Cosmic‐ray exposure ages of pallasites

Abstract

We analyzed cosmogenic nuclides in metal and/or silicate (primarily olivine) separated from the main‐group pallasites Admire, Ahumada, Albin, Brahin, Brenham, Esquel, Finmarken, Glorieta Mountain, Huckitta, Imilac, Krasnojarsk, Marjalahti, Molong, Seymchan, South Bend, Springwater, and Thiel Mountains and from Eagle Station. The metal separates contained an olivine fraction which although small, <1 wt% in most cases, nonetheless contributes significantly to the budgets of some nuclides (e.g., up to 35% for 21Ne and 26Al). A correction for olivine is therefore essential and was made using model calculations and/or empirical relations for the production rates of cosmogenic nuclides in iron meteoroids and/or measured elemental concentrations. Cosmic‐ray exposure (CRE) ages for the metal phases of the main‐group pallasites range from 7 to 180 Ma, but many of the ages cluster around a central peak near 100 Ma. These CRE ages suggest that the parent body of the main‐group pallasites underwent a major break‐up that produced most of the meteorites analyzed. The CRE age distribution for the pallasites overlaps only a small fraction of the distribution for the IIIAB iron meteorites. Most pallasites and IIIAB irons originated in different collisions, probably on different parent bodies; a few IIIABs and pallasites may have come out of the same collision but a firm conclusion requires further study. CRE ages calculated from noble gas and radionuclide data of the metal fraction are higher on average than the 21Ne exposure ages obtained for the olivine samples. As the metal and olivine fractions were taken in most cases from different specimens, the depth‐dependency of the production rate ratio 10Be/21Ne in metal, not accounted for in our calculations, may explain the difference.

Carbonaceous and noncarbonaceous iron meteorites: Differences in chemical, physical, and collective properties

“…This resulted in a small iron core encircled by a narrow spherical shell of pallasitic material. The 36 Ar/ 36 Cl CRE age of Eagle Station metal is 43 ± 5 Ma; this is shorter than the CRE ages of metal from all other pallasites analyzed by Herzog et al. () except Esquel (11 ± 1 Ma). The average CRE ages of the metal in all pallasites in their data set (excluding Eagle Station and Esquel) is 109 ± 32 ( n = 15).…”

“…The clustering of ages near 100 Ma suggests that the main‐group pallasite parent asteroid suffered a major collision at that time and likely disrupted (Herzog et al. ). The shorter CRE age of Eagle Station compared to main‐group pallasites is not consistent with the tendency of CC‐iron groups to have longer CRE ages than NC‐iron groups (Table ).…”

Experimentally determined subsolidus metal-olivine element partitioning with applications to pallasites

“…The only gap in the olivine element record is for Glorieta Mountain, for which no Co data are published. The high-Co and high-Ni values for Brenham were reported from ICP-OES analysis (Herzog et al, 2015), and may have incidentally incorporated some metal in analyses.…”

“…McKibbin et al (2013) investigated a Brenham olivine grain and reported all elements of interest. Additional single point or average olivine analyses come from a variety of studies (Buseck and Goldstein, 1969;Leitch et al, 1979;Lugmair and Shukolyukov, 1998;Wasson et al, 1999;Lucas, 2011;Boesenberg et al, 2012;Herzog et al, 2015;Greenwood et al, 2015). The only gap in the olivine element record is for Glorieta Mountain, for which no Co data are published.…”

“…Further, Cr in Springwater and Imilac from Wasson and Choi (2003) are subject to large uncertainties due to the possibility that their samples may have included tiny chromite grains. Here, we use the lowest reported Cr values from duplicate analyses (Table 2 of Wasson and Choi, 2003) for Imilac (12 ppm) and Springwater (14 ppm), which are similar to typical Cr concentrations in pallasite metal (~12 ppm, average of several main group pallasites from Herzog et al 2015).…”