⁴⁰Ar/³⁹Ar ages of Northwest Africa 7034 and Northwest Africa 7533

Abstract

The Martian breccias NWA 7034, NWA 7533, and paired meteorites record events ranging in age from 4.47 Ga to <200 Ma. Published ages indicate a period of major disturbance at~1.4 Ga, examined in detail here through 40 Ar/ 39 Ar dating of handpicked grains and two small chips. Argon diffusion parameters were obtained for six samples. Also presented are He, Ne, Ar, Kr, and Xe contents of two small (<100 µg), handpicked mineral separates, a felsic "Light" sample and a mafic/pyroxene-rich "Dark" sample. The 40 Ar/ 39 Ar ages of five samples, four containing >1 wt% K and thought to be rich in feldspar and one containing <~1 wt% K, cluster near 1.4 Ga. The 40 Ar/ 39 Ar ages of nine grains with low K contents have a wide range of apparent ages from 0.3 AE 0.1 Ga to 2.9 AE 0.1 Ga for individual temperature steps, and from 0.74 AE 0.06 Ga to~2.1 Ga for plateau ages. Isochron ages are less precise, but generally agree with plateau ages. Only two isochrons have the significantly positive intercepts expected in the presence of terrestrial or Martian atmospheric argon. At higher release temperatures, activation energies for diffusion obtained from 39 Ar data for six samples are generally 160-200 kJ mol −1 , consistent with published values for feldspathic minerals. For three of these samples, lower temperature data on Arrhenius plots are best fit with a much lower activation energy of <100 kJ mol −1 . We attribute the low values to the effects of varying degrees of shock on feldspathic minerals and/or the presence of phases in vitrophyric spherules produced by hydrothermal alteration. The low activation energies place an upper limit of~14 ka on the terrestrial age of NWA 7034. Much lower concentrations of cosmogenic (c) 3 He and 21 Ne in the Light than in the Dark separate indicate substantial losses concurrent with or postdating cosmic ray irradiation. A one-stage, cosmic ray exposure (CRE) age for the Dark separate from NWA 7034 is estimated to be between 7 and 10 Ma from the concentrations of 3 He c and † Deceased June 14, 2017.

40Ar/39Arages ofL4,H5,EL6,and feldspathic ureilitic clasts from the Almahata Sitta polymict ureilite (asteroid 2008TC3)

“…Lindsay et al. (2014, 2021) and references therein provide details. Taken together, the energy‐ and wavelength‐dispersive measurements are regarded as a rough guide to the composition of a grain but may not be fully representative.…”

In situ 87Rb–87Sr analyses of terrestrial and extraterrestrial samples by LA-MC-ICP-MS/MS with double Wien filter and collision cell technologies

“…The solid horizontal line is the isochron calculated using all in situ data ($2.2 Ga; Fig. 11), while the dashed horizontal line is the 40 Ar/ 39 Ar plateau age obtained for K-rich grains by Lindsay et al 81 As shown, the grains show no obvious sign of resetting at 1.4 Ga.…”

“…Chronometers more prone to thermal disturbance indicate much more recent resetting, with most 40 Ar- 39 Ar ages clustering around 1.4 Ga. 78,80,81 U + Th + Sm/He systematics point to even more recent resetting within the last $150 Ma. 78,[80][81][82] The 1.4 Ga age has been interpreted as the age of lithication (formation of the breccia) 81 or resetting by protracted heating associated with plume magmatism. 78 The 87 Rb- 87 Sr system complicates chronological inferences even further as the isochron obtained by Agee et al 70 on mineral separates yielded an age of 2.089 AE 0.081 Ga.…”

“…Individual spot analyses are consistent with an 87 Rb-87 Sr age of $2.2 Ga and there is no indication of resetting at 1.4 Ga even for the smaller grains. Lindsay et al 81 calculated the temperature-time conditions needed for radiogenic 40 Ar to be lost and 40 Ar/ 39 Ar ages to be reset at 1.4 Ga in K-rich feldspar. Their estimate that heating was short-lived, ranging between $5 h at 1000 C and 10 year at 500 C is likely to be a minimum as they did not account for the discrete distribution of diffusion domain sizes apparent in the degassing of virtually all their samples.…”

“…69,72,74-77 U-Pu-Xe, 78 146 Sm-142 Nd and 147 Sm-143 Nd 79 also point to very early formation of some components within Black Beauty. Chronometers more prone to thermal disturbance indicate much more recent resetting, with most 40 Ar- 39 Ar ages clustering around 1.4 Ga. 78,80,81 U + Th + Sm/He systematics point to even more recent resetting within the last $150 Ma. 78,[80][81][82] The 1.4 Ga age has been interpreted as the age of lithication (formation of the breccia) 81 or resetting by protracted heating associated with plume magmatism.…”