Oxygen isotope compositions of chondrules in enstatite chondrites: insights from relict olivine, chondrule size and redox state
Yves Marrocchi, Laurette Piani, Dorian Thomassin, Emmanuel Jacquet
Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 17 December 2025
“The recent discovery in ordinary chondrites of a population of small, sub-TFL (i.e., negative Δ17O; TFL = Terrestrial Fractionation Line) chondrules isotopically similar to relict grains in larger chondrules offered a glimpse into the sub-TFL precursors of inner disk chondrules. This prompts a search for similar relicts and small chondrules in enstatite chondrites (EC; here the EH3 Sahara 97096, Qingzhen, Caleta el Cobre 025, NWA 14425, and the EL3-an NWA 8785). Our SIMS oxygen isotopic analyses reveal that most olivine grains in E3 chondrites are isotopically indistinguishable from enstatite and must have crystallized during the same thermal event. Yet one analyzed chondrule contains relict olivine grains, one isotopically similar to refractory inclusions (a first in EC) and others with Δ17O values ranging from −9.5 to −7.7 ‰. The latter, and similar discoveries in the literature, must have derived from preexisting chondrules. Small porphyritic chondrules or isolated pyroxene grains tend to be 16O-enriched (and more reduced), as in ordinary chondrites, but their Δ17O never goes below –2 ‰. Cryptocrystalline chondrules found in Sahara 97,096 do extend to −7.3 ‰, but are olivine-free. This thus provides evidence for sub-TFL reservoirs ancestral to the E reservoir, although more than one may have existed.”
