Nonporphyritic chondrules and chondrule fragments in enstatite chondrites: Insights into their origin and secondary processing

Abstract

Sixteen nonporphyritic chondrules and chondrule fragments were studied inpolished thin and thick sections in two enstatite chondrites (ECs): twelve objects fromunequilibrated EH3 Sahara 97158 and four objects from equilibrated EH4 Indarch. Bulkmajor element analyses, obtained with electron microprobe analysis (EMPA) and analyticalscanning 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 (K2O + Na2O versus Al2O3) scatter roughly around the CIline, indicating equilibration with the chondritic reservoir. All lithophile trace elementabundances in the chondrules from Sahara 97158 and Indarch are within the range ofprevious analyses of nonporphyritic chondrules in unequilibrated ordinary chondrites(UOCs). The unfractionated (solar-like) Yb/Ce ratio of the studied objects and the mostlyunfractionated refractory lithophile trace element (RLTE) abundance patterns indicate anorigin by direct condensation. However, the objects possess subchondritic CaO/Al2O3 ratios;superchondritic (Sahara 97158) and subchondritic (Indarch) Yb/Sc ratios; and chondriticnormalizeddeficits in Nb, Ti, V, and Mn relative to RLTEs. This suggests a unique nebularprocess for the origin of these ECs, involving elemental fractionation of the solar gas by theremoval of oldhamite, niningerite, and/or another phase prior to chondrule condensation. Alayered chondrule in Sahara 97158 is strongly depleted in Nb in the core compared to therim, suggesting that the solar gas was heterogeneous on the time scales of chondruleformation. Late stage metasomatic events produced the compositional diversity of thestudied objects by addition of moderately volatile and volatile elements. In the equilibratedIndarch chondrules, this late process has been further disturbed, possibly by apostaccretional process (diffusion?) that preferentially mobilized Rb with respect to Cs inthe studied objects.