Komatiitic Magma Research Articles

Komatiite: NICHOLAS ARNDT, C. MICHAEL LESHER, AND STEVE J. BARNES. 488 Pp. Cambridge University Press, New York. 2008. ISBN: 978-0521874-748. Price CDN$150.00.

Komatiite, by Nicholas Arndt and the team of Michael Lesher and Stephen Barnes, demonstrates how much has been learned about this rock type since publication of the similarly named 1982 volume edited by N. Arndt and E.G. Nesbit. As far as possible, the new book focuses on the igneous aspects of komatiitic magmatism, although metamorphic processes cannot be avoided entirely. It is divided into two main parts, each containing seven chapters. The first part is entirely written by Arndt and is generally more descriptive in nature. The second part, with two chapters contributed by the team of Barnes and Lesher, is labelled “Interpretation,” but also contains a wealth of observations. The authors are all experienced and capable writers and they have produced a book that is highly readable and informative. Arndt occasionally allows a wry sense of humor to show through, which only makes the reading more enjoyable. Komatiite is richly illustrated with black-and-white photos, detailed line drawings, and tables and charts on almost every second page. The essential color plates of outcrop exposures and textures in thin section are also provided. Chapter 1 …

Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle

The origin of komatiites, volcanic rocks formed by unusually extensive melting of mantle rocks, mainly during the Archaean, is the subject of much debate. They are thought to have been produced either by anhydrous melting of anomalously hot mantle, or by hydrous melting at temperatures only a little higher than those of today. Berry et al. have now determined the oxidation state of iron in pristine samples of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe. Their findings are consistent with near-anhydrous melting of a source with an oxidation state similar to that of present-day ocean-floor basalt. The results suggest that the Belingwe melt was a product of high mantle temperatures of about 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archaean. The original iron isotope ratios of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe have been determined. These measurements are consistent with near-anhydrous melting of a source with similar oxidation state to that of present-day ocean-floor basalt. The results support the identification of the Belingwe komatiite as a product of high mantle temperatures of ∼ 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archean. Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle1,2,3 or hydrous melting at temperatures only modestly greater than those found today4,5. Here we determine the original Fe3+/ΣFe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe6, preserved as melt inclusions in olivine, to be 0.10 ± 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe3+/ΣFe value, together with a water content of only 0.2–0.3 wt% (ref. 7), excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe3+/ΣFe ratio would be ∼1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (∼1,700 °C), rather than melting under hydrous conditions (3–5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.

Deducing a reducing mantle

Increasingly sophisticated techniques are being used to persuade ancient rocks to yield information about conditions on and in the early Earth — for instance, about the oxidation state of the mantle. The origin of komatiites, volcanic rocks formed by unusually extensive melting of mantle rocks, mainly during the Archaean, is the subject of much debate. They are thought to have been produced either by anhydrous melting of anomalously hot mantle, or by hydrous melting at temperatures only a little higher than those of today. Berry et al. have now determined the oxidation state of iron in pristine samples of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe. Their findings are consistent with near-anhydrous melting of a source with an oxidation state similar to that of present-day ocean-floor basalt. The results suggest that the Belingwe melt was a product of high mantle temperatures of about 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archaean.

No Water, No Cyanobacteria—No Calc-alkaline Magmas: Progressive Oxidation of the Early Oceans May Have Contributed to Modernize Island-Arc Magmatism

A remarkable change in Earth history took place in Late Archean time. The time-span 3.0-2.5 Ga marks the progressive end of the low-P, high-T subduction regime and komatiitic magmatism. This coincides with a period of rapid crustal growth, when up to 70% of the present continental masses formed. Most authors would agree that such change indeed took place, although the why and how remain a controversial issue. By the end of the Archean, calc-alkaline magmatism was firmly established, and a key point relates to the nature of these arc-related igneous rocks. These magmas are not only characterized by silica enrichment, but are also more oxidized than their primitive counterparts (e.g., komatiites, MORBs). Thus, an increase in the oxygen fugacity of the upper mantle is here regarded as the turning point that allowed formation of calc-alkaline complexes during Archean time. In this respect we suggest that the appearance of cyanobacteria more than 3.7 billion years ago, may have played a decisive role in changing the old Archean tectonomagmatic scenario, by increasing the oxidizing potential of the oceans, and therefore inducing iron oxide precipitation onto the ocean floors in the form of FeO(OH)-like minerals. If subduction processes, whatever the type, were already active by Early—Meso Archean time (as seems to be the case), it follows that a massive amount of iron oxides must have been introduced into the mantle wedge. The melting of these chemical sediments would have resulted in liberation of oxygen, thus contributing to the oxidation of this realm, and therefore, triggering the onset of "modern" calc-alkaline, magmatism.

Komatiitic Sills and Multigenerational Peperite at Dundonald Beach, Abitibi Greenstone Belt, Ontario: Volcanic Architecture and Nickel Sulfide Distribution

The density and the tectonic stresses in the deep crust and the physical properties of komatiitic magmas determine the level to which they will rise, but once in the near-surface environment, the density and rheology of the near-surface rocks (consolidated and dense lava flows and sedimentary rocks versus unconsolidated sedimentary or volcaniclastic deposits) govern whether they will be emplaced as lava flows, invasive flows, or sills. Where near-surface strata are competent (i.e., flow-dominated or consolidated sedimentary and/or volcaniclastic successions), komatiitic magma erupts as flows that form extensive lava shields (e.g., Kambalda). However, where near-surface strata are not competent (i.e., unconsolidated volcaniclastic- and/or sediment-dominated successions), komatiitic magmas typically are emplaced as high-level sills that increase the bulk density of the volcano-sedimentary pile and eventually allow the eruption of lava and the construction of complex sub-volcanic-volcanic lava shields (e.g., Dundonald and Shaw dome, Abitibi belt; Raglan, Cape Smith belt; Pechenga, Kola Peninsula; Thompson Ni belt, Manitoba). The latter environment is illustrated in the volcano-sedimentary succession in Dundonald Township, which is only weakly metamorphosed and deformed and superbly exposed in glacially polished outcrops. The volcano-sedimentary succession in this area comprises (from base to top) (1) the McIntosh formation, composed of a succession of pillowed and massive intermediate volcanic flows; (2) the Dundonald formation, composed of a lower section of komatiite sills, argillites, and felsic volcaniclastic deposits and an upper section of komatiite flows, komatiitic sills, and pillowed intermediate volcanic flows; and (3) the Frederick House Lake formation, composed of massive and pillowed mafic flows. The distribution and thickness of argillites and felsic volcaniclastic rocks define a synvolcanic graben in which the Dundonald South and Alexo Ni-Cu-(PGE) deposits occur within the center and the margin, respectively. Sills and peperites in the lower komatiitic succession at Dundonald Beach exhibit a multigenerational emplacement history recording progressive lithification and increases in the bulk density and rheological strength of the unconsolidated argillites, which ultimately permitted the eruption of lavas at Alexo. Importantly, the nature of the near-surface rocks also influences the localization of Ni-Cu-(PGE) deposits. In lava shields (e.g., Kambalda), the initial eruptions are typically most voluminous and, if erupted at sufficient flow rates, form channelized flows conducive to thermomechanical erosion of sulfur-rich footwall rocks. In sub-volcanic-volcanic lava shields, however, channelized units may occur within the subvolcanic plumbing system and/or within overlying lavas. Where only sills are channelized (e.g., Thompson), mineralization will occur only within the subvolcanic environment; where only the flows are channelized (e.g., Damba-Silwane, Zimbabwe), mineralization will occur only within the volcanic environment. Where both sills and lava flows are channelized (e.g., Dundonald, Shaw dome, Raglan), the distribution of the mineralization is more diverse and may occur as subsea- or sea-floor Ni-Cu-(PGE) deposits.

Archean Komatiitic Sill-hosted Chromite Deposits in the Zimbabwe Craton

Lenticular chromite deposits of widely varying age, metamorphic grade and strain rate, as well as size, shape, and ore quality, are widespread in greenschist to amphibolite grade greenstonegneiss terrain in the south-central part of the Zimbabwe craton and in granulite grade equivalents in the Northern Marginal zone of the Limpopo belt in the far south. In order of age and, very approximately, decreasing abundance, they form three groups: (1) early Archean (3.5–3.2 Ga) deposits associated with amphibolite grade greenstone remnants in the 3.6 to 3.35 Ga Tokwe gneisses (e.g., Hornet, Valley) and on the amphibolite grade eastern flank of the Shurugwi greenstone belt, and granulite grade greenstone remnants in the Northern Marginal zone (e.g., Inyala, Rhonda); (2) mid-Archean deposits within the ca. 3.0 Ga Shurugwi Group in the main greenschist part of the Shurugwi belt (e.g., Peak, Railway Block) and associated with the ca. 3.0 Ga Belingwean Supergroup in several other belts; and (3) late Archean (ca. 2.74 Ga) deposits mostly in the western segment of the Masvingo belt (e.g., Prince). With their host and country rocks, the deposits together preserve geologic and chromite compositional evidence of formation within poorly differentiated, komatiitic sills intruded into predominantly siliciclastic, BIF platformal sedimentary sequences associated with mafic and/or ultramafic lavas. Unlike the thin, laterally extensive chromitite layers associated with macrocyclic units of large, layered intrusions, the komatiitic sill-associated deposits are the remnants of laterally discontinuous (up to several tens of meters) thick chromitite layers or dunite-chromitite zones located at high stratigraphic levels of their host intrusions not far below the upper pyroxenite differentiates. In at least some deposits—for example, at Railway Block, where the chromitites form narrow, elongate bodies at least 1,000 m long—the lateral discontinuities are most likely primary; in many others, later deformation obscures the primary geometry. Chromitite formation may have been delayed by the late attainment of equilibrium crystallization and the reduced nature of the magma. The crystallization of thick, massive, cyclically layered, Cr-rich chromitites was most probably controlled by the stabilization of chromite relative to olivine by absorption of connate water derived from wet host sediments and by episodic, flow-through processes which, at Railway Block, may have been focused along narrow pathways across the magma chamber floor. The remarkable abundance of such chromitites and their formation in repeated episodes over 1,000 m.y. (cf. other cratons) can be attributed to generation of komatiitic magmas in a mantle source that was both Cr rich and strongly reduced. Archean komatiitic sill-hosted deposits constitute a third type of chromite deposit in addition to stratiform and podiform deposits in layered intrusions and ophiolites, respectively. The economically important variability of bulk ore compositions (especially Cr, Fe, Al, Mg) is due to several factors: magnetite introduction during serpentinization and talc-carbonate alteration at greenschist grade, subsolidus fluid-related and other metamorphic cation exchange processes (dependant on chromite body size), and, possibly, exsolution of Cr-Al and magnetite phase pairs, both at midamphibolite to granulite grade, as well as fluid-related retrogression and partial conversion of chromite to (mainly) chlorite during amphibolite metamorphism.

Geochemistry of komatiites in the Eastern Goldfields Superterrane, Western Australia and the Abitibi Greenstone Belt, Canada, and implications for the distribution of associated Ni–Cu–PGE deposits

The Abitibi Greenstone Belt (AGB) of the Superior Province in Canada, and the extensive greenstone sequences of the Eastern Goldfields Superterrane (EGS) of the east Yilgarn Craton in Western Australia, both contain widespread and well-studied komatiite sequences. Both of these 2˙7 Ga belts contain broadly similar assemblages of volcanic rocks ranging from komatiites to rhyolites, and both contain abundant lode gold mineralisation. Despite this similarity, the two belts have widely differing metal endowments. The EGS is heavily endowed with magmatic Ni–Cu–PGE deposits, but contains only minor volcanogenic massive Cu–Zn sulphide occurrences, whereas the situation in the AGB is the reverse. The komatiite assemblages of the two belts are compared using a locality-weighted analysis of large whole-rock geochemical datasets. The EGS contains a higher relative proportion of highly olivine-rich cumulate rocks relative to the AGB, and shows more extensive evidence for crustal contamination of komatiite magmas. The most highly mineralised region of the AGB, the Shaw and Bartlett Dome area, is the most similar to the EGS in the distribution of komatiite rock types and also the presence of a crustal contamination signature. The bulk of samples in both data sets show no evidence for extensive PGE depletion attributable to sulphide liquid segregation, indicating that the komatiite suites in both belts were undersaturated in sulphide during partial melting, ascent and eruption. The combination of abundant thick cumulate-rich komatiite units and widespread crustal contamination in the EGS can be attributed to a predominance of rapid, high volume eruptions, relative to generally lower eruption rates in the AGB. Differences in lithospheric structure are likely to be the cause.

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

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

COTECTIC PRECIPITATION OF OLIVINE AND SULFIDE LIQUID FROM KOMATIITE MAGMA AND THE ORIGIN OF KOMATIITE-HOSTED DISSEMINATED NICKEL SULFIDE MINERALIZATION AT MOUNT KEITH AND YAKABINDIE, WESTERN AUSTRALIA

An empirical equation for the relationship of sulfur solubility to silicate magma composition and temperature has been combined with numerical simulations of the fractional crystallization of olivine from komatiite magma to predict the relative proportions of olivine and sulfide liquid which should segregate at equilibrium from a fractionating komatiite. The predicted cotectic ratio of olivine to sulfide is higher and the S content of the cumulate assemblage lower by a factor of about three compared with the peak observed in type II (Mount Keith-type) dunite-hosted disseminated Ni sulfide deposits for the observed range of olivine compositions in the dunites. This implies that a purely “cotectic” precipitation model for this deposit type is incorrect, and that most of the sulfide liquid found in type II deposits is transported and mechanically deposited.

Comparative lithogeochemistry of komatiites in the Norseman-Wiluna and Abitibi Greenstone Belts, and implications for nickel sulfide targeting

This paper uses large sets of whole-rock geochemical data to make comparisons between the komatiitic rocks of Abitibi (AGB) and Norseman-Wiluna (NWGB) greenstone terranes. The NWGB komatiite suite has a much higher proportion of highly olivine-enriched cumulates than the AGB suite, as indicated by data-density distributions on plots of MgO vs FeO and MgO vs Cr, although average compositions of the komatiite magmas in the two belts were not significantly different. NWGB komatiites appear generally more contaminated, on the basis of various ratios of strongly to moderately incompatible low-mobility trace elements. Both factors which are likely contributors to the much higher Ni sulfide resource endowment of the NWGB. The combination of high degrees of contamination and presence of olivine adcumulates in the NWGB attests to the presence of exceptionally high-intensity, prolonged eruptions, capable of forming long-lived entrenched magma pathways, represented by highly olivine-enriched cumulates, and capable of melting substrates to form orebodies. This is in contrast with more episodic, lower volume eruptions in the AGB. The contrast is interpreted as the result of crustal structure and tectonic setting, rather then the size and intensity of mantle plume sources.

Chalcophile Element Geochemistry and Metallogenesis of Komatiitic Rocks in the Abitibi Greenstone Belt, Canada

Komatiitic rocks occur in four lithotectonic assemblages in the Abitibi greenstone belt of the Archean Superior province: the 2750 to 2735 Ma Pacaud assemblage, the 2723 to 2720 Ma Stoughton-Roquemaure assemblage, the 2719 to 2710 Ma Kidd-Munro assemblage, and the 2710 to 2703 Ma Tisdale assemblage. There are significant volcanological and geochemical differences between the komatiitic rocks in these assemblages. Those in the older (Pacaud and Stoughton-Roquemaure) assemblages are primarily Ti- and Al-depleted-Ti-enriched komatiites, respectively, that appear to be uncontaminated by upper continental crustal rocks, form unchannelized lava flow facies, and are not underlain by sulfur-rich metasedimentary rocks. In contrast, those in the younger (Kidd-Munro and Tisdale) assemblages are primarily Al-undepleted komatiites, locally contaminated by upper continental crustal rocks, locally form lava channels and channelized sheet flow facies, and locally overlie sulfur-rich metasedimentary rocks. Sixty-four unmineralized samples from all four komatiite-bearing assemblages in the Abitibi greenstone belt have similar unfractionated platinum-group element (PGE) abundances at 0.5 to 5 times chondritic values. Minor variations in PGE abundances are attributable to variable (but uniformly high) degrees of partial melting, fractional crystallization and/or accumulation of olivine, chromite, or clinopyroxene and, rarely, loss of sulfide or Ir-bearing alloys. Almost all of the komatiitic magmas and lavas represented by the samples, regardless of komatiite type (including Al-undepleted, Al-depleted–Ti-enriched, and Ti-depleted komatiites) and depth of melt extraction, appear to have been derived by high-degree partial melting of sources with essentially identical chalcophile element contents and to have remained undersaturated in sulfide during melt extraction, ascent, eruption, and emplacement. This is similar to most komatiites worldwide. Type I (Kambalda-type) stratiform, basal massive-disseminated and type II (Mt. Keith-type), strata-bound, internally disseminated Ni-Cu-(PGE) deposits in the Abitibi greenstone belt are hosted mainly by Al-undepleted cumulate komatiites that are associated with sulfur-rich country rocks in the Kidd-Munro (e.g., Alexo-Dundonald, Marbridge, Dumont) and Tisdale (e.g., Langmuir-Redstone, Texmont, Sothman, Bannockburn) assemblages. Although many other deposits of this type are also hosted by Al-undepleted komatiites (e.g., Damba-Silwane-Shangani, Kambalda, Mt. Keith, Perseverance), some are not (e.g., Boa Vista, Forrestania, Ruth Well), confirming that the depth of melt extraction or the precise degree of partial melting do not appear to be important controls on the genesis of magmatic Ni-Cu-(PGE) deposits. However, virtually all deposits of this type are hosted by thick, cumulate komatiite units interpreted to represent lava channels or channelized sheet flows and most are associated with sulfur-rich country rocks, suggesting that high magma and/or lava flux, lava channelization, and sulfur-rich country rocks are the most critical features in determining the prospectivity of komatiites in the Abitibi greenstone belt and elsewhere.

The Geochemistry of Ultramafic to Mafic Volcanics from the Belingwe Greenstone Belt, Zimbabwe: Magmatism in an Archean Continental Large Igneous Province

The evolution of the late Archean Belingwe greenstone belt, Zimbabwe, is discussed in relation to the geochemistry of the ultramafic to mafic volcanic rocks. Four volcanic types (komatiite, komatiitic basalt, D-basalt and E-basalt) are distinguished in the 2� 7Ga Ngezi volcanic sequence using a combination of petrography and geochemistry. The komatiites and D-basalts are rocks in which isotopic systems and trace elements are depleted. Chemical variations in komatiites and D-basalts can be explained by fractional crystallization from the parental komatiite. In contrast, komatiitic basalts and E-basalts are siliceous and display enriched isotopic and trace element compositions. Their chemical trends are best explained by assimilation with fractional crystallization (AFC) from the primary komatiite. AFC calculations indicate that the komatiitic basalts and E-basalts are derived from komatiites contaminated with � 20% and � 30% crustal material, respectively. The volcanic stratigraphy of the Ngezi sequence, which is based on field relationships and the trace element compositions of relict clinopyroxenes, shows that the least contaminated komatiite lies between highly contaminated komatiitic basalt flows, and has limited exposure near the base of the succession. Above these flows, D- and E-basalts alternate. The komatiite appears to have erupted on the surface only in the early stages, when plume activity was high. As activity decreased with time, komatiite magmas may have stagnated to form magma chambers within the continental crust. Subsequent komatiitic magmas underwent fractional crystallization and were contaminated with crust to form D-basalts or E-basalts.

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

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

Discovery of Archean continental and mantle fragments inferred from xenocrysts in komatiites, the Belingwe greenstone belt, Zimbabwe

Research Article| April 01, 2004 Discovery of Archean continental and mantle fragments inferred from xenocrysts in komatiites, the Belingwe greenstone belt, Zimbabwe Kenji Shimizu; Kenji Shimizu 1Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan Search for other works by this author on: GSW Google Scholar Eizo Nakamura; Eizo Nakamura 1Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan Search for other works by this author on: GSW Google Scholar Katsura Kobayashi; Katsura Kobayashi 1Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan Search for other works by this author on: GSW Google Scholar Shigenori Maruyama Shigenori Maruyama 2Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8551, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Kenji Shimizu 1Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan Eizo Nakamura 1Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan Katsura Kobayashi 1Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori 682-0193, Japan Shigenori Maruyama 2Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8551, Japan Publisher: Geological Society of America Received: 31 Aug 2003 Revision Received: 19 Dec 2003 Accepted: 22 Dec 2003 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2004) 32 (4): 285–288. https://doi.org/10.1130/G20162.1 Article history Received: 31 Aug 2003 Revision Received: 19 Dec 2003 Accepted: 22 Dec 2003 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kenji Shimizu, Eizo Nakamura, Katsura Kobayashi, Shigenori Maruyama; Discovery of Archean continental and mantle fragments inferred from xenocrysts in komatiites, the Belingwe greenstone belt, Zimbabwe. Geology 2004;; 32 (4): 285–288. doi: https://doi.org/10.1130/G20162.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Controversy exists as to whether the 2.7 Ga Belingwe greenstone belt, Zimbabwe, is autochthonous or allochthonous. In this study we report direct evidence for an autochthonous continental setting for the Belingwe greenstone belt. Garnet and clinopyroxene xenocrysts were discovered in the fresh ultramafic komatiites. Major and trace element compositions of these xenocrysts suggest that they originated from mafic lower crust, presumably garnet granulite in composition, at a low temperature of ∼600 or ∼700 °C. Furthermore, in a komatiitic basalt sample, we have discovered orthopyroxene and clinopyroxene xenocrysts that may have originated from lithospheric mantle beneath the continental margin. Discoveries of these xenocrysts indicate that the komatiite magma was transported to the surface and quenched rapidly enough to prevent the complete melting of continental fragments. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Re–Os systematics of the ca. 2.7-Ga komatiites from Alexo, Ontario, Canada

The Re–Os isotope systematics of a suite of komatiitic flows from Alexo Township, in the Abitibi greenstone belt, Ontario were examined in order to characterize the partitioning behaviors of Re and Os during the generation and subsequent differentiation of komatiitic magma, and also to determine the initial Os isotopic composition of the late-Archean mantle source of these rocks. This suite of komatiites was suitable for these objectives because of their high Os concentrations, ≥∼1 ppb, and low 187Re/ 188Os ratios (ranging between 0.4 and 2.4). The Re–Os isotope results for whole-rock komatiites and chromite separates yield a model 3 isochron age of 2762±76 Ma. This age is in general agreement with previously determined U–Pb ages of zircons from associated felsic units and also with Sm–Nd and Pb–Pb isochron ages of these flows. This suite of Alexo komatiites, thus, demonstrates relatively limited mobility of Re and Os at the whole-rock scale, even after large-scale alteration and ambient lower greenschist facies metamorphism. The initial Os isotopic composition ( 187Os/ 188Os (I)=0.1080±0.0012) obtained from the regression is essentially chondritic ( γ Os=−0.1±1.0). Our results, therefore, suggest an absence of any large-scale deviation in initial Os isotopic composition of the mantle source of these komatiites relative to the putative chondritic composition of the contemporaneous convective upper mantle. For these komatiites, Os was moderately compatible with the mantle residue ( D Os mantle-melt∼2.1), whereas Re was incompatible ( D Re∼0.20). These D's are consistent with those previously estimated for other Archean komatiites. The apparent D Os olivine/liquid and D Re olivine/liquid in Alexo komatiites are 1.7 and 0.66, respectively. Our results, therefore, suggest that Os is slightly compatible, whereas Re is moderately incompatible in olivine or a coprecipitating phase during magmatic differentiation of komatiitic liquids. The concentrations of Re and Os in Alexo komatiites are also comparable to those reported for komatiites from other greenstone belts. The apparent magmatic behaviors of both Re and Os in the whole-rock komatiites indicate that the Re–Os elemental systematics are primary features of these rocks. The Os isotopic results obtained for the Alexo komatiites, combined with those from the spatially associated Munro Township komatiites suggest a source for komatiites in the southern volcanic zone of the Abitibi greenstone belt that was dominated by Os with a chondritic isotopic composition. Also, the Os isotopic results, combined with previous Nd and Pb isotopic results, are consistent with derivation of the komatiites from either the contemporaneous convective upper mantle or a deep mantle source with isotopically similar characteristics. These results imply that if the chondritic Os isotopic composition of the mantle sources for the komatiites were set by a chondritic “late veneer”, most late accretion and subsequent homogenization within the mantle must have occurred prior to their eruption at ∼2.7 Ga.

Spatial and temporal variations in the geochemistry of komatiites and komatiitic basalts in the Abitibi greenstone belt

Komatiites and komatiitic basalts in the Abitibi greenstone belt exhibit pronounced spatial and temporal variations in major and trace element geochemistry that provide important constraints on their petrogenesis and tectonic setting. Ti-depleted komatiites exhibit high Al 2O 3/TiO 2 (25–35) and low [Gd/Yb] MN (∼0.6–0.8), suggesting derivation by (1) dynamic melting of a refractory source leaving a refractory harzburgite residue, similar to that which produced Gorgona picrites, or (2) relatively high degree melting of a garnet-rich source leaving a peridotite residue. Al-depleted–Ti-enriched komatiites exhibit low Al 2O 3/TiO 2 (6–14) and high [Gd/Yb] MN (∼1.2–2.0), suggesting derivation by partial melting of a garnet peridotite source leaving a garnet-bearing residue. Al-undepleted komatiites exhibit moderate Al 2O 3/TiO 2 (15–25) and moderate [Gd/Yb] MN (∼0.8–1.2), suggesting derivation by shallower or higher-degree partial melting of a garnet peridotite source leaving a garnet-free residue. Those in the 2750–2735 Ma Pacaud assemblage are all Ti-depleted komatiites, those in the 2725–2720 Ma Stoughton–Roquemaure assemblage are predominantly Al-depleted–Ti-enriched komatiites with lesser Al-undepleted komatiites, those in the 2718–2710 Ma Kidd–Munro assemblage are predominantly Al-undepleted komatiites with lesser Al-depleted–Ti-enriched komatiites, and those in the 2710–2703 Ma Tisdale assemblage are predominantly Al-undepleted komatiites. Some of the komatiites in the younger assemblages (Kidd–Munro and Tisdale) are enriched in highly incompatible lithophile elements, exhibit low [Nb/Th] MN, and felsic-intermediate rocks in these assemblages contain inherited zircons, suggesting contamination by upper crustal rocks or sediments derived from upper crustal rocks. The high Mg contents of the komatiites suggest that they formed in mantle-derived plumes. The number of plumes required to generate the komatiitic magmatism in the Abitibi greenstone belt is not clear; it may be possible to derive all of the different magma types from different parts of a single plume at different times, but the long duration of komatiitic magmatism (≥47 My) suggests that multiple plumes are more likely. In any case, the temporal changes in composition are consistent with a decreasing influence of garnet in the source region of the plume(s) with time. There are several possible mechanisms to account of these variations, none of which are mutually exclusive: (1) a decrease in the depth of melt extraction as the plume(s) ascended; (2) a decrease in the amount of magma derived from deeper levels in the plume relative to that derived from shallower levels in the plume, possibly representing a change in plume morphology with time, for example as it was dragged by the overlying plate; (3) a decrease in the rate of plume ascent due to interaction with subducting slabs; or (4) a decrease in the thickness of the lithosphere during plume ascent, perhaps related to lithospheric thinning by the rising plume(s). The restriction of contamination to individual flow units or individual parts of flow units in the Kidd–Munro and Tisdale assemblages suggests that crustal contamination occurred locally, via assimilation of country rocks during emplacement.

Cr-spinel, an excellent micro-container for retaining primitive melts – implications for a hydrous plume origin for komatiites

Ultramafic melt inclusions were discovered in Cr-spinels of 2.7 Ga Al-undepleted komatiites from the Belingwe Greenstone Belt, Zimbabwe. The inclusions consist of glass and sub-micrometer-size quench crystals of olivine and clinopyroxene. Homogenized melt inclusions are highly magnesian, ranging from 12.5 to 19.5 wt% in MgO content, and are also close to host komatiitic magma in other compositions. This fact indicates entrapment of melt into host spinel during the early stages of crystallization. The water content of two melt inclusions was determined using an ion probe; the high magnesian melt inclusion, 17.5 wt% in MgO, contains 1.1 wt% H2O, whereas the moderately magnesian melt inclusion, 11.8 wt% in MgO, has 1.7 wt% H2O. This evidence suggests that the primary komatiite melt contained 0.8–0.9 wt% H2O and 23.4–25.0 wt% MgO. The water content is about five times greater than previous estimates from melt inclusions in olivine [McDonough and Danyushevsky, EOS Trans. AGU 76 (1995) S266]. In addition, even the high H2O content preserved in melt inclusions within Cr-spinel may represent the minimum estimates of the parental composition, because part of the water should be dehydrated from parental magma during crystallization in the magma chamber. If the komatiite melt was formed by a high degree of partial melting of a peridotite, the source mantle should contain considerable amounts of water (∼0.5 wt%). However, recent melting experiments of hydrous peridotite indicate that the addition of 0.5 wt% H2O to mantle peridotite would not significantly decrease komatiite liquidus temperature [Asahara et al., Geophys. Res. Lett. 25 (1998) 2201–2204]. Petrological study of melt inclusions together with experimental data suggest that Belingwe komatiites were formed from a hydrous plume at high temperatures.

Melting relations of hydrous pyrolite in CaO‐MgO‐Al2O3‐SiO2‐H2O System at the transition zone pressures

Phase relations and melt compositions in CaO‐MgO‐Al2O3‐SiO2‐pyrolite under hydrous (+2% of H2O) and anhydrous conditions have been determined at 13–20 GPa and 1600–2220°C. Liquidus and solidus temperatures for the hydrous system are about 50–100°C and 180–240°C lower than those for the dry system, respectively. Majorite is a liquidus phase of the hydrous pyrolite from 13 to 20 GPa. Olivine is a liquidus phase at 13 GPa and both periclase and majorite are the liquidus phases at 20 GPa in the dry pyrolite. We observed expansion of the stability field of anhydrous phase B in hydrous experiments. Compositions of partial melts at 13–20 GPa are generally similar in dry and hydrous systems, but hydrous melts contain more SiO2 at 13–17 GPa. The melts formed by low degree of melting have Al2O3‐depleted and CaO‐rich compositions. Trends of hydrous melt compositions are generally consistent with those of aluminum‐depleted komatiite magmas.

Late Archaean Ti–rich, Al–depleted komatiites and komatiitic volcaniclastic rocks from the Murchison Terrane in Western Australia

Greenstone belts in the northern Murchison Terrane of the Yilgarn Craton contain an extensive suite of 2.9–3.0 Ga, porphyritic komatiites and komatiitic volcaniclastic rocks. These unusual Ti–rich Al–depleted komatiites have been sampled at Gabanintha and are characterised by higher incompatible‐element abundances than most suites of Barberton‐type Al–depleted komatiites. They form a petrogenetically related group with similar Ti– and incompatible‐element‐rich, Al–depleted porphyritic komatiites and komatiitic volcaniclastic rocks from Karasjok in Norway, Dachine in French Guiana and Steep Rock‐Lumby Lake in Canada (here called Karasjok‐type komatiites). Their Al–depletion results from magma generation at depths of >250 km in the presence of residual majorite‐garnet. The porphyritic textures and abundance of amygdales and volcaniclastic rocks typical of this type of komatiite are features of hydrous ultramafic magmas. The incompatible‐element‐rich ultramafic rocks from Dachine contain diamonds that were most likely picked up as parent magmas interacted with mantle lithosphere that had been hydrated and chemically modified. Consequently the interaction of Karasjok‐type komatiite magmas with thick, island arc or continental mantle lithosphere may have resulted in their elevated water and incompatible‐element contents. The occurrence of Karasjok‐type komatiite lavas and volcaniclastic rocks in the northern Murchison Terrane suggests that during the Late Archaean that terrane had a hydrated, metasomatised or subduction‐modified mantle lithosphere.

Dacitic volcanism in the course of the Rio das Velhas (2800-2690 Ma) orogeny: a brazilian archean analogue (TTD) to the modern adakites

The evolution of the Rip das Velhas Greenstone Belt followed two main episodes of crustal addition. An initial phase of ocean- floor spreading is associated with tholeiitic to komatiitic magmatism. This early accreted, short-lived oceanic crust underwent rapid subduction, giving rise to widespread arc-related, felsic calc-alkaline volcanism, triggered by the direct melting of the oceanic slab. The felsic rocks comprise lavas and pyroclastic facies, extruded at ca. 2775 Ma and rapidly dispersed through syn- and post-eruptive submarine resedimentation processes. The sequence is coeval with a series of granitoids intrusive into the belt margins, characterizing a tonalite-trondhjemite-dacite (TTD) magmatism. The LILE depleted, Na-rich, chemical signature shown by the volcanic rocks fits well with a genetic model which incorporates aspects of the evolution of adakites in the modern Circum-Pacific arcs, and also the ancient FI dacites from the Barberton Greenstone Belt. Keywords: Rio das Velhas Greenstone Belt, evolution, Archean felsic volcanism, adakites, TTD association