Ultramafic Lavas Research Articles

Petrogenesis and tectonic significance of mafic–ultramafic rocks from Southwest Yunnan, China

This study focuses on the mafic–ultramafic lavas of the Early Carboniferous outcrop in Mangxin, southwestern Yunnan, China. Picrites with 26–32 wt% MgO and a quenched texture are the most significant components of this rock association. This article divides the Mangxin picrites into two types. The mantle potential temperature (T p ) of these picrites is higher than the T p range of mid-ocean ridges and reaches that of mantle plumes. According to the geochemical characteristics, the Type-1 picrites probably formed from the melting of the mantle plume head and were contaminated by ambient depleted mantle, whereas the Type-2 picrites formed from the melting of mantle plume tails. These plume-related mafic–ultramafic rocks in Mangxin and the ocean island basalt (OIB)-carbonate rock associations in many areas of the Changning–Menglian belt provide significant evidence for the improvement of previous models of the Palaeotethyan oceanic plateau.

Multi-stage alteration, rheological switches and high-grade gold mineralization at Sheba Mine, Barberton Greenstone Belt, South Africa

Structural, petrographic and geochemical data from Sheba Gold Mine in the Barberton Greenstone Belt of South Africa indicate that gold mineralization formed at the end, and as the result of a multi-stage structural, fluid flow and alteration history. This multi-stage evolution is a prerequisite for the formation of high-grade (>40 g/t Au) ore bodies. The Sheba Mine is situated in well-bedded metaturbidites of the Mesoarchaean Fig Tree Group and structurally infolded ultramafic lavas in the first-order, refolded Ulundi syncline in the northern parts of the belt. Original komatiitic lava flows are pervasively altered to talc-carbonate greyschist. This regionally widespread alteration relates to an early phase of deformation and associated fluid flow. The reaction softening associated with this alteration accentuates rheological anisotropies within the heterogeneous wall-rock sequence. This results in the localization of strain along the contact between talc-carbonate greyschist and overlying, massive greywacke and chert units during later-stage refolding and associated flexural slip. Pervasive quartz veining, silicification, K-enrichment and the progressive replacement of the earlier talc-carbonate assemblages by a high-variance quartz-fuchsite green schist alteration testify to the localized fluid flow along this contact. The newly formed, finely intergrown green schist assemblage leads, in turn, to a reaction hardening and embrittlement of the former greyschist-chert contact. High-grade gold mineralization is hosted by brittle quartz-carbonate vein networks and breccias developed along brittle-ductile structures that cross-cut the competent green schist at high angles. The siting of gold and predominance of free gold suggest that gold precipitation is related to extreme fluid pressure cycling and phase separation of the mineralizing fluid during the formation of the brittle fracture networks. The thrust kinematics of the late-stage controlling structure studied here indicate that the mineralized structure accommodated regional NW-SE subhorizontal shortening when further tightening of the Ulundi syncline passed the lock-up stage could no longer be accommodated by flexural slip. The results highlight that the formation of favourable conditions for high grade mineralization, such as those at Sheba Mine, may be the result of the interaction of temporally distinct fluid flow and alteration events and lithological heterogeneities during the progressive structural evolution of a region.

Trace Element Distribution in Magnetite Separates of Varying Origin: Genetic and Exploration Significance

Magnetite is a widespread mineral, as disseminated or massive ore. Representative magnetite samples separated from various geotectonic settings and rock-types, such as calc-alkaline and ophiolitic rocks, porphyry-Cu deposit, skarn-type, ultramafic lavas, black coastal sands, and metamorphosed Fe–Ni-laterites deposits, were investigated using SEM/EDS and ICP-MS analysis. The aim of this study was to establish potential relationships between composition, physico/chemical conditions, magnetite origin, and exploration for ore deposits. Trace elements, hosted either in the magnetite structure or as inclusions and co-existing mineral, revealed differences between magnetite separates of magmatic and hydrothermal origin, and hydrothermal magnetite separates associated with calc-alkaline rocks and ophiolites. First data on magnetite separates from coastal sands of Kos Island indicate elevated rare earth elements (REEs), Ti, and V contents, linked probably back to an andesitic volcanic source, while magnetite separated from metamorphosed small Fe–Ni-laterites occurrences is REE-depleted compared to large laterite deposits. Although porphyry-Cu deposits have a common origin in a supra-subduction environment, platinum-group elements (PGEs) have not been found in many porphyry-Cu deposits. The trace element content and the presence of abundant magnetite separates provide valuable evidence for discrimination between porphyry-Cu–Au–Pd–Pt and those lacking precious metals. Thus, despite the potential re-distribution of trace elements, including REE and PGE in magnetite-bearing deposits, they may provide valuable evidence for their origin and exploration.

Geophysical expression of the Meyers Crater, a new meteorite impact crater discovered in the Coolgardie Goldfield of Western Australia

SummaryDetailed airborne electromagnetic (VTEM), gravity and passive seismic HVSR surveying, followed up by RC and diamond drilling, has identified an isolated circular and geologically young meteorite impact crater filled with sediments and located next to the town of Coolgardie in Western Australia. It was discovered in 2017 (Meyers, 2017), is 800 m in diameter and estimated to be >140 m deep based on passive seismic results, because only angled drilling was carried out for gold exploration along the western side of the crater. The crater has been filled with impact debris and sedimentary deposits to form a flat modern topographic feature. The pre-impact bedrock is Archean greenstone, which hosts numerous gold occurrences and mines surrounding the crater. The crater is filled with a polymict impact breccia of mixed greenstone rocks from below and surrounds near its base. A thin (<2 m) tuffaceous looking ejecta layer occurs at the boundary between the impact breccia and underlying in situ brecciated greenstone bedrock (high Mg basalt and ultramafic lava). This transition zone is also imaged as two “bedrock” layers in passive seismic HVSR cross-sections. The impact breccia is overlain by a very thick deposit of anoxic peat and carbonised wood fragments containing framboidal pyrite, as well as slump blocks of greenstone rocks, starting from over 120 m deep to a depth of 14 m from surface, where this organic layer is capped by a fine clay deposit, which was likely transported into the crater after its raised rim was breached by erosion. The crater and its fill deposits cannot explain the entire circular gravity anomaly low of -5mGal, and the brecciated in situ greenstone bedrock around and below the crater is modelled to contribute to the gravity anomaly low. It is estimated that the meteorite projectile was about 40 m in diameter, came from an easterly trajectory, and impacted Coolgardie between the Miocene to Pleistocene, with age dating and Ir analysis of the impact layer and overlying plant material soon to be carried out. A high-resolution ground magnetic survey using 10 m line spacing was carried out to try and detect large iron meteorite fragments, but only a weak and diffuse anomaly pattern occurs at the crater centre, indicating that the meteorite was highly fragmented on impact and/or later demagnetised by weathering. A vertical diamond drillhole down the centre of the crater is required to properly study this relatively young impact structure and analyse the preserved plant material filling the crater over a vertical column of 100 m or more, continuously recording local environmental changes over a considerable time period.

Petrological, mineralogical and geochemical peculiarities of Archaean cratons

The most outstanding features of Archaean cratons are their extraordinary thickness and enduring longevity. Seismically, Archaean cratonic fragments are sharply-bounded deep roots of mechanically strong, cold lithospheric mantle, clearly distinguishable from non-cratonic lithosphere. Rhenium-depletion of deep cratonic xenolith whole rocks and sulphide inclusions in diamond indicate that melting was broadly coeval with formation of the overlying proto-cratonic crust, which was of limited mechanical strength.A very important process of proto-cratonic development was vertical crustal reorganisation that eventually yielded a thermally stable, cratonised crust with a highly K-U-Th-rich uppermost crust and much more depleted deeper crust. Clastic sedimentary rocks available for geochemical study are predominantly found in the youngest parts of supracrustal stratigraphies and over-represent the highly evolved rocks that appeared during cratonisation. Vertical crustal reorganisation was driven by crustal radiogenic heat and emplacement of proto-craton-wide, incubating and dense supracrustal mafic and ultramafic volcanic rocks. Statistical analysis of these cover sequences shows a preponderance of basalt and a high abundance of ultramafic lavas with a dearth of picrite. The ultramafic lavas can be grouped into Ti-enriched and Ti-depleted types and high pressure and temperature experimental data indicate that the latter formed from previously depleted mantle at temperatures in excess of 1700 °C.Most mantle harzburgite xenoliths from cratonic roots are highly refractory, containing very magnesian olivine and many have a high modal abundance of orthopyroxene. High orthopyroxene mode is commonly attributed to metasomatic silica-enrichment or a non-pyrolitic mantle source but much of the excess silica requirement disappears if melting occurred at high pressures of 4–6 GPa. Analysis of experimental data demonstrates that melting of previously depleted harzburgite can yield liquids with highly variable Si/Mg ratios and low Al2O3 and FeO contents, as found in komatiites, and complementary high Cr/Al residues. In many harzburgites, there is an intimate spatial association of garnet and spinel with orthopyroxene, which indicates formation of the Al-phase by exsolution upon cooling and decompression. New and published rare earth element (REE) data for garnet and orthopyroxene show that garnet has inherited its sinusoidal REE pattern from the orthopyroxene. The lack of middle-REE depletion in these refractory residues is consistent with the lack of middle- over heavy-REE fractionation in most komatiites. This suggests that such pyroxene or garnet (or precursor phases) were present during komatiite melting. In the Kaapvaal craton, garnet exsolution upon significant cooling occurred as early as 3.2 Ga and geobarometry of diamond inclusions from ancient kimberlites also supports cool Archaean cratonic geotherms. This requires that some mantle roots have extended to 300 to possibly 400 km and that early cratons must have been much larger than 500 km in diameter.We maintain that the Archaean-Proterozoic boundary continues to be of geological significance, despite the recognition that upper crustal chemistry, as sampled by sedimentary rocks, became more evolved from ca. 3 Ga onwards. The boundary coincides with the disappearance of widespread komatiite and marks the end of formation of typical refractory cratonic lithosphere. This may signify a fundamental change in the thermal structure of the mantle after which upwellings no longer resulted in very high temperature perturbations. One school of thought is that the thermal re-ordering occurred at the core-mantle boundary whereas others envisage Archaean plumes to have originated at the base of the upper mantle. Here we speculate that Archaean cratonic roots may contain remnants of older domains of non-convecting mantle. These domains are potential carriers of isotope anomalies and their base could have constituted a mechanical and thermal boundary layer. Above laterally extensive barriers, emerging proto-cratons were protected from the main mantle heat loss. The eventual collapse of these mechanical barriers terminated very high temperature upwellings and dismembered portions of the barrier were incorporated into the cratonic mantle during the final Neoarchaean ‘superplume’ event. The surviving cratons may therefore preserve biased evidence of geological processes that operated during the Archaean.

Ultra-magnesian olivine-bearing ultramafic lava blocks within Cenozoic accretionary sediments at Shizuoka, Japan: Implications for young, hot plume activity beneath the western Pacific Plate

Ultra-magnesian igneous olivine is thought to crystallize from ultramafic melts and can therefore provide information about physicochemical conditions and the geochemistry and lithology of the source in the context of extremely hot mantle activity. This study focuses on the petrology and mineralogy of the Shizuoka meimechites, a series of ultra-magnesian olivine-bearing ultramafic lava blocks embedded in middle Eocene–early Miocene accretionary sediments at Shizuoka, Japan. Olivine phenocrysts within the Shizuoka meimechites have Fo compositions of ca. 81–92.8 mol%, where Fo = 100 Mg/(Mg + Fe). These ultra-magnesian olivines have trace-element compositions that are indicative of derivation from ultramafic melts that were in equilibrium with peridotitic mantle olivine. Al-in-olivine geothermometry yielded crystallization temperatures of ~1450 °C for Fo92.5–92.8 olivine, higher than the potential temperature of present-day mid-ocean ridge basalt source mantle material and similar to the potential temperatures of some large igneous provinces and ocean island basalts. Primary melt compositions in equilibrium with Fo92.5–92.8 olivine were estimated using the whole-rock compositions of the ultramafic lavas and are meimechitic, with MgO concentrations of ~21.4–27.0 wt% and SiO2 concentrations of 45.4–44.5 wt%. The potential temperature of the source mantle region for these meimechites (Tp) is estimated to be ~1486 °C–1705 °C, which is consistent with the Al-in-olivine geothermometry when considering adiabatic cooling during magma ascent. The relationship between Tp and Al-in-olivine temperature estimates for these lavas is consistent with global examples of ultra-magnesian olivine-bearing lavas. These lavas provide evidence of hitherto unknown Late Cretaceous–early Miocene deep mantle activity beneath the western Pacific region.

Rodingitization and carbonation, associated with serpentinization of Triassic ultramafic cumulates and lavas in Othris, Greece

The Othris region in Central Greece includes sub-oceanic crustal rocks of Middle-Late Triassic age that have undergone low to moderate degrees of metasomatic/metamorphic alteration processes. Alteration documented in these rocks includes rodingitization, serpentinization and carbonation in cumulate wehrlites, ultramafic lavas, mafic dykes and pillow-lavas. Wehrlites and ultramafic lavas are partially to completely serpentinized during ocean floor metamorphism. They were additionally affected by rodingitization, mostly expressed with formation of hydroandradites and hydrogrossulars, closely associated with the breakdown of primary phases of clinopyroxene and plagioclase. We estimate that rodingitization occurred within relatively mildly oxidizing physicochemical conditions, with an increased CO2/H2O ratio.More significant rodingitization affected mafic doleritic dykes that intrude wehrlites: this alteration occurred in parallel to serpentinization of hosting ultramafics via subsequent circulation of relatively alkaline and hydrous fluids. We have calculated that this occurred under low to moderate P-T conditions (180–320 °C, P ≈ 2–5 kbar). Circulation of Ca-rich fluids at shallower crustal levels was responsible for local carbonation phenomena, mostly affecting ultramafic and mafic pillow lavas.

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.

Geochemistry and isotopic signatures of metavolcanic and metaplutonic rocks of the Faina and Serra de Santa Rita greenstone belts, Central Brazil: Evidences for a Mesoarchean intraoceanic arc

The Archean-Paleoproterozoic Terrane of Goiás, Central Brazil, is an allochthonous block within the Neoproterozoic Tocatins Province and consists of an association of Archean TTG complexes and gold-bearing Archean-Paleoproterozoic greenstone belts. The Faina and Serra Santa Rita greenstone belts, located in the southern portion of the terrane, are investigated using geochemistry and isotope geology to establish the time of magmatism and tectonic environment. Our data show that the ultramafic rocks have some chemical characteristics similar to modern boninites, whereas the amphibolites are subdivided into two groups: the type 1 basalts group are tholeiites with flat REE patterns and are similar to back-arc basin basalts; the type 2 basalts group have high Nb contents and are comparable to Nb-enriched basalts. Felsic to intermediate rocks present some of the main chemical diagnostic features of adakites, in which the metandesites and metatonalites are comparable to high-SiO2 adakites, and the metadiorites, characterized by very high MgO, Cr and Ni contents, are comparable to low-SiO2 adakites or high-Mg andesites. Metavolcanic and metaplutonic rocks show two main periods of magmatic crystallization ages with juvenile and slightly crustal contaminated rocks. The first occurred at 2.96–2.92Ga with positive ƐNd (t) values of +2.16 to +2.77, while the second formed at 2.8Ga with slightly negative ƐNd (t) value of −0.15. The volcanic and plutonic protoliths of the two greenstone belts were formed in an intraoceanic forearc-arc-back-arc system. The initial stage corresponds to ultramafic lava eruption in the forearc region of a proto-island arc, at 2.96Ga. The evolution of the island arc and subduction progression led to oceanic slab-melting and generation of adakites. At 2.92Ga, the adakitic melt was totally consumed by peridotite mantle and the subsequent melting of these hybridized mantle wedge generated high-Mg andesites that lodged in the crust as dioritic intrusions with high MgO, Cr and Ni contents. The late-stage corresponds to a continental arc formation at 2.8Ga, marked by tonalitic magmatism and amalgamation with other island arcs and continental arcs of the TTG complexes of the Archean-Paleoproterozoic Terrane of Goiás.

Sluggish Hadean geodynamics: Evidence from coupled 146,147Sm–142,143Nd systematics in Eoarchean supracrustal rocks of the Inukjuak domain (Québec)

The discovery of deficits in 142Nd/144Nd in mafic rocks of the Nuvvuagittuq supracrustal belt (NSB) has triggered a debate about the possible preservation of Hadean (pre-3.85 Ga) crustal remnants in the little-known but areally extensive Innuksuac complex (3.6–3.8 Ga, Inukjuak domain, Northeast Superior Province, Canada). Geochronological investigations in the NSB, however, are hampered by the poor preservation and highly disturbed isotopic record of various mafic (amphibolite) lithologies that host the 142Nd anomalies. Here we present 146Sm–142Nd and 147Sm–143Nd data for rocks of extrusive magmatic and sedimentary protoliths from the Ukaliq supracrustal belt, a newly discovered volcano-sedimentary enclave enclosed in granitoid gneisses of the Inukjuak domain. Our study also includes the first 146Sm–142Nd data for quartz-magnetite rocks (banded iron-formation; BIF) of the NSB and the Eoarchean Isua supracrustal belt (ISB) in southern West Greenland. We show that Ukaliq amphibolites carry variably negative 142Nd anomalies, ranging from 0 to −10 ppm, which are positively correlated with their Sm/Nd ratio. If considered as an isochron relationship, the 146Sm–142Nd array yields an apparent Hadean emplacement age of 4215−76+50 Ma. The negative 142Nd anomalies, however, appear to be mainly restricted to amphibolites with boninitic affinities, likely reflecting inheritance from an enriched mantle source. In contrast, tholeiitic and ultramafic lavas have normal μ142Nd regardless of their Sm/Nd ratio. Furthermore, BIF from Ukaliq and Nuvvuagittuq lack the negative 142Nd anomalies that should have been produced by in situ decay of 146Sm had these sediments been deposited prior to ca. 4.1 Ga. Instead, they exhibit μ142Nd identical to that measured in Isua BIF. Collectively, our results suggest that the 146Sm–142Nd array characterizing mafic lithologies of Ukaliq and Nuvvuagittuq is an inherited signature with doubtful chronological significance. We interpret the volcanic protoliths of the Innuksuac complex to have been produced by metasomatically triggered melting of a variably enriched Eoarchean mantle, following addition of felsic melts and/or fluids derived from a foundering Hadean mafic crust. Application of coupled 146,147Sm–142,143Nd chronometry to Ukaliq lavas yields a model age of differentiation of 4.36−0.06+0.05 Ga for this Hadean precursor. This is similar to late-stage crystallization ages inferred for the lunar and terrestrial magma oceans. The long-term preservation of Earth's primordial crust points to subdued lithospheric recycling in the post-magma ocean Earth.

Determination of eruption temperature of Io's lavas using lava tube skylights

Determining the eruption temperature of Io's dominant silicate lavas would constrain Io's present interior state and composition. We have examined how eruption temperature can be estimated at lava tube skylights through synthesis of thermal emission from the incandescent lava flowing within the lava tube. Lava tube skylights should be present along Io's long-lived lava flow fields, and are attractive targets because of their temporal stability and the narrow range of near-eruption temperatures revealed through them. We conclude that these skylights are suitable and desirable targets (perhaps the very best targets) for the purposes of constraining eruption temperature, with a 0.9:0.7-µm radiant flux ratio ≤6.3 being diagnostic of ultramafic lava temperatures. Because the target skylights may be small – perhaps only a few m or 10s of m across – such observations will require a future Io-dedicated mission that will obtain high spatial resolution ( < 100m/pixel), unsaturated observations of Io's surface at multiple wavelengths in the visible and near-infrared, ideally at night. In contrast to observations of lava fountains or roiling lava lakes, where accurate determination of surface temperature distribution requires simultaneous or near-simultaneous ( < 0.1s) observations at different wavelengths, skylight thermal emission data are superior for the purposes of temperature derivation, as emission is stable on much longer time scales (minutes, or longer), so long as viewing geometry does not greatly change during that time.

Komatiites reveal a hydrous Archaean deep-mantle reservoir.

Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth's mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth's history.

Evolution of a ∼2.7 Ga large igneous province: A volcanological, geochemical and geochronological study of the Agnew Greenstone Belt, and new regional correlations for the Kalgoorlie Terrane (Yilgarn Craton, Western Australia)

The thick package of ∼2.7Ga mafic and ultramafic lavas and intrusions preserved among the Neoarchean of the Kalgoorlie Terrene in Western Australia provides valuable insight into geological processes controlling the most prodigious episode of growth and preservation of juvenile continental crust in Earth's history. Limited exposure of these rocks results in uncertainty about their age, physical and chemical characteristics, and stratigraphic relationships. This in turn prevents confident correlation of regional occurrences of mafic and ultramafic successions (both intrusive and extrusive) and hinders the interpretation of tectonic setting and magmatic evolution. A recent stratigraphic drilling program of the Neoarchean stratigraphy of the Agnew Greenstone Belt in Western Australia has provided continuous exposures through a c. 7km thick sequence of mafic and ultramafic units. In this study, we present a volcanological, lithogeochemical and chronological study of the Agnew Greenstone Belt, and provide the first pre-2690Ma regional correlation across the Kalgoorlie Terrane. The Agnew Greenstone Belt records ∼30m.y. of episodic ultramafic–mafic magmatism that includes two cycles, each defined by a komatiite that is overlain by units that become more evolved and contaminated with time. The sequence is divided into nine conformable packages, each consisting of stacked subaqueous lava flows and comagmatic intrusions, as well as two sills without associated extrusions. Lavas, with the exception of intercalations between two units, form a layer-cake stratigraphy and were likely erupted from a system of fissures tapping the same magma source. The komatiites are not contaminated by continental crust ([La/Sm]PM∼0.7) and are of the Al-undepleted Munro-type. Crustal contamination is evident in many units (Songvang Basalt, Never Can Tell Basalt, Redeemer Basalt, and Turrett Dolerite), as judged by [La/Sm]>1, negative Nb and Ti anomalies, and geochemical mixing trends towards felsic contaminants. Crystal fractionation was also significant, with early olivine and chromite (Mg#>65) followed by plagioclase and clinopyroxene removal (Mg<65), and in the most evolved case, titanomagnetite accumulation. Three new TIMS dates on granophyric zones of mafic sills and one ICP-MS date from an interflow felsic tuff are presented and used for regional stratigraphic correlation. Cycle I magmatism began at ∼2720Ma and ended ∼2705Ma, whereas cycle II began ∼2705Ma and ended at 2690.7±1.2Ma. Regional correlations indicate the western Kalgoorlie Terrane consists of a remarkably similar stratigraphy that can be recognised at Agnew, Ora Banda and Coolgardie, whereas the eastern part of the terrane (e.g., Kambalda Domain) does not include cycle I, but correlates well with cycle II. This research supports an autochthonous model of greenstone formation, in which one large igneous province, represented by two complete cycles, is constructed on sialic crust. New stratigraphic correlations for the Kalgoorlie Terrane indicate that many units can be traced over distances >100km, which has implications for exploration targeting for stratigraphically hosted ultramafic Ni and VMS deposits.

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions, emplacement mechanisms and water contents

Two Archaean komatiitic flows, Fred’s Flow in Canada and the Murphy Well Flow in Australia, have similar thicknesses (120 and 160 m) but very different compositions and internal structures. Their contrasting differentiation profiles are keys to determine the cooling and crystallization mechanisms that operated during the eruption of Archaean ultramafic lavas. Fred’s Flow is the type example of a thick komatiitic basalt flow. It is strongly differentiated and consists of a succession of layers with contrasting textures and compositions. The layering is readily explained by the accumulation of olivine and pyroxene in a lower cumulate layer and by evolution of the liquid composition during downward growth of spinifex-textured rocks within the upper crust. The magmas that erupted to form Fred’s Flow had variable compositions, ranging from 12 to 20 wt% MgO, and phenocryst contents from 0 to 20 vol%. The flow was emplaced by two pulses. A first ~20-m-thick pulse was followed by another more voluminous but less magnesian pulse that inflated the flow to its present 120 m thickness. Following the second pulse, the flow crystallized in a closed system and differentiated into cumulates containing 30–38 wt% MgO and a residual gabbroic layer with only 6 wt% MgO. The Murphy Well Flow, in contrast, has a remarkably uniform composition throughout. It comprises a 20-m-thick upper layer of fine-grained dendritic olivine and 2–5 vol% amygdales, a 110–120 m intermediate layer of olivine porphyry and a 20–30 m basal layer of olivine orthocumulate. Throughout the flow, MgO contents vary little, from only 30 to 33 wt%, except for the slightly more magnesian basal layer (38–40 wt%). The uniform composition of the flow and dendritic olivine habits in the upper 20 m point to rapid cooling of a highly magnesian liquid with a composition like that of the bulk of the flow. Under equilibrium conditions, this liquid should have crystallized olivine with the composition Fo94.9, but the most magnesian composition measured by electron microprobe in samples from the flow is Fo92.9. To explain these features, we propose that the parental liquid contained around 32 wt% MgO and 3 wt% H2O. This liquid degassed during the eruption, creating a supercooled liquid that solidified quickly and crystallized olivine with non-equilibrium textures and compositions.

Gold concentrations in metamorphic fluids: A LA-ICPMS study of fluid inclusions from the Alpine orogenic belt

Vein and shear-zone hosted gold deposits in orogenic terrains of Archean to Phanerozoic age are formed from dominantly metamorphic fluids. It is debated, however, whether normal crustal source rocks are adequate to generate economic deposits, or whether selectively gold-enriched sources such as mafic to ultramafic lavas, black shales or an input of magmatic fluid make a decisive difference between the formation of high-grade deposits and barren vein systems. As an essential baseline study in this debate, we have analyzed the metal and sulfur content of fluid inclusions in barren veins across a prograde sequence from greenschist to amphibolite facies in the Central Alpine metamorphic belt. Comparison of the analyzed fluid compositions with thermodynamic solubility calculations shows that the fluids record metamorphic dehydration, decarbonation and desulfidation of the continental crust. However, gold with concentrations of 0.003 to 0.03ppm is increasingly undersaturated in the highest-temperature aqueous–carbonic fluids, which otherwise resemble those forming major gold deposits elsewhere. Our results show that the regional-metamorphic fluids could carry 10–1000 times higher gold concentrations, implying that pre-enriched source rocks or hydrous magmas may play an essential role in generating highly gold-endowed orogenic belts.

Near-infrared monitoring of Io and detection of a violent outburst on 29 August 2013

We present initial data from our campaign to monitor Io in the near-infrared, beginning in August 2013, using 3.8-μm adaptive optics imaging at Gemini N and 2–5μm disk-integrated spectroscopy at NASA’s IRTF. Conducted during 2013–2014, these observations are coincident with the ISAS/JAXA EXCEED mission’s continuous monitoring of the Io plasma torus and will enable the speculated effects of volcanic outgassing on the torus to be observed directly, in addition to enabling an assessment of the frequency and properties of large-scale outbursts. On 29 August 2013 we detected a powerful eruption (designated 201308C) on Io at 223.5±2.6°W, 29.1±1.8°N. Emitting between 15 and >25 TW, this event is one of the most powerful eruptions ever seen on Io and falls into the rare “outburst” class. This was the third eruption of this type seen on Io in August 2013, an unprecedented occurrence. Also unprecedented was the charting of the decay in thermal emission over the subsequent days and weeks. Modeling of the outburst spectrum places a lower bound of 1200–1300K on the eruption temperature, and is suggestive of temperatures 1900K or higher, typically associated with ultramafic lava composition. The eruption is likely a highly energetic, high-volume lava fountain event.

Trevorite: Ni-rich spinel formed by metasomatism and desulfurization processes at Bon Accord, South Africa?

AbstractThe 3.5 Ga Bon Accord Ni deposit occurs within the lowest serpentinized mafic–ultramafic lavas of the Barberton Greenstone Belt (South Africa). Though now completely mined out, it comprised a suite of rare Ni-rich minerals that led to its interpretation as either an extraterrestrial body or as an oxidized fragment of Fe-Ni alloy originating from the terrestrial core. In this study, we draw on detailed petrographic observation and mineral chemical data, as well as previous work, to re-evaluate these ideas. The balance of evidence, from thin section (&lt;1 mm) to regional (∼10s of km) scales, appears to support an alternative origin for Bon Accord, possibly as an oxidized Ni-sulfide deposit formed in association with ocean floor komatiite eruptions.

Crustal evolution, intra-cratonic architecture and the metallogeny of an Archaean craton

Abstract The generation of the Earth's continental crust modified the composition of the mantle and provided a stable, buoyant reservoir capable of capturing mantle material and ultimately preserving ore deposits. Within the continental crust, lithospheric architecture and associated cratonic margins are a first-order control on camp-scale mineralization. Here we show that the evolving crustal architecture of the Archaean Yilgarn Craton, Western Australia, played a key role in controlling the localization of camp-scale gold, iron and nickel mineralized systems. The age and source characteristics of Archaean lithosphere are heterogeneous in both space and time and are recorded by the varying Nd isotopic signature of crustal rocks. Spatial and temporal variations in isotopic character document the evolution of an intra-cratonic architecture through time, and in doing so map transient lithospheric discontinuities where gold, nickel and iron mineral systems were concentrated. Komatiite-hosted nickel deposits cluster into camps localized within young, juvenile crust at the isotopic margin with older lithosphere; orogenic gold systems are typically localized along major structures within juvenile crust; and banded iron formation (BIF)-hosted iron deposits are localized at the edge of, and within, older lithospheric blocks. Furthermore, this work shows that crustal evolution plays an important role in the development and localization of favourable sources of nickel, gold and iron by controlling the occurrence of thick BIFs, ultramafic lavas and fertile (juvenile) crust, respectively. Fundamentally, this study demonstrates that the lithospheric architecture of a craton can be effectively imaged by isotopic techniques and used to identify regions prospective for camp-scale mineralization.

Panxi region (South-West China): Tectonics, magmatism and metallogenesis. A review

Abstract The Panxi region of SW China makes up most of the western margin of the Yangtze craton. Its structural pattern, defined by NS-trending deep faults, is superimposed on a zone of late Proterozoic crustal extension. The Panxi region seems to have evolved as a continental rift in the Permian, and was subsequently subjected to compression, caused mainly by the Himalayan collision. This induced the uplift of its axial and western parts, generating a horst-type structure within the former rift graben. At ca. 260 Ma, mantle plume-related magmatism in the Panxi region generated the Emeishan flood basalts together with ultramafic and silicic volcanic rocks and with numerous intrusive bodies (peridotites, pyroxenites, layered gabbros, syenites, granites). A geochemical distinction between high-Ti and low-Ti Emeishan basalts can be made, but just with the significance of compositional end-members since the published data define a continuous variation of the entire compositional range. Similar compositional variation occurs in the ultramafic lavas (picrites and komatiites) intercalated in the sequence of the Emeishan basalts, which are considered to be the products of undifferentiated primary magmas. Based on the geochemical data, the primary magmas could have been generated from distinct source materials (mantle plume, asthenosphere, lithospheric mantle), from compositionally heterogeneous mantle plume source or from a common source but with various degrees of mantle melting and crustal contamination. Considering the picritic composition of the primary magmas, a deep-seated differentiation of large volume of magma is needed to produce the extrusion of the relatively evolved Emeishan basalts. World class Fe–Ti–V oxide ore deposits are associated with the layered gabbros, while the ultramafic intrusions can host small Ni–Cu and PGE sulfide deposits. The intrusions containing Fe–Ti–V oxide deposits show remarkably continuous layering and probably derived from mafic magmas differentiated in relatively large chambers. The ultramafic intrusions hosting Ni–Cu(–PGE) sulfide ores most likely formed from sulfide saturated magma moving through conduits.

The carbonated source region of Cenozoic mafic and ultra-mafic lavas from western Qinling: Implications for eastern mantle extrusion in the northeastern margin of the Tibetan Plateau

Cenozoic Indo-Asian collision caused significant crustal shortening in central Tibet. The strike-slip faults around the Tibetan Plateau (TP) are generally attributed to extrusion tectonics, resulting from lower crust flow. Therefore, the mantle extrusion site corresponding to the Cenozoic elevation of the TP needs to be identified. This paper reports the petrology and geochemistry of Cenozoic mafic and ultra-mafic volcanic rocks in the Xiahe and Lixian areas, at the northeastern margin of the TP. Detailed analysis indicates a regular change in partial melting conditions and source regions of the volcanic rocks from west to east, revealing a Cenozoic eastward mantle extrusion in the eastern margin of the TP. The Xiahe volcanic rocks display ocean island basalt affinity with negative K anomalies and positive Nb and Ta anomalies. They are alkaline with extremely high Na2O/K2O ratios and relatively enriched Sr–Nd–Pb isotopic compositions, indicating that these basalts were derived from partial melting of carbonated pyroxenite. The Lixian picro-basalts are closely associated with igneous carbonatites. They have relatively high TiO2 (3.47–4.66%) and MgO (11.24–18.88%) contents and low SiO2 (41.14–44.82%) and Al2O3 (5.84–9.18%) contents. Based on the depleted Sr–Nd–Pb isotopic compositions, we propose that the Lixian picro-basalts may have originated from the partial melting of carbonated lithosphere mantle peridotites at relatively high pressure (>3GPa). Minor hornblendite in their source region can account for the high TiO2 and Na2O+K2O contents. Thus, we argue that these volcanic rocks were formed by episodic decompression melting of the carbonated mantle lithosphere during the eastward extrusion of the Tibetan lithosphere, in contrast to the conventional view that they were formed in a continental rift setting. Their partial melting and eruption processes may be closely related to the Cenozoic strike-slip fault activities in the northeastern margin of the TP.