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• Chemical interactions at the core-mantle boundary (CMB) lead to oxygen transfer to the core. • Lower mantle temperatures may have been supersolidus after core formation, facilitating rapid oxygen transfer. • We assess this transfer using a new model of coupled core-magma ocean evolution. • Oxygen transfer following core formation can account for 10-50% of present-day core concentration. • FeO loss to the core hastens the complete crystallization of the magma ocean. Chemical interactions between metal and silicates at the core-mantle boundary (CMB) are now thought to lead to transfer of oxygen into Earth's liquid core. Establishing the nature and extent of this transfer is important for constraining the conditions under which the core formed, the origin of a stably stratified region below the CMB and the possible precipitation of oxides within the core. Previous models of FeO transfer have considered a solid mantle; however, several lines of evidence suggest that the lowermost mantle could have remained above its solidus long after core formation was complete, which would allow much faster mass transfer. We investigate this scenario by developing a time-dependent model of FeO exchange between a diffusive stratified layer at the top of the core and a long-lived molten magma ocean. Core FeO concentration, c ¯ F e O c , is evolved subject to a time-dependent mass flux at the CMB, radius r cmb , which depends on the FeO concentration at the bottom ( c ¯ F e O m (r cmb)) and top ( c ¯ F e O m (r bulk)) of the chemical boundary layer above the CMB. Coupled core-magma ocean evolution arises because c ¯ F e O m (r cmb) and c ¯ F e O c (r cmb) are linked through the partition coefficient P = c ¯ F e O c (r cmb) / c ¯ F e O m (r cmb). c ¯ F e O m (r bulk) is held constant in No Crystallization (NC) models and evolves in Middle-Out Crystallization (MOC) models according to the basal magma ocean model of Labrosse et al. (2007) , generalised to account for FeO loss to the core. In the first 1 Gyr, FeO transfer in all models with ≥10% FeO in the magma ocean and P ≥ 5 produces pure FeO compositions at the CMB, stably stratified layers of 60 − 80 km and accounts for 15 − 50 % of the total present-day core oxygen content. In NC models the magma ocean does not completely freeze in 4 Gyr, in which time the stable layer reaches 120 − 150 km and FeO transfer can account for all of the present-day O in the core. However, in MOC models FeO loss to the core causes the magma ocean to completely freeze in the first 1-3 Gyrs following core formation. Our results suggest that the present-day core composition may not provide a strong constraint on models of core formation and that FeO could have precipitated at the top of the core. [ABSTRACT FROM AUTHOR]

Highlights • Spectral methods are applied to products of dynamo simulations. • Dipole energy at the surface linked with magnetic energy in the core at long period. • Skewed magnetic dipole growth and decay linked to induction and dissipation. • Transition from low frequency steady state to higher frequency dynamo operation. • Variations in diffusion linked to changes in field strength rather than lengthscale. Abstract The geomagnetic field displays complicated variations over a broad range of frequencies. These variations can be decomposed by frequency and linked to physical processes using frequency domain spectral methods. These spectral methods are well developed but have not previously been applied to study the energy balance of geodynamo simulations. We illustrate their potential by analyzing output from numerical dynamo simulations that have previously been studied for their apparently Earth-like properties. We show that high coherence between variations in axial dipole energy at the outer boundary of the simulation and total magnetic energy within the fluid shell occur at frequencies below ∼0.1 kyr−1. This suggests that paleomagnetically-observable signals with periods exceeding 10 kyrs contain information about magnetic energy changes in the bulk core. We then use spectral analysis to investigate differences in the rate of growth and decay of the axial dipole field. This behaviour, characterised by rapid growth and slow decay, is observed when signals with frequencies higher than 0.03 kyr−1 have been filtered out. The origin of this asymmetric growth and decay is assessed using coherence spectra between rates of change in kinetic and magnetic energy, ohmic and viscous dissipation, and work done by the buoyancy and Lorentz forces. We show that asymmetry is associated with an imbalance between ohmic dissipation and work done by the Lorentz force; when changes in magnetic energy are more coherent with ohmic dissipation the field grows rapidly and decay slowly. Variations in Ohmic dissipation reflect changes in field strength in our models, while changes in viscous dissipation are associated with amplitude fluctuations of the large-scale flow that exists on millennial timescales. Our work shows that spectral analysis coupling observable and global products of the dynamo process can elucidate the physical origin of periodic processes occurring on timescales exceeding 10 kyrs. [ABSTRACT FROM AUTHOR]

The Pacific–Australian plate boundary in the South Island of New Zealand is a transpressive boundary through continental lithosphere consisting of multiple terranes that were amalgamated during previous periods of subduction and plate reorganization. The style and locus of deformation within the present-day plate boundary is controlled by the mechanical behaviour and distribution of these different lithospheric blocks. Geological studies are limited when it comes to illuminating lithospheric structure and rheology at depth. Imaging the 3-D seismic velocity and attenuation (1/ Q), with distributed local earthquakes, helps unravel regional structure and variations in strength, fractures and fluids. We determine the 3-D distribution of Qp and Qs, which show much more variation than seismic velocity (Vp), underlining the utility of Q (1/attenuation). The Haast schist belt, previously shown as ca. 25-km thick dry unit with moderate Vp and low Vp / Vs, is imaged with high Qs, and the highest Qs areas correlate with zones of higher grade schist. Below 25-km depth, the distribution of high Qp and Qs is markedly different from that of the overlying geological terranes. Both the strike and depth of the high Q regions indicate that they represent the subducted Hikurangi Plateau and its adjacent Cretaceous oceanic crust. The thickest part of the plateau, previously identified by Vp  > 8.5 km s−1 from seismic tomography and P -wave precursors and associated with an eclogite layer at the base of the plateau, also has the highest Q. This confirms that the strong plateau extends southwestwards as a narrow salient to the northern Fiordland subduction zone, where moderate- Q Eocene oceanic crust on the Australian plate is being subducted and bent to vertical. In the ductile crust, Q results suggest fluid saturation and elevated temperature conditions in the crustal root of the Southern Alps, and confirm that the shape of this crustal root is influenced by both the orientation and depth of the underlying plateau. Q also provides insight into the failed rifting that occurred in oceanic crust at the edges of the Hikurangi Plateau, with a region of relatively low Q at the on-land extension of the Bounty Trough and Canterbury Basin, at the narrowest part of the South Island. In the brittle crust above 10-km depth, low Q is related to regions of active recent seismicity that have high fracture density, with low Qs where fluids are present. In contrast, the locked Alpine Fault does not exhibit low Q in the brittle crust. [ABSTRACT FROM AUTHOR]