Global resources of heavy Rare Earth Elements (REE) are dominantly sourced from Chinese regolith-hosted ion-adsorption deposits in which the REE are inferred to be weakly adsorbed onto clay minerals. Similar deposits elsewhere might provide alternative supply for these high-tech metals, but the adsorption mechanisms remain unclear and the adsorbed state of REE to clays has never been demonstrated in situ. This study compares the mineralogy and speciation of REE in economic weathering profiles from China to prospective regoliths developed on peralkaline rocks from Madagascar. We use synchrotron X-ray absorption spectroscopy to study the distribution and local bonding environment of Y and Nd, as proxies for heavy and light REE, in the deposits. Our results show that REE are truly adsorbed as easily leachable 8- to 9-coordinated outer-sphere hydrated complexes, dominantly onto kaolinite. Hence, at the atomic level, the Malagasy clays are genuine mineralogical analogues to those currently exploited in China.

The layered agpaitic nepheline syenites (kakortokites) of the Ilímaussaq complex, South Greenland, host voluminous accumulations of eudialyte-group minerals (EGM). These complex Na-Ca-zirconosilicates contain economically attractive levels of Zr, Nb and rare-earth elements (REE), but have commonly undergone extensive autometasomatic/hydrothermal alteration to a variety of secondary mineral assemblages. Three EGM alteration assemblages are recognized, characterized by the secondary zirconosilicates catapleiite, zircon and gittinsite. Theoretical petrogenetic grid models are constructed to assess mineral stabilities in terms of component activities in the late-stage melts and fluids. Widespread alteration of EGM to catapleiite records an overall increase in water activity, and reflects interaction of EGM with late-magmatic Na-, Cl- and F-rich aqueous fluids at the final stages of kakortokite crystallization. Localized alteration of EGM and catapleiite to the rare Ca-Zr silicate gittinsite, previously unidentified at Ilímaussaq, requires an increase in CaO activity and suggests post-magmatic interaction with Ca-Sr bearing aqueous fluids. The pseudomorphic replacement of EGM in the kakortokites was not found to be associated with significant remobilization of the primary Zr, Nb and REE mineralization, regardless of the high concentrations of potential transporting ligands such as F and Cl. We infer that the immobile behaviour essentially reflects the neutral to basic character of the late-magmatic fluids, in which REE-F compounds are insoluble and remobilization of REE as Cl complexes is inhibited by precipitation of nacareniobsite-(Ce) and various Ca-REE silicates. A subsequent decrease in F– activity would furthermore restrict the mobility of Zr as hydroxyl-fluoride complexes, and promote precipitation of the secondary zirconosilicates within the confines of the replaced EGM domains.

Earth’s surface and mantle sulphur reservoirs are connected via subduction, crustal recycling and volcanism. Although oceanic hotspot lavas currently provide the best constraints on the deep sulphur cycle, their restricted age range (<200 Ma) means they cannot reveal temporal variations in crustal recycling over Earth history. Sulphur-rich alkaline magmas offer the solution because they are associated with recycled sources (i.e. metasomatized lithospheric mantle and plumes) and, crucially, are found throughout the geological record. Here, we present a detailed study of sulphur isotope fractionation in a Mesoproterozoic alkaline province in Greenland and demonstrate that an enriched subduction-influenced source (δ³⁴S of +1 to +5‰) can be reconstructed. A global δ³⁴S compilation reveals secular variation in alkaline magma sources which support changes in the composition of the lithospheric mantle and/or Ga timescales for deep crustal recycling. Thus, alkaline magmas represent a powerful yet underutilized repository for interrogating crustal recycling through geological time.

The kakortokites of the Mesoproterozoic Ilímaussaq complex, South Greenland, comprise a rhythmically layered series of agpaitic nepheline syenites that crystallized at the base of a shallow crustal magma chamber. They host eudialyte-group minerals (EGM), i.e. structurally complex Na-zirconosilicates, as a major cumulate phase, and have attracted considerable interest as a potential resource for rare earth elements (REE), Zr, Nb, Hf and Ta. The origin of the macrorhythmic (c. 8 m) layering has been the subject of much debate, and both open system processes including nucleation cycles induced by periodic replenishment of the magma chamber, and closed system mechanisms involving gravitational sorting leading to crystal mat formation, have recently been hypothesized. Here we present new compositional data on eudialyte cores and overgrowths from the full layered series and part of the overlying lujavrites to reflect on the proposed models for the kakortokite layering and overall evolution of the complex. Based on these data we argue for continuous bulk liquid fractionation and in situ fractionation in macrorhythmic compartments of kakortokite mush gradually building upwards from the floor of the magma chamber. Eudialyte in the kakortokites displays complex magmatic zoning patterns, typically comprising a sector- and oscillatory-zoned core with subhedral concentric overgrowths. Sector-zoned eudialyte cores reveal stratigraphical fractionation trends of decreasing Ca/(REE+Y), Fe/Mn, Ti, Nb and Cl contents upwards through the layered series. These are interpreted to reflect continuous differentiation of a single agpaitic bulk melt and support models for closed system evolution of the kakortokites. Upward trends become more pronounced in the overlying lujavrites (decreasing Ca/(REE+Y), Fe/Mn, Zr/Hf and Cl), while others remain constant (Ti), or are even reversed (Nb). Eudialyte overgrowths have compositions that diverge from the overall fractionation trends recorded in the cores, and also vary systematically across the sequence. These overgrowths are interpreted to reflect in situ fractionation trends of intercumulus mush melts that were chemically isolated from the bulk magma following compartmentalization of the crystal mush. As such, EGM overgrowths are interpreted to record changing layering dynamics as well as varying co-crystallizing intercumulus phase assemblages at the kakortokite-lujavrite transition. The data provide new insights into the geochemical evolution of the Ilímaussaq complex, with broader implications on the emplacement and layering mechanisms operating in peralkaline systems, and demonstrate the importance of detailed petrographic and in situ mineral chemical analyses where zoned minerals record contrasting evolution of bulk and mush melts. Deciphering such records is fundamental to understanding the full complexity of magma chamber processes.

We describe the field demonstration of astrobiology instruments and research methods conducted in and from the Mars Desert Research Station (MDRS) in Utah during the EuroGeoMars campaign 2009 coordinated by ILEWG, ESA/ESTEC and NASA Ames, with the contribution of academic partners. We discuss the entire experimental approach from determining the geological context using remote sensing, in situ measurements, sorties with sample collection and characterization, analysis in the field laboratory, to the post sample analysis using advanced laboratory facilities.We present the rationale for terrestrial field campaigns to strengthen astrobiology research and the link between in situ and orbital remote sensing data. These campaigns are supporting the preparation for future missions such as Mars Science Laboratory, ExoMars or Mars Sample Return. We describe the EuroGeoMars 2009 campaign conducted by MDRS crew 76 and 77, focused on the investigation of surface processes in their geological context. Special emphasis was placed on sample collection and pre-screening using in-situ portable instruments. Science investigations included geological and geochemical measurements as well as detection and diagnostic of water, oxidants, organic matter, minerals, volatiles and biota.EuroGeoMars 2009 was an example of a Moon–Mars field research campaign dedicated to the demonstration of astrobiology instruments and a specific methodology of comprehensive measurements from selected sampling sites. We discuss in sequence: the campaign objectives and trade-off based on science, technical or operational constraints. This includes remote sensing data and maps, and geological context; the monitoring of environmental parameters; the geophysical context and mineralogy studies; geology and geomorphology investigations; geochemistry characterization and subsurface studies.We describe sample handling (extraction and collection) methods, and the sample analysis of soils and rocks performed in the MDRS laboratory using close inspection, initial petrological characterization, microscopy, Visible-NIR spectrometry, Raman spectrometry, X-ray diffraction/X-ray fluorescence spectrometry, soil analysis, electrochemical and biological measurements.The results from post-mission analysis of returned samples using advanced facilities in collaborator institutes are described in companion papers in this issue. We present examples of in-situ analysis, and describe an example investigation on the exploration and analysis of endolithic microbial mats (from reconnaissance, in-situ imaging, sampling, local analysis to post-mission sample analysis).