Carl Spandler

Survival times of anomalous melt inclusions from element diffusion in olivine and chromite

The chemical composition of basaltic magma erupted at the Earth’s surface is the end product of a complex series of processes, beginning with partial melting and melt extraction from a mantle source and ending with fractional crystallization and crustal assimilation at lower pressures. It has been proposed that studying inclusions of melt trapped in early crystallizing phenocrysts such as Mg-rich olivine and chromite may help petrologists to see beyond the later-stage processes and back to the origin of the partial melts in the mantle. Melt inclusion suites often span a much greater compositional range than associated erupted lavas, and a significant minority of inclusions carry distinct compositions that have been claimed to sample melts from earlier stages of melt production, preserving separate contributions from mantle heterogeneities. This hypothesis is underpinned by the assumption that melt inclusions, once trapped, remain chemically isolated from the external magma for all elements except those that are compatible in the host minerals. Here we show that the fluxes of rare-earth elements through olivine and chromite by lattice diffusion are sufficiently rapid at magmatic temperatures to re-equilibrate completely the rare-earth-element patterns of trapped melt inclusions in times that are short compared to those estimated for the production and ascent of mantle-derived magma or for magma residence in the crust. Phenocryst-hosted melt inclusions with anomalous trace-element signatures must therefore form shortly before magma eruption and cooling. We conclude that the assumption of chemical isolation of incompatible elements in olivine- and chromite-hosted melt inclusions is not valid, and we call for re-evaluation of the popular interpretation that anomalous melt inclusions represent preserved samples of unmodified mantle melts.

Origin of chromitites in layered intrusions: Evidence from chromite-hosted melt inclusions from the Stillwater Complex

Chromitites from layered mafic intrusions are of great economic importance, yet the origin of these deposits remains enigmatic. We describe multiphase silicate inclusions trapped within chromite grains from the G chromitite seam of the Stillwater Complex, Montana, United States. These inclusions are interpreted to represent melt trapped during chromite growth and hence provide information on chromitite formation. Most reheated inclusions have variable quench textures and chemical compositions that are consistent with variable degrees of mixing between a high-Mg basaltic parental magma and a Na-rich trondhjemitic melt. The trondhjemite is suggested to derive from partial melting of mafic or metasedimentary country rocks. Based on these inclusions, we outline a model for chromitite formation involving ponding of a new pulse of primitive magma at the roof of the Stillwater magma chamber, followed by localized partial melting and assimilation of the country rock. The newly formed hybrid melts become oversaturated in chromite, leading to extensive chromite crystallization. Chromitite horizons are proposed to form from dense chromite-rich plumes that periodically sink down from the roof zone to settle out as layers at the basal cumulate mush zone. Numerous radiogenic isotope studies, together with the widespread occurrence of similar multiphase inclusions in chromite from other cumulate complexes, indicate that assimilation of country rock by primitive magma may be a critical mechanism for forming chromitites in many layered intrusions.

The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa

New ages for flowstone, sediments and fossil bones from the Dinaledi Chamber are presented. We combined optically stimulated luminescence dating of sediments with U-Th and palaeomagnetic analyses of flowstones to establish that all sediments containing Homo naledi fossils can be allocated to a single stratigraphic entity (sub-unit 3b), interpreted to be deposited between 236 ka and 414 ka. This result has been confirmed independently by dating three H. naledi teeth with combined U-series and electron spin resonance (US-ESR) dating. Two dating scenarios for the fossils were tested by varying the assumed levels of 222Rn loss in the encasing sediments: a maximum age scenario provides an average age for the two least altered fossil teeth of 253 +82/–70 ka, whilst a minimum age scenario yields an average age of 200 +70/–61 ka. We consider the maximum age scenario to more closely reflect conditions in the cave, and therefore, the true age of the fossils. By combining the US-ESR maximum age estimate obtained from the teeth, with the U-Th age for the oldest flowstone overlying Homo naledi fossils, we have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphology. DOI: http://dx.doi.org/10.7554/eLife.24231.001

Exsolution of thortveitite, yttrialite, and xenotime during low-temperature recrystallization of zircon from New Caledonia, and their significance for trace element incorporation in zircon

Abstract Recrystallized zircon grains in a phengite-epidote-chlorite schist from north-eastern New Caledonia contain as inclusions a mineral assemblage consisting of celadonite, kaolinite, quartz, Fe-oxy-hydroxide, smectite, chlorite, xenotime-(Y), thortveitite, yttrialite, and allanite-(Ce). This assemblage formed during low-temperature (<100 °C) seafloor alteration of a plagioclase-rich mafic rock and represents the first documented evidence of this alteration event in the high-P belt of northeastern New Caledonia. The survival of this low-temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite-facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions. Thortveitite (Sc2Si2O7) and yttrialite (Y2Si2O7) inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized. The altered zircon cores generally have lower trace-element contents than the pristine igneous zircon cores and we suggest that thortveitite, yttrialite, and xenotime-(Y) formed as a result of trace-element expulsion from zircon during low-temperature recrystallization. Trace-element concentrations in the zircon cores indicate that trivalent cations (REE, Y, Sc) in zircon cannot be charge-balanced by xenotime substitution alone. Pentavalent cation concentrations (Nb, As) are also insufficient for charge balance. The presence of thortveitite (Sc2Si2O7) and yttrialite (Y2Si2O7) suggests that trivalent cations in zircon might be charge-balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as H occurring interstitially in the zircon lattice.

Remnants of ancient Australia in Vanuatu: Implications for crustal evolution in island arcs and tectonic development of the southwest Pacific

We report new geochemical and geochronological data from igneous rocks of the little studied western belt of the Vanuatu intraoceanic arc. Ar-Ar dating of igneous hornblende from hornblende andesites and U-Pb dating of zircon from a tonalite place the time of formation of these rocks in the late Eocene to Miocene; therefore, they represent part of the earliest arc development at Vanuatu. The petrological and geochemical characteristics of these rocks are typical of island arc magmas, except they contain inherited zircon grains with significant age populations at ca. 2.8–2.5 Ga, 2.0–1.8 Ga, 1.75–1.5 Ga, 850–700 Ma, 530–430 Ma, and 330–220 Ma. This inheritance signature is unlike anything recognized from the oceanic realm of the southwest Pacific, but in general matches the age of major crustal blocks of the Australian continent. An exception is the significant proportion of zircons of Rodinia breakup age (ca. 800 Ma) that previously have not been found in such amounts in eastern Australia or the southwest Pacific. We propose that part of the Vanuatu arc basement comprises continental material that was rifted and transported thousands of kilometers from northeastern Australia prior to the Cenozoic. The presence of hitherto-unrecognized ancient continental material within an intraoceanic arc provides an alternative source for the crustal trace element and isotopic signature of island arc magmas, and may help reconcile the relatively large thickness and low density of the crust of Vanuatu and possible other intraoceanic arcs.

Using melt inclusions to determine parent-magma compositions of layered intrusions: Application to the Greenhills Complex (New Zealand), a platinum group minerals–bearing, island-arc intrusion

The Permian Greenhills Complex of Southland, New Zealand, is a small ultramafic to mafic layered intrusion that hosts primary magmatic platinum-group minerals (PGMs) as inclusions in cumulus chromian spinel grains. We have determined the major and trace element chemistry of the parent magmas of the complex, using the composition of rehomogenized melt inclusions trapped in cumulus spinel grains from dunite. These melt inclusions have compositions similar to those of primitive ankaramite dikes that cut the complex, verifying that our data are representative of the parent-melt chemistry. These magmas have high MgO and CaO and are enriched in Pb, Sr, and large ion lithophile elements compared to high field strength and rare earth elements. We suggest that the parent magmas to the complex were primitive low-K island-arc tholeiite basalts that formed by high degrees of melting of a spinel peridotite source. The platinum-group element contents of such magmas are expected to be very high. Therefore, the primary PGMs at Greenhills are suggested to have precipitated directly from the melt following differentiation and cooling in a shallow magma chamber.

Continuous eclogite melting and variable refertilisation in upwelling heterogeneous mantle

Large-scale tectonic processes introduce a range of crustal lithologies into the Earths mantle. These lithologies have been implicated as sources of compositional heterogeneity in mantle-derived magmas. The model being explored here assumes the presence of widely dispersed fragments of residual eclogite (derived from recycled oceanic crust), stretched and stirred by convection in the mantle. Here we show with an experimental study that these residual eclogites continuously melt during upwelling of such heterogeneous mantle and we characterize the melting reactions and compositional changes in the residue minerals. The chemical exchange between these partial melts and more refractory peridotite leads to a variably metasomatised mantle. Re-melting of these metasomatised peridotite lithologies at given pressures and temperatures results in diverse melt compositions, which may contribute to the observed heterogeneity of oceanic basalt suites. We also show that heterogeneous upwelling mantle is subject to diverse local freezing, hybridization and carbonate-carbon-silicate redox reactions along a mantle adiabat.

Igneous rocks of the Brook Street Terrane, New Zealand: implications for Permian tectonics of eastern Gondwana and magma genesis in modern intra-oceanic volcanic arcs

Abstract The Brook Street Terrane of South Island, New Zealand, is a remnant of a primitive intra‐oceanic arc system of Permian age. The terrane consists largely of volcanogenic sequences that contain plagioclase‐ and clinopyroxene‐phyric basalts, high‐MgO ankaramite dikes, and basaltic to andesitic volcaniclastic and sedimentary rocks. Dacites and rhyolites are relatively rare. Intruding the sequences are thick dolerite dikes, trondhjemite plutons, and numerous small cumulate complexes. The cumulate complexes contain early‐formed olivine‐ and clinopyroxene‐rich ultramafic cumulates overlain by anorthite and hornblende‐bearing gabbros. There is convincing geological evidence to support earlier interpretations of a direct correlation between the Brook Street Terrane and the Gympie Terrane of Queensland, and we present geochemical data to support correlation with the Teremba Terrane of New Caledonia. It is likely that these terranes are exposed sections of an extensive island‐arc system that was active in the Pacific in Permian times. Dislocation of the arc probably occurred during accretion to the Gondwana margin and subsequent Gondwana breakup. The major and trace element geochemistry of a range of mafic dikes and flows from along the terrane precludes significant geochemical variation along the terrane and shows that the majority of magmas were primitive island‐arc tholeiites. The Bluff Complex is an exception and may have formed in a back‐arc or arc‐rift environment. The geochemistry, petrology, and field relations indicate that most of the intrusive and volcanic rocks are directly related products of upper crustal magmatic differentiation. Primary magma types include high‐MgO ankaramites and trondhjemites that are suggested to have formed by partial melting of lower crustal clinopyroxene‐rich cumulates and gabbros, respectively. The parental ankaramites fractionated to form the bulk of the Brook Street Terrane, including the mafic‐ultramafic cumulates and evolved melts of high‐Al basalt to andesite composition. The Brook Street Terrane is an excellent analogue for modern island‐arc systems and allows for the evaluation of magmatic processes that operate at the subvolcanic level of arcs. The wide distribution of ankaramites in the Brook Street Terrane indicates that parental magmas in island arcs may be more primitive than is currently recognised. Furthermore, partial melting of arc lower crustal cumulates before delamination may be crucial to the development of arcs and the evolution of the continental crust.

Igneous Layering in Basaltic Magma Chambers

Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as cross-bedding, trough structures or layer termination. It is now accepted that basaltic magma chambers mostly crystallize in situ in slightly undercooled boundary layers formed at the margins of the chamber. As a consequence, most known existing layering cannot be ascribed to a simple crystal settling process. Based on detailed field relationships, geochemical analyses as well as theoretical and experimental studies, other potential mechanisms have been proposed in the literature to explain the formation of layered igneous rocks. In this study, we review important mechanisms for the formation of layering, which we classify into dynamic and non-dynamic layer-forming processes.

Mineral supertrumps: a new card game to assist learning of mineralogy

ABSTRACT Mineralogy is an essential component of Earth Science education, yet many students struggle to obtain adequate comprehension and knowledge of mineralogy during tertiary (postsecondary) degree programs. The use of educational games can be an effective strategy for science teaching as games provide an active learning environment that enhances student engagement and motivation. This paper introduces a new card game called “Mineral Supertrumps” that can be used to counter the challenge of learning mineralogy at either secondary or tertiary level. The card game includes information on the properties of 54 minerals, which include the most important rock-forming minerals as well as minerals of industrial and economic significance. The game is easy to learn and play, and it is designed to motivate learning of mineral properties through active and competitive game-play in a group setting. Group play also helps to build identity and culture around student cohorts, which may also promote learning outcomes. Most students in the second year of a tertiary geology program surveyed after playing the game found it enjoyable to play and considered it to be effective for enhancing learning about mineral properties and their application to society and other Earth Science disciplines. Nevertheless, our survey results also indicate that student engagement with the game (and hence, learning benefits) may be limited if the game is not integrated with other course content, and/or it is not linked to incentive-based exercises (e.g., assessment).