Paucity of sulfide in a large slab of Esquel: New perspectives on pallasite formation

Autor: Finn Ulff-Møller, Jacquelyn Tran, Byeon-Gak Choi, Alan E. Rubin, John T. Wasson
Rok vydání: 1998
Předmět:
Zdroj: Meteoritics & Planetary Science. 33:221-227
ISSN: 1945-5100
1086-9379
DOI: 10.1111/j.1945-5100.1998.tb01627.x
Popis: — The 0.7 ton Esquel meteorite, found in Patagonia before 1951, is a typical main-group pallasite in most respects. We examined petrographically a slab ∼1 m long having an area of 3000 cm2 that shows the typical pallasitic texture with fragmental olivine. Phase abundances (in vol%) are olivine (66%), metal (32%), schreibersite (0.76%), troilite (0.46%) and chromite (0.31%). Esquel can be divided into four lithologies: (1) “pallasitic” matrix consisting of olivine fragments embedded in metal (81%); (2) large (>5 cm) olivine nodules having low metal contents (18%); (3) massive metal (0.3%); and (4) zones dominated by FeS and fine olivine (0.7%). Main-group pallasites appear to have formed by the intrusion of a highly evolved (low Ir, high Ni, Au and S) metallic magma into fragmented olivine. This model implies that FeS should be abundant in main-group pallasites, and we had speculated that examination of an exceptionally large slab might reveal a high troilite content. We found instead an exceptionally low FeS content. New compositional data confirm that Esquel has a lower Au content than other main-group pallasites having similar Ir contents. Literature data (based, however, on relatively small sections) suggest that high-Au pallasites have higher S contents than Esquel but have lower S contents than expected from a trapped-melt model. We conclude that a relatively complex model is required to explain the origin of main-group pallasites. After intrusion, the degree of crystallization of the metallic magma varied from location to location but, in almost all cases, an FeS-rich liquid either escaped or formed FeS-rich pallasitic rocks that are underrepresented in the meteorite inventory.
Databáze: OpenAIRE
Externí odkaz: