Comparisons of Antarctic and non-Antarctic achondrites and possible origin of the differences

Abstract

In order to discuss differences between Antarctic and non-Antarctic achondrites, we investigated pairing of mainly the Yamato achondrites by mineralogical techniques. In addition to five paired specimens of the Y74159-type polymict eucrites, three new groups distinct from the Y74159-type have been recognized. One group (Y791960) of howarditic polymict eucrites is similar to those of non-Antarctic origin. Some Antarctic monomict eucrites differ in texture and chemistry from the non-Antarctic specimens. More than 50 diogenite specimens recovered from the Yamato bare ice field can be grouped into two falls of separate locations, compared with 8 from the USA collection and 9 from the non-Antarctic groups. Thirty Antarctic ureilites are separate falls (2 pairings), and their individual masses are small. These observations suggest that discussion of differences between the Antarctic and non-Antarctic achondrites largely depends on detection of pairings of the Antarctic specimens. The pairings of the specimens from Victoria Land have not been as well characterized as those of the Yamato Mountains. Comparisons with non-Antarctic meteorites show that the Antarctic HEDs and ureilites differ from non-Antarctic specimens by the following criteria: 1. 1) Polymict eucrites excluding those with howarditic affinity have not been found in the non-Antarctic collections. 2. 2) Magnesian ureilites and augite-bearing ureilites were found only in Antarctica. The differences may not be attributed to just one factor. The above evidence of two unique diogenite falls preserved on the restricted ice fields suggests that the Yamato diogenites may represent falls on restricted areas in the distant past. The discovery of many small unique eucrites and ureilites suggests that within the large number of small specimens on a bare ice field there is a higher chance of finding unique varieties.