Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history, exposure history, and pre‐atmospheric size

Abstract

Ar concentrations indicate a major impact on the parent body between 300 and 400 Myr ago, which must have preceded the impacts that brought the Gold Basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 Myr ago.

The complex exposure history of the Jiddat al Harasis 073 L‐chondrite shower

“…The most remarkable observation is that the 10 Be and 26 Al concentrations are not correlated, but seem to vary randomly, with 26 Al/ 10 Be ratios ranging from ~3.5 to ~5.0. These 26 Al/ 10 Be ratios are significantly higher than typical 26 Al/ 10 Be production rate ratios of 3.0-3.5 in large chondrites (Vogt et al 1992;Welten et al 2003). The high 26 Al/ 10 Be ratios in JaH 073 indicate that 10 Be and (to a lesser extent) 26 Al were not saturated at the time of fall.…”

“…To compare the results with model calculations we thus normalize all 10 Be and 26 Al results to bulk L-chondrites, using the normalization factors, F10 and F26, given in Table 3. Although the variations in 10 Be and 26 Al are small compared to those (factors of [15][16][17][18][19][20] found in the large Gold Basin Lchondrite shower (Welten et al 2003), they are significantly larger than those found in Frontier Mountain (FRO) 90174, an Antarctic H-chondrite shower with a pre-atmospheric radius of 80-100 cm (Welten et al 2001). The most remarkable observation is that the 10 Be and 26 Al concentrations are not correlated, but seem to vary randomly, with 26 Al/ 10 Be ratios ranging from ~3.5 to ~5.0.…”

“…Measured 20 Ne/ 22 Ne ratios in most JaH 073 samples are 0.85-0.97, indicating that Ne in JaH 073 is almost purely cosmogenic (see Table 3). Welten et al 2003). We will discuss the contributions of trapped Ar, cosmogenic Ar (= spallogenic + neutron-capture), and radiogenic 40 Ar in more detail below.…”

“…e.g., Welten et al 2003). Briefly, the primary and secondary particle spectra were calculated for L-chondrites with various radii using the LAHET code system (Prael and Lichtenstein 1989;Briesmeister 1993).…”

“…Cosmogenic nuclide studies show that many of the large chondrites or chondrite showers, such as Bur Gheluai (Vogt et al 1993), Gold Basin (Welten et al 2003), Jilin (Honda et al 1982;Heusser et al 1985), and Tsarev (Herzog et al 1997) experienced a complex exposure history with a first exposure stage on or close to the surface of the asteroid parent body followed by a second exposure stage as a relatively large object in space. The high percentage of complex exposure histories among large chondrites (R > 50 cm) is interesting because complex exposures are rare among small to medium-sized objects (at least they are rarely recognized).…”