Download ON DISCRIMINATION OF THERMAL VERSUS MECHANICAL

79th Annual Meeting of the Meteoritical Society (2016)
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ON DISCRIMINATION OF THERMAL VERSUS MECHANICAL EFFECTS OF SHOCK ON ROCK
MAGNETIC PROPERTIES OF SPHERICALLY SHOCKED UP TO ∼10-140 GPA
BASALT AND DIABASE.
N. S. Bezaeva1,2,3, N. L. Swanson-Hysell4, S. M. Tikoo4,5, M. Kars6, R. Egli7
1
Faculty of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, 119991 Moscow, Russia. Email:
[email protected]. 2Ural Federal University, 19 Mira Str., 620002 Ekaterinburg, Russia. 3Kazan Federal
University, 18 Kremlyovskaya Str., 420008 Kazan, Russia. 4University of California, Berkeley, CA, 94720, USA.
5
Department of Earth and Planetary Sciences, Rutgers University, Piscataway Township, NJ, 08854, USA. 6Center
for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, 783-8502, Japan. 7Central Institute for Meteorology and Geodynamics, Hohe Warte 38, A-1190 Vienna, Austria
Introduction: Hypervelocity impacts represent a major mechanism for the evolution of the solid matter in our
solar system. Shock waves generated during impacts can modify the properties of magnetic minerals in rocks and
their remanent magnetization [1]. Understanding the physical mechanisms associated with shock-induced changes in
bulk magnetic properties is important for interpreting the paleomagnetic records of lunar rocks, meteorites, and
cratered planetary surfaces. Laboratory simulations of impact events show that ultra-high shock experiments induce
non-negligible post-shock heating of the target material. At high pressures (>10 GPa), shock heating occurs in tandem with mechanical effects, such as grain fracturing and creation of crystallographic defects and dislocations within magnetic grains. This makes it difficult to conclude whether shock-induced changes in the rock magnetic properties of target materials, such as magnetic hardening (i.e., shock-induced increase in remanent coercivity Bcr) are
primarily associated with mechanical or thermal effects. Here we present a new experimental method to discriminate
between mechanical and thermal effects of shock on mineral magnetic properties. We illustrate this method with two
examples of spherically shocked terrestrial basalt and diabase.
Following ref. [2], we conducted spherical shock experiments on basaltic lava flow (og-1) and diabase dike (og2) samples from the Osler Volcanic Group of the Mid-Continent Rift near Lake Superior, Canada [3]. Our goal was
to characterize the magnetic changes induced in these rocks by shock pressures ranging between ∼10-140 GPa [4].
In the previous work [4], we described spherical shock experiments on og-1 and og-2 that resulted in partial to total
melting of the innermost parts of the spherical samples, along with a variety of solid-state shock effects such as
thermal resetting of remanent magnetizations and an increase in remanent coercivity (Bcr). In this study, we investigate possible explanations for the observed post-shock increase in Bcr.
Methods and Results: To test whether the shock-induced Bcr increase in og-1 and og-2 was predominantly due
to ferromagnetic mineral microfracturing rather than shock-induced heating, we conducted new stepwise heating
experiments under vacuum up to 700-850°C on unshocked specimens using slow (10°C/min) and fast (>60°C/min)
heating-cooling rates. The cooling rates employed in the vacuum heating experiments were similar to the cooling
timescales associated with the spherical shock experiments. Apart for one specimen (which partially consisted of
shock melt and was therefore petrologically distinct from the unshocked sample), Bcr values of our shocked og-1 and
og-2 specimens were distinctly higher than the Bcr values of heated unshocked specimens. Because purely temperature-induced changes in Bcr (in unshocked specimens) were modest compared to the changes in the shocked specimens, we favor a mechanical origin for the increase in Bcr in the spherically shocked og-1 and og-2 specimens.
First order reversal curves (FORCs) provide detailed information about the distribution of microscopic coercivities within a sample (not reflected by mean Bcr values) and about the type of irreversible magnetization processes
occurring during hysteresis. In order to further characterize rock magnetic changes resulting from mechanical and
thermal effects, we conducted high-resolution FORC measurements using in-field steps of 0.5-0.7 mT on selected
shocked (post-shock temperature: ∼550°C), heated (up to ∼700°C) and untreated samples of og-2. Visual inspection
of FORC contours (in addition to detailed analyses) reveals a “stretching” the FORC distribution of shocked sample
towards higher coercivities, consistent with shock-induced hardening. However, shock does not alter the intrinsic
shape of coercivity and the shape of FORC contours (apart from field scaling) while heating does, for instance
through the growth of new magnetic particles, which is seen as a significant alteration of FORC contours. Therefore,
we conclude that, with exception of material that underwent melting, the shock-induced coercivity hardening in our
shocked samples is predominantly due to solid-state, mechanical effects of shock rather than alteration associated
with shock heating. Acknowledgements: The work is supported by Act 211 Government of the Russian Federation, agreement №
02.A03.21.0006, IRM Visiting Fellowships to NSB, NLS-H and SMT, and is performed according to the Russian Government Program
of Competitive Growth of Kazan Federal University. We are grateful to D.D. Badyukov (GEOKHI RAS) and D.A. Chareev (IEM RAS).
References: [1] Weiss B. P. et al. 2010. Space Science Reviews 152:341-390. [2] Bezaeva N. S. et al. 2010. Meteoritics & Planetary Science 45:1007-1020. [3] Swanson-Hysell N. L. et al. 2014. Geochemistry Geophysics Geosystems 15:2039-2047. [4] Tikoo et al. 2015. Meteoritics & Planetary Science 50 (S1):A326.