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Abstract: LA-ICP-MS U–Pb zircon dating reveals that the Phu Kham adakites in the Truong Son Belt were emplaced during the Late Carboniferous (ca. 306 to 304Ma), whereas the Puthep 1 (PUT 1) adakites in the Loei Fold Belt were formed during the Middle Triassic (ca. 244 to 241Ma). These rock formation ages are largely coeval to Re–Os molybdenite ages (Phu Kham: ca. 304Ma; PUT 1: ca. 246Ma), suggesting close temporal links between adakite formation and copper–gold mineralization. New geochemical results show that the Phu Kham and PUT 1 adakites are characterized by low HREE and Y contents, but elevated LREE, Sr, Sr/Y, and La/Yb values. In addition, the two adakites have relatively high 143Nd/144Nd ratios (0.512648 to 0.512719), Mg# (Phu Kham: 37–68; PUT 1: 40–65), and Cr and Ni contents. These results are best interpreted as representing slab melts that have interacted with supra-subduction zone mantle wedge during ascent. Tectonically, the Phu Kham and PUT 1 adakites were most likely formed during the initiation of subduction of the Ailaoshan–Song Ma and the Main Paleo-Tethys ocean plates respectively. Close temporal relations between the adakite formation and the copper–gold mineralization suggest strong genetic links between the two in the mainland SE Asia region. [Copyright &y& Elsevier]

Mount Polley is a Late Triassic (~205 Ma) alkalic porphyry Cu-Au-Ag deposit (226.3 thousand tonnes (t) Cu, 21.5 t Au, and 65.11 Ag), hosted by silica-undersaturated to silica-saturated monzonitic intrusions of the Mount Polley Complex, located in British Columbia, Canada. The Northeast ore zone at Mount Polley is hosted by magmatic-hydrothermal breccia. Copper and precious metals occur in sulfide minerals primarily as coarse- to fine-grained breccia cement. Local wall rocks include equigranular to porphyritic diorite, monzodiorite, and monzonite. Alteration, breccia cement, and veins of the Northeast ore zone formed in five paragenetic stages: prebreccia (stage 1), brecciation and main-stage mineralization (stage 2), late-stage mineralization (stage 3), unmineralized postbreccia dikes and veins (stage 4), and epithermal-style veins (stage 5). Intense pervasive K and Fe metasomatism ± calcite and calc-silicate alteration occurred prior to brecciation caused by the intrusion of mega-crystic K-feldspar-phyric monzonite. Stage 2 fluids were silica undersaturated, high temperature (>350°C), CO2 enriched, and of near-neutral to alkaline pH. Potassic, sodic, and cale-potassic assemblages precipitated with mineralization during stage 2 with moderate temperatures at the deposit periphery and in stages 3 and 4. Evidence for more acidic and lower-temperature conditions is preserved in stage 5 veins. The δ34Ssuifide isotope compositions of stages 2 and 3 chalcopyrite, pyrite, and bornite range from -7.1 to +1.4%c. Sulfur isotope compositions of anhydrite and gypsum are mostly between 6.2 and 9.8%o. These values, together with the presence of hematite, are consistent with deposition from an oxidized, sulfate-dominant, high-temperature magmatic-hydrothermal fluid. Limited sulfur isotope geothermometry indicates that Cu sulfides precipitated at temperatures from -480° to ~250°C. Hydrothermal calcite occurs in all paragenetic stages at Mount Polley. Calcite δ13C values range from -0.2 to -10.5%, and δ18O values from 4.0 to 20.9%. The enriched C-O isotope values are not consistent with simple precipitation from an entirely magmatic source of hydrothermal fluid. Interaction of the fluid and/or magma with limestone is considered a likely process to explain the C and O isotope signature. Lead isotope data suggest mixing of mantle and crustal sources during mineralization. Main-stage chalcopyrite and pyrite as well as late-stage galena have 206/204pb values of 18.77 to 18.92, 207/204pb of 15.56 to 15.59, and 208/204pb of 38.22 to 38.32. Strontium isotope data (0.70331-0.70371) provide evidence of a strongly depleted mantle source of Sr with minor crustal input. Epsilon Nd values for main-stage apatite range between 5.9 and 6.5, also indicating a depleted mantle source. Stage 5 carbonate 206/204pb values of 18.96 to 19.04, 207/2044pb of 15.57 to 15.59, and 207/204pb of 38.26 to 38.36 suggest superposition of an epithermal system onto the Northeast ore zone, potentially as late as -100 m.y. after breccia formation. The data presented are consistent with the hypothesis that the silica-undersaturated alkalic Mount Polley Complex formed due to carbonate assimilation prior to mineralization. This process can explain the 1δ13C-δ18O isotope data, calcite precipitation concurrent with Cu-Au mineralization, and silica undersaturation of the magma. The CO2 released during assimilation of carbonate also could have promoted magmatic-hydrothermal brecciation. Silica-undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate-bearing substrate. [ABSTRACT FROM AUTHOR]

Ulleung Island is the top of a 3000 m (from sea floor) intraplate alkalic volcanic edifice in the East Sea/Sea of Japan. The emergent 950 m consist of a basaltic lava and agglomerate succession (Stage 1, 1·37–0·97 Ma), intruded and overlain by a sequence of trachytic lavas and domes, which erupted in two episodes (Stage 2, 0·83–0·77 Ma; Stage 3, 0·73–0·24 Ma). The youngest eruptions, post 20 ka bp, were explosive, generating thick tephra sequences of phonolitic composition (Stage 4), which also entrained phaneritic, porphyritic and cumulate accidental lithics. Major element chemistry of the evolved products shows a continuous spectrum of trachyte to phonolite compositions, but these have discordant trace element trends and distinct isotopic characteristics, excluding a direct genetic relationship between the two end-members. Despite this, the Stage 3 trachytes and some porphyritic accidental lithics have chemical characteristics transitional between Stage 2 trachytes and Stage 4 phonolites. Within the phonolitic Stage 4 tephras three subgroups can be distinguished. The oldest, Tephra 5, is considerably enriched in incompatible elements and chondrite-normalized rare earth element (REE) patterns display negative Eu anomalies. The later tephras, Tephras 4–2, have compositions intermediate between the early units and the trachyte samples, and their REE patterns do not have significant Eu anomalies. The last erupted, Tephra 1, from a small intra-caldera structure, has a distinct tephriphonolite composition. Trace element and isotopic chemistry as well as textural characteristics suggest a genetic relationship between the phaneritic lithics and their host phonolitic pumices. The Stage 4 tephras are not related to earlier phases of basaltic to trachytic magmatism (Stages 1–3). They have distinct isotopic compositions and cannot be reliably modelled by fractional crystallization processes. The differences between the explosive phonolitic (Stage 4) and effusive trachytic (Stage 2–3) eruptions are mainly due to different pre-eruptive pressures and temperatures, causing closed- versus open-system degassing. Based on thermodynamic and thermobarometric modelling, the phonolites were derived from deeper (subcrustal) magma storage and rose quickly, with volatiles trapped until eruption. By contrast, the trachytes were stored at shallower crustal levels for longer periods, allowing open-system volatile exsolution and degassing before eruption. [ABSTRACT FROM PUBLISHER]

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Jeju is a volcanic field that has erupted from around 1·8 Myr to c. 1 kyr ago. Activity began with dispersed, basaltic, monogenetic, phreatomagmatic eruptions. Continuing monogenetic volcanism was later joined by more voluminous lava effusion events building a central composite shield. Samples from older (>0·7 Ma) and younger (<0·2 Ma) monogenetic centres were analysed for their whole-rock major element, trace element and Sr–Nd–Pb isotopic compositions. Pyroclastic products from the monogenetic centres are dominantly alkali basalt to trachybasalt, whereas the more voluminous lava flows and domes of the central edifice consist of subalkali basalt and alkali basalt to trachyte. Lavas from the Early Pleistocene monogenetic centres are depleted in MgO, Cr and Ni, reflecting considerable olivine fractionation. By contrast, Late Pleistocene–Holocene monogenetic centre magmas fractionated clinopyroxene + olivine at deeper levels. Isotopic compositions show little variation across the suite; however, the Late Pleistocene–Holocene monogenetic centres have generally lower 87Sr/86Sr and 208Pb/204Pb and higher 143Nd/144Nd than the older centres and subalkali lavas. Major and trace element and isotope data suggest a common, shallower source for the high-Al alkali and subalkali lavas, in contrast to a deeper source for the low-Al alkali magmas. We propose that mantle melting was initiated under partially hydrous conditions at a pressure of near 2·5 GPa, followed by drier conditions and extension of the melting zone to 3–3·5 GPa, with a concomitant increase in the volume of melt derived from the shallower part of the system to produce subalkaline magmas. Increasing melt production at shallow depths may be related to accelerated heat transfer resulting from deepening of the melting zone, or increased mantle upwelling. Mantle lenses were uplifted, probably lubricated by shear zones created during the opening of the Sea of Japan c. 15 Myr ago, and reactivated during rotation of the Philippine Sea plate direction of subduction at around 2 Ma. This is the first hypothesized link between subduction processes and intraplate volcanism at Jeju. [ABSTRACT FROM PUBLISHER]

[Display omitted] • Sm-Nd dating yields robust chronologies for several carbonate associations. • First Lu-Hf isotopic dates for hydrothermal carbonates in an ore deposit. • Three carbonate-forming events confirmed (~1.59–1.55 Ga, 0.8–1.0 Ga, 0.5 Ga). • Sr isotope ratios in carbonates support multi-stage, protracted evolution of deposit. Mineralization at the supergiant Cu-U-Au-Ag Olympic Dam deposit (South Australia), the 'uranium endmember' of the iron-oxide copper–gold (IOCG) spectrum of ore deposits, is hosted in a breccia complex developed entirely within granite of the 1.59 Ga Hiltaba Suite (Gawler Craton). Earlier studies suggested brecciation and mineralization occurred within a magmatically-driven hydrothermal system at 1.59 Ga, with a critical role for mafic–ultramafic intrusions. In contrast, recent radiometric dating of the breccia complex indicates a prolonged, multi-stage history of brecciation and mineralization from 1.59 to 0.5–0.4 Ga. Ca-Fe-Mg-Mn-carbonate gangue minerals are associated with ore minerals at virtually every stage of mineralization. In a companion study (Apukhtina et al., 2020), this mineralogically, texturally and compositionally diverse carbonate mineral suite was assigned to seven associations defined on the basis of host lithology and texture. Here we report Sm-Nd, Pb-Pb and Lu-Hf isotope ages for these carbonates, which are used to examine the chronology of carbonate deposition. Initial Sr-Nd isotopic compositions are used to place constraints on fluid sources. Sm-Nd and Pb-Pb isotope systematics of calcite veins in ~1.59 Ga IOCG ore indicate 1.59–1.55 Ga deposition ages. Likewise, locally abundant laminated siderites have Sm-Nd ages in this age interval. A world-first attempt to apply Lu-Hf dating to carbonate gangue in an ore deposit yields ages that are 70–100 Ma younger than corresponding Sm-Nd ages, presumably reflecting isotopic exchange of carbonate Lu-Hf isotope systems with host rocks. Sm-Nd ages for carbonates assigned to other carbonate associations (hosted in highly altered inferred 1.59 Ga basalt and picrite; diverse settings within granite-dominated breccia; locally abundant megaclasts of green and red bedded sandstone/mudstone sequences; ~0.82 Ga doleritic dykes) are more diverse and range from ~1.59 to 0.5 Ga. The structurally youngest carbonates (unbrecciated fluorite-barite veins; carbonate matrix in polymict conglomerate above the breccia complex) yield ~0.50 Ga Sm-Nd ages. Inferred carbonate ages are broadly consistent with radiometric dates for other hydrothermal minerals (e.g., hematite, uraninite, apatite, fluorite). They suggest that mineralization initiated at 1.59 Ga was reworked and possibly increased in size in response to large-scale tectonic, magmatic, sedimentary and hydrothermal events. Initial 87Sr/86Sr in the carbonates is higher and more variable (0.710–0.752, average ~0.721) than could be explained by ore and gangue mineral formation from magmatic-hydrothermal fluids during a single event at 1.59 Ga, a model favored in several earlier studies. By contrast, carbonate formation over a long period, as inferred from the Sm-Nd chronology presented here, would allow ingrowth of 87Sr in the granitic host rocks to develop the heterogeneous initial 87Sr/86Sr recorded in the carbonates. Carbonate-bearing fluids appear to have sourced Nd (and most likely also Sr) locally, within the host granite and breccia, with contributions from mafic rocks. The emerging evidence for protracted, multi-stage mineralization implies that single-stage models for Olympic Dam need to be revisited and that all studies of sulfide and gangue minerals in this deposit require careful radiometric dating. We speculate that the polymetallic nature and unusually large metal reserves of the Olympic Dam mineralization are related to its multi-stage formation history. [ABSTRACT FROM AUTHOR]

Abstract: Building upon the work of Richards et al. [1998. U–Pb dating of a speleothem of Quaternary age. Geochimica et Cosmochimica Acta 62, 3683–3688], we have developed a method for precise dating of speleothems beyond the range of the U–Th technique using the U–Pb decay scheme. By coupling low-blank sample preparation procedures and multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) analytical methodologies developed for low-level Pb-isotope analysis, we find that, under ideal circumstances (radiogenic speleothems with very low common Pb), U–Pb dating of speleothems is not only possible, but also produces excellent age resolution—often comparable to or better than U–Th studies. Corrections for initial isotopic disequilibrium are necessary and exert a strong control on the achievable age uncertainty. The technique will be of immediate benefit in extending speleothem-based climate proxy records beyond ∼500ka and will also find other uses, such as the dating of associated sub-fossil remains, and providing constraints on rates of landscape evolution and neo-tectonic processes. Here we present initial results for speleothems from the Nullarbor Plain, Western Australia, and the Alpi Apuane, Italy. The Nullarbor samples provide important new constraints on the development of aridity in Australia during the late Tertiary/early Quaternary, while the Apuane samples offer insights into the landscape history and uplift of that region. [Copyright &y& Elsevier]

The kimberlite rocks of the Udachnaya-East pipe (Siberia) are uniquely fresh and contain very high abundances of primary volatiles (Cl, CO2, S). Alkali elements and chlorine are extremely abundant in the reconstructed kimberlite melt compositions, and this enrichment is very important for our understanding of deep-mantle melting and melt transport. Here we present new isotopic data that confirm a mantle origin for these kimberlitic chlorides and carbonates, and constrain the kimberlite emplacement age as ca. 347 Ma. The initial Nd and Pb isotope ratios in a large salt aggregate, in a Cl-S-enriched water leachate of the groundmass, and in the silicate fraction of the groundmass are very similar (ϵNd = +3 to +4, 206Pb/204Pb = 18.6, 207Pb/204Pb = 15.53), implying a comagmatic origin of the chlorides and carbonates and the silicates. Combined Sr, Nd, and Pb isotope data are used to rule out any significant contributions to the kimberlite chlorine budget from crustal sources, such as the Cambrian evaporite sequences of the Siberian platform. Our data support the interpretation that exsolved Na-K chloride and Na-K-Ca carbonate formed directly from original uncontaminated kimberlite magma. High Cl abundances in kimberlites suggest the presence of a Cl-rich reservoir in the deep sublithospheric mantle. [ABSTRACT FROM AUTHOR]

The South Auckland Volcanic Field is a Pleistocene (1·59–0·51 Ma) basaltic intraplate, monogenetic field situated south of Auckland City, North Island, New Zealand. Two groups of basalts are distinguished based on mineralogy and geochemical compositions, but no temporal or spatial patterns exist in the distribution of various lava types forming each group within the field: Group A basalts are silica-undersaturated transitional to quartz-tholeiitic basalts with relatively low total alkalis (3·0–4·6 wt %), Nb (7–29 ppm), and (La/Yb)N (3·4–7·6); Group B basalts are strongly silica-undersaturated basanites to nepheline-hawaiites with high total alkalis (3·3–7·9 wt %), Nb (32–102 ppm), and (La/Yb)N (12–47). Group A has slightly higher 87Sr/86Sr, similar ɛNd, and lower 206Pb/204Pb values compared with Group B. Contrasting geochemical trends and incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) are consistent with separate evolution of Groups A and B from dissimilar parental magmas derived from distinct sub-continental lithospheric mantle sources. Differentiation within each group was controlled by olivine and clinopyroxene fractionation. Group B magmas were generated by <8% melting of an ocean island basalt (OIB)-like garnet peridotite source with high 238U/204Pb mantle (HIMU) and enriched mantle (EMII) characteristics possibly inherited from recycled oceanic crust. Group A magmas were generated by <12% melting of a spinel peridotite source also with HIMU and EMII signatures. This source type may have resulted from subduction-related metasomatism of the sub-continental lithosphere modified by a HIMU plume. These events were associated with Mesozoic or earlier subduction- and plume-related magmatism when New Zealand was at the eastern margin of the Gondwana supercontinent. [ABSTRACT FROM PUBLISHER]

Accessory gangue scheelite (CaWO4) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique.Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial 87Sr/86Sr and &epsiv;Nd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have 87Sr/86Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone.The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial 87Sr/86Sr ratios and &epsiv;Nd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that 87Sr/86Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite.A positive correlation between Na and REE suggests that REE3+ are accommodated by the coupled substitution REE3++Na+=2 Ca2+ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (

Stratigraphic ages from conventional glauconite geochronology are commonly younger than those obtained via high temperature chronometers. The widely used glauconite reference material GL‐O, for example, has a K‐Ar age (95.03 ± 1.11 Ma) ~ 5 Ma younger than its expected stratigraphic age. To identify the influences on glauconite ages and assess the suitability of GL‐O as a reference material for in situ Rb‐Sr geochronology, we separated GL‐O grains based on colour and morphology. Each fraction was characterised petrographically and compositionally before in situ Rb‐Sr dating. Separate aliquots were dated via conventional isotope dilution (ID) Rb‐Sr geochronology. We find a ~ 10 Ma spread in the in situ Rb‐Sr ages of GL‐O fractions, where more rapid maturation and isotopic closure of darker grains yields ages closer to the depositional age, whereas smaller, more porous light green grains show evidence for delayed maturation leading to continued Rb uptake during burial as well as Sr isotope exchange with connate fluids. Discrepancies between ID and in situ Rb‐Sr ages are explained by (i) core‐rim age zonation, (ii) the presence of alteration resistant, Sr‐rich apatite inclusions, (iii) differences in laser‐induced fractionation. We recommend additional purification steps before use of GL‐O as reference material for in situ Rb‐Sr geochronology. [ABSTRACT FROM AUTHOR]

The Deddick Granodiorite, a mafic S-type pluton in the Lachlan Fold Belt, contains abundant enclaves. Most are derived from high-grade metasediments of pelitic–psammitic composition, and migmatites are common. Among these, discrete fragments of melanosomes rich in cordierite and garnet are restites from partial melting. However, compositional and Nd–Sr isotopic data indicate they are not restite in equilibrium with the melt component of the host magma, but may have formed part of a spectrum of diverse magma source lithologies. Alternatively, they may be accidental xenoliths. Regardless of their origin, data for these enclaves suggest their precursors could represent deeply buried age equivalents of the ubiquitous Ordovician turbidites of the Lachlan Fold Belt. Three types of microgranular enclaves with igneous textures are distinguished. The most common type are small and rounded mafic enclaves of tonalitic composition. They carry xenocrysts derived from the host magma, and some contain high-Mg pyroxene derived from a mafic magma. Isotopic data form an array with εNd from −6 to −12, and 87Sr/86Sr from 0.7130 to 0.7167 (host rock −10 and 0.715, respectively). These enclaves formed as globules of hybrid mafic magma commingled with, and contaminated by, the more felsic host magma. Other enclave types were derived from disrupted syn-plutonic dykes having a distinct isotopic composition, and from a cogenetic marginal facies of the host. [ABSTRACT FROM AUTHOR]

Large-scale mantle convective processes are commonly reflected in the emplacement of Large Igneous Provinces (LIPs). These are high-volume, short-duration magmatic events consisting mainly of extensive flood basalts and their associated plumbing systems. One of the most voluminous LIPs in the geological record is the ~ 2.06 billion-year-old Bushveld Igneous Complex of South Africa (BIC), one of the most mineralised magmatic complexes on Earth. Surprisingly, the known geographic envelope of magmatism related to the BIC is limited to a series of satellite intrusions in southern Africa and has not been traced further afield. This appears inconsistent with the inferred large size of the BIC event. Here, we present new radiometric ages for alkaline magmatism in the Archean Yilgarn Craton (Western Australia), which overlap the emplacement age of the BIC and indicate a much more extensive geographic footprint of the BIC magmatic event. To assess plume involvement at this distance, we present numerical simulations of mantle plume impingement at the base of the lithosphere, and constrain a relationship between the radial extent of volcanism versus time, excess temperature and plume size. These simulations suggest that the thermal influence of large plume events could extend for thousands of km within a few million years, and produce widespread alkaline magmatism, crustal extension potentially leading to continental break-up, and large ore deposits in distal sectors. Our results imply that superplumes may produce very extensive and diverse magmatic and metallogenic provinces, which may now be preserved in widely-dispersed continental blocks. [ABSTRACT FROM AUTHOR]

• Isotope dilution analyses will likely remain the standard for precision and accuracy in carbonate U-Pb studies. • The "stacked resin" chromatography reduces analysis time by factor of two. • Loading samples in dilute HBr did not adversely affect U retention on TRU-resin. • Larger resin beds increased Pb blank (4.5 pg Pb to 12 pg Pb). • Protocol increases isotope dilution sample throughput while still utilizing acids and resins common to geochemical labs. U-Pb dating can provide age constraints on carbonate minerals from a wide range of geological settings. A major practical limitation, however, is the need for rapid and efficient extraction of Pb and U from large numbers of typically low-Pb (≤10 ng/g) calcite samples, while maintaining low blanks (∼10 pg Pb) and high Pb-U yields and purity. Here we describe a well-tested ion exchange procedure able to extract Pb and U from large (up to 200 mg) calcite samples using a single pass over a mini-column filled with small (∼0.1 ml) volumes of AG1-X8 anion exchange and Eichrom TRU-resins. This 'stacked resin' technique halves the time spent on elemental extractions and provides MC-ICPMS-ready Pb and U fractions in a single day. The method results in considerable savings in laboratory time and allows higher sample throughput, without negative impacts on data quality. It is ideally suited for high-precision U-Pb dating of speleothems where large numbers of samples need to be processed, but it is equally adaptable to calcites from other settings. While not explored as part of this work, the stacked resin technique should also be applicable to other carbonates (dolomite, ankerite, siderite). [ABSTRACT FROM AUTHOR]

Chemical and lead isotope analyses aided by metallographic examination of copper axes found at the Late Neolithic settlement of Kietrz in southwestern Poland provide new evidence for the origin of copper and metal trade routes in the region. Our results indicate that metal used for the axes could be sourced from copper mines in modern Slovakia, Bulgaria and perhaps Serbia. The evidence from this study confirms that the Funnel Beaker people from Poland became parties to a metal trading network that connected much of continental Europe in the mid‐4th millennium bce and provides a better understanding of how these contacts provided the background for the technological and socio‐economic developments of the Baden era. [ABSTRACT FROM AUTHOR]

Speleothems represent important archives of terrestrial climate variation that host a variety of proxy signals and are also highly amenable to radiometric age determination. Although speleothems have been forming on Earth for at least 400 million years, most studies rely upon the U-Th chronometer which extends only to the mid Pleistocene, leaving important questions over their longer-term preservation potential. To date, older records, exploiting the advantages of the U-Pb chronometer, remain fragmentary 'snapshots in time'. Here we demonstrate the viability of speleothems as deep time climate archives by showing that a vast system of shallow caves beneath the arid Nullarbor plain of southern Australia, the world's largest exposed karst terrain, formed largely within the Pliocene epoch, with a median age of 4.2 Ma, and that, in these caves, even the most delicate formations date from this time. The long-term preservation of regional-scale cave networks such as this demonstrates that abundant speleothem archives do survive to permit the reconstruction of climates and environments for much older parts of Earth history than the ~600 ka period to which most previous studies have been limited. [ABSTRACT FROM AUTHOR]

We present the first national-scale lead (Pb) isotope maps of Australia based on surface regolith for five isotope ratios, 206Pb/204Pb , 207Pb/204Pb , 208Pb/204Pb , 207Pb/206Pb , and 208Pb/206Pb , determined by single-collector sector field inductively coupled plasma mass spectrometry after an ammonium acetate leach followed by aqua regia digestion. The dataset is underpinned principally by the National Geochemical Survey of Australia (NGSA) archived floodplain sediment samples. We analysed 1219 samples (0–10 cm depth, <2 mm grain size), collected near the outlet of 1119 large catchments covering 5.647×106 km 2 (∼75% of Australia). The samples consist of mixtures of the dominant soils and rocks weathering in their respective catchments (and possibly those upstream) and are therefore assumed to form a reasonable representation of the average isotopic signature of those catchments. This assumption was tested in one of the NGSA catchments, within which 12 similar samples were also taken; results show that the Pb isotope ratios of the NGSA catchment outlet sediment sample are close to the average of the 12 upstream sub-catchment samples. National minimum, median, and maximum values were 15.56, 18.84, and 30.64 for 206Pb/204Pb ; 14.36, 15.69, and 18.01 for 207Pb/204Pb ; 33.56, 38.99, and 48.87 for 208Pb/204Pb ; 0.5880, 0.8318, and 0.9847 for 207Pb/206Pb ; and 1.4149, 2.0665, and 2.3002 for 208Pb/206Pb , respectively. The new dataset was compared with published bedrock and ore Pb isotope data, and it was found to dependably represent crustal elements of various ages from Archaean to Phanerozoic. This suggests that floodplain sediment samples are a suitable proxy for basement and basin geology at this scale, despite various degrees of transport, mixing, and weathering experienced in the regolith environment, locally over protracted periods of time. An example of atmospheric Pb contamination around Port Pirie, South Australia, where a Pb smelter has operated since the 1890s, is shown to illustrate potential environmental applications of this new dataset. Other applications may include elucidating details of Australian crustal evolution and mineralisation-related investigations. The new regolith Pb isotope dataset for Australia is publicly available (Desem et al., 2023; 10.26186/5ea8f6fd3de64). [ABSTRACT FROM AUTHOR]

The article offers information on the early Cretaceous subduction-modified mantle underneath the ultraslow spreading Gakkel Ridge, Arctic Ocean. It mentions the Earth's upper mantle, as sampled by mid-ocean ridge basalts (MORBs) at oceanic spreading centers, has developed chemical and isotopic heterogeneity over several years through focused melt extraction and re-enrichment by recycled crustal components.

We describe a technique for high-precision analysis of 109Ag/107Ag ratios in natural and archaeological gold (1.0–120 mg/g Ag) with particular emphasis on the control of Ag yield and effects of instrumental mass bias. After dissolution of mg-sized gold samples in aqua regia and conversion to chlorides, a miniature column filled with the anion exchange resin AG@1-X8 was used for removal of Au from the sample solutions. In a second miniature column made of the Triskem TBP resin Ag was converted from the chloride to the nitrate form. Isotopic analyses on the MC-ICP-MS employed the combination of Pd-doping and standard bracketing and considering the measurements of replicate dissolutions and chromatographic separations for SRM978a and CEZAg reference solutions, the combined analytical uncertainty (2 s) of the analytical method is better than 0.016‰. A solution prepared from several gold nuggets was characterised isotopically (CEZAg with δ109Ag = 0.043 ± 0.015‰) for use as a silver-in‑gold reference material in Ag isotope studies of natural and processed gold. Results for gold nuggets from three continents indicate a wide variation in Ag concentrations (5 to 123 mg/g) and this is matched by an equally wide range in isotopic compositions (δ109Ag −0.58 to +0.83‰). This is larger than the variation found so far in in native gold from primary deposits (−0.42 to +0.5‰). Although small isotopic effects to lower δ109Ag during strong Ag loss are possible within a single grain, most analysed nuggets are internally isotopically homogeneous. This suggests preservation of the primary δ109Ag in the supergene environment and that gold was sourced from several distinct gold deposits in the catchment. Recent studies, however, indicate that even single primary deposits may be isotopically heterogeneous implying that Ag isotope fractionation is caused by numerous deposition-dissolution cycles in hydrothermal systems. Fine-grained detrital gold from three placers along the Rhine river in Germany show only small differences in δ109Ag (−0.005 ± 0.047 to +0.124 ± 0.007‰, despite being distributed along 480 km of river length. This may indicate thorough mixing of gold grain populations during transport. The small sample size required for Ag isotope work on gold opens the way for detailed micro-sampling approaches. These may be used to further examine the potential for within-grain isotopic variability related to fluviatile processing and can be used to correlate the composition of the placers with that of grains from primary sources. Unlabelled Image • Ag isotope ratios determined on natural single gold grains as small as 0.03 mg. • Gold nuggets in placer show a wide range of 109 Ag (−0.58 to +0.83‰) • Larger than the variation found in native gold from primary deposits • Most nuggets are internally isotopically homogeneous. • Isotopic heterogeneity within single grains due to Ag loss is possible [ABSTRACT FROM AUTHOR]

The Nxau Nxau kimberlites in northwest Botswana belong to the Xaudum kimberlite province that also includes the Sikereti, Kaudom and Gura kimberlite clusters in north-east Namibia. The Nxau Nxau kimberlites lie on the southernmost extension of the Congo Craton, which incorporates part of the Damara Orogenic Belt on its margin. The Xaudum kimberlite province is geographically isolated from other known clusters but occurs within the limits of the NW-SE oriented, Karoo-aged Okavango Dyke Swarm and near NE-SW faults interpreted as the early stages of the East African Rift System. Petrographic, geochronological and isotopic studies were undertaken to characterise the nature of these kimberlites and the timing of their emplacement. The Nxau Nxau kimberlites exhibit groundmass textures, mineral phases and Sr-isotope compositions (87Sr/86Sri of 0.7036 ± 0.0002; 2σ) that are characteristic of archetypal (Group I) kimberlites. U-Pb perovskite, 40Ar/39Ar phlogopite and Rb-Sr phlogopite ages indicate that the kimberlites were emplaced in the Cretaceous, with perovskite from four samples yielding a preferred weighted average U-Pb age of 84 ± 4 Ma (2σ). This age is typical of many kimberlites in southern Africa, indicating that the Xaudum occurrences form part of this widespread Late Cretaceous kimberlite magmatic province. This time marks a significant period of tectonic stress reorganisation that could have provided the trigger for kimberlite magmatism. In this regard, the Nxau Nxau kimberlites may form part of a NE-SW oriented trend such as the Lucapa corridor, with implications for further undiscovered kimberlites along this corridor. [ABSTRACT FROM AUTHOR]

We present the first national-scale lead (Pb) isotope maps of Australia based on surface regolith for five isotope ratios, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 207Pb/206Pb, and 208Pb/206Pb, determined by single collector Sector Field-Inductively Coupled Plasma-Mass Spectrometry after an Ammonium Acetate leach followed by Aqua Regia digestion. The dataset is underpinned principally by the National Geochemical Survey of Australia (NGSA) archived floodplain sediment samples. We analysed 1219 'top coarse' (0-10 cm depth, <2 mm grain size) samples, collected near the outlet of 1119 large catchments covering 5.647 million km2 (~75% of Australia). The samples consist of mixtures of the dominant soils and rocks weathering in their respective catchments (and possibly those upstream) and are therefore assumed to form a reasonable representation of the average isotopic signature of those catchments. This assumption was tested in one of the NGSA catchments, within which 12 similar 'top coarse' samples were also taken; results show that the Pb isotope ratios of the NGSA catchment outlet sediment sample are close to the average of the 12 sub-catchment, upstream samples. National minimum, median and maximum values reported for 206Pb/204Pb were 15.558, 18.844, 30.635; for 207Pb/204Pb 14.358, 15.687, 18.012; for 208Pb/204Pb 33.558, 38.989, 48.873; for 207Pb/206Pb 0.5880, 0.8318, 0.9847; and for 208Pb/206Pb 1.4149, 2.0665, 2.3002, respectively. The new dataset was compared with published bedrock and ore Pb isotope data, and was found to dependably represent crustal elements of various ages from Archean to Phanerozoic. This suggests that floodplain sediment samples are a suitable proxy for basement and basin geology at this scale, despite various degrees of transport, mixing, and weathering experienced in the regolith environment, locally over protracted periods of time. An example of atmospheric Pb contamination around Port Pirie, South Australia, where a Pb smelter has operated since the 1890s, is shown to illustrate potential environmental applications of this new dataset. Other applications may include elucidating detail of Australian crustal evolution and mineralisation-related investigations. The new regolith Pb isotope dataset for Australia is publicly available (Desem et al., 2023; http://dx.doi.org/10.26186/5ea8f6fd3de64). [ABSTRACT FROM AUTHOR]

'Transitional kimberlite' is a collective term previously used to classify rocks occurring in southern Africa that show bulk rock geochemical and Sr–Nd isotope features intermediate between (cratonic) lamproites and kimberlites. However, it is now well established that detailed petrographic and mineral chemical criteria represent a more robust approach towards the classification of kimberlites, lamproites and related rocks. Here, we re-assess the classification of southern African 'transitional kimberlites' by combining new petrographic observations and mineral compositional results for samples from six localities (Leicester, Frank Smith, Wimbledon, Melton Wold, Droogfontein, and Silvery Home) straddling the southwestern margin of the Kaapvaal Craton. These new data indicate that Leicester and Frank Smith are archetypal kimberlites, whereas Wimbledon, Melton Wold, Droogfontein, and Silvery Home represent bona fide olivine lamproites. We combine the mineral chemical results with new (Wimbledon) and existing bulk rock trace element and Nd–Hf isotope compositions, and emplacement ages, to assess whether the previously documented trends in Nd–Hf isotope vs time for these 'transitional kimberlites' constrain their petrological evolution. Modal groundmass mineralogy, bulk rock K/La and chromite compositions, the latter being a proxy for primitive melt composition, are linearly correlated with emplacement age and initial Nd–Hf isotope compositions. These observations suggest derivation of both older lamproites (181–115 Ma) and younger kimberlites (114–93 Ma), from a common evolving source. The temporal evolution of Nd–Hf isotope compositions in these rocks converge to values typical of archetypal Cretaceous kimberlites elsewhere in the Kaapvaal Craton, but are clearly different from the isotopic compositions of on-craton Kaapvaal lamproites (previously known as orangeites). This observation distinguishes the petrogenesis of the Wimbledon, Melton Wold, Droogfontein, and Silvery Home lamproites from those of 'typical' Kaapvaal lamproites. We hypothesize that progressive consumption of enriched and hence fertile K-bearing components in a common sub-lithospheric (i.e. convective mantle) source beneath the southwestern margin of the Kaapvaal Craton might represent a plausible scenario to explain the temporal evolution of petrographic and geochemical traits of the examined lamproites and kimberlites. A source in the lithospheric mantle is considered at odds with the contrasting location of the current localities as they occur both off- and on-craton. Migration of the African plate between 180 and 90 Ma over a relatively stationary convective mantle (plume?) source is not compatible with the spatial–temporal distribution of 'transitional kimberlites'. Instead, we invoke viscous coupling between an upper asthenospheric source and the lithosphere to reconcile a single evolving source with the geographic and age distribution of these rocks. This work supports the hypothesis that olivine lamproites occurring in intra-continental settings share similar genetic features with kimberlites. [ABSTRACT FROM AUTHOR]

Reference materials (RMs) with well‐characterised composition are necessary for reliable quantification and quality control of isotopic analyses of geological samples. For in situRb‐Sr analysis of silicate minerals via laser ablation inductively coupled plasma tandem mass spectrometry (LA‐ICP‐MS/MS) with a collision/reaction cell, there is a general lack of mineral‐specific and matrix‐matched RMs, which limits wider application of this new laser‐based dating technique to certain minerals. In this work, pressed nano‐powder pellets (NP) of four RMs, GL‐O (glauconite), Mica‐Mg (phlogopite), Mica‐Fe (biotite) and FK‐N (K‐feldspar), were analysed and tested for in situRb‐Sr dating, complemented by isotope dilution (ID) MC‐ICP‐MS Rb‐Sr analyses of GL‐O and Mica‐Mg. In addition, we attempted to develop alternative flux‐free and fused 'mineral glasses' from the above RMs for in situRb‐Sr dating applications. Overall, the results of this study showed that among the above RMs only two NP (Mica‐Mg‐NP and GL‐O‐NP) were suitable and robust for in situ dating applications. These two nano‐powder reference materials, Mica‐Mg‐NP and GL‐O‐NP, were thus used as primary RMs to normalise and determine Rb‐Sr ages for three natural minerals: MDC phlogopite and GL‐O glauconite grains, and also Mica‐Fe‐NP (biotite). Our in situ analyses of the above RMs yielded Rb‐Sr ages that are in good agreement (within 8%) of published ages, which suggests that both Mica‐Mg‐NP and GL‐O‐NP are suitable RMs for in situRb‐Sr dating of phlogopite, glauconite and biotite. However, using secondary RMs is recommended to monitor the quality of the obtained ages. [ABSTRACT FROM AUTHOR]

The Mbarga itabirite deposit in the Mbalam iron district on the northwest edge of the Congo Craton (CC) hosts two main types of iron ore enrichments: supergene and specularite ores. This study presents mineralogical, geochemical, and isotopic datasets on these ores to determine their genesis. Ore microscopic studies indicate that the itabirites are of the oxide facies type, with magnetite showing partial to extensive alteration to hematite-martite. The supergene ores consist of hematite + martite + goethite ± gibbsite ± magnetite ± quartz, while the specularite ores are mainly composed of hematite + martite ± quartz. Magnetite microchemistry suggests formation under low- T hydrothermal conditions (~200–300 °C) with high f O 2. Geochemical analyses show that the supergene and specularite ores have higher Fe 2 O 3 (88.27 to ~100 wt%) and lower SiO 2 (<0.01 to 0.18 wt%) contents than the itabirites (31.95 wt% Fe 2 O 3 , 67.16 wt% SiO 2). The enrichment of Fe in the supergene ores is attributed to the depletion of major oxides and trace elements due to weathering and supergene enrichment, while the high Fe content in the specularite ores stems from the precipitation of iron-rich, but trace- and rare earth elements (REE)-deficient hydrothermal fluids. The slightly higher Al 2 O 3 content and positive Ce anomalies in the supergene ores suggest the retention of Al-bearing minerals (gibbsite) and reveal highly oxidative conditions during martitization. Stable isotope analyses reveal that the supergene and specularite ores have δ18O values of −2.5 to −0.3 ‰ and − 2.0 to −3.4 ‰, and δ2H values of −75 to −123 ‰ and − 70 to −119 ‰, respectively, suggesting the involvement of isotopically light-evolved meteoric water in their formation. In contrast, the itabirites exhibit heavier δ18O (8.5 to 10.2 ‰) and δ2H (−85 to −91 ‰) values, suggesting formation from mixed magmatic and metamorphic fluid sources. A "polygenic-supergene-hydrothermal" model is suggested for the formation of the Mbarga itabirite-hosted iron ores. • Itabirite at Mbarga is linked with supergene and specularite ores. • Magnetite microchemistry suggests formation under low- T hydrothermal conditions. • The itabirite's isotopic signature suggests mixed magmatic and metamorphic fluid sources. • Itabirite enrichment was aided by isotopically light-evolved meteoric water. • A polygenic-supergene-hydrothermal model is proposed for the iron ores formation. [ABSTRACT FROM AUTHOR]

Kimberlites represent magmas derived from great mantle depths and are the principal source of diamonds. Kimberlites and their xenolith cargo have been extremely useful for determining the chemical composition, melting regime and evolution of the subcontinental mantle. The late-Devonian Udachnaya (means Fortuitous ) pipe hosts the largest diamond deposit in Russia (> 60% diamond quantity and value) and one of the largest in the world, supplying gem-quality diamonds (~ 12% of world production). Since its discovery in 1956, the Udachnaya kimberlite pipe has become a “type locality” for geochemists and petrologists studying mantle rocks and mantle physical–chemical conditions. Apart from hosting a diverse suite of extremely well-preserved mantle xenoliths, the host kimberlite (East body) is the only known occurrence of fresh kimberlite, with secondary serpentine almost absent and uniquely high Na 2 O and Cl (up to 6.2 wt.%) and low H 2 O (< 1 wt.%) contents. The discovery of such compositional features in the only unaltered kimberlite has profound implications for models of parental kimberlite magma compositions, and the significance of the high Na and Cl abundances in the Udachnaya-East pipe has therefore been subjected to vigorous criticism. The main argument against a primary magmatic origin of high Na-Cl levels involves the possibility of contamination by salt-rich sedimentary rocks known in the subsurface of the Siberian platform, either by assimilation into the parental magma or by post-intrusion reaction with saline groundwaters. In this paper we review evidence against crustal contamination of Udachnaya-East kimberlite magma. This evidence indicates that the kimberlitic magma was not contaminated in the crust, and the serpentine-free varieties of this kimberlite owe their petrochemical and mineralogical characteristics to a lack of interaction with syn- and post-magmatic aqueous fluids. The groundmass assemblage of this kimberlite, as well as earlier-formed melt inclusions, contains alkali carbonate, chloride and other Na- and Cl-bearing minerals. This mineralogy reflects enrichment of the parental melt in carbonate, chlorine and sodium. The combination of low H 2 O, high alkali-Cl abundances, lack of serpentine, and the presence of alteration-free mantle xenoliths all indicate that the Udachnaya-East kimberlite preserves pristine compositions in both kimberlite and mantle xenoliths. Evidence for broadly similar chemical signatures is found in melt inclusions from kimberlites in other cratons (South Africa, Canada and Greenland in our study). We demonstrate that two supposedly “classic” characteristics of kimberlitic magmas – low sodium and high water contents – relate to postmagmatic alteration. A “salty” carbonate composition of the kimberlite parental melt can account for trace element signatures consistent with low degrees of partial melting, low temperatures of crystallisation and exceptional rheological properties that enable kimberlite magmas to rise with high ascent velocities, while carrying a large cargo of entrained xenoliths and crystals. Our empirical studies are now supported by experimental data which suggest that carbonate-chloride fluids and melts derived by liquid immiscibility are a crucial factor of diamond formation. [ABSTRACT FROM AUTHOR]

• Western side of study area consists of ∼300 × 50 km corridor of variably sediment-fluxed mantle wedge-sourced volcanic rocks. • Eastern side of study area consists of ∼400 km corridor of back arc-like volcanic rocks. • Multiple 2820–2838 Ma magmatic units show evidence for hydrous melt compositions. • AncientAncient isotopic input into sub-Youanmi mantle interpreted as subduction of Narryer Terrane crust under the Youanmi Terrane. This study defines a comprehensive chemostratigraphic framework for the 2820–2738 Ma magmatic rocks of the northwestern Youanmi Terrane of the Yilgarn Craton. Sixteen chemostratigraphically distinct units can be correlated between greenstone belts that are separated by later voluminous granitic plutons. On a regional scale, the chemostratigraphic units define discontinuous, broadly NNE striking zones up to 350 km long and 50 km wide (including later granitic plutons). Neodymium isotope and trace element systematics of magmatic rocks require an increasing component of metasomatised mantle from c. 2820 to 2738 Ma and variable recycling of a 3050–2920 Ma crustal component. We interpret the first magmatic episode at 2820–2782 Ma to reflect lithospheric thinning and asthenosphere upwelling in a back-arc-like setting and attribute the metasomatism of sources for multiple mafic units to subduction of 3050–2920 Ma, predominantly isotopically juvenile, mafic crust. Following an approximate 16 Ma hiatus, a second magmatic episode at 2766–2738 Ma is dominated by crustal-derived transitional TTGs with lower volumes of mantle-derived shoshonitic and sanukitoid-like rocks, and LILE-LREE enriched tholeiitic mafic to felsic rocks. The compositions of the transitional TTG and shoshonitic rocks in this study area are similar to potassic granites and shoshonitic rocks (including shoshonitic lamprophyres) from post-orogenic continental collision settings, such as the Mesozoic Tibetan Plateau. We interpret this later episode as reflecting post-orogenic melting of crustal and upper (modified) mantle sources caused by upwelling mantle related to delamination of arc-crust or slab breakoff. The northwestern Youanmi Terrane preserves an extensive package of Archean magmatic rocks that can be confidently attributed to subduction-magmatism and provides a reference point to show that some form of subduction occurred in the Meso-Neoarchean. [ABSTRACT FROM AUTHOR]

Island-arc basalts (IAB) from convergent plate margins tend to have both higher iron oxidation states (higher Fe3+/Fe2+) and higher H 2 O contents than the mid-ocean ridge basalts (MORB) produced by seafloor spreading, which raises the question of whether these two characteristics are causally related. Back-arc basin basalts (BABB) may help with this question, in that they are products of seafloor spreading, like MORB, but sourced from supra-subduction mantle, like IAB. Here we examine the relationship between Fe3+/Fe2+ (determined by XANES spectroscopy) and H 2 O contents in BABB glasses from the North Fiji Basin (NFB). The glasses cover a range of compositions from 6.1 to 8.5 wt% MgO, and from 0.16 to 1.6 wt% H 2 O, and have also been analysed for trace elements and Sr-Nd-Pb-isotopes. Measured Fe3+/Fe2+ range from MORB-like to slightly higher values and are positively correlated with H 2 O and heavy-halogen contents (Cl, Br and I). The correlation is due to lower Fe2+ rather than higher Fe3+ compared to the MORB array. This suite of BABBs is not significantly enriched in trace elements other than H 2 O and the heavy halogens, with MORB-like Ba/Th and other incompatible-element ratios, suggesting that the H 2 O and halogens may have come from dehydration of subducted lithospheric serpentinites under subsolidus conditions. Na 2 O is unusually variable for a suite of samples from such a limited area, and is not correlated with H 2 O, precluding a relationship between H 2 O and degree of melting. The link between H 2 O and low Fe2+ may be explained by H 2 O-rich fluids entering the transcrustal magma plumbing system, where they increase the proportion of olivine and augite to plagioclase crystallizing during the crustal evolution of the magmas. In this situation, relatively small additions of H 2 O can have an enhanced effect on major-element chemistry if the crustal evolution proceeds by cycles of replenish-mix-tap-crystallize (RMTX) rather than by simple fractional crystallization without repeated replenishment and tapping. The absence of any increase in Fe3+ with H 2 O, together with the lack of any correlation between H 2 O and Fe3+/Fe2+ in MORB glasses generally, suggest that various mechanisms, including more extensive olivine fractionation, were responsible for elevating Fe3+/Fe2+ in H 2 O-rich basalts from convergent margin settings. Proxy methods based on concentrations of elements with redox-variable partition coefficients (V, Eu) are not sensitive enough to record the subtle variations in Fe3+/Fe2+ observable by XANES spectroscopy. • Fe3+/Fe2+ in back-arc-basin glasses from the North Fiji Basin increases with H 2 O contents. • The higher Fe3+/Fe2+ reflects lower Fe2+, not higher Fe3+. • Higher H 2 O in the NFB suite is not correlated with a geochemical subduction features like elevated Ba/Th. • The H 2 O addition did not cause oxidation. [ABSTRACT FROM AUTHOR]

Abstract: In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of ∼2030–2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as ∼2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS 207Pb–206Pb age of 2030.9±1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and high-resolution geochemical studies to accurately match mineral ages to distinct crustal processes. [Copyright &y& Elsevier]

The origins of many supergiant ore deposits remain unresolved because the factors responsible for such extreme metal enrichments are not understood. One factor of critical importance is the timing of mineralization. However, timing information is commonly confounded by the difficulty of dating ore minerals. The world's largest uranium resource at Olympic Dam, South Australia, is exceptional because the high abundance of U allows U-Pb dating of ore minerals. The Olympic Dam U(-Cu-Au-Ag) ore deposit is hosted in ca. 1.59 Ga rocks, and the consensus has been that the supergiant deposit formed at the same time. We argue that, in fact, two stages of mineralization were involved. Paired in situ U-Pb and trace element analyses of texturally distinct uraninite populations show that the supergiant size and highest-U-grade zones are the result of U addition at 0.7-0.5 Ga, at least one billion years after initial formation. This conclusion is supported by a remarkable clustering of thousands of radiogenic 207Pb/206Pb model ages of Cu sulfide grains at this time. Upgrading of the original ca. 1.59 Ga U deposit to its present size at 0.7-0.5 Ga may have resulted from perturbation of regional fluid flow triggered by global climatic (deglaciation) and tectonic (breakup of Rodinia) events. [ABSTRACT FROM AUTHOR]

The geological setting of extensive ~3.5 Ga greenstone sequences in the Pilbara Craton (Australia) has been controversial for many years. Models range from a purely oceanic origin to emplacement over a pre-existing, possibly Hadean, sialic basement. Here we present new isotopic (Nd Hf) and trace element data for a geographically extensive suite of komatiitic-tholeiitic metavolcanic rocks which comprise much of the Pilbara Supergroup. Most of the samples have flat to mildly enriched normalised trace element patterns, often associated with a slight Nb Ta depletion. A small subset of samples, represented throughout the stratigraphy, show strong LREE enrichment and pronounced HFSE anomalies (low Nb/Th, high La/Nb and Zr/Y) either requiring a different magma source or the addition of an enriched component (possibly pre-existing continental crust). Despite this compositional diversity, the Sm Nd and Lu Hf data for all samples define ~3.5 Ga isochrons and yield homogeneous initial ratios (ε Nd = 1.15 ± 0.19; ε Hf = 2.29 ± 0.65) suggesting a magma source — or sources — slightly more depleted than the putative primitive (chondritic) mantle but consistent with recent observations of other primordial, deep mantle reservoirs. Published Sm Nd data for the Dresser Formation obtained during the Pilbara Drilling Project appear to be affected by alteration processes and are considered unreliable for the estimation of an accurate initial isotopic composition. The data for the enriched samples are inconsistent with assimilation of known examples of locally exposed continental crust as a mechanism responsible for their trace element characteristics. If an alternate crustal contaminant is invoked it cannot be much older than the basaltic magmatism itself. As a consequence, we see no evidence for an extensive Hadean (>4.0 Ga) sialic basement in the region. • Coherent Sm Nd and Lu Hf isotopic systems evolving from a common initial ratio • Sm Nd data from the Dresser Formation likely unreliable • Isotope data allow no possibility of Hadean crustal involvement • Derivation from a near chondritic primordial mantle source • Enriched samples are not crustally contaminated [ABSTRACT FROM AUTHOR]

Rocks with chemical compositions similar to Cenozoic boninites occur in many Archean cratons (boninite-like rocks), but they are rarely well-preserved, well-sampled, or presented within chrono- and chemo-stratigraphic context. This study provides a detailed description of the most extensive and well-preserved Archean boninite-like rocks reported to date. Within the 2820 to 2740 Ma magmatic suites of the northwest Youanmi Terrane, Yilgarn Craton, boninite-like rocks occur as two distinct units. The first boninite-like unit is thinner (several 10 s of m thick), occurs close to the base of the 2820–2800 Ma Norie Group and includes both volcanic flows and subvolcanic intrusions. The second boninite-like unit is thicker (locally several 100 s m), occurs near the base of the 2800–2740 Ma Polelle Group and consists of mainly fine-grained volcanic flows with local cumulate units. On average, major and trace element compositions for Youanmi Terrane boninite-like rocks are marginal between basalt, picrite and boninite and they have asymmetrically concave REE patterns, and Th–, Zr–Hf enrichments, similar to many Phanerozoic low-Si boninite suites, but at generally higher MREE–HREE contents. We report over 300 new whole-rock geochemical analyses, and 16 new Sm–Nd isotopic analyses, and associated petrographic evidence, including representative mineral compositions, which we support with published geochemical analyses and several decades of fieldwork in our study area. Comparison between Archean boninite-like rocks and Cenozoic boninites shows that most Archean examples had less depleted sources. We consider two possible petrogenetic models for the Youanmi Terrain examples: (1) they reflect variably contaminated komatiites, or (2) they reflect melts of metasomatised refractory mantle, analogous to Phanerozoic boninites. Trace element modelling indicates that crustal contamination could potentially produce rocks with boninite-like compositions, but requires an Al-enriched komatiitic parent liquid, for which there is no field evidence in our study area. Initial εNdT values in pre-2800 Ma rocks (εNdT -0·4 to +1·2) are on average slightly higher than those in 2800–2733 Ma examples (εNdT -3·2 to +1·2), compatible with increasing mantle metasomatism involving recycling of ≥ 2950 Ma crust. Integration of trace element and Nd isotopic data demonstrates that significant direct crustal assimilation was restricted to felsic magmas. The Th–Nb and Ba–Th systematics of mafic-intermediate rocks reflect fluid- and sediment-derived processes in the mantle, with boninite-like examples being linked primarily to fluid metasomatism. We compare the well-preserved igneous textures and mineralogy of Youanmi Terrane boninite-like rocks with those of their Phanerozoic counterparts, and based on studies of the latter, suggest that former had similarly hot, H2O-rich parent magmas. The association of boninite-like rocks in the Norie and Polelle Groups with coeval high-Mg andesites, sanukitoids and hydrous mafic intrusions of the Narndee Igneous Complex strongly suggests a metasomatised mantle source and subduction operating in the Yilgarn between 2820 and 2730 Ma. [ABSTRACT FROM AUTHOR]

Kimberlites are often closely associated, both in time and space, with a wide variety of alkaline ultramafic rock types, yet the question of a genetic relationship between these rock types remains uncertain. One locality where these relationships can be studied within the same cluster is the Karelian craton in Finland. In this study we present the first petrographic, mineral and whole-rock geochemical results for the most recently discovered kimberlite cluster on this craton, which represents an example of the close spatial overlap of kimberlites with ultramafic lamprophyres. The Kuusamo cluster incorporates seven bodies [Kasma 45, Kasma 45 south, Kasma 47, Kalettomanpuro (KP), Kattaisenvaara (KV), Dike 15 and Lampi] distributed along a 60 km NE–SW corridor. Hypabyssal samples from KV, KP, Kasma 45 and Kasma 47 consist of altered olivine macrocrysts and microcrysts and phlogopite phenocrysts in a groundmass of perovskite, apatite, spinel, ilmenite, serpentine, and calcite. These petrographic features combined with mineral (e.g. Mg-rich ilmenite, Al–Ba-rich, Ti–Fe-poor mica) and whole-rock incompatible trace element compositions (La/Nb = 0·8 ± 0·1; Th/Nb = 0·07 ± 0·01; Nb/U = 66 ± 9) are consistent with these rocks being classified as archetypal kimberlites. These Kuusamo kimberlites are enriched in CaO and poor in MgO, which, combined with the absence of chromite and paucity of olivine macrocrysts and mantle-derived xenocrysts (including diamonds), suggests derivation from differentiated magmas after crystal fractionation. Samples from Lampi share similar petrographic features, but contain mica with compositions ranging from kimberlitic (Ba–Al-rich cores) to those more typical of orangeites–lamproites (increasing Si–Fe, decreasing Al–Ti–Ba), and have higher bulk-rock SiO2 contents than the Kuusamo kimberlites. These features, combined with the occurrence of quartz and titanite in the groundmass, indicate derivation from a kimberlite magma that underwent considerable crustal contamination. This study shows that crustal contamination can modify kimberlites by introducing features typical of alkaline ultramafic rock types. Dike 15 represents a distinct carbonate-rich lithology dominated by phlogopite over olivine, with lesser amounts of titaniferous clinopyroxene and manganoan ilmenite. Phlogopite (Fe–Ti-rich) and spinel [high Fe2+/(Fe2+ + Mg)] compositions are also distinct from the other Kuusamo intrusions. The petrographic and geochemical features of Dike 15 are typical of ultramafic lamprophyres, specifically, aillikites. Rb–Sr dating of phlogopite in Dike 15 yields an age of 1178·8 ± 4·1 Ma (2σ), which is considerably older than the ∼750 Ma emplacement age of the Kuusamo kimberlites. This new age indicates significant temporal overlap with the Lentiira–Kuhmo–Kostomuksha olivine lamproites emplaced ∼100 km to the SE. It is suggested that asthenospheric aillikite magmas similar to Dike 15 evolved to compositions akin to the Karelian orangeites and olivine lamproites through interaction with and assimilation of MARID-like, enriched subcontinental lithospheric mantle. We conclude that the spatial coincidence of the Kuusamo kimberlites and Dike 15 is probably the result of exploitation of similar trans-lithospheric corridors. [ABSTRACT FROM AUTHOR]

• We suggest a near-continuous model of hydrothermal reworking in the Olympic Cu-Au Province resulting in recrystallisation of the Cu-(Fe)-sulfides, and Pb (re)mobilisation at nm- to deposit-scales. • Pb-isotope ratios in galena, clausthalite, altaite hosted within the Cu-(Fe)-sulfides, and the Cu-(Fe)-sulfides themselves, are highly radiogenic and are indistinguishable from one another. • Hydrothermal autoclave experiments showed that the Pb-chalcogenide inclusions likely formed within porous zones of the Cu-(Fe)-sulfides during coupled dissolution reprecipitation reactions. • The data presented here are consistent with other studies in the Olympic Cu-Au Province which record the impact of tectonothermal events younger than the 1600–1585 Ma 'main' mineralisation event. Lead-isotope data were obtained for a population of Pb-chalcogenides (galena, clausthalite and altaite) and Cu-(Fe)-sulfides in Cu-(Fe)-sulfide mineral separates from the Prominent Hill iron-oxide copper gold deposit, Mt Woods Inlier, South Australia. Each mineral displays wide variability in 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, ranging from moderately radiogenic Pb signatures to strongly radiogenic. Any contribution from non-radiogenic (common) lead at the time of initial deposit formation (1600–1585 Ma) is minor. Lead is overwhelmingly derived from the decay of U, with a minor contribution from the decay of Th also present within the ores. The heterogenous nature of the recorded Pb isotope values suggests a mixing of isotope signatures during repeated cycles of fluid-assisted dissolution, remobilisation and reprecipitation of U- and Pb-bearing phases. Hydrothermal experiments showed that the incorporation of Pb-chalcogenide (galena) occurs rapidly (days) at relatively mild conditions (≤150 °C), via precipitation in reaction-induced porosity formed during coupled dissolution reprecipitation reactions of the host Cu-(Fe)-sulfides. Hence, even minor hydrothermal activity may result in some trapping of radiogenic Pb by pre-existing Cu-(Fe)-sulfides. Construction of Pb-Pb pseudo-isochrons from the data supports these assessments, returning a range of possible (theoretical) ages (0–0.535 Ga) for Pb-chalcogenide formation. An additional (two-stage) Pb evolution model is preserved by the data with separation of each reservoir from the bulk silicate earth at ∼ 830 Ma and mixing of the two reservoirs at ∼ 500 Ma. Although the Pb-isotope data can be interpreted as resulting from a number of discrete events that generated regional-scale fluid flow, near-continuous Pb (re)-mobilisation at the nm- to deposit-scales is an equally valid interpretation, and is also consistent with observations of modern radionuclide mobility in the ores. The observations made within this study should be considered when discussing the mobility of elements within and between mineral phases in ore-forming systems. They show that many mineral phases, particularly the sulphide mineral phases discussed here, should not be considered as closed systems, but potentially, as systems in which chemistry and mineral stoichiometry evolve continuously over geological time. [ABSTRACT FROM AUTHOR]

Mafic and ultramafic magmas that intrude into the lower crust can preserve evidence for metal and sulfur transfer from the lithospheric mantle into the lower continental crust. Here we focus on a series of ultramafic, alkaline pipes in the Ivrea Zone (NW Italy), which exposes deeply buried (6–11 kbar), migmatitic metasedimentary rocks intruded by voluminous basaltic magmas of the Mafic Complex, a major crustal underplating event precisely dated via U/Pb CA-IDTIMS on zircon at 286.8 ± 0.4 Ma. The ultramafic pipes postdate the Mafic Complex and from 100 to 300 m wide cumulate-rich conduits. They are hydrated and carbonated, have unusually high incompatible element concentrations and contain blebby and semi-massive Ni-Cu-PGE sulfide mineralisation. The sulfides occur as coarse intergranular nodules (>10 mm) and as small intragranular blebs (<1 mm) hosted in olivine, and have homogeneous, mantle-like δ 34 S (+1.35 ± 0.25‰). This homogeneity suggests that the pipes reached sulfide supersaturation without addition of crustal sulfur, and that the δ 34 S signature is representative of the continental lithospheric mantle. One of the pipes, the 249 Ma Valmaggia pipe, carries a very distinctive Sr-Nd-Hf-Pb isotopic composition in its core ( 87 Sr/ 86 Sr 0.70250, ε Nd-18 , ε Hf-18 , 206 Pb/ 204 Pb 16.0, 207 Pb/ 204 Pb 15.16, 208 Pb/ 204 Pb 35.87), very different from the margin of this pipe and from other pipes that have higher 87 Sr/ 86 Sr, ε Nd and 206 Pb/ 204 Pb. The unusual isotopic composition of the Valmaggia pipe requires a source with long-term (2500–1500 million years) U-, Th- and Rb-depletion and LREE enrichment. Such compositions are found in Late Archean/Early Proterozoic granulites and lower crustal xenoliths. We suggest that the unusual isotopic composition of the Valmaggia pipe reflects contamination of the mantle source of the pipe with a crustal component that is neither represented in the local Paleozoic crust nor in the isotopically anomalous hydrated mantle inferred as the source of the large-volume mafic underplate that formed the Mafic Complex. During post-collisional gravitational collapse of the Variscan Orogen, this source produced the alkaline, metal (Ni, Cu, PGE)- and volatile (H 2 O, CO 2 , S)-rich mafic–ultramafic magma that formed the deep-crustal intrusion at Valmaggia. U/Pb dating of other chemically and geologically comparable pipes in the area shows that this process was active over at least 40 Ma. The Ivrea pipes illustrate how the lower continental crust can be fertilised with mantle-derived metals and volatiles, which are available for later remobilisation into upper-crustal ore systems. World-class mineral deposits along the margins of lithospheric blocks may thus be the result of both favourable crustal architecture (focussing of magmas and fluids) and localised volatile and metal enrichment of the lower crust related to mantle-derived hydrous metasomatism. [ABSTRACT FROM AUTHOR]

The Man Craton in West Africa hosts two clusters of highly micaceous kimberlites at Koidu and Tonguma with mineralogical and geochemical features transitional towards those of cratonic lamproites. To develop a better understanding of the genesis of these peculiar rocks, which have also been documented elsewhere (e.g., South Africa, China, Russia, India), we have undertaken a geochemical and isotopic study of representative fresh samples from Koidu and Tonguma. New mica Rb Sr dating by isotope dilution confirms the Jurassic age of Koidu (∼145 Ma) and establishes that the Tonguma dykes are marginally younger (∼138 Ma). Bulk-rock analyses of samples from both localities show strong enrichment in K 2 O (up to 4.4 wt%), but also CaO and CO 2 (up to 24 wt% and 18 wt%, respectively), which is entirely consistent with the micaceous nature of these rocks and their variable abundance of calcite. Despite their highly potassic compositions, the incompatible trace element systematics (e.g., Ba/Nb) of these rocks are typical of archetypal kimberlites and unlike those of cratonic lamproites. The Sr-Nd-Hf isotopic composition of the bulk samples range between moderately depleted values (age-corrected 87Sr/86Sr ∼ 0.7032; εNd = +3.8; εHf = +3.9) resembling the common depleted (Prevalent Mantle or PREMA-like) kimberlite component and more geochemically enriched compositions (87Sr/86Sr ∼ 0.7058; εNd = −0.4; εHf = +0.3). The direct relationship between bulk-rock Nd isotopes and average Mg# of magmatic olivine, combined with the previously reported correlation between average Mg# of magmatic and xenocrystic olivine of lithospheric mantle derivation in these kimberlites, suggests that the observed spread in radiogenic isotope compositions stems from the following petrogenetic scenario. Primary kimberlite melts of similar asthenospheric origin with unradiogenic Sr and moderately radiogenic Nd and Hf isotopes interacted with variably metasomatised (i.e., containing phlogopite) and hence geochemically enriched lithospheric mantle wall rocks. Mass balance calculations suggest that the transfer of incompatible elements to the asthenospheric melts was mediated by incongruent melting of metasomatic components in the lithospheric mantle rather than their bulk addition. This study reinforces previous suggestions that interaction between carbonate-rich asthenospheric melts and the sub-continental lithospheric mantle can generate the range of potassic magmas observed within cratons. • Highly micaceous kimberlites with petrological features transitional to lamproites at Koidu and Tonguma. • New geochemical and radiogenic isotope data confirm an affinity to kimberlites. • Sr-Nd-Hf isotope variations from PREMA-like to more geochemically enriched compositions. • Enrichment in mica and isotopic variability due to interaction of asthenospheric melts with metasomatised lithosphere. • Incongruent melting of lithospheric mantle components favoured over their bulk assimilation. [ABSTRACT FROM AUTHOR]

The petrogenesis of kimberlites is commonly obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during fluid interaction and the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa). Sample BK consists of abundant macrocrysts (> 1 mm) and (micro-) phenocrysts of olivine and lesser phlogopite, smaller grains of apatite, serpentinised monticellite, spinel, perovskite, phlogopite and ilmenite in a matrix of calcite, serpentine and dolomite. As in kimberlites worldwide, BK olivine grains consist of cores with variable Mg/Fe ratios, overgrown by rims that host inclusions of groundmass phases (spinel, perovskite, phlogopite) and have constant Mg/Fe, but variable Ni, Mn and Ca concentrations. Primary multiphase inclusions in the outer rims of olivine and in Fe-Ti-rich (‘MUM’) spinel are dominated by dolomite, calcite and alkali carbonates with lesser silicate and oxide minerals. Secondary inclusions in olivine host an assemblage of Na-K carbonates and chlorides. The primary inclusions are interpreted as crystallised alkali-Si-bearing Ca-Mg-rich carbonate melts, whereas secondary inclusions host Na-K-rich C-O-H-Cl fluids. In situ Sr-isotope analyses of groundmass calcite and perovskite reveal similar 87 Sr/ 86 Sr ratios to perovskite in the Bultfontein and the other Kimberley kimberlites, i.e. magmatic values. The δ 18 O composition of the BK bulk carbonate fraction is above the mantle range, whereas the δ 13 C values are similar to those of mantle-derived magmas. The occurrence of different generations of serpentine and occasional groundmass calcite with high 87 Sr/ 86 Sr, and elevated bulk carbonate δ 18 O values indicate that the kimberlite was overprinted by hydrothermal fluids, which probably included a significant groundwater component. Before this alteration the groundmass included calcite, monticellite, apatite and minor dolomite, phlogopite, spinel, perovskite and ilmenite. Inclusions of groundmass minerals in olivine rims and phlogopite phenocrysts show that olivine and phlogopite also belong to the magmatic assemblage. We therefore suggest that the crystallised kimberlite was produced by an alkali-bearing, phosphorus-rich, silica-dolomitic melt. The alkali-Si-bearing Ca-Mg-rich carbonate compositions of primary melt inclusions in the outer rims of olivine and in spinel grains with evolved compositions (MUM spinel) support formation of these melts after fractionation of abundant olivine, and probably other phases (e.g., ilmenite and chromite). Finally, the similarity between secondary inclusions in kimberlite olivine of this and other worldwide kimberlites and secondary inclusions in minerals of carbonatitic, mafic and felsic magmatic rocks, suggests trapping of residual Na-K-rich C-O-H-Cl fluids after groundmass crystallisation. These residual fluids may have persisted in pore spaces within the largely crystalline BK groundmass and subsequently mixed with larger volumes of external fluids, which triggered serpentine formation and localised carbonate recrystallisation. [ABSTRACT FROM AUTHOR]