Fig. 3.

YDB spherules from 18 sites. SEM images illustrate the wide variety of sizes, shapes, and microstructures of YDB spherules. Diameters are in yellow. EDS compositional percentages corresponding to the letter designations of these 36 YDB spherules are in SI Appendix, Table S5. Most spherules are rounded, but there are also dumbbells (D), bottle shapes (H), gourd shapes (J), and ovoids (P). Most small spherules are solid, although a few are hollow (F and J), whereas most large spherules are vesicular and/or hollow (A, E, and M). A large number of spherules were cross-sectioned (n = 137 EDS; A, E, I, and M); all others were analyzed whole (n = 335 EDS). Lechatelierite and flow marks (schlieren) that formed at >2,200 °C were observed in spherules from three sites (A, E, and M). Many large spherules display accretion with other spherules (E, M, and Q) and microcratering by smaller spherules (Q). Interior and exterior compositions of both spherules are similar, but occasionally, Fe-rich material (thin, light-colored bands) migrated or accreted to the outside of the spherule while molten (M). Some spherules have high percentages of TiO₂ (B, L, and R; averaging 42 wt%), inconsistent with anthropogenic and most cosmic origins, but consistent with impact melting of titanomagnetite or ilmenite.