ALBERT

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: 〈span〉Oxygen and iron isotope variations have been investigated in three compositionally distinct garnet samples to assess natural variations and search for suitable reference material. We report 〈span〉in situ〈/span〉 major, trace element and O isotope analyses for mantle-derived garnet xenocrysts from Kakanui, New Zealand, as well as magmatic and hydrothermal garnets (skarn) from two different localities in Erongo, Namibia. The 〈span〉in situ〈/span〉 analyses are complemented by bulk mineral separate Fe isotope analyses for all samples and CO〈sub〉2〈/sub〉 laser fluorination oxygen isotope analysis for Kakanui garnet. Mantle-derived pyrope-rich garnet megacrysts from Kakanui are chemically homogeneous in major and trace elements, and in O isotopes, (δ〈sup〉18〈/sup〉O〈sub〉VSMOW〈/sub〉 = 5.67 ± 0.02‰). Magmatic garnet from Erongo, Namibia, is rich in Mn and Fe〈sup〉2+〈/sup〉 and very poor in Ca showing minor variations along the almandine–spessartine join [(Fe,Mn)〈sub〉3〈/sub〉Al〈sub〉2〈/sub〉Si〈sub〉3〈/sub〉O〈sub〉12〈/sub〉]. Although rare earth elements vary over one order of magnitude, no resolvable O isotope zoning is observed (δ〈sup〉18〈/sup〉O = 9.3 ± 0.3‰, 1σ). Hydrothermal garnet from Namibia is rich in Ca and Fe〈sup〉3+〈/sup〉 and shows strong zonation along the andradite–grossular join [Ca〈sub〉3〈/sub〉(Fe〈sup〉3+〈/sup〉,Al)〈sub〉2〈/sub〉Si〈sub〉3〈/sub〉O〈sub〉12〈/sub〉] with a considerable spread in trace-element contents, accompanied by a limited, but resolvable, spread in O isotopes values between cores (8.3 ± 0.3‰, 1σ) and rims (7.4 ± 0.3‰, 1σ). Iron isotopes (expressed as δ〈sup〉57〈/sup〉Fe〈sub〉IRMM-014〈/sub〉) within bulk garnet separates are heterogeneous in both crustal garnet from Erongo with a large spread ranging from −0.15 to +0.30‰ in igneous garnet and from +0.4 to +1.1‰ in hydrothermal garnet. Igneous garnet from Kakanui are homogeneous with an average δ〈sup〉57〈/sup〉Fe〈sub〉IRMM-014〈/sub〉 of +0.09 ± 0.01, 1σ. The Fe〈sup〉3+〈/sup〉-dominated andradite shows very heavy Fe isotopes, suggesting a link between preferential ferric iron incorporation into garnet and Fe isotope signatures. Combined O and Fe isotope analyses in garnet can provide potentially important insights into the nature of parental medium from which the garnet forms (based on O isotopes) and associated petrogenetic processes (〈span〉e.g.〈/span〉, redox conditions based on Fe isotopes), though more systematic studies are required to further assess these proxies in natural systems. Finally, we propose that Kakanui garnet might represent a suitable reference material for both, O and Fe isotope analyses.〈/span〉

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: 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: The Archean Windimurra Igneous Complex consists of distinct components, including a thick layered series, with a cumulate mineral stratigraphy similar to the zones identified in the well-studied Bushveld Complex, South Africa. The complex is part of the plume-related and laterally extensive 2.81 Ga Meeline Suite, the intrusive component of a large igneous province. It is an anhydrous tholeiitic suite consisting of five layered mafic–ultramafic intrusions 25–85 km in the long dimension. These intrusions host significant V–Ti mineralization in their fractionated, Fe-rich upper zones. Recent mapping, combined with aeromagnetic, gravity and seismic surveys, has provided unparalleled three-dimensional constraints on the largest of these intrusions. The results of three-dimensional modelling show that it is thicker than previously recognized. At c. 11 km, it is the thickest layered mafic–ultramafic intrusion identified globally and one of the largest such intrusions volumetrically. The mineral zone stratigraphy and many other features associated with this complex share similarities with the c. 800 myr younger Bushveld Complex. On a large scale, three discordant units are delineated geometrically, providing fundamental constraints on a multi-stage genetic model for magma emplacement. The indication of a thick, subsurface Ultramafic Zone provides a potential target for Ni–Cr–platinum group element mineralization.

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.