Barberton Komatiites Research Articles

Hf and Nd isotope systematics of early Archean komatiites from surface sampling and ICDP drilling in the Barberton Greenstone Belt, South Africa

![][1] To constrain the origin of komatiites from the Barberton Greenstone Belt, South Africa, we measured 147Sm-143Nd and 176Lu-176Hf compositions for 18 komatiites from core obtained during the International Continental Drilling Program in the Komati Formation of the Barberton Belt, as well as 33 komatiites from surface outcrops of the Komati, Hooggenoeg, and Weltevreden Formations, these latter for purposes of comparison between core and surface samples. Magmatic clinopyroxene from surface samples near the drill site was also analyzed. For the Lu-Hf isotope system, the Komati Formation core and surface samples including the clinopyroxene define a linear array whose slope corresponds to an age of 3426 ± 16 Ma (MSWD = 118; ɛHf(T) = +2.2), which is slightly younger than the accepted age of the rocks (3.48 Ga). The Sm-Nd isotope data for the same set of samples likewise fall along a linear array also yielding a younger age of 3339 ± 12 Ma (MSWD = 42; ɛNd(T) = +2.8). The high MSWD for both isotope systems indicate substantial scatter at variance with normal magmatic processes, likely implying element mobility disturbing even these relatively robust isotopic systems shortly after eruption of the lavas. The average initial ɛNd and ɛHf of the core samples at 3.48 Ga are +0.45 and +1.4, respectively, in overall accordance with the positive errorchron intercepts and a depleted mantle source at 3.5 Ga. In contrast, the clinopyroxene and their host rocks have strongly positive ɛHf(T) of about +5 and negative ɛNd(T) of about −2. Given the overall scatter of the whole-rock data, the most robust constraint on the composition of the komatiite source comes from the clinopyroxene. Their positive ɛHf(T) is in line with, though somewhat higher than other results from komatiites from the Komati Formation, but their negative ɛNd(T) is unexpected in that it indicates a source with long-term low Sm/Nd, which is at odds with its long-term high Lu/Hf. This signature is also found in the trace element compositions of some of the komatiites, such as moderately enriched LREE, negative Hf anomalies, and low Hf/Sm ratios. The origin of these features is uncertain but one possibility is that the discordance between the Hf and Nd isotope systems reflects the presence of deep-sea sediments in the source of some of the Barberton komatiites. The possible presence of a surface component in an ancient deep mantle source has wide-ranging implications for mantle-crust interaction and dynamics in the early Earth and for constraining a minimum age for the onset of plate tectonics. [1]: /embed/graphic-1.gif

Platinum-group element geochemistry of komatiite-derived 3.1 Ga ultramafic–mafic rocks and chromitites from the Nuggihalli greenstone belt, Western Dharwar craton (India)

The 3.1Ga Nuggihalli greenstone belt in the Western Dharwar craton (southern India) comprises a sill-like layered ultramafic–mafic igneous complex with associated metasedimentary and metavolcanic (komatiitic to komatiitic basalt) schists that are enclosed by the tonalite–trondhjemite granodiorite suite of rocks (TTG). The sill-like layered complex is represented by a succession of chromitite-bearing serpentinite (after dunite) and peridotite, anorthosite, pyroxenite, and gabbro hosting magnetite bands. Extensive bulk-rock trace element and platinum-group element (PGE) geochemical study of the plutonic sill-like layered complex and the metavolcanic schists, suggest immobility of most trace elements (except La and Cu) and the PGEs, despite greenschist facies metamorphism and hydrothermal alteration experienced by the rocks. Their immobile nature is understood from good correlation of the trace elements and PGE with MgO and Cr. Other than chromitites and serpentinites all plutonic rocks show PPGE (Pd, Pt, Rh) enriched primitive-mantle normalized PGE patterns (Pd/IrN=3.9–81.1) that are suggestive of fractionation of IPGEs (Ir, Os, Ru) by the early crystallizing chromite mineral, and the incompatible nature of PPGEs in the same. The chromitites show high PGE abundances (∑PGE=96–296ppb), especially IPGEs (∑IPGE=63–223ppb), due to the presence of inclusions of IPGE-bearing minerals. In the primitive-mantle normalized PGE plot the chromitites show an IPGE enriched pattern. The PPGE enriched pattern (Pd/IrN=7.7–26) of the komatiitic to komatiitic basalt schists in a primitive-mantle normalized PGE plot indicates retention of IPGEs in the mantle or IPGE-bearing alloy saturation in the melt, while incompatible behavior of the PPGEs implies the sulfide undersaturated nature of the mantle source.The PGE pattern of the metavolcanic schists resembles the pattern of early Archean (3.5Ga) Barberton komatiites (Pd/IrNBarberton=1–40.7; Pd/IrNNuggihalli=6.3–21.3), which corroborates our previous results based on REE study, and also resembles the pattern of komatiites from the 2.9Ga Sandstone greenstone belt in the Youanmi Terrane of Western Australia (Pd/IrNSandstone=6). The metavolcanic schists exhibit the typical PGE depleted character observed in early Archean komatiites (∑PGEschist=0.4–27.2ppb; ∑PGEBarberton=15.0–20.8ppb; ∑PGEYouanmi Terrane, Western Australia=4.2–7.0ppb) which is explained to be a result of progressive mixing of late veneer matter in the Earth's mantle with time. Pt fractionation in the Nuggihalli metavolcanic schists and in early or late Archean komatiites indicates Pt alloy dispersal in the lower mantle during crystallization of the primary magma ocean and a consequent formation of Pt-enriched and Pt-depleted isolated upper mantle domains that did not homogenize and mix away by 2.7Ga.In the plutonic layered sequence, pyroxenite represents a change from sulfide-undersaturation to sulfide-saturation. The pyroxenite represents a break in trend from the negative correlation of Pt and Pd with MgO displayed by the serpentinites and peridotites due to incompatible behavior of the PPGEs during lava differentiation, to the positive pattern displayed by the gabbro and metavolcanic schists due to attainment of sulfide saturation. Sulfide-saturation was probably triggered by fractional crystallization of olivine, chromite and pyroxenes. Chondrite-normalized REE patterns and a plot of incompatible elements negate the role of crustal contamination of the parental komatiitic magma. In addition, the absence of ambient sulfidic sediments rules out assimilation of crustal sulfur in the Nuggihalli rocks. The immiscible sulfides segregated from the Al-depleted komatiitic parental magma concentrating the PGEs during crystallization of the pyroxenes that accumulated to form pyroxenite.

Insights into early Earth from the Pt–Re–Os isotope and highly siderophile element abundance systematics of Barberton komatiites

Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance and Pt–Re–Os isotopic data are reported for well-preserved komatiites from the Komati and Weltevreden Formations of the Barberton Greenstone Belt in South Africa. The Re–Os data for whole-rock samples and olivine and chromite separates define isochrons with ages of 3484±38 and 3263±12Ma for the Komati and Weltevreden systems, respectively. The respective initial 187Os/188Os=0.10335±15 (γ187Os=+0.34±0.15) and 0.10442±4 (γ187Os=−0.14±0.04) are well within the range defined by chondritic meteorites. When considered together with the Re–Os data for late Archean komatiite systems, these data indicate that the mantle sources of most Archean komatiites evolved with essentially uniform long-term Re/Os that is well within the chondritic range. By contrast, the initial 186Os/188Os=0.1198283±9 (ε186Os=−0.12±0.08) and 0.1198330±8 (ε186Os=+0.22±0.07) for the Komati and Weltevreden systems, respectively, are outside of known chondritic evolution paths, indicating that the mantle sources of these two komatiite systems evolved with fractionated time-integrated Pt/Os. The new 186,187Os isotopic data for these early Archean komatiite systems, combined with published 142,143Nd and 176Hf isotopic data for these systems, are consistent with formation and long-term isolation of deep-seated mantle domains with fractionated time-integrated Sm/Nd, Lu/Hf, and Pt/Os ratios, at ca. 4400Ma. These domains may have been generated as a result of late-stage crystallization of a primordial magma ocean involving Mg-perovskite, Ca-perovskite and Pt-alloys acting as the fractionating phases. The inferred fractionated mantle domains were sampled by the early Archean komatiites, but were largely mixed away by 2.7Ga, as evidenced by uniform time-integrated Sm/Nd, Lu/Hf, and Pt/Os ratios inferred for the sources of most late Archean komatiite systems. The calculated total Pt+Pd abundances present in the sources of the early Archean komatiite systems fall only 7–14% short of those present in estimates for the modern primitive mantle. These are also within the range of the total Pt+Pd abundances present in the sources of late Archean komatiite systems, indicating little change in the HSE abundances in the Archean mantle between 3.5 and 2.7Ga. The new HSE data for the early Archean komatiite systems may implicate late accretion of HSE to the mantle prior to completion of crystallization of a final terrestrial magma ocean, followed by sluggish mixing of diverse, post-magma ocean domains characterized by variably fractionated lithophile element and HSE abundances.

Insights into early Earth from Barberton komatiites: Evidence from lithophile isotope and trace element systematics

Major, minor, and lithophile trace element abundances and Nd and Hf isotope systematics are reported for two sets of remarkably fresh, by Archean standards, samples of komatiitic lavas from the 3.48Ga Komati and the 3.27Ga Weltevreden Formations of the Barberton Greenstone Belt (BGB) in South Africa. These data are used to place new constraints on the thermal history of the early Archean mantle, on the timing of its differentiation, and on the origin and chemical nature of early mantle reservoirs and their evolution through time.Projected moderate to strong depletions of highly incompatible lithophile trace elements and water in the mantle sources of both komatiite systems, combined with the partitioning behavior of V during lava differentiation, are consistent with anhydrous conditions during generation of the komatiite magmas. Komati and Weltevreden lavas are inferred to have erupted with temperatures of ∼1600°C, and, thus, represent the hottest known lavas on Earth. The calculated mantle potential temperatures of ∼1800°C for both komatiite systems are 150–200°C higher than those of contemporary ambient mantle. Combined, these observations are consistent with the origin of these BGB komatiite magmas in mantle plumes in the lower mantle.New Sm–Nd and Lu–Hf isotopic data allow precise determination of initial ε143Nd=+0.46±0.10 and +0.50±0.11 and initial ε176Hf=+1.9±0.3 and +4.7±0.8 for the Komati and the Weltevreden system komatiites, respectively. These positive initial values reflect prior fractionation of Sm/Nd and Lu/Hf in the mantle early in Earth history. Conversely, μ142Nd values are 0.0±2.4 and +2.2±4.1 for the Komati and the Weltevreden systems, respectively. These values overlap, within uncertainties, those of modern terrestrial rocks, thus, limiting the magnitudes of possible Sm/Nd fractionations generated by early Earth processes in the sources of these rocks. Combined 142,143Nd and Hf isotope and lithophile trace element systematics are consistent with formation and long-term isolation of deep-seated mantle domains with fractionated Sm/Nd and Lu/Hf at ca. 4400Ma. These domains were likely generated as a result of crystallization of a primordial magma ocean, with Mg-perovskite and minor Ca-perovskite acting as fractionating phases. The inferred mantle domains were evidently mixed away by 2.7Ga on the scale of mantle reservoirs sampled by late Archean komatiite lavas emplaced worldwide.

Mantle melting in equilibrium with an Iron–Wüstite–Graphite buffered COH-fluid

Partial melting experiments on a San Carlos peridotite were done in a Walker type multi-anvil press at pressures from 5 to 12.5 GPa. Experiments were done in the presence of a COH-fluid and at oxygen fugacity controlled by the Fe–FeO buffer. Olivine, clinopyroxene, garnet and orthopyroxene are stable in all but the highest temperature 10 GPa experiments where olivine and garnet coexist, and the highest temperature 5 GPa experiments where olivine is the single crystalline phase. The solidus at 5 GPa was found to be at approximately 1,200°C and the liquidus was estimated to be at 1,325°C, which is ∼500°C lower than has been reported for dry melting of peridotite. The aluminum concentration of the melts decreases with increasing melt fraction and decreases also with increasing pressure. At 5 GPa the melts have a CaO/Al2O3-ratio of 0.85–1.0, which is similar to that of undepleted komatiites; major element concentrations are also identical to those of undepleted komatiites such as the Munro komatiites. At 10 and 12.5 GPa the partial melts have CaO/Al2O3-ratios above 1.5 and major element composition almost identical to aluminum depleted komatiites such as the Barberton komatiites. We therefore conclude that in the presence of a reducing COH-fluid both aluminum-depleted and -undepleted komatiites could have formed at temperatures much lower than generally accepted.

Hf isotopic measurements on Barberton komatiites: effects of incomplete sample dissolution and importance for primary and secondary magmatic signatures

Komatiites from a single lava flow, Carl's Flow, in the Schapenburg Greenstone Remnant, near Barberton in South Africa, yielded very different Hf isotope compositions, depending on the manner in which the samples were treated analytically. Data from samples dissolved in Savillex beakers define a statistically significant (MSWD=0.77) isochron with an age of 3.4±0.2 Ga, which is identical within error to the probable eruption age, and an initial ε Hf of −1.3. Analyses of samples dissolved in bombs, on the other hand, plot on a linear array which, if interpreted as an isochron, gives an age of 2.9 Ga. Their initial 176Hf/ 177Hf, calculated using the probable eruption age of 3.45 Ga, are positive and variable, ranging from ε Hf of +1.3 to +6.3. Hf contents are much lower and Lu/Hf ratios systematically higher in the Savillex beaker-dissolved batch of komatiites, indicating that a Hf-rich phase (zircon or baddeleyite?) remained undissolved in these samples. The negative ε Hf of the Savillex beaker-dissolved samples is attributed to interaction with crust-derived fluids that affected the komatiite soon after its eruption. The anomalously low isochron age of the bomb-dissolved samples is due to the presence of a single olivine cumulate sample that appears to have lost Hf during post-emplacement alteration. Without this sample, initial ε Hf values cover a much narrower range between +4.3 and +6.3, which is believed to be that of the mantle source of the komatiites. The combination of highly positive ε Hf and lower-than-chondritic Lu/Hf indicates that the mantle source of the Carl's Flow was garnet-enriched for a substantial period of time prior to komatiite formation and that garnet was retained in the source during the melting event that eventually produced the komatiite magma.

A geochemical and Nd isotopic study of Barberton komatiites (South Africa): implication for the Archean mantle

The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al 2O 3/TiO 2 and CaO/Al 2O 3 ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb) N ratios. In contrast, the Al-depleted komatiites show significantly lower Al 2O 3/TiO 2 ratios of 8–12, highly variable CaO/Al 2O 3 (0.19–2.81) ratios combined with (Gd/Yb) N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. 147Sm/ 144Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite ε i,Nd (3.45) values cluster at +1.9±0.7. Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites. The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al 2O 3/TiO 2, (Gd/Yb) N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb) PMN, (Sm/Yb) PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites.

Paleomagnetic constraints on the Archean geomagnetic field intensity obtained from komatiites of the Barberton and Belingwe greenstone belts, South Africa and Zimbabwe

The intensity of the geomagnetic field during Archean is an important source of information about the evolution of the Earth’s core. Hale [Earth Planet. Sci. Lett. 86 (1987b) 354] reported a very low equatorial paleointensity of 5 μT at ca. 3.5 Ga obtained from komatiites of the Barberton greenstone belt, South Africa, and regarded their remanences as thermal overprints due to a metamorphic event. However, this result has a major drawback since remanences of the Barberton komatiites are carried by magnetites formed during serpentinization that typically results in acquisition of chemical remanent magnetization (CRM). We report the results of detailed paleomagnetic, rock magnetic, and paleointensity investigations using komatiite samples from the Barberton (ca. 3.5 Ga) and Belingwe (ca. 2.7 Ga) greenstone belts in South Africa and Zimbabwe, respectively. Paleodirectional data for Barberton are consistent with results of previous work, and the characteristic remanent magnetization (ChRM) of the Belingwe samples reveals a positive fold test, suggesting that samples from both areas record the Archean geomagnetic field. The main carriers of ChRM are Ti-free magnetites presumed to be akin to single domain grains, and their rock magnetic properties are very stable when subjected to laboratory heating up to 600–700 °C. Our microscopic observations provide support that these are secondary minerals formed during serpentinization. Thellier–Thellier paleointensity experiments for the Barberton and Belingwe samples yielded very low mean virtual dipole moment (VDM) estimates of (1.8±1.3)×1022 A m2 and (1.1±0.9)×1022 A m2, respectively. These correspond to about 24 and 15% of the present day value. Considering the low metamorphic grade for the sampling areas, however, it seems to be somewhat difficult to regard the entire ranges of komatiite ChRM as thermal overprints acquired by reheating during metamorphic events. We hence propose as the most likely scenario the possibility that most fractions of ChRM are survivors of primary grain growth CRM. Taking the theoretical and experimental estimates of the ratio of CRM to thermoremanent magnetization (TRM) into account, this suggests that these low mean VDM values can provide constraints on the lower limits of the geomagnetic field intensities in both greenstone belts at ca. 3.5 and 2.7 Ga.

The Concentration of the Platinum-Group Elements in South African Komatiites: Implications for Mantle Sources, Melting Regime and PGE Fractionation during Crystallization

We have analysed 18 samples of komatiite from five consecutive lava flows of the Komati Formation at Spinifex Creek, Barberton Mountain Land. Our samples include massive komatiite, various types of spinifex-textured komatiite, and flow-top breccias. The rocks have low platinum-group element (PGE) contents and Pd/Ir ratios relative to komatiites from elsewhere, at 0·45–2 ppb Os, 1–1·4 ppb Ir, <1–5 ppb Ru, 0·33–0·79 ppb Rh, 1·7–6 ppb Pt, 1·6–6·1 ppb Pd, and Pd/Ir 3·3. Pt/Pd ratios are c. 1·1. Platinum-group elements are depleted relative to Cu (Cu/Pd = 15 300). They display a tendency to increase in the less magnesian samples, suggesting that the magmas were S-undersaturated upon eruption and that all PGE were incompatible with respect to crystallizing olivine. Komatiites from the Westonaria Formation of the Ventersdorp Supergroup and the Roodekrans Complex near Johannesburg have broadly similar PGE patterns and concentrations to the Komati rocks, suggesting that the PGE contents of South African ultrabasic magmas are controlled by similar processes during partial mantle melting and low-P magmatic crystallization. Most workers believe that the Barberton komatiites formed by relatively moderate-degree batch melting of the mantle at high pressure. Based on the concentration of Zr in the Komati samples, we estimate that the degree of partial melting was between 26 and 33%. We suggest that the low PGE contents and Pd/Ir ratios of all analysed South African komatiites are the result of sulphides having been retained in the mantle source during partial melting. The difference in Pd/Ir between our samples and Al-undepleted komatiites from elsewhere further suggests that the PGE are fractionated during progressive partial melting of the mantle. Thus, our data are in agreement with other recent studies showing that the PGE are hosted by different phases in the mantle, with Pd being concentrated by interstitial Cu-rich sulphide, and the IPGE (Os, Ir, Ru) and Rh resting in monosulphide solid solution included within silicates. Pt is possibly controlled by a discrete refractory phase, as Pt/Pd ratios of most komatiites worldwide are sub-chondritic.

Constraints on the pre-metamorphic trace element composition of Barberton komatiites from ion probe analyses of preserved clinopyroxene

The trace element compositions of igneous augite preserved in metamorphosed komatiite samples from the lower Komati Formation, Barberton Mountainland, South Africa (3.49 Ga), were analyzed in order to estimate the pre-metamorphic bulk composition of the samples. The chemical effects of metamorphism are evaluated by calculating the trace element (TE) composition of a hypothetical augite in equilibrium with the bulk sample composition, and comparing it with ion probe analyses of the cores of augite crystals preserved in that sample. Differences between the calculated and measured composition of the augite indicate chemical gain or loss from the bulk sample during metamorphism. The data indicate that the rare earth elements (REE) and Y were immobile during metamorphism. Immobility is also indicated for Ti and Zr, though chromite fractionation makes this conclusion less firm. Sr and Eu were highly mobile (>50% loss) in all samples studied. Because the REE were immobile, the light REE (LREE) enrichments [(La/Sm)n>1] present in some Barberton komatiite samples are a primary igneous feature, and not the result of post-emplacement metamorphic processes. The variations in (La/Sm)n correlate with Ti/Zr in a strikingly similar manner to variations present in modern boninites, which are high Mg, hydrous subduction zone magmas. Plume-based models that appeal to majorite garnet fractionation cannot produce the low Ti/Zr of komatiites or their wide variation in La/Sm. Thus, we propose that a subduction origin for the Barberton komatiites and basaltic komatiites is more consistent with the available trace element data.

The production of Barberton komatiites in an Archean Subduction Zone

Based upon their geochemical similarity, we propose that the 3.5 Ga Barberton basaltic komatiites (BK) are the Archean equivalents of modern boninites, and were produced by the same melting processes (i.e. hydrous melting in a subduction zone). The Barberton komatiites also share some geochemical characteristics with boninites, including petrologic evidence for high magmatic H2O contents. Experimental data indicates that the Archean sub‐arc mantle need only be 1500–1600°C to produce hydrous komatiitic melts. This is considerably cooler than estimates of mantle temperatures assuming an anhydrous, plume origin for komatiites (up to 1900°C). The depleted mantle residue that generates the Barberton komatiites and BK will be cooled and metasomatised as it resides beneath the fore‐arc, and may represent part of the material that formed the Kaapvaal cratonic keel.

Hf isotope compositions of komatiites

Many of the oldest ∼3.5 Ga komatiites have low Al2O3/TiO2 and depleted HREE, features that are attributed to garnet fractionation at some stage during their formation. It is not known, however, whether this fractionation took place at the time of melting, or well before, perhaps during the crystallization of an early magma ocean. We report here the Hf isotope compositions of four Al-depleted komatiites from the 3.45 Ga Barberton Greenstone Belt in South Africa and six Al-undepleted komatiites from 2.7 to 1.9 Ga belts in Canada and Zimbabwe. We also measured the Hf isotope compositions of five tholeiites from Barberton and two tholeiites from the 1.9 Ga Canadian Belt, to provide information about the composition of ambient Precambrian mantle. All the komatiites have high initial 176Hf/177Hf ratios. All initial ɛHf values are positive, in the range of +2.6 to +7.8. Almost this entire range is also present in the 3.45 Ga Barberton komatiites (+2.6 to +7.3). ɛHf(T) correlates with ɛNd(T) and defines a crude linear array that coincides with compositions of both modern oceanic basalts and of Phanerozoic to Archean juvenile crustal rocks. The composition of Bulk Silicate Earth [J. Blichert-Toft, F. Albarède, Earth Planet. Sci. Lett. 148 (1997) 243–258] plots below this array, at slightly lower ɛHf. To explain the positive initial ɛHf values of 3.45 Ga Barberton komatiites, the source must have had high Lu/Hf. This implies a source enriched in garnet, a result contrary to earlier models in which the low Al2O3/TiO2 and low Lu/Hf of this type of komatiite was thought to be inherited directly from mantle depleted in garnet. The low Al2O3/TiO2 and low Lu/Hf are features of the primary komatiite magmas, not of their mantle source, and result from garnet fractionation during komatiite formation. To explain the bias to high initial ɛHf values of the komatiites and other terrestrial rocks, we consider two alternatives: (a) a reservoir with low Lu/Hf formed very early in Earth history; (b) accepted values for the Hf isotopic composition of Bulk Silicate Earth are inappropriate and the actual composition has higher 176Hf/177Hf and lower 176Lu/177Hf. A problem with the latter interpretation is that the proposed composition lies off the achondrite meteorite isochron, which implies that the Earth has a different age, or accreted from material with composition different from that of meteorites.

Emplacement conditions of komatiite magmas from the 3.49 Ga Komati Formation, Barberton Greenstone Belt, South Africa

This paper provides new constraints on the crystallization conditions of the 3.49 Ga Barberton komatiites. The compositional evidence from igneous pyroxene in the olivine spinifex komatiite units indicates that the magma contained significant quantities of dissolved H2O. Estimates are made from comparisons of the compositions of pyroxene preserved in Barberton komatiites with pyroxene produced in laboratory experiments at 0.1 MPa (1 bar) under anhydrous conditions and at 100 and 200 MPa (1 and 2 kbar) under H2O-saturated conditions on an analog Barberton composition. Pyroxene thermobarometry on high-Ca clinopyroxene compositions from ten samples requires a range of minimum magmatic water contents of 6 wt.% or greater at the time of pyroxene crystallization and minimum emplacement pressures of 190 MPa (6 km depth). Since high-Ca pyroxene appears after 30% crystallization of olivine and spinel, the liquidus H2O contents could be 4 to 6 wt.% H2O. The liquidus temperature of the Barberton komatiite composition studied is between 1370 and 1400°C at 200 MPa under H2O-saturated conditions. When compared to the temperature-depth regime of modern melt generation environments, the komatiite mantle source temperatures are 200°C higher than the hydrous mantle melting temperatures inferred in modern subduction zone environments and 100°C higher than mean mantle melting temperatures estimated at mid-ocean ridges. When compared to previous estimates of komatiite liquidus temperatures, melting under hydrous conditions occurs at temperatures that are ∼ 250°C lower than previous estimates for anhydrous komatiite. Mantle melting by near-fractional, adiabatic decompression takes place in a melting column that spans ∼ 38 km depth range under hydrous conditions. This depth interval for melting is only slightly greater than that observed in modern mid-ocean ridge environments. In contrast, anhydrous fractional melting models of komatiite occur over a larger depth range (∼ 130 km) and place the base of the melting column into the transition zone.

The influence of alteration on the trace-element and Nd isotopic compositions of komatiites

To investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites, we studied samples from the Alexo and Texmont regions in the 2.7-Ga Abitibi belt of Canada and from the Weltevreden, Mendon and Komati Formations of the 3.2–3.5-Ga Barberton belt of South Africa. Particular emphasis was placed on multiple samples from individual layered spinifex-olivine cumulate flows, the argument being that if these flows showed variations in the ratios of elements incompatible with olivine, then these variations were most likely due to chemical mobility during alteration. Data for rare-earth (REE) and high-field-strength elements (HFSE) reveal anomalies (non-chondritic HFSE/REE ratios), particularly in the most altered samples. After taking into account the limits of analytical precision, these anomalies are attributed to element mobility, not to fractionation of high-pressure minerals, as suggested by other authors for Abitibi komatiites. In Barberton komatiites three separate processes produced HFSE anomalies: element mobility during alteration, crust assimilation in some samples, and majorite fractionation. The effects of the latter process were recognized from systematic relationships between HFSE/REE and Al/Ti that coincided with calculated majorite fractionation trends. Initial Nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes. The initial ϵ Nd of pyroxene from the 2.7-Ga Alexo komatiite is +3.8, slightly higher than in whole rocks ( +0.6 to +3.5, average ∼ +2.5). In the carbonatized Texmont samples, the range in initial ϵ Nd is far greater ( −6.2 to +8.8). The variable initial ϵ Nd-values in the rocks are attributed to isotopic exchange of Nd with surrounding rocks during early alteration, and fractionation of Sm/Nd during later events. Pyroxene from 3.3-Ga Barberton komatiitic basalt had initial ϵ Nd of + 2.3.

The origin of fluids and the effects of metamorphism on the primary chemical compositions of Barberton komatiites: New evidence from geochemical (REE) and isotopic (Nd, O, H, [formula omitted]) data

Numerous greenstone relics, all containing the two lowermost formations of the Onverwacht Group, occur in the Archean trondhjemitic/tonalitic gneiss terrains south of the Barberton Greenstone Belt. In this study, we report detailed petrological, geochemical and isotopic (Nd, O, H, 40Ar 39Ar ) data obtained on komatiites from the Schapenburg Greenstone Remnant (SGR), the largest and best-preserved greenstone relic. The main goals are 1. (1) to date the metamorphism affecting the SGR using the 40Ar 39Ar dating method on amphiboles, 2. (2) to evaluate the effect of metamorphism on the preservation of primary isotopic and chemical signatures, 3. (3) to estimate the temperature and water/rock ratios that prevailed during metamorphic recrystallization in order to constrain the composition and origin of the reacting fluid phase. 40Ar 39Ar ages of 2.9 Ga obtained on two amphibole separates from the Schapenburg metavolcanics reveal the existence of a metamorphic event younger than the emplacement age (3.5 Ga). This metamorphic event belongs to a series of discrete periods of thermal activity from 3.4 to 2 Ga, each of which coincides with a major episode of magmatic activity. The ultrabasic lava flows acquired their δ 18O values (from +3.2 to +5%.) at high temperature (≈450°C) under high water/rock ratios. The reacting water had initial isotopic values typical of metamorphic fluids ( δ 18 O = +5 to +7%.; δD = −65 to −50%.). REE patterns were not disturbed by metamorphic recrystallization. Despite the long time interval between emplacement and metamorphism (≈600 Ma), ϵ Nd ( T) values are uniform throughout the whole magmatic suite, indicating that the Sm-Nd system was closed on the sample scale during metamorphism. The mantle source of these greenstones was depleted in LREE as evidence by ϵ Nd ( T) ≈ +2.5. Chemical fluxes during metamorphism were calculated for elements unfractionated by olivine removal (e.g. Na, Ca, Ti, Al, and Sr), by normalizing to Nd. They suggest a significant mobility of most major elements in the cumulate zones of the lava flows. By contrast, spinifex zones appear to have preserved most of their primary chemical signatures during metamorphic recrystallization. Their CaO Al 2O 3 and Al 2O 3 TiO 2 ratios can be used with confidence to determine the PT conditions of melting in the mantle source. A general model of water-rock interactions applied to the sedimentary and magmatic rocks of the Onverwacht Group is also presented. The model involves conditions of metamorphism deduced from this study and available data from the literature. In addition to metavolcanic rocks, most of the oxygen isotope compositions of carbonates and cherts (except the highest values) can be explained by reequilibration with metamorphic fluids under greenschist-amphibolite facies conditions.

Depth and degree of melting of komatiites

High pressure melting experiments have permitted new constraints to be placed on the depth and degree of partial melting of komatiites. Komatiites from Gorgona Island were formed by relatively low degrees of pseudoinvariant melting (&lt; 30%) involving L + Ol + Opx + Cpx + Gt on the solidus at 40 kbar, about 130 km depth. Munro‐type komatiites were separated from a harzburgite residue (L + Ol + Opx) at pressures that are poorly constrained, but were probably around 50 kbar, about 165 km depth; the degree of partial melting was &lt;40%. Komatiites from the Barberton Mountain Land were formed by high degrees (∼50%) of pseudoinvariant melting (L + Ol + Gt + Cpx) of fertile mantle peridotite in the 80‐ to 100‐kbar range, about 260‐ to 330‐ km depth. Secular variations in the geochemistry of komatiites could have formed in response to a reduction in the temperature and pressure of melting with time. The 3.5 Ga Barberton komatiites and the 2.7 Ga Munro‐type komatiites could have formed in plumes that were hotter than the present‐day mantle by 500° and 300°, respectively. When excess temperatures are this size, melting is deeper and volcanism changes from basaltic to komatiitic. The komatiites from Gorgona Island, which are Mesozoic in age, may be representative of komatiites that are predicted to occur in oceanic plateaus of Cretaceous age throughout the Pacific [Storey et al., 1991].

The petrography and geochemistry of komatiite flows from the Abitibi Greenstone Belt and a model for their formation — Comments

To investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites, we studied samples from the Alexo and Texmont regions in the 2.7-Ga Abitibi belt of Canada and from the Weltevreden, Mendon and Komati Formations of the 3.2–3.5-Ga Barberton belt of South Africa. Particular emphasis was placed on multiple samples from individual layered spinifex-olivine cumulate flows, the argument being that if these flows showed variations in the ratios of elements incompatible with olivine, then these variations were most likely due to chemical mobility during alteration.Data for rare-earth (REE) and high-field-strength elements (HFSE) reveal anomalies (non-chondritic HFSE/REE ratios), particularly in the most altered samples. After taking into account the limits of analytical precision, these anomalies are attributed to element mobility, not to fractionation of high-pressure minerals, as suggested by other authors for Abitibi komatiites.In Barberton komatiites three separate processes produced HFSE anomalies: element mobility during alteration, crust assimilation in some samples, and majorite fractionation. The effects of the latter process were recognized from systematic relationships between HFSE/REE and Al/Ti that coincided with calculated majorite fractionation trends.Initial Nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes. The initial ϵNd of pyroxene from the 2.7-Ga Alexo komatiite is +3.8, slightly higher than in whole rocks ( +0.6 to +3.5, average ∼ +2.5). In the carbonatized Texmont samples, the range in initial ϵNd is far greater ( −6.2 to +8.8). The variable initial ϵNd-values in the rocks are attributed to isotopic exchange of Nd with surrounding rocks during early alteration, and fractionation of Sm/Nd during later events. Pyroxene from 3.3-Ga Barberton komatiitic basalt had initial ϵNd of + 2.3.