Description: The Mg- and Si-rich nature of the sub-cratonic lithospheric mantle (SCLM) beneath the Kaapvaal Craton indicates extensive melt depletion, followed by a Si-enrichment process. Six highly silica enriched peridotites from Kimberley containing high amounts of orthopyroxene (Opx) or garnet (Grt) that are locally concentrated in clots, were investigated to constrain the timing and nature of the Si-enrichment process. A clinopyroxene-bearing lherzolite containing an Opx-clot was studied to quantify the effects of recent metasomatism on the Si-enriched samples. Minerals from the lherzolite, together with Opx from harzburgites and Opx- and Grt-clots have Hf–Nd isotope ratios at the time of kimberlite eruption, 90 Ma, comparable to group I kimberlites and are close to trace element equilibrium with kimberlitic melts. This implies the xenoliths underwent major interaction with kimberlitic melts close to the time of kimberlite eruption. Harzburgites and mineral clots record equilibration pressures and temperatures of, respectively, between 3.5–4.3 GPa and 930–1060 °C. The garnets in Opx-clots have low Lu/Hf and εHf(t) −15, whereas garnets from Grt-clots have high Lu/Hf and εHf(t) +10. In contrast, Grt from both Grt- and Opx-clots have low Sm/Nd and εNd −10. The whole rock platinum group element (PGE) concentrations are an order of magnitude higher in the Grt-clot than the Opx-clot. Measured 187Os/188Os range from 0.1085 to 0.1222. The Grt-clot bearing sample yields Nd–Hf–Os isotope model ages that suggest formation in the Neoproterozoic (∼650 Ma). In contrast, an Opx-clot yields TRD ages of 2.8 Ga, which is interpreted as the time of formation of the host harzburgite. The Opx-clots and host harzburgites have comparable Lu–Hf isotope systematics that imply Opx growth at ∼1.3 Ga and hence their formation is not related to the Grt-clots. Garnets from Opx- and Grt-clots have elevated high-field strength element (HFSE) concentrations, and lack HFSE depletion relative to other trace elements with comparable degrees of incompatibility in the mantle (La/Nb 〈 0.5). In addition, calculated melts in equilibrium with Grt have strongly fractionated REE (Nd/Yb 〉 300) and HREE depletion (YbN 〈 0.1) suggesting equilibration with a hydrous melt that is more HREE depleted than a kimberlitic melt. Previous models that related Si-enrichment to subduction are inconsistent with the lack of HFSE depletion (La/Nb 〈 0.5). Therefore the favoured model for Opx- and Grt-clot formation is infiltration of a hydrous melt in a within plate geodynamical environment associated with volcanism in the Mid-proterozoic and Neoproterozoic, respectively. This implies that Si-enrichment of the Kaapvaal SCLM may be a consequence of numerous localised magmatic events rather than a single craton-wide process.

Description: Geophysical observations suggest sub-arc convective flow transports melt-exhausted and metasomatized wedge mantle into deeper mantle regions. Reciprocally, asthenospheric, fertile mantle may supply back-arc ridges distal to the trench by shallow, lateral mantle ingress, insinuating initial wedge mantle depletion in its back-arc region. Here we show that light Fe isotope compositions of the Central Lau Spreading Centre located in the Lau back-arc basin on the farside of the Tonga-Kermadec arc are indicative for derivation from a modified arc-front mantle with elemental and Nd-isotopic memory of former slab fluid addition. We propose that this shallow wedge material has been transported from the sub-arc mantle to the back-arc either convectively or in a buoyant diapir. This implies that melt-depleted mantle in subduction zones is, at least in parts, recycled in a resurfacing loop. This can explain the depletion in back-arc regions, and the progressively depleted nature of island arc sources in maturing arc systems.

Description: Leucocratic granites of the Proterozoic Kaoko Belt, northern Namibia, now preserved as meta-granites, define a rock suite that is distinct from the surrounding granitoids based on their chemical and isotopic characteristics. Least evolved members of this ~1.5–1.6-Ga-old leucogranite suite can be distinguished from ordinary calc-alkaline granites that occur elsewhere in the Kaoko Belt by higher abundances of Zr, Y, and REE, more radiogenic initial εNd values and unradiogenic initial 87Sr/86Sr. The leucogranites have high calculated zircon saturation temperatures (mostly 〉 920°C for least fractionated samples), suggesting that they represent high-temperature melts originating from deep crustal levels. Isotope data (i.e., εNdi: +2.3 to –4.2) demonstrate that the granites formed from different sources and differentiated by a variety of processes including partial melting of mantle-derived meta-igneous rocks followed by crystal fractionation and interaction with older crustal material. Most fractionation-corrected Nd model ages (TDM) are between 1.7 and 1.8 Ga and only slightly older than the inferred intrusion age of ca. 1.6 Ga, indicating that the precursor rocks must have been dominated by juvenile material. Epsilon Hf values of zircon separated from two granite samples are positive (+11 and +13), and Hf model ages (1.5 and 1.6 Ga) are similar to the U–Pb zircon ages, again supporting the dominance of juvenile material. In contrast, the Hf model ages of the respective whole rock samples are 2.3 and 2.4 Ga, demonstrating the involvement of older material in the generation of the granites. The last major tectonothermal event in the Kaoko Belt in the Proterozoic occurred at ca. 2.0 Ga and led to reworking of mostly 2.6-Ga-old rocks. However, the presence of 1.6 Ga “post-collisional” granites reflects addition of some juvenile mantle-derived material after the last major tectonic event. The results suggest that similar A-type leucogranites are potentially more abundant in crustal terranes but are masked by AFC processes. In the case of the Kaoko Belt, it is suggested that this rock suite indicates a yet unidentified period of mantle-derived crustal growth in the Proterozoic of South Western Africa.

Description: Combined in situ U–Pb and Hf–O isotope analyses for zircon are often used to date igneous events precisely and to gain insights into the origin of the magma from which the zircon crystallized. In conjunction with its resistance to weathering, zircon can therefore be considered a unique crystal toolbox and an ideal crustal archive. This concept, however, relies on the basic assumption that each zircon crystal is in Hf isotope equilibrium with its host magma. Here we test this hypothesis for zircon crystals from mafic–ultramafic layered intrusions and show that this assumption may not always be correct. We find Hf isotope disequilibrium between zircon crystals and their host-rocks in three Neoproterozoic mafic–ultramafic layered intrusions from the northwestern margin of the Yangtze Block, Central China, formed as part of convergent margin magmatism along the Hannan–Panxi subduction zone. Zircon crystals separated from diorite samples from these three intrusions confirm prolonged magmatism for over 90 Myr for the Beiba (869 ± 5 Ma), Wangjiangshan (822 ± 4 Ma) and Bijigou (785 ± 5 Ma) intrusions, with a chronologically progressive decrease in 18 O values from 7·4 to 6·3 and 6·0, respectively. We interpret the transition from an isotopically evolved (high 18 O) towards a progressively more primitive mantle source (lower 18 O) as the fading influence of subducted sediment-derived melts in a subduction zone, consistent with a reconstructed change in subducting plate motions from the northern to the western margin of the Yangtze Block. Unlike the coherent O isotopes, the Hf ( t ) values of zircon populations from each intrusion show a range of several Hf units (Beiba: –1·0 to+3·0; Wangjiangshan: +2·7 to +8·3; Bijigou: +2·3 to +7·8), outside analytical uncertainty and inconsistent with an origin from a single magma batch. Whole-rock Hf isotope analyses obtained by high-pressure dissolution indicate that the diorite samples from the Beiba, Wangjiangshan and Bijigou intrusions have Hf ( t ) of +8·2, +7·5 and +9·3, respectively. In contrast, table-top dissolutions for the same samples yield Hf ( t ) of +9·7, +10·0 and +11·7, respectively. The apparent systemic offset in Hf ( t ) values towards more crustal compositions in high-pressure dissolutions is interpreted here to reflect mixing of zircon-hosted Hf isotopes with less evolved Hf isotopes in associated mineral phases. The more crustal character of in situ Hf isotope determinations in zircons and their range of several Hf units are interpreted here to reflect progressive crustal contamination in magma chambers at the time of zircon saturation. This implies that Hf isotope compositions of zircon crystals can be biased towards crustal signatures, particularly in mafic–ultramafic intrusions that are more susceptible to crustal contamination. In such cases, source interpretation as well as Hf model ages calculated from these isotopic mixing pools are geologically meaningless. Inevitably, contaminated igneous suites, mafic–ultramafic complexes in particular, and detrital zircon populations derived therefrom may have a complex Hf isotopic history that cannot be resolved by fast in situ analyses of Hf isotopes alone. This history may be revealed only by an atypical range of Hf isotope compositions within single magmatic suites and, if unidentified, can lead to biased geological interpretations.

Description: Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting ( F ) and pressure ( P ) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton-Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P /moderate F and moderate P /high F , respectively, on the basis of their Al/Ti and Gd/Yb ratios. Following calculation of the primary magma composition of each suite, we show that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F , independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows. The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P , 25% melting at 5 GPa (mantle potential temperature, T P = 1750°C) whereas the ADKs of Barberton segregated at the highest F and P (40%, 9 GPa, T P = 1950°C). The petrogenetic type is a combination of P and F , where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2·7 Ga komatiites tend to have sources that are more depleted than those of older, 3·5 Ga komatiites. Komatiites are invaluable records of the mantle’s chemical and physical evolution during the Archean.

Description: Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc lavas. Furthermore, many arc picrites and boninites show evidence for late-stage differentiation prior to or during eruption, masking their primary, mantle-derived geochemical signatures. We report textural and chemical data on spinel-hosted melt inclusions of mantle origin in amphibole-bearing websterite veins cross-cutting spinel harzburgite xenoliths from the active andesitic Avacha volcano (south Kamchatka, Russia). The data are used to constrain the composition and origin of melts that formed the websterite veins in the sub-arc lithospheric mantle. The melt inclusions typically contain euhedral orthopyroxene and clinopyroxene and occasionally minor amphibole in silicate glass. The melt inclusions were homogenized using heating stages and gas-mixing furnaces. The homogenized glasses range from subalkaline primitive silica-rich picrite and high-Ca boninite (〉15 wt % MgO, 48–54 wt % SiO 2 ) to rhyolite. High-Ca boninite glasses have moderate volatile and low heavy rare earth element contents and elevated Cs, Rb, Ba, U, Sr, and Li abundances, with extremely high U/Th. In turn, the glasses display no negative spikes in the high field strength elements Nb, Ta, Zr, Hf, and Ti. We show that the silica-rich picrite and high-Ca boninite liquids in this study formed by high degrees of melting (〉25%), at volatile under-saturation, of hybrid melt-depleted but silica-rich mantle sources at ≥1·5 GPa. The hybrid sources formed in two stages: first, by extraction of ~15% melt from the convecting mantle to form a refractory protolith, which was subsequently enriched in silica via interaction with subduction-derived components prior to or during remelting in the mantle wedge. The subduction-derived components were enriched in fluid-mobile elements and probably oxidized. Overall, our results suggest that silica-rich picrites and high-Ca boninites can be primary melts in mature subduction zones and differentiate within the mantle wedge and the deep arc crust to form more evolved andesite magmas.

Description: The majority of 〉3 Ga metabasalts have chemical features, such as high field strength element (HFSE) depletions, that are characteristic of modern island-arc basalts. These compositions have been interpreted as evidence for subduction of oceanic crust early in Earth’s history. Alternatively, the apparent absence of Archean mafic rocks with mid-oceanic ridge basalt (MORB) and ocean island basalt (OIB) compositions and the ubiquitous occurrence of metabasalts with HFSE anomalies suggest that these chemical features may instead be a widespread characteristic of the Archean mantle related to early chemical differentiation and unrelated to modern-style recycling of crust. Here we present major- and trace-element data for a suite of metabasalts from Innersuartuut Island, southwest Greenland, which have a minimum age constraint of 3.75 Ga and are likely as old as ≥3.85 Ga. Samples from Innersuartuut show no evidence for crustal contamination or subduction-related magmatism, and have a petrogenesis comparable to modern OIB. The new data demonstrate that a compositional range for volcanic rocks comparable to that seen in the Phanerozoic existed in the Eoarchean. Therefore, rather than a global anomaly, subduction-related processes are the likely origin for the compositions of the most commonly preserved Archean mafic rocks with island-arc basalt characteristics.

Description: Establishing if and when South Atlantic hotspots interacted with surface processes during rifting and continental breakup is important for understanding the mechanisms that control the evolution of passive margins and their adjacent continents. One approach is to reconstruct the volcanic history of hotspot trails located on the African Superswell in order to find the locations of hotspots during rifting and breakup to determine if, for example, they caused extreme fluxes of magma and post-rift uplift along the continental margin. However, because hotspot trails located south of the classical Tristan-Gough are virtually un-sampled we don’t know how many hotspots might have existed or for how long, and whether they originated from the core-mantle boundary or much shallower depths. In 2006 we dredge sampled hotspot trails located on the African Superswell using the RV Polarstern, an icebreaker capable of working in the poor weather conditions in the Southern Ocean. Combining new and existing Ar/Ar isotopic ages shows that volcanism migrated synchronously along co-parallel hotspot trails consistent with northeastern African plate motion relative to the leading edges of the African Superswell and an underlying stable Superplume (large low-shear-velocity province) extending from the core-mantle boundary. Between roughly 132 and 100 million years ago only the Tristan-Gough hotspot trail developed where rifting and breakup facilitated the rise of hotspot melts to the surface, while along rest of the leading edge hotspot volcanism was suppressed by the African continent. Such a notion implies that the African passive continental margin was migrating relative to the leading edge of the African Superplume for as long as 30 million years after continental rifting and breakup had facilitated the 132 Ma Parana-Etendeka continental flood basalts and initiation of the Tristan-Gough hotspot trail. This provides a mechanism for extended late stage interplay between deep mantle processes and the passive margin and adjacent continents that might explain extensive magmatism, lithospheric thinning and phases of postrift uplift.

Description: The origin of hotspot trails ranges controversially1 from deep mantle plumes rising from the core-mantle boundary2 to shallow plate cracking. But these mechanisms cannot explain uniquely the scattered hotspot trails on the 2,000 km-wide southeast Atlantic hotspot swell3, which projects down to one of the Earth’s two largest and deepest regions of slower-than-average seismic wave speed – the Africa Low Shear Wave Velocity Province, which marks a massive thermo-chemical ‘pile’ at the core-mantle boundary4,5,6. Here we use 40Ar/39Ar isotopic ages – and crustal structure and seafloor ages – to show that age progressive hotspot trails formed synchronously across the swell, consistent with African plate motion over plumes rising from the stable edge of a Low Shear Wave Velocity Province. We show also that hotspot trails formed initially only at spreading boundaries at the outer edges of the swell until roughly 44 million years ago, when they started forming across the swell, far from spreading boundaries in lithosphere that was sufficiently weak (young) for plume melts to reach the surface. We conclude that if plume melts formed synchronous age progressive hotspot trails wherever and whenever they could penetrate the swell lithosphere then hotspot trails in the South Atlantic are controlled by an interplay between deep plumes and the motion and structure of the African plate.