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Nonporphyritic chondrules and chondrule fragments in enstatite chondrites: Insights into their origin and secondary processing
Author(s) -
Varela M. E.,
Sylvester P.,
Brandstätter F.,
Engler A.
Publication year - 2015
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/maps.12468
Subject(s) - chondrule , chondrite , enstatite , lithophile , volatiles , empa , electron microprobe , geology , trace element , pyroxene , meteorite , formation and evolution of the solar system , analytical chemistry (journal) , geochemistry , mineralogy , astrobiology , chemistry , olivine , crust , partial melting , physics , chromatography
Sixteen nonporphyritic chondrules and chondrule fragments were studied in polished thin and thick sections in two enstatite chondrites ( EC s): twelve objects from unequilibrated EH 3 Sahara 97158 and four objects from equilibrated EH 4 Indarch. Bulk major element analyses, obtained with electron microprobe analysis ( EMPA ) and analytical scanning electron microscopy ( ASEM ), as well as bulk lithophile trace element analyses, determined by laser ablation inductively coupled plasma–mass spectrometry ( LA ‐ ICP ‐ MS ), show that volatile components (K 2 O + Na 2 O versus Al 2 O 3 ) scatter roughly around the CI line, indicating equilibration with the chondritic reservoir. All lithophile trace element abundances in the chondrules from Sahara 97158 and Indarch are within the range of previous analyses of nonporphyritic chondrules in unequilibrated ordinary chondrites ( UOC s). The unfractionated (solar‐like) Yb/Ce ratio of the studied objects and the mostly unfractionated refractory lithophile trace element ( RLTE ) abundance patterns indicate an origin by direct condensation. However, the objects possess subchondritic CaO/Al 2 O 3 ratios; superchondritic (Sahara 97158) and subchondritic (Indarch) Yb/Sc ratios; and chondritic‐normalized deficits in Nb, Ti, V, and Mn relative to RLTE s. This suggests a unique nebular process for the origin of these EC s, involving elemental fractionation of the solar gas by the removal of oldhamite, niningerite, and/or another phase prior to chondrule condensation. A layered chondrule in Sahara 97158 is strongly depleted in Nb in the core compared to the rim, suggesting that the solar gas was heterogeneous on the time scales of chondrule formation. Late stage metasomatic events produced the compositional diversity of the studied objects by addition of moderately volatile and volatile elements. In the equilibrated Indarch chondrules, this late process has been further disturbed, possibly by a postaccretional process (diffusion?) that preferentially mobilized Rb with respect to Cs in the studied objects.