Spinifex Textures Research Articles

Quenching of basaltic melts by volatile exsolution

Normally, volatiles in silicate melts are ephemeral components that escape as gases when the melt reaches fluid saturation. When fluid saturation occurs at elevated pressure, magmatic fluids may have large amounts of oxide solute dissolved, are less volatile, and may resemble viscous gels. In Cyprus we have the rare case that solutes of a magmatic fluid coexist with H2O saturated basaltic to boninitic glasses. Quenching of the melts and fluid solutes was induced by fluid segregation. When the fluids exsolved, the liquidus temperature was raised and the melts were left supercooled, while the system temperature remained ± constant. Quenching rates deduced from the morphologies and compositions of quench crystals were high. We analyzed coexisting glasses and fluid solutes for major and trace elements. The fluid mobile trace elements (Rb, K, Pb, Sr) are enriched in both the glasses and fluid solutes. Both endmembers (melt and fluid) have a common parentage and originated within a hydrous mantle source. The glasses have 2.5 ± 0.25 wt.% H2O and record residual H2O contents left after fluid exsolution was completed. Water contents in glasses correspond to an H2O partial pressure (pH2O) of 65 ± 10 MPa and an emplacement depth on the seafloor of 6500 ± 1000 m, provided equilibrium was reached between the pH2O imposed by the melts and the seawater column. Following fluid exsolution, the degree of supercooling ∆T of the melts relative to the dry MgO-in-melt liquidus temperature was – 65 ± 10 °C. The cooling rate ∆T/t at the time of crystallization of dendritic clinopyroxene crystals can be semi-quantified from the distribution of Al2O3 between metastable clinopyroxene dendrites and melt, to at least – 50 °C h−1. Toward the end of the article we speculate if other cases exist where quenching was triggered by fluid exsolution. A possible example are spinifex textures deep inside komatiite flows where quenching rates by conductive cooling did not exceed 0.3 to 1 °C h−1. Our proposition assumes that many spinifex-textured komatiites were hydrous, that they contained H2O in quantities sufficient to reach fluid saturation at emplacement pressure, and that spinifex textures formed as a result of supersaturation by fluid loss.

The discovery and petrogenetic significance of komatiites

This review paper reports on the discovery of komatiites and the impact this has had on the geosciences internationally. Komatiites were first discovered by Richard and Morris Viljoen in the Barberton greenstone belt of South Africa and proposed as a new class of ultramafic volcanic rock with >18 wt% MgO. Komatiites are in places associated with spinifex textures (elongated bladed and plumose olivine crystals). The discovery and petrological study of komatiites and understanding of the field controls have given rise to intense debate about their mantle origin, water content and geodynamic setting of eruption. The observation that komatiites are found predominantly in the Archean and to some extent in the Palaeoproterozoic, has led to the proposition that the early Earth was much hotter (by c. 400 °C) than the present day. However, some workers have challenged this model involving secular cooling of the Earth since Archean times. These workers argue that komatiites formed in a cooler Archean subduction zone, rather than in a mantle plume. Komatiites of the mid-to late-Archean have also proven to be important economically (e.g. Yilgarn Craton, Australia; Superior Craton, Canada) as they host important nickel-sulphide deposits. Today, novel petrological techniques (e.g. secondary ion microprobe analyses of olivine melt inclusions) are being applied to komatiites to address important outstanding questions in the geosciences, including the question of whether or not plate tectonics was operating in the early Archean.

Architectural and Compositional Diversity of Early Earth Ocean Floor Evidenced by the Paleoarchean Nondweni Greenstone Belt, South Africa

Abstract The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km2 in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43–3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau.

La Queglia carbonatitic melnöite: a notable example of an ultra-alkaline rock variant in Italy

Very primitive ultramafic igneous rocks occur at Mt. La Queglia (Abruzzo, Italy). They form a strongly deformed sill–dyke system now tilted vertically. These rocks were initially classified as alnöite and, subsequently, have been suggested to be a carbonatitic olivine melilitite. However, further investigation and interpretation of these rocks is needed due to the presence of hand-specimen-scale textural variation suggesting a complex petrogenesis. We study the texture, mineral chemistry, and whole-rock geochemistry to define three main rock-types. (1) A brecciated rock with an ocellar texture composed of calcite pseudomorphs after olivine and melilite, plus fresh diopside in a groundmass of mica, aegirine, garnet, calcite, apatite, perovskite, titanate and chlorite. Zoned ocelli in this rock show an amoeboid shape, agglutination, and menisci typical of a plastic state. (2) A quenched rock showing a spinifex texture containing long feathery phenocrysts of cpx and mica suspended in a groundmass of nepheline, aegirine, apatite, Ti–rich magnetite, plus abundant calcite and some K-feldspar and zeolites. (3) A coarse-grained rock is composed of calcite plus intergranular glauconite, a mixture of spinel mineral group and Ti–rich magnetite, accessory barite, pyrite, and chabazite-K. The igneous rocks at Mt. La Queglia show extreme SiO2-undersaturation (33.5–37.3 wt% SiO2), high MgO contents and TiO2/Al2O3 ratios. Rock-type 1 has a lower Mg number Mg# = 100 × [Mg/(Mg + Fe2+)], higher Ca number Ca# = 100 × [Ca/(Ca + Mg)], high Cr (up to 720 ppm) Ni (up to 379 ppm), higher rare earth elements (REE) contents as well as La/Lu ratio, compared to rock-type 2. Perovskite and chromite accumulation seems an important agent during rock differentiation. Rock-type 3 shows REE cross-over with rock-type 2 suggesting light (L)REE concentration in a carbothermal residuum. Mt. La Queglia rocks are an end-member compared to other Upper Cretaceous and Paleogene Italian lamprophyres, suggesting a low degree of melting of a HIMU (a colloquialism for “high-μ”; referring to mantle domains with high 238U/ 204Pb) garnet-bearing mantle source.

Geochemistry of the Abiete-Toko komatiite-like ultrabasites: Petrogenesis, evolution and geodynamic implications

The Greenstone Belt of the Paleoproterozoic Nyong Unit, in the northwestern Congo Craton, consists of metasediments and basic-to ultrabasic magmatic rocks. Ultrabasic rocks are of two types in the Abiete-Toko portion of the Nyong Unit. Type 1 has a mesh texture and consists of olivine (∼40 vol %), serpentine (∼30 vol %), orthopyroxene (∼15 vol %), clinopyroxene (∼5 vol %), brucite (∼5 vol %) and magnetite (∼5 vol %), while type 2 typified by spinifex texture is composed of serpentine (∼30 vol %), clinopyroxene (∼25 vol %), magnetite (∼20 vol %), hornblende (∼10 vol %), plagioclase (∼10 vol %) and pyrite (∼10 vol %). These petrographic features indicate that Abiete-Toko ultrabasic rocks have experienced both alteration of the main petrogenic minerals (olivine and pyroxene) and greenschist facies metamorphism. The behaviour of incompatible elements (HFSE and LREE) combined with chemical index of alteration values (35.6 = CIA ≤75.9), suggest that the investigated rocks have retained much of their original chemistry. Olivine is highly forsteritic (Mg# between 89.73 and 90.65), and contains high FeO (9.14–9.89 wt %), low NiO (0.45–0.47 wt %) and CaO (˂ 0.1 wt %), thus suggesting a mantle source. Based on bulk-rock chemical composition, the ultrabasites of the Paleoproterozoic Nyong Unit are hyper-magnesian [(MgO)adj ˃ 39 wt %] and have the Al-undepleted komatiites signature (Munro type). The low (0.17–0.70) (CaO/Al2O3)adj, low (0.60–0.94) (Gd/Yb)N and high (18.18–62.5) (Al2O3/TiO2)adj ratios, associated with the high ƩHREE (0.38–2.03 ppm), Y (0.87–3.57 ppm) and Zr (2–55 ppm) contents, suggest a shallow upper mantle source. The observed chemical characters including, (MgO)adj, (FeO)adj, (Al2O3/TiO2)adj, (CaO/Al2O3)adj, and Na, Ni, Cr, Y, Zr, Hf and REE contents reveal that the parent magma of these rocks was generated through partial melting of a heterogeneous and depleted mantle, at temperatures close to 1550 °C. Multi-element discrimination diagrams are consistent with crystallisation of these rocks in the arc setting, thus suggesting a Paleo subduction zone in the north-western part of the Congo Craton. These results are in favour of plate tectonic dynamics already operating in a hotter and ancient Earth environment point of view.

Geochemical characteristics of the early Neoarchean komatiite from the North China Craton: Evidence for plume–craton interaction

Komatiite is characterized by extremely high MgO contents and H2O-poor nature and plays an important role in understanding the early mantle-derived magma processes, crust–mantle differentiation of the Archean earth, but it is sparse in the North China Craton. Although komatiitic rocks have ever been discovered in the Mengyin area, their formation ages and types are still in debate. Here, we firstly report a new komatiite outcrop near Nanbaita village (Jinan city) of the western Shandong Province and present new major and trace element contents, as well as Sr–Nd and Re–Os isotopic compositions. The rocks show spinifex texture with primary olivine and pyroxene skeletal needles have been transformed into secondary minerals (e.g., serpentine, tremolite). The komatiites display extremely high MgO, but low TiO2 and Na2O + K2O. Their chondritic Al2O3/TiO2 and GdN/YbN ratios suggest they are Aluminium-undepleted komatiites which are generated by larger degrees of melting (~50%) in relatively lower pressure (ca. 5–7 GPa) with garnet absence than Aluminium-depleted komatiites. The formation age of these komatiites was no later than 2.69 Ga constrained by the Re depletion age. Flat heavy rare earth element patterns and low 187Re/188Os and Th/Yb ratios show the primary komatiitic magma was not contaminated by substantial crustal material. The γOs value of sample 18LH51 is 0.09, which is adjacent with chondrite and other contemporaneous komatiites in the world, indicating that the Nanbaita komatiites were originated from a chondritic mantle. Furthermore, the widely distribution of ~2.7 Ga ultramafic–mafic rocks in the western Shandong Province might be the reflection of a contemporaneous worldwide superplume event.

Iron and zinc stable isotope evidence for open-system high-pressure dehydration of antigorite serpentinite in subduction zones

Subducted serpentinites have the potential to control the exchange of volatile and redox sensitive elements (e.g., Fe, S, C, N) between the slab, the mantle wedge and the deep mantle. Here we examine the mobility of iron and zinc in serpentinite-derived fluids by using their stable isotopes (δ56Fe and δ66Zn) in high-pressure subducted meta-serpentinites from the Cerro del Almirez massif (Spain). This massif preserves a metamorphic front between antigorite (Atg-serpentinite) and antigorite-olivine-orthopyroxene (transitional lithologies) -bearing serpentinites, and chlorite-bearing harzburgite (Chl-harzburgite), displaying granofels, spinifex and fine-grained recrystallized textures. Those rocks were formed at eclogite facies conditions (1.6–1.9 GPa and 680–710 °C). The mean δ56Fe of all the Cerro del Almirez meta-serpentinites (+0.05 ± 0.01‰) is identical within an error to that of primitive mantle (+0.03 ± 0.03‰). A positive correlation between δ56Fe and indices of peridotite protolith fertility (e.g., Al2O3/SiO2) suggests that the δ56Fe values of Cerro del Almirez samples predominantly reflect protolith compositional variations, likely produced by prior episodes of melt extraction. In contrast, the Zn concentrations ([Zn] = 34–67 ppm) and isotope signatures (δ66Zn = +0.18 – +0.55‰) of the Cerro del Almirez samples show a broad range of values, distinct to those of the primitive mantle ([Zn] = 54 ppm; δ66Zn = +0.16 ± 0.06‰). The Atg-serpentinites ([Zn] = 34–46 ppm; δ66Zn = +0.23 ± 0.06‰) display similar [Zn] and δ66Zn values to those of slab serpentinites from other high-pressure meta-ophiolites. Both [Zn] and δ66Zn increase in transitional lithologies ([Zn] = 45–67 ppm; δ66Zn = +0.30 ± 0.06‰) and Chl-harzburgites with granofels ([Zn] = 38–59 ppm; δ66Zn = +0.33 ± 0.04‰) or spinifex ([Zn] = 48–66 ppm; δ66Zn = +0.43 ± 0.09‰) textures. Importantly, Cerro del Almirez transitional lithologies and Chl-harzburgites display abnormally high [Zn] relative to abyssal peridotites and serpentinites (29–45 ppm) and a positive correlation exists between [Zn] and δ66Zn. This correlation is interpreted to reflect the mobilization of Zn by subduction zone fluids at high pressures and temperatures coupled with significant Zn stable isotope fractionation. An increase in [Zn] and δ66Zn from Atg-serpentinite to Chl-harzburgite is associated with an increase in U/Yb, Sr/Y, Ba/Ce and Rb/Ce, suggesting that both [Zn] and δ66Zn record the interaction of the transitional lithologies and the Chl-harzburgites with fluids that had equilibrated with metasedimentary rocks. Quantitative models show that metasediment derived fluids can have isotopically heavy Zn as a consequence of sediment carbonate dissolution and subsequent Zn complexation with carbonate species in the released fluids (e.g., [ZnHCO3(H2O)5+] or [ZnCO3(H2O)3]). Our models further demonstrate that Zn complexation with reduced carbon species cannot produce fluids with heavy δ66Zn signature and hence explain the δ66Zn variations observed in the Chl-harzburgites. The most straightforward explanation for the heavy δ66Zn of the Cerro del Almirez samples is thus serpentinite dehydration accompanied by the open system infiltration of the massif by oxidized, carbonate-rich sediment-derived fluids released during prograde subduction-related metamorphism.

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.

Textural evolution during high-pressure dehydration of serpentinite to peridotite and its relation to stress orientations and kinematics of subducting slabs: Insights from the Almirez ultramafic massif

The Almirez ultramafic massif (SE Spain) preserves the transformation of high-P antigorite (Atg-) serpentinite to chlorite (Chl-) harzburgite (1.6–1.9 GPa; 680–710 °C), a metamorphic reaction that is the primary source of water at the intermediate depth of subducting slabs. We present a detailed μ-CT and EBSD study of oriented samples across the Atg-serpentinite dehydration isograd to investigate the textural evolution during serpentinite dehydration to peridotite and its relation to stress orientations and the kinematics of subducting slabs.Above the Atg-out isograd, Atg-serpentinite shows a prograde mylonitic foliation and a weak Shape Preferred Orientation (SPO) of oxide aggregates defining a N—S stretching lineation. The antigorite Crystal Preferred Orientation (CPO) is characterized by [001]Atg perpendicular to the foliation, and the poles to (100)Atg and (010)Atg distributed in a girdle-like symmetry with [100]Atg nearly parallel to the stretching lineation. The antigorite microstructure and CPO are consistent with deformation by dislocation creep, twinning, and dissolution-precipitation creep. These textures record the long-term shear deformation near the slab interface where the main compressive stress, σ1, was at an acute angle to the foliation.Below the Atg-out isograd, Atg-serpentinite dehydrated to unfoliated, coarse-grainedChl-harzburgite with granofels or spinifex textures distributed in alternating decameter-sized lenses. Crystallization of granofels and spinifex Chl-harzburgite records, respectively, a sequence of slow and fast fluid draining events during serpentinite dehydration under the same orientation of the principal stresses that resulted in the shear deformation of the Atg-serpentinite. Both textural types exhibit coarse-grained textures with systematic mineral CPOs and SPOs and microstructures without evidence of major ductile deformation. The texture of the granofels Chl-harzburgite formed by a topotactic dehydration reaction after Atg-serpentinite coupled to compaction leading to an olivine layering subparallel to the Atg-serpentinite foliation. The olivines of granofels Chl-harzburgite are rounded and display a weak CPO that can be accounted for by the topotactic reaction 〈100〉Atg||〈100〉Ol and (001)Atg||(010)Ol after Atg-serpentinite. Similarly, orthopyroxene shows a marked CPO consistent with the topotactic reaction (100)Opx||(001)Atg and [001]Opx||[100]Atg.Spinifex Chl-harzburgite displays systematic mineral SPO and CPO. Spinifex olivines are tabular on (100)Ol and elongated along [001]Ol (c > b >> a), and define a ESE–WNW platelet lineation. The average texture is characterized by [001]Ol,Opx subparallel to a strong ESE–WNW oxide aggregate lineation, and [100]Ol,Opx and [001]Ol,Opx within a plane of similar orientation to the Atg-serpentinite foliation. The SPOs and CPOs of spinifex Chl-harzburgites are composed of up to four orientation populations of tabular olivines, where one population is volumetrically dominant in all samples. Relative to the main orientation population, the CPO of olivine shows clustered distribution of [100]Ol and [010]Ol maxima rotated at systematic angles around [001]Ol. These clustered CPOs and SPOs show a remarkable correlation with the orientation of the principal paleostresses. The preponderant orientation population of tabular olivines lies on the plane of maximum compression (σ2–σ3 plane) and has [010]Ol and its (100)Ol tabular faces nearly perpendicular to the Atg-serpentinite foliation. This population can be accounted for by oriented growth of platy crystals perpendicular to σ1. The other populations lie in the plane perpendicular to the least compressive stress (σ3) and the planes of maximum shear. The driving force and causes for the oriented crystallization of tabular olivine in these planes is uncertain, but their correlation with paleostresses suggests a cause-effect relationship.Spinifex and granofels Chl-harzburgites show a marked ESE–WNW oxide aggregate lineation that differs in orientation from that of Atg-serpentinite and is approximately parallel to the intermediate compressive stress (σ2). These lineations and the platelet lineation of spinifex Chl-harzburgite may be due to along-strike fluid flow below the permeability barrier that constituted the Atg-out dehydration isograd. Our study shows that the kinematics of the slab, paleostresses, and fluid flow exert a dynamic control on the textures of Atg-serpentinite dehydrating to peridotite in subducting slabs.

Geochemical and geochronological evidence for a Middle Permian oceanic plateau fragment in the Paleo-Tethyan suture zone of NE Iran

The mafic–ultramafic Fariman complex in northeastern Iran has been interpreted as a Paleo-Tethyan ophiolitic fragment with subduction- and plume-related characteristics as well as a basin deposit on an active continental margin. Contributing to this issue, we present geochemical, geochronological, and mineralogical data for transitional and tholeiitic basalts. Thermodynamic modeling suggests picritic parental magmas with 16–21 wt% MgO formed at plume-like mantle potential temperatures of ca. 1460–1600 °C. Rare pyroxene spinifex textures and skeletal to feather-like clinopyroxene attest to crystallization from undercooled magma and high cooling rates. Chromium numbers and TiO2 concentrations in spinel are similar to those in intraplate basalts. 40Ar–39Ar dating of magmatic hornblende yielded a plateau age of 276 ± 4 Ma (2σ). Transitional basalt with OIB-like trace element characteristics is the predominant rock-type; less frequent are tholeiitic basalts with mildly LREE depleted patterns and picrites with intermediate trace element characteristics. All samples show MORB-OIB like Pb/Ce, Th/La, and Th/Nb ratios which preclude subduction-modified mantle sources and felsic crustal material. Tholeiitic basalts and related olivine cumulate rocks show MORB-like initial eNd values of + 9.4 to + 6.2 which define a mixing line with the data for the transitional basalts (eNd ca. + 2.6). Initial 187Os/188Os ratios of 0.124–0.293 support mixed sources with a high proportion of recycled mafic crust in the transitional basalts. High concentrations of highly siderophile elements are in agreement with the high mantle potential temperatures and inferred high-melting degrees. It is argued that the Fariman complex originated by melting of a mantle plume component as represented by the OIB-like transitional basalt and entrained asthenosphere predominant in the MORB-like tholeiites. Two lines of evidence such as association of the Fariman complex with pelagic to neritic sedimentary rocks and the tectonic position at the boundary of two continental blocks defined by ophiolites and accretionary complexes of different ages suggest formation in an oceanic domain. Thus, we interpret it as a fragment of an oceanic plateau, which escaped subduction and was accreted as exotic block in the Paleo-Tethyan suture zone.

Vestiges of older greenstone in Mesoarchaean Chakradharpur granite gneiss, Singhbhum Craton, India: Implications for plume‐lithosphere interaction at rifted cratonic margin

The Chakradharpur granite gneiss (CGG) represents an uplifted Archaean basement within the North Singhbhum Orogen (NSO) of Singhbhum‐Orissa Craton in the eastern Indian shield and shares a Mesoarchaean 3.12 Ga age equivalent to the emplacement of Singhbhum granite (SBG) phase III. The CGG hosts numerous enclaves of tonalitic gneisses, metamorphosed ultramafic and mafic volcanics, metagabbros, metasediments, and conglomerates that collectively represent vestiges of an older greenstone belt preserved within it. The mineralogical composition of the ultramafic volcanic enclaves of CGG has been overprinted by greenschist facies metamorphism presenting a talc‐tremolite‐phlogopite‐actinolite assemblage with prominent development of crenulation cleavage, pressure solutions and relict spinifex texture. These are characterised by restricted low SiO2 (39.7–46.8 wt.%) at consistently high MgO (20.7–28 wt.%) contents, and geochemically classified as: (a) subalkaline (Na2O + K2O < 2) komatiite and meimechite and (b) an alkaline (Na2O + K2O > 2) type picrobasalt‐foidite‐basanite assemblage. Depleted LILE and LREE chemistry, Al2O3/TiO2 versus (Gd/Yb)N and HFSE abundances suggest that the studied Al‐undepleted Munro‐type komatiite and meimechite were derived by high degree (30%–50%), shallow level (<90 km) partial melting of a hot, anhydrous mantle plume carrying depleted components. It is suggested that the komatiite‐meimechite melts were deflected by an Archaean cratonic keel and erupted through thinned continental lithosphere at a rifted cratonic margin. Geochemical evidence of crustal contamination preserved in these komatiites are equated with minor input from reworking of older continental materials and interaction between plume‐derived asthenospheric melts and metasomatized subcontinental lithospheric mantle (SCLM). Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, and Nb/Th ratios for the picrobasalt, high Mg basalt, foidite and basanite rocks corroborate their generation through variable degrees of partial melting of a heterogeneous lithospheric mantle source carrying depleted plume‐derived melts and enriched components from metasomatized lithosphere. Mantle processes involved assimilation of metasomatic components of SCLM by ascending melts of plume incubating at the base of lithosphere ensued by melt generation at different mantle depths extending from garnet to spinel peridotite domains. Therefore, the geochemical signatures of ultramafic enclaves in CGG invoke distinct magmatic episodes during plume‐craton interaction, melt generation at thinned craton margin to rifted continental interior and melting of metasomatized continental lithosphere that collectively contributed to the evolution of Archaean mantle lithosphere system, volcanism with diverse chemical compositions and greenstone generation in the Singhbhum Craton.

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.

DIFFERENTIATED MESOARCHEAN KOMATIITE LAVA FLOWS: MINERALOGICAL AND GEOCHEMICAL CHARACTERISTICS, OUTFLOW AND CRYSTALLIZATION CONDITIONS

The paper presents the results of the study of thin differentiated Mesoarchean komatiite lava flows from the Koikary and Sovdozero greenstone domains. Several zones were recognized in the section of differentiated lava flows: autobreccia, spinifex texture of olivine and pyroxene types, massive, cumulate (ortho- and mesocumulates), as well as the lower chilled contact at the basement of the flow. The stratification of the lava flows is due to magmatic differentiation in situ coupled with the continuing injection of more molten rock until the stage of complete crystallization.

Ultramafic lavas and high-Mg basaltic dykes from the Othris ophiolite complex, Greece

We evaluate the petrography and geochemistry of an unusual suite of subduction-related Phanerozoic high-MgO rocks from the Othris ophiolite complex in Greece, some of which have previously been described as komatiitic lavas. In particular, we study ultramafic, olivine-phyric lavas from the Agrilia area and high-Mg basaltic dykes from the Pournari area. We seek to define primary magmatic MgO contents and initial liquidus temperatures as well as the differentiation sequence and cooling rates experienced by the lavas and dykes. One of our goals is to relate the Othris case to known komatiite and boninite occurrences and to address whether Othris documents an important new constraint on the temporal evolution of ambient mantle temperature, plume-related magmatism, and subduction of oceanic lithosphere. We conclude that, despite whole-rock MgO contents of 31–33wt%, the olivine-phyric lavas at Agrilia had an upper limit liquid MgO content of 17wt% and are therefore picrites, not komatiites. The Agrilia lavas contain the unusual Ti-rich pyroxenoid rhönite; we discuss the significance of this occurrence. In the case of the Pournari high-Mg dykes, the distinctive dendritic or plumose clinopyroxene texture, though it resembles in some ways the classic spinifex texture of komatiites, is simply evidence of rapid cooling at the dyke margin and not evidence of extraordinarily high liquidus temperatures. We correlate the dendritic texture with disequilibrium mineral chemistry in clinopyroxene to constrain the cooling rate of the dyke margins.

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

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

Viscous flow behavior of tholeiitic and alkaline Fe-rich martian basalts

The chemical compositions of martian basalts are enriched in iron with respect to terrestrial basalts. Their rheology is poorly known and liquids of this chemical composition have not been experimentally investigated. Here, we determine the viscosity of five synthetic silicate liquids having compositions representative of the diversity of martian volcanic rocks including primary martian mantle melts and alkali basalts. The concentric cylinder method has been employed between 1500°C and the respective liquidus temperatures of these liquids. The viscosity near the glass transition has been derived from calorimetric measurements of the glass transition. Although some glass heterogeneity limits the accuracy of the data near the glass transition, it was nevertheless possible to determine the parameters of the non-Arrhenian temperature-dependence of viscosity over a wide temperature range (1500°C to the glass transition temperature). At superliquidus conditions, the martian basalt viscosities are as low as those of the Fe–Ti-rich lunar basalts, similar to the lowest viscosities recorded for terrestrial ferrobasalts, and 0.5 to 1 order of magnitude lower than terrestrial tholeiitic basalts. Comparison with empirical models reveals that Giordano et al. (2008) offers the best approximation, whereas the model proposed by Hui and Zhang (2007) is inappropriate for the compositions considered.The slightly lower viscosities exhibited by the melts produced by low degree of mantle partial melting versus melts produced at high degree of mantle partial melting (likely corresponding to the early history of Mars), is not deemed sufficient to lead to viscosity variations large enough to produce an overall shift of martian lava flow morphologies over time. Rather, the details of the crystallization sequence (and in particular the ability of some of these magmas to form spinifex texture) is proposed to be a dominant effect on the viscosity during martian lava flow emplacement and may explain the lower range of viscosities (102–104Pas) inferred from lava flow morphology. Further, the differences between the rheological behaviors of tholeiitic vs. trachy-basalts are significant enough to affect their emplacement as intrusive bodies or as effusive lava flows. The upper range of viscosities (106–108Pas) suggested from lava flow morphology is found consistent with the occurrence of alkali basalt documented from in situ analyses and does not necessarily imply the occurrence of basaltic-andesite or andesitic rocks.

Rochas ultramáficas plutônicas do greenstone belt Rio das Velhas na porção central do Quadrilátero Ferrífero, Minas Gerais, Brasil

Em Amarantina, distrito de Ouro Preto, encontram-se rochas ultramáficas expostas em duas áreas com cerca de 500 m² cada. As rochas afloram no Complexo do Bação, que é o embasamento gnáissico do greenstone belt Rio das Velhas, na porção central do Quadrilátero Ferrífero (QF). O interesse no estudo petrogenético desses corpos deve-se à preservação parcial de minerais ígneos, ausentes na maior parte das rochas ultramáficas totalmente metamorfizadas do QF. Entre essas rochas, destacam-se os esteatitos e os serpentinitos, devido a sua importância econômica. As rochas ultramáficas de Amarantina possuem textura equigranular, fato que as caracteriza como tendo origem plutônica, isto é, trata-se de metaperidotitos. Possuem grãos maiores de olivina, piroxênio e espinélio da rocha ígnea original distribuídos em matriz metamórfica fina com talco, serpentinas, cloritas, anfibólios e minerais opacos. Escassas arita (NiSbAs) e breithauptita (NiSb) foram formadas a partir de pentlandita durante o metamorfismo associado a hidrotermalismo. A comparação da composição química com a de um metakomatiito com textura spinifex do QF, bem como com rochas komatitiiticas de outras partes do mundo, mostra que os metaperidotitos são, quimicamente, semelhantes aos komatiitos não-desfalcados em alumínio. Portanto é provável que as rochas ultramáficas estudadas correspondam à porção plutônica do magmatismo komatitiitico do Grupo Nova Lima, que é a unidade basal do greenstone belt Rio das Velhas.

Geochemical constraints on komatiite volcanism from Sargur Group Nagamangala greenstone belt, western Dharwar craton, southern India: Implications for Mesoarchean mantle evolution and continental growth

We present field, petrographic, major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt, western Dharwar craton. Field evidences such as crude pillow structure indicate their eruption in a marine environment whilst spinifex texture reveals their komatiite nature. Petrographic data suggest that the primary mineralogy has been completely altered during post-magmatic processes associated with metamorphism corresponding to greenschist to lower amphibolite facies conditions. The studied komatiites contain serpentine, talc, tremolite, actinolite and chlorite whilst tremolite, actinolite with minor plagioclase in komatiitic basalts. Based on the published Sm-Nd whole rock isochron ages of adjoining Banasandra komatiites (northern extension of Nagamangala belt) and further northwest in Nuggihalli belt and Kalyadi belt we speculate ca. 3.2–3.15 Ga for komatiite eruption in Nagamangala belt. Trace element characteristics particularly HFSE and REE patterns suggest that most of the primary geochemical characteristics are preserved with minor influence of post-magmatic alteration and/or contamination. About 1/3 of studied komatiites show Al-depletion whilst remaining komatiites and komatiite basalts are Al-undepleted. Several samples despite high MgO, (Gd/Yb)N ratios show low CaO/Al2O3 ratios. Such anomalous values could be related to removal of CaO from komatiites during fluid-driven hydrothermal alteration, thus lowering CaO/Al2O3 ratios. The elemental characteristics of Al-depleted komatiites such as higher (Gd/Yb)N (>1.0), CaO/Al2O3 (>1.0), Al2O3/TiO2 (<18) together with lower HREE, Y, Zr and Hf indicate their derivation from deeper upper mantle with minor garnet (majorite?) involvement in residue whereas lower (Gd/Yb)N (<1.0), CaO/Al2O3 (<0.9), higher Al2O3/TiO2 (>18) together with higher HREE, Y, Zr suggest their derivation from shallower upper mantle without garnet involvement in residue. The observed chemical characteristics (CaO/Al2O3, Al2O3/TiO2, MgO, Ni, Cr, Nb, Zr, Y, Hf, and REE) indicate derivation of the komatiite and komatiite basalt magmas from heterogeneous mantle (depleted to primitive mantle) at different depths in hot spot environments possibly with a rising plume. The low content of incompatible elements in studied komatiites suggest existence of depleted mantle during ca. 3.2 Ga which in turn imply an earlier episode of mantle differentiation, greenstone volcanism and continental growth probably during ca. 3.6–3.3 Ga which is substantiated by Nd and Pb isotope data of gneisses and komatiites in western Dharwar craton (WDC).

A Melt origin for spinifex-textured metaperidotite in the Cerro del Almirez massif, southern Spain

Outcrops of spinifex-textured metaperidotite in the Cerro del Almirez ultramafic massif, S. Spain, are located a few meters upgrade from a unique eclogite-facies (1.6–1.9 GPa, ≈680 °C), antigorite-out (orthopyroxene-in) isograd. Olivine in the spinifex rock is pleochroic brown in thin section, and microprobe analysis (carefully avoiding accidental inclusions) shows contents of Cr2O3 and TiO2 in the range 0 to 0.4 and 0 to 0.1 weight percent respectively. These amounts of Cr2O3 and TiO2 are matched by olivine in peridotite xenoliths from the mantle, komatiites, and pseudotachylytes. According to laboratory experiments, at 2 GPa and a likely redox state near QFM, these levels of Cr2O3 signify equilibration at temperatures in excess of 1600 °C. Curvature of the elongate olivines, and outcrop-scale alignment of olivine splays suggest a connection with komatiites. Two early studies at Cerro del Almirez emphasized this connection, but all papers from the late-1990s to the present time have advocated a metamorphic origin, analogous to jackstraw-olivine rocks. Some 5 to 6 weight percent or 13 to 15 volume percent of H2O was released at the isograd. We hypothesize that this dehydration water induced embrittlement of chlorite-metaharzburgite, and frictional slip in shear zones raised the temperature by several hundred degrees C above the ambient, more than enough to melt wet or dry ultramafic rock and form spinifex texture on rapid cooling. The amounts of crystallized melt so formed are orders-of-magnitude larger than found in peridotite pseudotachylyte, but are perhaps not totally inconsistent with dehydration-induced seismic events repeated over a protracted period of time during Miocene subduction of an oceanic plate. The transient ultramafic melts yielded on rapid cooling up to 12 cm long olivines and interstitial aggregates of parallel to radiating orthopyroxene prisms, followed by crystallization from glass of assorted eclogite-facies minerals (chlorite, tremolite, talc).

Synthesis of a spinifex‐textured basalt as an analog to Gusev crater basalts, Mars

Abstract– Analyses by the Mars Exploration Rover (MER), Spirit, of Martian basalts from Gusev crater show that they are chemically very different from terrestrial basalts, being characterized in particular by high Mg‐ and Fe‐contents. To provide suitable analog basalts for the International Space Analogue Rockstore (ISAR), a collection of analog rocks and minerals for preparing in situ space missions, especially, the upcoming Mars mission MSL‐2011 and the future international Mars‐2018 mission, it is necessary to synthesize Martian basalts. The aim of this study was therefore to synthesize Martian basalt analogs to the Gusev crater basalts, based on the geochemical data from the MER rover Spirit. We present the results of two experiments, one producing a quench‐cooled basalt (&lt;1 h) and one producing a more slowly cooled basalt (1 day). Pyroxene and olivine textures produced in the more slowly cooled basalt were surprisingly similar to spinifex textures in komatiites, a volcanic rock type very common on the early Earth. These kinds of ultramafic rocks and their associated alteration products may have important astrobiological implications when associated with aqueous environments. Such rocks could provide habitats for chemolithotrophic microorganisms, while the glass and phyllosilicate derivatives can fix organic compounds.