Paucity of sulfide in a large slab of Esquel: New perspectives on pallasite formation
Abstract
The 0.7-ton Esquel meteorite, found in Patagonia before 1951, is in most respects, a typical main-group pallasite. We examined petrographically a slab nearly 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 instead found 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 lower 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.
- Publication:
-
Meteoritics and Planetary Science
- Pub Date:
- March 1998
- DOI:
- 10.1111/j.1945-5100.1998.tb01627.x
- Bibcode:
- 1998M&PS...33..221U