Meteoritics & Planetary Science 48, Nr 5, 758–770 (2013) doi: 10.1111/maps.12100
Thermal modeling of shock melts in Martian meteorites: Implications for preserving Martian atmospheric signatures and crystallization of high-pressure minerals from shock melts Cliff S. J. SHAW1 and Erin WALTON2,3* 1
Department of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada Department of Physical Sciences, MacEwan University, City Centre Campus, Edmonton, Alberta T5J 4S2, Canada 3 Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta T6G 2E3, Canada * Corresponding author. E-mail:
[email protected]/
[email protected]
2
(Received 21 June 2012; revision accepted 30 January 2013)
Abstract–The distribution of shock melts in four shergottites, having both vein and pocket geometry, has been defined and the conductive cooling time over the range 2500 °C to 900 °C calculated. Isolated 1 mm2 pockets cool in 1.17 s and cooling times increase with pocket area. An isolated vein 1 9 7 mm in Northwest Africa (NWA) 4797 cools to 900 °C in 4.5 s. Interference between thermal haloes of closely spaced shock melts decreases the thermal gradient, extending cooling times by a factor of 1.4 to 100. This is long enough to allow differential diffusion of Ar and Xe from the melt. Small pockets (1 mm2) lose 2.2% Ar and 5.2% Xe during cooling, resulting in a small change in the Ar/Xe ratio of the dissolved gas over that originally trapped. With longer cooling times there is significant fractionation of Xe from Ar and the Ar/Xe ratio increases rapidly. The largest pockets show less variation of Ar/ Xe and likely preserve the original trapped gas composition. Considering all of the model calculations, even the smallest isolated pockets have cooling times greater than the duration of the pressure pulse, i.e., >0.01 s. The crystallization products of these shock melts will be unrelated to the peak shock pressure experienced by the meteorite.
INTRODUCTION Isolated regions of silicate glass containing a variety of microlites are found heterogeneously distributed throughout the groundmass of strongly shocked chondrite and achondrite meteorites (Dodd and Jarosewich 1979, 1982; Chen et al. 1996; Gillet et al. 2000; Malavergne et al. 2001; Xie et al. 2002, 2006; Beck et al. 2004). These features are called shock-melt veins or shock-melt pockets to indicate their origin via impact on the parent body. They are interpreted to have formed in local hot spots (up to 2500 K) by shock impedance contrasts or frictional melting along shear bands as shock waves traveled through heterogeneous, cracked, and/or porous materials (Langenhorst and Poirier 2000; Beck et al. 2004, 2007). The hot spots are distinct from the bulk rock in which the temperature increase, by shock compression and the nonadiabatic deposition of heat
© The Meteoritical Society, 2013.
after decompression, was limited to a few hundred degrees (Sharp and DeCarli 2006). This study focuses on shock-melt veins and pockets (hereafter referred to simply as veins and pockets) in shergottites: mafic, permafic, or ultramafic igneous rocks from Mars having subophitic, porphyritic, or poikilitic textures (Walton et al. 2012). Shock melts are ubiquitous among shergottites, comprising up to 14 vol% of the host rock (Allan Hills [ALH] 77005; Treiman et al. 1994). Shock melts in shergottites are of particular interest because they host a nearly pure sample of the Martian atmosphere, defined by isotopic ratios and abundances of N2, CO2, and noble gases (Bogard and Johnson 1983; Marti et al. 1995; Walton et al. 2007). In this study, we present a detailed analysis of the postshock thermal history of four shergottites using the 2D mode of the HEAT model developed by K. Wohletz (Wohletz et al. [1999] and http://geodynamics.
758
Thermal modeling of shock melts in Martian meteorites
lanl.gov/Wohletz/Heat.htm). The goals are twofold: (1) to resolve the discrepancy between shock-melt cooling times, i.e., the time required for cooling to the solidus of the melt, derived from previous calculations of heat flow and those from dynamic crystallization experiments, and (2) to assess the thermal history of natural meteorites with a range of shock-melt distributions and abundances. The models in this study provide refined estimates for the rate of meteorite cooling after a shock event. RATIONALE FOR CURRENT STUDY Shock veins and shock-melt pockets comprise material that was locally melted (Fredriksson et al. 1963) and then cooled by conduction of heat to the surrounding host rock (Langenhorst and Poirier 2000; Leroux et al. 2000; Sharp et al. 2003; Xie et al. 2006). Calculations by Beck et al. (2007) indicate cooling rates for a 1 mm diameter shock melt of 5000 °C s 1 over the cooling interval 2500–500 °C, giving a cooling time for this interval of 0.2 s. This is considerably shorter than cooling rates of 0.2–0.3 °C s 1 determined by Walton et al. (2006) from dynamic crystallization experiments. These longer cooling times correspond to a cooling duration of 8 16 min to hours for the largest cm-size shock melts found in shergottites. Reaction textures between shock melts and host rock minerals are consistent with cooling times longer than those estimated by Beck et al. as indicated by experimental data (Walton and Shaw 2009). The cooling times of shergottite shock melts are of particular importance for sampling of Martian atmosphere. The longer cooling times of Walton et al. (2006) support diffusion of Martian atmosphere to the host rock, which has the potential to erase or modify that atmospheric signature, especially if the diffusion rates of the gases involved are significantly different. Cooling times of shock melts also have implications for the pressure at which shock melts crystallize and cool below the solidus. This is important because the mineral assemblages that crystallize within shock melts, when compared with phase diagrams obtained from static high-pressure experiments, can be used to constrain the pressure conditions of crystallization (see discussions in Sharp and DeCarli 2006; Gillet et al. 2007). How the crystallization pressure relates to the shock history of the meteorite will depend on two factors: the shock duration, defined as the time lag between the arrival of the initial shock wave and the production of the release wave, and the quench time of the melt (Xie et al. 2006). Constraints on the shock duration in one shergottite, Zagami, have been obtained by studying trace-element concentrations in liquidus aggregates of K-hollandite in a shock-melt pocket (Beck
759
et al. 2005). This method assumes that the trace-element partitioning took place during the shock pulse so the time required for a trace element to diffuse from the melt into the K-hollandite can be used to calculate a minimum value for the duration of shock pressure. Based on their measurements of Cs, Ba, and Rb the equilibrium shock pressure duration was found to be of the order of 10 ms (0.01 s). Similar time estimates for the shock duration are derived based on formation and preservation of high-pressure phases in Chassigny (Fritz and Greshake 2009). If the shock duration exceeds the quench time, crystallization occurs at the peak pressure and the mineral assemblage that crystallizes will be directly related to the peak shock pressure experienced by the meteorite. If the shock duration is shorter than the quench time, only part of the cooling path will be at high pressure with remainder occurring after the passage of the release wave. Finally, if the quench time is much longer than the shock duration the shock melt will remain molten after pressure release to crystallize a mineral assemblage whose formation conditions are unrelated to the shock-pressure conditions. PETROGRAPHY OF SHOCK MELTS IN SHERGOTTITES Shock-melt veins are easily observed in polished sections as black to brown veins cutting across the entire meteorite sample. Their widths vary from 1–2 lm up to several millimeters and they may be interconnected or occur as single, straight features. Shock-melt pockets are rounded or amoeboid features, varying in size from
