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Age and Temperature of Shock Metamorphism of Martian Meteorite Los Angeles from (U-Th)/He Thermochronometry

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Age and Temperature of Shock Metamorphism of Martian Meteorite Los Angeles from (U-Th)/He Thermochronometry Age and temperature of shock metamorphism of Martian meteorite Los Angeles from (U-Th)/He thermochronometry Kyoungwon Min Peter W. Reiners Stefan Nicolescu James P. Greenwood Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA ABSTRACT Mineralogic features attributed to impact-induced shock metamorphism are commonly observed in meteorites and terrestrial impact craters. Partly because the duration of shock metamorphism is very short, constraining the timing and temperature of shock events has been problematic. We measured (U-Th)/He ages of single grains of merrillite and chlor- apatite from the Martian meteorite Los Angeles (LA). Merrillite and chlorapatite ages cluster at 3.28 6 0.15 Ma (2s) and 2.18 6 0.19 (2s) Ma, respectively. The mean age of the merrillites, which are larger than chlorapatites, is indistinguishable from cosmic-ray exposure ages (3.1 6 0.2 Ma), suggesting that impact-induced shock metamorphism was coeval with ejection of the LA precursor from Mars. To constrain the initial temperature of shock metamorphism in the LA precursor body, we modeled diffusive loss of He from merrillite as a function of diffusion domain size, LA precursor body size, and ablation depth. From these calculations, we suggest that the metamorphic temperature of the shock event was higher than 450 8C. These results support the idea that shock pressures of the Martian meteorite Shergotty were higher than 45 GPa, as inferred from the presence of post-stishovite SiO2 polymorphs. Single-grain (U-Th)/He dating of phosphates may pro- vide unique constraints on the timing and pressure-temperature dynamics of shock meta- morphism in a wide variety of extraterrestrial materials. Keywords: helium, diffusion, merrillite, chlorapatite, Los Angeles Martian meteorite, shock metamorphism. INTRODUCTION led to the meteorite's ejection from Mars is ,350 8C for the metamorphism thought to Impact processes among planets or aster- only indirectly constrained by cosmic expo- have occurred ca. 15 Ma. Compared with oids play a key role in solar system dynamics, sure ages from several different systems that 40Ar/39Ar, (U-Th)/He dating has only rarely changing original morphologies of parent bod- yield ages ranging from 1.9 Ma (Garrison and been applied to meteorites (Paneth et al., ies and sometimes creating new satellites (e.g., Bogard, 2000) to 3.1 Ma (Nishiizumi et al, 1930, 1952; Arrol et al., 1942; Heymann, formation of the Moon). Understanding im- 2000; Terribilini et al., 2000), with a preferred 1967; summarized in Wasson and Wang, pact processes is also important for investi- age of 3.1 6 0.2 Ma (Nyquist et al., 2001). 1991; Min et al., 2003). The unique low- gating hypothesized biological interactions be- To better estimate the timing of impact- temperature thermal sensitivity of the phos- tween Earth and other planets such as Mars, induced shock metamorphism and compare its phate (U-Th)/He system may provide more because extreme conditions of impact-induced age with that of cosmic exposure, we applied detailed constraints on relatively low temper- shock metamorphism may have obliterated bi- the low-temperature (U-Th)/He thermochron- ature and short duration shock episodes. In ological entities or their signatures. Many me- ometer to phosphates from LA. particular, (U-Th)/He dating may be useful for teorites bear evidence of shock metamorphic In addition to cooling ages, thermochrono- this purpose because: (1) U- and Th-rich min- effects resulting from at least one impact metry can also constrain metamorphic thermal erals (e.g., phosphates) are common in many event, providing valuable information on the histories. The 40Ar/39Ar thermochronometry meteorites, (2) He diffusion in apatites is well dynamics of impact processes that are useful has been widely applied to meteorites and lu- characterized (Farley, 2000) and sensitive to for addressing these and other issues. nar samples for this purpose. Bogard and temperatures and durations previously inferred The differentiated shergottite meteorite Los Hirsch (1980) documented correlations be- for shock events, and (3) the accessible age Angeles (LA) has a basaltic composition and tween 40Ar/39Ar ages and the intensity of range of the method spans from at least the comprises mainly maskelynite (shock-induced shock metamorphism in several chondrites, beginning of the solar system (ca. 4.5 Ga: Min glass with plagioclase composition), pyrox- suggesting that 40Ar/39Ar systems are at least et al., 2003) to relatively modern human his- ene, SiO2, fayalite, felsic glass, sul®de, iron- partially in¯uenced by shock events. They tory (ca. AD 79: Aciego et al., 2003). We ap- titanium oxides, and phosphates (merrillites also deduced postshock cooling rates based on plied single-grain (U-Th)/He dating to merril- and chlorapatites) (Rubin et al., 2000). The age spectra and diffusion experiments. Using lites and chlorapatites in LA to better Sm/Nd and Rb/Sr isotopic data suggest a crys- Bogard and Garrison's (1999) 40Ar/39Ar data understand several aspects of impact process- tallization age of ca. 175 Ma (Nyquist et al., from Martian meteorite ALH84001, Weiss et es, particularly timing and temperature con- 2001), but the timing of the impact event that al. (2002) inferred a maximum temperature ditions of the shock metamorphism. Basic pet- q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; August 2004; v. 32; no. 8; p. 677±680; doi: 10.1130/G20510.1; 2 ®gures; 1 table; Data Repository item 2004111. 677 TABLE 1. (U-Th)/He DATA FOR MERRILLITES AND CHLORAPATITES IN LOS ANGELES quist et al., 2001). The results also imply that MARTIAN METEORITE LA had not been exposed to cosmic rays on 4 4 #4 ²² Sample U Th U/Th He total (U-Th)/He Hecosmo (U-Th)/He 1s He loss the Martian surface or at shallow depths be- ² § (pg) (pg) (fmol) age (%) agecorrected (%) (Ma) (Ma) fore ejection, like other shergottites (Nishi- izumi et al., 1986), because prolonged expo- Merrillites LA1IIE36 4.45 33.2 0.134 2.17 32.5 0.6 32.4 0.90 83.4 sure on Mars would have yielded LA1IIF33 8.24 49.3 0.167 1.11 10.3 1.7 10.1 0.32 96.0 metamorphic ages younger than cosmic-ray LA1IIE35 22.8 148 0.154 1.10 3.51 5.1 3.34* 0.10 99.9 LA1IIF22 8.20 79.0 0.104 0.502 3.46 5.9 3.25* 0.18 99.9 exposure ages. LA1IIF14 3.26 32.2 0.101 0.202 3.44 6.0 3.24* 0.39 99.9 LA1IIE23 7.79 66.8 0.117 0.426 3.34 5.9 3.14* 0.20 100 LA1IIE41 4.77 36.3 0.131 0.238 3.29 5.8 3.10* 0.38 100 PEAK TEMPERATURE OF SHOCK LA1IIF18 4.53 40.5 0.112 0.215 2.83 7.1 2.63 0.31 100 METAMORPHISM LA1IIF12 21.9 59.7 0.367 0.477 2.45 4.7 2.36 0.13 100 The simplest interpretation of the abundant Mean of the 5 ages: 3.28 0.08 ca. 3 Ma He ages is that they represent vir- Chlorapatites tually complete resetting of the phosphate (U- LA1IIE43 6.65 7.17 0.927 0.261 5.79 2.9 5.62 0.56 LA1IIE21 4.53 8.41 0.539 0.137 3.88 6.5 3.63 0.73 Th)/He systems from a much older preimpact LA1IIE66 17.3 20.0 0.867 0.346 2.92 6.1 2.74 0.20 age (ca. 175 Ma, if the target rock resided near LA1IIF32 6.28 5.94 1.06 0.107 2.58 5.8 2.43* 0.59 the Martian surface at relatively low temper- LA1IIE68 8.73 26.5 0.330 0.196 2.42 14 2.08* 0.29 LA1IIF19 39.9 33.6 1.19 0.599 2.32 5.9 2.19* 0.10 atures after crystallization). Thus, assuming LA1IIE51 34.2 31.7 1.08 0.415 1.85 8.1 1.70 0.11 diffusion characteristics of He in the phos- Mean of the 3 ages: 2.18 0.10 phates and other assumptions related to the *Samples used to calculate mean age. impact event and the LA precursor body, we ²Uncorrected (U-Th)/He age. §(U-Th)/He ages corrected for cosmogenic 4He. The contents of cosmogenic 4He were calculated from (1) 4He can constrain the minimum peak metamorphic production rate (8.05 3 1028 cm3´g21´m.y.21: Eugster, 1988; Lorenzetti et al., 2003), (2) exposure age (3.1 Ma: temperature of the shock metamorphism. In Nyquist el al., 2001), and (3) mass of the samples (see text footnote 1 for details). this model we assume that the entire (spheri- #Uncertainties propagated from analytical errors in U, Th, and He analyses. ²²He loss during the shock metamorphism at 3.1 Ma calculated for crystallization age of 175 Ma. cal) precursor body reached a peak metamor- phic temperature (Ti) instantaneously and ho- mogeneously, and then cooled conductively to rographic and oxygen isotope studies of LA In most cases the analyzed merrillite sam- an ambient surface temperature of 270 8C phosphates (Greenwood et al., 2003) provided ples were fragments of originally larger, an- (the equilibrium temperature of light-colored the petrographic basis for this study. hedral crystals (Fig. DR1; see footnote 1). If chondrites at Mars' distance from the Sun: the fragments were originally located .;20 Butler, 1966). We assumed a thermal diffusiv- (U-Th)/He AGES mm from grain boundaries, and intragranular ity of 0.01 cm2/s, typical of terrestrial basaltic Nine merrillite grains ;100 mm in average U and Th zonation was not strong, then the rocks.
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