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

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

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

The effect of fluorine on clinopyroxene/melt trace-element partitioning

International audienceThe impact of fluorine on phase relations and clinopyroxene-melt element partitioning has been explored experimentally to better understand the effect of this halogen on the residual enrichment of the REE and HFSE during crystallisation of alkaline and peralkaline magmas. Clinopyroxene was grown from three H2O -saturated synthetic glasses of tephriphonolite-to-phonolite composition in a rapid quench internally heated pressure vessel at 650-800 ∘C and 200 MPa, with Δ log f O2 fixed to ca. FMQ + 1. The fluorine content in the charges was varied and produced quenched melts from fluorine-free to 1.6 wt.% F. The experiments yield an assemblage of melt, fluid, sodic clinopyroxene, biotite, magnetite, ± K-feldspar, ± titanite, ± fluorite, and ± hiortdahlite (a Na-Ca-Ti-F sorosilicate). Addition of fluorine markedly increases the mode of biotite without significantly affecting the mode of clinopyroxene. Relative to fluorine-free compositions, experiments with 1.6 wt.% fluorine in the melt show a strong decrease in clinopyroxene-melt partition coefficients for the trivalent REE La-Dy and Y with a lesser decrease for the DHREE and DHFSE 4 +. The diminished uptake of these metals by clinopyroxene may reflect changes to their speciation in the melt phase, consistent with the formation of REE-F complexes and with modifications to the medium-range structural environment around HFSE4+ ions in the melt. An increase in the fluorine content of the melt will thus make the REE and HFSE4+ progressively less compatible and, therefore, available for residual enrichment

The amphiboles of the REE-rich A-type peralkaline Strange Lake pluton – fingerprints of magma evolution

Major and trace element compositions of amphibole in igneous environments commonly reflect evolving magma compositions. In this study, we use the amphibole-group minerals from the Strange Lake, REE-enriched peralkaline granitic pluton to gain insights into the evolution of the magma. This 1240 Ma old pluton consists of two main intrusive facies, an early hypersolvus granite, which occurs as separate northern and southern intrusions, and a more evolved transsolvus granite. [...] Owing to their small ionic radius and their compatibility with the amphibole structure, HREE concentrations were more controlled by partitioning (crystal chemical effects) than by the concentrations in the corresponding magma. Large proportions of the bulk HREE content (up to 70%) reside in the amphibole, and their later release through hydrothermal replacement helps to explain the extreme and unusual HREE enrichment of the Strange Lake pluton

The geochemical composition of serpentinites in the Mesoarchaean Tartoq Group, SW Greenland:harzburgitic cumulates or melt-modified mantle?

Large slivers and enclaves (up to 100 × 2000 m) of serpentinites form tectonic imbricates within mafic supracrustal rocks of the Mesoarchaean Tartoq Group in SW Greenland. In this study we present new bulk-rock major, trace and platinum-group element data, and preliminary Re–Os isotope data for these serpentinites. Additionally, we present in situ major and trace element data for chromite and magnetite. Bulk-serpentinite major and trace element compositions allow us to rule out komatiitic, picritic, boninitic or residual mantle origins. Rather, the fractionated platinum-group patterns for the Tartoq Group serpentinites suggest a cumulate origin. The bulk-rock Re–Os isotope data shows Re-depletion ages (TRD) of ca. 2300, 2800 and 2950 Ma for the three analysed samples. These ages are younger than the 2986 ± 4 Ma minimum age of the Tartoq Group, and suggest disturbance of the Re–Os system during metamorphism or post-magmatic metasomatic events. Late resetting and metamorphic overprinting are in agreement with oxide mineral compositions, which are not primary as indicated by negligible Mg and Al contents and elevated Fe3 +# (> 55). Although the major and trace element data do not provide conclusive evidence for the petrogenesis of these Mesoarchaean serpentinites, we observe remarkable similarities with ultramafic rocks associated with the lower crustal cumulates. In particular, ultramafic cumulates of the Kohistan and Talkeetna island arc sections are compositionally comparable, if we allow for slight loss of MgO and addition of SiO2 during early alteration, serpentinisation and metamorphism. Based on these similarities, we propose an arc origin for the Tartoq Group serpentinites. Previous geochemical and structural studies of the enclosing Tartoq Group supracrustal rocks also point to formation in a subduction zone geodynamic setting. This suggests that this assemblage of imbricated mafic to ultramafic rocks and minor sediments represents a Mesoarchaean arc-related ophiolite