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Geoscience Canada

Igneous Associations 10. Jaroslav Dostal

Volume 35, Number 1, March 2008 Article abstract Komatiites are ultramafic volcanic rocks that occur mainly in Archean and URI: https://id.erudit.org/iderudit/geocan35_1ser02 Paleoproterozoic greenstone belts. These -rich rocks are assumed to have crystallized from that have about 28–30 wt% MgO. They are See table of contents characterized by spinifex texture - platy or skeletal of olivine set in a glassy . Chemically, komatiites resemble and have high MgO but low SiO2, TiO2 (<1 wt%), K2O (<0.5 wt%) and incompatible trace element Publisher(s) contents. Most of their compositional variations can be accounted for by olivine fractionation. Komatiites are conventionally considered to be derived The Geological Association of Canada from high temperature melts that have eruption temperatures of about 1600EC and are produced by high degrees of anhydrous melting of plumes. The ISSN abundance of Archean komatiites, their decrease through the Proterozoic and extreme rarity in the Phanerozoic have been taken as evidence for secular 0315-0941 (print) cooling of the mantle. However, the plume model has recently been challenged. 1911-4850 (digital) In marked contrast, it has been proposed that komatiites originate via hydrous melting at a shallow depth in environments at significantly lower Explore this journal melting temperatures than those invoked by the plume hypothesis. This new model thus challenges traditional views of the early evolution of the . Nevertheless, it appears that many komatiites are plume-related. In addition to Cite this document the information they provide about the tectonics and the thermal evolution of Archean Earth, komatiites are economically important because they host Dostal, J. (2008). Associations 10. Komatiites. Geoscience Canada, locally significant magmatic Ni-sulfide (Ni-Cu-PGE) mineralization. 35(1), 21–31.

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This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ GEOSCIENCE CANADA Volume 35 Number 1 March 2008 21

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extreme rarity in the Phanerozoic have 1600°C, et qui proviennent d0un haut been taken as evidence for secular degré De fusion de panaches man- cooling of the mantle. However, the téliques anhydres. L’abondance des plume model has recently been chal- komatiites archéennes, leur moindre lenged. In marked contrast, it has been abondance au Paléozoïque et leur proposed that komatiites originate via extrême rareté au Phanérozoïque ont hydrous melting at a shallow depth in été interprétés comme étant l’effet d’un subduction environments at signifi- refroidissement séculaire du manteau. cantly lower melting temperatures than Par-centre, récemment, le modèle des those invoked by the plume hypothesis. panaches mantéliques a été remis en This new model thus challenges tradi- question. On a proposé qui au con- tional views of the early evolution of traire les komatiites proviendraient de Igneous Rock Associations the Earth. Nevertheless, it appears that fusions hydratées de faibles pro- 10. many komatiites are plume-related. In fondeurs en milieux de subduction, et Komatiites addition to the information they pro- à des températures de fusion significa- vide about the tectonics and the ther- tivement plus basses que celles sup- mal evolution of Archean Earth, posées par l’hypothèse des panaches. Jaroslav Dostal komatiites are economically important Ce nouveau modèle remet donc en Department of Geology because they host locally significant question la perspective traditionnelle Saint Mary’s University magmatic Ni-sulfide (Ni-Cu-PGE) sur les premiers stades de l’évolution Halifax, NS, Canada, B3H 3C3 mineralization. de la Terre. Ceci dit, il semble que E-mail: [email protected] nombre de komatiites soit relié à des SOMMAIRE panaches. Au-delà des informations SUMMARY Les komatiites sont des roches vol- qu’elles fournissent sur la tectonique et Komatiites are ultramafic volcanic caniques ultramafiques que l’on retrou- l’évolution thermique de la Terre rocks that occur mainly in Archean and ve principalement dans des bandes de archéenne, les komatiites sont impor- Paleoproterozoic greenstone belts. roches vertes archéennes et paléopro- tantes économiquement, étant les These olivine-rich rocks are assumed térozoïques. On suppose que ces roches hôtes de minéralisations mag- to have crystallized from magmas that roches à fort contenu en olivine ont matiques de sulfures de nickel (Ni-Cu- have about 28–30 wt% MgO. They are cristallisé à partir de magmas ayant des EGP) en certains endroits. characterized by spinifex texture - platy teneurs de 28 à 30% en poids de MgO. or skeletal crystals of olivine set in a Typiquement, elles présentent une tex- INTRODUCTION glassy matrix. Chemically, komatiites ture spinifex; c’est-à-dire en cristaux Komatiites are rare ultramafic volcanic resemble peridotites and have high lamellaires ou squelettiques d’olivine and subvolcanic rocks that occur, pre- MgO but low SiO2, TiO2 (<1 wt%), dans une matrice vitreuse. Chimique- dominantly, in Archean and Paleopro- K2O (<0.5 wt%) and incompatible ment, les komatiites ressemblent aux terozoic greenstone belts. These rocks trace element contents. Most of their péridotites et présentent des teneurs contain more than 18 wt% MgO (Le compositional variations can be élevées en MgO mais basses en SiO2, Bas 2000) and are distinguished from accounted for by olivine fractionation. TiO2 (<1% en poids), K2O (<0.5% en other magnesium-rich rocks, such as Komatiites are conventionally consid- poids) ainsi qu’en éléments traces picrites and meimechites, by having ered to be derived from high tempera- incompatibles. La plupart de leurs vari- spinifex texture (characterized by spec- ture melts that have eruption tempera- ations compositionnelles peuvent s’ex- tacular arrays of subparallel or ran- tures of about 1600EC and are pro- pliquer par le fractionnement de l’o- domly-oriented skeletal, platy and blad- duced by high degrees of anhydrous livine. Par convention, on considère ed olivine crystals set in a glassy melting of mantle plumes. The abun- que les komatiites proviennent de mag- groundmass; Fig. 1). Because all dance of Archean komatiites, their mas de hautes températures qui ont komatiites do not display spinifex tex- decrease through the Proterozoic and des températures d’éruption d’environ ture, Arndt and Fowler (2004) defined 22

Figure 1. a) flow showing a a zone of spinifex-textured olivine blades overlying a cumulate zone (Pyke Hill, Munro Township, Ontario, Abitibi greenstone belt; photo courtesy of A. Fowler); b) Boulder of olivine spinifex-textured flow top komatiite. Top of flow is toward hammer handle. Olivine textures become rapidly coars- er away from upper chilled margin of the flow and evolve from < 1 mm equant crystals (not visible) to radiate spinifex needles (2-10 mm long) to coarse bladed spinifex crystals (10-30 cm long; plate spinifex A3) in flow interior. Olivine textures are preserved by metamorphic assemblage of serpentine + chlorite + tremolite (Spinifex stream area, Barberton Mountainland, Mpumulunga, South Africa; Dann and Grove 2007; photo courtesy of T. Grove). Hammer han- dle is ~ 0.5 m in length; c) Spinifex texture in (Pyke Hill). b Note skeletal spinifex-textured olivine crystals largely altered to serpentine with clinopyroxene (photo courtesy of A. Fowler).

komatiites as ultramafic rocks that either contain spinifex texture or are related to rocks that do (see Kerr and Arndt 2001). Komatiitic are assumed to crystallize from magmas that have ultramafic compositions. Komatiites were first identi- fied by Viljoen and Viljoen (1969) in the Archean greenstone belt in the c Barberton Mountainland of South Africa as a distinctive and “new” class of -rich (20–30 wt% MgO) volcanic rocks. Up until then, genuine ultramafic lavas were not known. Viljoen and Viljoen (1969) recognized that many high-Mg rocks in the Bar- berton greenstone belt were flows of significant lateral extent and thick- ness, with chilled or brecciated tops, , pillows and the distinctive quench textures that were subsequently named “spinifex” (after an Australian spiky grass - Triodia spinifex). The Viljoen brothers inferred that the rocks had characteristics of erupted with distinctive chemical composition and named them after the Komati GEOSCIENCE CANADA Volume 35 Number 1 March 2008 23

Figure 2. Proportions of komatiites in volcanic packages of greenstone belts as a function of age (modified after de Wit and Ashwal 1997, and Condie 2001). River flowing through the type locality. aqueous volcanic successions of Pre- Figure 3. Variations in texture across These 3.5 Ga flows are among the old- cambrian greenstone belts. De Wit and a several metre-thick komatiite flow est known ultramafic rocks. Ashwal (1997) estimated that komati- showing well developed layering. The Similar ultramafic lavas were ites constitute typically less than 10% idealized section includes a thin (1-5 soon described from other Archean of the total volume of volcanic rocks cm) chilled flow top (A1), a ~5–50 cm and Paleoproterozoic belts particularly in most greenstone belts although in thick spinifex-textured subzone that from Canada, Africa, and some areas they form up to 30% of has small randomly oriented spinifex Finland. Most are Neoarchean to Pale- the volcanic (Fig. 2). blades usually millimetres to centime- oproterozoic (Fig. 2) but extend into Komatiite units can be tens or even tres in length (A2 or random spinifex), the Phanerozoic, including hundreds of metres thick and can be and coarse-grained platy spinifex (A3) Permian–Triassic komatiites from NW traced continuously for up to 20 km, in which the olivine blades (centime- Vietnam (Glotov et al. 2001; Hanski et indicating that komatiite eruptions tres to decimetres in length) are orient- al. 2004) and Cretaceous (~ 90 Ma) were major events during the Precam- ed perpendicular to flow margins and komatiites from Gorgona Island off brian. a cumulate zone (B). Zone B displays the coast of Columbia (Kerr et al. Komatiites occur either as lava characteristics of accumulation 1996; Kerr 2005; Brandon et al. 2003). flows or subvolcanic bodies; rarely, and inflation (ballooning). At the top However, komatiites have no known they are also pyroclastic. Komatiite of the Zone B, there may be a thin modern analogues. The predominance lavas range from thin (a few cm) to layer of foliated skeletal olivine (B1- of komatiites in the Archean, their massive to thick (> 100 m) and are dis- horizontal spinifex; probably rip-ups decreasing occurrence in the Protero- tinctly layered. Individual flows display- from A3). The rest of the Zone B zoic, and extreme rarity in the ing well-developed layering (Arndt et includes a B2 subzone of massive Phanerozoic have been interpreted to al. 2004; Grove et al. 1997) show a tex- medium- to fine-grained cumulate reflect secular cooling of the mantle tural division that includes an upper composed mainly of equant olivine (by up to 400EC; Nisbet et al. 1993). part (Zone A), characterized by crystals; B3 - coarse-grained equant Although komatiites are considered to spinifex-textured rock, and a lower part olivine cumulate; B4 - massive cumu- be important windows into Earth’s (Zone B) containing a high proportion late composed of olivine crystals with early mantle and therefore have major of equant olivine crystals resembling chilled basal margin. Note that thick- implications for constraining models of various to cumulates nesses of the zones may not be pro- the thermal evolution of Earth, the (Fig. 3). Both zones have been further portionally accurate. temperatures, sources and depths of subdivided into several subzones, their formation remain controversial. although the individual subzones may In addition, komatiites are of econom- not be continuous and may occasional- more than 11 km and compositionally ic interest as they are associated with ly be absent. Many thin and even some and texturally homogeneous. Massive significant magmatic nickel sulfide thick komatiite flows are not layered lavas may pass laterally into flows with deposits in Australia and Canada. and do not have spinifex zones. They well-developed spinifex textures. The may have a massive texture, and olivine variation in the textures and layering of GEOLOGICAL ENVIRONMENT are commonly concentrat- komatiite units has been attributed to Although, where present, they occur ed in the centre of such flows. Dann variations in flow volume, lava flow only in minor/subordinate amounts, (2001) reported massive komatiite velocity, effusion rate, topography and komatiites are an integral part of sub- sheets, 12 to 50 m thick, traceable for cooling rate (Sylvester et al. 1997; 24

Dann 2001; Dann and Grove 2007), all komatiites, including those from the ORIGIN OF SPINIFEX TEXTURE of which might also be related to dis- quartzite–komatiite association that is Petrologists who initially studied tance from an eruptive site. widespread in the western Superior spinifex texture noticed that the skele- Komatiites are associated with Province, overlie older granitic base- tal olivine phenocrysts in komatiites komatiitic (12–18 wt% MgO; ment and are inferred to be autochtho- (Fig. 1) resemble quench crystals Arndt et al. 1997), which have similar nous (Thurston and Chivers 1990; (formed at very rapid cooling rates) in flow and textures as komatiites, Bickle et al. 1994; Sylvester et al. 1997; experimental melts (Donaldson 1982). except that the dominant phenocrysts Shimizu et al. 2004). Solidification under the conditions of are . Their differentiated In Canada, some of the most supercooling (low nucleation rates and flows contain upper spinifex prominent komatiitic occurrences are rapid crystal growth rates) produces a zones and lower cumulate zones. in the Abitibi belt, which contains sev- few large skeletal or dendritic crystals Komatiite lavas are highly eral komatiitic successions that were (Fig. 1). However, this model cannot fluid because of their low and emplaced over ca. 20 Ma (2724–2703 explain the occurrence of spinifex tex- high liquidus temperature, so they Ma). Komatiitic rocks of the Abitibi ture within thick komatiite flows well would be expected to form rather thin belt occur mainly in the Timmins– below the upper chilled . Large flows. The thick komatiitic flows were Kirkland Lake–Lake Abitibi region of skeletal crystals may have crystallized at probably ponded (lava lakes and rivers; Ontario and Québec. depths, many metres below the sur- Hill et al. 1995) or were thickened by faces of the flows (Fig. 3) where cool- flow inflation (flow ballooning) due to MINERALOGY AND PETROGRAPHY ing rates must have been low. To over- repeated flow pulses, like pahoehoe Komatiites and related rocks have been come this problem, Viljoen and Viljoen basalts of Kilauea, Hawaii (Dann 2001; affected to variable degrees by meta- (1969), among others, suggested that Dann and Grove 2007). Komatiite morphism, hydrothermal and seafloor komatiites, most of which were related lavas are inferred to have the ability to alteration, and deformation, which to submarine eruptions, were rapidly flow over considerable distances from have, at least in part, obliterated the cooled by sea water. However, present eruption sites (Dann 2001; Sproule et original textures and primary mineralo- day submarine basalts have only nar- al. 2002). Once a solid crust formed gy. Hence, komatiites generally contain row rims that show features of rapid around a flow, it would create an effi- metamorphic in place of their cooling and neither Archean basalts cient insulating surface that would primary assemblages, relics of which coeval with komatiites nor modern facilitate the flow of hot komatiitic may nonetheless be fortuitously pre- submarine basalts have spinifex tex- lava. served in some instances. Low-grade tures. The origin of spinifex textures Some komatiitic rocks are of komatiites produced thus presents a dilemma: whereas the intrusive; they usually form high-level mineral assemblages dominated by ser- shape of crystals is suggestive of fast dikes and sills and even layered differ- pentine–antigorite, chlorite, talc, cooling near the flow margins, the entiated bodies. These rocks have tex- tremolite, magnesite–dolomite and blades formed deep within the flows. tures and compositions similar to those . At higher metamorphic As the spinifex texture is con- of their volcanic equivalents and might grades, metamorphosed komatiites fined to MgO-rich basaltic to ultramaf- be the hypabyssal parts of the komati- contain , , olivine ic rocks, a partial explanation is that itic volcanic suites (Arndt et al. 2004). and . spinifex texture is related to the tem- Small amounts (typically <1%) of The primary mineralogy of perature difference between the liq- komatiitic volcaniclastic rocks have komatiites is simple: phenocrysts of uidus and solidus, which is very large been documented in some greenstone olivine and / (+/- for komatiites and Mg-rich rocks belts (Sylvester et al. 1997). pyroxene) are enclosed in a ground- (400–500EC; Faure et al. 2006) com- A variety of geological envi- mass composed of glass, calcic pared with typical basaltic rocks ronments has been proposed for the clinopyroxene and minor orthopyrox- (<100EC). However, in detail, the ori- emplacement of komatiites including ene. In the spinifex-textured komatiites gin of the spinifex texture remains mid-ocean ridges, plumes, oceanic (Fig. 1), elongate and skeletal olivine problematic. Because olivine crystals plateaus, giant impacts and (or pyroxene in komatiitic basalts) are thermally anisotropic, Shore and oceans (Grove and Parman blades ranging in length from mm to Fowler (1999) suggested that heat 2004). There has been a suggestion tens of cm are set in a fine-grained transfer increases the cooling rate in that some komatiites are allochtho- matrix that originally contained a large front of the crystal tips leading to the nous, i.e. that they have been tectoni- proportion of ranging in formation of platy crystals. Alterna- cally transported and interleaved with composition from komatiite to komati- tively, a recent experimental study of rocks of different environments itic (Donaldson 1982). Olivine is Faure et al. (2006) inferred that (Sylvester et al. 1997). Most komatiites highly magnesian and shows normal spinifex texture is a result of slow are associated with Archean and Paleo- zoning from cores of Fo95-90 to rims of cooling of ultramafic magma in a ther- proterozoic greenstone belts (Arndt et Fo92-84. Their contents of Ni and Cr mal gradient within a layer that sepa- al. 1997) where komatiites occur main- are high (up to > 4000 ppm and rates magma from the solid outer ly in lithological assemblages that are >2000 ppm, respectively; Donaldson crust. Another explanation has been considered to be remnants of ocean 1982). put forward by Grove et al. (1997), plateaus or arcs (Condie 2001). Some who proposed that elevated water con- GEOSCIENCE CANADA Volume 35 Number 1 March 2008 25

Figure 4. MgO vs. Al2O3 (wt.%) diagram showing composi- tional variations in an idealized komatiite flow (Fig. 3) due to olivine fractionation and accumulation (modified after Arndt et al. 1997, and Rollinson 2007). tent in komatiitic magmas would lead (Jochum et al. Figure 5. Variations of MgO vs. Al2O3, TiO2, La, Gd, Th to the rapid growth of large crystals, 1991). Similarly, and Nb in Al-depleted (ADK) and Al-undepleted (AUK) and accompanying degassing of radiogenic iso- komatiites (modified after Dostal and Mueller 2004). The hydrous komatiites would generate a topes, particularly solid and dashed lines delineate the ranges for AUK and strongly supercooled . However, Nd, provide useful ADK, respectively. AUK includes Abitibi, Belingwe and this hypothesis has been challenged constraints on con- Kambalda greenstone belts while ADK encompasses samples (Arndt et al. 1998). tamination by from the Barberton and Abitibi belts. older continental crust. The geo- Virtually all komatiites and associated chemical data suggest that crustal con- lizing mineral, olivine, involving either rocks have been metamorphosed and tamination did not play a major role in the gain or the loss of olivine phe- chemically altered. Many elements used the genesis of most komatiites (e.g. nocrysts. In differentiated flows, in discussion of modern volcanic rocks Sproule et al. 2002). olivine cumulates at the base of the such as Na, K, Rb, Sr and Ba as well as Komatiites have a chemical flows have 30–45 wt% MgO (Figs. 4, H2O and CO2, have commonly been composition similar to peridotite or 5) whereas the spinifex-textured redistributed. Thus geochemical/ dunite, with high MgO, Ni and Cr but komatiites of the upper zone (residual petrological investigations of komati- low contents of SiO2, TiO2 (<1 wt%), liquids) have lower MgO (Fig. 4). This ites have been based upon elements K2O (< 0.5 wt%), Na2O and incom- decrease of MgO in the differentiated that are considered to be less mobile, patible trace elements. Arndt et al. komatiite flows from the basal cumu- including Al, Ti, high-field-strength (1997, 2004) inferred that original lates to the spinifex zone is accompa- elements (HFSE) and rare-earth ele- komatiitic liquids (i.e. lavas without nied by an increase in Al2O3, TiO2, ments (REE). olivine phenocrysts) had about 28–30 CaO and incompatible trace elements Another potential complica- wt % MgO. This is close to the com- because these components are exclud- tion is crustal contamination. Komati- position of aphyric komatiites and ed from olivine (Fig. 5). itic magmas are prone to contamina- chilled margins of komatiitic flows. of olivine does not modify the element tion because of their high tempera- Olivine that crystallized from a ratios as these elements will be tures. Crustal contamination of parental magma of 28–30 wt% MgO enriched in the residual liquid to the komatiite enriches light REE (LREE) would have a composition of ~ Fo94, same degree. The chondrite- and man- and Th relative to heavy REE (HREE) similar to in the lava flows tle-normalized trace-element patterns and HFSE, particularly Nb and Ta (Arndt et al. 2004; Lesher and Arndt of the related spinifex and cumulate (Jochum et al. 1991). Because of a sig- 1995). rocks are parallel although the former nificant compositional contrast Many komatiites show signifi- have higher absolute concentrations between and komati- cant compositional variations, even (Figs. 6, 7). Pyroxene spinifex-textured ites, element ratios such as Nb/Th, within a single lava flow (Fig. 4). Most komatiitic basalts have lower MgO and Nb/U, Th/La and Nb/LREE are sen- of these variations can be accounted Ni and frequently compositionally sitive indicators of contamination for by fractionation of the first crystal- grade into komatiites. 26

Figure 6. Chondrite-normalized REE abundances in ADK and AUK komatiites. a) AUK from Pyke Hill, Munro Town- ship (Fan and Kerrich 1997) compared to N-type mid-ocean ridge basalts (N-MORB; Sun and McDonough 1989); S - spinifex AUK (Samples P-4, P-5, P-6 and P-8), C - cumulate AUK (Samples P-2 and P-3); b) ADK from Komati Forma- tion, Barberton greenstone belt (Lahaye et al. 1995). S - spinifex ADK (Samples 166-1, 229-2, 229-3, B12 and B14), Figure 7. Primitive mantle-normalized trace-element abun- C- cumulate ADK (samples 229-5 and B15). Normalizing dances in ADK and AUK komatiites. a) AUK from Alexo, values after Sun and McDonough (1989). Ontario (Lahaye and Arndt 1996) compared to N-type MORB (Sun and McDonough 1989). S - plate spinifex AUK According to their chemical the Abitibi (samples 656, 657, 665, 668), C - basal cumulates (samples composition, komatiites are usually (Ontario– 712, 714 and 715); b) ADK from Komati Formation, Barber- subdivided into two major types (Nes- Québec), Belingwe ton greenstone belt (Lahaye et al. 1995). S - spinifex ADK bitt et al. 1979; Arndt et al. 1997): alu- () and (samples 166-1, 229-2, 229-3, B12 and B14), C - cumulate minum (Al)-undepleted (or Munro- Norseman–Wiluna ADK (samples 229-5 and B15). Normalizing values after type, named after their type locality in (Australia) but are Sun and McDonough (1989). the Abitibi greenstone belt) and alu- rare in pre-3.0 Ga minum-depleted (or Barberton-type). greenstone belts. They include the belt of South Africa and the ~3.5 Ga Al-undepleted komatiites (AUK) are 2718–2710 Ma Kidd– Munro assem- greenstone belts of the Pilbara craton characterized by ratios of Al2O3/TiO2 blage (eastern Ontario), which contains (Australia) but in other greenstone ~ 20 and CaO/Al2O3 ~ 1, values that the ~1000 m-thick Munro komatiite belts, particularly those of post-3.0 Ga are similar to those of chondritic mete- flows. Some AUKs are also of Pro- age, they are rare. An association of orites and primitive mantle. Their REE terozoic and Phanerozoic ages such as both AUK and ADK is uncommon patterns (Fig. 6) are typically slightly those from Gorgona Island (Kerr et al. but has been documented in the depleted in LREE and have a flat 1996). Abitibi greenstone belt (e.g. Dostal and

HREE segment with (Gd/Yb)n ~ 1 (n- Al-depleted komatiites (ADK) Mueller 1997; Fan and Kerrich 1997) chondrite-normalized). They resemble are lower in Al2O3 and have low among other places. Komatiitic basalts the patterns of recent N-type mid- Al2O3/TiO2 (typically <12; Fig. 8) but include both Al-undepleted and Al- ocean ridge basalts (MORB) although high CaO/Al2O3 (~2–2.5). Their REE depleted types and their characteristics the absolute REE abundances in patterns (Fig. 6) have fractionated match those of komatiites. In addition komatiites are significantly lower than HREE with (Gd/Yb)n >1.3. Al-deplet- to the two komatiite groups, several in MORB (Fig. 6). The mantle-normal- ed komatiites have higher contents of other komatiite subtypes have been ized patterns of AUK are also charac- strongly incompatible trace elements recognized in particular regions but terized by near chondritic Ti/Zr (Th, LREE) than the AUK but their they are only subordinate (e.g. Sproule (~110) and depletion of Th, Nb and mantle-normalized patterns typically et al. 2002). LREE (Fig. 7). Al-undepleted komati- show small negative Zr and Hf anom- The Nd isotopic data on rela- ites are the most widespread komatiitic alies (Fig. 7). Al-depleted komatiites are tively fresh komatiites as well as their lavas and occur predominantly in 2.7 the most abundant komatiitic lavas in fresh pyroxenes from the Abitibi belt Ga Archean greenstone belts including the 3.5–3.0 Ga Barberton greenstone yielded GNd values of 2.8 to 3.8 (e.g. GEOSCIENCE CANADA Volume 35 Number 1 March 2008 27

Figure 9. The secular cooling curve for the temperature of the Earth mantle calculated by Richter (1988). Also shown Figure 8. Element ratios distinguishing AUK and ADK. The are the dry melting temperature for 3.5 Ga ADK from Bar- data for the AUK and ADK fields were compiled from litera- berton and 2.7 Ga AUK from the Belingwe greenstone belts ture, mainly by Dostal and Mueller (2004). Solid lines repre- (Nisbet et al. 1993), and Grove and Parman's (2004) estimat- sent chondritic (primitive mantle) values (n-chondrite-nor- ed komatiite temperatures for wet melting (graph modified malized; chondritic values after Sun and McDonough 1989). after Herzberg 1995, and Rollinson 2007).

Lahaye et al. 1995). Some komatiites presence of water significantly GPa (~370 km), whereas ADK with have slightly higher GNd values (e.g. decreased the melting temperature about 29 wt% MgO, had an eruption Lesher and Arndt 1995). (Grove et al. 1997; Grove and Parman temperature of about 1580EC and 2004). This model challenges the view melted at 1900EC at a depth of 18 PETROGENESIS that the Archean mantle was unusually GPa (~ 560 km; i.e. in the majorite hot (Fig. 9). The controversy regarding stability field; majorite garnet is Melting Conditions “wet” (subduction-related origin) ver- a high Mg- mineral From the time of their discovery, there sus “dry” ( origin) melt- which occurs in the mantle transition has been active debate about the melt- ing is still very much alive. zone, below 400 km). ing conditions required to produce liq- Currently, the conventional Most Archean plumes are uids of komatiitic composition. model among petrologists is that inferred to have been composed of Because the MgO contents of anhy- komatiites are high temperature, low depleted mantle material (Campbell et drous magmas are proportionally relat- viscosity melts produced by high al. 1989; Lesher and Arndt 1995). The ed to their melting temperature (higher degrees of anhydrous melting of man- low concentrations of incompatible MgO results from higher melting tem- tle plumes. They erupted at tempera- trace elements and their patterns (Fig. peratures), komatiite melts require sig- tures close to 1600EC. Because 7), and element ratios in both komati- nificantly higher temperatures (Green CaO/Al2O3 is strongly pressure itic types are, in general, consistent et al. 1975) than the 1250–1350EC dependent, Herzberg (1995) used an with their derivation from sources responsible for recent MORB. Tradi- Al2O3-CaO/Al2O3 plot to estimate the depleted in incompatible elements rela- tionally, most petrologists have accept- depth of melting of komatiite magmas. tive to a primitive mantle composition, ed that during the Archean and Paleo- Figure 10 shows that the Paleoarchean i.e. depleted by a previous melt extrac- proterozoic, komatiite melting temper- komatiites (mainly ADK), which have tion event (low degree [1–2%] partial atures ranged from 1600–1900EC, so low Al2O3 and high CaO/Al2O3, were melts; Hofmann 1988). Likewise, Nd komatiitic magmas were therefore generated at depths of 300–450 km isotopic characteristics of komatiites related to deep-seated mantle plumes (9–14 GPa), Neoarchean komatiites point to derivation from a depleted (Campbell et al. 1989). Plumes are hot (mainly AUK) from depths of mantle source (Fig. 11). The high con- upwelling jets in the mantle that origi- 150–200 km (5–6.5 GPa), and young tents of MgO (up to 30 wt%) coupled nate as temperature instabilities at ther- komatiites (< 100 Ma old) from depths with low abundances of incompatible mal boundary layers, and possibly may of 100–130 km (3–4 GPa). Nisbet et trace elements in komatiites suggest include material from the core–mantle al. (1993) proposed similar although that the komatiitic magma was generat- boundary and . Alterna- slightly higher pressure conditions; ed by a high degree of tively, these high temperatures led they inferred that komatiitic (AUK) when Mg-rich olivine and orthopyrox- some researchers to propose that the magmas that have about 26wt % MgO ene were dominant phases entering the komatiitic melts were produced by would have an eruption temperature of melts. Herzberg (1992) inferred that hydrous melting of the mantle wedge 1520EC and have melted at a tempera- the komatiites were formed by 30–50% above subduction zones, where the ture of 1800EC at a depth of about 12 partial melting of mantle peridotite. 28

Figure 11. Initial GHf vs. intial GNd diagram showing the man- tle array, which includes the fields for MORB and ocean island basalts (OIB), and Archean komatiites (solid circles) (after

Blichert-Toft and Arndt 1999). Positive Hf and Nd values are Figure 10. Al2O3 (wt%) vs. CaO/Al2O3 diagram showing the G G distribution of komatiites relative to the mantle solidus conventionally thought be derived from a depleted mantle (heavy solid line), which is depicted as a function of the vari- source. Solid (vertical and horizontal) lines correspond to the ation of pressure given in GPa (1 GPa is approximately values of bulk Earth. equivalent to 10 kb). The komatiites are subdivided according to their age. Paleoarchean komatiites are mainly ADK, (Fig. 9), probably not retain dissolved volatiles. Also, a Neoarchean komatites are predominantly AUK. The group by only about depletion of incompatible elements in of Phanerozoic rocks (< 100 Ma) includes picrites. The 100EC (Grove and most komatiites requires a depleted graph is modified after Herzberg (1995) and Condie (2001). Parman 2004). mantle source (i.e. a source which Solidus represents conditions at which the mantle will begin Grove and underwent previous melt extraction). to melt. Parman (2004) As water behaves as an incompatible argued that the element during melting, it would be An alternative model invokes presence of vesicles in komatiites and expected to have been consumed dur- wet mantle melting in subduction envi- the occurrence of pyroclastic komati- ing the depletion event and would not ronments. Modern plume magmas typ- ites is evidence for the degassing of be present during melting producing ically contain only very small amounts erupting hydrous komatiites, whereas komatiite magma (Sproule et al. 2002). of water (<0.5 wt%; Dixon et al. 2002) Parman et al. (1997) concluded that Bouquain et al. (2006) also disputed and modern hydrous melts are related high pre-eruptive water contents are the pyroxene argument of Parman et to volatiles released from the subduct- needed to reproduce observed pyrox- al. (1997). Thus there is a continuing ing slabs. Grove and his colleagues ene compositions in the Barberton debate on the origin of komatiites. (Grove and Parman 2004; Grove et al. komatiites. They inferred that pyrox- Several processes have also 1997; Parman et al. 1997) argued that enes from Barberton komatiites crys- been invoked to explain the relation- wet mantle melting can produce tallized from a magma having about 6 ship between komatiites and komatiitic komatiitic melts at temperatures well wt% water. The wet melting model is basalts. Komatiitic basalts can be below 1600EC as compared to 1800 also supported by analyses of melt derived from komatiites by fractional and 1900EC advocated by Nisbet et al. inclusions that have escaped metamor- crystallization or by fractional crystal- (1993). Experimental studies (e.g. phism. Shimizu et al. (2001) docu- lization accompanied by crustal con- Inoue et al. 2000) have shown that mented the presence of water in tamination. However, Sproule et al. hydrous melting can generate ADK at komatiitic melts. Because some (2002) argued that the relatively uni- relatively low temperatures Archean komatiites are associated with form compositions of komatiitic (1300–1500EC). The experiments of calc-alkaline volcanic rocks and basalts in the Abitibi greenstone belt, Barr and Grove (2006) suggest that the boninites, Grove and Parman (2004) accompanied by a lack of evidence of Barberton komatiites were formed by argued that like recent boninites, contamination, do not support an wet melting at depths lower than those komatiites may result from wet melting assimilation–fractional crystallization corresponding to of 1.7 GPa above a subduction zone. However, the model. Alternatively, komatiitic basalts (< 55 km). This would also imply that wet melting model has been fiercely can be derived from a similar source as the mantle during the Archean was not contested. Arndt et al. (1998) noted komatiites but by a lower degree of significantly hotter that at the present that low viscosity komatiite melts can- melting (Dostal and Mueller 1997). GEOSCIENCE CANADA Volume 35 Number 1 March 2008 29

Al-ddepleted vs. Al-uundepleted MINERALIZATION ASSOCIATED CONCLUSIONS Komatiites WITH KOMATIITES Komatiites provide a window into the Most recent models for the origin of Komatiites and komatiitic basalts local- composition and thermal dynamics of komatiites postulate that differences ly host magmatic Ni-sulfide (Ni-Cu- the Archean mantle. They are usually between ADK and AUK are related to PGE) mineralization. Famous deposits considered to be the product of man- the role of garnet in their sources (e.g. include those in the Archean Yilgarn tle plumes generated at depth, possibly Herzberg 1995; Arndt et al. 1997), and Pilbara cratons of Western Aus- near the core–mantle boundary although the sources of both komatiite tralia, the Abitibi greenstone belt, Zim- (Campbell et al. 1989; Arndt et al. types are isotopically (Lu-Hf and Nd- babwe craton, the Proterozoic (1.8 Ga) 1998; Sproule et al. 2002). The chemi- Sm) similar (Blichert-Toft and Arndt Cape Smith (Ungava) belt of Québec cal compositions of komatiites and 1999). Al-undepleted komatiites, which and the Proterozoic Thompson belt of related Mg-rich rocks appear to vary have flat HREE patterns and chondrit- Manitoba. Principal ore minerals are with time. For example, with respect to ic values of Al2O3/TiO2 (Fig. 8) and pyrrhotite, pentlandite, pyrite and chal- the MgO contents of common ultra- Ti/Zr, did not have garnet in the melt- copyrite. The sulfides formed as magmas, komatiites characteristic ing residue. This implies that garnet, immiscible liquids and thus they are of of the Archean have >18 wt% MgO, which can fractionate these elements, primary magmatic origin. The produc- komatiitic basalts dominant in the Pro- was either incorporated into the liquid tion of large amounts of immiscible terozoic have 12–18 wt% MgO and during melting or was not present in sulfide droplets that settle in the host picrites in the Phanerozoic have the source. Al-undepleted komatiites magma to form an orebody requires 10–13% wt% MgO. Because the MgO are thought to be produced by a large the attainment of sulfide saturation in contents of dry magmas are related to amount of melting of a garnet peri- the komatiitic melts, mainly through their melting temperature, the trend is dotite leaving only olivine (+/-or the assimilation of sulfur from sulfur- consistent with secular cooling of the orthopyroxene) in the residue (Arndt rich country rocks. Nickel, copper and Earth’s interior. However, recently, et al. 1997). Alternatively, AUK could platinoid elements as well as some researchers (Parman et al. 1997; have been generated by melting of a would then be scavenged by the sulfur Grove and Parman 2004; Wilson et al. garnet-free source either at a shallow to form sulfide droplets. 2003) have challenged the plume depth (above the garnet stability field; Naldrett (2004) subdivided model and attendant volatile-free man- < 3 GPa) or at great depth, below the these komatiite-hosted deposits into tle melting, and postulated that komati- mantle transition zone (> 660 km) three groups. The first type includes ites may be produced by hydrous melt- where (a major silicate min- ore bodies that are generally small (1 to ing at shallow mantle depth in a sub- eral in the lower mantle) becomes the 5 x 106 tonnes) but high grade (1.5–3.5 duction (arc) environment. In this sce- major mantle phase (Xie and Kerrich wt% Ni) and occur where massive sul- nario, temperatures would be consider- 1994). fides are concentrated at the base of ably lower than those required by the

Low Al2O3/TiO2 in ADK the host ultramafic flows in zones up plume model. Since many current (Fig. 8), as well as the fractionated to about 50 m thick. Examples include models of chemical differentiation and HREE patterns, indicate that either the 2.7 Ga deposits of the Kambalda thermal evolution of the Earth are their mantle source was depleted in Al, district (with 0.8–1.4 wt% Cu), which based on mantle plume-generated or garnet remained in the residue after lie in the Yilgarn craton, and the Lang- komatiites, the subduction model, if the melting and extraction of ADK. muir deposit of the Abitibi greenstone correct, would significantly change cur- The latter is supported by Lu-Hf iso- belt, about 40 km southeast of Tim- rent views of the evolution of Archean topic data (Blichert-Toft and Arndt mins, Ontario. Earth. Although recent data imply that 1999) which show that the Barberton The second deposit type many komatiites are derived from a dry ADK was derived from a garnet-bear- encompasses large (100 to 250 x 106 mantle source, there are still some ing source and their residuum was gar- tonnes) but low grade (~0.6 wt% Ni) rocks that could be subduction-related, net-rich. Thus ADK formed either in deposits of disseminated sulfides in such as high-SiO2 komatiites from the the presence of garnet at depths of olivine-rich cumulate dikes/sills that ~3.33 Ga Commondale greenstone 100–400 km or majorite garnet at feed komatiitic flows. Examples of belt in South Africa (Wilson et al. depths of 400–660 km (possibly a these deposits are the Six Mile and Mt. 2003), and some komatiitic basalts that plume tail; Campbell et al. 1989). Xie Keith deposits near Yakabindi, Western resemble boninites. More studies of and Kerrich (1994) inferred that the Australia and the Dumont deposit of the trace element and isotopic charac- trace-element signatures, particularly Québec (~ 60 km NE of Rouyn). The teristics of komatiites and their miner- the ratios of HFSE (Zr, Hf, Nb) to the third type includes deposits related to als as well as experimental studies, par- REE can distinguish between olivine Proterozoic komatiitic basalts that ticularly under wet melting conditions, (shallow mantle), majorite (>14 GPa; occur in rifted continental margin envi- are needed to constrain their origin. >~ 400 km; Herzberg 1995) and per- ronments. They host deposits of the Obviously, there is much left to learn ovskite (lower mantle) fractionation. Thompson (Manitoba) area and the about these fascinating rocks. The negative Zr-Hf anomalies Cape Smith belt (Ungava Peninsu- observed in ADK (Fig. 7) were inter- la, Québec). ACKNOWLEDGEMENTS preted to be due to melting with resid- I am grateful to Victor Owen for read- ual majorite garnet. ing an early draft of the manuscript, to 30

Andrew Hynes and Tony Fowler for core: Earth Planetary Science Letters, gent margin environment: Geochimica thorough, constructive reviews and to v. 206, p. 411-426. et Cosmochimica Acta, v. 61, p. 4723- Georgia Pe-Piper for editorial advice Campbell, I.H., Griffiths, R.W., and Hill, 4744. and continuous encouragement. I have R.I., 1989, Melting in an Archean Faure, F., Arndt, N. and Libourel, G., 2006, benefited greatly from discussions with mantle plume: Heads it is basalts and Formation of spinifex texture in Wulf Mueller. Many thanks go to Tim tails it is komatiites: Nature, v. 339, p. komatiites: An experimental study: 697-699. Journal of , v. 47, p. 591- Grove, Tony Fowler and Michel Houle Condie, K.C., 2001, Mantle Plumes and 1610. for the photos of komatiites and their Record in Earth history: Cam- Glotov, A.L., Polyakov, G.V., Hoa, T.T., Randy Corney for cheerful technical bridge University Press, Cambridge, Balykin, P.A., Akimtsev, V.A., assistance. 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