The Twannberg (Switzerland) IIG iron meteorites: Mineralogy, chemistry, and CRE ages

Abstract

Al activities. Bulk concentrations of 5.2% Ni and 2.0% P were calculated. The preatmospheric mass is estimated to have been at least 11,000 kg. The average CRE age for the different Twannberg samples is 230 ± 50 Ma. Detrital terrestrial mineral grains in the oxide rinds of the three larger masses indicate that they oxidized while they were incorporated in a glacial till deposited by the RhÙne glacier during the last glaciation (Würm). The find location of mass I is located at the limit of glaciation where the meteorite may have deposited after transport by the glacier over considerable distance. All evidence indicates pairing of the six masses, which may be part of a larger shower as is indicated by the large inferred pre-atmospheric mass.

The IIG iron meteorites: Probable formation in the IIAB core

“…With the exception of Twannberg (for which Wasson et al (1989) and Hofmann et al (2009) published data) this is the first report of INAA data for IIG irons. As shown in Table A1, most of the data were gathered between 2001 and 2008.…”

“…Hofmann et al (2009) listed our INAA data for Twannberg masses I and II; we now have data for Twannberg III and an additional replicate on Twannberg I. As discussed by Hofmann et al (2009) and recognizable in the replicate data in Table A1, there appear to be resolvable compositional differences between Twannberg I and Twannberg II and III. However, the range is small compared to the total range in IIG.…”

“…All visible inclusions are schreibersite, and the dark hole on the upper left resulted from the loss of schreibersite. As discussed by various authors and in detailed fashion by Hofmann et al (2009), the schreibersite in IIG irons is present in three forms: (1) the irregular coarse inclusions common on the right side of Fig. 1 are the most abundant;…”

“…The La Primitiva estimate is from Buchwald (1975) and the Twannberg estimate is from Hofmann et al (2009). Our Bellsbank and Tombigbee estimates are based on the descriptive material in Buchwald (1975).…”

“…Analyses of additional Twannberg specimens are provided in Hofmann et al (2009). The Guanaco meteorite was discovered in Chile in 2000 (Russell et al, Meteoritical Bull.…”