MODELS of magma genesis at mid-ocean ridges¹, together with recent experimental data² and observations of trace element abundances in clinopyroxenes from abyssal peridotites³, suggest that small-volume melt fractions can be efficiently extracted from the melting mantle. As shown in ref. 3, residues of this type of melting (fractional melting) display extremely low abundances of incompatible trace elements and extreme fractionation amongst them, especially at advanced stages of the process because of the compounded effects of differences in the partition coefficients. If this process operates beneath mid-ocean ridges, one would expect to sample melts that are correspondingly depleted and fractionated in trace elements. Indeed, the existence of very depleted melts in mid-ocean ridges was predicted previously^4–6 in order to explain the presence of magnesian pyroxene and calcic plagioclase in mid-ocean-ridge basalts. We report here the discovery of melt that has major and trace element characteristics consistent with these predictions^4–6, occurring as an inclusion in an olivine phenocryst in a typical mid-ocean-ridge basalt from the Mid-Atlantic Ridge. Although our preferred model for the origin of this 'ultra-depleted' melt is critical (continuous) melting, we cannot at this stage rule out other models. Our results underscore the importance of trapped melt inclusions as recorders of the processes involved in melting and melt extraction, and also as pointers to primary melt compositions.

• Olivine and melt inclusion chemical constraints on the nature and origin of the common mantle component beneath eastern Asia

  • Jian-Qiang Liu
  • Li-Hui Chen
  • Zhong-Yuan Ren

  Contributions to Mineralogy and Petrology (2022)

• Mixing between chemically variable primitive basalts creates and modifies crystal cargoes

  • David A. Neave
  • Philipp Beckmann
  • François Holtz

  Nature Communications (2021)

• Deep roots for mid-ocean-ridge volcanoes revealed by plagioclase-hosted melt inclusions

  • Emma N. Bennett
  • Frances E. Jenner
  • C. Johan Lissenberg

  Nature (2019)

• Crystal scavenging from mush piles recorded by melt inclusions

  • Penny E. Wieser
  • Marie Edmonds
  • Barbara E. Kunz

  Nature Communications (2019)

• Ubiquitous ultra-depleted domains in Earth’s mantle

  • Andreas Stracke
  • Felix Genske
  • Janne M. Koornneef

  Nature Geoscience (2019)