Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanicrock characterized by spinifex texture, there is a growing recognitionthat similar textures can also form in high-level dykes andsills. Here, we report the results of a petrological and geochemicalinvestigation of a 5 m thick komatiite sill in Dundonald Township,Ontario, Canada. This unit forms part of a series of komatiitesand komatiitic basalts, some of which clearly intruded unconsolidatedsediments. The komatiite sill is differentiated into a spinifex-texturedupper part and an olivine cumulate lower part. Features characteristicof the upper sections of lava flows, such as volcanic brecciaand a thick glassy chilled margin, are absent and, instead,the upper margin of the sill is marked by a layer of relativelylarge (1–5 mm) solid, polyhedral olivine grains that gradesdownwards over a distance of only 2 cm into unusually large,centimetre-sized, skeletal hopper olivine grains. This is underlainby a 1 m thick zone of platy spinifex-textured olivine and coarse,complex, dendritic, spinifex-textured olivine. The texture ofthe olivine cumulate zone in the overlying unit is uniform rightdown to the contact and a lower chilled margin, present at thebase of all lava flows, is absent. The textures in the silland the overlying unit are interpreted to indicate that thesill intruded the olivine cumulate zone of the overlying unit.Thermal modelling suggests that soon after intrusion, a narrowinterval of the overlying cumulate partially melted and thatthe liquid in the upper part of the sill became undercooled.The range of olivine morphologies in the spinifex-textured partof the sill was controlled by nucleation and crystallizationof olivine in these variably undercooled liquids. KEY WORDS: komatiite; intrusion; spinifex texture; olivine     

A comparative study of olivine and clinopyroxene spinifex flows from Alexo,abitibi greenstone Belt,Ontario, Canada

A petrological and geochemical study of an olivine and of a clinipyroxene spinifex textured flow, from Alexo, indicates that the initial liquid in both flows probably came from the same mantle melting event and that the source was incompatible element depleted. The starting liquid of the clinopyroxene flow had experienced more olivine fractionation (10%) prior to its emplacement at Alexo, than the initial liquid of the olivine spinifex flow. The development of each of the textural and compositional zones in the flows can be modelled by means of crystal fractionation. In the case of the clinopyroxene flow the B-zone is formed by the fractionation of olivine, low-Ca pyroxene and chromite. An unusual feature of the Alexo clinopyroxene flow is presence of a peridotitic komatiite above the pyroxene cumulate layer, where a basaltic komatiite would usually be present. The presence of the peridotitic komatiite suggests an influx of new magma and hence a dynamic model for the flow. The composition of the clinopyroxene spinifex zone represents a mixture of clinopyroxene plus liquid, rather than simply a frozen liquid. This could happen if the clinopyroxene needles grew stalactitelike from the chilled upper surface of the flow into a flowing basaltic liquid. In the olivine spinifex flow the zones can be modelled as frozen liquids in the A₂-zone, as initial liquid which has fractionated 30% olivine in the A₃-zone and as liquid plus 50% olivine in the B-zone. But, if the clinopyroxene spinifex developed by stalactite growth of clinopyroxene needles into the a flowing liquid, the possibility that the olivine spinifex represent fractionated liquid plus stalactite olivines arises.     

Spinifex-textured komatiites in the south border of the Carajas ridge,Selva Greenstone belt,Carajás Province,Brazil

Spinifex-textured komatiites in the Selva greenstone belt are the first unequivocal examples of komatiites in the Transition Subdomain of the Carajás Mineral Province. Outcrops of spinifex-textured komatiites, located ∼1.5 km to the south of the Carajás ridge, were discovered during regional exploration for Ni–Cu–(PGE) sulfide deposits by VALE. They are associated with a 3.8 km long unit consisting of variable types of ultramafic rocks (talc schist, serpentinite and spinifex-textured komatiite). This ultramafic unit follows the steep dipping NW–SE trending Selva greenstone belt composed mainly by quartz-chlorite schists (interpreted as metasediments) and chlorite-actinolite schists (interpreted as metabasalts). Greenschist facies metamorphic parageneses characterize all rock types in the Selva greenstone belt.The komatiitic rocks in the Selva belt comprise a sequence of flows consisting of an upper spinifex-textured layer and a lower olivine cumulate layer. Although the spinifex and cumulus textures are well preserved in the field, the primary mineralogy of the komatiites has been completely replaced by greenschist facies metamorphic minerals. Platy olivine spinifex texture, consisting of an array of roughly parallel olivine plates, and random spinifex texture, consisting of randomly oriented olivine plates, are the most common primary volcanic textures in komatiites in the Selva greenstone belt. Platy and random spinifex texture is defined by former plates of olivine replaced by serpentine with minor actinolite, chlorite and magnetite, alternating with former matrix replaced by abundant actinolite and minor chlorite, talc, serpentine, and magnetite. The domains between olivine plates in both platy and random spinifex-textured rocks contain irregular arrays of fine-grained parallel crystals, representing primary fine-grained “quench” clinopyroxene crystals replaced by actinolite.Spinifex-textured komatiites have MgO contents bracket between 22.8 and 26.9 wt.%, and cumulate textured komatiites have MgO contents up to 40.6 wt.%. When plotted vs MgO contents, most major and minor elements fall on well-defined linear trends indicating control by olivine fractionation or accumulation. Komatiites from the Selva and Seringa (located in the Rio Maria Domain) belts are Al-undepleted with Al₂O₃/TiO₂ ratios close to 20. Results for CaO, Na₂O, and REE suggest that these elements were mobile and their abundances have been modified during metasomatic alteration. REE contents in some samples are very high (up to 40 times primitive mantle values) and REE patterns vary from flat (La/Yb_MN ∼ 1) to highly enriched in LREE (La/Yb_MN up to ∼ 10). The REE mobility may be related to hydrothermal alteration associated to Cu–Au mineralization in the region.The identification of spinifex-textured komatiites close to the Carajás Basin suggests the continuation of 3.0–2.9 Ga greenstone belts of the Rio Maria Domain within the Transition Subdomain, and enlarges the area with potential to host komatiite-associated Ni–Cu–PGE deposits.     

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton,southern India: implications for Archaean mantle evolution and crustal growth

ABSTRACT

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al₂O₃ and (Gd/Yb)_N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.     

Bizarre geochemistry of komatiites from the Crixás greenstone belt,Brazil

Komatiite lava flows in the Crixás greenstone belt, Goiás, Brazil, have textures and volcanic structures typical of Archean komatiites, but are geochemically most unusual. The flows are porphyritic and massive, or layered with spinifex upper parts and olivine cumulate lower parts. MgO contents range from 18 to 40%. In such lavas, only olivine (and minor chromite) can have crystallized, but neither major nor trace elements fall on olivine control lines. In MgO variation diagrams, CaO and Sr fall on lines with slopes steeper than olivine control lines; SiO₂, FeO, Na₂O, K₂O and Y show little systematic variation; Zr shows a large variation that does not correlate with MgO; and Al₂O₃ decreases markedly with decreasing MgO. The aberrant behaviour is highlighted by the REE (rare earth elements) in spinifex and olivine cumulate layers from three flows: in the spinifex layers, chondrite-normalized REE patterns are hump-shaped with maxima at Nd or Sm ((La/Sm)_N=0.6, (Gd/Yb)_N=1.6–2.3), whereas cumulate zones in the same flows have steadily sloping patterns, with LREE enriched relative to HREE ((La/Sm)_N=1.3, (Gd/Yb)_N=1.4). Neither normal magmatic processes acting during emplacement of the komatiites, nor thermal erosion and wall-rock assimilation can explain these effects, and we speculate that elements commonly thought of as “immobile” (e.g. Al, Zr, REE) migrated during hydrothermal alteration or metamorphism. A Pb-Pb whole rock isochron gave an age of 2,728±140 Ma and selected Sm-Nd analyses an apparent isochron age of 2,825±98 Ma (ɛ_Nd≈0). The Pb-Pb age is believed to be the approximate time of emplacement. Interpretation of the Sm-Nd data is complicated by the evidence of mobility of REE.     

The petrogenesis of the Lac Guyer komatiites and basalts and the nature of the komatiite-komatiitic basalt compositional gap

A sequence of ultramafic rocks in the Lac Guyer Archean greenstone belt exhibit brecciated flow tops, pillow structures, and spinifex textures testifying to their volcanic origin. Massive, spinifex-textured and differentiated flows in the sequence have the chemical characteristics of peridotitic komatiite, with MgO ranging from 19–25 wt.%. Associated pillowed flows have compositions that straddle the conventional boundary between komatiite and komatiitic basalt with MgO contents ranging from 16 to 19 wt.% MgO and are best termed pyroxenitic komatiites. Unlike other komatiitic occurrences, the peridotitic and pyroxenitic komatiites at Lac Guyer constitute a continuous chemical spectrum with no evidence of population minimum near 18 wt.% MgO. The contrasting behaviour of highly compatible elements, such as Ni and Cr, versus incompatible elements, such as Zr, indicate that this compositional spectrum was produced by a variation in the extent of partial melting (10–40%) of a garnet lherzolite source in the Archean mantle. The pyroxenitic komatiites represent liquids produced during lower (10–20%) degrees of melting during which garnet remained in the mantle residue. However, a change in slope in the distribution of Zr vs. Y between the pyroxenitic and the peridotitic komatiites indicates that garnet was completely consumed at the more extensive degrees of melting which produced the peridotitic komatiites. The Lac Guyer volcanic rocks display a population minimum at 15 wt.% MgO separating komatiitic magmas whose compositions are controlled by partial melting from basalts whose composition is controlled by crystal fractionation. The population minimum near 18 wt.% MgO which is taken as the boundary between komatiite and komatiitic basalt may have a similar origin.     

Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario

Munro Township, in the Archean Abitibi greenstone belt of northeastOntario, contains volcanic and hypabyssal rocks of two magmaseries: (1) an Fe-rich tholeiitic series of basaltic to daciticlava flows (3–10 m thick), layered peridotite-pyroxenite-gabbroflows (120 m thick), and layered sills (700 m thick); (2) anultramafic-mafic komatiitic series, comprising discrete lavaflows of peridotitic to andesitic composition (1–17 mthick), layered peridotite-gabbro flows (120 m thick), and layeredsills (500 m thick). The komatiitie lavas form a successionabout 1000 m thick that is both underlain and overlain by thickersuccessions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic,and basaltic komatiites. The most ultramafic are peridotiticcumulates rich in forsteritic olivine (Fo_89–94), at thebases of flows and sills. Many less mafic peridotitic komatiites(MgO: 20–30 per cent), which typically form the upperparts of flows and the marginal parts of small intrusions, exhibitspinifex textures indicative of their formation from ultrabasicliquids. Pyroxenitic komatiites (MgO: 12–20 per cent)also may contain olivine, but are dominated by clinopyroxene,usually in spinifex textures. Basaltic komatiites (MgO <12per cent) are composed mainly of clino-pyroxene and plagioclaseor devitrified glass, rarely in spinifex texture and more commonlyin equigranular textures. Peridotitic komatiite with roughly30 per cent MgO appears to represent a parental liquid fromwhich the more ultramafic komatiites formed by accumulationof olivine, and the less mafic types were derived by fractionationof olivine, joined and finally succeeded in later stages byclinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristicsof the komatiites from the Barberton Mountain Land, South Africa(Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al₂O₃ratios that distinguish the Barberton rocks. The Munro komatiitesare identical in this respect to ultramafic volcanic rocks inAustralia, Canada, Rhodesia, and India. It is proposed thatthe definition of the term komatiite be broadened so that itincludes all members of this ultramafic-mafic rock series, notonly those from Barberton Mountain Land. The proposed criteriaare: (1) highly ultramafic compositions in noncumulate lavas;(2) unusual volcanic structures such as spinifex texture andpolyhedral jointing; (3) low Fe/Mg ratios at given Al₂O₃ valuesor high CaO/Al₂O₃ ratios; low TiO₂ at given SiO₂; and high MgO,NiO, and Cr₂O₃.     

Comparison of Archean dunites and komatiites associated with nickel mineralisation in Western Australia: Implications for dunite genesis

The two important classes of Archean Fe-Ni-Cu sulphide deposits in Western Australia are those hosted by (1) coarse-grained adcumulate dunites (e.g. Agnew, Mount Keith) and (2) komatiitic lavas (e.g. Kambalda). Close similarities in compositions of relict olivine, pyroxene, and chromite from dunitic and komatiitic host rocks and compositional and textural similarities between chilled margins of some dunites and some komatiites indicate origins from similar komatiitic liquids which contained 20%–32% MgO. As field relations suggest that the dunites are probably lateral stratigraphic equivalents of spinifextextured komatiites, they are interpreted as the slowcooled end member of a continuum from spinifex-textured komatiites (former liquids) through cumulate komatiites (40%–90% olivine) to adcumulate komatiitic dunite (>90% olivine). Previous classification of these Ni deposits as intrusive- and volcanic-related deposits now appears unwarranted, and the terms komatiitic dunite-hosted and komatiite-hosted deposits are preferred.     

Evidence of water degassing during emplacement and crystallization of 2.7 Ga komatiites from the Agnew-Wiluna greenstone belt,Western Australia

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old, which formed through >30% partial melting of the mantle. This study addresses the crucial relationship between volcanology and physical manifestation of primary magmatic water content in komatiites of the Agnew-Wiluna greenstone belt, Western Australia, and documents the degassing processes that occurred during the emplacement and crystallization of these magmas. The Agnew-Wiluna greenstone belt of Western Australia contains three co-genetic komatiite units that (1) display laterally variable volcanological features, including thick cumulates and spinifex-textured units, and (2) were emplaced as both lava flows and intrusions at various locations. Komatiite sills up to 500 m thick contain widespread occurrence of hydromagmatic amphibole in orthocumulate- and mesocumulate-textured rocks, which contain ca. 40–50 wt% MgO and <3 wt% TiO₂. Conversely, komatiite flows do not contain any volatile-bearing mineral phases: ~150-m-thick flows only contain vesicles, amygdales and segregation structures, whereas <5–10-m-thick flows lack any textural and petrographic evidence of primary volatile contents. The main results of this study demonstrate that komatiites from the Agnew-Wiluna greenstone belt, irrespective of their initial water content, have degassed upon emplacement, flow and crystallization. More importantly, data show that komatiite flows most likely degassed more water than komatiite intrusions. Komatiite degassing may have indirectly influenced numerous physical and chemical parameters of the water from the primordial oceans and hence indirectly contributed to the creation of a complex zonation at the interface between water and seafloor.     

桂北四堡群中科马提岩系及其成因类型

桂北四堡群中许多镁铁质—超镁铁质岩体,实际上是火山岩或次火山岩。有的还具有科马提岩岩流的结构分层,其堆积带上部橄榄石和辉石晶体间隙中充填有基质物质。这些基质中发育了典型的橄榄石鬣刺结构。科马提岩与其伴生玄武岩属具成因联系的同一演化系列。在微量元素及REE特征上,区内科马提岩具有较好的Ⅱ类科马提岩属性,它们系亏损地幔经较小程度部分熔融产生。     

Emplacement,crystallization and alteration of spinifex-textured komatiitic basalt flows in the Archaean Nondweni greenstone belt,southern Kaapvaal Craton,South Africa

A well exposed succession of spinifex-textured komatiite flows is reported from the Archaean Nondweni greenstone belt located near the southern margin of the Kaapvaal Craton. The flows are relatively thin (1–5 m) compared to similar occurrences in other greenstone belts. They are characterised by well developed cone structures of highly elongate amphibole crystals (after augite) which fan downwards from the tops of the flows. Extreme development of coned spinifex has not been reported from other greenstone belts and points to specific thermal conditions prevailing in the Nondweni environment. The zones of bladed spinifex are contained between layers of random spinifex and overlie a lower cumulus layer originally of augite, orthopyroxene and minor olivine. The observed major and trace element distributions through a 1.7 m thick spinifex-textured flow are consistent with a model involving concentration of phenocryst phases resulting in significant fractionation upwards in the flow. Approximately 40% of the spinifex-textured phenocrysts grew in situ after the lithological units were established. Collapse and displacement of the coned crystal networks, originally attached to the top of the flow, are shown to have influenced the distribution of liquid within the flow and accentuated the fractionation. Associated with the spinifex-textured units are massive aphyric and brecciated flows which show distinct chemical cycles through the succession. The brecciated zones have compositions with <18% MgO and are characterised by ovoid bodies that are not pillows and may represent magmatic reworking and movement of a partly congealed flow. Post-solidus alteration is considered to have caused early hydration of the original mineralogy and also introduced SiO₂ and Na₂O into the upper part of the flow by way of microfractures. The observed alteration is different to that of Mid-Ocean Ridge basalts, and a subaerial/shallow water environment is suggested.     

Contrasting komatiite belts,associated Ni–Cu–(PGE) deposit styles and assimilation histories

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is the most nickel-sulfide-endowed komatiite belt in the world. The Agnew–Wiluna greenstone belt contains two mineralised units/horizons that display very different volcanological and geochemical features. The Mt Keith unit comprises >500 m-thick spinifex-free adcumulate-textured lenses, which are flanked by laterally extensive orthocumulate-textured units. Spinifex texture is absent from this unit. Disseminated nickel sulfides, interstitial to former olivine crystals, are concentrated in the lensoidal areas. Massive sulfides are locally present along the base or margins of the lenses or channels. The Cliffs unit is locally >150 m thick and comprises a sequence of differentiated spinifex-textured flow units. The basal unit is the thickest, and contains basal massive nickel-sulfide mineralisation. The Mt Keith and Cliffs units display important common features: (i) MgO contents of 25–30% in inferred parental magmas; and (ii) Al/Ti ratios of ～20 (Munro-type). However, the Mt Keith unit is highly crustally contaminated (e.g. LREE-enriched, high HFSEs), whereas the Cliffs unit does not display evidence of significant crustal assimilation. We argue that the distinct trace-element concentrations and profiles of the two komatiite units reflect their different emplacement style and country rocks: the Mt Keith unit is interpreted to have been emplaced as an intrusive sill into dacitic volcanic units whereas the Cliffs unit was extruded as lava flow onto tholeiitic basalts in a subaqueous environment. The mode of emplacement and nature of country rock is the single biggest factor in controlling mineralisation styles in komatiites. On the other hand, evidence of crustal contamination does not necessarily provide information of the prospectivity of komatiites to host Ni–Cu–(PGE) mineralisation, despite being a good proxy for the style of komatiite emplacement and the nature of country rocks.     

Formation of Spinifex Texture in Komatiites: an Experimental Study

The formation of platy olivine spinifex, the texture that characterizeskomatiite lavas, has long been enigmatic. A major problem isthat the dendritic morphology of the olivine resembles thatof crystals grown in laboratory experiments at high coolingrates (>50°C/h), but at the position where these texturesform, up to several meters below the komatiite flow top, thecooling rate cannot have been greater than 1–5°C/h.We performed experiments that demonstrate that the platy habitof spinifex olivine or pyroxene is a consequence of slow coolingof ultramafic magma in a thermal gradient (7–35°C/cm).The charges were cooled at rates between 2 and 1428°C/hand, even at the low cooling rates, the thermal gradient ledto constrained growth and the development of preferentiallyoriented dendritic crystals with morphologies like those innatural platy spinifex-textured lavas. Under these conditions,olivine starts to crystallize at temperatures well below theequilibrium liquidus temperature (37°C < –T<56°C) depending on the composition of the starting material.When the cooling rate is high, the thermal gradient has a negligibleeffect on the texture and the crystals have a random orientation,like that in the upper parts of komatiite flows. KEY WORDS: komatiite; spinifex; cooling rate; experimental petrology; thermal gradient     

Komatiite Flows from the Reliance Formation, Belingwe Belt, Zimbabwe: I. Petrography and Mineralogy

The 2.7-Ga Reliance Formation of the Ngezi Group, Belingwe GreenstoneBelt, Zimbabwe, contains extremely fresh komatiite lavas. Detailedfield mapping and a 200-m deep drill-hole, with excellent corerecovery, demonstrated the existence of a suite of lava flows.Each major flow is 10 m thick and characteristically exhibitschilled top and bottom margins, a spinifex zone dominated byrandom spinifex, a B1 zone, and a thick cumulate zone that typicallycomposes two-thirds of the flow thickness. Preservation of olivineand pyroxene mineralogy is superb by Archaean standards, tothe extent that even the tips of skeletal crystals survive.The matrix, although devitrified, is well preserved. Detailedstudy of two flows shows that skeletal grains from the spinifexzone have maximum Fo contents of 91.4. The Fo contents of microphenocrystsfrom the cumulate zone range from Fo_91.2 to Fo_91.6, but rarelarge phenocrysts ( 5 vol.%) have maximum Fo contents of 93.6.The Fo contents of the cumulate olivines do not vary with stratigraphicheight, implying that the cumulate zone formed rapidly, by accumulationof transported crystals. The cumulate zones contain 42–57%modal olivine and display reverse size grading of the olivinemicrophenocrysts. This grain-size variation is believed to resultfrom adcumulus growth within a cumulate pile formed by the formedby the gravitational settling of clusters of olivine crystals.Textural relationships indicate that the final part of the flowto start to crystallize was the lowermost part of the spinifexzone. Reprint requests.     

Differentiation of Komatiite Flows

Although layered komatiite flows with spinifex-textured upperparts and olivine-enriched lower parts are commonly thoughtto provide firm evidence of magmatic differentiation by olivinesettling, there are reasons to suspect that this may not betrue. Komatiite flows almost certainly convect vigorously asthey cool, with convective velocities that greatly exceed thesettling velocities of olivine grains. Other explanations forthe differentiation of komatiite flows into layers with differentolivine contents need to be investigated. One such explanation is provided by a detailed study of a komatiiteflow from Alexo, Ontario, Canada. This flow is about 16 m thickand has a spinifex-textured upper layer, and a lower layer,called the B-layer, which is composed of closely-packed, equantolivine phenocrysts. The composition of the initial liquid inthe flow is given by an upper chill sample, which has 28 percent MgO. In underlying spinifex lavas, MgO contents range from20 to 35 per cent MgO, and the B-layer has about 42 per centMgO. Olivines are most Fo-rich in the chill margin (Fo_94.1)and in the B-layer (Fo_93.8). In spinifex lavas, olivines aregenerally less magnesian, ranging from a high of Fo_93.8 in unusuallyMgO-rich chevron spinifex lavas, down to Fo₈₉ in the lower platespinifex lavas. It is believed that solidification of the upper part of theflow started only after it had become ponded, and that crystallizationproceeded with growth of spinifex olivines downwards from theroof of the flow. The manner in which the composition of silicateliquid within the flow changed during growth of the spinifexlayer can be calculated using the compositions of olivines inspinifex lavas. These calculations show that all the spinifexlavas are more magnesian than the liquids from which they formed:i.e. they all contain a component of excess olivine. Furthercalculations indicate, however, that liquid compositions inthe lower part of the flow changed more rapidly than can beexplained by accumulation of olivine in the spinifex lavas.This additional olivine must have crystallized in the lowerpart of the flow, and remained there as phenocrysts suspendedin the convecting liquid. As the spinifex-textured upper partcontinued to grow and thicken, olivine continued to crystallizewithin the flow, and the growing phenocrysts became more andmore concentrated in an ever-decreasing volume of liquid. Whenthe concentration exceeded about 50 vol. per cent, the viscosityof the lava became high enough to inhibit convection. The flowthen solidified completely, maintaining its olivine distributionwith a spinifextextured upper layer and an olivine-enrichedlower layer.     

Morphology and textures of komatiite flows of J.C. Pura schist belt,Dharwar craton

Study of komatiites for their structures and textures in cratonic blocks could provide more insights into the early Archaean volcanism, mantle processes and associated metallogeny. Jayachamarajapura (J.C.Pura) belt in Western Dharwar craton is a komatiitc milieu, where outcrop features display several flow characteristics and sub-volcanic emplacement features typical of well known komatiitic areas of the world. In spite of deformation, metamorphism and alterations the komatiites still preserve many of the primary cooling structures, which stand testimony for their extrusive volcanic nature. Distinct features like pillows, flow-top polyhedral joints, ocelli, vesicular, flow-top breccia and cumulate segregations and crude layering are observed. However, massive, undifferentiated nature of komatiitic flows is more predominant. Because of serpentinisation, carbonitization and chloritization, the original mineralogy and textures are obliterated and scantily preserved. Still, these observed features provide vital clues to imply the formation of komatiite sequences in a submarine to subaerial conditions when episodic pulses of komatiite lava piled up (about 3.35 Ga ago) to form the ultramafic milieu of J.C. Pura belt.     

Highly siderophile element geochemistry of Os-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.     

Komatiite hosted Hunters Road nickel deposit,central Zimbabwe: physical volcanology and sulfide genesis

The nickel sulfide bearing Main Flow at Hunters Road is a thick komatiite unit situated at the base of a well developed lava channel complex overlying a prominent banded iron formation that caps a thick sequence of felsic lavas and volcaniclastic rocks intruded by a probably comagmatic feeder sill. The 300–350 m thick inner flank comprises a 200–250 m thick central olivine meso to adcumulate, relatively narrow lower and upper ortho to mesocumulates and a 2–10 m thick olivine spinifex zone at the top. Approximately 700 m towards the 90 m thick outer flank, the spinifex zone is up to 30 m thick, the central meso to adcumulate lenses out between the upper and lower mesocumulates, and the lower orthocumulate is more rich in clinopyroxene. In places, the flanks are directly overlain by rubbly hyaloclastite. The less well preserved, 500 m thick central axis coincides with a floor rock embayment of demonstrably primary origin, which is 200 m deep and at least 800 m wide. Nickel sulfide mineralisation is disseminated, texturally cocumulus to olivine and confined to the meso to adcumulate, the highest nickel grades being located towards the top. Large floor rock xenoliths occur in the mineralised meso to adcumulate of the inner flank within reaction envelopes of barren, pyroxene bearing, olivine ortho to mesocumulate. Formation of the embayment and xenoliths (clear evidence of large scale thermomechanical floor rock erosion) and of the olivine sulfide meso to adcumulate (the sulfur probably derived by the assimilation of sulfidic wall rocks adjacent to the feeder sill) is attributed to prolonged focused flow of hot turbulent lava close to the vent. The Main Flow is interpreted as the product of a thick channelised sheet flow comprising: (i) a turbulent lava river, open along the central axis and partly tubed over along the inner flanks; and (ii) a tubed over levée facies along the outer flanks accommodating overflow from the central axis by inflationary growth under laminar flow conditions.     

Crystallochemistry and origin of pyroxenes in komatiites

We present a detailed mineralogical and major- and trace-element study of pyroxenes in two Archean komatiitic flows in Alexo, Canada. The pyroxenes in spinifex-textured lavas commonly are zoned with cores of magnesian pigeonite and rims of augite. Concentrations of incompatible trace elements are low in pigeonite and jump to higher values in the augite mantles, a variation that can be modelled using accepted partition coefficients and assuming crystallization from komatiitic liquids. Crystallization sequences are very different in different parts of both flows. In the flow top, the sequence is olivine followed by augite: deeper in the spinifex sequence, pigeonite crystallizes after olivine, followed by augite; in lower cumulates, orthopyroxene or augite accompany olivine. In spinifex lavas, pigeonite crystallizes sooner than would be predicted on the basis of equilibrium phase relations. We propose that contrasting crystallization sequences depend on the position in the flow and on the conditions of crystal growth. In the flowtop, rapid cooling causes quench crystallization. Deeper in the spinifex layer, constrained growth in a thermal gradient, perhaps augmented by Soret differentiation, accounts for the early crystallization of pigeonite. The cumulus minerals represent a near-equilibrium assemblage. Augites in Al-undepleted Archean komatiites in various localities in Canada and Zimbabwe have high moderate to high Wo contents but their Mg# (Mg/(Mg + Fe) are lower than in augites in komatiites from Barberton, South Africa. We attribute the combination of high Wo and high Mg# in Barberton rocks to the unusually high CaO/Al₂O₃ of these Al-depleted komatiites.     

Komatiites from west Shandong, North China craton: Implications for plume tectonics

The Mengyin komatiites are located at the base of the Taishan Complex ( 2.7 Ga), in the western Shandong greenstone belt of the eastern block of North China craton. The Mengyin komatiites have high-magnesian, low-titanium, and aluminum-undepleted chemical characteristics. Although the ultramafic rocks underwent amphibolite to greenschist facies metamorphism, they preserve remnant igneous spinifex textures. Some komatiite samples even preserve rare fresh olivine. According to the mass equilibrium between the olivine and melt, the potential eruption temperature of the Mengyin komatiites at 1 atm is about 1270 °C which might indicate a plume tectonic setting. It is inferred that komatiites in Mengyin, west Shandong may be part of a plume-related global magmatic event related to the break-up of a supercontinent that included the eastern block of the North China craton.