PAUCITY OF SULFIDE IN A LARGE SLAB OF ESQUEL - NEW PERSPECTIVES ON PALLASITE FORMATION

The 0.7 ton Esquel meteorite, found in Patagonia before 1951, is a typ ical main-group pallasite in most respects. We examined petrographical ly a slab similar to 1 m long having an area of 3000 cm(2) that shows the typical pallasitic texture with fragmental olivine. Phase abundanc es (in vol%) are olivine (66%), metal (32%), schreibersite (0.76%), tr oilite (0.46%) and chromite (0.31%). Esquel can be divided into four l ithologies: (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 dominate d 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, An and S) metallic magma into fragmented olivine. This model implies that FeS should be abundant in main-group pallasites, and we had speculated th at examination of an exceptionally large slab might reveal a high troi lite content. We found instead an exceptionally low FeS content. New c ompositional data confirm that Esquel has a lower Au content than othe r main-group pallasites having similar Ir contents. Literature data (b ased, however, on relatively small sections) suggest that high-Au pall asites have higher S contents than Esquel but have lower S contents th an expected from a trapped-melt model. We conclude that a relatively c omplex model is required to explain the origin of main-group pallasite s. After intrusion, the degree of crystallization of the metallic magm a varied from location to location but, in almost all cases, an FeS-ri ch liquid either escaped or formed FeS-rich pallasitic rocks that are underrepresented in the meteorite inventory.