Abstract A new thermodynamic model for silicate melt in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–Fe2O3–Cr2O3 model system is presented, building on the tholeiitic through to granitic melt model of Holland et al. (2018) [Journal of Petrology, 59, 881–900] but extending for the first time into anhydrous alkaline systems. The new melt model is accompanied by new thermodynamic models for nepheline, kalsilite, leucite, melilite and ilmenite. Collectively these models enable pseudosection modelling of alkaline-silicate magmatic systems, providing a new tool for investigating this geologically- and economically-important compositional space. The models are calibrated with respect to experimental data on phase relations among minerals and melt, and the fit is benchmarked here via detailed comparison with seven experimental datasets, which encompass a range of pressure (0–22 kbar), temperature (680–1350°C), oxygen fugacity (log fO2 ΔFMQ-3 to +1), total alkali (3–16 wt%) and silica (37–70 wt%) conditions. The calculated pseudosections successfully reproduce experimental crystallisation sequences and phase compositions, indicating that the thermodynamic models are well calibrated across this spectrum of conditions. Redox buffered experimental conditions are simulated using oxygen buffered pseudosections. Contouring of oxygen buffered pseudosections with XFe3+ (mol. Fe3+ /Fetotal), or pseudosections of varying XFe3+ with ΔFMQ, reveals (i) often complex and non-intuitive relationships between these two representations of oxidation state, and (ii) substantial variation in ferric iron over narrow temperature intervals in some oxygen buffered sets of experiments. An implication is that simulating oxygen buffering is vital when benchmarking thermodynamic models using experimental results. Furthermore, because natural igneous systems likely feature a near-constant XFe3+, it is important to assess experimental results in this framework when making inferences about natural systems, recognising that oxygen fugacity is a consequence not a control of phase equilibria in nature. Overall, our new models provide a novel tool to explore the role of variables such as pressure, fractional crystallisation and crustal assimilation in the petrogenesis of alkaline-silicate magmatic systems and their associated mineralisation.
Recent studies of non-traditional stable isotope systems (e.g., Fe, Ni, Zn, Ti, Ca, Cr, V) have exploited variations in mineraland redox-specific equilibrium fractionation effects to link observed variations to source mineralogy and processes, such as partial melting, magmatic differentiation, and the tectonic recycling of surface material [e.g., 1-4].In this presentation, I will review some examples of how novel stable isotope systems, such as Fe, can be used to place constraints on the mineralogy and chemistry of the mantle source regions of ocean island basalts and Archean komatiites, and what the implications of these findings could be for the mineralogical and chemical evolution of the Earth's upper and lower mantle.I will also discuss recent studies exploiting quantitative combined phase equilibria and equilibrium melt isotope fractionation models [7][8] and the extent that these can be used to predict equilibrium stable isotope partitioning during upper mantle melting of enriched and depleted lithologies.
Geophysical analysis of the Earth’s lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core-mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep-seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep-seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in OIBs provides constraints on the structure of the Earth’s lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high-MgO basalts, that the isotopically enriched south-western region of the Galápagos mantle – assigned to melting of LLSVP material – displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north-central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.
Oceanic basalts show variation in their iron and magnesium isotope compositions. One hypothesis for the origin of this is source variation: radiogenic isotope and trace element abundance studies have long argued that the Earth's upper mantle is geochemically heterogeneous and that subducted crust is a major contributor to this diversity. In contrast, a recent hypothesis posits that stable isotopes record disequilibrium during melt transport and so provide novel insight into the melting process. In this study we investigate the first of these hypotheses, that source heterogeneity explains global Fe-Mg isotope systematics. We compile a global dataset of oceanic basalt Fe and Mg isotopes and complement this with new Fe-Mg isotope data from locations possessing some of the most extreme radiogenic isotope ratios for their setting: ocean island basalts from the Cook-Austral and Society islands and a Mid-Atlantic Ridge basalt. Despite both Fe and Mg isotope systems having the ability to trace recycled crustal material in the mantle, their global systematics are very different in this dataset. The global compilation of primitive oceanic basalts records heavier Fe (higher δ57Fe) isotope compositions than bulk silicate earth (BSE), but a mixture of heavier and lighter Mg isotope compositions than BSE. By employing a coupled Fe-Mg equilibrium isotope fractionation model during mantle melting we show that much of this isotopic variability can be generated by the mixed melts produced by melting of peridotite mantle containing moderate amounts of recycled crust as a discrete lithology. The Fe isotope composition of the melts is controlled by the bulk isotope composition of the recycled crust (expected to be considerably heavier than BSE, but variable). In contrast, the Mg isotope composition is controlled by source mineralogy. Olivine-poor lithologies such as recycled crust are able to generate large Mg isotope fractionations during melting, both positive and negative (± 0.1‰) relative to the mantle source, depending on the presence of spinel, clinopyroxene or garnet. These melt Mg isotope fractionations are consistent with the Mg isotope compositions of mid-ocean ridge basalts generated by variable depths of mantle melting. Our equilibrium model provides a baseline to test hypotheses of Fe-Mg isotope variability in basalts: our results show that contributions from recycled crust-derived melts, generated in spinel-, pyroxene-, and garnet-bearing mineral assemblages in the mantle, would be able to produce much of the Fe-Mg isotope variability seen in the global compilation of primitive oceanic basalts, without requiring isotopically extreme mantle components (e.g., carbonate with a light Mg isotope signature) or disequilibrium fractionation. However some basalt variability in ocean island settings may indeed fall outside the paradigm of pyroxenite heterogeneity – whilst we consider carbonates unlikely to be important, disequilibrium processes may in these cases play a role.
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