Early Eocene Arctic volcanism from carbonate-metasomatized mantle

Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New 40Ar/39Ar chronology indicates their emplacement between ~ 56 and ~ 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at ~ 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component (“A”; 3He/4He ~ 8 RA, εNdi ~ + 3 εHfi ~ + 7, γOsi ~ 0). The second was an enriched component (“B” 3He/4He + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at ~ 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere

Tracking the Late Jurassic apparent (or true) polar shift in U-Pb-dated kimberlites from cratonic North America (Superior Province of Canada)

Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (∼30°) gap in mean paleopoles in the Late Jurassic, between about 160 and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high-precision U-Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9 ± 1.1 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5 ± 1.2 Ma Triple B kimberlite. The combined ∼156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95 = 2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites), showing that the polar motion was at a relatively steady yet rapid (∼1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria-Africa and Iran-Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander

Kimberlites from Source to Surface: Insights from Experiments

High-pressure experiments are unconvincing in explaining kimberlites as direct melts of carbonated peridotite because the appropriate minerals do not coexist stably at the kimberlite liquidus. High-pressure melts of peridotite with CO2 and H2O have compositions similar to kimberlites only at pressures where conditions are insufficiently oxidizing to stabilize CO2: they do not replicate the high K2O/Na2O of kimberlites. Kimberlite melts may begin their ascent at ≈300 km depth in reduced conditions as melts rich in MgO and SiO2 and poor in Na2O. These melts interact with modified, oxidized zones at the base of cratons where they gain CO2, CaO, H2O, and K2O and lose SiO2. Decreasing CO2 solubility at low pressures facilitates the incorporation of xenocrystic olivine, resulting in kimberlites' characteristically high MgO/CaO

Quantification of dissolved CO2 in silicate glasses using micro-Raman spectroscopy

International audienceThis study investigates the potential use of confocal micro-Raman spectroscopy for the quantification of CO2 in geologically relevant glass compositions. A calibration is developed using a wide range of both natural and synthetic glasses that have CO2 dissolved as carbonate (CO32−) in the concentration range from 0.2 to 16 wt%. Spectra were acquired in the 200 and 1350 cm−1 frequency region that includes the ν1 Raman active vibration for carbonate at 1062-1092 cm−1 and the intensity of this peak is compared to various other peaks representing the aluminosilicate glass structure. The most precise and accurate calibration is found when carbonate peaks are compared to aluminosilicate spectral features in the high-frequency region (HF: 700-1200 cm−1), which can be simulated with several Gaussian peaks, directly related to different structural species in the glass. In some samples the "dissolved" CO32− appears to have two different Raman bands, one sharper than the other. This may be consistent with previous suggestions that CO32− has several structural environments in the glass, and is not related to any precipitation of crystalline carbonate from the melt during quenching. The calibration derived using the HF peaks appears linear for both the full range of glass composition considered and the range of CO2 concentrations, even when multiple carbonate peaks are involved. We propose the following, compositionally independent linear equation to quantify the CO2 content in glass with micro-Raman spectroscopy Formula where CO3/HF is the area ratio of the fitted ν1 carbonate peak(s) at 1062-1092 cm−1 to the remaining area of the fitted aluminosilicate envelope from 700-1200 cm−1. This is similar to the Raman calibration developed for water, but is complicated by the overlapping of these two fitted components. Using error propagation, we propose the calibration accuracy is better than ±0.4 wt% CO2 for our data set. The ν1 Raman peak position for carbonate is not constant and appears to be correlated with the density of the melt (or glass) in some way rather than the chemical composition

Constraints on the depth and thermal history of cratonic lithosphere from peridotite xenoliths, xenocrysts and seismology

Despite the relatively long-standing availability of numerical approaches for estimating palaeogeotherms using peridotite xenolith Pressure–Temperature (P–T) data, the practise of fitting xenolith P–T arrays to simple models of lithospheric heat generation, in a non-quantitative manner, remains widespread. The lack of quantification in both the magnitude and uncertainty of heat flow and lithosphere thickness estimates leads to difficulty in evaluating proposed models for lithosphere evolution on a local and regional scale. Here, we explore the advantages of using a numerical approach to palaeogeotherm fitting, in terms of the ability to make objective comparisons of the effect that differing thermobarometer combinations and varying states of mineral and textural equilibrium have on the shape of the palaeogeotherm, and the resulting estimates of lithospheric thickness and heat flow. We also make quantitative comparisons between lithospheric mantle properties estimated using peridotite xenoliths versus single mineral xenocrysts. Using two reference peridotite xenolith databases from Bultfontein (S. Africa) and Somerset Island (Canada) we show that the same lithospheric mantle properties are predicted using harzburgite versus lherzolite thermobarometry methods. Filtering mineral data for the effects of inter-mineral disequilibrium does not produce significantly different palaeogeotherms but does increase the quality of fit of the palaeogeotherm to the P–T data, allowing more confidence to be placed in comparisons between locations. Palaeogeotherms calculated using xenocryst data, screened for peridotitic affinities, show misfits that are 2–3 times greater than those obtained using xenoliths. Lithospheric properties calculated from the Somerset Island xenocryst-based geotherm yield results that are within error of the xenolith estimate. A mutually consistent and quantitative palaeogeotherm fitting approach is used to evaluate existing hypotheses for the evolution of the southern African lithosphere. We find very similar estimates for the heat flow and thickness of the lithosphere between SW Namibia (off-craton) and Bultfontein (on-craton). This supports suggestions of a cratonic thermal regime and equivalent lithospheric thickness across that region of southern Africa at the time of kimberlite sampling, with concurrent local thermal disturbance evident in Namibia. Complimentary, novel, seismically-obtained geotherm estimates show that the lithosphere in Namibia is now significantly thinner than the estimate at 70 Ma obtained from xenolith thermobarometry

Age and evolution of the deep continental root beneath the central Rae craton, northern Canada

Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton. Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0–2.6 Ga overall; 2.8–2.6 Ga for Pelly Bay and 3.0–2.7 Ga for Repulse Bay), Paleoproterozoic (2.1–1.7 Ga), and “Recent” (<1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7–2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77–1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (Snyder et al., 2013, Snyder et al., 2015) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ∼200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (Snyder et al., 2013, Snyder et al., 2015, Spratt et al., 2014). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures

Studies in stoichiometry with compositional data

Geochemical data that is derived from the whole or partial analysis of various geologic materialsrepresent a composition of mineralogies or solute species. Minerals are composed of structuredrelationships between cations and anions which, through atomic and molecular forces, keep the elementsbound in specific configurations. The chemical compositions of minerals have specific relationships thatare governed by these molecular controls. In the case of olivine, there is a well-defined relationshipbetween Mn-Fe-Mg with Si. Balances between the principal elements defining olivine composition andother significant constituents in the composition (Al, Ti) have been defined, resulting in a near-linearrelationship between the logarithmic relative proportion of Si versus (MgMnFe) and Mg versus (MnFe),which is typically described but poorly illustrated in the simplex.The present contribution corresponds to ongoing research, which attempts to relate stoichiometry andgeochemical data using compositional geometry. We describe here the approach by which stoichiometricrelationships based on mineralogical constraints can be accounted for in the space of simplicialcoordinates using olivines as an example. Further examples for other mineral types (plagioclases andmore complex minerals such as clays) are needed. Issues that remain to be dealt with include thereduction of a bulk chemical composition of a rock comprised of several minerals from which appropriatebalances can be used to describe the composition in a realistic mineralogical framework. The overallobjective of our research is to answer the question: In the cases where the mineralogy is unknown, arethere suitable proxies that can be substituted?Kew words: Aitchison geometry, balances, mineral composition, oxidesGeologische Vereinigung; Institut d’Estadística de Catalunya; International Association for Mathematical Geology; Càtedra Lluís Santaló d’Aplicacions de la Matemàtica; Generalitat de Catalunya, Departament d’Innovació, Universitats i Recerca; Ministerio de Educación y Ciencia; Ingenio 2010