Ar‐Ar ages and thermal histories of enstatite meteorites

Abstract

Abstract-Compared with ordinary chondrites, there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies. In this study, we report 39 Ar-40 Ar dating results for five EL chondrites: Khairpur, Pillistfer, Hvittis, Blithfield, and Forrest; five EH chondrites: Parsa, Saint Marks, Indarch, Bethune, and Reckling Peak 80259; three igneous-textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies: Zaklodzie, Queen Alexandra Range 97348, and Queen Alexandra Range 97289; and three aubrites, Norton County, Bishopville, and Cumberland Falls Several Ar-Ar age spectra show unusual 39 Ar recoil effects, possibly the result of some of the K residing in unusual sulfide minerals, such as djerfisherite and rodderite, and other age spectra show 40 Ar diffusion loss. Few additional Ar-Ar ages for enstatite meteorites are available in the literature. When all available Ar-Ar data on enstatite meteorites are considered, preferred ages of nine chondrites and one aubrite show a range of 4.50-4.54 Ga, whereas five other meteorites show only lower age limits over 4.35-4.46 Ga. Ar-Ar ages of several enstatite chondrites are as old or older as the oldest Ar-Ar ages of ordinary chondrites, which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies, or were metamorphosed to lower temperatures compared to other chondrite types. Many enstatite meteorites are brecciated and ⁄ or shocked, and some of the younger Ar-Ar ages may record these impact events. Although impact heating of ordinary chondrites within the last 1 Ga is relatively common for ordinary chondrites, only Bethune gives any significant evidence for such a young event.

Shock effects in “EH6” enstatite chondrites and implications for collisional heating of the EH and EL parent asteroids

“…The Blithfield brecciated EL impact-melt rock has a corrected Ar-Ar plateau age of 4.534 Ga (Bogard et al, 2010). These dates range from 31 to 135 Ma after accretion occurred $4.565 Ga ago (Carlson and Lugmair, 2000;Wadhwa et al, 2008;Kleine et al, 2009) and are long after any 26 Al (t ½ = 730,000 years) that was initially present had decayed away.…”

“…Argon-argon data (corrected for errors in the 40 K decay constant) show that the Abee and RKP 80259 EH impactmelt breccias have Ar-Ar ages of $4.52 and 4.24 Ga, respectively (Bogard et al, 2010 and references therein). The Blithfield brecciated EL impact-melt rock has a corrected Ar-Ar plateau age of 4.534 Ga (Bogard et al, 2010).…”

“…This correspondence is suggestive that these F-rich phases are themselves petrographic indicators of impact processing. Bogard et al (2010) reviewed literature data and determined Ar-Ar ages for a suite of enstatite meteorites. They concluded that several EH and EL chondrites had experienced collisional heating early in solar system history that was sufficiently intense to have disturbed both the Ar-Ar and Rb-Sr chronometers.…”

Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body

“…However, some samples, notably Bishopville, Cumberland Falls, and Norton County, show signs of later disturbance, resulting in younger Rb‐Sr and Ar‐Ar ages or precluding obtainment of statistically meaningful ages (Compston et al. 1965; Bogard et al. 2010).…”

40Ar/39Ar thermochronology of the fossil LL6-chondrite from the Morokweng crater, South Africa

“…The 40 Ar/ 39 Ar chronometer typically has a low closure temperature of a few hundred degree celsius and thus has the potential to determine heating events such as large collisions between asteroids (Bogard, 2002). When applied to the Morokweng meteorite, 40 Ar/ 39 Ar thermochronology provides an opportunity to study (1) effects associated with pre-impact and post-impact processes and (2) collision events within a potentially unsampled and distinct asteroid population from the LL category.…”