Reidar G. Trønnes

Deep mantle structure as a reference frame for movements in and on the Earth

Significance Since the Pangea supercontinent formed about 320 million years ago, plumes that sourced large igneous provinces and kimberlites have been derived from the edges of two stable thermochemical reservoirs at the core–mantle boundary. We test whether it is possible to maintain this remarkable surface-to-deep Earth correlation before Pangea through the development of a new plate reconstruction method and find that our reconstructions for the past 540 million years comply with known geological and tectonic constraints (opening and closure of oceans, mountain building, and more). These results have important implications for Earth history, including the style of mantle convection in the deep past and the long-term stability of mantle reservoirs. Earth’s residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core–mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth’s surface, correlate with the reconstructed positions of large igneous provinces and kimberlites since Pangea formed about 320 million years ago. Using this surface-to-core–mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander, we have developed a model for absolute plate motion back to earliest Paleozoic time (540 Ma). For the Paleozoic, we have identified six phases of slow, oscillatory true polar wander during which the Earth’s axis of minimum moment of inertia was similar to that of Mesozoic times. The rates of Paleozoic true polar wander (<1°/My) are compatible with those in the Mesozoic, but absolute plate velocities are, on average, twice as high. Our reconstructions generate geologically plausible scenarios, with large igneous provinces and kimberlites sourced from the margins of the large low shear-wave velocity provinces, as in Mesozoic and Cenozoic times. This absolute kinematic model suggests that a degree-2 convection mode within the Earth’s mantle may have operated throughout the entire Phanerozoic.

Recent volcanic rocks from Jan Mayen: Low‐degree melt fractions of enriched northeast Atlantic mantle

Isotopic and trace element analyses of recent alkaline volcanic rocks from Jan Mayen were used to characterize their mantle source. The samples, ranging in composition from ankaramites to trachytes, are isotopically homogeneous with 11 of 12 samples within the following ranges: 87 Sr/ 86 Sr 0.7035-0.7036, 143 Nd/ 144 Nd: 0.51285-0.51290, 206 Pb/ 204 Pb: 18.54-18.76, 207 Pb/ 204 Pb 15.45-15.49, and 208 Pb/ 204 Pb: 38.19-38.51. Geophysical data indicate that the Jan Mayen Ridge represents a microcontinent rifted off the east Greenland continental margin at 36 Ma. The isotopic homogeneity and poor correlation between the degree of magmatic differentiation and Sr-Nd isotopic composition show that the recent Jan Mayen magmas escaped significant contamination by Precambrian or Lower Paleozoic continental crust. A pervasive Tertiary to recent magmatic infrastructure may explain the lack of contamination with old crustal material. The Jan Mayen alkali basalts have similar trace element and isotopic composition to other oceanic plume basalts and are dominated by low-degree melts from an enriched mantle component prevalent in the NE Atlantic. The Icelandic alkali basalts are derived from similar sources but are more diluted with picritic or tholeiitic melt fractions formed by progressive melting at shallower levels. Minimum dilution of the incipient low-degree melts occurs when the melting column is truncated by a thick lithosphere. The mantle source sampled by the Jan Mayen and other alkaline volcanic systems in the NE Atlantic is strongly enriched in high field strength elements like Nb, has a relatively high μ value, and has 207 Pb/ 206 Pb ratios below and 208 Pb/ 206 Pb ratios above the Northern Hemisphere Reference Line. Such a Pb isotopic composition fits well with an old mantle domain with a time-integrated low μ value, enriched by a Paleozoic high μ (HIMU) component. The young HIMU-component in the NE Atlantic upper mantle could represent recycled oceanic crust entrained in the ancestral Iceland plume and distributed laterally by the ancestral plume head.

Continental crust beneath southeast Iceland

Significance The Iceland hotspot is widely thought to be the surface expression of a deep mantle plume from the core–mantle boundary that can be traced back in time at least 62 My. However, some lavas contain continental material, which has previously been proposed to have been recycled through the plume. Here, we argue that the plume split off a sliver of continent from Greenland in the Early Eocene. This sliver is now located beneath southeast Iceland where it locally contaminates some of the plume-derived magmas. The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland—and especially the Öræfajökull volcano—is characterized by a unique enriched-mantle component (EM2-like) with elevated 87Sr/86Sr and 207Pb/204Pb. Here, we demonstrate through modeling of Sr–Nd–Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2–6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume.

Partitioning of platinum group elements in the Fe‐O‐S System to 11 GPa and their fractionation in the mantle and meteorites

The partitioning of minor amounts of platinum group elements (PGE: Ru, Rh, Pd, Os, Ir, Pt) in the Fe-O-S system has been investigated to 11 GPa. The marked fractionation of Pt from Pd in the presence of coexisting alloy and sulfide liquid, reported earlier for the Fe-Ni-S system at low pressure, is extended to high pressure. The experiments were carried out at 1000°–1375°C and 4.5–11 GPa with graphite capsules using 18-mm octahedral pressure cells in a uniaxial split-sphere multianvil apparatus. Oxygen fugacities corresponded to wustite stability. Phases present included sulfide liquid (close to M3S2 stoichiometry), FeS (troilite, pyrrhotite, HPP), alloys (PtFe, PtFeIr, OsIr, FePtIrOs, OsFeIr), and wustite. Partition coefficients for OsIr alloy/Fe1−x S at 1200°C, 11 GPa are Os (1400), Ir (160), Pt (32), Ru (0.6), Rh (0.2), and Pd (0.04) and for FePtOs alloy/liquid at 1100°C, 8.5 GPa are Os (120), Ir (82), Pt (20), Ru (5.0), Rh (1.6), and Pd (0.1). Geochemical processes which involve separation of a metal-rich phase from sulfide therefore would result in fractionation of PGE, with Os, Ir, and Pt concentrated in the former phase and Pd in the latter. These results are further support for the presence of a discrete metal-sulfide liquid during the primary differentiation of planetary material and preclude an explanation for the fractionation of PGE in chromitites and mafic crustal rocks based on equilibrium partitioning between refractory alloys in residual assemblages and immiscible sulfide liquid in silicate melts.

Equations of state of CaIrO3 perovskite and post-perovskite phases

Abstract Unit-cell lattice parameters have been measured to ~8 GPa in a diamond anvil cell for two single crystals of CaIrO3: one with the perovskite (Pbnm) and the other with the post-perovskite (Cmcm) structure. The CaIrO3 post-perovskite structure is more compressible than the perovskite. A third-order Birch Murnaghan equation of state has been used to fit the measured P-V data with the following refined parameters: V0 = 229.463(8) Å3, K0 = 198(3) GPa, K’ = 1.2(8); and V0 = 226.38(1) Å3, K0 = 181(3) GPa, K’ = 2.3(8) for CaIrO3 perovskite and post-perovskite, respectively. The compressibility of the unit-cell axes of the perovskite structure is highly anisotropic with βa >> βc >> βb. In contrast, the b axis is the most compressible in the post-perovskite structure, whereas the a and c axes have similar compressibilities (with c slightly less compressible than a) and are much stiffer. A comparison between the compressibility of CaIrO3 perovskite and post-perovskite with the isostructural MgSiO3 phases, reveals a similar general behavior, although in detail CaIrO3 perovskite is more and the postperovskite less anisotropic than the corresponding MgSiO3 compounds.

Phase diagram and P-V-T equation of state of Al-bearing seifertite at lowermost mantle conditions

Abstract We investigated the properties of Al-bearing SiO2 (with 4 or 6 wt% Al2O3) at pressures and temperatures corresponding to the lowermost mantle, using laser-heated diamond-anvil cell coupled with synchrotron-based in situ X-ray diffraction. The phase transition from CaCl2-structured to α-PbO2- structured (seifertite) polymorphs occurs between 113 and 119 GPa at 2500 K. The range of pressure where the two phases coexist is small. There is a slight decrease of the transition pressure with increasing Al-content. We propose a tentative phase diagram reporting the minerals composition as a function of pressure in the SiO2-Al2O3 system. We also refine the P-V-T equation of state of Al-bearing seifertite based on volume measurements up to more than 160 GPa and 4000 K [V0 = 92.73(10) Å3, K0 = 304.2(3.0) GPa, K′0 = 4.59 (fixed), ΘD0 = 1130 K (fixed), γ0 = 1.61(3)]. At 300 K, the volume decrease at the CaCl2 to α-PbO2 transition is 0.5(1)%, a value slightly lower than the 0.6% reported previously for Al-free samples. At high temperature, the Grüneisen parameter of seifertite is found to be similar to that of stishovite. Nevertheless, the ΔV/V across the CaCl2-form to seifertite transition is found to increase slightly with increasing temperature. Across the phase transition, volume changes can be translated into density changes only when the Al substitution mechanisms in both CaCl2-form and seifertite are defined. The analysis of all available data sets suggests different substitution mechanisms for the two SiO2 polymorphs. Al-substitution could occur via O-vacancies in the CaCl2-form and via extra interstitial Al in seifertite. This would result in a density increase of 2.2(3)% at 300 K for SiO2 in basaltic lithologies. Alternatively, the same Alsubstitution mechanism in both of the SiO2-dominated phases would yield a density increase of 0.5(1)%.

Evidence for slab material under Greenland and links to Cretaceous High Arctic magmatism

Understanding the evolution of extinct ocean basins through time and space demands the integration of surface kinematics and mantle dynamics. We explore the existence, origin, and implications of a proposed oceanic slab burial ground under Greenland through a comparison of seismic tomography, slab sinking rates, regional plate reconstructions, and satellite-derived gravity gradients. Our preferred interpretation stipulates that anomalous, fast seismic velocities at 1000-1600 km depth imaged in independent global tomographic models, coupled with gravity gradient perturbations, represent paleo-Arctic oceanic slabs that subducted in the Mesozoic. We suggest a novel connection between slab-related arc mantle and geochemical signatures in some of the tholeiitic and mildly alkaline magmas of the Cretaceous High Arctic Large Igneous Province in the Sverdrup Basin. However, continental crustal contributions are noted in these evolved basaltic rocks. The integration of independent, yet complementary, data sets provides insight into present-day mantle structure, magmatic events, and relict oceans.

Stabilizing Effect of Compositional Viscosity Contrasts on Thermochemical Piles

The large low shear velocity provinces observed in the lowermost mantle are widely accepted as chemically distinct thermochemical “piles,” but their origin and long-term evolution remain poorly understood. The survival time and shape of the large low shear velocity provinces are thought to be mainly controlled by their compositional density, while their viscosity has been considered less important. Based on recent constraints on chemical reactions between mantle and core, a more complex viscosity structure of the lowermost mantle, possibly including high viscosity thermochemical pile material, seems reasonable. In this study, we use numerical models to identify a trade-off between compositional viscosity and density contrasts required for long-term stability of thermochemical piles, which permits lower-density and higher-viscosity piles. Moreover, our results indicate more restrictive stability conditions during periods of strong deformation-induced entrainment, for example, during initial pile formation, which suggests long-term pile survival. Plain Language Summary Seismic images of the Earth’s mantle show two anomalous continent-sized regions close to the core-mantle boundary. The inferred properties of these regions suggest that they have a different composition than the surrounding mantle. Two possible candidate materials have been proposed: accumulated oceanic crust from the Earth’s surface or an iron-rich residue remaining from Earth’s original magma ocean. Although both materials are denser than the surrounding mantle, it remains unclear whether piles of these chemical heterogeneities can survive at the core-mantle boundary beneath vigorous mantle convection. Numerical models show that the excess density required to preserve these structures is typically larger than indicated by seismological and gravitational observations. In this study, we show that the excess density used in numerical models can be reduced toward the observed values if the pile material is also stiffer than the surrounding mantle. Furthermore, we show that piles must be denser and/or stiffer to avoid destruction during episodes of strong deformation. Because pile formation probably includes vigorous deformation, we expect long-term survival of the piles after their formation is completed.