A sentence in the article's Acknowledgment was inadvertently cut off. The corrected paragraph is presented here: This work was supported by NSF grant EAR0948549. We thank the Smithsonian Institute for providing the MORB glass samples. Special thanks to Roberta Rudnick for discussions and Yu Huang for help with coding. We also appreciate the critical reviews of Bill White and Albrecht Hofmann, Catherine Chauvel, Yaoling Niu and Charlie Langmuir, which helped us to clarify and strengthen the paper. Editors Stefan Weyer and Marc Norman are thanked for handling this manuscript. The color clarity of Fig. 5’s y-axis was also compromised in the final article version. The correct image appears below. The publisher apologises for any inconvenience caused.