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Paucity of sulfide in a large slab of Esquel: New perspectives on pallasite formation
Author(s) -
ULFFMØLLER FINN,
CHOI BYEONGAK,
RUBIN ALAN E.,
TRAN JACQUELYN,
WASSON JOHN T.
Publication year - 1998
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1998.tb01627.x
Subject(s) - olivine , troilite , geology , parent body , meteorite , geochemistry , chromite , mineralogy , intrusion , slab , chondrite , geophysics , astrobiology , physics
— 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 cm 2 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.