Fig. 2: Iron isotope composition (δ⁵⁶Fe) versus MgO/FeO^T for different lithologies from ocean lithosphere.

a Global MORB^1,7,9 (orange circles). b Gabbro (green squares) and felsic vein (red circle) samples, as well as mineral, separate from Hole 735B of this study. c Global AP² (blue squares). MORB are considered to have uniform Fe isotope composition (δ⁵⁶Fe = +0.050 to +0.176‰) with a mean of +0.105 ± 0.006‰¹ (orange dashed line). AP, as MORB mantle melting residues, have lighter iron isotope composition (δ⁵⁶Fe = −0.094 to +0.108‰) with a mean of +0.010 ± 0.014‰² (blue dashed line). The bulk rock gabbro and felsic vein samples have Fe isotope compositions varying as low as those of AP and as high as and even higher than those of MORB. The modeled compositions (purple squares) of melt (Supplementary Data 4) in equilibrium with gabbroic samples and olivine, pyroxene therein are plotted a for comparison (see “Methods” section for model details). The mineral separates such as olivine (Ol, purple diamonds), clinopyroxene (Cpx, yellow diamonds), amphibole (Am, brown diamonds), and Fe–Ti oxides (black diamonds) also show a similar negative correlation between δ⁵⁶Fe and MgO/FeO^T (b). The best-fit lines at 95% confidence intervals with R² values are given for the calculated melt (a) and bulk rock samples (b). Error bars are ±1 SD.