Clinopyroxene/melt trace element partitioning in sodic alkaline magmas

Clinopyroxene is a key fractionating phase in alkaline magmatic systems, but its impact on metal enrichment processes, and the formation of REE + HFSE mineralisation in particular, is not well understood. To constrain the control of clinopyroxene on REE + HFSE behaviour in sodic (per)alkaline magmas, a series of internally heated pressure vessel experiments was performed to determine clinopyroxene–melt element partitioning systematics. Synthetic tephriphonolite to phonolite compositions were run H2O-saturated at 200 MPa, 650–825°C with oxygen fugacity buffered to log f O2 ≈ ΔFMQ + 1 or log f O2 ≈ ΔFMQ +5. Clinopyroxene–glass pairs from basanitic to phonolitic fall deposits from Tenerife, Canary Islands, were also measured to complement our experimentally-derived data set. The REE partition coefficients are 0·3–53, typically 2–6, with minima for high-aegirine clinopyroxene. Diopside-rich clinopyroxene (Aeg5–25) prefer the MREE and have high REE partition coefficients (DEu up to 53, DSm up to 47). As clinopyroxene becomes more Na- and less Ca-rich (Aeg25–50), REE incorporation becomes less favourable, and both the VIM1 and VIIIM2 sites expand (to 0·79 Å and 1·12 Å), increasing DLREE/DMREE. Above Aeg50 both M sites shrink slightly and HREE (VIri ≤ 0·9 Å ≈ Y) partition strongly onto the VIM1 site, consistent with a reduced charge penalty for REE3+ ↔ Fe3+ substitution. Our data, complemented with an extensive literature database, constrain an empirical model that predicts trace element partition coefficients between clinopyroxene and silicate melt using only mineral major element compositions, temperature and pressure as input. The model is calibrated for use over a wide compositional range and can be used to interrogate clinopyroxene from a variety of natural systems to determine the trace element concentrations in their source melts, or to forward model the trace element evolution of tholeiitic mafic to evolved peralkaline magmatic systems

Pressure--Temperature History of the> 3 Ga Tartoq Greenstone Belt in Southwest Greenland and Its Implications for Archaean Tectonics

CITATION: Van Hinsberg, V. et al. 2018. Pressure–temperature history of the >3 Ga Tartoq Greenstone belt in Southwest Greenland and its implications for Archaean tectonics. Geosciences, 8:367, doi:10.3390/geosciences8100367.The original publication is available at https://www.mdpi.comThe Tartoq greenstone belt of southwest Greenland represents a well-preserved section through >3 Ga old oceanic crust and has the potential to provide important constraints on the composition and geodynamics of the Archaean crust. Based on a detailed structural examination, it has been proposed that the belt records an early style of horizontal convergent plate tectonics where elevated temperatures, compared to the modern-day, led to repeated aborted subduction and tonalite–trondhjemite–granodiorite (TTG) type melt formation. This interpretation hinges on pressure–temperature (P–T) constraints for the belt, for which only preliminary estimates are currently available. Here, we present a detailed study of the pressure–temperature conditions and metamorphic histories for rocks from all fragments of the Tartoq belt using pseudosection modelling and geothermobarometry. We show that peak conditions are predominantly amphibolite facies, but range from 450 to 800 °C at up to 7.5 kbar; reaching anatexis with formation of TTG-type partial melts in the Bikuben segment. Emplacement of the Tartoq segments into the host TTG gneisses took place at approximately 3 Ga at 450–500 °C and 4 kbar as constrained from actinolite–chlorite–epidote–titanite–quartz parageneses, and was followed by extensive hydrothermal retrogression related to formation of shear zone-hosted gold mineralisation. Tourmaline thermometry and retrograde assemblages in mafic and ultramafic lithologies constrain this event to 380 ± 50 °C at a pressure below 1 kbar. Our results show that the convergent tectonics recorded by the Tartoq belt took place at a P–T gradient markedly shallower than that of modern-day subduction, resulting in a hot, weak and buoyant slab unable to generate and transfer ‘slab pull’, nor sustain a single continuous downgoing slab. The Tartoq belt suggests that convergence was instead accomplished by under-stacking of slabs from repeated aborted subduction. The shallow P–T path combined with thermal relaxation following subduction stalling subsequently resulted in partial melting and formation of TTG melts.https://www.mdpi.com/2076-3263/8/10/367Publisher's versio

Geological processes defining the formation of plumasite-type corundum in the Paleoproterozoic Isertoq Terrane, South-East Greenland

Plumasite-type corundum occurrences in the Nattivit area in South-East Greenland offer a unique opportunity to study corundum formation in-situ where pegmatites intruded into metamorphosed lherzolite and dunite of the Archean-Paleoproterozoic continental crust. The Nattivit area, located in the Isertoq Terrane of North Atlantic Craton, forms part of the overriding plate during convergence of the Nagssugtoqidian orogen (1910-1840 Ma). New field observations and elemental and isotopic geochemical analysis provide further insights in the history of crustal convergence, its exhumation and how corundum was formed. The continental crust in the area consists of metamorphosed mafic to ultramafic rocks and tonalite-trondhjemite-granodiorite (TTG) gneisses, where the mafic rocks in the Isertoq Terrane yield a εNd T$_{DM}$ model age of 3000–2800 Ma. Dunite and lherzolite sills/dikes intruded the mafic rocks before the intrusion of the TTG sheets. The intrusion ages for the TTG obtained from zircon U-Pb geochronology are 2818 ± 8 Ma, 2760 ± 13 Ma to 2667 ± 7 Ma. U-Pb zircon data, zircon textures and Th/U ratios indicate metamorphism occurred at 2698 ± 7 Ma to 2629 ± 11 Ma, 2500–2400 Ma and 1900–1600 Ma. Whole rock geochemical data of mafic to ultramafic rocks show a continental arc affinity, with negative Ta, Nb and positive Pb anomalies. A metasomatic event at 2390 ± 70 Ma partly reset the isotopic signature in the mafic to ultramafic rocks. A marked absence of ages between 2350 and 2100 Ma in the TTG zircon age populations exists, indicating a period with minimal magmatic and/or metamorphic activity. The metamorphic mineral assemblages of the schist, amphibolite, ultramafic rocks and metasomatic reaction zones in ultramafic rocks indicate upper to medium–high amphibolite facies conditions. Kyanite in the metasomatic reaction zones in ultramafic rocks indicate the higher end of the temperature and pressure range above 4.2–10 kbar and 530–800 °C, similar to estimates from dolerite dikes in the Kitak area. The syn-tectonic pegmatites with an intrusion age of 1843 ± 4 Ma formed corundum. The new data indicate that the pegmatite melt/fluid and the geotectonic setting are defining factors for generating plumasite-type corundum

Remnants of Mesoarchaean oceanic crust in the Tartoq Group, South-West Greenland

The Tartoq Group is located in the Sermiligaarsuk fjord region in South-West Greenland in an area of approximately 20 × 50 km (Fig. 1). The Tartoq Group consists of several discrete, fault-bound blocks of metavolcanic rocks, surrounded by Archaean tonalite-trondhjemite-granodioritetype (TTG) gneisses. A zircon age of 2996.3 ± 5.9 Ma of a TTG intrusion provides a minimum age for the formation of the Tartoq Group (Fig. 2). The metavolcanic rocks probably show the lowest degree of metamorphism found anywhere in the Archaean craton of Greenland. Here we present a new model for the origin of the metavolcanic rocks of the Tartoq Group based on geochemical, metamorphic and structural data. The samples used for this study were collected by the Geological Survey of Denmark and Greenland (GEUS) in 2009 and 2010. The study is part of a joint project between the Greenland Bureau of Minerals and Petroleum and GEUS on the mineral potential of south-western Greenland

Tourmaline : monitor of the P-T-X evolution in rocks

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Editorial: The Early Earth Crust and Its Formation

International audienceEditorial on the Research Topic The Early Earth Crust and Its Formation The geochemical and petrological nature of the early Earth crust, and the processes involved in its formation and stabilization, are critical questions to understand the earliest evolution of our planet, and have yet to be resolved. The ancient rock archives in stable cratons provide the foundation for our understanding of the early formation and evolution of Earth's crust, but it is unlikely that these archives are representative of average early crustal composition or its evolution. Geodynamic modeling and isotope tracers provide key complementary constraints, as well as tests for hypotheses proposed based on the rock record. One of the most fundamental questions that is still unresolved is the timing of the onset of plate tectonics, which is a feature that is unique to the Earth among the known rocky planets. Subduction zones represent the geological environment in which crustal fractionation currently takes place, but it could be argued that this particular setting is not conducive to the long-term preservation of crust due to recycling. Moreover, the style of subduction, or horizontal tectonics more broadly, may have changed during early Earth history, and hence the nature of crusts generated over time. Additionally, the onset of subduction could well have pre-dated global plate tectonics, as the required assemblage of global plates may not have been a stable configuration on the hot young Earth. Interpretations of the ancient rock record are strongly debated and divided among those who support horizontal plate tectonic processes throughout the Archean Eon, and those who invoke a plume-dominated, stagnant lid scenario and infracrustal differentiation with a transition (gradual or abrupt) to modern-style plate tectonics, likely towards the end of the Archean. This Research Topic brings together articles that explore the earliest part of Earth's geological history; from mantle-derived magmas and their fractionation, all the way through partial melting and crustal differentiation to form stable continental crust. The invited review by Hawkesworth et al. comprehensively combines observations from metamorphism, tectonics, geochemistry, petrology and geophysics to infer the nature and secular evolution of the continental crust, and its implications for the onset of plate tectonics. They purposely provide a global picture, which advocates for a transition in the nature of the crust towards a more felsic flavor, coincidental with a proposed onset of plate tectonics at ∼3.0 Ga. The formation and evolution of the early Earth's crust is further evaluated by Garde et al. who reviewed the geological history of the North Atlantic craton of West Greenland and present their conclusions regarding the geodynamical context of its construction from the Eoarchean to Mesoarchean. The authors conclusions advocate for the existence of horizontal tectonics since the Eoarchean

Evaluating the biosignature potential of nitrogen concentrations in graphite and associated K-silicates

Funding: This study was financially supported by a NERC Frontiers grant (NE/V010824/1) to EES and an Osisko research stipend to VvH.The oldest remnants of life on Earth from various localities in the Isua supracrustal belt in Greenland date back to >3.7 billion years ago (Ga). They are in the form of graphite, whose biogenicity is controversial. Previous studies used the presence and isotopic composition of nitrogen in graphite from along the Isua belt to argue both for and against biogenicity. To determine if the nitrogen chemistry of graphite can indeed serve as a biosignature, we investigated a hydrothermal graphite deposit from south-east Greenland (1.87–1.82 Ga). We found indications that molar C/N ratios of hydrothermal graphite may be similar to those of biogenic graphite from the Archean rock record, meaning that the nitrogen content of graphite is itself perhaps not diagnostic of ancient life, requiring caution in future studies. However, the hydrothermal graphite deposit also revealed unusually low N concentrations in associated silicates, despite a wide range of K concentrations up to 5 wt%. Using a thermodynamic model of nitrogen speciation in the presence of graphite, paired with previously published partition coefficients for ammonium in K-silicates, we show that abiotic process can explain these low N-concentrations of around 1 μg/g in potassic silicates. Higher concentrations of >10 μg/g, such as those found in graphitic metapelites from the Isua supcracrustal belt, would, however, require an unusually N-rich fluid. Such a N-rich fluid is most easily derived from the breakdown of biomass within sediments prior to graphitization. We therefore conclude that potassic silicates associated with graphite can serve as an indirect biosignature. Our approach supports previous inferences of life on Earth back to at least 3.7 Ga.Publisher PDFPeer reviewe

Tourmaline: An ideal indicator of its host environment

Tourmaline-supergroup minerals are ubiquitous accessory minerals in rocks of the Earth\u27s crust. They can adjust their composition to suit a wide variety of environments, and therefore display a remarkable range in stability in terms of pressure, temperature, fluid composition, and host-rock composition. Because of this compositional sensitivity, tourmaline is an excellent indicator of the environmental conditions in its host. This is further enhanced by negligible diffusion up to high temperatures and a strongly refractory character during subsequent host-rock alteration and weathering, as well as mechanical transport of grains. Whereas most prior research on tourmaline has focused on chemical and crystallographic characterizations and systematics of the tourmaline-supergroup minerals, recent studies are shifting the focus to a quantitative reconstruction of environmental conditions in the host using a combination of structural, compositional and crystallographic characteristics of the tourmaline. This thematic issue, which follows a special session at the 2009 GAC-MAC-AGU meeting in Toronto, highlights these exciting advances; here we discuss some of the obstacles that will need to be overcome to insure the practical applicability of tourmaline. The papers presented in this thematic issue of The Canadian Mineralogist show that we are standing on the brink of a major breakthrough in the use of tourmaline as a quantitative indicator of the chemical and physical properties of its host environment these properties may well make tourmaline the prime mineral for this purpose