outcrops in this area, including a variety of komatiite flows, sedimentary rocks and pyroclastic rocks. Despite the pervasive alteration of primary mineral assemblages due to greenschist to amphibolite facies metamorphism, the ultramafic rocks exhibit diverse igneous textures and structures. Volcanic textures are recognizable due to the morphology of the original igneous olivine and pyroxene, which underwent pseudomorphosis by hydrous minerals – serpentine and chlorite. These features include distinctive spinifex and cumulate textures that provide clues of the conditions operating during emplacement and crystallization of the ultrabasic liquids. The predominance of thick-packages of massive flows suggests channelized lava flows, with subordinate thin marginal layered (spinifex-textured), pillowed, brecciated and pyroclastic flows. Volcanic breccias, pillow lavas and chemical sedimentary rocks indicate that these rocks erupted underwater. These multiple volcanic products characterize a unique komatiite flow field and their remarkable preservation distinguishes the Quebra Osso Group from the other ultramafic associations within the São Francisco Craton.

Sulfur in Archean komatiite implies early subduction of oceanic lithosphere

Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ33S from −0.20‰ to −0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ33S < 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.

Early global mantle chemical and isotope heterogeneity revealed by the komatiite-basalt record: The Western Australia connection

Although the heterogeneous nature of the chemical composition of Earth's mantle is now well established, the origin and longevity of the heterogeneities continue to be debated. In order to further study early-Earth heterogeneities, we present a set of Sm-Nd, Lu-Hf, Re-Os, and Hf-W isotope and lithophile and siderophile element abundance data for komatiites and basalts from the ∼3.53 Ga Coonterunah, ∼3.34 Ga Kelly, and ∼3.18 Ga Ruth Well and Regal systems of the Pilbara Craton in Western Australia. The Sm-Nd, Lu-Hf, and Re-Os isotope data yield isochrons consistent with the accepted emplacement ages of the respective komatiite-basalt lavas. The mantle sources evolved with long-term 147Sm/144Nd = 0.200 to 0.214 and 176Lu/177Hf = 0.0355 to 0.0395, spanning the entire range of the time-integrated Sm/Nd and Lu/Hf measured in the Archean and Proterozoic komatiite-basalt systems to-date. Unlike with the other early Archean komatiites and basalts, the coupled 143Nd-176Hf isotope systematics of the Pilbara lavas provide no evidence for the involvement of early magma ocean processes in the evolution of their mantle sources. Episodes of variable degrees of partial mantle melting and melt extraction can account for the observed large variations in the time-integrated Sm/Nd and Lu/Hf ratios in the early Archean mantle domains.In contrast to the highly variable Nd-Hf systematics, the initial γ187Os values vary within a narrow range from +0.9 to −0.4 indicating that the Pilbara mantle sources evolved with chondritic time-integrated Re/Os. The apparent discrepancy between the depletions in incompatible lithophile trace elements and near-chondritic Re/Os observed globally is reconciled via a model whereby early low-degree mantle melting events fractionated Sm from Nd and Lu from Hf, but had little effect on the Re/Os ratio. This in turn would imply early formation and long-term isolation of a basaltic crust highly enriched in incompatible lithophile trace elements.The calculated total HSE abundances in the komatiite mantle sources range from ∼30% in the Coonterunah to ∼70% in the Regal system, of those in the estimates for the modern BSE, indicative of a 2.4× increase in HSE abundances from 3.53 to 3.18 Ga.All four komatiite-basalt systems exhibit positive 182W anomalies ranging between +11.4 and +7.7 ppm. The 182W/184W compositions and calculated HSE abundances in the Pilbara komatiite-basalt sources are inversely correlated and are most consistent with grainy late accretion of large differentiated planetesimals. Regression of the combined 182W-HSE data for the komatiite systems allows an estimate of the W isotopic composition of the pre-late accretion BSE of +17 ± 7. This estimate is similar to that of the Moon of +25 ± 5 and lends further support to the notion regarding an initially common W isotopic composition in the Earth-Moon system.Regression of the available HSE abundance data for komatiite mantle sources worldwide provides an estimate for the time of complete homogenization of late accreted materials within the mantle by 2.5 ± 0.2 Ga. Calculations indicate an average survival time of late accreted planetesimals in the Earth's mantle of 1.9 ± 0.2 Ga, which constrains the average mantle stirring rates for the HSE in the Hadean and Archean.

Olivine in komatiite records origin and travel from the deep upper mantle

Abstract The deep upper mantle is the main source of high-temperature magmatism, but the only known naturally occurring samples of high-pressure mantle constituents are mineral inclusions in diamonds. Trace elements in olivine crystals from the 3.33 Ga Commondale Greenstone Belt in South Africa reveal that these crystals formed in the deep upper mantle as high-pressure phenocrysts, and some perhaps even formed in the mantle transition zone (410–600 km) where they began as wadsleyite. The crystals were entrained within ascending komatiite magma and conveyed to the surface. The olivine crystals have the highest contents of Al2O3 (0.3 wt%) recorded in any terrestrial olivine, which is indicative of formation at high pressure. The deep mantle gave rise to Archean komatiites, extraordinarily hot magmas (up to 1700 °C), which provide insight into Earth's early mantle evolution and the formation of most ancient continental and oceanic crust. In spite of extensive research since their discovery over 50 years ago, the origins of komatiites have remained contentious. Plumes—thermochemical instabilities originating at the core-mantle boundary—are the most likely source, but no direct evidence of a deep mantle origin of komatiite has yet been recognized.

Stratigraphy and U–Pb detrital zircon provenance, Crixás Greenstone Belt, Goiás-Brasil: Gold-bearing vs barren siliciclastic rocks

The Crixás Greenstone Belt, located in an Archean-Paleoproterozoic terrane within the Neoproterozoic Tocantins Province, hosts an important gold deposit in Brazil with an endowment of 7 Moz currently operated by AngloGold Ashanti. Its stratigraphy is composed of an Archean komatiite and basalt basal sequence followed by a Paleoproterozoic sedimentary sequence composed of carbonaceous phyllites, dolomite-rich marbles with minor siliciclastic lenses (Ribeirão das Antas Formation) followed by a uniform siliciclastic sequence (Córrego Geral Formation). At the mining area, the entire sequence is stratigraphically overturned and metamorphosed under greenschist to lower amphibolite facies conditions. The orebodies are structurally controlled by thin-skinned low-angle thrust faults, and are mainly hosted by the sedimentary sequence in dilation zones named locally, from the shallowest to the deepest, as Structure Palmeiras, Structure IV and Structure III. The gold mineralization occurs in association with disseminated sulfides, massive sulfide lenses or quartz veins. Structure III is the deepest mineralized structure with gold mostly occurring in quartz veins cutting carbonaceous phyllites. Siliciclastic lenses occurring within the Structure III interval show variable gold grades, while siliciclastic rocks from the uniform deeper sequence just below Structure III are barren. The chronology of the sedimentary deposition, metamorphism and mineralization is still poorly understood. Detailed drill core logging, petrography and U–Pb geochronology of detrital zircons from siliciclastic rocks were carried out in order to better understand the relationship between the two metasedimentary packages and gold distribution. Petrographically there are no relevant differences between Au-bearing siliciclastic rocks (Ribeirão das Antas Formation) and barren siliciclastic rocks (Córrego Geral Formation), except for the presence of sulfides in the former. The new dataset of detrital zircon U–Pb ages revealed a ubiquitous Paleoproterozoic peak in all samples, with nearly no Archean contribution. K-feldspar and lithic angular fragments, zircons with low to medium roundness, and the U–Pb results indicate a Rhyacian provenance from a proximal source. Neoproterozoic overgrowths are found on detrital zircons from the barren siliciclastic interval of the Córrego Geral Formation, providing unequivocal evidence of a Neoproterozoic event around 600 Ma that affected the rocks. The Neoproterozoic partial reset, possibly related to the overriding of the Mara Rosa Magmatic Arc onto the Crixás Greenstone Belt, is unlikely to be linked to the mineralization considering that zircons from the barren rocks are the most thermally affected by this event. The Au mineralization is more likely related to Paleoproterozoic events, a time period known as a major global episode of orogenic gold deposit formation.

Chlorine in the Earth’s Mantle as an Indicator of the Global Recycling of Oceanic Crust

Abstract —Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but are significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth’s history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle.

How mafic was the Archean upper continental crust? Insights from Cu and Ag in ancient glacial diamictites

The concentrations of Cu and Ag, both insoluble chalcophile elements, can be used to place tight constraints on the proportion of basalts in the upper continental crust (UCC) through time via analyses of fine-grained terrigenous sedimentary rocks. Copper and Ag concentrations in magmas are largely controlled by sulfide dissolution during melting and sulfide fractionation during differentiation. We show that Cu is high in basalts but low in komatiites and felsic rocks, making Cu useful for constraining the proportion of basalts in the UCC. Furthermore, Cu/Ag ratios are high in basalts and komatiites but decrease with differentiation. The fine-grained matrix of ancient glacial diamictites shows dramatic declines in both Cu concentrations and Cu/Ag ratios at 3.0–2.4 Ga, after which diamictite Cu concentrations and Cu/Ag ratios remain roughly constant. Mass-balance calculations using average Cu concentrations of Archean komatiites, basalts and felsic rocks require that a high proportion basalt (65–75%) was present in the UCC at ∼3.0 Ga, and this crust transitioned to a felsic-rock-dominated crust at ∼2.4 Ga ago and has remained basically unchanged since then. This conclusion is supported by the consistency between the Cu/Ag ratios in the diamictites and the weighted average Cu/Ag ratios based on the lithological proportions obtained using Cu mass-balance calculations. Our observations document the emergence of felsic continental crust in the late Archean, which, in turn, indicates a fundamental change in Earth’s dynamic regime at that time.

Geochemistry of komatiites and basalts in Archean greenstone belts of Russian Karelia with emphasis on platinum-group elements

Archean komatiites form an important host rock for Ni-Cu sulfide deposits in the world. In this study, we examined Archean komatiites and komatiitic basalts from four areas in the Vedlozero-Segozero greenstone belt and two areas in the Tikshozero greenstone belt, Russian Karelia, to constrain their Ni-Cu sulfide mineralization potential using chalcophile element geochemistry. The platinum-group element (PGE) concentrations in these Munro-type komatiitic rocks are at a level of other similar S-undersaturated komatiites and komatiitic basalts globally, with Pt and Pd concentrations in the range of 5–20 ppb and Pd/Ir varying from < 10 (komatiites) to > 15 (komatiitic basalts and basalts). Generally, the metals of the iridium-group elements (IPGEs; Ir, Ru, Os) show a compatible behavior, decreasing in abundance with decreasing MgO, whereas the metals of the palladium group (PPGEs; Pt, Pd, Rh) exhibit an incompatible behavior. The poor correlation between Ir and MgO suggests that olivine fractional crystallization is not the main control on the behavior of IPGE. In contrast, Ir, Ru, and Os show positive correlations with Cr, which is consistent with the compatible behavior of IPGEs in chromite or during the co-precipitation of chromite and platinum-group minerals (PGMs). Palladium, Cu, and Au have been variably mobile during alteration and metamorphism whereas Pt appears to have been less mobile. Some samples from the Khizovaara area show low (Pt/Ti)N ratios, low Ni, and high La/Sm and La/Nb, suggesting localized sulfide saturation in response to crustal contamination. However, the potential of the Russian Karelian greenstone belts for Ni-Cu sulfide mineralization is considered relatively low because most samples show neither enrichment nor depletion of chalcophile elements, and the paucity of dynamic lava channel environments, as indicated by the scarcity of olivine-rich adcumulates. In addition, there appears to be a lack of exposed sulfidic sedimentary rocks in the region.

Акцессорные зональные хромиты из архейских коматиитов Карельского кратона: признаки метаморфических преобразований

SEM and LA-ICP-MS analyses were used for the study of accessory chromite in Archean (3.0—2.8 Ga) komatiites from the most representative greenstone belts of the Karelian Craton. Zoned chromite, subjected to varying degrees of metamorphic alteration, was identified in the komatiites. Two types of zoned chromite, differing in the composition of their cores, were recognized. Al-rich zoned chromite of type I shows the low extent of metamorphic alteration and has cores with Cr# 0.70—0.86, Mg# 0.03—0.12, and Fe# 0.06—0.23. It is enriched in Zn, Co, and Mn but strongly depleted in Ga, Ni, and V. The study reveals that the primary magmatic distribution of minor and trace elements in the cores was strongly redisturbed during prograde metamorphism of greenschist to low amphibolite facies. Zoned chromite with the low Al 2 O 3 content of type II has cores with higher Fe# (0.42—0.49), Cr# (0.88—1), and lower Mg# (0—0.04) as compared to cores of Al-rich chromite, that corresponds to the more intensive metamorphic alteration. Since the primary composition of studied zoned chromite was significantly obliterated during metamorphism, established patterns can be used as a guide for the reconstruction of metamorphic processes.

Bridging the depleted MORB mantle and the continental crust using titanium isotopes

The mechanisms driving the chemical complementarity between depleted MORB mantle (DMM) and continental crust (with an average 'andesitic' composition) remain unclear. By investigating Archean komatiites, and modern enriched (E) and normal (N) MORB samples, we demonstrate that partial melting of the mantle does not fractionate Ti isotopes, whereas intracrustal differentiation causes significant Ti isotopic fractionation between melts and minerals, specifically Fe-Ti oxides. Thus, Ti isotope ratios are tracers of these two magmatic regimes. N-MORB and late Archean (2.9-2.7 Ga) komatiites are depleted in the heavier Ti isotopes compared to E-MORB and middle Archean (3.5-3.3 Ga) komatiites. We show that the depletion in the heavier Ti isotopes of the DMM is due to mantle recycling of the isotopically light residues from the generation of felsic continental crust over 3.5-2.7 Ga. This process must have reached a steady state by ≈ 2.5 Ga, based on the uniform Ti isotopic composition of contemporary N-MORBs and late Archean komatiites. This change is likely due to a decrease in the mantle potential temperature related to the emergence of plate tectonics.

Nickel isotopic variation in black shales from Bohemia, China, Canada, and Finland: a reconnaissance study

We present δ60Ni values for black shales, determined by double-spike MC-ICP-MS. The samples comprise Paleoproterozoic Talvivaara Ni–Zn–Co–Cu black shales from Finland, Neoproterozoic black shales from the Tepla-Barrandian Unit, Czech Republic, Early Cambrian Ni–Mo-rich black shales from the Yangtze Craton, and Devonian Ni–Zn–PGE black shales from Yukon, Canada. In addition, the sample set includes a black smoker sample from the Logatchev hydrothermal field, Mid-Atlantic Ridge. The δ60Ni values vary from − 0.84 ± 0.09 to + 0.62 ± 0.04‰ (2SD) with a median of − 0.10‰ (n = 28). Ni isotopic compositions were predominantly lighter than those of abiotic terrestrial and extraterrestrial samples (0.15 and 0.27‰), mantle (0.23‰, Gall et al. 2017), present-day seawater (1.44‰, Cameron and Vance 2014), dissolved Ni from riverine input (0.84‰, Cameron and Vance 2014), ferromanganese crusts (0.9–2.5‰, Gall et al. 2013), Devonian/Mississippian organic-rich marine sediments, lower Jurassic organic-rich marine sediments (0.2–2.5‰, average 0.92‰, n = 18, Porter et al. 2014), and euxinic sediments of the Black Sea (0.14–0.51‰, Vance et al. 2016). However, the range of δ60Ni values in our black shale samples was close to that of the weathering products of mafic/ultramafic rocks (ore and soil samples) ranging from − 0.60 to + 0.30‰ (Ratie et al. 2015; Spivak-Birndorf et al. 2018), Ni-sulfide ores hosted by Archean komatiites from Australia and Canada (− 0.10 to − 1.03‰, average − 0.70‰, n = 8, Gueguen et al. 2013), and Archean Ni-rich magmatic sulfides from Zimbabwe (− 0.28 to − 0.47‰, n = 6, Hoffman et al. 2014). Based on our observations and considering the extremely low contribution of direct biological uptake of Ni, and a dominant Ni residence in early Fe/Ni-sulfides, we suggest that our mostly light Ni isotopic compositions in metal-rich black shales result from sulfidization of organic matter and Ni removal into sulfides.

182W and HSE constraints from 2.7 Ga komatiites on the heterogeneous nature of the Archean mantle

While the isotopically heterogeneous nature of the terrestrial mantle has long been established, the origin, scale, and longevity of the heterogeneities for different elements and isotopic systems are still debated. Here, we report Nd, Hf, W, and Os isotopic and highly siderophile element (HSE) abundance data for the Boston Creek komatiitic basalt lava flow (BCF) in the 2.7 Ga Abitibi greenstone belt, Canada. This lava flow is characterized by strong depletions in Al and heavy rare earth elements (REE), enrichments in light REE, and initial ε143Nd = +2.5 ± 0.2 and intial ε176Hf = +4.2 ± 0.9 indicative of derivation from a deep mantle source with time-integrated suprachondritic Sm/Nd and Lu/Hf ratios. The data plot on the terrestrial Nd-Hf array suggesting minimal involvement of early magma ocean processes in the fractionation of lithophile trace elements in the mantle source. This conclusion is supported by a mean μ142Nd = −3.8 ± 2.8 that is unresolvable from terrestrial standards. By contrast, the BCF exhibits a positive 182W anomaly (μ182W = +11.7 ± 4.5), yet is characterized by chondritic initial γ187Os = +0.1 ± 0.3 and low inferred source HSE abundances (35 ± 5% of those estimated for the present-day Bulk Silicate Earth, BSE). Collectively, these characteristics are unique among Archean komatiite systems studied so far. The deficit in the HSE, coupled with the chondritic Os isotopic composition, but a positive 182W anomaly, are best explained by derivation of the parental BCF magma from a mantle domain characterized by a predominance of HSE-deficient, differentiated late accreted material. According to the model presented here, the mantle domain that gave rise to the BCF received only ∼35% of the present-day HSE complement in the BSE before becoming isolated from the rest of the convecting mantle until the time of komatiite emplacement at 2.72 Ga. These new data provide strong evidence for a highly heterogeneous Archean mantle in terms of absolute HSE abundances and W isotopic composition, and also indicate slow mixing, on a timescale of at least 1.8 billion years. Additionally, the data are consistent with a stagnant-lid plate tectonic regime in the Hadean and Archean, prior to the onset of modern-style plate tectonics.

Zinc isotope composition of the Earth and its behaviour during planetary accretion

The terrestrial planets are depleted in volatile elements with respect to chondritic meteorites, their possible building blocks. However, the timing, extent and origin of volatile depletion is debated. Zinc is a moderately volatile element (MVE), whose stable isotopic composition can distinguish when and where this depletion took place. Here, we report data for 40 ultramafic rocks comprising pristine upper mantle peridotites from the Balmuccia orogenic lherzolite massif and Archean komatiites that together define the Zn isotope composition of the Earth's primitive mantle. Peridotites and komatiites are shown to have indistinguishable Zn isotopic compositions of δ66Zn=+0.16±0.06‰ (2SD), (with δ66Zn the per mille deviation of 66Zn/64Zn from the JMC-Lyon standard), implying a constant Zn isotope composition for the silicate Earth since 3.5Ga. After accounting for Zn sequestration during core formation, the Earth falls on the volatile-depleted end of a carbonaceous chondrite array in δ66Zn-Zn/Mg space, implying Earth avoided modification of its MVE budgets during late accretion (e.g. during a giant impact), in contrast to the Moon. The Moon deviates from the chondritic array in a manner consistent with evaporative loss of Zn, where its δ66Zn co-varies with Mn/Na, implying post-nebular volatile loss is more pronounced on smaller bodies. Should the giant impact deliver the Earth's volatile complement of Pb and Ag, it cannot account for the budget of lithophile MVEs (e.g. Zn, Rb, Mn), whose abundances reflect those of Earth's nebular building blocks. The Earth initially accreted from material that experienced chemical- and mass-dependent isotopic fractionation akin to carbonaceous chondrites, though volatile depletion was more pronounced on Earth.

Scanning electron microscopy and Raman spectroscopy as combined methods for studying zoning in minerals: The case of spinels from Archean komatiites

Several types of both magmatic and metamorphic spinels have been found in Archean komatiites of the Sovdozero and Kostomuksha greenstone belts in the eastern part of the Fennoscandian Shield. Scanning electron microscopy and Raman spectroscopy revealed relics of cores of primary magmatic chrome-spinels with high Cr and Al contents. In the Sovdozero structure, the relics are better retained than those in the Kostomuksha structure, which is caused by a different degree of metamorphic transformation. The comparable 100 · Cr/(Al + Cr) values of spinel cores from Sovdozero and Kostomuksha reflect similar conditions of partitional melting in the mantle. These data agree with the fact that both komatiite complexes belong to the Al-undepleted petrogenic type. Wide variations in the Cr and Al contents in primary chrome-spinel cores together with a constant Mg/(Fe2+ + Mg) ratio correspond to low oxygen fugacity during magma crystallization. In general, the composition of these primary chrome-spinels is similar to that of accessory phases in peridotites from suprasubduction zones and agrees with hypothesis of komatiite complex formation in back-arc basins.

A Calcium-in-Olivine Geohygrometer and its Application to Subduction Zone Magmatism

High-precision electron microprobe analyses were obtained on olivine grains from Klyuchevskoy, Shiveluch and Gorely volcanoes in the Kamchatka Arc; Irazú, Platanar and Barva volcanoes of the Central American Arc; and mid-ocean ridge basalt (MORB) from the Siqueiros Transform. Calcium contents of these subduction zone olivines are lower than those for olivines from modern MORB, Archean komatiite and Hawaii. A role for magmatic H2O is likely for subduction zone olivines, and we have explored the suggestion of earlier workers that it has affected the partitioning of CaO between olivine and silicate melt. We provide a provisional calibration of DCaOOl/L as a function of magmatic MgO and H2O, based on nominally anhydrous experiments and minimally degassed H2O contents of olivine-hosted melt inclusions. Application of our geohygrometer typically yields 3–4 wt % magmatic H2O at the Kamchatka and Central American arcs for olivines having ∼1000 ppm Ca, which agrees with H2O maxima from melt inclusion studies; Cerro Negro and Shiveluch volcanoes are exceptions, with about 6% H2O. High-precision electron microprobe analyses with 10–20 μm spatial resolution on some olivine grains from Klyuchevskoy and Shiveluch show a decrease in Ca content from the core centers to the rim contacts, and a sharp increase in Ca in olivine rims. We suggest that the zoning of Ca in olivine from subduction zone lavas may provide the first petrological record of temporal changes that occur during hydration of the mantle wedge and dehydration during ascent, and we predict olivine H2O contents that can be tested by secondary ionization mass spectrometry analysis.

Nickel–cobalt contents of olivine record origins of mantle peridotite and related rocks

Olivine is distinguished from all other minerals in providing a remarkable chemical narrative about magmatic processes that occurred in Earth’s crust, mantle, and core over the entire age of Earth history. Olivines in mantle peridotite have Ni contents and Mg numbers that were largely produced by equilibrium crystallization in an early turbulently convecting magma ocean; subsequent stages of partial melting operated to slightly elevate Ni and Mg number in residual olivines. Olivines from Archean komatiites from the Abitibi greenstone belt have Ni contents and Mg numbers that are consistent with an extensively melted peridotite source at great depths in the mantle. Olivines from basaltic oceanic crust, the Icelandic mantle plume and other Phanerozoic occurrences have compositions that record magma chamber crystallization, recharge, mixing, and partial melting. Olivines from the present-day Icelandic mantle plume have compositions that are consistent the melting of a peridotite source; unlike Hawaii, the melting of recycled crust as a distinct pyroxenite lithology is not evident in the olivine chemistry of Iceland. Paleocene picrites from Baffin Island and West Greenland from the ancient Icelandic plume have olivines with Ni contents that are consistent with either Ni-rich peridotite that formed by core-mantle interaction or by low-pressure crystallization of hot and deep magmas. In general, hot magma oceans, mantle plumes, and ambient mantle magmatism form in ways that are captured by the compositions of the olivine crystals that they contain.

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.