Peridotitic Komatiites Research Articles

Mantle heterogeneity and crust-mantle interaction in the Singhbhum craton, India: New evidence from 3340 Ma komatiites

The Singhbhum Craton in eastern India preserves the records of Paleoarchean - Mesoarchean oceanic crust dominated by ultramafic-mafic extrusive rocks (greenstones), and sialic rafts composed of tonalite-trondhjemite-granodiorite, together with younger granitoids constituting the composite Singhbhum granite batholith. The greenstone sequences, intruded by younger granitoids are entrained as synformal keels in the batholith. The lower greenstone sequence of eastern Iron Ore Group (IOG) is composed of peridotitic komatiite, and komatiitic basalt with minor chert and BIF, whereas the upper sequence has tholeiite to tholeiitic andesite. Here we report results from first direct dating of the Karanjharan spinifex-textured komatiite from the lower part of Badampahar greenstone sequence of IOG. LA-ICPMS U-Pb analysis of zircon grains in the komatiites yielded consistent weighted mean 207Pb/206Pb age of 3340 Ma, with closely similar Hf depleted mantle age (TDM) between 3357 Ma and 3432 Ma. The high MgO (25–35%), and low Al2O3/TiO2 (<10) and CaO/Al2O3 (2.3–3.7) ratios in the serpentinized Karanjharan komatiites suggest strong similarity with the Al-depleted komatiites from Barberton Mountainland, South Africa. The Badampahar komatiites have Gd/YbN in the range 2–3, and show depletion in incompatible elements including LREE, slight enrichment of MREE and near chondritic HREE, suggesting derivation from a depleted mantle source with garnet in the residue. Juvenile Paleoarchean mantle source is also suggested by positive εHf(t) (3.0–4.6) and broadly similar 207Pb/206Pb age and TDM age. Geochemical features of basalts from the upper greenstone sequence overlap with those of the lower greenstone sequence, but are more enriched in HFSE (Th, U) and LREE possibly due to contamination with continental crust or oceanic sediments. The variation in Al2O3/TiO2 ratios as well as Gd/YbN ratios from the Karanjharan komatiite and other coeval komatiites in the eastern IOG suggest Paleoarchean mantle heterogeneity and/or melt extraction from different depths.

Mineralogy and Geochemistry of Hydrothermally altered Talcose rocks from Ila Orangun-Oyan areas, part of Southwestern Nigeria

Background/Objectives: Talcose rocks within Precambrian Basement Complex serve as relics of Archean greenstones. alterations associated with polycyclic-orogenies that affected this complex is studied to understand mineralogical and geochemical alterations. Methods: Five fresh samples of talcose rocks were collected during field mapping. These samples were cut into thin sections to reveal modal mineralogy, altered minerals and degree of alteration of such minerals. Mineral phase identification of the talcose rocks was conducted using X-ray Broker D8 ADVANCE diffractometer while whole rock analysis was carried out using Inductively Coupled Plasma Mass Spectrometry. Findings: Lithological relationship revealed from field evidence showed that the talc bodies occurred in close association with micaceous schist. The mineral assemblage of talc, tremolite, actinolite, chlorite and calcite suggest low grade greenschist metamorphic facies from possible hydrothermal alteration. Geochemical results revealed the following range of concentrations; SiO2 42.19-59.03%; Al2O3 1.1 - 11.8%; Fe2O3 7.64-9.56%; MgO 24.47-26.639%; Ni 594-1207ppm; Co 43.2-113.9ppm; Sn 6-41ppm; V 32-75ppm and Zr 1.3-58.7ppm, and these are typical of talcose rocks. Petrogenetic studies suggest a komatiitic origin with a peridotitic komatiite precursor for the talc-chlorite-tremolite schist. Enrichment in LREE, depletion in HREE and a negative Eu anomaly suggest alteration of the parent magma for the talcose rock and plagioclase fractionation. The trends observed for the LILE, HFSE and REE suggest possible contamination or mixing of crustal and mantle materials during the formation of the protolith. Ni and Co concentrations are higher than average crustal values with implication for ultrabasic to basic magma composition for the komatiitic progenitor and also suggestive of possible mineralisation. Conclusion: Mineralogical examination has revealed a talc-chlorite-tremolite composition for the talcose rocks with peridotitic komatiite precursory while geochemical composition supported ultrabasic magmatism similar to those with the Ilesha schist belt.

Compositional Features and Industrial Assessment of Talcose Rocks of Itagunmodi-Igun Area, Southwestern Nigeria

Talcose rocks from Itagunmodi and Igun areas within the Ilesa Schist Belt, southwestern Nigeria, were studied to determine their compositional affinities and industrial properties. The talcose bodies occurred as lensoid enclaves closely associated with amphibolites and quartz-mica schist. Petrographic and X-ray diffraction-aided mineralogical studies reveal that the talcose rocks are of the talc-chlorite schist variety composed predominantly of talc (average 73.63%), subordinate chlorite (average 16.08%) and accessory lizardite (average 6.5%). Total whole rock characterization of the samples using Lithium Fusion digestion revealed high mean concentrations of SiO2 (57.53%), MgO (24.84%) and Fe2O3 (7.73%) with significant enrichment of Ni, Co and Cr; and low values of Ba, Rb, Sr, V, Cu and Pb. The talc-chlorite schists plotted in the peridotitic komatiite field of the Al2O3-(FeO + TiO2)-MgO diagram. Water Absorption Capacity ranges from 6.25% - 8.20%, Loss on Ignition is 4.8% - 6.1% while Linear Shrinkage is 1.25% - 1.70% and firing colour is brown to dark-brown. The overall compositional features of the talcose rocks of the area, which were hitherto uncharacterized, show their suitability as raw materials for paint, coloured pottery, ceramic insulation, textile, rubber and plastic manufacture with varying beneficiation requirements. Their high trace element content however, makes them unsuitable for use in pharmaceutical and cosmetic applications.

Volcanological and petrogenetic characteristics of komatiites of the 3.3 Ga Saw Mill Complex, Weltevreden Formation, Barberton Greenstone Belt, South Africa

Abstract The Saw Mill Complex (SMC) is a 1 275 m thick layered komatiitic sequence in the 3.3 Ga Weltevreden Formation, uppermost stratigraphic unit of the Onverwacht Group in the northern part of the Barberton Greenstone Belt, South Africa. A series of ultramafic complexes in the Weltevreden Formation have previously been interpreted as layered ultramafic intrusions, consisting of thick, layered ultramafic units of peridotite, pyroxenite, dunite, and gabbro. However, recent work on the Pioneer complex of the Weltevreden Formation has demonstrated an extrusive origin of komatiitic flows and tuffs. The Weltevreden Formation has not been studied in the detail of other Onverwacht Group units, but it is likely composed of a number of individual tectonically juxtaposed terrains brought in by thrust faulting during deposition of Fig Tree Group sediments and felsic volcanics in a magmatic arc setting post-dating the Onverwacht mafic and ultramafic units, which are currently regarded as products of plume-based eruptive centers. This study represents an ongoing effort to elucidate the structure, stratigraphy, and petrogenesis of the Weltevreden Formation. A new 1:1000 scale map and stratigraphic section of the SMC reinterpret the igneous complex as extrusive, with lithologic units interpreted as layered komatiitic flows and interbedded tuffs. Flows accumulated olivine, forming distinct layers from bottom to top of: olivine adcumulate (dunitic lithologies),poikilitic orthopyroxene with abundant olivine inclusions (pyroxenitic lithologies), andolivine orthocumulate (peridotitic lithologies), and sometimes with a cap ofolivine-depleted pyroxene-rich orthocumulates (komatiitic basalts). The chemical variation of flows is almost entirely controlled by olivine accumulation, with dunitic komatiites representing up to 70% accumulated olivine and 30% trapped liquid, and peridotitic komatiites representing only 25% accumulated olivine, while spinifex-textured komatiites are representative of quenched liquid. Minor (up to 1%) chromite accumulation partly affects bulk chemistry, while orthopyroxene accumulation may have an effect on bulk chemistry of SMC pyroxentitic komatiites. Layered flows are up to 120 m thick and some display spinifex-textured chilled upper margins. The original liquid was near primitive mantle composition that likely represents a two-stage melting process. The SMC was rapidly emplaced and was not near a major sedimentary depositional system, as no sediment other than tuff was deposited. Flows preserved in the SMC dominantly formed as open flow pathways, allowing for large quantities of olivine to accumulate in the lower portions of flows. Some flows formed as closed systems, possibly as a result of deforming tuffs below the flows to form lava lakes. Large cm-sized lapilli, including those with aerodynamic shapes, suggest subaerial explosions and possibly near-vent environments (though littoral rootless vents can develop 100’s of km’s from actual volcanic vents). Pillow structures have not been observed in the SMC. Geochemically, the SMC is very similar to Weltevreden Formation komatiites recently studied in the Pioneer complex. They have Al2O3/TiO2 near 30, Gd/Lu normalized values between 0.8 and 1.2, maximum liquid compositions of approximately 33% MgO, and olivines up to Fo94. Oxygen fugacity of the mantle source is most likely to have change in log(fO2) of 0.00 relative to nickel-nickel-oxide buffer. SMC komatiites likely erupted at temperatures in excess of 1615°C based on MELTS modeling, and represent some of the hottest volcanism experienced on the early Earth.

Magnesium‐rich crustal compositions on Mercury: Implications for magmatism from petrologic modeling

We have conducted petrologic modeling of MESSENGER‐derived compositions and analog compositions to gain a better understanding of the petrogenesis of the crust of Mercury. Analog compositions included a 1425°C partial melt of the Indarch (EH4) meteorite and a range of Mg‐rich terrestrial rocks (magnesian basalt, basaltic komatiite, and peridotitic komatiite). All models were held at the iron‐wüstite buffer to simulate the reducing conditions that likely existed during Mercury's formation. We then compared modeled mineral compositions and abundances, liquidus temperatures, and viscosities to better constrain the characteristics of the lavas that erupted on Mercury's surface. Our results show that the surface composition of Mercury is most similar to that of a terrestrial magnesian basalt (with lowered FeO), composed mainly of Mg‐rich orthopyroxene and plagioclase. Because the model magmas are Mg‐rich, their counterparts on Mercury would have erupted at high temperatures and displayed low viscosities. Producing melts of these compositions would have required high temperatures at the mantle source regions on Mercury. The inferred low‐viscosity lavas would have erupted as thin, laterally extensive flows (depending upon their effusion rate) and would be expected to display surficial flow features that might be preserved to the present.

Komatiite within Chhotanagpur Gneissic Complex at Semra, Palamau District, Jharkhand: Petrological and geochemical fingerprints

Komatiite near Semra village, southwest of Daltonganj in Palamau District of Jharkhand, occurs within tremolite actinolite schist of ultramafic parentage. The fragmented olivine phenocrysts show mutually parallel as well as angular alignment, representing relict spinifex texture. Mineralogically the Semra ultramafic is lehrzolite in composition. The cumulates lack visible deformation suggesting original magmatic crystallization of these ultramafic rocks. The present occurrence of Spinifex Textured Peridotitic Komatiite (STPK) in Chhotanagpur Gneissic Complex (CGC) at northwestern part of Eastern Indian Shield is of great significance as it opens up a possibility of the presence of Archaean rock in CGC, which is yet to be established. These ultramafics are geochemically characterised by distinctive high MgO, Ni, Cr and poor in alkali, TiO2, Ba, Cs, Rb, Nb, Hf and Y contents. It has low abundance of incompatible elements and is LREE depleted [(La/Yb)n= 0.74–1.07] with enriched flat HREE profile, representing chondrite like composition. This shows diagnostic convex upward REE profile. All these together with relict but distinct spinifex texture confirm the komatiitic character of Semra ultramafic. The chemical data plots for ultramafics confine to komatiite field in discriminate diagrams. The Mg# (>74) of the Semra ultramafics are comparable to primitive upper mantle Mg# (89.8) and high enough for rocks derived from ultramagnesian liquid of the mantle derived source. Considerably high Zr concentration in Semra ultramafic is attributed to Zr-enriched mantle source. Geochemically Semra ultramafics is comparable with “Al-undepleted” Munro type komatiites. Depleted incompatible trace elements also point towards Al-undepleted nature for the investigated ultramafics, which are characterised by distinctive low SiO2 content in comparison to many well known komatiites of the world. The low K2O content indicate plume related magmatism for its generation. Tectonic setting of this STPK with distinctive cumulus nature is suggestive of its emplacement in an extensional tectonic regime.

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D 1, D 2 and D 3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D 3. The orebodies constitute sulfide-bearing D 1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage. Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H 2O-rich containing CO 2 + Na + and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H 2O is indicated. The CO 2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H 2O to CO 2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.

Petrology and Geochemistry of MORB and OIB in the Mid-Archean North Pole Region, Pilbara Craton, Western Australia: Implications for the Composition and Temperature of the Upper Mantle at 3.5 Ga

It is necessary and important to determine the tectonic setting of mafic volcanism, because compositions of magma are strongly related to tectonic setting. Our recent geological investigation of the North Pole (NP) region (3.5 Ga), Pilbara, showed that the greenstone belt was an accretionary complex, defined by oceanic-plate stratigraphy and duplex structure. Moreover, application of accretionary geology reveals that there are two types of tholeiitic basalts from different tectonic settings: midocean ridge (MORB) and oceanic island (OIB). This work presents geochemistry of major, trace, and rare-earth elements (REE) of greenstones originating as MORB and OIB in the mid-Archean. Relict igneous clinopyroxenes (Cpx) in NP MORB and OIB have compositional variations equivalent to chemical variations of Cpx in modern MORB and OIB. In addition, TiO2 content and Mg# = (MgO/(MgO+FeO*)) in the host magma calculated from those in Cpx are consistent with whole-rock compositions, indicating preservation of original compositions in terms of TiO2, FeO, and MgO contents. The Cpx in NP MORB and OIB show LREE-depleted and slightly LREE-depleted REE patterns, respectively. Calculated REE patterns of host magmas also are in agreement with the wholerock compositions. Analyses of 64 alteration-free greenstones selected from 2662 samples indicate that NP MORB is enriched in Al2O3 and depleted in TiO2, FeO*, Y, and Zr at any MgO content, compared with NP OIB. In addition, NP MORB show a slightly LREE-depleted, whereas NP OIB forms a slightly LREE-enriched REE patterns. Such differences suggest that the Archean mantle already was differentiated. However, the compositional difference between the source mantles is relatively small, and differentiation into MORB and OIB source mantles was in the process of development. NP MORB are enriched in FeO, compared with modern MORB. Comparison of NP MORB with recent melting experiments shows that the source mantle of the NP MORB had Mg# = 85-87 and was enriched in FeO relative to the modern upper mantle. A possible mechanism to decrease the FeO content in the mantle is precipitation of metallic iron at the point of slab penetration into the lower mantle through time. The potential temperature of the source mantle of NP MORB is estimated to be ~1500 °C, indicating that even the mid-Archean mantle was hotter by at most ~150-200°C than the modern mantle, in contrast to the estimate from peridotitic komatiites.

Heavy metal behavior associated with soil cover used in the Bandeirantes Sanitary landfill, São Paulo District

spilitization, epidotization, keratophyrization and silicification. Basaltic andesites show actinolite (substituting clinopyroxene), chlorite, albite, epidote, quartz and opaques. Porphyritic types bear relic phenocrysts of magmatic hornblende. Keratophyrized dacites and rhyolites show quartz, albite, chlorite, epidote, zircon and opaques. Spinifex-textured MBVU-rocks have been referred to previously as komatiites or basaltic komatiites; this is not sustained mineralogically nor geochemically. These rocks present tremolite-actinolite, chlorite, albite, talc, epidote, carbonate and opaques. Pseudomorphosed acicular clinopyroxene spinifex textures predominate by far over those of bladed olivine, typical of (peridotitic) komatiites. SiO2 reaching the range of intermediate rocks, low TiO2 (0.25-0.40 wt-%), and Ti/Zr∼ 60 (50-75) relegate these rocks to siliceous magnesian basalts or low-Ti tholeiites linked to boninites, rather than komatiites with Ti/Zr > 100 (Coish 1989). These rocks may represent extrusive equivalents of the most primitive magmas, from whichAVU-andesites differentiated in infracrustal magma chambers. Differentiated magnesian basaltic flows show a spinifex-textured andesitic top and massive ultramafic cumulate base made up of pseudomorphosed euhedric medium-grained olivines and rare relic orthopyroxenes in a tremolite-chlorite intercumulus matrix with chromite as a constant magmatic intercumulus phase. The BVU consists mainly of basic Fe(-Mg) tholeiitic flows. All three volcanic units contain subordinate volcaniclastic and chemical intertrap metasediments. — ( December 14, 2001 ) .

PALEOSEA-FLOOR VOLCANIC-ASSOCIATED MASSIVE SULFIDE MINERALIZATION RELATED TO A COOLING KOMATIITE FLOW, ABITIBI SUBPROVINCE, CANADA

Paleosea-floor, volcanic-associated massive sulfide mineralization has been discovered 50 to 150 m upsection from a Late Archean peridotitic komatiite flow, where the flow is thickest in a thermal erosion channel. The Terminus massive sulfide occurrence is within carbonaceous argillites that are intercalated with basaltic komatiite flows and flow breccias. It contains appreciable Zn and Cu contents and is associated with silicification and chlorite-epidote alteration. Ultramafic magmas are ideal heat sources for hydrothermal convective systems because of their extremely high liquidus temperatures and their tendency to become less permeable during serpentinization, which impedes advective cooling. Heuristic calculations indicate that there is sufficient enthalpy in the peridotitic komatiite flow to drive the hydrothermal system responsible for the Terminus mineralization. This hypothesis is tested using two-dimensional, finite element heat and fluid-flow simulations. The results suggest that, under the right circumstances (1) relatively thick komatiite flows may take hundreds of years to cool; (2) komatiite flow-driven hydrothermal systems can form sea-floor or subsea-floor deposits comparable in mass to those along the mid-Atlantic Ridge; (3) much of the metal leaching by hydrothermal fluids occurs below, and adjacent to, the flow; and (4) the shape of heat source komatiite flows, or subsea-floor magma chambers, affects the locus and distribution of hydrothermal upwelling. Similar, base metal-precipitating paleohydrothermal systems are present above ponded komatiite flows broadly along strike at the Potter Zn-Cu deposit in Munro township, Ontario. These paleohydrothermal systems, and the calculations presented here, suggest that thick, ponded basaltic flows on the modern sea floor have sufficient enthalpy to form significant hydrothermal sulfide accumulations.

METAKOMATIITIC AND META-ULTRAMAFIC ROCKS FROM THE RIO MANSO REGION, MINAS GERAIS: GEOLOGY TEXTURES AND METAMORPHISM

The study area is located in the southwestern part of the "Quadrilátero Ferrífero", south of Serra do Curral, Minas Gerais (Brazil). Three major units occur in the area: Granite-Gneiss Complex, Nova Lima Group and intrusive bodies. The Nova Lima Group consists of metakomatiite and metakomatiitic basalt, meta-ultramafic rocks devoid of spinifex texture, rare iron formation, amphibolites and metasedimentary rocks. It comprises mainly meta-ultramafic rocks without primary igneous texture relicts and subordinate metakomatiitic rocks. Metakomatiites occur only in Morro da Onga, and display spinifex texture, especially of the random type. Thin layers interpreted as probable metacumulate parts of flows (B zone) occur locally. A structure that may correspond to a komatiite lava tube is also exposed, Metakomatiites are typically peridotitic komatiites and subordinately komatiitic metabasalts; the latter may exhibit pillow-lava structures. Progressive metamorphic evolution began with a Mg-chlorite + tremolite-actinolite paragenesis. In a second stage orthopyroxene and olivine porphyroblasts were developed in a Mg-hornblende groundmass. Progressive increase of Al in chlorite contributed to olivine and rare anthophyllite blastesis in the initial phases of the progressive metamorphism. Late metasomatism led to serpentinization and talcification. Amphibole plays an important role when the modal/textural relations of the highest-grade metamorphic parageneses are considered. Al enrichment of Ca-amphibole through tschermak and edenite replacements led to consumption of olivine to yield pyroxene and to spinel consumption through its incorporation as the aluminous component of amphibole. Thus the area consists of abundant meta-ultramafic rocks devoid of igneous texture relicts and subordinate komatiitic flows with thin cumulate zones. The former may partly correspond to an ultramafic layered sequence.

METAKOMATIITIC AND META-ULTRAMAFIC ROCKS FROM THE RIO MANSO REGION, MINAS GERAIS: GEOLOGY TEXTURES AND METAMORPHISM

The study area is located in the southwestern part of the Quadrilatero Ferrifero, south of Serra do Curral, Minas Gerais (Brazil). Three major units occur in the area: Granite-Gneiss Complex, Nova Lima Group and intrusive bodies. The Nova Lima Group consists of metakomatiite and metakomatiitic basalt, meta-ultramafic rocks devoid of spinifex texture, rare iron formation, amphibolites and metasedimentary rocks. It comprises mainly meta-ultramafic rocks without primary igneous texture relicts and subordinate metakomatiitic rocks. Metakomatiites occur only in Morro da Onga, and display spinifex texture, especially of the random type. Thin layers interpreted as probable metacumulate parts of flows (B zone) occur locally. A structure that may correspond to a komatiite lava tube is also exposed, Metakomatiites are typically peridotitic komatiites and subordinately komatiitic metabasalts; the latter may exhibit pillow-lava structures. Progressive metamorphic evolution began with a Mg-chlorite + tremolite-actinolite paragenesis. In a second stage orthopyroxene and olivine porphyroblasts were developed in a Mg-hornblende groundmass. Progressive increase of Al in chlorite contributed to olivine and rare anthophyllite blastesis in the initial phases of the progressive metamorphism. Late metasomatism led to serpentinization and talcification. Amphibole plays an important role when the modal/textural relations of the highest-grade metamorphic parageneses are considered. Al enrichment of Ca-amphibole through tschermak and edenite replacements led to consumption of olivine to yield pyroxene and to spinel consumption through its incorporation as the aluminous component of amphibole. Thus the area consists of abundant meta-ultramafic rocks devoid of igneous texture relicts and subordinate komatiitic flows with thin cumulate zones. The former may partly correspond to an ultramafic layered sequence.

99/03117 Kinetic behavior of the Ru/TiO 2 catalyst in the reaction of partial oxidation of methane

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.

GEOLOGY AND GEOCHEMISTRY OF THE BOA VISTA NICKEL SULFIDE DEPOSIT, CRIXÁS GREENSTONE BELT, CENTRAL BRAZIL.

The Boa Vista nickel-sulfide deposit is located at the NW edge of the late Archean Crixas greenstone belt. The nickel-sulfide deposit is associated with a narrow 7 km-long EW-trending sequence of meta-ultramafic and metabasic rocks. The volcanic sequence was overprinted by regional ductile deformation and associated metamorphism but primary volcanic structures and textures are preserved within low-strain zones. Two types of ultramafic flows are recognized in these zones (spinifex-textured flows and unsettled flows). The ultramafic rocks exhibit some of the classical geochemical and textural features that become the main criterion for the recognition of komatiitic lavas. The MgO content (from 22.1 to 28.7 wt. %) on spinifex-textured and unsettled flows is typical for peridotitic komatiites. Geochemical variations of major elements indicate the importance of olivine fractional crystallization. Scattering of the data suggests that regional metamorphism has chemically modified the ultramafic rocks. The sulfide mineralization is always present at the lowest part of an ultramafic sequence overlying a mafic sequence. The thickness of the mineralized horizon is variable but it is usually less than few-meters thick. Four types of ore are recognized in the Boa Vista deposit. In order of decreasing abundance they are: stringer ore, disseminated ore, massive ore and matrix ore. The sulfide ore mineralogy is typical of magmatic sulfides associated with ultramafic komatiites. It consists mainly of pyrrhotite ( 70 vol. %) associated with pentlandite and chalcopyrite and minor magnetite and sphalerite. The Fe-Ni-S variation of the ore is consistent with a primary magmatic origin for the nickel mineralization. Analyses of sulfide ore recalculated to 100 % sulfides plot within the field of the monosulfide solid solution at high temperature. The Ni/Cu ratio of the ore is high (± 10) and comparable to most Archean KHNS deposits. The Boa Vista deposit provided the first opportunity to study nickel-sulfide mineralization processes in the Crixas greenstone belt. Considering that KHNS deposits occuring in greenstone belts worldwide tend to be restricted to specific ultramafic units, geological and petrological studies of the Boa Vista deposit provide important clues for future nickel-sulfide exploration work in the extensive greenstone belt terranes of central Brazil.

Asteroids and early precambrian crustal evolution

Major enigmas arising from the Archaean crustal record are capable of resolution in terms of the combined role of episodic mega-impacts and of long-term mantle processes. The terrestrial impact record suggests that, for very large impact structures, the degree of preservation of proximal deformation effects decreases with the increase in the magnitude of the associated thermal effects—a consequence of extensive recrystallization and melting. This focuses attention on thermal igneous and metamorphic peaks as possible clues of impact events. Isotopic age studies define principal thermal events about 3.5, 3.0 and 2.7 (±0.1) Gyr and less well defined episodes, succeeded by protracted thermal fluctuations in infracrustal high-grade metamorphic zones. The bulk of the igneous activity associated with these episodes took place over relatively short time spans of c.20–30 × 10 6 years. Interpretations of these episodes in terms of internal dynamics of the Earth are inconsistent with the thermal behavior of silicate rheologies in a continuously convecting mantle regime. A triggering of these episodes by the mantle rebound response to intermittent extra-terrestrial asteroid impacts is considered on the basis of (1) identification of major impacts from distal ejecta horizons marked by iridium anomalies and impact/earthquake-triggered diamictite breccia and olistostromes; (2) the relationships between microtektite horizons and volcanic activity; (3) geochemical and experimental evidence for mantle upwelling from the transition zone and catastrophic adiabatic melting required to generate peridotitic komatiites. Episodic differentiation/accretion growth of sial consequent on these events is capable of resolving the volume problem which arises from comparisons between modern continental crust and sial produced by continuous two-stage mantle melting processes, taking the temporal decline in heat flow into account. Impact shock effects on the Archaean crust have been largely obscured by (1) outpouring of voluminous basic/ultrabasic lavas, inundating shock-deformed crust and extending beyond the perimeters of impact excavated basins: (2) gravity subsidence and downfaulting of terrestrial mare, accounting for the anatexis of sub-greenstones basement; (3) extensive shearing and recrystallization of impact structures, breccias and mineral deformation features beneath impact-excavated basins, relics of which may be retained in structural windows in high grade metamorphic terrains. Isostatic subsidence and anatexis of thick mare-type piles and underlying impacted crust resulted in intracrustal comagmatic plutonic and volcanic suites within periods in the order of15–30 × 10 6 years, as controlled by post-impact mantle convection cooling. Subsequent isostatic updoming of remobilized basement gneiss terrains and the rise of their anatectic products along major normal shears, bearing an analogy to metamorphic core complexes, resulted in the multiple gneiss-granite complexes amidst greenstone-dominated terrains. Recurrent post-tectonic thermal fluctuations and magmatic activity reflect a long-term existence of anomalous mantle regions beneath original mega-impact sites and thermal perturbations related to distal impacts. A broad age zonation in some Archaean shields suggests lateral migration and terrane accretion in a convection driven plate tectonic regime.

Speculations on the Archean Mantle: Missing link between komatiite and depleted garnet peridotite

Like in the modern Earth, thermal and tectonic regime of the planet in the Archean (≥2.5 Ga) may have been dominated by those activities on the ocean floor. Because of the continuous operation of plate tectonics, however, the geologic record of the Archean ocean floor has been eliminated a long time ago. Using high‐pressure melting experiment data on a mantle peridotite and some key observations on Archean rocks (DGPs and PKs), a model for mid‐oceanic ridge in the Archean (AMOR) is constructed. High magnesian peridotitic komatiites (PKs, MgO ≥ 33 wt %) which are limited only on the oldest cratons (≥3.3 Ga) are considered to have been produced by partial melting of mantle peridotite at ≥7 GPa and ≥ 1800°C. Less magnesian PKs (MgO ≥ 25 wt %) which are characteristic of the latest Archean (≥2.5 Ga) may have been produced at ≥ 4 GPa and ≥ 1650°C. Potential mantle temperatures (PMTs) of the Earth were estimated from the above two constraints on PKs and that for the genesis of mid‐ocean ridge basalts (MORBs) in the modern Earth; PMT = 1750°C at 3.5 Ga, 1600°C at 2.5 Ga, and 1280°C at present. If estimated cooling rate of the Earth's mantle (1.3°C/107 years) is valid for another 5×108 years or so, PMT will become lower than mantle solidus, and then plate tectonics would cease. Very depleted garnet peridotites (DGPs) found as xenoliths in south African kimberlites show distinct chemical trends compared with spinel and garnet peridotites found in young orogenic terrains and oceanic regions. Re‐Os isotopic analyses on bulk xenoliths and Sm‐Nd model ages of garnets in diamonds in them indicate that DGPs have undergone extensive partial melting and melt extraction in the early Archean. The bulk chemical trends of DGPs can be explained by extraction of PK magma from primitive mantle peridotite. A 40–60 wt % extraction of PK magma ranging from 25 to 35 wt % MgO would suffice to yield entire spectrum of DGPs. A hypothesis for their origin by global melting (magma ocean) of chondritic source mantle is not preferred because the KD(Fe/Mg) values between majorite and magma postulated from this model is 2–3 times greater than experimental determination. On the other hand, KD values between olivine in DGP (residue) and PKs (magma) are consistent with experiments. If DGPs (which are supposed to be major constituents of the lithospheric mantle beneath the Kaapvaal craton in southern Africa) are complementary residue for PK magma, total volume of extracted PKs should be at least the size of the craton. In order to allow such high degree of partial melting in a steady state, existence of MOR in the Archean (AMOR) is postulated. At the AMOR, where PMT was ∼300°C higher than today, extensive partial melting of adiabatically rising mantle material would have initiated at ∼200 km depth. Net production of partial melt at the AMOR axis was equivalent to yield ∼80‐km‐thick oceanic crust (roughly 10 times greater than today). Total latent heat dissipation from the Archean mantle by generation of the ∼200‐km‐thick chemically differentiated oceanic lithosphere may have been comparable or even greater than total heat conduction through oceanic and continental lithospheres. The average Archean geotherm in the area away from the AMOR, therefore, may have been even lower than modern Earth.

Underplating of oceanic lithosphere in the Archean: a possible mechanism for the formation of Archean komatiites

The thermal evolution of a subducting Archean oceanic lithosphere suggests that amphibole and phlogopite can transport water to depths of about 100 km. Water released from the oceanic crust hydrates the overlying depleted peridotitic lithosphere, lowering its melting temperature and causing a high degree of partial melting. Temperatures at the top of the Archean asthenosphere only about 150°C higher than the present value are sufficient to produce over 50% melting in the bottom of the overlying lithosphere under wet conditions, and generate peridotitic komatiites.

The intensity of the geomagnetic field at 3.5 Ga: paleointensity results from the Komati Formation, Barberton Mountain Land, South Africa

Paleointensity measurements have been carried out on 3.5 Ga samples from the Komati Formation type locality using both the Thellier and Van Zijl methods. These samples contain a single steeply-directed negative TRM component acquired during metamorphism of the Komati lavas. Thellier experiments yielded values ranging from 12 to 37 μT but an average paleofield intensity for the four best determinations is 20 ± 3 μT. A slightly lower average paleointensity of 15 ± 3 μT was obtained using Van Zijl experiments. Preheating was used to chemically stabilize seven samples used in Van Zijl determinations and these produced nearly ideal plots with an average paleointensity of 13 ± 2 μT. A single basaltic komatiite sample gave a nearly ideal Van Zijl plot indicating about 21 μT, nearly the same paleointensity as the peridotitic komatiite samples even though its NRM intensity was several orders of magnitude lower. Since the Komati characteristic remanence was acquired during a slow cooling, the data must be reduced by a factor of 1.55 to account for the difference between laboratory and natural cooling rates. Calculation of an equivalent equatorial paleointensity using the paleolatitude implied by the steep Komati characteristic remanence then gives value of 5 μT for the intensity of the geomagnetic field at 3.5 Ga, lower than the present value of about 30 μT.

Chemical composition of Archaean komatiites: Implications for early history of the earth and mantle evolution

Estimates of the chemical composition of the Archaean mantle, derived from elemental abundance ratios in komatiites combined with ultramafic xenolith data, support a model of a multistage heterogeneous accretion history of the Earth and synchronous core formation, 4.6 Ga ago. Most refractory lithophile element abundance ratios in komatiites and xenoliths are close to chondritic except for V/Ti and Ca/Al. Depletion of vanadium is likely due to its partial incorporation into the iron core; whereas fractionation of Ca/Al observed in Archaean Al-undepleted komatiites (1.20 times chondrites) and in some modern fertile spinel lherzolite xenoliths (1.15 times chondrites) could be due to small amounts of garnet (rich in Al but poor in Ca) segregation into the lower mantle during partial or complete melting of the upper mantle in the very early history of the Earth. However, this process may have had only a small effect on the overall chemical composition of the upper mantle. Simultaneous occurrence of early Archaean Al-undepleted (Al/Ti ∼ chondrites) and Al-depleted (Al/Ti ∼ 0.5 chondrites, and depletion of Sc and heavy REE) peridotitic komatiites in the Barberton area, S. Africa, and late Archaean Newton Township, Canada, argue against derivation of peridotitic komatiites from a circum-global magma ocean. Garnet separation from a mantle diapir which intersects the solidus at great depth (⩾ 200 km) in a hotter early Archaean mantle could explain the chemical characteristics of Al-depleted komatiites. Alternatively, these two types of komatiites could have been derived from different layers in a fractionated mantle. A limited amount of Hf isotope data for Archaean komatiites seems to suggest that both mechanisms are important. This chemically and minerallogically layered mantle, if it existed, was homogenized by mantle convection after early Archaean times. Constant P 2O 5/TiO 2, Ni/Mg, Co/Mg, Fe/Mg ratios (siderophile/lithophile) and PGE abundances, estimated for the mantle sources of komatiites from Archaean to modern times, strongly argue against continuous growth of the Earth's core since the early Archaean. Extensive crustal contamination might have been involved in the generation of Archaean-early Proterozoic siliceous high magnesian basalts with “boninite affinity”. However, involvement of chemically modified ancient continental lithosphere may also be important in the generation of these basalts.

Melting of a dry peridotite KLB‐1 up to 14 GPa: Implications on the Origin of peridotitic upper mantle

Melting phase relations of a fertile lherzolite KLB‐1 have been studied in the pressure range from 1 atm to 14 GPa (140 kbar). Olivine is the liquidus phase at all pressures studied. The second mineral to crystallize changes with increasing pressure; chromian spinel (1 atm), Ca‐poor orthopyroxene (up to 3 GPa), pigeonitic clinopyroxene (up to 7 GPa), pyrope‐rich garnet (above 7 GPa). The melting temperature interval of the peridotite is more than 600°C wide at 1 atm but narrows to about 150°C at 14 GPa. The partial melts along the peridotite solidus become increasingly more MgO‐rich as pressure increases throughout the pressure range studied. At 5–7 GPa, the partial melts formed within 50°C of the solidus contain more than 30 wt % MgO and are very similar to Al‐undepleted‐type peridotitic komatiite which is common in Archean volcanic terrains. Due to the increase of enstatite component in clinopyroxene solid solution at high pressure and temperature, the orthopyroxene liquidus field narrows as pressure increases and disappears at 3.5 GPa. Harzburgites which are common in the basal peridotite in ophiolite suites may have been produced as residues by partial melting at relatively shallower depths (&lt; 100 km). Lherzolites showing protogranular textures might have been produced as single‐pyroxene peridotite residue by partial melting at relatively greater depths (&gt; 100 km). A diapiric model is consistent with the genesis of komatiite magma by partial melting of mantle peridotite at 150–200 km depth. Based on the following observations, (1) convergence of the liquidus and solidus of the peridotite at pressures &gt; 14 GPa, (2) the near solidus partial melt composition very close to the bulk rock at 14 GPa, and (3) change in liquidus mineral from olivine to majorite garnet at pressures between 16 and 20 GPa in preliminary experiments, it is proposed that the upper mantle peridotite was generated originally as a magma (or magmas) by partial melting of the primitive earth at 400–500 km depth.