THE HETEROGENEOUS RESPONSE OF MARTIAN METEORITE ALLAN HILLS 84001 TO PLANAR SHOCK

Introduction: Martian meteorite Allan Hills 84001 (ALH 84001) was ejected from the surface of Mars around 14 million years ago (Ma) [1]. Predominantly orthopyroxenite, the meteorite contains coarse-grained inclusions of chromites, carbonates and plagioclase feldspar in addition to fine-grained iron oxides and sulfides that host a heterogeneously oriented natural remanent magnetization (NRM) [2]–[5]. The meteorite contains several shock-induced textures and mineral thermometers indicative of one or more impact events. Petrologic and paleomagnetic studies have shown shock metamorphism to be fundamental to the mineralogical development of ALH 84001 [6]. However, the strength of these impacts is poorly understood as there are contradictory interpretations of evidence for temperature excursions within the meteorite [6]–[9]. The NRM hosted in the iron oxides and sulfides embedded within chromites is understood to be a thermoremanent magnetization (TRM) and has two strongly magnetized components that do not share common alignment, in addition to several incoherent, weakly magnetized grains, indicative of an underlying mechanism capable of localized (~200 nm) heating [5]. To place new constraints on the shock pressures associated with multiple impacts suffered by the meteorite up to and including its ejection from Mars and attempt to reconcile independently reported thermal histories of the meteorite, we have simulated planar shock wave propagation through computational analogs of two samples of ALH 84001. Modeling: Using the iSALE-2D shock physics code [10]–[12], we have performed a suite of ‘mesoscale’ simulations to quantify the effects of impact-induced shockwaves likely to have been experienced by the meteorite. The initial simulation geometry is a ‘sandwich’ design whereby a 2D planar impactor with a given velocity collides with an initially stationary cover plate. A shock front is generated in the cover plate and travels into the sample, while a release wave forms at the rear of the impactor. After the shock front travels through the sample, it dissipates through a buffer plate at the bottom of the computational mesh. The simulation was run until the release wave had passed through the sample and the thermodynamic response of different components within the meteorite is recorded by Lagrangian tracers placed throughout the Eulerian mesh. Material Models: The constituent materials that make up the meteorite are each described by an equation of state (EOS) and strength model. As the availability of accurate equations of state for meteoritic materials is limited, we used the closest analog materials possible. A main criterion when representing the meteorite was to maintain the natural heterogeneity of the system while eliminating the smallest inclusions (<<10 μm). Therefore, we divided ALH 84001 into four constituent parts: chromites, carbonates, plagioclase and orthopyroxene, which are described by material models for chromite (developed for this study), calcite [13], gabbroic anorthosite (gabbro) [14] and dunite [15, 16], respectively. The chromite and gabbro are described using a Tillotson EOS [17], while the calcite and dunite are represented with an ANEOS-derived tabular EOS. A pressure-, temperatureand strain-dependent strength model was used to describe the resistance of the geologic materials to shear deformation, while a pressure-independent ductile strength model was used to describe the shear strength of chromite [10]. Paleomagnetic studies have placed constraints on the thermal history of ALH 84001 based on measurements of iron sulfides and oxides present within the meteorite. We represented all these iron compounds using our material model for chromite. The Tillotson parameters (a, b, α, β, B) were fitted to shock data for magnetite, while other material constants were chosen to be appropriate for chromite. Results: Following the passage of the shock wave, we find strong and complex material shear that causes intense and well-defined thermal gradients across the

Introduction: Allan Hills 84001 (ALH 84001) is an orthopyroxenite Martian meteorite containing coarse-grained inclusions of chromites, carbonates and plagioclase feldspar, in addition to fine-grained iron oxides and sulfides that host a heterogeneously oriented natural remanent magnetization (NRM) [1]- [4].The meteorite contains several shockinduced textures and mineral thermometers indicative of one or more impact events.
The NRM hosted in the iron oxides and chromite-sulfide assemblages within the meteorite is understood to be a thermoremanent magnetization (TRM) and has two strongly magnetized components that do not share common alignment, in addition to several incoherent, weakly magnetized grains, indicative of an underlying mechanism capable of localized (~200 nm) heating [5].
We have developed a methodology, using thermal constraints from paleomagnetism and petrologic observation, where we are able to place new constraints on the shock pressures associated with multiple impacts suffered by the meteorite up to and including its ejection from Mars.To reconcile the reported thermal histories of the meteorite, we have simulated planar shock wave propagation through computational analogs of two samples of ALH 84001.
Modeling: Using the iSALE-2D shock physics code [6]-[8], we have performed a suite of 'mesoscale' simulations to quantify the effects of impact-induced shockwaves likely to have been experienced by the meteorite.The materials used in our simulations are each described by an equation of state and strength model.As the availability of accurate equations of state for meteoritic materials is limited, we have used the closest analog materials possible.
Results: We found strong and complex material shear responsible for steep thermal gradients throughout the sample.Shearing occurs principally in the rock matrix, using the (weaker) inclusions as nucleation points (Fig. 1).We see both intraand inter-material variations in temperature on length scales of tens of microns.
Subsequent modeling of post-impact thermal equilibration reveals that the constituent materials reach equilibrium ~3 seconds after the release wave has passed through the meteorite (Fig. 1).This has implications for paleomagnetism: small fractions of the meteorite may be remagnetized in low-pressure impacts, meaning the meteorite is capable of hosting NRMs recorded at different times.
Implications for Paleomagnetism: Palaeomagnetic studies of this meteorite have found a heterogeneously oriented pattern of remanent magnetization, indicative of remagnetization in the meteorite on the sub-millimeter scale, but the mechanism for such heterogeneous heating was unclear.We observe that portions of chromite grains close to shear zones will experience temperatures significantly higher than those elsewhere in the meteorite which only warm up to the equilibrium temperature.Since the meteorite was magnetized in an initial extensive thermal event where the whole meteorite was heated above the curie point of the chromite-sulfide assemblages, our simulations suggest that a subsequent impact with a bulk shock pressure between 25-45 GPa would achieve partial remagnetization.
Figure 1: Thermal experience of a sub-volume of the cross-sections we examined of the cross-section close to a shear zone in a 33 GPa impact.Time zero is immediately after the release wave passes through the sample.We see the chromite experiences a maximum temperature after the impact due to heating from a nearby shear zone before settling to thermal equilibrium approximately 3 seconds after the impact.