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Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites
No full textDusty olivine (olivine containing multiple sub-micrometer inclusions of metallic iron) in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence, capable of maintaining a faithful record of pre-accretionary magnetization acquired during chondrule formation. Here we show how the magnetic architecture of a single dusty olivine grain from the Semarkona LL3.0 ordinary chondrite meteorite can be fully characterized in three dimensions, using a combination of focused ion beam nanotomography (FIB-nT), electron tomography, and finite-element micromagnetic modeling. We present a three-dimensional (3D) volume reconstruction of a dusty olivine grain, obtained by selective milling through a region of interest in a series of sequential 20 nm slices, which are then imaged using scanning electron microscopy. The data provide a quantitative description of the iron particle ensemble, including the distribution of particle sizes, shapes, interparticle spacings and orientations. Iron particles are predominantly oblate ellipsoids with average radii 242 ± 94 × 199 ± 80 × 123 ± 58 nm. Using analytical TEM we observe that the particles nucleate on sub-grain boundaries and are loosely arranged in a series of sheets parallel to (001) of the olivine host. This is in agreement with the orientation data collected using the FIB-nT and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host. The shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets. Individual particle geometries are converted to a finite-element mesh and used to perform micromagnetic simulations. The majority of particles adopt a single vortex state, with “bulk” spins that rotate around a central vortex core. We observed no particles that are in a true single domain state. The results of the micromagnetic simulations challenge some preconceived ideas about the remanence-carrying properties of vortex states. There is often not a simple predictive relationship between the major, intermediate, and minor axes of the particles and the remanence vector imparted in different fields. Although the orientation of the vortex core is determined largely by the ellipsoidal geometry (i.e., parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids), the core and remanence vectors can sometimes lie at very large (tens of degrees) angles to the principal axes. The subtle details of the morphology can control the overall remanence state, leading in some cases to a dominant contribution from the bulk spins to the net remanence, with profound implications for predicting the anisotropy of the sample. The particles have very high switching fields (several hundred millitesla), demonstrating their high stability and suitability for paleointensity studies
Cambrian suspension-feeding tubicolous hemichordates
No full textThe combination of a meager fossil record of vermiform enteropneusts and their disparity with the tubicolous pterobranchs renders early hemichordate evolution conjectural. The middle Cambrian Oesia disjuncta from the Burgess Shale has been compared to annelids, tunicates and chaetognaths, but on the basis of abundant new material is now identified as a primitive hemichordate
The magmatic system beneath the Tristan da Cunha Island: Insights from thermobarometry, melting models and geophysics
No full textThis study provides new insights on the conditions of melt generation and of magma transport and storage beneath Tristan da Cunha Island in the South Atlantic. Situated at the seaward end of the Walvis Ridge-guyot hotspot track, this island is related to the evolving magmatic system of the Tristan plume. Much is known about the geochemical and isotopic composition of the alkaline lavas on Tristan, but the pressure-temperature conditions of the hotspot magmas are under-explored. This contribution reports new data from a suite of 10 samples collected during a geologic-geophysical expedition in 2012. The focus of this study is on the least-evolved, phenocryst-rich basanite lavas but we also included a sample of trachyandesite lava erupted in 1961. Mineral-melt equilibrium thermobarometry uses the composition of olivine, clinopyroxene and plagioclase phenocrysts. In addition to bulk-rock data we also analysed olivine-hosted melt inclusions for the P-T calculations. The results for olivine-melt and clinopyroxene-melt calculations suggest crystallization conditions of around 1200-1250. °C and 0.8-1.3. GPa for the least-evolved magmas (ankaramitic basanites). Combined with seismological evidence for a Moho depth of about 19. km, these results imply magma storage and partial crystallization of Tristan magmas in the uppermost mantle and at Moho level. The trachyandesite yielded values of about 1000. °C and 0.2-0.3. GPa (6 to 10. km depth), indicating further crystallization within the crust.Constraints on the depth and degree of melting at the source of Tristan basanites were derived from REE inverse modelling using our new trace element data. The model predicts 5% melt generation from a melting column with its base at 80-100. km and a top at 60. km, which is consistent with the lithospheric thickness resulting from cooling models and seismological observations. The thermobarometry and melting models combined suggest a mantle potential temperature of about 1360. °C for the Tristan hotspot
Local artifacts in ice core methane records caused by layered bubble trapping and in situ production: a multi-site investigation
No full textAbstract. Advances in trace gas analysis allow localised, non-atmospheric features to be resolved in ice cores, superimposed on the coherent atmospheric signal. These high-frequency signals could not have survived the low-pass filter effect that gas diffusion in the firn exerts on the atmospheric history and therefore do not result from changes in the atmospheric composition at the ice sheet surface. Using continuous methane (CH4) records obtained from five polar ice cores, we characterise these non-atmospheric signals and explore their origin. Isolated samples, enriched in CH4 in the Tunu13 (Greenland) record are linked to the presence of melt layers. Melting can enrich the methane concentration due to a solubility effect, but we find that an additional in situ process is required to generate the full magnitude of these anomalies. Furthermore, in all the ice cores studied there is evidence of reproducible, decimetre-scale CH4 variability. Through a series of tests, we demonstrate that this is an artifact of layered bubble trapping in a heterogeneous-density firn column; we use the term “trapping signal” for this phenomenon. The peak-to-peak amplitude of the trapping signal is typically 5 ppb, but may exceed 40 ppb. Signal magnitude increases with atmospheric CH4 growth rate and seasonal density contrast, and decreases with accumulation rate. Significant annual periodicity is present in the CH4 variability of two Greenland ice cores, suggesting that layered gas trapping at these sites is controlled by regular, seasonal variations in the physical properties of the firn. Future analytical campaigns should anticipate high-frequency artifacts at high-melt ice core sites or during time periods with high atmospheric CH4 growth rate in order to avoid misinterpretation of such features as past changes in atmospheric composition
Age of the Laschamp excursion determined by U-Th dating of a speleothem geomagnetic record from North America
No full textThe Laschamp geomagnetic excursion was the first short-lived polarity event recognized and described in the paleomagnetic record, and to date remains the most studied geomagnetic event of its kind. In addition to its geophysical significance, the Laschamp is an important global geochronologic marker. The Laschamp excursion occurred around the time of the demise of Homo neanderthalensis, in conjunction with high-amplitude, rapid climatic oscillations leading into the Last Glacial Maximum, and coeval with a major supervolcano eruption in the Mediterranean. Thus, precise determination of the timing and duration of the Laschamp excursion would help in elucidating major scientific questions situated at the intersection of geology, paleoclimatology, and anthropology. Here we present a North American speleothem geomagnetic record of the Laschamp excursion that is directly dated using a combination of high-precision 230Th dates and annual layer counting using confocal microscopy. We have determined a maximum excursion duration that spans the interval 42,250-39,700 yr BP, and an age of 41,100 ± 350 yr BP for the main phase of the excursion, during which the virtual geomagnetic pole was situated at the southernmost latitude in the record. Our chronology provides the first age bracketing of the Laschamp excursion using radioisotopic dating, and improves on previous age determinations based on 40Ar/39Ar dating of lava flows, and orbitally-tuned sedimentary and ice-core records
Zinc isotope evidence for sulfate-rich fluid transfer across subduction zones
No full textSubduction zones modulate the chemical evolution of the Earth’s mantle. Water and
volatile elements in the slab are released as fluids into the mantle wedge and this process
is widely considered to result in the oxidation of the sub-arc mantle. However, the
chemical composition and speciation of these fluids, which is critical for the mobility of
economically important elements, remain poorly constrained. Sulfur has the potential to
act both as oxidizing agent and transport medium. Here we use zinc stable isotopes
( δ 66 Zn) in subducted Alpine serpentinites to decipher the chemical properties of slab-
derived fluids. We show that the progressive decrease in δ 66Zn with metamorphic grade is correlated with a decrease in sulfur content. As existing theoretical work predicts that
Zn-SO42- complexes preferentially incorporate heavy δ 66Zn, our results provide strong
evidence for the release of oxidized, sulfate-rich, slab serpentinite-derived fluids to the
mantle wedge
Strike-slip Faulting during the 2014 Bárðarbunga-Holuhraun Dike Intrusion, Central Iceland.
No full textOver a 13 day period magma propagated laterally from the sub-glacial Bárðarbunga volcano in the northern rift zone, Iceland. It created > 30,000 earthquakes at 5–7 km depth along a 48 km path before erupting on 29 August 2014. The seismicity, which tracked the dike propagation, advanced in short bursts at 0.3–4.7 km/h separated by pauses of up to 81 hours. During each surge forward, seismicity behind the dike tip dropped. Moment tensor solutions from the leading edge show exclusively left-lateral strike-slip faulting sub-parallel to the advancing dike tip, releasing accumulated strain deficit in the brittle layer of the rift zone. Behind the leading edge, both left- and right-lateral strike-slip earthquakes are observed. The lack of non double-couple earthquakes implies that the dike opening was aseismic
Evolution and function of anterior cervical vertebral fusion in tetrapods
No full textThe evolution of vertebral fusion is a poorly understood phenomenon that results in the loss of mobility between sequential vertebrae. Non-pathological fusion of the anterior cervical vertebrae has evolved independently in numerous extant and extinct mammals and reptiles, suggesting that the formation of a ‘syncervical’ is an adaptation that arose to confer biomechanical advantage(s) in these lineages. We review syncervical anatomy and evolution in a broad phylogenetic context for the first time and provide a comprehensive summary of proposed adaptive hypotheses. The syncervical generally consists of two vertebrae (e.g. hornbills, porcupines, dolphins) but can include fusion of seven cervical vertebrae in some cetaceans. Based on the ecologies of taxa with this trait, cervical fusion most often occurs in fossorial and pelagic taxa. In fossorial taxa, the syncervical likely increases the out-lever force during head-lift digging. In cetaceans and ricochetal rodents, the syncervical may stabilize the head and neck during locomotion, although considerable variation exists in its composition without apparent variability in locomotion. Alternatively, the highly reduced cervical vertebral centra may require fusion to prevent mechanical failure of the vertebrae. In birds, the syncervical of hornbills may have evolved in response to their unique casque-butting behaviour, or due to increased head mass. The general correlation between ecological traits and the presence of a syncervical in extant taxa allows more accurate interpretation of extinct animals that also exhibit this unique trait. For example, syncervicals evolved independently in several groups of marine reptiles and may have functioned to stabilize the head at the craniocervical joint during pelagic locomotion, as in cetaceans. Overall, the origin and function of fused cervical vertebrae is poorly understood, emphasizing the need for future comparative biomechanical studies interpreted in an evolutionary context
An X-ray magnetic circular dichroism (XMCD) study of Fe ordering in a synthetic MgAl₂O₄ – Fe₃O₄ (spinel – magnetite) solid solution series; implications for magnetic properties.
Get PDFFe L₂,₃-edge XAS and XMCD studies have been used to unravel structural trends in the MgAl₂O₄-Fe₃O₄ solid solution where thermodynamic modelling has presented a challenge due to the complex ordering arrangements of the end-members. Partitioning of Fe³⁺ and Fe²⁺ between tetrahedral (Td) and octahedral (Oh) sites has been established. In the most Fe-rich samples, despite rapid quenching from a disordered state, Fe²⁺_Td is not present, which matches the ordered, inverse spinel nature of end-member magnetite (Mgt) at room-T. However, in intermediate compositions Al and Mg substantially replace Fe and small amounts of Fe²⁺_Td are found, stabilized or trapped by decreasing occurrence of the continuous nearest neighbour Fe – Fe interactions which facilitate charge redistribution by electron transfer. Furthermore, in the composition range ~Mgt₀.₄₋₀.₉, XAS and XMCD bonding and site occupancy data suggest that nano- scale, magnetite-like Fe clusters are present. By contrast, at the spinel-rich end of the series, Mgt₀.₁₇and Mgt₀.₂₃ have a homogeneous long-range distribution of Fe, Mg and Al. These relationships are consistent with the intermediate and Fe-rich samples falling within a wide solvus in this system such that the Fe- clusters occur as proto-nuclei for phases which would exsolve following development of long-range crystalline order during slow cooling.
Unit cell edges calculated from the spectroscopy-derived site occupancies show excellent agreement with those measured by X-ray powder diffraction on the bulk samples. Calculated saturation magnetic moments (M_s) for the Fe-rich samples also show excellent agreement with measured values but for the most Mg-rich samples are displaced to slightly higher values; this displacement is due to the presence of abundant Mg and Al disrupting the anti-parallel alignment of electron spins for Fe atoms
Estimates of fault strength from the Variscan foreland of the northern UK
No full textWe provide new insights into the long-standing debate regarding fault strength, by studying structures active in the late Carboniferous in the foreland of the Variscan Mountain range in the northern UK. We describe a method to estimate the seismogenic thickness for ancient deformation zones, at the time they were active, based upon the geometry of fault-bounded extensional basins. We then perform calculations to estimate the forces exerted between mountain ranges and their adjacent lowlands in the presence of thermal and compositional effects on the density. We combine these methods to calculate an upper bound on the stresses that could be supported by faults in the Variscan foreland before they began to slip. We find the faults had a low effective coefficient of friction (i.e. 0.02–0.24), and that the reactivated pre-existing faults were at least 30% weaker than unfaulted rock. These results show structural inheritance to be important, and suggest that the faults had a low intrinsic coefficient of friction, high pore-fluid pressures, or both