The sulfur isotope evolution of magmatic-hydrothermal fluids : insights into ore-forming processes

This project was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 689909. W.H. also acknowledges support from a UKRI Future Leaders Fellowship (MR/S033505/1). A.J.B. is funded by the NERC National Environment Isotope Facility award (NE/S011587/1) and the Scottish Universities Environmental Research Centre.Metal-rich fluids that circulate in magmatic-hydrothermal environments form a wide array of economically significant ore deposits. Unravelling the origins and evolution of these fluids is crucial for understanding how Earth’s metal resources form and one of the most widely used tools for tracking these processes is sulfur isotopes. It is well established that S isotopes record valuable information about the source of the fluid, as well as its physical and chemical evolution (i.e. changing pH, redox and temperature), but it is often challenging to unravel which of these competing processes drives isotopic variability. Here we use thermodynamic models to predict S isotope fractionation for geologically realistic hydrothermal fluids and attempt to disentangle the effects of fluid sources, physico-chemical evolution and S mineral disequilibrium. By modelling a range of fluid compositions, we show that S isotope fingerprints are controlled by the ratio of oxidised to reduced S species (SO42−/H2S), and this is most strongly affected by changing temperature, fO2 and pH. We show that SO42−/H2S can change dramatically during cooling and our key insight is that S isotopes of individual sulfide or sulfate minerals can show large fractionations (up to 20 ‰) even when pH is constant and fO2 fixed to a specific mineral redox buffer. Importantly, while it is commonly assumed that SO42−/H2S is constant throughout fluid evolution, our analysis shows that this is unlikely to hold for most natural systems. We then compare our model predictions to S isotope data from porphyry and epithermal deposits, seafloor hydrothermal vents and alkaline igneous bodies. We find that our models accurately reproduce the S isotope evolution of porphyry and high sulfidation epithermal fluids, and that most require magmatic S sources between 0 and 5 ‰. The S isotopes of low sulfidation epithermal fluids and seafloor hydrothermal vents do not fit our model predictions and reflect disequilibrium between the reduced and oxidised S species and, for the latter, significant S input from seawater and biogenic sources. Alkaline igneous fluids match model predictions and confirm magmatic S sources and a wide range of temperature and redox conditions. Of all these different ore deposits, porphyry and alkaline igneous systems are particularly well-suited to S isotope investigation because they show relationships between redox, alteration and ore mineralogy that could be useful for exploration and prospecting. Ultimately, our examples demonstrate that S isotope forward models are powerful tools for identifying S sources, flagging disequilibrium processes, and validating hypotheses of magmatic fluid evolution.Publisher PDFPeer reviewe

Layering in peralkaline magmas, Ilímaussaq Complex, S Greenland

EJH acknowledges funding from a NERC PhD studentship and the work was completed at the University of St Andrews, UK.The peralkaline to agpaitic Ilímaussaq Complex, S. Greenland, displays spectacular macrorhythmic (> 5 m) layering via the kakortokite (agpaitic nepheline syenite), which outcrops as the lowest exposed rocks in the complex. This study applies crystal size distribution (CSD) analyses and eudialyte-group mineral chemical compositions to study the marker horizon, Unit 0, and the contact to the underlying Unit − 1. Unit 0 is the best-developed unit in the kakortokites and as such is ideal for gaining insight into processes of crystal formation and growth within the layered kakortokite. The findings are consistent with a model whereby the bulk of the black and red layers developed through in situ crystallisation at the crystal mush–magma interface, whereas the white layer developed through a range of processes operating throughout the magma chamber, including density segregation (gravitational settling and flotation). Primary textures were modified through late-stage textural coarsening via grain overgrowth. An open-system model is proposed, where varying concentrations of halogens, in combination with undercooling, controlled crystal nucleation and growth to form Unit 0. Our observations suggest that the model is applicable more widely to the layering throughout the kakortokite series and potentially other layered peralkaline/agpaitic rocks around the world.Publisher PDFPeer reviewe

A high sensitivity system for luminescence measurement of materials

The authors would like to thank the support of the Fundamental Research Funds for the Central Universities of China, the National Science Foundation of China (No.11205134) and Beijing Higher Education Young Elite Teacher Project (YETP0640). The refurbishment of the RLTLCL system at St Andrews was funded by NERC grant NE/H002715/1.A unique combined and multi-disciplinary wavelength multiplexed spectrometer is described. It is furnished with high-sensitivity imaging plate detectors, the power to which can be gated to provide time-resolved data. The system is capable of collecting spectrally resolved luminescence data following X-ray excitation [radioluminescence (RL) or X-ray excited optical luminescence (XEOL)], electron irradiation [cathodoluminescence (CL)] and visible light from light emitting diodes (LEDs) [photoluminescence (PL)]. Time-resolved PL and CL data can be collected to provide lifetime estimates with half-lives from microsecond timeframes. There are temperature stages for the high and low temperature experiments providing temperature control from 20 to 673 K. Combining irradiation, time resolved (TR) and TR-PL allows spectrally-resolved thermoluminescence (TL) and optically stimulated luminescence (OSL). The design of two detectors with matched gratings gives optimum sensitivity for the system. Examples which show the advantages and multi-use of the spectrometer are listed. Potential future experiments involving lifetime analysis as a function of irradiation, dose and temperature plus pump-probe experiments are discussed.PostprintPostprintPostprintPostprintPeer reviewe

Optical determination of the width of the band-tail states, and the excited and ground state energies of the principal dosimetric trap in feldspar

SR performed the measurements during her M.Sc., which was financially supported by the Deutschlandstipendium of the Bundesministerium für Bildung und Forschung – Stiftung Studium und Lehre (Ministry for Education and Research of the German government). The paper was written, during SR's PhD, which is financed by an AberDoc PhD Scholarship of Aberystwyth University. An Erasmus + student mobilitygrant enabled SR's research stay at the Center for Nuclear Technologies, Technical University of Denmark, DTU Risø Campus, Roskilde, Denmark. GEK acknowledges support from SNSF grant number PZ00P2_167960. Samples HAM-5 and JSH1-13 were taken in the framework of the QuakeRecNankai project, funded by the Belgian Science Policy Office (BELSPO BRAIN-be BR/121/A2). We thank Benny Guralnik for the provision of KTB-383-C, Renske Lambert for MBT-I-2430 and MBT-F-5704, Javier Garcia-Guinea for Cleavelandite and David Sanderson for F1.We constrain parameters that determine thermal stability of the infrared stimulated luminescence (IRSL) signal in a suite of 13 compositionally different feldspar samples by optical probing. We focus specifically on the excited and ground state of the principal trap and the width of the sub-conduction band-tail states. Excitation spectra measured at room temperature result in approximate trap depth of about 2.04 eV and the excited state energy at 1.44 ± 0.02 eV, irrespective of feldspar composition for the sample's measured here. Fitting the non-resonant rising continuum of the excitation spectra suggests that the width of the band-tail states accessible from the ground state of the trap (ΔE) ranges from 0.21 to 0.47 eV at room temperature between the different samples. Photoluminescence measurements are used to constrain the full sub-conduction band-tail width (Urbach width, Eu) using the excitation-energy-dependent emission (EDE), resulting in values ranging from 0.26 to 0.81 eV. While the depth of the principal trap and its main excited state seem to be independent of feldspar composition, the difference between ΔE and Eu seems to be related to sample K-content.PostprintPeer reviewe

New insights from field observations of the Younger giant dyke complex and mafic lamprophyres of the gardar province on Tuttutooq island, South Greenland

LK, RW, RC, LM and AM received funding from the Mining Institute of Scotland, Institute of Materials, Minerals and Mining, the Edinburgh Geological Society, the Augustine Courtauld Trust and the Scott Polar Research Institute. LK received funding from the Society of Economic Geology Hickok-Radford Fund.The Gardar Province of south Greenland is defined by the products of alkaline igneous magmatism during the Mesoproterozoic. The most laterally extensive Gardar intrusions are a series of giant dyke complexes best exposed on the Tuttutooq archipelago. We present new field observations and a geological map of north-east Tuttutooq island that provide fresh insights into the temporal evolution of the Younger giant dyke complex and two associated ultramafic lamprophyres. Our data demonstrate that distinctive crystallisation regimes occurred in different sectors of the dyke complex, leading to the formation of marginal gabbros and ovoid pod-like domains displaying lamination, modal layering and/or more evolved differentiates. We infer that at least two pulses of magma contributed to the formation of the Younger giant dyke complex. In addition, the relative ages of two ultramafic lamprophyre diatremes are constrained and attributed to two distinct phases of rifting in the Gardar Province.Publisher PDFPeer reviewe

The KD Sr/Ca in cultured massive Porites spp. corals are reduced at low seawater pCO2

This work was supported by the UK Natural Environment Research Council (award NE/I022973/1) to AAF and NA.Coral skeletal Sr/Ca has valuable potential as a proxy of sea surface temperatures (SSTs). However seawater pCO2 can influence skeletal Sr incorporation and Sr/Ca-SST calibrations derived from present day corals may not be applicable to ancient specimens or older sections of modern corals deposited under lower seawater pCO2 than the present day. In this study we analysed skeletal Sr/Ca in multiple genotypes of massive Porites spp. cultured over a range of seawater pCO2 (from 180 to 750 μatm) and temperature (25°C and 28°C). Multiple linear regression analysis indicates that the Sr/Ca aragonite partition coefficient, KD Sr/Ca is inversely related to seawater temperature and positively related to seawater pCO2 (equivalent to changes in skeletal Sr/Ca of 0.046 mmol mol-1 °C-1 and 0.0002 mmol mol–1 µatm-1 respectively). Applying present day Sr/Ca-SST equations to older coral skeletons growing at lower pCO2 could underestimate seawater temperatures. However KD Sr/Ca vary significantly between some coral genotypes cultured at the same seawater pCO2 indicating that other unidentified processes also influence skeletal Sr/Ca and it is unknown how these processes varied when ancient corals were deposited. We do not observe a significant relationship between KD Sr/Ca and coral calcification rate after combining all coral genotypes to allow identification of the correct KD Sr/Ca to apply to coral records.PostprintPeer reviewe

Mantle sources and magma evolution in Europe's largest rare earth element belt (Gardar Province, SW Greenland) : new insights from sulfur isotopes

This work is a contribution to the HiTech AlkCarb project and was funded by the European Union's Horizon 2020 research and innovation programme under grant agreement No. 689909. W.H. also acknowledges support from a UKRI Future Leaders Fellowship (MR/S033505/1). A.J.B. is funded by the NERC National Environment Isotope Facility award (NE/S011587/1) and the Scottish Universities Environmental Research Centre.Alkaline igneous complexes are often rich in rare earth elements (REE) and other metals essential for modern technologies. Although a variety of magmatic and hydrothermal processes explain the occurrence of individual deposits, one common feature identified in almost all studies, is a REE-enriched parental melt sourced from the lithospheric mantle. Fundamental questions remain about the origin and importance of the mantle source in the genesis of REE-rich magmas. In particular, it is often unclear whether localized enrichments within an alkaline province reflect heterogeneity in the mantle source lithology (caused by prior subduction or plume activity) or variations in the degree of partial melting and differentiation of a largely homogeneous source. Sulfur isotopes offer a means of testing these hypotheses because they are unaffected by high temperature partial melting processes and can fingerprint different mantle sources. Although one must be careful to rule out subsequent isotope fractionation during magma ascent, degassing and crustal interactions. Here, we present new S concentration and isotope (δ34S) measurements, as well as a compilation of major and trace element data, for a suite of alkaline magmatic units and crustal lithologies from the Mesoproterozoic Gardar Province. Samples span all phases of Gardar magmatism (1330–1140 Ma) and include regional dykes, rift lavas and the alkaline complexes Motzfeldt and Ilímaussaq, which represent two of Europe's largest REE deposits. We show that the vast majority of our 115 samples have S contents >100 ppm and δ34S of −1 to 5‰. Only 8 samples (with low S contents, <100 ppm) show evidence for crustal interactions, implying that the vast majority of Gardar melts preserve the S isotopic signature of their magma source. Importantly, samples from across the Gardar Province δ34S have above the canonical mantle range (≤−1.4‰) and therefore require recycled surface S in their mantle source. Elevated values are explained by a period of Andean-style subduction and mantle metasomatism which took place ∼500 Ma before rift onset and are also supported by trace elements signatures (e.g. Ba/La) which match modern subduction zones. Comparing the various generations of Gardar magmas, we find that δ34S  values, large ion lithophile elements (K, Ba, P) and selective incompatible elements (Nb and light REE) are particularly enriched in the Late Gardar dykes, alkaline complexes and clusters of silica-undersaturated dykes spatially associated with the alkaline complexes. These data indicate that subduction-related metasomatism of the Gardar mantle was spatially heterogeneous, and that alkaline complexes are sourced from localized mantle domains highly enriched in 34S, REE, alkalis and volatiles (particularly, F). Since alkalis and volatiles play an essential role in driving extreme differentiation of alkaline melts and fluids, we suggest the co-location of these species plus incompatible metals at high concentrations in the lithospheric mantle is a critical first-step in the genesis of a world-class alkaline REE deposit. S isotopes are powerful tools for identifying enriched mantle domains and the sources of mineralized alkaline igneous bodies.Publisher PDFPeer reviewe

Structural state of rare earth elements in eudialyte-group minerals

This work was carried out under the NERC-funded SOS RARE consortium [grant NE/M010856/1 to AMB, NJH and AF]. We thank Diamond Light Source and KIT Karlsruhe Light Source for beam time at I18 [grants SP14793 and SP15903 to AMB and AF] and SUL-X, respectively. PG was funded by the People Programme (Marie Curie Actions) in the EU Seventh Framework Programme (FP7/2007-2013), REA grant agreement no. 609405 (COFUNDPostdocDTU).Eudialyte-group minerals (EGM) attract global interest as potential resources for high-field-strength elements (e.g. Zr, Nb, Ta, and rare-earth elements), i.e. critical materials for modern technologies. They are particularly valued for their relative enrichment in the most critical lanthanides, i.e. Nd and heavy rare earth elements (Gd–Lu). However, rare earth element (REE) substitution mechanisms into the EGM structure are still poorly understood. Light and heavy REE may occupy different sites and there may be ordering and/or defect clustering in the structure. This study uses X-ray absorption spectroscopy to determine the structural state of REE in EGM from prospective eudialyte-bearing complexes. Yttrium K-edge and Nd L3-edge spectra were collected as proxies for heavy and light REE, respectively, and compared to natural and synthetic REE-bearing standards. Extended X-ray absorption fine structure data yield best fits for Y in six-fold coordination with Y–O distances of 2.24–2.32 Å, and a second coordination sphere comprising Fe, Na, Ca, Si and O at radial distances of 3.6–3.8 Å. These findings are consistent with dominant Y3+ substitution for Ca2+ on the octahedral M1 site in all the samples studied, and exclude preferential substitution of Y3+ onto the smaller octahedral Z site or the large low-symmetry N4 site. Using lattice strain theory, we constructed relative partitioning models to predict site preferences of lanthanides we have not measured directly. The models predict that all REE are favoured on the Ca-dominant M1 site and that preferential partitioning of heavy over light REE increases in EGM containing significant Mn in the M1-octahedral rings (oneillite subgroup). Thus, the flat REE profiles that make EGM such attractive exploration targets are not due to preferential partitioning of light and heavy REE onto different sites. Instead, local ordering of Mn- and Ca-occupied M1 sites may influence the capacity of EGM to accommodate heavy REE.Publisher PDFPeer reviewe

Insights into the response of coral biomineralisation to environmental change from aragonite precipitations in vitro

Funding: This work was supported by the UK Natural Environment Research Council (NE/S001417/1) to NA, KP, RK, MC and AF. We thank Gavin Peters, University of St Andrews, for assistance with BET analyses. Electron microscopy was carried out in the Aberdeen Centre for Electron Microscopy, Analysis and Characterisation (ACEMAC).Precipitation of marine biogenic CaCO3 minerals occurs at specialist sites, typically with elevated pH and dissolved inorganic carbon, and in the presence of biomolecules which control the nucleation, growth, and morphology of the calcium carbonate structure. Here we explore aragonite precipitation in vitro under conditions inferred to occur in tropical coral calcification media under present and future atmospheric CO2 scenarios. We vary pH, ΩAr and pCO2 between experiments to explore how both HCO3- and CO32- influence precipitation rate and we identify the effects of the three most common amino acids in coral skeletons (aspartic acid, glutamic acid and glycine) on precipitation rate and aragonite morphology. We find that fluid ΩAr or [CO32-] is the main control on precipitation rate at 25°C, with no significant contribution from HCO3- or pH. All amino acids inhibit aragonite precipitation at 0.2-5 mM and the degree of inhibition is inversely correlated with ΩAr and, in the case of aspartic acid, also inversely correlated with seawater temperature. Aspartic acid inhibits precipitation the most, of the tested amino acids (and generates changes in aragonite morphology) and glycine inhibits precipitation the least. Previous work shows that ocean acidification increases the amino acid content of coral skeletons and probably reduces calcification media ΩAr, both of which can inhibit aragonite precipitation. This study and previous work shows aragonite precipitation rate is exponentially related to temperature from 10-30°C and small anthropogenic increases in seawater temperature will likely offset the inhibition in precipitation rate predicted to occur due to increased skeletal aspartic acid and reduced calcification media ΩAr under ocean acidification.Publisher PDFPeer reviewe

Insights into the response of coral biomineralisation to environmental change from aragonite precipitations in vitro

This work was supported by the UK Natural Environment Research Council (NE/S001417/1) to NA, KP, RK, MC and AF. We thank Gavin Peters, University of St Andrews, for assistance with BET analyses. Electron microscopy was carried out in the Aberdeen Centre for Electron Microscopy, Analysis and Characterisation (ACEMAC).Precipitation of marine biogenic CaCO3 minerals occurs at specialist sites, typically with elevated pH and dissolved inorganic carbon, and in the presence of biomolecules which control the nucleation, growth, and morphology of the calcium carbonate structure. Here we explore aragonite precipitation in vitro under conditions inferred to occur in tropical coral calcification media under present and future atmospheric CO2 scenarios. We vary pH, ΩAr and pCO2 between experiments to explore how both HCO3- and CO32- influence precipitation rate and we identify the effects of the three most common amino acids in coral skeletons (aspartic acid, glutamic acid and glycine) on precipitation rate and aragonite morphology. We find that fluid ΩAr or [CO32-] is the main control on precipitation rate at 25°C, with no significant contribution from HCO3- or pH. All amino acids inhibit aragonite precipitation at 0.2-5 mM and the degree of inhibition is inversely correlated with ΩAr and, in the case of aspartic acid, also inversely correlated with seawater temperature. Aspartic acid inhibits precipitation the most, of the tested amino acids (and generates changes in aragonite morphology) and glycine inhibits precipitation the least. Previous work shows that ocean acidification increases the amino acid content of coral skeletons and probably reduces calcification media ΩAr, both of which can inhibit aragonite precipitation. This study and previous work shows aragonite precipitation rate is exponentially related to temperature from 10-30°C and small anthropogenic increases in seawater temperature will likely offset the inhibition in precipitation rate predicted to occur due to increased skeletal aspartic acid and reduced calcification media ΩAr under ocean acidification.Publisher PDFPeer reviewe