Fig. 4: Plots of the oxygen isotope composition versus the δ⁵⁶Fe of I-type cosmic spherules.

Displayed are the δ¹⁸O (A) and Δ’¹⁷O (B) versus δ⁵⁶Fe of the spherules investigated in this study, together with modern I-type cosmic spherules that are displayed as ref. ¹¹ (gray triangles). The relation between oxygen and iron isotopes of modern I-type cosmic spherules from Fischer et al.¹¹ is represented as gray solid curves. The oxygen isotope composition of modern atmospheric O₂ is shown for comparison at a pre-evaporative δ⁵⁶Fe of 0‰. The vertical offset of sample MDC-A5 in Δ’¹⁷O (B) is depicted with a grey dashed arrow. Error bars for oxygen isotopes are 1SD and for δ⁵⁶Fe they are 2SD.