RESEARCH ARTICLE10.1002/2016GC006583

The effects of 10 to >160 GPa shock on the magneticproperties of basalt and diabaseN. S. Bezaeva1,2, N. L. Swanson-Hysell3, S. M. Tikoo3,4, D. D. Badyukov5, M. Kars6, R. Egli7,D. A. Chareev1,2,8, L. M. Fairchild3, E. Khakhalova9, B. E. Strauss9, and A. K. Lindquist10,11

1Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia, 2Institute of Geology and PetroleumTechnologies, Kazan Federal University, Kazan, Russia, 3Department of Earth and Planetary Science, University ofCalifornia, Berkeley, California, USA, 4Department of Earth and Planetary Sciences, Rutgers University, New Brunswick,New Jersey, USA, 5V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,Moscow, Russia, 6Center for Advanced Marine Core Research, Kochi University, Nankoku, Japan, 7Central Institute forMeteorology and Geodynamics, Vienna, Austria, 8Institute of Experimental Mineralogy, Russian Academy of Sciences,Moscow, Russia, 9Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota, USA, 10Department ofGeosciences, University of Arkansas, Fayetteville, Arkansas, USA, 11Department of Environmental Studies, St. Olaf College,Northfield, Minnesota, USA

Abstract Hypervelocity impacts within the solar system affect both the magnetic remanence and bulkmagnetic properties of planetary materials. Spherical shock experiments are a novel way to simulate shockevents that enable materials to reach high shock pressures with a variable pressure profile across a single sam-ple (ranging between �10 and >160 GPa). Here we present spherical shock experiments on basaltic lava flowand diabase dike samples from the Osler Volcanic Group whose ferromagnetic mineralogy is dominated bypseudo-single-domain (titano)magnetite. Our experiments reveal shock-induced changes in rock magneticproperties including a significant increase in remanent coercivity. Electron and magnetic force microscopysupport the interpretation that this coercivity increase is the result of grain fracturing and associated domainwall pinning in multidomain grains. We introduce a method to discriminate between mechanical and thermaleffects of shock on magnetic properties. Our approach involves conducting vacuum-heating experiments onuntreated specimens and comparing the hysteresis properties of heated and shocked specimens. First-orderreversal curve (FORC) experiments on untreated, heated, and shocked specimens demonstrate that shock andheating effects are fundamentally different for these samples: shock has a magnetic hardening effect thatdoes not alter the intrinsic shape of FORC distributions, while heating alters the magnetic mineralogy asevident from significant changes in the shape of FORC contours. These experiments contextualize paleomag-netic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters.

1. Introduction

Hypervelocity impacts represent a major mechanism for the evolution of solid matter in our solar system.Impact shock waves can modify bulk magnetic properties and the remanent magnetization of rocksthrough a variety of mechanisms [Cisowski and Fuller, 1978; Gattacceca et al., 2007]. Shock-induced heatingat sufficiently high pressures may induce total or partial melting accompanied by crystallization of new fer-romagnetic grains, as well as subsolidus thermal remagnetization of existing grains. Shock can also producephase transformations of ferromagnetic minerals, grain fracturing, and crystallographic defects (such aspoint defects and dislocations) within magnetic grains [e.g., Reznik et al., 2016]. Consequently, the investiga-tion of magnetic records in solid bodies in the solar system often involves analyzing rocks whose primarymagnetization has been partially erased or completely overprinted by impact events. Understanding theproperties of shock-induced remagnetization and changes in magnetic properties of rocks is thereforeimportant for correctly interpreting the crustal magnetism of cratered surfaces on Earth, Mars, the Moon,and asteroids, as well as paleomagnetic records of extraterrestrial materials [Weiss et al., 2010] and rocksfrom terrestrial impact craters [e.g., Halls, 1975; Louzada et al., 2008; Elbra et al., 2009].

One way to experimentally assess the effects of shock on magnetic minerals is to conduct shock experi-ments wherein rocks are briefly taken to high pressures as an analog for the effect of the passage of shock

Special Section:Magnetism From Atomic toPlanetary Scales: PhysicalPrinciples and InterdisciplinaryApplications in Geo- andPlanetary Sciences

Key Points:� Spherical shock experiments result in

a range of pressures across a singlesample� We observed and characterized

shock-induced and temperature-induced magnetic hardening� The described effects likely occur in

target rocks during large impacts onterrestrial bodies

Supporting Information:� Supporting Information S1

Correspondence to:N. S. Bezaeva,bezaeva@physics.msu.ru

Citation:Bezaeva, N. S., et al. (2016), The effectsof 10 to >160 GPa shock on themagnetic properties of basalt anddiabase, Geochem. Geophys. Geosyst.,17, doi:10.1002/2016GC006583.

Received 8 AUG 2016

Accepted 3 NOV 2016

Accepted article online 7 NOV 2016

VC 2016. American Geophysical Union.

All Rights Reserved.

BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 1

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waves during a hypervelocity impact. In spherical shock experiments, explosively generated shock wavestravel through a prepared spherical rock sample [Bezaeva et al., 2010]. The spherical shock approach hasseveral advantages relative to other types of shock experiments. One advantage is that target rocks are notcontaminated by impactor materials. Another advantage is that a single shock experiment produces a widerange of shock pressures and temperatures within the same sample (with shock pulse duration �8 ls),including ultra-high pressure (from �10 to >160 GPa) and temperature (from �200 to >15008C) regimes.Due to the maximum shock wave amplitude in the center of the sphere, shock pressures and temperaturesdecrease from the center of the sample toward the periphery such that the different regimes can be sub-sampled along the radius of the sphere. In this study, we conducted spherical shock experiments on naturalbasalt and diabase samples, which can be considered to be analogs of basaltic crusts on differentiated plan-etary bodies. Our goal was to characterize the mechanisms and extent of shock-induced changes in rema-nent magnetization and bulk rock magnetic parameters by comparing these properties to those ofunshocked sister samples prepared from the same parent rocks.

2. Materials and Methods

2.1. SamplesUnoriented block samples were collected from a basalt flow, og-1, (previously studied by Swanson-Hysellet al. [2014] as site SI1-11.8 to 26.4) and a diabase dike, og-2, both from the Osler Volcanic Group, Ontario,Canada (Figure 1). Both the lava flow and the dike were emplaced during magmatic activity associated withthe development of the late Mesoproterozoic North American Midcontinent Rift [Cannon, 1992].

These targets were chosen because of the largely single-component nature of their natural remanent mag-netization (NRM) [Swanson-Hysell et al., 2014] and because they are unshocked analogs of lithologies thatexperienced shock within the nearby Slate Islands impact crater (Figure 1). In a study of the remanence androck magnetic properties of lithologies within the Slate Island central uplift, Halls [1979] hypothesized a rela-tionship between shock pressure and coercivity of remanence, but stated: ‘‘to test whether [remanent coer-civity] gives a measure of shock pressure, further experiments, including laboratory shock wave studies, onSlate Islands rocks are obviously required.’’ Low-temperature magnetic properties of unshocked og-1 and

Figure 1. Simplified geological map of the Lake Superior Archipelago showing the locations of sampling sites for og-1 and og-2 (modified from Carter et al. [1973], Sage [1990], and theOntario Geological Survey [2011]). The location of the geological map is shown in the inset map of North America. Sites og-1 and og-2 belong to the Osler Group, which consists of lavaflows and intrusions related to the ca. 1110 to 1085 Ma North American Midcontinent Rift. The outline of the Slate Islands impact structure as inferred by bathymetry is shown by thedashed circle. The Slate Islands archipelago is the remnant of the crater’s central uplift [Halls and Grieve, 1976] and dominantly comprises of Superior Province metamorphic rocks withsome exposure of Osler Volcanic Group intrusions and flows [Sage, 1990].

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og-2 samples, along with thermomagnetic experiments, reveal ferromagnetic mineral assemblages domi-nated by near-stoichiometric magnetite, in the case of og-1, and low-Ti titanomagnetite, in the case of og-2(estimated as TM08, see section S2 in Supporting Information). Minor hematite contributions in og-1 can bededuced from thermal demagnetization of NRM and are independently confirmed by petrographic investi-gations (see Supporting Information).

2.2. Spherical Shock ExperimentsCubic 1000 cm3 samples from the og-1 basalt and og-2 diabase were cut from the original blocks andshaped into spheres of 49 mm (og-1) and 50 mm (og-2) diameter for the spherical shock experiments. Eachsample was encased in a stainless steel spherical container that was vacuum-degassed. The initial (pre-shock) bulk densities of the spherical samples were 2.80 g/cm3 for og-1 and 2.84 g/cm3 for og-2 [Nikolaevet al., 2015]. Spherically convergent shock waves were generated through detonation of a layer of explosivematerials surrounding the stainless steel casing. During and after shock loading, the container remainedhermetically sealed and intact, such that the investigated samples were not contaminated by products ofthe explosion (Figure 2).

Figure 2. Microtomographic images of the spherically shocked samples of basaltic lava flow og-1 (a) and diabase dike og-2 (c), and photo-graphs of equatorial slices of the same samples og-1 (b) and og-2 (d) with labeled specimens. The shock zones discussed in the text arelabeled as I, II, III, and IV. Note that the spherical steel container was thinned down prior to cutting the equatorial slice imaged in Figure 2b,so that its initial thickness in Figure 2a does not correspond to its altered thickness in Figure 2b. The total volume of the inner cavity inog-1 is �330 mm3 and the total volume of the inner voids in zone I of og-2 is �196 mm3.

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2.3. Peak Shock Pressure EstimationsThe spherical shock experiments explored two different pressure regimes that were achieved by varyingthe thickness of the stainless steel container, the thickness of the explosive layer, and the chemical compo-sition of the explosive agent. The ‘‘high-intensity’’ regime, used for sample og-2, is similar to that imple-mented by Bezaeva et al. [2010] and Kozlov and Sazonova [2012]. The ‘‘low-intensity’’ regime, used forsample og-1, was previously implemented by Dobromyslov et al. [2013]. Numerical simulations using the‘‘VOLNA’’ software package [Kuropatenko et al., 1988a, 1988b] and experimental data on the shock com-pressibility of og-1 and og-2 [Nikolaev et al., 2015] were collectively used to estimate the peak shock pres-sures and temperatures experienced by different regions within the og-1 and og-2 samples (Figure 3)[Kozlov et al., 2015; E. A. Kozlov, personal communication, 2015]. Calculations were based on the Mie-Gr€uneisen equation of state with ultimate compression values according to Zababakhin [1997]. The parame-ters for the Mie-Gr€uneisen equation of state were selected from the following relation between mass veloci-ty u and shock wave propagation velocity D: D 5 3.01 1 1.482u for 2 km/s< u< 3.5 km/s [Nikolaev et al.,2015], and initial bulk densities as reported in section 2.2.

During spherical shock experiments, the shock wave converges toward the sample center and progressivelycompresses the samples along the radius as it focuses into the center of the sphere. After the initial passageof the wave, a second wave propagates from the center outward and increases the compression state ofthe already shock-loaded matter, thereby producing higher peak pressures than the initial incoming wave(Figure 3). Given that regions with higher postshock temperatures have much smaller volumes than thosewith lower temperatures, the contribution of conductive heating to postshock temperatures is relativelyminor. The errors for the reported pressure and temperature estimates are likely 65% and 610%,respectively.

3. Results

3.1. Petrographic Effects of ShockThe shocked sample spheres, still encased in their steel jackets, were imaged using X-ray microtomographywith a spatial resolution of 39 lm (og-1; Figure 2a) and 33 lm (og-2; Figure 2c), using a XT H 450 microfocusX-ray and CT tomography system by Nikon Metrology in Leuven (Belgium) with assistance from Neva Tech-nology (Moscow, Russia). Three-dimensional reconstructions based on 2400 angle scans per sample weredeveloped with the X-Tek Reconstruction Software. Equatorial planes extracted from 3-D reconstructions ofog-1 and og-2 via VolumeGraphics StudioMax 2.2 software are shown in Figures 2a and 2c. Subsequently,wedges taken from slabs cut along equatorial planes were used for scanning electron microscopy (SEM)and magnetic force microscopy (MFM; Figure 4).

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Figure 3. Pressure (solid lines) and temperature (dashed lines) profiles along radii of shocked (a) og-1 and (b) og-2 samples along with position of the zonal divisions used in the text[Kozlov et al., 2015; E. A. Kozlov, personal communication, 2015]. Diamonds and circles correspond to calculations for radially positioned Lagrangian particles. Solid gray lines correspondto the estimated peak pressure values of the incoming spherically convergent shock wave, and solid black lines correspond to the estimated overall maximum peak pressure. Postshocktemperatures are indicated by dashed lines.

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MFM imagery enables visualization of the grain structure and magnetic domain patterns of unshocked andshocked samples. MFM images of og-2 reveal the structure of exsolved magnetite and ilmenite lamellae(Figures 4c and 4e) with complex magnetization patterns. Magnetic domains are small relative to the size of

Figure 4. SEM backscattered electron micrographs of (a) unshocked og-1 and (b) shocked og-1 from zone IV (see Figure 2 for zonationlabeling). (c, d) MFM topographic images of unshocked and shocked og-2, respectively. (e, g) Corresponding MFM magnetic images takenover exactly the same area as Figures 4c and 4d. The magnetic state of these images corresponds to NRM. Labeling: ‘‘Ilm’’ designatesilmenite and ‘‘Tmt’’ designates titanomagnetite.

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the grain and their orientation generally perpendicular to lamellae boundaries can be attributed to internalstress. SEM and MFM images for both og-1 (Figure 4b) and og-2 (Figure 4d) also reveal numerous shock-induced cracks in the shocked specimens.

The X-ray microtomography images reveal that the shocked sample og-1 has a central cavity with an aver-age diameter of �9 mm, whereas the central region of sample og-2 contains numerous bubbles of up tofew mm in size (Figure 2). We suggest that these bubbles are remnants of a central cavity that has been par-tially filled with shock melt.

Petrographic shock effects observed with transmitted light microscopy confirm that the intensity of shockmetamorphism decreases from the center to the margins of the shocked spheres. Accordingly, we define fourconcentric petrographic shock zones in og-1 and og-2 (Table S2): I—total melt zone; II—zone of partial melt-ing; III—zone of total conversion of plagioclase to diaplectic glass; IV—zone of solid-state shock features inminerals. Zone I features opaque bands containing newly formed magnetite and olivine (Figure S6a). Zones IIand III are characterized by plagioclase recrystallization (Figure S6b) and total conversion to glass (Figures S6cand S6d), respectively. Plagioclase crystals in Zone IV contain planar deformation features (Figure S6e and seesection S4 in Supporting Information for further details), and typical shock-induced grain fracturing of Fe-Ti-oxides in Zone IV is shown in Figures 4b and 4d. Zone IV of og-2 can be further divided into subzones IVa, IVb,and IVc that correspond to variable development of shock effects from strong to very weak.

3.2. Rock Magnetic Effects of ShockSpecimens for rock magnetic analyses were cut from og-1 and og-2 spheres along radial transects using adiamond wire saw (Figures 2b and 2d). These specimens cover all shock zones, and have been used, togeth-er with unshocked sister specimens, to document the nature and extent of shock-induced changes of mag-netic minerals.3.2.1. Ferromagnetic Mineralogy and Verwey Transition Changes Inferred FromLow-Temperature DataStoichiometric magnetite is characterized by a crystal symmetry transition from cubic to monoclinic uponcooling through the Verwey transition temperature Tv � 120 K [Verwey, 1939]. This transition affects manyphysical properties [e.g., Honig, 1995], including magnetocrystalline anisotropy [Abe et al., 1976], so thatcooling and warming cycles of remanent magnetization across Tv are characterized by an inflection in rema-nent magnetization [Feinberg et al., 2015]. Accordingly, Tv is conventionally defined as the peak on the firstderivative of the warming curve of a zero-field-cooled (ZFC) remanent magnetization acquired below Tv

[€Ozdemir and Dunlop, 1999] (Table 1). Another method for determining the Verwey transition is to deter-mine the median demagnetization temperature of ZFC remanence during zero-field warming [Carporzenand Gilder, 2010], referred to as Td [Sato et al., 2016] (Table 1).

Carporzen and Gilder [2010] observed a static pressure-induced increase of Td in stoichiometric magnetitespecimens after pressure release, which was found to be 1 K/GPa for pressures up to 5 GPa. This increasewas attributed to internal stress associated with crystal defects caused by the nonhydrostatic component ofapplied pressures, since stress is known to affect the Verwey transition temperature and the choice of amagnetic easy axis [Coe at al., 2012]. Reznik et al. [2016] reported a shock-induced increase in Verwey transi-tion temperature Tv* (determined from the maximum in the first derivative of temperature dependence oflow-field magnetic susceptibility acquired in 83–300 K range) by 6 K for all shocked samples of magnetitewith regard to their unshocked analogue; however, Tv* remained essentially constant within the samplesshocked to 5, 10, 20, and 30 GPa. The authors attribute such changes in Tv* to a combination of grain frac-turing, plastic deformation mechanisms, and amorphization but argue that in the 5–30 GPa dynamic pres-sure range Tv* cannot be used as a geobarometer because strain memory saturation occurs by 5 GPa.

In order to assess possible shock-induced changes in Verwey transition temperature in magnetite-bearingog-1 samples, we imparted a saturation magnetization at 5 K and measured magnetization as they warmedto 300 K. In the field-cooled experiments (FC), the sample was cooled from 300 to 5 K in a 2.5T field. In theZFC experiments the sample was cooled to 5 K in near-zero field and then pulsed with a 2.5T field.Low-temperature cycling of room temperature saturation magnetization (RT-SIRM) experiments were alsoconducted. These experiments were conducted with a Quantum Design Magnetic Property MeasurementSystem (MPMS)22 at the Institute for Rock Magnetism (IRM) and a Quantum Design MPMS-XL5 at KochiUniversity. ZFC warming curves are presented in Figure 5a.

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Low-temperature remanence experiments reveal shock-induced increases in Td and Tv (Figure 5 and Table 1).Td is �104 K for the unshocked og-1 specimen and increases in the shocked specimens og-1s_a-e to �107–108 K (Table 1). These data reveal shock-induced increases of Td by 3–4 K from specimens that only havesolid-state shock effects. As in Carporzen and Gilder [2010], we see an increase in Td following shock althoughin our experiment shock pressures reach far above 5 GPa (�10–53 GPa for the solid-state specimens). Similarto Reznik et al. [2016] we observe that Td and Tv increase in the shocked specimens with regard to unshockedanalogs, but are similar for samples that experienced shock from 11 to 53 GPa (Figure 5b).

Memory ratio is defined as the fraction of remanence remaining after a RT-SIRM cooling-warming cycle(Figures S3c and S3d), divided by the initial RT-SIRM before low-temperature cycling. For sample og-1, thememory ratio after shock is similar but slightly less than that in the unshocked samples: the memory ratio

Figure 5. (a) ZFC warming curves (dots) with corresponding model fits (lines) for unshocked and shocked og-1 specimens (see Figure 2 and Table 1), which include a contribution repre-senting the magnetization loss across the Verwey transition (dashed lines). (b) Verwey transition temperatures estimated from 50% loss (Td or median, lower limit) and maximum slope(Tv or mode, upper limit) of the dashed curves in Figure 5a. The shaded region is a guide for the eye.

Table 1. Main Rock Magnetic Parameters of Unshocked and Spherically Shocked og-1 and og-2 Specimens (See Figure 2 for SpecimenIdentification)a

Specimen D (mm)ShockZone Pb (GPa) Tb (8C) Bc (mT) Bcr (mT) Bcr/Bc Mrs/Ms Td/Tv (K)

MemoryRatio (%)

Unshocked og-1og-1un (13) 25 7.2 6 0.9 21 6 2 2.9 6 0.1 0.082 6 0.008 104.4/1063c 58

Shocked og-1og-1s_a 6.4 II1III 53 574 13.7 34 2.5 0.134 107.6/108.3 54og-1s_b 10.0 IV 23 278 12.8 34 2.6 0.120 107.6/108.0 54og-1s_c 14.8 IV 12 216 10.9 31 2.8 0.110 107.0/107.4 54og-1s_d 19.4 IV 11 242 10.9 31 2.9 0.102 107.5/107.9 57og-1s_e 23.2 IV 12 265 10.9 31 2.8 0.105 107.5/107.7 57

Unshocked og-2og-2un (15) 25 15.6 6 0.3 29 6 0.3 1.9 0.164 6 0.003 63

Shocked og-2og-2s_a 2.6 I >160 >1500 29.6 47 1.6 0.290 95og-2s_b 8.4 II 97 974 41.3 74 1.8 0.258 94og-2s_c 12.9 III 44 620 31.4 61 2.0 0.212 85og-2s_d 16.0 IV 38 548 27.6 54 2.0 0.202og-2s_e1 19.1 IV 33 541 26.9 54 2.0 0.201og-2s_e2 19.7 IV 32 540 26.6 53 2.0 0.204og-2s_f2 23.4 IV 25 532 28.4 55 1.9 0.214 73d

aD is distance of specimen center to sphere center; P is peak shock pressure; T is postshock temperature; Bc is coercive field; Bcr isremanent coercivity (i.e., field required to demagnetize a saturation remanent magnetization); Ms is saturation magnetization; Mrs issaturation remanent magnetization; Td is Verwey transition temperature determined from the median of ZFC warming curves (alsoreferred to as median demagnetization temperature of ZFC); Tv is Verwey transition temperature determined from the peak on the firstderivative of ZFC warming curves. Numbers in brackets in the first column after og-1un and og-2un designate the number of unshockedspecimens used for measurements (mean values with standard deviations are presented).

bErrors in P and T estimates are within 65% and 610%, respectively.cMeasured on a single specimen.dDetermined for a sister sample og-2s_f1 from the same position on the radius.

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for the unshocked specimen og-1un_a sample is 58% while it is 57% for shocked specimens og-1s_d andog-1s_e and 54% for og-1s_a-c. The data for the og-2 sample show a significant shock-induced increase inmemory ratio from 63% for the unshocked specimen og-2un_a up to 73–95% for the shocked specimensog-2s_f2, c, b, and a. Given that the remanence that demagnetizes through low-temperature cycling is asso-ciated with multidomain grains, this memory ratio increase indicates a shock-induced change in domainstate.3.2.2. Changes in Bulk Domain State/Domain Wall Pinning as Inferred by Hysteresis and First-OrderReversal CurvesAs a means to assess shock-induced changes in bulk domain state, we ran major hysteresis loops and back-field remanence demagnetization experiments using Princeton Micromag Vibrating Sample Magnetometers(VSMs) at the Institute for Rock Magnetism and at Kochi University. Domain state changes are inferred frombulk hysteresis parameters Ms (saturation magnetization), Mrs (saturation remanence), Bc (coercivity), and Bcr

(coercivity of remanence) [Day et al., 1977; Dunlop, 2002] (Table 1 and Figure 6).

Shock produced increases in Mrs/Ms and in Bcr relative to the unshocked sister specimens (Figure 6). The neteffect of shock is equivalent to a shift of the bulk magnetic properties toward the single-domain end-mem-ber, as seen in the Day plot (Figure 6). This trend away from end-member multidomain behavior is likelyrelated to domain wall pinning and fracturing of multidomain ferromagnetic grains.

For the innermost specimen of og-2, changes in rock magnetic properties are attributed to melting andcrystallization of new ferromagnetic phases in the form of dendritic (titano) magnetite grains (Figure 7).Such dendrites are typical for basaltic glass samples in natural lava flows [Shaar and Feinberg, 2013].

We acquired room-temperature first-order reversal curves (FORCs) for all unshocked (untreated), shocked,and vacuum-heated og-1 and og-2 specimens to further fully characterize domain state and interactions(Figures 8, S7, and S8). First-order reversal curve distributions provide information about the distribution ofmicroscopic coercivities (represented along the horizontal axis of a FORC diagram) and the magnetic inter-actions between domains or particles within magnetic mineral assemblages (which are represented as abias field along the vertical axis). Coercivity and bias field distributions allow for better discrimination ofdomain states and their contribution to the bulk magnetization with respect to bulk magnetic measure-ments [Roberts et al., 2000].

Stacked, high-resolution FORC measurements using field steps of 0.5–0.7 mT were performed on selectedsamples and processed with the VARIFORC software package [Egli, 2013; http://www.conrad-observatory.at/zamg/de] (Figure 8). Coercivity distributions have been calculated with VARIFORC from the first derivative ofthe backfield or DC-demagnetization curve, which coincides with the set of remanent magnetizationsobtained from the FORC measurements.

FORC diagrams (Figure 8) are dominated by the typical signature of pseudo-single-domain (PSD) magnetiteparticles, consisting of triangular-shaped contour lines with maximum vertical spread along Bc 5 0 [Robertset al., 2000; Muxworthy and Dunlop, 2002; Carvallo et al., 2006; Ludwig et al., 2013]. Additional featuresappear in addition to this general trend over the lower quadrant, breaking the Preisach-type of symmetryabout Bu 5 0 seen in synthetic PSD particle assemblages [Muxworthy and Dunlop, 2002].

The most evident feature is a small-amplitude, negative peak located between the Bu 5 2Bc diagonal andthe Bu 5 0 axis. Its appearance in Figures 8a and 8b is enhanced by a nonlinear color scale; nevertheless, itis highly significant, having a signal-to-noise ratio >50 [Heslop and Roberts, 2012; Egli, 2013]. In addition, thecentral peak of all FORC diagrams is slightly offset toward positive Bu values. These features are slightly rem-iniscent of the ‘‘wishbone’’ structure seen in FORC diagrams of single-domain particle arrays affected by astrong negative mean interaction field [Pike et al., 2005]. In presence of a mean interaction field, the effec-tive field experienced by the magnetic carriers is given by Beff 5 B 2 a M/Ms, where B is the field applied dur-ing the measurements, M is the corresponding magnetization, Ms is the saturation magnetization, and a is aconstant expressing the maximum amplitude of the mean internal (i.e., interaction) field. The correct valueof a can be empirically found by replacing the FORC measurement fields with Beff, until the symmetrybetween lower and upper quadrant is restored as far as possible. This type of correction has been per-formed with VARIFORC by choosing the value of a for which the central peak offset is eliminated (Figure8c). This correction reduces the extent of the asymmetry, but does not eliminate it completely becausebasalt is a heterogeneous material for which different internal fields might be required to describe different

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parts of the specimen. Hence, a should be regarded as a statistical distribution, rather than the single valueused for the corrections. In any case, the untreated and the shocked specimens required the same meanfield correction given by a 5 25 mT, while the heated specimen has been corrected with a 5 10 mT.

The mean fields of og-2 samples are too large to be explained by a demagnetizing field associated with thebulk magnetization and we tentatively attribute them to the interaction between two phases in ilmenite-titanomagnetite intergrowths (Figures 4c and 4e) (see Supporting Information).

The effect of shock on the FORC properties of og-2 is equivalent to a widening of the FORC distributionover both the Bc and Bu axes, whereby the distribution shape itself does not change significantly (Figure

Figure 6. Summary of hysteresis parameters associated with unshocked and shocked specimens of og-1 basalt and og-2 diabase. All specimens are labeled accordingly (see Table 1 andFigure 2). Bc is coercivity, Bcr is coercivity of remanence, Ms is saturation magnetization, Mrs is saturation remanent magnetization. Horizontal lines in Figures 6a, 6b, 6c, and 6d depict themean value and range of data from the unshocked specimens. In the (e) Day plot, the labels SD, PSD, and MD are for single domain, pseudo-single-domain and multidomain, respective-ly. A theoretical SD-MD mixing curve for magnetite after [Dunlop, 2002] is also shown for reference.

Figure 7. MFM (a) topographic and (b) magnetic images from the shock melt zone (zone I) of the og-2 diabase dike sample. Notice thedendritic texture of ferrimagnetic grains recrystallized from the shock melt.

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8b). A better assessment of magnetic property changes induced by shock and heat may be based on aquantitative comparison of the FORC distributions obtained after mean field correction (Figures 8c and 8d).This comparison is based on the superposition of the contours of one distribution to the reference distribu-tion of the untreated sample, after rescaling the Bc and Bu axes for best contour match over the upper

Figure 8.

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quadrant. The upper quadrant has been chosen because it is less affected by reversible magnetizations dueto unremoved mean field contributions. As seen in Figure 8c, a good match between untreated andshocked specimens is obtained if Bc and Bu of the latter are scaled by a factor 1/1.65 and 1/1.35, respectively.If Bc is identified with coercivity in the framework of a Preisach interpretation of the FORC diagram [Preisach,1935; Pike et al., 1999], this means that the shocked specimen is �65% harder than the untreated specimen.This result compares well with the observed increase of Bc and Bcr values listed in Table 1. On the other hand,the vertical spread of the FORC distribution increases only by 35%, so that the overall aspect ratio of the distri-bution is decreased. This change in aspect ratio is equivalent to an apparent PSD grain size decrease, as seenin synthetic samples [Muxworthy and Dunlop, 2002]. With respect to coercivity, the opposite trend is obtainedwith the heated specimen (Figure 8d). In this case, the FORC distribution appears to be fundamentally differ-ent from that of the untreated specimen, with differences related to (1) a rounding of the contours near Bc 5 0in the upper quadrant (labeled as 1 in Figure 8d), (2) enhanced high-coercivity contributions just below Bu 5 0(labeled as 2 in Figure 8d), and (3) an upward shift of all contours near Bc 5 0 in the lower quadrant (labeledas 3 in Figure 8d). Interestingly, the same differences (albeit in a much reduced form) are responsible for theresidual mismatches between the FORC diagrams of shocked and untreated specimens (Figure 8c). In summa-ry, the FORC distribution of the shocked specimen may be considered to be a rescaled version of theunshocked material with changes due to increase in coercivity and decreases in effective grain size and withsmall residual differences being similar to the changes observed upon heating.

These conclusions are supported by the backfield coercivity distributions of unshocked, shocked, and heat-ed specimens (Figures 8e and 8f). The distributions of unshocked and shocked specimens become identicalwhen calculated from mean-field-corrected data, after rescaling fields and magnetization values such thatthe distribution peaks coincide. This result cannot be obtained without mean field correction. On the otherhand, the coercivity distribution of the vacuum-heated specimen appears fundamentally different, as nomatch is obtained upon rescaling. Fundamental differences in the shape of a coercivity distribution can beintroduced by the addition of a new magnetic component [Egli, 2003], as suggested by the appearance of acentral ridge toward the high-coercivity end of the FORC diagram (Figure 8d).

An important result of the detailed FORC analysis is that the existence of a mean internal field alters themeasured magnetic properties and introduces differences between unshocked and shocked specimens,which do not reflect intrinsic changes of the magnetic particles. This can be explained by the fact that themean field is controlled by the total magnetization, which includes the reversible component, while coerciv-ity distributions and the upper quadrant of the FORC diagram are governed by irreversible processes [Pike,2003; Carvallo et al., 2005].

The overall conclusion from this analysis is that the experimental shock did not alter the intrinsic shape of coer-civity and FORC distributions while heating experiment did—likely due to the growth and chemical modifica-tion of magnetic particles. Possible causes for the observed shock hardening and apparent grain size decreasethrough shock include the introduction of crystal defects and cracking [Reznik et al., 2016]. As far as magneticproperties are concerned, the formation of cracks is not equivalent to the partition of the crystals into smallerparticles. For instance, vortex structures originally occupying the whole crystal will still be able to form acrossthe cracks, as seen in finely exsolved submicron magnetite blocks [Harrison et al., 2002]. Therefore, the apparentgrain size decrease is likely related to the larger fields required to nucleate magnetic vortex states.

Figure 8. FORC analyses of sample og-2. FORC distributions of (a) an untreated og-2 specimen and (b) shocked specimen og-2s_d (collect-ed from shock zone IV, see Table 1). Color scales and field ranges are identical for each sample for ease of comparison. Note that the cen-tral peak of both distributions is slightly offset toward positive Bu values. (c) FORC distribution of an untreated og-2 specimen (color map)after a mean field correction with a/Ms 5 25 mT, used to bring the central peak on Bu 5 0. This distribution is compared with that ofog-2s_d with the same mean field correction (blue contours). The Bc and Bu axes have been rescaled to obtain the best match of the twoFORC distributions over the upper quadrant. Circled numbers refer to differences between the two distributions as explained in the text.(d) Same as Figure 8c for the comparison of the untreated specimen with a specimen heated to 7008C (fast heating and cooling in vacuumas described in section S3.3). (e) Backfield coercivity distributions derived from the original FORC measurements for the untreated,shocked, and heated og-2 specimens, respectively. All curves are normalized by the specimen’s Ms, so that the unit of the vertical axis isT21. The insert shows the same curves, after the horizontal and vertical axes have been rescaled by the field and coercivity distribution val-ue corresponding to the curve maximum. (f) Same as the inset of Figure 8e for backfield coercivity distributions calculated from FORCmeasurements after mean field corrections with a/Ms 5 25 mT (untreated and shocked specimens) and a/Ms 5 10 mT (heated specimen).All diagrams have been generated using VARIFORC built-in functions [Egli, 2013].

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Finally, standard-resolution (i.e., 5 mT field step) FORC distributions acquired for all shocked specimens ofog-1 (Figure S7 in Supporting Information) and og-2 (Figure S8) demonstrate progressive shock-inducedcoercivity hardening and changes toward more single-domain (SD) like behavior with increasing peak shockpressure toward the sphere center (as the distributions spread out from the origin along the Bc axis).3.2.3. Magnetic RemanenceAll shocked specimens were stepwise demagnetized by alternating magnetic fields (AF) prior to other rockmagnetic experiments (Figure 9). All (mutually oriented) specimens from both og-1 and og-2 contained alow-coercivity overprint that was unidirectional within each parent sample and that was removed at AF lev-els <20 mT, which we interpret to be an isothermal remanent magnetization (IRM) (Figure 9). Application ofREM0 method [Gattacceca and Rochette, 2004] (REM0(AF) 5 DNRM(AF)/DSIRM(AF)) indicates for this low-coercivity component a REM0 � 10–20%, which is typical of IRM. Such IRM could be acquired between thetime of shock experiment and the time of magnetic measurements as a result of magnetic contaminationand complicates interpretation of the magnetic remanence.

Above AF levels of 20 mT, we observed two different classes of remanence behavior. Specimens og-2s_a,og-2s_b, and og-2s_c, which were sampled from the interior of the og-2 sphere (Figure 2d), have a unidirec-tional high-coercivity component that is likely a thermoremanent magnetization (TRM) acquired as a resultof shock-induced heating to temperatures in excess of the (titano)magnetite Curie temperature (531–5808C,see section S2 and Figure S2 in Supporting Information) and subsequent cooling in the ambient field. Theremanence of specimens in the shock melt zone (e.g., og-2s_a in Figure 9) must be a TRM and therefore thefact that the same direction is observed in the interior-most unmelted specimens (e.g., og-2s_c in Figure 9)is best explained by that remanence being a full TRM as well. This interpretation is consistent with the esti-mated postshock temperatures (Table 1) and the corresponding REM0 values of 2–3% for the high-coercivity component, consistent with TRM acquired in the Earth’s magnetic field. In contrast, specimensfrom the outer portion of the sphere (e.g., og-2s_e and og-2s_f) do not appear to contain this shock-inducedTRM as the remanence direction isolated after removal of the IRM overprint is distinct from that of theimpact melt (e.g., og2s_e2 in Figure 9). This interpreted absence of a preserved shock-related TRM or partialTRM (pTRM) in the outer specimens suggests that shock heating did not reach far into the range of blocking

Figure 9. Zijderveld diagrams for the mutually oriented shocked specimens of og-2 (Figure 2d): (a) og-2s_e2 (zone IV), (b) og-2s_c (zone III), (c) og-2s_a (zone I) (see Table 1). Note thatthe Zijderveld diagram for the least shocked sample og-2s_f2 is almost identical to (a) presented here.

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BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 12

temperatures for the ferromagnetic grains. For example, if specimen og-2s_e2 (Figure 9a) was heated to�4508C then there would be little remanence acquired by the heating as the unblocking temperature spec-trum of the og-2 sample reveals (un)blocking temperatures dominantly above 4508C (Figure S1). This inter-pretation suggests that the calculated temperatures of shock-related heating (estimated to be 5408C for og-2s_e2 with an uncertainty of 610%) associated with the experiments are at the low end, or below, thereported uncertainty of the calculations. Temperature measurements on basalt samples experimentallyshocked to pressures of 20–30 GPa reveal peak temperatures between 330 and �4708C [Stewart et al.,2007]. If peak shock temperatures experienced by samples such as og-2s_e2 were �470–4858C or below,pTRM acquired during the spherical shock experiment would have been more readily overprinted by theobserved IRM.

The unidirectional magnetizations observed at high AF demagnetization levels in shocked og-2 d-f2 specimensare likely preshock remanence. Magnetizations at AF demagnetization levels of 20–40 mT are �40% weaker inthe shocked specimens than in unshocked equivalents. The weaker magnetization intensities observed in the

high coercivity fraction of the shockedsamples are likely the result of shockdemagnetization. It is unlikely that thisremanence is a result of shock remanentmagnetization (SRM). REM0 values for thishigh-coercivity component in og-2 speci-mens d-f2 are consistent with those for theunshocked samples with NRM and are like-ly incompatible with SRM, because Mid-continent Rift basaltic dikes within theSlate Islands have low SRM acquisition effi-ciencies (�1% of TRM acquired in thesame magnetic field), at least for shockpressures �2 GPa [Tikoo et al., 2015]. Thislow SRM acquisition efficiency may beattributed to the fact that these sampleshave relatively high coercivity magneticcarriers in the PSD size range, whereas SRMis acquired most easily by multidomaingrains with low coercivities (often<20 mT)[Gattacceca et al., 2007; Tikoo et al., 2015].SRM does not appear to be preserved inthe remanence of samples including thosethat were not thermally overprinted (e.g.,shock levels of �35 GPa). However, SRMcould have been acquired by low coercivitygrains and subsequently overprinted byTRM and IRM.

4. Discussion

4.1. Petrographic Effects inComparison to Shock MagnitudePetrographic observations of the shockedsamples show the progressive develop-ment of shock effects with increasingpeak shock pressure from the edgesof the og-1 and og-2 spheres to theircenters. The observed shock effects incomparison to shock magnitude are sum-marized in Table 2.

Table 2. Shock Effects in og-1 Basalt and og-2 Diabase With Shock PressureCalibration

Shock Effect Shock Pressure (GPa)

Shock Wave Barometry for og-1 Basalt/og-2 DiabaseZone IV (Partial Overlap With

Classes 1 and 2a13–39/40–41

Plagioclase� Rare planar deformation

features (PDFs)� Partial transformation

into diaplectic glassPyroxene� Mechanical twinning�Weak to moderate mosaicism� Irregular and planar fractures

Magnetite/Titanomagnetite� Irregular fractures

Zone III (�Class 3a) 39–44/41–63Plagioclase� Total transformation into

diaplectic glass�39/�41

Pyroxene� Mechanical twinning� Planar fractures� Strong mosaicism

Magnetite/Titanomagnetite� Strong irregular fracturing

Zone II (Classes 4–5a) 44–70/63–158Plagioclase� Incipient melting

Pyroxene� Mechanical twinning� Numerous planar fractures� Strong mosaicism� Incipient melting

Magnetite/titanomagnetite� Fragmentation� Incipient melting

Zone Ib 70–101c {>29}/>158 {>42}Whole-rock melting >70c/>158� Melt glass with pores

and bubbles� lm-dendrites of olivine

and magnetite

aClassification after Kieffer et al. [1976], Schaal and H€orz [1977], and Schaalet al. [1979].

bZone I: peak pressure for the incoming spherically convergent shockwave is also indicated in brackets {} (first peak pressure in Figure 3).

cIndicated pressure values may be underestimated as the material waslikely shocked to higher pressures in the center and then moved to its finalposition on the radius.

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BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 13

Pyroxene grains in marginal parts of the samples display brittle and weak plastic deformation features,including irregular fractures, planar features, mosaicism, and mechanical polysynthetic twinning. Closer tothe sphere centers, plagioclase crystals lose the birefringence and have planar deformation features (PDFs)while secondary minerals do not show any pleochroism, and intensity of shock damages in pyroxeneincreases. Further pressure increases resulted in total conversion of plagioclase to diaplectic glass, decom-position of secondary minerals, partial melting, and total melting of the rocks.

These shock effects are roughly consistent with previously described petrographic effects [see St€offler et al., 2006,Table 5.3] and five shock metamorphic classes [Kieffer et al., 1976; Schaal and H€orz, 1977; Schaal et al., 1979] (seesection S4 in Supporting Information for details). Due to the specific geometry of shock waves in spherical shockexperiments, our samples experienced mechanical damage to a lesser extent than in planar shock experiments,which makes our experiments more analogous to natural conditions of giant impacts where shocked rocks aresupported by surrounding matrices. This difference may be the source of the observed discrepancy in shockmagnitude, needed for the total conversion of plagioclase into diaplectic glass between this work and previousstudies (other possible reasons are discussed in section S4 of Supporting Information).

4.2. Changes in Rock Magnetic PropertiesTwo opposed shock effects have been reported in the literature in rocks with a range of magnetic mineralo-gies: shock-induced magnetic hardening (i.e., an increase in bulk coercivity) [Cisowski and Fuller, 1983; Peso-nen et al., 1997; Langenhorst et al., 1999; Gattacceca et al., 2007; Louzada et al., 2007; Bezaeva et al., 2011;Mang et al., 2013; Reznik et al., 2016] and shock-induced magnetic softening (i.e., a decrease in bulk coercivi-ty) [Bezaeva et al., 2010; Kohout et al., 2012]. Halls [1979] observed that target rocks from the most shockedcentral zone of the Slate Islands impact structure had somewhat higher remanent coercivities than targetrocks from the periphery of the exposed central uplift (see Figure 13c of their paper). Shock-induced mag-netic hardening may be observable in target rock samples from other impact craters as well. Shock-induceddecreases in bulk coercivity are less often observed and takes place as a result of either magnetic mineralphase transformations, such as the disordering of tetrataenite into taenite [Bezaeva et al., 2010] or as theresult of shock wave propagation in magnetite-bearing porous targets [Kohout et al., 2012]. Kohout et al.[2012], who studied rocks with preshock bulk densities of �2.1 g/cm3, argue that their observed shock-induced magnetic softening is likely due to higher initial porosity, causing higher shock temperatures (as aresult of pore crushing) accompanied by partial or localized total melting of the sample, melt migration,and possibly recrystallization of magnetite aggregates into larger grains.

With the exception of magnetite in the innermost melt, no magnetic mineral phase transformations wereobserved in our experiments and our targets were not particularly porous (our bulk densities fall between 2.80and 2.84 g/cm3 range, as indicated above in section 2.2). In this work, peak shock pressures P between �10 and>160 GPa produced increases in Bc and Bcr within all specimens of og-1 and og-2 (Table 1 and Figures 6 and 8), upto total melting. For the central glass specimen og-2s_a, the increase in coercivity is attributed to the creation ofnew ferromagnetic minerals during cooling of the shock-induced melt (Figure 7). For other specimens that didnot contain melt glass, coercivity hardening is likely caused by irreversible solid-state changes such as fracturing,domain wall pinning, and introduction of planar defects and dislocations within ferromagnetic grains, and thedevelopment of share bands and twins [Gattacceca et al., 2007; Reznik et al., 2016], which are evident in MFM andSEM images of the shocked samples and their unshocked precursors (Figure 4). Magnetic hardening is accompa-nied by an increase in domain stability as indicated by the shift of bulk hysteresis parameters of shocked speci-mens toward the SD field on the Day plot (Figure 6e) and changes in the FORC distribution (Figure 8).

We did not observe shock-induced formation of superparamagnetic grains in our shocked specimens, asseen by the lack of major frequency-dependent in-phase and out-of-phase magnetic susceptibility compo-nents between 5 and 300 K (Figure S9 in Supporting Information). This result is consistent with [Reznik et al.,2016], but contrasts with the findings of Van de Moortele et al. [2007].

4.3. Discrimination of Thermal Versus Mechanical Effects of Shock on Rock Magnetic PropertiesAs is evident from the presence of shock melt and thermal resetting of magnetic remanence near the cen-ter of the og-1 and og-2 sample spheres, high levels of shock can produce substantial shock-induced heat-ing (Figure 3). Hysteresis data and backfield remanence curve measurements from unshocked og-1specimens after heating under Ar and He atmospheres revealed an increase of Bcr with respect to the

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BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 14

unheated material, so that the discrimination of shock and heat-induced magnetic hardening effects is notstraightforward (Figure 10a). To test whether the shock-induced hardening in og-1 and og-2 is predominant-ly due to mechanical shock-related changes such as microfracturing of ferromagnetic grains (Figure 4), rath-er than shock-induced heating, we conducted stepwise vacuum-heating experiments with unshocked og-1and og-2 specimens using fast (>608C/min) and slow (108C/min) heating-cooling rates (see section S3 inSupporting Information for details). The cooling rate in the heating experiments under vacuum was similarto the cooling timescale associated with the spherical shock experiments. With an exception of specimenog-1s_a, which partially contains shock melt, Bcr values for all shocked og-1 specimens are distinctly higherthan Bcr values of heated unshocked equivalents (Figure 10a). For og-2, temperature-induced changes in Bcr

are far below Bcr values for shocked samples (Figure 10b).

We conclude that with exception of material that underwent melting and associated crystallization of newferromagnetic grains, the shock-induced coercivity hardening in our shocked samples is predominantly dueto solid-state, mechanical effects of shock rather than alteration associated with shock heating. This conclu-sion is supported by the analyses of high-resolution FORCs and visual inspection of their contours (see

Figure 10. Results of heating experiments: remanent coercivity versus postshock temperature for (a) og-1 and (b) og-2. Data labeled as‘‘untreated’’ correspond to unshocked/unheated specimens. Slow-heating experiments (108C/min, black circles) were conducted stepwiseon the same specimens. Heating under argon and helium (red circles) were conducted on separate specimens. Fast-heating experiments(>608C/min, red crosses) were conducted on sister specimens (one specimen per temperature step). Shocked specimens (yellow squares)are shown for comparison. Atmospheric composition does not significantly impact Bcr changes in og-1 and og-2.

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BEZAEVA ET AL. SHOCKED BASALT AND DIABASE 15

above, section 3.2.2). Quantitative comparison of high-resolution FORC diagrams for shocked and heatedog-2 specimens against their untreated analog demonstrate that shock and heating effects are fundamen-tally different: shock does not alter the intrinsic shape of coercivity distributions and of the FORC function(apart from field scaling consistent with the observed magnetic hardening) while heating does as seenfrom the alteration of the FORC contours.

Overall, (i) conducting heating experiments at temperatures resembling those experienced during high-pressure shock events on untreated equivalents of shocked rocks and (ii) quantitative comparison of high-resolution FORC diagrams for shocked, heated, and untreated specimens represent novel, independentmethods to discriminate between thermal and mechanical effects of shock on rock magnetic properties attheir simultaneous action.

4.4. Remanence PropertiesSRM is acquired during shock experiments in ambient magnetic field as a result of shock wave propagationat shock pressures starting at <0.045–0.25 GPa [Nagata, 1971; Pohl et al., 1975; Gattacceca et al., 2010; Tikooet al., 2015]. Above several tens of GPa the postshock temperature increase is high enough to exceed thecorresponding Curie points of ferromagnetic minerals in host rocks [St€offler et al., 1991; Nyquist et al., 2001;Fritz et al., 2005], such that the impact-acquired remanence is a full TRM (and thus no more SRM acquisitionis observed) [Cisowski and Fuller, 1978].

In these experiments, samples that reached pressures >40 GPa experienced a full thermal resetting of theinitial remanence (indicative of heating above the Curie temperatures of low-Ti titanomagnetite-bearingog-2) (Figures 9b and 9c). Thus, shock-induced thermal remagnetization is fully developed at >40 GPa. Weinterpret the remanence of (titano)magnetite-bearing rocks that experienced the pressure range 11–38 GPa(Figure 9a and Table 1) to preserve a preexperiment remanence after removal of a pervasive secondary IRMobserved in all of our shocked specimens. This preexperiment remanence is weaker than in unshocked sam-ples likely as a result of shock demagnetization. Any shock-induced partial thermal overprint or shock rema-nent magnetization that was acquired in the experiment may have resided in low-coercivity grains (<20 mT)that were overprinted by the IRM. Overall, in the pressure range >10 GPa, occurrence of shock demagnetiza-tion versus thermal remagnetization depends on peak shock pressure as well as associated postshock tem-peratures and their relation to the Curie temperature of the dominant remanence carriers in the host rock.

5. Conclusions

The abundance of large, complex craters on terrestrial bodies within our solar system such as Earth, Mars, andthe Moon indicates that large regions of planetary crusts have experienced shock. Unraveling the magneticcharacteristics of planetary materials such as meteorites thereby requires an understanding of the effects ofshock. These effects are intertwined with the effects of temperature as seen in the remanence of the samplessubjected to the highest pressures (>40 GPa), which we attribute to a full thermal remagnetization. Followingour spherical shock experiments to pressures ranging between �10 and 160 GPa, we observed increases inbulk coercivity and the coercivity of remanence in the shocked specimens. Through stepwise heating experi-ments of unshocked samples and quantitative comparison of high-resolution FORCs for shocked, heated, anduntreated specimens, we demonstrate that these changes are due to the mechanical effects of shock ratherthan the accompanying shock-induced heating. Therefore, these rock magnetic changes can primarily beascribed to grain fracturing, domain wall pinning, and the introduction of crystallographic defects within ferro-magnetic grains. Microscopy reveals pervasive fracturing of the FeTi-oxide grains, which is consistent with thismechanism. The observed increase in coercivity suggests that rocks that have been previously shocked topressures ranging between �10 and 160 GPa will be more resistant to acquiring subsequent low coercivityoverprints such as shock remanent magnetization than unshocked analogs.

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