Komatiitic Lavas Research Articles

Depleted mantle sources through time: Evidence from Lu–Hf and Sm–Nd isotope systematics of Archean komatiites

In this study, we present Lu–Hf isotope systematics and Lu and Hf abundances for komatiites from the lowermost part of the 2.8 Ga Kostomuksha greenstone belt in the Baltic Shield and compare these, as well as available Sm–Nd isotope data, with those for the best characterized Archean komatiite systems. The Lu–Hf isotope compositions of four spatially associated differentiated lava flows from the Kostomuksha greenstone belt yield an isochron (MSWD = 1.6) with an age of 2931 ± 300 Ma, which represents the first Lu–Hf isochron obtained for a suite of co-magmatic komatiite lavas. The calculated mean initial 176Hf/ 177Hf for the Kostomuksha komatiite samples is 0.281107 ± 3 (2 σ mean ), which corresponds to an initial ε 176Hf of + 4.9 ± 0.1 (2 σ mean ). Assuming that mantle differentiation occurred 10 Ma after Earth's accretion at 4.558 Ga, this precise initial ratio requires a time-integrated 176Lu/ 177Hf = 0.03759 ± 8, which is identical to the average time-integrated 176Lu/ 177Hf = 0.0375 ± 6 calculated for the best characterized late Archean komatiite systems. Together with the calculated average time-integrated 147Sm/ 144Nd = 0.2091 ± 4 for the same late Archean komatiite systems, these parameters represent our best estimate of the Lu/Hf and Sm/Nd properties in the late Archean mantle and indicate derivation of komatiite magmas from around the globe from long-term melt-depleted sources that were remarkably homogenous in terms of lithophile trace element systematics. These time-integrated ratios are identical to the respective values of 0.0375 and 0.209 calculated by Boyet and Carlson (2006) for the so-called Early Depleted Reservoir (EDR), and may indicate that the late Archean mantle was similar in composition to the putative EDR, whereas early Archean systems had higher, and Proterozoic systems lower time-integrated Lu/Hf and Sm/Nd ratios. The observed decrease in time-integrated Lu/Hf and Sm/Nd in komatiite sources over time is interpreted as strong evidence for the existence of a hidden enriched reservoir complementary to the EDR that has been gradually mixed back into the mantle over time. The overall depletion of the early mantle likely occurred very early in Earth's history as a result of either global magma ocean differentiation or extraction and subsequent long-term isolation of primordial terrestrial crust.

The Fe-rich clay microsystems in basalt-komatiite lavas: importance of Fe-smectites for pre-biotic molecule catalysis during the Hadean eon.

During the Hadean to early Archean period (4.5-3.5 Ga), the surface of the Earth's crust was predominantly composed of basalt and komatiite lavas. The conditions imposed by the chemical composition of these rocks favoured the crystallization of Fe-Mg clays rather than that of Al-rich ones (montmorillonite). Fe-Mg clays were formed inside chemical microsystems through sea weathering or hydrothermal alteration, and for the most part, through post-magmatic processes. Indeed, at the end of the cooling stage, Fe-Mg clays precipitated directly from the residual liquid which concentrated in the voids remaining in the crystal framework of the mafic-ultramafic lavas. Nontronite-celadonite and chlorite-saponite covered all the solid surfaces (crystals, glass) and are associated with tiny pyroxene and apatite crystals forming the so-called "mesostasis". The mesostasis was scattered in the lava body as micro-settings tens of micrometres wide. Thus, every square metre of basalt or komatiite rocks was punctuated by myriads of clay-rich patches, each of them potentially behaving as a single chemical reactor which could concentrate the organics diluted in the ocean water. Considering the high catalytic potentiality of clays, and particularly those of the Fe-rich ones (electron exchangers), it is probable that large parts of the surface of the young Earth participated in the synthesis of prebiotic molecules during the Hadean to early Archean period through innumerable clay-rich micro-settings in the massive parts and the altered surfaces of komatiite and basaltic lavas. This leads us to suggest that Fe,Mg-clays should be preferred to Al-rich ones (montmorillonite) to conduct experiments for the synthesis and the polymerisation of prebiotic molecules.

Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine

Komatiites from Alexo, Canada, are well preserved and represent high-degree partial mantle melts (∼50%). They are thus well suited for investigating the Mg and Fe isotopic compositions of the Archean mantle and the conditions of magmatic differentiation in komatiitic lavas. High precision Mg and Fe isotopic analyses of 22 samples taken along a 15-m depth profile in a komatiite flow are reported. The δ25Mg and δ26Mg values of the bulk flow are −0.138 ± 0.021‰ and −0.275 ± 0.042‰, respectively. These values are indistinguishable from those measured in mantle peridotites and chondrites, and represent the best estimate of the composition of the silicate Earth from analysis of volcanic rocks. Excluding the samples affected by secondary Fe mobilization, the δ56Fe and δ57Fe values of the bulk flow are +0.044 ± 0.030‰, and +0.059 ± 0.044‰, respectively. These values are consistent with a near-chondritic Fe isotopic composition of the silicate Earth and minor fractionation during komatiite magma genesis. In order to explain the early crystallization of pigeonite relative to augite in slowly cooled spinifex lavas, it was suggested that magmas trapped in the crystal mush during spinifex growth differentiated by Soret effect, which should be associated with large and coupled variations in the isotopic compositions of Mg and Fe. The lack of variations in Mg and Fe isotopic ratios either rules out the Soret effect in the komatiite flow or the effect is effaced as the solidification front migrates downward through the flow crust. Olivine separated from a cumulate sample has light δ56Fe and slightly heavy δ26Mg values relative to the bulk flow, which modeling shows can be explained by kinetic isotope fractionation associated with Fe-Mg inter-diffusion in olivine. Such variations can be used to identify diffusive processes involved in the formation of zoned minerals.

Elemental and Nd-isotope systematics of the Upper Basalt Unit, 2.7 Ga Kambalda Sequence: Quantitative modeling of progressive crustal contamination of plume asthenosphere

The 2.7 Ga Kambalda Sequence comprises a mafic and ultramafic volcanic rock-dominated sequence of the Kalgoorlie Terrane, Yilgarn Craton, Western Australia. The Sequence is divided into the Lower Basalt and Upper Basalt Units, separated by the Kambalda komatiite lava units. Upper Basalts have a total range of ε Nd from + 3.0 to + 1.5, defining a trend in ε Nd versus Nb/Th from the uncontaminated Victorious Basalt ( ε Nd + 3, Nb/Th 8.8–9.7), through the Big Dick, Devon Consols, and Coolgardie komatiite basalts, to the most contaminated Bent Tree Basalt ( ε Nd + 1.5, Nb/Th 2.9–3.5). Recent geological and geochronological results show that the Black Flag Group (BFG) tonalite–trondhjemite–dacite has an early eruptive phase coeval with the Upper Basalts, as well as a later phase overlying the Kambalda Sequence. Modeling of assimilation-fractional crystallisation (AFC) of the Kambalda Komatiite by 10–15% addition of average BFG, followed by 30–40% crystallisation, generates the best fit to the compositional spectrum of the LREE-enriched Devon Consols Basalts. Modeling of Archean average lower-, middle-, or upper continental crust generates a poor fit. Collectively, the results are consistent with decompressional melting at ≤ ∼ 100 km, of an asthenosphere plume impinging on the base of thinned, rifted, cratonic lithosphere where the most primitive, hottest liquids were contaminated by the Black Flag Group, or its melts, whereas the more evolved, cooler liquids were not. Contamination was dominantly by crust rather than cratonic mantle lithosphere.

Geochemical systematics of basalts of the Lower Basalt Unit, 2.7 Ga Kambalda Sequence, Yilgarn craton, Australia: Plume impingement at a rifted craton margin

The 2.7 Ga Kambalda Sequence is a mafic and ultramafic volcanic sequence in the Kalgoorlie Terrane of the Yilgarn craton, Western Australia. The Sequence is divided into the Lower Basalt and Upper Basalt Units, separated by the komatiite lava flow units, and was erupted onto an autochthonous volcanic sequence, the 2.8 Ga Penneshaw Formation. The Lower Basalt Unit includes six basalt suites or formations with well preserved pillows and vesicles, at mid-greenschist facies: these are the Woolyeenyer and Burbanks Formations, and Lunnon, Wongi, Scotia, and Missouri basalts. Basaltic flows of the Woolyeenyer Formation, and Lunnon and Wongi Basalts, are compositionally restricted tholeiites with Mg# and Ni contents of 56–52, 110 ppm, 57–51, 190–160 ppm, and 63, 300–210 ppm, respectively. Heavy-REE (HREE) are flat at 4–6 times chondrite, with mild LREE depletion at (La/Sm) N = 0.77–0.94, and Nb/Th ratios of 8–17 signifying that these basalts were erupted through the autochthon without crustal contamination as there is no correlation of Nb/Th with (La/Sm) N . The Scotia Basalt is distinctive in having a range of HREE fractionation where (Gd/Yb) N = 1.1–1.4 indicative of melting over a range of depths from above to below the garnet stability field at ∼ 90 km. Basalts of the Burbanks and Penneshaw Formations have major element compositions akin to the Woolyeenyer Formation, but near-flat REE at 9–11 times chondrite. Both formations have two populations: the most primitive have greater Th contents but Nb/Th < 8, whereas the more evolved show lower Th in conjunction with Nb/Th > 8, in keeping with the hottest liquids having assimilated crust but the evolved flows being uncontaminated. Missouri basalts are all crustally contaminated. Collectively, the uncontaminated Woolyeenyer, Lunnon, Wongi, Scotia basalts and Burbanks Formation have ε Nd values spanning from 4.5 to 1.5, whereas ε Nd values for the contaminated Missouri Basalt and Penneshaw Formation span between 2 and 1; there is a weak trend of ε Nd with Nb/Th. Collectively, the results are interpreted in terms of a mantle plume impinging at the edge of rifted cratonic lithosphere mantle, melting over a range of depths, where hotter liquids assimilated crust but cooler liquids did not.

Pt–Re–Os and Sm–Nd isotope and HSE and REE systematics of the 2.7 Ga Belingwe and Abitibi komatiites

High-precision Pt–Re–Os and Sm–Nd isotope and highly siderophile element (HSE) and rare earth element (REE) abundance data are reported for two 2.7 b.y. old komatiite lava flows, Tony’s flow (TN) from the Belingwe greenstone belt, Zimbabwe, and the PH-II flow (PH) from Munro Township in the Abitibi greenstone belt, Canada. The emplaced lavas are calculated to have contained ∼25% (TN) and ∼28% (PH) MgO. These lavas were derived from mantle sources characterized by strong depletions in highly incompatible lithophile trace elements, such as light REE (Ce/Sm N = 0.64 ± 0.02 (TN) and 0.52 ± 0.01 (PH), ε 143Nd(T) = +2.9 ± 0.2 in both sources). 190Pt– 186Os and 187Re– 187Os isochrons generated for each flow yield ages consistent with respective emplacement ages obtained using other chronometers. The calculated precise initial 186Os/ 188Os = 0.1198318 ± 3 (TN) and 0.1198316 ± 5 (PH) and 187Os/ 188Os = 0.10875 ± 17 (TN) and 0.10873 ± 15 (PH) require time-integrated 190Pt/ 188Os and 187Re/ 188Os of 0.00178 ± 11 and 0.407 ± 8 (TN) and 0.00174 ± 18 and 0.415 ± 5 (PH). These parameters, which by far represent the most precise and accurate estimates of time-integrated Pt/Os and Re/Os of the Archean mantle, are best matched by those of enstatite chondrites. The data also provide evidence for a remarkable similarity in the composition of the sources of these komatiites with respect to both REE and HSE. The calculated absolute HSE abundances in the TN and PH komatiite sources are within or slightly below the range of estimates for the terrestrial Primitive Upper Mantle (PUM). Assuming a chondritic composition of the bulk silicate Earth, the strong depletions in LREE, yet chondritic Re/Os in the komatiite sources are apparently problematic because early Earth processes capable of fractionating the LREE might also be expected to fractionate Re/Os. This apparent discrepancy could be reconciled via a two-stage model, whereby the moderate LREE depletion in the sources of the komatiites initially occurred within the first 100 Ma of Earth’s history as a result of either global magma ocean differentiation or extraction and subsequent long-term isolation of early crust, whereas HSE were largely added subsequently via late accretion. The komatiite formation, preceded by derivation of basaltic magmas, was a result of second-stage, large-degree dynamic melting in mantle plumes.

Endogenous growth in channelized komatiite lava flows: evidence from spinifex-textured sills at Pyke Hill and Serpentine Mountain, Western Abitibi Greenstone Belt, Northeastern Ontario, Canada

Spinifex-textured sills (i.e., veins) characterized by komatiitic magmas that have intruded their own volcanic-piles have long been recognized. For instance, in the early 1970s, Pyke and coworkers, in their classic work at Pyke Hill in Munro Township, noted that not all spinifex-bearing ultramafic rocks formed as lava flows, rather some were clearly emplaced as small dikes and sills. Several hypotheses have been proposed to explain spinifex-textured sills: intrusion into a cold host, filter pressing, or drainage of residual liquid. However, these do not satisfactorily explain the phenomenon. Field and petrographic observations at Pyke Hill and Serpentine Mountain demonstrate that spinifex-bearing komatiite sills and dikes were emplaced during channel inflation processes when new magma was intruded into a cooler, semi-consolidated but permeable cumulate material. Komatiitic liquids were intruded into the olivine cumulate rocks near the boundary between the spinifex and the cumulate zones of well-organized to organized komatiite flows. Spinifex-textured sills are generally tabular in morphology, stacked one above another, with curviplanar contacts sub-parallel to stratigraphy. Some sills exhibit complex digitated apophyses. Thinner sills typically have a random olivine spinifex texture similar, though generally composed of coarser crystals, to that of komatiite lava flows. Thicker sills exhibit more complex organization of their constituent crystals characterized by zones of random olivine spinifex, overlying zones of organized coarse spinifex crystals similar to those found in lava flows. They have striking coarse dendritic spinifex zones composed of very large olivine crystals, up to several centimetres long and up to 1 cm wide that are not observed in lava flows. Typically, at the sill margins, the cumulate material of the host flow is composed of euhedral to subhedral olivine crystals that are larger than those distal to the contact. Many of these margin-crystals have either concentric overgrowth shells or dendritic olivine overgrowths that grew from the cumulate-sill contact toward the sill interior. The dendrites grew on pre-existing olivine cumulate at the contact in response to a sharp temperature gradient imposed by the intrusion of hot material, whereas the concentric overgrowths formed as new melt percolated into the unconsolidated groundmass of the host-flow cumulate material. Spinifex-textured sills and dikes occur in well-organized to organized flows that are interpreted to have formed by “breakouts” above and peripheral to lava pathways (channels/conduits) as a result of inflation that accompanied voluminous komatiitic eruptions responsible for the construction and channelization of komatiitic flow fields. The spinifex-textured dikes and sills represent komatiitic lava that was originally emplaced into the channel roof during periods of episodic inflation that resulted in lava breakouts and was subsequently trapped in the “roof rocks” during periods of channel deflation. Accordingly, the occurrence of spinifex-textured sills and dikes may indicate proximity to, and aid in the identification and delineation of lava channel-ways that could potentially host Ni–Cu–(PGE) mineralization within komatiitic lava flow-fields.

Re–Os isotope systematics and HSE abundances of the 3.5 Ga Schapenburg komatiites, South Africa: Hydrous melting or prolonged survival of primordial heterogeneities in the mantle?

We report Re–Os isotope and highly siderophile element (HSE) abundance data for komatiites from the Schapenburg Greenstone Remnant (SGR), South Africa, an equivalent to the lowermost formations of the Onverwacht Group in the Barberton Greenstone Belt (BGB). The Re–Os isotopic data for 13 whole-rock samples define a regression line with an age of 3549 ± 99 Ma, consistent with the ~ 3.5 Ga age of the Onverwacht Group. The immobility of Os during alteration and the correct Re–Os age provide evidence that the initial γ 187Os = + 3.7 ± 0.3 derived from the regression reflects a time-integrated suprachondritic 187Re/ 188Os in the source of the SGR komatiites. The HSE abundances in the emplaced SGR komatiite lavas are 2× to 3× lower than those in other well-studied komatiites. Compared to the Primitive Upper Mantle (PUM) estimate, the calculated mantle source of the SGR komatiites was moderately depleted in HSE and was characterized by a fractionated HSE pattern. The enrichment in 187Os, HSE depletion, and fractionated HSE pattern in the SGR komatiite source could be the result of fluid transport of radiogenic Os from the subducting slab, incorporation in the overlying mantle, and hydrous melting of the modified mantle to produce the komatiites. This would imply that plate tectonic processes operated as early as 3.5 Ga and that at least some komatiites formed via wet melting in island arc settings at relatively shallow depths and temperatures not exceeding 1400 °C. An alternative model would include dry melting of a chemically distinct, majorite-enriched mantle domain formed very early in Earth's history as a result of an initial stratification of the mantle during crystallization of a magma ocean. Preferential partitioning of Re into the majorite-rich domain would have resulted in its acquiring suprachondritic Re/Os. In order to grow the radiogenic Os isotopic composition, the majorite-rich domain would have to have remained isolated from the rest of the mantle for a billion years. If this model is correct, the data would require a deep plume origin for the SGR komatiites at temperatures ~ 300 °C higher than the ambient mantle temperatures. Both models have their shortcomings in reconciling the available geochemical and petrologic data for the SGR komatiites. Because BGB komatiites are geochemically closer to island arc tholeiites and boninites than the SGR komatiites, it will be especially important to determine the Re–Os isotope and HSE systematics of the BGB komatiites and of typical boninites.

Archean plate tectonics, rise of Proterozoic supercontinentality and onset of regional, episodic stagnant-lid behavior

Intense devolatilization and chemical-density differentiation attended late-stage accretion of the primitive Earth; it lessened after crystallization of a magma mush ocean during continued cooling. By ∼ 4.3Ga, shallow seas were present, so surface temperatures had fallen far below the ∼ 1300, ∼ 1120, and ∼ 950°C low-pressure solidi of peridotite, basalt, and granite, respectively. At temperatures less than about half their solidi, such materials existed as lithosphere in the near-surface Hadean realm. Stagnant-lid convection probably did not occur because massive heat transfer necessitated vigorous crust–mantle overturn in the early, hot Earth. Instead, bottom-up mantle convection, including voluminous plume ascent, efficiently rid the planet of heat, but lessened over time. Plate thickening and broadening is reflected in the post-Hadean rock record. Stages of geologic evolution included: (a) 4.5–4.4Ga, early, chaotic magma mush ocean overturn and ephemeral lithospheric platelets; (b) 4.4–2.7Ga, growth of oceanic and diminutive continental plates, obliterated by return mantle flow prior to ∼ 4.0Ga, but the latter enlarging and gradually accumulating as largely submarine, sutured, sialic crust-capped lithospheric collages; (c) 2.7–1.0Ga, progressive assembly of old shields and younger orogenic belts into supercratonal plates characterized by continental freeboard, sedimentary differentiation, and episodic glaciation during transpolar drift, as well as onset of regionally, temporally limited stagnant-lid convection beneath supercontinents; (d) 1.0Ga-present, modern, laminar-flowing asthenospheric cells capped by giant, stately moving plates. Restriction of komatiitic lavas to the Archean, and of multicycle sediments, most ophiolite complexes ± alkaline igneous rocks, and high-pressure and ultrahigh-pressure metamorphic belts to progressively younger Proterozoic–Phanerozoic orogens reflects increasingly negative buoyancy of the cooler oceanic lithosphere. Attending supercontinent assembly, density instabilities of thickening oceanic plates increasingly began to dominate overturn of the suboceanic mantle as cold, top-down convection. Scales and dynamics of hot asthenospheric upwelling versus lithospheric foundering and asthenospheric return flow (bottom-up versus top-down) changed gradually over geologic time in response to planetary thermal relaxation.

Triassic rift-related komatiite, picrite and basalt, Pelagonian continental margin, Greece

A wide range of Triassic rift-related volcanic rocks is preserved on the extensional continental margin of the central part of the Pelagonian micro-continental block of Greece, in the region of northern Evia Island and Othris. This part of the continental margin is unusual for the abundance of lavas, the wide range of lava types, and the presence of komatiitic lavas. The predominant rock types are subalkaline basalt and basaltic andesite, mildly alkaline basalt and picrite, and minor komatiite. Four groups of mafic rocks are distinguished from the 32 samples analyzed on the basis of variation in incompatible trace elements, with all but one group including both picritic and basaltic rocks. The geochemical character of the volcanic rocks suggests derivation from inhomogenous, spinel-bearing, lithospheric mantle. This mantle source was variably depleted and moderately refertilized by subduction-related fluids during earlier Hercynian subduction. The picrites were generated by variable degrees of partial melting, which are inferred to inversely correlate with pressure, thus suggesting decompression melting of their source. Evidence is lacking for a major mantle plume, but the enhanced magmatism in this sector of the rifted margin suggests that melting was induced by a large amount of hydration, which is appropriate to generate melts at lower temperatures. Consequent fractionation coupled with crustal assimilation generated the wide range of subalkaline to mildly alkaline mafic rocks.

3.35 Ga komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-rock geochemistry

We present field, petrographic, Sm–Nd whole-rock isochron and whole-rock geochemical data for komatiites from Sargur Group greenstone belts of the western Dharwar craton. Field evidence such as pillow structure indicates their eruption in a marine environment. Petrographic data reveal that the igneous mineralogy has been altered during post-magmatic hydrothermal alteration processes corresponding to greenschist- to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene. A 16-point Sm–Nd whole-rock isochron gives an age of 3352 ± 110 Ma for the timing of eruption of komatiite lavas. About 60% of the studied komatiite samples show Al-depletion whilst the remaining are Al-undepleted. The Al-depleted komatiites are characterised by high CaO/Al 2O 3 ratios (1.01–1.34) and low Al 2O 3/TiO 2 (5–16) whereas Al-undepleted komatiites show lower CaO/Al 2O 3 ratios (0.59–0.99) and higher Al 2O 3/TiO 2 (17–26). Trace element distribution patterns of komatiites suggest that most of the primary geochemical and Nd isotopic compositions are preserved with only minor influence of post-magmatic alteration processes and negligible crustal contamination. The chemical characteristics of Al-depleted komatiites, such as high (Gd/Yb) N together with lower HREE, Y, Zr and Hf, imply their derivation from deeper upper mantle with garnet (majorite?) involvement, whereas lower (Gd/Yb) N slightly higher HREE, Y, Zr and Hf suggest derivation from shallower upper mantle without garnet involvement. The observed chemical characteristics (CaO/Al 2O 3, Al 2O 3/TiO 2, MgO, Ni, Cr, Nb, Zr, Y, Hf, REE) indicate derivation of the komatiite magmas from different depths in a plume setting, whereas sub-contemporaneous felsic volcanism and TTG accretion can be attributed to an arc setting. In order to explain the spatial association of komatiite volcanism with contemporaneous mafic-felsic volcanism and TTG accretion we propose a combined plume-arc setting. Nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early (>4.53 Ga) global differentiation of the silicate Earth as suggested by Boyet and Carlson [Boyet, M., Carlson, R.W., 2005. 142Nd evidence for early (>4.53 Ga) global differentiation of silicate Earth. Science 309, 577–581] or extraction of continental crust during the early Archaean.

Speculations on evolution of the terrestrial lithosphere–asthenosphere system—Plumes and plates

In the early Earth, accretionary impact heating, including collision with a large, Mars-sized object, decay of short-lived radioisotopes, and (after an initial thermal run-up) continuous segregation of the liquid Fe–Ni core resulted in extensive the melting of the silicate mantle and in the formation of a near-surface magma mush ocean. Progressive, continuous degassing and chemical–gravitational differentiation of the crust–mantle system accompanied this Hadean stage, and has gradually lessened during the subsequent cooling of the planet. Mantle and core overturn was vigorous in the Hadean Earth, reflecting deep-seated chemical heterogeneities and concentrations of primordial heat. Hot, bottom-up mantle convection, including voluminous plume ascent, efficiently rid the planet of much thermal energy, but gradually decreased in importance with the passage of time. Formation of lithospheric scum began when planetary surface temperatures fell below those of basalt and peridotite solidi. Thickening and broadening of lithospheric plates are inferred from the post-Hadean rock record. Developmental stages of mantle circulation included: (a) 4.5–4.4 Ga, early, chaotic magma ocean circulation involving an incipient or pre-plate regime; (b) 4.4–2.7 Ga, growth of small micro-oceanic and microcontinental platelets, all returned to the mantle prior to 4.0 Ga, but increasing in size and progressively suturing sialic crust-capped lithospheric amalgams at and near the surface over time; (c) 2.7–1.0 Ga, assembly of cratons surmounting larger, supercontinental plates; and (d) 1.0 Ga–present, modern, laminar-flowing asthenospheric cells capped by gigantic, Wilson-cycle lithospheric plates. Restriction of komatiitic lavas to the Archean, and of ophiolite complexes ± alkaline igneous rocks, high-pressure and ultrahigh-pressure metamorphic terranes to progressively younger Proterozoic–Phanerozoic orogenic belts supports the idea that planetary thermal relaxation promoted the increasingly negative buoyancy of cooler oceanic lithosphere. The Thickening of oceanic plates enhanced the gravitational instability and the consequent overturn of the outer Earth as cold, top-down oceanic mantle convection. The scales and dynamics of deep-seated asthenospheric circulation, and of lithospheric foundering + shallow asthenospheric return flow evidently have evolved gradually over geologic time in response to the progressive cooling of the Earth.

Formation of Spinifex Texture in Komatiites: an Experimental Study

The formation of platy olivine spinifex, the texture that characterizes komatiite lavas, has long been enigmatic. A major problem is that the dendritic morphology of the olivine resembles that of crystals grown in laboratory experiments at high cooling rates (>50 C/h), but at the position where these textures form, up to several meters below the komatiite flow top, the cooling rate cannot have been greater than 1–5 C/h. We performed experiments that demonstrate that the platy habit of spinifex olivine or pyroxene is a consequence of slow cooling of ultramafic magma in a thermal gradient (7–35 C/cm). The charges were cooled at rates between 2 and 1428 C/h and, even at the low cooling rates, the thermal gradient led to constrained growth and the development of preferentially oriented dendritic crystals with orphologies like those in natural platy spinifex-textured lavas. Under these conditions, olivine starts to crystallize at temperatures well below the equilibrium liquidus temperature (37 C < –DT< 56 C) depending on the composition of the starting material.When the cooling rate is high, the thermal gradient has a negligible effect on the texture and the crystals have a random orientation, like that in the upper parts of komatiite flows.

Volcanological Controls on the Localization of the Komatiite-Hosted Ni-Cu-(PGE) Coronet Deposit, Kambalda, Western Australia

Coronet is a small komatiite-hosted Ni-Cu-(PGE) ore deposit within the northwestern part of the Kambalda dome. Coronet is unusual in that it is concealed beneath a faulted wedge of Lunnon Basalt, which also forms the footwall of the deposit. The orebody consists of four major ore surfaces, Coronet South, Coronet North, and two hanging-wall ore surfaces, SO1H and SO2H. The orientation of the two hanging-wall ore surfaces is oblique to the trend of Coronet North. The SO1H is the main mineralized ore shoot and is spatially associated with the base of the second flow unit. The mineralization is predominantly interspinifex in nature. The SO2H surface follows a similar trend to the SO1H and is inferred to represent the continuation of the SO1H. The SO2H ore is unusual in that the footwall is sedimentary. The orebody is locally hosted by shallow intrusive units, which are inferred to represent dense flows that burrowed into the wet, unconsolidated, sedimentary substrate. Adjacent to the Coronet ore shoot is the smaller McCloy ore shoot, which has a sedimentary footwall. The sedimentary units thin toward the ore shoot suggesting incomplete erosion by the ore-hosting flows. The contrast in distribution of sedimentary units at Coronet and McCloy illustrates the role of synemplacement erosion as a primary control on the distribution of the sedimentary units. The distribution of sedimentary units, the nature of the contacts with the komatiite flows, the abundance and distribution of vesicles in the flows, and the trend of primary volcanic facies at Coronet are consistent with the emplacement of komatiite lavas by early open channel flow followed by a transition to laminar emplacement.

Highly siderophile element geochemistry of 187Os-enriched 2.8 Ga Kostomuksha komatiites, Baltic Shield

New analyses of highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, and Pd) obtained by Carius tube digestion isotope dilution inductively coupled plasma mass-spectrometry (ID-ICPMS) technique are reported for 187Os-enriched 2.8 Ga komatiites from the Kostomuksha greenstone belt. As a result of a significant improvement in the yield over our previous digestions by the NiS fire-assay technique, these komatiites have now been shown to contain 22 to 25% more Os, Ir, and Pt and 34% more Ru. The emplaced komatiite lavas at Kostomuksha thus had siderophile element abundances comparable to those of the Abitibi belt. The discrepancies observed between the two techniques are interpreted to be the result of incomplete digestion of HSE carriers (particularly chromite) during the NiS fire-assay procedure. Our results for UB-N peridotite reference material agree well with those obtained by the high-pressure ashing digestion ID-ICPMS technique reported in the literature. Two types of komatiite lavas have been distinguished in this study based on the IPGE (Os, Ir, and Ru) behavior during lava differentiation. The Kostomuksha type is unique and is characterized by an incompatible behavior of IPGEs, with bulk solid-liquid partition coefficients for IPGEs being close to those for olivine. Cumulate zones in this type of komatiite lava occupy <20% of the total thickness of the flows. The Munro type exhibits a compatible behavior of IPGEs during lava differentiation. The cumulate zone in this type of komatiite occupies >20% of the total thickness of the flows. The calculated bulk partition coefficients indicate that, as with the other Munro-type komatiite lavas, the bulk cumulate contained an IPGE-rich minor phase(s) in addition to olivine. The non-CI chondritic HSE pattern for the source of the Kostomuksha komatiites calculated here is similar to that of Abitibi komatiites and to average depleted spinel lherzolite (ADSL) and supports the hypothesis of a non-CI chondritic HSE composition of the Earth’s mantle. The absolute HSE abundances in the source of the Kostomuksha komatiite have been demonstrated to be comparable to those of the source of Abitibi komatiites, even though the two komatiites contrast in their Os isotopic compositions. This supports the earlier hypothesis that if core-mantle interaction produced the 187Os/ 188Os radiogenic signature in the Kostomuksha source, it must have occurred in the form of isotope exchange at the core-mantle boundary. Other explanations of the radiogenic Os signature are similarly constrained to conserve the elemental abundance pattern in the mantle source of Kostomuksha komatiites.

Komatiites and nickel sulfide ores of the Black Swan area, Yilgarn Craton, Western Australia. 3: Komatiite geochemistry, and implications for ore forming processes

The Black Swan komatiite sequence is a package of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. A large body of whole rock analyses on komatiitic rocks from the Black Swan area has been filtered for metasomatic effects. With the exception of mobile elements such as Ca and alkalis, most samples retain residual igneous geochemistry, and can be modelled predominantly by fractionation and accumulation of olivine. Whole rock MgO–FeO relationships imply a relatively restricted range of olivine compositions, more primitive than the olivine which would have been in equilibrium with the transporting komatiite lavas, and together with textural data indicate that much of the cumulus olivine in the sequence was transported. Flow top compositions show evidence for chromite saturation, but the cumulates are deficient in accumulated chromite. Chromite compositions are typical of those found in compound flow-facies komatiites, and are distinct from those in komatiitic dunite bodies. Incompatible trace element abundances show three superimposed influences: control by the relative proportion of olivine to liquid; a signature of crustal contamination and an overprint of metasomatic introduction of LREE, Zr and Th. This overprint is most evident in cumulates, and relatively insignificant in the spinifex rocks. Platinum and palladium behaved as incompatible elements and are negatively correlated with MgO. They show no evidence for wholesale depletion due to sulfide extraction, which was evidently restricted to specific lava tubes or pathways. The lack of correspondence between PGE depletion and contamination by siliceous material implies that contamination alone is insufficient to generate S-saturation and ore formation in the absence of sulfide in the assimilant. Contamination signatures in spinifex-textured rocks may be a guide to Ni-sulfide mineralisation, but are not entirely reliable in the absence of other evidence. The widespread vesicularity of the sequence may be attributable to assimilated water rather than to primary mantle-derived volatiles, and cannot be taken as evidence for primary volatile-rich magmas. The characteristic signature of the Black Swan Succession is the presence of highly localised disseminated sulfide within a sequence showing more widespread evidence for crustal contamination and interaction with its immediate substrate. This has important implications for the applicability of trace element geochemistry in exploration for komatiite-hosted nickel deposits.

Komatiites and nickel sulfide ores of the Black Swan area, Yilgarn Craton, Western Australia. 4. Platinum group element distribution in the ores, and genetic implications

The Black Swan komatiite sequence, in the Eastern Goldfields province of the Archaean Yilgarn Craton in Western Australia, is a body of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. The massive sulfide orebodies of the Black Swan Succession are pervasively depleted in all platinum group elements (PGEs), particularly Pt and Pd, despite very high Ni contents. This depletion cannot be explained by R-factor variations, which would also require relatively low Ni tenors. The PGE depletion could be explained in part if the ores are enriched in a monosulfide solid solution (MSS) cumulate component, but requires some additional fractional segregation of sulfide melt upstream from the site of deposition. The Silver Swan orebody shows a remarkably consistent vertical zonation in PGE contents, particularly in Ir, Ru, Rh, Os, which increase systematically from very low levels at the stratigraphic base of the sulfide body to maxima corresponding roughly with the top of a lower layer of the orebody rich in silicate inclusions. Platinum shows the opposite trend, but is somewhat modified by remobilisation during talc carbonate alteration. A similar pattern is also observed in the adjacent White Swan orebody. This zonation is interpreted and modelled as the result of fractional crystallisation of MSS from the molten sulfide pool. The strong IPGE depletion towards the base of the orebody may be a consequence of sulfide liquid crystallisation in an inverted thermal gradient, between a thin rapidly cooling upper rind of komatiite lava and a hot substrate.

Komatiites and nickel sulphide orebodies of the Black Swan area, Yilgarn Craton, Western Australia. 1. Petrology and volcanology of host rocks

The Black Swan Succession is a bimodal association of dacitic and komatiitic volcanic rocks located about 50 km NNE of Kalgoorlie, within the 2.7-Ga Eastern Goldfields greenstone province of the Yilgarn Craton. The komatiite stratigraphy comprises a steep dipping, east facing package about 700 m in maximum thickness and about 2.5 km in strike length (Fig. 1), which hosts a number of economically exploitable Ni sulphide orebodies including the Silver Swan massive ore shoot (approximately half a million tonnes at about 10.5% Ni). The sequence can be subdivided into a Lower Felsic Unit, comprising coherent and autobrecciated facies of multiple dacite lava flows; an upper Eastern and lower Western Ultramafic Unit, each showing marked lateral facies variation, and an Upper Felsic Unit coeval with the Eastern Ultramafic Unit. The komatiite sequence has been metamorphosed at sub-greenschist facies in the presence of high proportions of CO2-rich fluid, giving rise to pervasive talc–carbonate and talc–carbonate–quartz assemblages, with extensive preservation of pseudomorphed igneous textures. Cores of lizardite serpentinite are present in the thickest parts of the ultramafic succession. The degree of penetrative deformation is generally very low, and original stratigraphic relationships are largely intact in much of the sequence. The Eastern Ultramafic Unit and Western Ultramafic Unit are interpreted as components of a single large komatiite flow field, representing overlapping stages in the emplacement of a series of distributory lava pathways and flanking sheet flows. The Western Ultramafic Unit which hosts the bulk of the high-grade massive and disseminated ores is a sequence dominated by coarse-grained olivine cumulates, 2 km wide and up to 500 m thick, with major magma pathways represented by thick, homogenous olivine mesocumulate piles at its northern and southern ends: respectively 400 and 200 m thick. The sequence between the two major pathways consists of olivine orthocumulates (oOC) with minor spinifex-textured intervals. The Unit is capped by a persistent spinifex-textured crust less than 1 m thick, and is locally vesicular. The Eastern Ultramafic Unit contains the Black Swan Cumulate Zone, a 500-m thick sequence of very coarse-grained hopper-textured, locally vesicular oOC containing disseminated sulphides in its lower 200 m. The zone is flanked to the north and south by complexly interdigitated sequence of highly irregular, spinifex-capped, olivine cumulate-rich flow lobes between 1 and 100 m thick, and dacitic lavas and tuffs. The complexity of the 3-D spatial relationship of these units suggests a combination of simultaneous eruption of dacite and komatiite, combined with thermal or thermomechanical erosion. The Eastern and Western Units are interpreted as the result of more or less continuous prolonged eruption of olivine charged komatiite lava, which developed localised thermo-mechanical erosion channels in the dacitic substrate. Komatiite and dacite eruption was synchronous, giving rise to complex interdigitation and extensive contamination and hybridisation.

Precise Pt–Re–Os isotope systematics of the mantle from 2.7-Ga komatiites

Precise Pt–Re–Os isotopic and highly siderophile element (HSE) data are reported for 10 drill core samples collected across a 2.7 m thick, differentiated komatiite lava flow at Pyke Hill in Ontario, Canada. The Re–Pt–Os isotopic data for five cumulate samples yield Re–Os and Pt–Os isochrons with ages of 2725±33 and 2956±760 Ma and initial 187Os/188Os=0.10902±16 and 186Os/188Os=0.1198318±18, respectively. These ages are consistent with the time of lava emplacement determined by other geochronometers. The initial 187Os/188Os isotopic composition of the Pyke Hill source is similar to the 187Os/188Os isotopic compositions of enstatite and ordinary chondrites and to that of PUM at 2725 Ma. Using the Pt–Os isotopic data for the Pyke Hill source, the present-day 186Os/188Os isotopic composition of PUM was calculated to be 0.1198387±18. The regression of global 187Os/188Os isotopic data for terrestrial materials shown to represent Os isotopic composition of the contemporary mantle indicates that the LILE-depleted mantle evolved from the solar system starting composition and, over Earth's history, was on average ∼8% depleted in Re vs. Os relative to PUM. From the HSE composition of the emplaced Pyke Hill komatiite lava, the mantle source was inferred to contain Re (0.30 ppb), Os (3.9 ppb), Ir (3.6 ppb), Ru (5.4 ppb), Pt (6.4 ppb), and Pd (6.3 ppb) in relative proportions similar to those in average enstatite chondrites, except for Pd, which was enriched over Pt by ∼30% in the Pyke Hill source compared to enstatite chondrites. From these data, the 190Pt/188Os of PUM was calculated to be 0.00157. If the HSE abundances in the terrestrial mantle were inherited from chondritic material of a late veneer, the latter should have had the HSE composition of ordinary or enstatite chondrites. Alternatively, if the reenrichment of the mantle with HSE was the result of an open-system transport of material across the core–mantle boundary, it was likely a global event that should have occurred shortly after core formation.

Contemporaneous ultramafic and felsic intrusive and extrusive magmatism in the Archaean Boorara Domain, Eastern Goldfields Superterrane, Western Australia, and its implications

Two komatiite emplacement styles are recognized in the Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia. (1) Large volume, dominantly cumulate-textured komatiite bodies, interpreted as high-level intrusions based on the presence of symmetrical spinifex-cumulate textural zonation, together with apophysy injection and xenolith incorporation along the top and basal margins of the komatiite. (2) Extrusive komatiite lavas exhibit asymmetrical textural zonation, with the top chilled flow margin preserving a fine-grained, fractured crust that coarsens downward into a supercooled former olivine spinifex zone and then into pseudomorphed olivine (now serpentine) orthocumulate texture above a thin basal chilled margin.The intrusive komatiite unit is observed to be intercalated with quartz- and feldspar-phyric dacite. The two lithologies exhibit primary irregular contacts against each other; komatiite apophyses have been injected into the dacite and magma mingling has occurred. These features are indicative of liquid–liquid interaction between the two units and komatiite magmatism is interpreted to have been contemporaneous with dacitic magmatism. The dacite was emplaced first and the komatiite intruded the dacite before it solidified.The reconstructed stratigraphy of the Boorara Domain comprises a basal unit of quartzofeldspathic turbidite beds, conformably overlain by high Mg tholeiite basalt, the intercalated dacite and komatiite units overlie the lower tholeiitic basalt with an unobserved contact, an upper komatiitic basalt unit overlies the komatiite and at the stratigraphic top of the sequence are felsic volcaniclastic turbidite units with felsic porphyry intrusions. These greenstone lithologies were emplaced within a deep submarine basin as indicated by carbonaceous metapelite units intercalated throughout the sequence.The tectonic-scale processes operating in, and around, the Boorara Domain depocentre include derivation of the komatiite unit from a likely mantle plume source. The contemporaneous dacite magmatism is interpreted to be the result of melting of pre-existing Archaean crust during plume ascension, although the exact composition of the primitive crust is yet to be determined.