The relationship between mantle potential temperature and oceanic lithosphere buoyancy

The Earth's mantle potential temperature (Tp) is thought to have cooled by ~250 °C since the Archean, causing a progressive change in both the structure and composition of oceanic lithosphere. These variables affect the negative buoyancy of subducting slabs, which is known to be an important force in driving plate motions. However, the relationship between Tp and slab buoyancy remains unclear. Here, we model the formation and subduction of oceanic lithosphere as a function of Tp, to investigate how Tp influences the buoyancy of subducting slabs, and by extension how buoyancy forces may have changed through time. First, we simulate isentropic melting of peridotite at mid-ocean ridges over a range of Tp (1300-1550 °C) to calculate oceanic lithosphere structure and composition. Second, we model the thermal evolution of oceanic plates undergoing subduction for a variety of scenarios (by varying lithospheric thickness, slab length and subduction velocity). Finally, we integrate the structural, compositional and thermal constraints to forward model subduction metamorphism of oceanic plates to determine down-going slab density structures. When compared with ambient mantle, these models allow us to calculate buoyancy forces acting on subducting slabs. Our results indicate that oceanic lithosphere derived from hotter mantle has a greater negative buoyancy, and therefore subduction potential, than lithosphere derived from cooler mantle for a wide range of subduction scenarios. With respect to the early Earth, this conclusion supports the viability of subduction, and models of subduction zone initiation that invoke the concept of oceanic lithosphere being primed to subduct. However, we also show that decreases to lithosphere thickness and slab length, and reduced crustal hydration, progressively reduce slab negative buoyancy. These results highlight the need for robust estimates of early Earth lithospheric properties when considering whether subduction was operative at this time. Nevertheless, our findings suggest that subduction processes on the early Earth may have been uniformitarian

Quantitative elemental mapping of granulite-facies monazite: Textural insights and implications for petrochronology

Texturally complex monazite grains contained in two granulite-facies pelitic migmatites from southern Baffin Island, Arctic Canada, were mapped by laser ablation-inductively coupled plasma-mass spectrometry (using spot sizes < 5 µm) to quantitatively determine the spatial variation in trace element chemistry (with up to 1883 analyses per grain). The maps highlight the chemical complexity of monazite grains that have experienced multiple episodes of growth, resorption and chemical modification by dissolution-precipitation during high-grade metamorphism. Following detailed chemical characterisation of monazite compositional zones, a related U-Pb dataset is re-interpreted, allowing petrologically-significant ages to be extracted from a continuum of concordant data. Synthesis of these data with pseudosection modelling of prograde and peak conditions allows for the temporal evolution of monazite trace element chemistry to be placed in the context of the evolving P-T conditions and major phase assemblage. This approach enables a critical evaluation of three commonly used petrochronological indicators: linking Y to garnet abundance, the Eu anomaly to feldspar content, and Th/U to anatectic processes. Europium anomalies and Th/U behave in a relatively systematic fashion, suggesting that they are reliable petrochronological witnesses. However, Y systematics are variable, both within domains interpreted to have grown in a single event, between grains interpreted to be part of the same age population, and between samples that experienced similar metamorphic conditions and mineral assemblages. These observations caution against generalised petrological interpretations on the basis of Y content, as it suggests Y concentrations in monazite are controlled by domainal equilibria. The results reveal a ~45 Myr interval between prograde metamorphism and retrograde melt crystallisation in the study area, emphasising the long-lived nature of heat flow in high-grade metamorphic terranes. Such long timescales of metamorphism would be assisted by the growth, retention and dominance of high-Th suprasolidus monazite, as observed in this study, contributing to the radiogenic heating budget of mid- to lower-crustal environments. Careful characterisation of monazite grains suggests that continuum-style U-Pb datasets can be decoded to provide insights into the duration of metamorphic processes.Natural Sciences and Engineering Research Council Visiting Fellowship Isaac Newton Trust Research Grant (RG74916

Estimating the carbon content of the deep mantle with Icelandic melt inclusions

Earth’s carbon budget is central to our understanding of the long-term co-evolution of life and the planet. Direct observations of surface reservoirs allow for the detailed quantification of their carbon content. However, the carbon content of Earth’s deep interior remains poorly constrained. Here we study olivine-hosted melt inclusions from two Icelandic eruptions, with those from the Miðfell eruption allowing us to investigate the carbon content of the deep mantle. Comparison with the previously studied Borgarhraun eruption highlights the presence of deep, plume-sourced mantle material within the Miðfell source region. Miðfell contains trace element-depleted melt inclusions undersaturated in CO2, which have high CO2/Ba (= 396 ± 48) and CO2/Nb (= 1832 ± 316), though some inclusions preserve even greater relative carbon enrichment. These observations allow us to reconstruct the CO2 content of the bulk Miðfell source as being > 690 ppm. By identifying that Miðfell is a mixture of depleted and deep mantle components, we can estimate a CO2 content for the deep mantle component of 1350 ± 350 ppm; a concentration that is over ten times higher than depleted MORB mantle estimates. Assuming that the deep mantle component identified in Miðfell is representative of a global reservoir, then with our new CO2 estimate and by considering a range of representative mantle fractions for this reservoir, we calculate that it contains up to 14 times more carbon than that of the atmosphere, oceans, and crust combined. Our result of elevated CO2/Ba and CO2/Nb ratios, and carbon enrichment support geochemical bulk Earth carbon models that call for the presence of carbon-rich deep mantle domains to balance Earth’s relatively carbon-poor upper mantle and surface environment

Comparable Ages for the Independent Origins of Electrogenesis in African and South American Weakly Electric Fishes

One of the most remarkable examples of convergent evolution among vertebrates is illustrated by the independent origins of an active electric sense in South American and African weakly electric fishes, the Gymnotiformes and Mormyroidea, respectively. These groups independently evolved similar complex systems for object localization and communication via the generation and reception of weak electric fields. While good estimates of divergence times are critical to understanding the temporal context for the evolution and diversification of these two groups, their respective ages have been difficult to estimate due to the absence of an informative fossil record, use of strict molecular clock models in previous studies, and/or incomplete taxonomic sampling. Here, we examine the timing of the origins of the Gymnotiformes and the Mormyroidea using complete mitogenome sequences and a parametric Bayesian method for divergence time reconstruction. Under two different fossil-based calibration methods, we estimated similar ages for the independent origins of the Mormyroidea and Gymnotiformes. Our absolute estimates for the origins of these groups either slightly postdate, or just predate, the final separation of Africa and South America by continental drift. The most recent common ancestor of the Mormyroidea and Gymnotiformes was found to be a non-electrogenic basal teleost living more than 85 millions years earlier. For both electric fish lineages, we also estimated similar intervals (16–19 or 22–26 million years, depending on calibration method) between the appearance of electroreception and the origin of myogenic electric organs, providing rough upper estimates for the time periods during which these complex electric organs evolved de novo from skeletal muscle precursors. The fact that the Gymnotiformes and Mormyroidea are of similar age enhances the comparative value of the weakly electric fish system for investigating pathways to evolutionary novelty, as well as the influences of key innovations in communication on the process of species radiation

The relationship between mantle potential temperature and oceanic lithosphere buoyancy

The Earth's mantle potential temperature (TP)is thought to have cooled by ∼250 ∘C since the Archean, causing a progressive change in both the structure and composition of oceanic lithosphere. These variables affect the negative buoyancy of subducting slabs, which is known to be an important force in driving plate motions. However, the relationship between TP and slab buoyancy remains unclear. Here, we model the formation and subduction of oceanic lithosphere as a function of TP, to investigate how TP influences the buoyancy of subducting slabs, and by extension how buoyancy forces may have changed through time. First, we simulate isentropic melting of peridotite at mid-ocean ridges over a range of TP (1300–1550 ∘C)to calculate oceanic lithosphere structure and composition. Second, we model the thermal evolution of oceanic plates undergoing subduction for a variety of scenarios (by varying lithospheric thickness, slab length and subduction velocity). Finally, we integrate the structural, compositional and thermal constraints to forward model subduction metamorphism of oceanic plates to determine down-going slab density structures. When compared with ambient mantle, these models allow us to calculate buoyancy forces acting on subducting slabs. Our results indicate that oceanic lithosphere derived from hotter mantle has a greater negative buoyancy, and therefore subduction potential, than lithosphere derived from cooler mantle for a wide range of subduction scenarios. With respect to the early Earth, this conclusion supports the viability of subduction, and models of subduction zone initiation that invoke the concept of oceanic lithosphere being primed to subduct. However, we also show that decreases to lithosphere thickness and slab length, and reduced crustal hydration, progressively reduce slab negative buoyancy. These results highlight the need for robust estimates of early Earth lithospheric properties when considering whether subduction was operative at this time. Nevertheless, our findings suggest that subduction processes on the early Earth may have been uniformitarian

The relationship between mantle potential temperature and oceanic lithosphere buoyancy

The Earth's mantle potential temperature (TP)is thought to have cooled by ∼250 ∘C since the Archean, causing a progressive change in both the structure and composition of oceanic lithosphere. These variables affect the negative buoyancy of subducting slabs, which is known to be an important force in driving plate motions. However, the relationship between TP and slab buoyancy remains unclear. Here, we model the formation and subduction of oceanic lithosphere as a function of TP, to investigate how TP influences the buoyancy of subducting slabs, and by extension how buoyancy forces may have changed through time. First, we simulate isentropic melting of peridotite at mid-ocean ridges over a range of TP (1300–1550 ∘C)to calculate oceanic lithosphere structure and composition. Second, we model the thermal evolution of oceanic plates undergoing subduction for a variety of scenarios (by varying lithospheric thickness, slab length and subduction velocity). Finally, we integrate the structural, compositional and thermal constraints to forward model subduction metamorphism of oceanic plates to determine down-going slab density structures. When compared with ambient mantle, these models allow us to calculate buoyancy forces acting on subducting slabs. Our results indicate that oceanic lithosphere derived from hotter mantle has a greater negative buoyancy, and therefore subduction potential, than lithosphere derived from cooler mantle for a wide range of subduction scenarios. With respect to the early Earth, this conclusion supports the viability of subduction, and models of subduction zone initiation that invoke the concept of oceanic lithosphere being primed to subduct. However, we also show that decreases to lithosphere thickness and slab length, and reduced crustal hydration, progressively reduce slab negative buoyancy. These results highlight the need for robust estimates of early Earth lithospheric properties when considering whether subduction was operative at this time. Nevertheless, our findings suggest that subduction processes on the early Earth may have been uniformitarian