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  • Journal article
    Shah J, Williams W, Almeida TP, Nagy L, Muxworthy AR, Kovacs A, Valdez-Grijalva MA, Fabian K, Russell SS, Genge M, Dunin-Borkowski REet al., 2018,

    The oldest magnetic record in our solar system identified using nanometric imaging and numerical modeling

    , Nature Communications, Vol: 9, ISSN: 2041-1723

    Recordings of magnetic fields, thought to be crucial to our Solar System’s rapid accretion, are potentially retained in unaltered nanometric low-Ni kamacite (~metallic Fe) grains encased within dusty olivine crystals, found in the chondrules of unequilibrated chondrites. However, most of these kamacite grains are magnetically non-uniform, so their ability to retain four-billion-year-old magnetic recordings cannot be estimated by previous theories, which assume only uniform magnetization. Here, we demonstrate that non-uniformly magnetized nanometric kamacite grains are stable over Solar System timescales and likely the primary carrier of remanence in dusty olivine. By performing in-situ temperature-dependent nanometric magnetic measurements using off-axis electron holography, we demonstrate the thermal stability of multi-vortex kamacite grains from the chondritic Bishunpur meteorite. Combined with numerical micromagnetic modeling, we determine the stability of the magnetization of these grains. Our study shows that dusty olivine kamacite grains are capable of retaining magnetic recordings from the accreting Solar System.

  • Journal article
    Davison TM, Derrick JG, Collins GS, Bland PA, Rutherford ME, Chapman DJ, Eakins DEet al., 2017,

    Impact-induced compaction of primitive solar system solids: The need for mesoscale modelling and experiments

    , Procedia Engineering, Vol: 204, Pages: 405-412, ISSN: 1877-7058

    Primitive solar system solids were accreted as highly porous bimodal mixtures of mm-sized chondrules and sub-μm matrix grains. To understand the compaction and lithification of these materials by shock, it is necessary to investigate the process at the mesoscale; i.e., the scale of individual chondrules. Here we document simulations of hypervelocity compaction of primitive materials using the iSALE shock physics model. We compare the numerical methods employed here with shock compaction experiments involving bimodal mixtures of glass beads and silica powder and find good agreement in bulk material response between the experiments and models. The heterogeneous response to shock of bimodal porous mixtures with a composition more appropriate for primitive solids was subsequently investigated: strong temperature dichotomies between the chondrules and matrix were observed (non-porous chondrules remained largely cold, while the porous matrix saw temperature increases of 100’s K). Matrix compaction was heterogeneous, and post-shock porosity was found to be lower on the lee-side of chondrules. The strain in the matrix was shown to be higher near the chondrule rims, in agreement with observations from meteorites. Chondrule flattening in the direction of the shock increases with increasing impact velocity, with flattened chondrules oriented with their semi-minor axis parallel to the shock direction.

  • Journal article
    Nagy L, Williams W, Muxworthy AR, Fabian K, Almeida TP, Ó Conbhuí P, Shcherbakov Vet al., 2017,

    Stability of Equidimensional Pseudo-Single Domain Magnetite Over Billion Year Time-Scales.

    , Proceedings of the National Academy of Sciences of the United States of America, Vol: 114, Pages: 10356-10360, ISSN: 1091-6490

    Interpretations of paleomagnetic observations assume that naturally occurring magnetic particles can retain their primary magnetic recording over billions of years. The ability to retain a magnetic recording is inferred from laboratory measurements, where heating causes demagnetization on the order of seconds. The theoretical basis for this inference comes from previous models that assume only the existence of small, uniformly magnetized particles, whereas the carriers of paleomagnetic signals in rocks are usually larger, nonuniformly magnetized particles, for which there is no empirically complete, thermally activated model. This study has developed a thermally activated numerical micromagnetic model that can quantitatively determine the energy barriers between stable states in nonuniform magnetic particles on geological timescales. We examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that, contrary to previously published theories, such nonuniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts. Hence, nonuniformly magnetized grains, which are commonly the main remanence carrier in meteorites and rocks, can record and retain high-fidelity magnetic recordings over billions of years.

  • Journal article
    Watters WA, Hundal CB, Radford A, Collins GS, Tornabene LLet al., 2017,

    Dependence of secondary crater characteristics on downrange distance: high-resolution morphometry and simulations

    , Journal of Geophysical Research: Planets, Vol: 122, Pages: 1773-1800, ISSN: 2169-9097

    On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (vi) increases with downrange distance (L). We have used high-resolution topography (1–2 m/pixel) to characterize the morphometry of secondary craters as a function of L for several well-preserved primary craters on Mars. The secondaries in this study (N = 2644) span a range of diameters (25 m ≤D≤400 m) and estimated impact velocities (0.4 km/s ≤vi≤2 km/s). The range of diameter-normalized rim-to-floor depth (d/D) broadens and reaches a ceiling of d/D≈0.22 at L≈280 km (vi= 1–1.2 km/s), whereas average rim height shows little dependence on vi for the largest craters (h/D≈0.02, D > 60 m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations (L< 110–160 km), taller downrange rims (L < 280 km), and cavities that are deeper uprange (L< 450–500 km). Populations of secondaries with lopsided ejecta were found to extend to at least L ∼ 700 km. Impact hydrocode simulations with iSALE-2D for strong, intact projectile and target materials predict a ceiling for d/D versus L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long-term evolution of small crater populations.

  • Journal article
    Shah J, Bates H, Muxworthy AR, Hezel DC, Russell SS, Genge MJet al., 2017,

    Long-lived magnetism in chondrite parent bodies

    , Earth and Planetary Science Letters, Vol: 475, Pages: 106-118, ISSN: 1385-013X

    We present evidence for both early- and late-stage magnetic activity on the CV and L/LL parent bodies respectively from chondrules in Vigarano and Bjurböle. Using micro-CT scans to re-orientate chondrules to their in-situ positions, we present a new micron-scale protocol for the paleomagnetic conglomerate test. The paleomagnetic conglomerate test determines at 95% confidence, whether clasts within a conglomerate were magnetized before or after agglomeration, i.e., for a chondritic meteorite whether the chondrules carry a pre- or post-accretionary remanent magnetization. We found both meteorites passed the conglomerate test, i.e., the chondrules had randomly orientated magnetizations. Vigarano's heterogeneous magnetization is likely of shock origin, due to the 10 to 20 GPa impacts that brecciated its precursor material on the parent body and transported it to re-accrete as the Vigarano breccia. The magnetization was likely acquired during the break-up of the original body, indicating a CV parent body dynamo was active ∼9 Ma after Solar System formation. Bjurböle's magnetization is due to tetrataenite, which transformed from taenite as the parent body cooled to below 320 °C, when an ambient magnetic field imparted a remanence. We argue either the high intrinsic anisotropy of tetrataenite or brecciation on the parent body manifests as a randomly orientated distribution, and a L/LL parent body dynamo must have been active at least 80 to 140 Ma after peak metamorphism. Primitive chondrites did not originate from entirely primitive, never molten and/or differentiated parent bodies. Primitive chondrite parent bodies consisted of a differentiated interior sustaining a long-lived magnetic dynamo, encrusted by a layer of incrementally accreted primitive meteoritic material. The different ages of carbonaceous and ordinary chondrite parent bodies might indicate a general difference between carbonaceous and ordinary chondrite parent bodies, and/or format

  • Journal article
    Muxworthy AR, Bland PA, Davison TM, Moore J, Collins GS, Ciesla FJet al., 2017,

    Evidence for an impact-induced magnetic fabric in Allende, and exogenous alternatives to the core dynamo theory for Allende magnetization

    , Meteoritics & Planetary Science, Vol: 52, Pages: 2132-2146, ISSN: 1086-9379

    We conducted a paleomagnetic study of the matrix of Allende CV3 chondritic meteorite, isolating the matrix’s primary remanent magnetization, measuring its magnetic fabric and estimating the ancient magnetic field intensity. A strong planar magnetic fabric was identified; the remanent magnetization of the matrix was aligned within this plane, suggesting a mechanism relating the magnetic fabric and remanence. The intensity of the matrix’s remanent magnetization was found to be consistent and low (~6 μT). The primary magnetic mineral was found to be pyrrhotite. Given the thermal history of Allende, we conclude that the remanent magnetization formed during or after an impact event. Recent mesoscale impact mode ling, where chondrules and matrix are resolved, has shown that low-velocity collisions can generate significant matrix temperatures, as pore-space compaction attenuates shock energy and dramatically increases the amount of heating. Non-porous chondrules are unaffected, and act as heat-sinks, so matrix temperature excursions are brief. We extend this work to model Allende, and show that a 1km/s planar impact generates bulk porosity, matrix porosity, and fabric in our target that match the observed values. Bimodal mixtures of a highly porous matrix and nominally zero-porosity chondrules, make chondrites uniquely capable of recording transient or unstable fields. Targets that have uniform porosity, e.g., terrestrial impact craters, will not record transient or unstable fields. Rather than a core dynamo, it is therefore possible that the origin of the magnetic field in Allende was the impact itself, or a nebula field recorded during transient impact heating.

  • Journal article
    Jourdan F, Timms NE, Eroglu E, Mayers C, Free A, Bland PA, Collins G, Davison T, Abe M, Yada Tet al., 2017,

    Collisional history of asteroid Itokawa

    , Geology, Vol: 45, Pages: 819-822, ISSN: 1943-2682

    In situ extrate rrestrial samples returned for study (e.g., from the Moon) are crucial in understanding the origin and evolution of the Solar System as, contrary to meteorites, they provide a known geological context for the samples and their analyses. Asteroid 25143 Itokawa is a rubble pile asteroid consisting of reaccumulated fragments from a catastrophically disrupted monolithic parent asteroid, and from which regolith dust particles have been recovered by the Hayabusa space probe. We analyzed two dust particles using Electron Backscatter Diffraction (EBSD) and 40 Ar/39 Ar dating techniques. One of the grains showing signs of 15–25 GPa impact shock pressure, yielded a 40 Ar/Ar plateau age of 2.3 ± 0.1 Ga. We develop a novel temperature -pressure-porosity model, coupled with diffusion models to show that the relatively low pressure and high temperature involved in the impact process can be reconciled only if the asteroid was already made of porous material at ~2.3 Ga and thus, if asteroid Itokawa was already formed, thereby providing a minimum age for catastrophic asteroid breakup. A second particle shows no sign of deformation indicating shock pressure of ˂ 10 GPa and a calculated maximum temperature of ~200 °C. This low temperature estimate is compatible with a lack of isotopic resetting for this particle. This suggests that the breakup of Itokawa’s parent was a relatively low-temperature process at the scale of the asteroid, and occurred on a pre-shattered parent body.

  • Journal article
    Berndt T, Paterson GA, Cao C, Muxworthy ARet al., 2017,

    Experimental test of the heating and cooling rate effect on blocking temperatures

    , Geophysical Journal International, Vol: 210, Pages: 255-269, ISSN: 1365-246X

    The cooling rates at which rocks acquire thermoremanent magnetizations (TRMs), affect their unblocking temperatures in thermal demagnetization experiments; similarly the heating rates at which the thermal demagnetization experiments are done also affect the unblocking temperature. We have tested the effects of variable cooling and heating rates on the unblocking temperatures of two natural non-interacting, magnetically uniform (single-domain, SD) (titano)magnetite samples and a synthetic SD magnetoferritin sample. While previous studies have only considered unblocking temperatures for stepwise thermal demagnetization data (i.e. the room-temperature magnetization after incremental heating), in this work we derive an expression for continuous thermal demagnetization of both TRMs and viscous remanent magnetizations (VRMs) and relate the heating rate to an effective equivalent hold time of a stepwise thermal demagnetization experiment. Through our analysis we reach four main conclusions: First, the theoretical expressions for the heating/cooling rate effect do not accurately predict experimentally observed blocking temperatures. Empirically, the relation can be modified incorporating a factor that amplifies both the temperature and the heating rate dependence of the heating/cooling rate effect. Using these correction factors, Pullaiah nomograms can accurately predict blocking temperatures of both TRMs and VRMs for continuous heating/cooling. Second, demagnetization temperatures are approximately predicted by published ‘Pullaiah nomograms’, but blocking occurs gradually over temperature intervals of 5–40 K. Third, the theoretically predicted temperatures correspond to ∼54–82 per cent blocking, depending on the sample. Fourth, the blocking temperatures can be used to obtain estimates of the atomic attempt time τ0, which were found to be 3 × 10−10 s for large grained (titano)magnetite, 1 × 10−13&t

  • Journal article
    Collins GS, Lynch E, McAdam R, Davison TMet al., 2017,

    A numerical assessment of simple airblast models of impact airbursts

    , Meteoritics & Planetary Science, Vol: 52, Pages: 1542-1560, ISSN: 1086-9379

    Asteroids and comets 10–100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy-based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving-source airburst model where the meteoroid's energy is partitioned as two-thirds internal energy and one-third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving-source model predicts overpressures two times greater than the static-source model, whereas the cylindrical line-source model based on the pancake model predicts overpressures two times lower than the static-source model. Given other uncertainties associated with airblast damage predictions, the static-source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.

  • Journal article
    Forman LV, Bland PA, Timms NE, Daly L, Benedix GK, Trimby PW, Collins GS, Davison TMet al., 2017,

    Defining the mechanism for compaction of the CV chondrite parent body

    , Geology, Vol: 45, Pages: 559-562, ISSN: 1943-2682

    The Allende meteorite, a relatively unaltered member of the CV carbonaceous chondrite group, contains primitive crystallographic textures that can inform our understanding of early Solar System planetary compaction. To test between models of porosity reduction on the CV parent body, complex microstructures within ~0.5-mm-diameter chondrules and ~10-μm-long matrix olivine grains were analyzed by electron backscatter diffraction (EBSD) techniques. The large area map presented is one of the most extensive EBSD maps to have been collected in application to extraterrestrial materials. Chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal-plastic deformation. Crystallographic preferred orientations (CPOs) preserved by matrix olivine grains are strongly coupled to grain shape, most pronounced in shortest dimension <a>, yet are locally variable in orientation and strength. Lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold. Significant local variability in the orientation and intensity of compaction is also inconsistent with a global process. Detailed microstructures indicative of crystal-plastic deformation are consistent with brief heating events that were small in magnitude. When combined with a lack of sintered grains and the spatially heterogeneous CPO, ubiquitous hot isostatic pressing is unlikely to be responsible. Furthermore, Allende is the most metamorphosed CV chondrite, so if sintering occurred at all on the CV parent body it would be evident here. We conclude that the crystallographic textures observed reflect impact compaction and indicate shock-wave directionality. We therefore present some of the first significant evidence for shock compaction of the CV parent body.

  • Journal article
    Collins GS, 2017,

    Moon Formation: Punch Combo or Knock-out Blow

    , Nature Geoscience, ISSN: 1752-0908
  • Journal article
    Morgan JV, 2016,

    The formation of peak rings in large impact craters

    , Science, Vol: 354, Pages: 878-882, ISSN: 0036-8075

    Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

  • Journal article
    Johnson BC, Blair DM, Collins GS, 2016,

    Formation of the Orientale lunar multiring basin

    , Science, Vol: 354, Pages: 441-444, ISSN: 0036-8075

    Multiring basins, large impact craters characterized by multiple concentric topographic rings, dominate the stratigraphy, tectonics, and crustal structure of the Moon. Using a hydrocode, we simulated the formation of the Orientale multiring basin, producing a subsurface structure consistent with high-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft. The simulated impact produced a transient crater, ~390 kilometers in diameter, that was not maintained because of subsequent gravitational collapse. Our simulations indicate that the flow of warm weak material at depth was crucial to the formation of the basin’s outer rings, which are large normal faults that formed at different times during the collapse stage. The key parameters controlling ring location and spacing are impactor diameter and lunar thermal gradients.

  • Journal article
    Kring DA, Kramer GY, Collins GS, Potter RWK, Chandnani Met al., 2016,

    Peak-Ring Structure and Kinematics from a Multi-disciplinary Study of the Schrödinger Impact Basin

    , Nature Communications, Vol: 7, ISSN: 2041-1723

    The Schrödinger basin on the lunar farside is ~320 km in diameter and the best-preservedpeak-ring basin of its size in the Earth–Moon system. Spectral and photogeologic analyses ofdata from the Moon Mineralogy Mapper instrument on the Chandrayaan-1 spacecraft and theLunar Reconnaissance Orbiter Camera (LROC) on the LRO spacecraft indicate the peak ring iscomposed of anorthositic, noritic, and troctolitic lithologies that were juxtaposed by severalcross-cutting faults during peak ring formation. Hydrocode simulations indicate the lithologieswere uplifted from depths up to 30 km, representing the crust of the lunar farside. Combining2geological and remote-sensing observations with numerical modeling, here we show a DisplacedStructural Uplift model is best for peak rings, including that in the K-T Chicxulub impact crateron Earth. These results may help guide sample selection in lunar sample return missions that arebeing studied for the multi-agency International Space Exploration Coordination Group.Determining which lunar landing site may yield information about the lunar interior is veryimportant with impact basins usually the best sites. Kring et al. provide a geological map of theSchrödinger basin on the moon via a multidisciplinary approach of remote sensing and numericalmodeling.

  • Conference paper
    Penny C, Muxworthy AR, Fabian K, 2016,

    The Curie temperature of magnetite nanoparticles (poster)

    , EMRS Fall 2016
  • Conference paper
    Shah J, Bates H, Muxworthy AR, Russell SS, Genge MJet al., 2016,

    A micro-CT conglomerate test (poster)

    , 15th Castle Meeting
  • Conference paper
    Penny C, Muxworthy AR, Fabian K, 2016,

    The Curie temperature of magnetite nanoparticles (poster)

    , 15th Castle Meeting
  • Conference paper
    Shah J, Muxworthy AR, Almeida TP, Kovacs A, Russell SS, Genge M, Williams W, Dunin-Borkowski REet al., 2016,

    Determining the magnetic recording fidelity of chondrule dusty olivine

    , 15th Castle Meeting
  • Journal article
    Almeida TP, Muxworthy AR, Kovacs A, Williams W, Nagy L, Conbhuí PC, Frandsen C, Supakulopus R, Dunin-Borkowski REet al., 2016,

    Direct observation of the thermal demagnetization of magnetic vortex structures in non-ideal magnetite recorders

    , Geophysical Research Letters, Vol: 43, Pages: 8426-8434, ISSN: 1944-8007

    The thermal demagnetization of pseudo-single-domain (PSD) magnetite (Fe3O4) particles, which govern the magnetic signal in many igneous rocks, is examined using off-axis electron holography. Visualization of a vortex structure held by an individual Fe3O4 particle (~ 250 nm in diameter) during in situ heating is achieved through the construction and examination of magnetic-induction maps. Step-wise demagnetization of the remanence-induced Fe3O4 particle upon heating to above the Curie temperature, performed in a similar fashion to bulk thermal demagnetization measurements, revealed its vortex state remains stable under heating close to its unblocking temperature, and is recovered upon cooling with the same or reversed vorticity. Hence, the PSD Fe3O4 particle exhibits thermomagnetic behavior comparable to a single-domain carrier, and thus vortex-states are considered reliable magnetic recorders for paleomagnetic investigations.

  • Conference paper
    Shah J, Muxworthy AR, Almeida T, Kovacs A, Russell SS, Genge M, Dunin-Borkowski Ret al., 2016,

    Hot Holography: Magnetic recording fidelity of dusty olivine (poster)

    , 13th UK Planetary Forum Early Career Scientists’ Meeting

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