Komatiitic Composition Research Articles

Sulfur in Archean komatiite implies early subduction of oceanic lithosphere

Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ33S from −0.20‰ to −0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ33S < 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.

The chromian spinels of the Lyavaraka ultrabasic complex, Serpentinite Belt, Kola Peninsula, Russia: Patterns of zoning, hypermagnesian compositions, and early oxidation

ABSTRACTThe maximum value of Mg# [= 100Mg/(Mg + Fe2+ + Mn)] in chromium-bearing spinel-group minerals (Chr) in the Ultrabasic Core Zone (UCZ) of the Lyavaraka orthopyroxenite – harzburgite – dunite complex of the Serpentinite Belt in the Kola Peninsula is 54.5–67.5. Such highly magnesian compositions of spinel are associated with notable enrichments of ferric iron (Fe3+# 58–63). There are two generations of accessory Chr in the UCZ unit. The first generation occurs as inclusions in olivine that is not unusually magnesian (Mg# 90.3), and the second is closely associated with serpentine. The compositional series of Chr at Lyavaraka attains more aluminous compositions than was observed in nearby intrusive bodies. The anomalously high level of Mg in Chr, also manifest in ilmenite, is mainly a result of the high intrinsic fugacity of oxygen attained locally in the melt. A progressive buildup in H2O and increase in fO2 likely resulted from efficient vesiculation and selective loss of H2 from the Al-undepleted komatiitic magma crystallizing in a shallow setting. The chromian spinel forming in such a modified magma is virtually unzoned in Mn, and a minor quantity of Mn is also present in olivine and orthopyroxene. In contrast, zinc is strongly partitioned in the core of Chr, as it is relatively incompatible in the coexisting olivine and orthopyroxene at that stage. Zinc efficiently partitioned into the H2O-enriched melt, which crystallized as the pegmatitic orthopyroxenite near the contacts at Lyavaraka. A high potential of oxidation appears to be characteristic of all orthopyroxenite – harzburgite – dunite suites of the Serpentinite Belt formed from a primitive melt of komatiitic composition.

Geochemical and isotopic constraints on the dynamics of the Earth: a Special Issue dedicated to Jan Dirk Kramers and his legacy to geochemistry

Chromium exists in two oxidation states Cr2+ and Cr3+ during high-temperature magmatic processes and changes in Cr redox are often associated with stable isotopic fractionation. Thus, the stable chromium isotope compositions of mantle-derived magmas bear the potential to trace the oxidation states of their mantle sources as well as any post melting changes in Cr redox (e.g. during magmatic differentiation), in a manner similar to magmatic stable Fe isotopic fractionation. However, these stable isotope fractionation effects are less understood for Cr relative to Fe. Komatiites and tholeiitic basalts represent a wide range of mantle-derived partial melts with variable fractional crystallization of phases with different affinity for Cr2+ and Cr3+. Thus, they offer potential archives to better understand high-temperature Cr isotope fractionation processes and mantle redox.Here, we report major and trace elements as well as coupled stable Cr and Fe isotope compositions of two well-characterized Archean komatiite-tholeiite suites from the Barberton Greenstone Belt, South Africa and Eswatini, and Belingwe Greenstone Belt, Zimbabwe. The sample suites range in MgO concentrations from 3.85 to 34.33 wt%, which allows investigation of the impact of large degrees of magmatic differentiation in a komatiite-basalt system. Whole-rock δ53/52CrSRM979 and δ56/54FeIRMM014 values range from −0.390 ± 0.016 to −0.061 ± 0.016‰ and −0.014 ± 0.018 to +0.192 ± 0.018‰, respectively. The komatiites have a very narrow range in their Cr isotopic composition with an average δ53/52CrSRM979 value of −0.122 ± 0.050‰ (2SD; n = 21), which supports previous estimates of the bulk silicate Earth δ53/52CrSRM979 value. However, high-Mg tholeiites and basaltic andesites exhibit significantly lighter δ53/52CrSRM979 and heavier δ56/54FeIRMM014 values than komatiites. These variations can be linked to crystallization and accumulation of mineral phases observed from fractionation trends of the two Archean komatiite-tholeiite suites. In detail, during crystallization and accumulation of olivine the Cr isotope compositions of komatiites stay invariant, whereas at the onset of Cr-bearing spinel and pyroxene crystallization the Cr isotope signatures of komatiitic basalts, high-Mg tholeiites and basaltic andesites become progressively lighter, which is attributed to the preferential incorporation of isotopically heavier Cr3+ in these mineral phases. The gradual increase of δ56/54FeIRMM014 with increasing magmatic differentiation, does not allow identifying the crystallization of particular mineral phases using Fe isotopes alone. Ultimately, this study demonstrates the power of combining stable Cr and Fe isotopic analyses to examine the effects of fractional crystallization on modifying melt source values and thus to ensure accurate mantle redox estimations.

Origin of Ni-Cu-(PGE + Au) sulfides in late-Archean komatiitic suite of rocks in the Shankaraghatta belt, Western Dharwar Craton (India)

The Shankaraghatta ultramafic–mafic suite of rocks (12 × 0.3 km) within the late ArcheanShimoga greenstone belt (Western Dharwar Craton, southern India) is constituted of sulfide-bearing serpentinized dunite flanked by metabasaltic schist rocks, which are probable analogues of the adcumulate-mesocumulate sheet flows in a komatiitic sequence, flanked by komatiitic basalts. Owing to limited field exposures in this study, the bulk concentrations of Ni, Cr, Ti, Fe, and Mg were used to classify the rocks into different volcanogenic facies observed in a komatiitic flow. The mineralized mesocumulate (serpentinized dunites) flanking its S-poor adcumulate counterpart host 5–8 modal % disseminated sulfide globules formed by an assemblage of pentlandite-millerite-pyrite-chalcopyrite. Millerite is the dominant sulfide mineral, whereas coarse euhedral pyrite grains are abundant in the serpentinites. The sulfide-bearing serpentinites have higher platinum-group element (PGE) abundances than those lacking visible sulfides (∑PGEsulfide-bearing samples = 440–1848 ppb; ∑PGEsulfide-lacking samples = 4.8–69 ppb) and show a strong PPGE (Pd, Pt, Rh) enriched trend with distinct negative Pt-anomalies in the primitive-mantle normalized PGE diagram. The mineralized serpentinites have higher Ni and Cu concentrations (0.5–2.2 wt% and 29–201 ppm, respectively) compared to their unmineralized counterparts (0.1–0.6 wt% and 22–72 ppm, respectively). Major element data indicate a komatiitic composition for the serpentinites, whereas the schists are komatiitic basalts. Chondrite-normalized REE plots for the serpentinites and schists show LREE-enriched patterns (La/SmNserp = 1.8–3.5; La/SmNschist = 2.5), with almost flat to slightly depleted HREEs (Gd/YbNserp = 1.1–1.7; Gd/YbNschist = 1.8). The rocks show prominent negative anomalies of Zr and Hf, which along with the subchondritic Al2O3/TiO2 ratios for serpentinites (~9.9), indicate derivation of the komatiitic magma from a plume that originated from a deep mantle source (>400 km) with the mantle residue retaining a majorite garnet phase. The komatiites and komatiitic basalts from which the serpentinites and schists were derived have a common mantle source. They are connected by fractional crystallization as understood from collinear and coherent patterns in bivariate plots between major oxides such as MgO, and alteration resistant immobile trace elements such as Nb, that are plotted with other major and trace elements, as well as from the coherent patterns observed in the chondrite-normalized REE and multi-element spider diagrams. The serpentinites and schists show signatures of crustal contamination as defined by negative anomalies of Nb, Ta, and Ti,high La/Sm ratios, along with higher concentrations of trace elements like La and Th, and lower Nb/Th ratios(Nb/Thavgserp = 6.6; Nb/Thavgschist = 3.7). AFC (Assimilation Fractional Crystallization) models indicate that the schists underwent a greater extent of crustal contamination (50% AFC) relative to the serpentinites (12% AFC). Sulfide saturation was achieved late, after emplacement of the komatiitic magma and crystallization of the olivine adcumulates, when the melt was slightly more differentiated to form the mesocumulates. The sulfides are later overprinted by low-temperature alteration processes, as evident from the significant enrichments in serpentinites for Au (AuNavg: 2.1), As (AsNavg: 20), Bi (BiNavg: 8.2), and Te (TeNavg: 1.5), which occurred perhaps during the replacement ofpentlandites by millerites.

Coupled stable chromium and iron isotopic fractionation tracing magmatic mineral crystallization in Archean komatiite-tholeiite suites

Chromium exists in two oxidation states Cr2+ and Cr3+ during high-temperature magmatic processes and changes in Cr redox are often associated with stable isotopic fractionation. Thus, the stable chromium isotope compositions of mantle-derived magmas bear the potential to trace the oxidation states of their mantle sources as well as any post melting changes in Cr redox (e.g. during magmatic differentiation), in a manner similar to magmatic stable Fe isotopic fractionation. However, these stable isotope fractionation effects are less understood for Cr relative to Fe. Komatiites and tholeiitic basalts represent a wide range of mantle-derived partial melts with variable fractional crystallization of phases with different affinity for Cr2+ and Cr3+. Thus, they offer potential archives to better understand high-temperature Cr isotope fractionation processes and mantle redox.Here, we report major and trace elements as well as coupled stable Cr and Fe isotope compositions of two well-characterized Archean komatiite-tholeiite suites from the Barberton Greenstone Belt, South Africa and Eswatini, and Belingwe Greenstone Belt, Zimbabwe. The sample suites range in MgO concentrations from 3.85 to 34.33 wt%, which allows investigation of the impact of large degrees of magmatic differentiation in a komatiite-basalt system. Whole-rock δ53/52CrSRM979 and δ56/54FeIRMM014 values range from −0.390 ± 0.016 to −0.061 ± 0.016‰ and −0.014 ± 0.018 to +0.192 ± 0.018‰, respectively. The komatiites have a very narrow range in their Cr isotopic composition with an average δ53/52CrSRM979 value of −0.122 ± 0.050‰ (2SD; n = 21), which supports previous estimates of the bulk silicate Earth δ53/52CrSRM979 value. However, high-Mg tholeiites and basaltic andesites exhibit significantly lighter δ53/52CrSRM979 and heavier δ56/54FeIRMM014 values than komatiites. These variations can be linked to crystallization and accumulation of mineral phases observed from fractionation trends of the two Archean komatiite-tholeiite suites. In detail, during crystallization and accumulation of olivine the Cr isotope compositions of komatiites stay invariant, whereas at the onset of Cr-bearing spinel and pyroxene crystallization the Cr isotope signatures of komatiitic basalts, high-Mg tholeiites and basaltic andesites become progressively lighter, which is attributed to the preferential incorporation of isotopically heavier Cr3+ in these mineral phases. The gradual increase of δ56/54FeIRMM014 with increasing magmatic differentiation, does not allow identifying the crystallization of particular mineral phases using Fe isotopes alone. Ultimately, this study demonstrates the power of combining stable Cr and Fe isotopic analyses to examine the effects of fractional crystallization on modifying melt source values and thus to ensure accurate mantle redox estimations.

Komatiitic Lamprophyre in West Sulawesi: First Evidence for &gt;1350°C and 3.5 - 3.8 GPa Mantle Melts

DOI: 10.17014/ijog.8.1.39-58The presence of lamprophyric lavas of Late Cenozoic in Talaya Volcanic Formation at the boundary between the subregencies of Mamuju and Tabulahan (Western Sulawesi) associated with the mantle enrichment rocks of the Adang Volcanics is the subject of this study. Petrologically, lamprophyre is composed of orthopyroxene (enstatite), clinopyroxene (augite), biotite, leucite, amphibole, magnetite, and autometasomatism of chlorite in grain minerals and groundmass. The lamprophyre is classified into monchiquite shoshonitic lamprophyre, and it has a komatiitic composition with the ratio of MgO/Al2O3 &gt; 0.7906 (in wt %). The komatiitic monchiquite lamprophyre is characterized by high MgO (10.02 - 12.67 %), relatively low alumina (Al2O3= 10.98 - 11.70 %), SiO2= 46.43 - 47.8 %, TiO2 (0.84 - 1.00 %), FeOt (7.75 - 7.88 %), and relatively high content of alkaline (Na2O: 2.20 - 2.59 %; K2O: 1.58 - 2.45 %; Total alkali: 4.00 - 4.89 %, and CaO (9.29 - 10.71 %). The geochemical trace element plots using various diagrams suggests the geotectonic setting of the lamprophyric rock was formed in suprasubduction alkaline continental-arc, and the proposed source of magmatism comes from the suprasubduction activities from the east. The protolith of magma was originated from partial melting of depleted MORB mantle (DMM), composed of pyroxene-peridotite (garnet-lherzolite). The partial melting conditions are suggested to occur at high pressure (3.5 - 3.8 GPa) and the depth of ~120 km with melting temperature of &gt;1350°C, and the magma is dominantly controlled by olivine fractional crystallization.

Volcanic succession, petrology, and geochemistry of the Sujiagou komatiite from the North China Craton

The ~2.7‐Ga Taishan greenstone belt from the Eastern Block of the North China Craton has well‐exposed komatiite sections. The komatiite outcrops at Sujiagou have well‐preserved primary volcanic texture although they have experienced amphibolite‐facies metamorphism. Two komatiite lava flows are identified in the Sujiagou outcrops: “Flow Ι,” with a thickness of 55 m, is characterized by poly‐sutured crust at the top, spinifex texture in the middle, and olivine cumulates at the flow bottom. “Flow ΙΙ” consists of abundant pahoehoe lava lobes with no interstitial materials. These volcanic characteristics, as well as the interbedding with terrestrial sediments, suggest that the Sujiagou komatiite erupted in a subaerial continental environment. The olivine–chromite geothermometer gives peak metamorphic temperatures from 684°C to 764°C. Phase equilibria modelling suggests that the mineral proportions and mineral compositions are consistent with a retrograde temperature of ~488°C, at which the komatiite would have low olivine Fo (79–89) and high bulk‐rock Mg# (83–90). Linear mixing of the in situ trace element compositions of the metamorphic olivine, augite, tremolite, serpentine, and chlorite from Sujiagou komatiite samples can reproduce the trace elements of representative komatiite compositions, suggesting that later metamorphism and fluid–rock interactions played key roles in trace element variations in the bulk‐rock komatiite samples. These results also imply that the “arc signature” of the Sujiagou komatiite may be an overprint from posteruption fluid alteration rather than a primary subduction geochemical feature. Therefore, the Sujiagou komatiites may belong to a part of the global plume‐related greenstone belts at ~2.7 Ga.

Deep hydrous mantle reservoir provides evidence for crustal recycling before 3.3 billion years ago.

Water strongly influences the physical properties of the mantle and enhances its ability to melt or convect. Its presence can also be used to trace recycling of surface reservoirs down to the deep mantle1, which makes knowledge of the water content in the Earth's interior and its evolution crucial for understanding global geodynamics. Komatiites (MgO-rich ultramafic magmas) result from a high degree of mantle melting at high pressures2 and thus are excellent probes of the chemical composition and water contents of the deep mantle. An excess of water over elements that show similar geochemical behaviour during mantle melting (for example, cerium) was recently found in melt inclusions in the most magnesium-rich olivine in 2.7-billion-year-old komatiites from Canada3 and Zimbabwe4. These data were taken as evidence for a deep hydrated mantle reservoir, probably the transition zone, in the Neoarchaean era(2.8 to 2.5 billion years ago). Here we confirm the mantle source of this water by measuring deuterium-to-hydrogen ratios in these melt inclusions and present similar data for 3.3-billion-year-old komatiites from the Barberton greenstone belt. From the hydrogen isotope ratios, we show that the mantle sources of these melts contained excess water, which implies that a deep hydrous mantle reservoir has been present in the Earth's interior since at least the Palaeoarchaean era(3.6 to 3.2 billion years ago). The reconstructed initial hydrogen isotope composition of komatiites is more depleted in deuterium than surface reservoirs or typical mantle but resembles that of oceanic crust that was initially altered by seawater andthen dehydrated during subduction. Together with an excess of chlorine and depletion of lead in the mantle sources of komatiites, these results indicate that seawater-alteredlithosphere recycling into the deep mantle, arguably by subduction, started before 3.3 billion years ago.

Carboniferous continental margin mafic-ultramafic sheeted dyke complex in the West Magnitogorsk zone (Southern Urals)

In the West of the aHochthon of Magnitogorsk zone thrusted onto the Precambrian complexes of the Uraltau zone, the Devonian island-arc complexes overlap stratigraphicalry the Ordovician and Silurian chert-basalt sequences and serpentine melange. Melange and Ordovician strata are intruded by dyke swarms and sheeted dykes («dyke in dyke») which are composed of mafic and ultramafic rocks. The dykes, composed by gabbro-dolerite, amphibole K-feldspar gabbro, hornblendite, picrite and lamprophyre, predominate. The composition of the ultramafic rocks corresponds to the composition of picrite and komatiite. The 40Ar/39Ar age of the magmatic amphibole from gabbro is 357 ± 8 m.y. The formation of dykes is related to the Early Carboniferous rift-related magmatism on an active continental margin.

Mineralogical and petrographic features of metakomatiites of the Kostomuksha greenstone structure (Karelia)

Object of study. The paper presents the mineralogical and petrographic study results of metamorphosed and metasomatized komatiites and komatiitic peridotites from the Ozerki soapstone deposit and Pentinsuo prospect, located in the Kostomuksha greenstone structure of the Karelian Craton, Fennoscandian Shield. Material and methods. Surface and drill core samples of various mineral and structural-textural varieties of altered komatiites were studied by optical microscopy, electron microscopy with an attachment for microanalysis, XRD phase, thermogravimetric and chemical analysis. Results. Soapstone formation in the investigated localities displays a multistage alteration and associates with the superimposed alteration of Mg-rich metakomatiite flows and olivine cumulates due to influx of carbon dioxide bearing fluids enriched in calcium and potassium. The chemical composition of initial komatiite (MgO content of the rock) and the degree of its fracture intensity are the main factors controlling soapstone formation. In differentiated lava flows soapstone is formed mainly in high-magnesium cumulate zones characterized by a high content of serpentine. In the flow top and spinifex zones an early amphibole-chlorite-magnetite mineral association is preserved in varying degrees. Talc and carbonate are formed by the decomposition of serpentine and amphibole. The chlorite content in soapstone is controlled by the Al2O3 concentration in the respective flow zones. Conclusion. The carbonate-chlorite-talc is the general natural type of soapstone associated with thin komatiite lava flows of distal volcanic facies. In more thick proximal lava flows the formation of soapstone of chloritecarbonate-talc and talc-carbonate composition is possible. The most prefered prospecting areas for soapstone are fields proximal to the eruptive vent, main lava conduits and subvolcanic analogues of komatiites.

Geochemistry of Mesoarchean felsic to ultramafic volcanic rocks of the Lac Guyer area, La Grande Subprovince (Canada): Evidence for plume-related magmatism in a rift setting

The Lac Guyer area of the La Grande Subprovince (Superior Province) hosts Mesoarchean east-trending volcano-sedimentary sequences subdivided into 3 stratigraphic units (Mintisch and Rouget Formations and Guyer Group), which were emplaced between 2847 and 2806 Ma on a gneissic-tonalitic basement. These approximately 35 to 175 km-long sequences comprise felsic to ultramafic volcanic rocks that have been metamorphosed to the amphibolite and granulite facies and record mainly two Archean episodes of ductile deformation.The Mesoarchean tholeiitic basalt, komatiitic basalt, and komatiite appear to be genetically related based on petrologic modeling, their close spatial association, and their major and trace element signatures. They are characterized by the following: depletion to enrichment in LREE relative to HREE; Zr/Ypm, Ti/Vpm, and La/Smpm ratios higher than or close to 1; Ti/Zrpm and Nb/Thpm ratios lower than or close to 1; common negative anomalies in Nb-Ta ± Ti; and local positive to negative anomalies in Hf-Zr. Komatiitic basalt and komatiite are mostly Al-undepleted and rarely Al-depleted. According to current komatiite melting models, this suggests that these rocks formed by a high degree of partial melting of a garnet-bearing lherzolite source that consumed any residual garnet and progressively depleted the primitive mantle source in incompatible elements. Basalt and komatiitic basalt compositions were likely produced by assimilation and fractional crystallization (AFC) processes from komatiitic magmas (∼21–23 wt% MgO), as suggested by AFC modeling and their geochemical characteristics, whereas komatiite compositions were controlled by partial melting processes and crustal contamination.The Mesoarchean calc-alkaline rhyolite, dacite, and andesite are enriched in LREE relative to HREE, enriched in Th relative to LREE, and have negative anomalies in Nb-Ta-Ti, negative to positive anomalies in Hf-Zr, as well as high Zr/Y and Ti/V ratios and low Ti/Zr ratios. Such geochemical signatures could be explained either by concurrent assimilation and fractional crystallization of the basaltic magmas in the lower crust or by magma mixing between basalt and lower crustal anatectic magmas.The spatial and temporal distribution of Mesoarchean volcanic rocks within the Lac Guyer area and their major and trace element characteristics suggest that they were emplaced into a continental rift associated with a mantle plume. The geochemical similarities between these rocks and the rift-related volcanic rocks of the younger Yasinski Group (2751–2732 Ma), and the presence of other Mesoarchean volcanic sequences with rift affinity indicate that the La Grande Subprovince was affected by two main plume-related rifting events during the Archean (ca. 2.88–2.81 and 2.75–2.71 Ga), each produced by a single or multiple magmatic plumes.

Petrogenesis of ultramafic rocks of komatiitic composition from the Central Zone of the Limpopo Belt, South Africa: Evidence from O and H isotopes

Komatiites are MgO-rich ultramafic volcanic rocks, mostly interpreted to be essentially anhydrous at the time of extrusion, and are often characterized by spinifex texture. They mainly occur in Archean or Proterozoic greenstone belts within cratons. However, in the Central Zone of the Limpopo Belt in South Africa, ultramafic rocks of komatiitic composition occur within a high-grade metamorphic terrane. Their chemical composition overlaps with that of Al-undepleted (Munro-type) komatiites from elsewhere, but unlike komatiites, they are mainly composed of pargasitic amphibole, along with olivine, orthopyroxene, Cr-rich spinel and magnetite. The studied rocks have a narrow range of whole rock δ18O values (+5.4 to +6.3‰), unlike δ18O values from komatiites, which are generally altered and have δ18O values from ∼+2 to +9‰. However, they are within the range estimated for unaltered komatiites (+5.4 to +6.0‰). Amphibole separates from the studied rocks have δ18O values between +5.5 and + 6.1‰, similar to that of the bulk rock. The δD values of amphiboles range between −78 and −83‰, similar to primary amphiboles from other komatiites and many other mantle-derived mafic igneous rocks. The δ18O values, along with textural and mineralogical evidence, are consistent with the ultramafic rocks of komatiitic composition from the Limpopo Belt being intrusive and unaffected by low-temperature alteration. The O and H isotope composition of the amphiboles are consistent with them being primary magmatic in origin. It cannot be distinguished with certainty whether the water was introduced to the magma through partial melting of hydrated mantle or during ascent through hydrous mantle subsequent to their formation. The minerals observed at present most likely crystallized from komatiite magma that intruded into the lower crust and subsequently re-equilibrated at the ambient P-T conditions.

Relict of Meso-Archean paleosol developed on the komatiites from Koikary Area, Central Karelia

A mineralogical and geochemical study was carried out to investigate the relict of Meso-Archean paleosol developed on the komatiites from Koikary Area, Central Karelia. The obtained data showed that the paleosol had the age of 3000–2930 million years and was not derived. Hugh values of CIA (91) and Al2O3/SiO2(0.73) parameters indicate the high extent of chemical weathering. The evolution of the komatiite composition during weathering and the manner of REE distribution correspond to the paleosol formation during the oxic weathering in subaerial conditions of humid and warm climate with oxic.

Modelling komatiitic melt accumulation and segregation in the transition zone

Komatiites are probably produced in very hot mantle upwellings or plumes. Under such conditions, melting will take place deep within the upper mantle or even within the mantle transition zone. Due to its compressibility at such pressures, melt might be denser than olivine, but would remain buoyant with respect to a peridotitic mantle both above and below the olivine–wadsleyite phase boundary because of the presence of its higher temperature and denser garnet. We studied the physics of melting and melt segregation within hot upwelling mantle passing through the transition zone, with particular emphasis on the effect of depth-dependent density contrasts between melt and ambient mantle. Assuming a 1D plume, we solved the two-phase flow equations of the melt-matrix system accounting for matrix compaction and porosity-dependent shear and bulk viscosity. We assumed a constant ascent velocity and melt generation rate. In a first model series, the level of neutral buoyancy zneutr is assumed to lie above the depth of onset of melting, i.e. there exists a region where dense melt may lag behind the solid phases within the rising plume. Depending on two non-dimensional numbers (accumulation number Ac, compaction resistance number Cr) we find four regimes: 1) time-dependent melt accumulation in standing porosity waves that scale with the compaction length. The lowermost of these waves broadens with time until a high melt accumulation zone is formed in steady state. During this transient solitary porosity waves may cross the depth of neutral density and escape. 2) steady-state weak melt accumulation near zneutr, 3) no melt accumulation due to small density contrast or, 4) high matrix viscosity. In regime 4 the high mantle viscosity prevents the opening of pore space necessary to accumulate melt. In a second series, the rising mantle crosses the olivine–wadsleyite phase boundary, which imposes a jump in density contrast between melt and ambient mantle. A sharp melt porosity contrast develops and a large melt porosity accumulates immediately above the phase boundary. Both model series show 1) that not only melt density, but also porosity-dependent matrix viscosity controls the melt ascent or accumulation, 2) that there are parameter ranges and physical conditions which may lead to the accumulation of very large melt porosities (> degree of melting), 3) that in spite of melt being denser than olivine at some depths, in general these melts escape these regions and continue to percolate upward faster than the rising mantle. Melting and melt transport under the conditions predicted by the numerical modelling is able to reproduce the compositions of the main types of komatiite. Thus, the accumulation of large melt fractions, and sequential escape of melt from porosity waves, explains several puzzling features of the geochemical compositions of komatiites.

Metamorphic evolution of ultramafic rocks of komatiitic composition from the Central Zone of the Limpopo Belt, South Africa

A large number of published accounts are available on the metamorphic evolution of the Central Zone of the Limpopo Belt. However, most investigations on the P-T evolution of this zone are restricted to pelitic and calc-silicate rocks. In this paper we present for the first time a detailed mineralogical and petrological investigation on an ultramafic granulite of komatiitic composition, which occurs in the vicinity of Musina in the Central Zone. These rocks are composed mainly of magnesium-rich Ca-amphibole (pargasite: MgO up to 18 wt%), together with Mg-rich olivine (X Mg = 0.82 to 0.83) and orthopyroxene (X Mg = 0.82 to 0.84), Cr-rich spinel, magnetite and some sulfide minerals. This set of minerals represents the peak metamorphic assemblage formed during the early stage of the metamorphic evolution of the Central Zone and is different from the mineral assemblage of less altered komatiites. This assemblage was subjected to minor retrogression and alteration to chlorite and serpentine during the waning stage of the metamorphic evolution of the Central Zone. Thermodynamic calculations combining classical conventional methods as well as equilibrium assemblage modelling using Theriak-Domino, in conjunction with textural relationships and mineral chemistry, indicate that the studied rocks went through granulite facies metamorphic conditions at a temperature of up to 820°C and an intermediate pressure of up to 8.5 kbar. These P-T conditions are in good agreement with previous investigations of metamorphic conditions on other rock types (mainly metapelites) in the Central Zone of the Limpopo Belt.

Liquidus temperatures of komatiites and the effect of cooling rate on element partitioning between olivine and komatiitic melt

Archean komatiites are the hottest magmas preserved on Earth and are thus unique probes of its thermal evolution. Estimating their eruption temperatures remains problematic, however, because the uppermost (A1, A2) zones of komatiite flows contain randomly oriented spinifex-textured olivines, indicative of rapid cooling and growth. Fe–Mg partitioning between olivine and assumed komatiitic liquid typically shows departures from equilibrium, extending towards higher \(K_{\text{D}}^{{{\text{Fe}}^{2 + } - {\text{Mg}}}}\). If these higher values are a disequilibrium effect, using them to calculate parental magma composition would lead to errors in estimated liquidus temperatures. In order to investigate this possibility, we have performed experiments on two komatiite compositions, the classic Barberton Aluminium Undepleted Komatiite (AUK) sample 49J (32.2 % MgO) and Munro AUK sample 422/95 (23 % MgO). Isothermal experiments to constrain phase equilibria on 49J at atmospheric pressure, between 1360 and 1600 °C at 1.7 log units below and 1.1 above the fayalite–magnetite–quartz (FMQ) buffer reveal a liquidus temperature (T liq) of 1616 °C, ~40 °C lower than a previous estimate. The \(K_{\text{D}}^{{\varSigma {\text{Fe}}{-}{\text{Mg}}}}\) ranges between 0.320 and 0.295 at FMQ − 1.7, with a slight negative dependence on temperature. To replicate the conditions that prevailed during the quenching of komatiites in their upper chill zones, experiments with a constant cooling rate at FMQ − 1.7 were performed on 422/95 (T liq = 1450 °C) at 0.5, 1.5, 2.5, 6.5 and 16 °C/min. Olivine morphology changes from euhedral to tabular at low cooling rates, hopper at intermediate, and skeletal and chain structures at high rates. Concurrently, the \(K_{\text{D}}^{{\Sigma {\text{Fe}}{-}{\text{Mg}}}}\) increases monotonically from an equilibrium value of 0.305 to 0.376 at 16 °C/min, reflecting the inability of unwanted cations to diffuse away from the growing olivine. The high \(K_{\text{D}}^{{\varSigma {\text{Fe}}{-}{\text{Mg}}}}\) between olivine and komatiitic liquid caused by rapid cooling accounts for the systematically lower Mg#s in spinifex olivines with respect to their euhedral counterparts in natural komatiites. The maximum MgO content of komatiite liquids in the Archean is revised upwards to 32 wt%, implying temperature excesses in Archean plumes relative to ambient mantle were greater than in their contemporary equivalents.

High‐Precision Fe and Mg Isotope Ratios of Silicate Reference Glasses Determined In Situ by Femtosecond LA‐MC‐ICP‐MS and by Solution Nebulisation MC‐ICP‐MS

In this study, a technique for high precision in situ Fe and Mg isotope determinations by femtosecond‐laser ablation‐multi collector‐ICP‐MS (fs‐LA‐MC‐ICP‐MS) was developed. This technique was employed to determine reference values for a series of common reference glasses that may be used for external standardisation of in situ Fe and Mg isotope determinations in silicates. The analysed glasses are part of the MPI‐DING and United States Geological Survey (USGS) reference glass series, consisting of basaltic (BIR‐1G, BCR‐2G, BHVO‐2G, KL2‐G, ML3B‐G) and komatiitic (GOR128‐G and GOR132‐G) compositions. Their Fe and Mg isotope compositions were determined by in situ fs‐LA‐MC‐ICP‐MS and by conventional solution nebulisation multi‐collector ICP‐MS. We determined δ56Fe values for these glasses ranging between ‐0.04‰ and 0.10‰ (relative to IRMM‐014) and δ26Mg values ranging between ‐0.40‰ and ‐0.15‰ (relative to DSM‐3). Our fs‐LA‐MC‐ICP‐MS results for both Fe and Mg isotope compositions agreed with solution nebulisation analyses within analytical uncertainties. Furthermore, the results of three USGS reference glasses (BIR‐1G, BHVO‐2G and BCR‐2G) agreed with previous results for powdered and dissolved aliquots of the same reference materials. Measurement reproducibilities of the in situ determinations of δ56Fe and δ26Mg values were usually better than 0.12‰ and 0.13‰ (2s), respectively. We further demonstrate that our technique is a suitable tool to resolve isotopic zoning in chemically‐zoned olivine crystals. It may be used for a variety of different applications on isotopically‐zoned minerals, e.g., in magmatic or metamorphic rocks or meteorites, to unravel their formation or cooling rates.

Komatiites as Mercury surface analogues: Spectral measurements at PEL

The elemental composition of Mercury's surface, which has been recently measured by the NASA MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, suggests a mineralogy dominated by magnesium-rich orthopyroxene and feldspar. The most magnesium-rich and aluminium-poor regions of Mercury's surface (which are presumably orthopyroxene-rich) have compositions, and possibly mineralogies, analogous to terrestrial boninites and basaltic komatiites. Unfortunately, little is known about the spectral properties of komatiites, especially at the high surface temperatures of Mercury. We therefore have collected three terrestrial komatiites with different compositions plus a synthetic komatiitic sample, and measured their reflectances in the visible and thermal infrared spectral ranges. Samples divided into four grain size ranges (when enough material was available) were measured fresh and after thermal processing in vacuum (10 Pa) at 500 °C, comparable to Mercury peak surface temperatures. Our measurements show that spectral changes between fresh and thermally processed samples occur in both spectral channels, but are stronger in the visible range, with reddening affecting all the samples, while darkening is more selective. It is important to note that darkening and reddening after thermally processing the samples are independent of the komatiites ferrous iron content. In fact the synthetic sample which is nearly iron-free is most strongly affected. From our study it turns out that thermally processing the samples in vacuum at Mercury surface temperature produces the removal of samples' colour centres. The results of our study show also that the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument on MESSENGER orbiting Mercury currently cannot distinguish between different compositions of komatiites, while the future MErcury Radiometer and Thermal infrared Imaging Spectrometer (MERTIS) on the upcoming ESA BepiColombo mission will resolve their differences in the 7−14μm spectral range.

Geochemical Constraints on the Petrogenesis and Tectonic Environment of Gabbroic Intrusives in the Siang Window of Eastern Himalaya, Northeast India

Abstract Carbonate and calcareous-quartzite of Miri-Buxa Group in the Siang Window of Eastern Himalaya intruded by mafic rocks of gabbroic affinity. These intrusive rocks are low-Ti tholeiites (Ti/Y = 379-478; Nb/La = 0.99-1.88) and characterized by enriched LILE-LREE, depleted in HFSE with minor REE fractionation [(La/Yb)N = 2.72-3.35)]. Geochemical behaviour of the incompatible trace elements with the rare earth elements abundances indicates their cogenetic nature and their emplacement in a continental rift tectonic environment. The liquidus olivine temperature of these mafic rocks ranges from 1262°C to 1380°C showing a gentle decrease of [Mg] with a steep increase of [Fe]. These charters thus imply that the rocks are either related to the extent of common source or fractionational crystallization of plagioclase and clinopyroxene from a single batch parental magma. Petrogenetic modeling of [Mg]-[Fe] and REE indicates that these mafic intrusives probably derived from a mantle source similar to komatiitic composition at moderate to high degree (8%-20%) of partial melting.

A Chill Sequence to the Bushveld Complex: Insight into the First Stage of Emplacement and Implications for the Parental Magmas

A rare occurrence of a chill sequence in drill core from the eastern Bushveld Complex has been discovered at the base of a thick succession of ultramafic rocks that forms part of the Lower Zone. The lowest 10 m of the section preserves a variety of rock types including a true chill against quartzite floor rock, crystalline quench-textured and spinifex-textured rocks as well as high-temperature olivine and orthopyroxene cumulates. It represents the first stage of magma emplacement into the Bushveld chamber and gives an insight into the nature of the parental magmas to the Bushveld, the processes that took place at that early stage, and how rocks with a high original interstitial liquid content give way to more normal cumulates higher in the section. The chill sequence also provides insight into the variety of rock types that are encountered in marginal sills that are regarded as representing early stage magmas intruded as the chamber developed but that are incompletely understood because of inadequate field exposures. Olivine compositions (up to Mg# 0·912) in a pyroxene dunite in this section are the highest recorded for the Bushveld Complex and cores of associated orthopyroxene have Mg# 0·93. Zoned orthopyroxenes in the quench- and spinifex-textured units range from Mg# 0·91 to 0·72 and preserve core compositions close to the original liquidus. Small single chromite crystals have Cr/(Cr + Al) of 0·85 and Cr/Fe(Total) of three; these are also the most primitive compositions found to date in the Bushveld Complex. The chill and quench zones represent the earliest magmas to be emplaced in the Bushveld Complex; however, these are relatively evolved and similar to the B1 liquid, long assumed to be the most primitive magma that gave rise to the Lower Zone. Major and trace element geochemistry and mineral compositions show that the true parental magmas to the Lower Zone were of komatiite composition, with the most primitive containing 19% MgO. The liquid compositions have a strong crustal signature but are also enriched in K, Rb, Pb and especially Cs and Cl. The olivines in this section are highly Ni enriched, consistent with a component of mantle pyroxenite derived from recycled ocean crust in the source of the parental magmas. It is suggested that this source, combined with PGE-fertile subcontinental lithospheric mantle, was melted within a rising mantle plume and the resultant melts then interacted with the basement rocks of the Kapvaal Craton.