Single crystal elasticity of clinopyroxene along Diopside–Hedenbergite solid solution

Clinopyroxene (Cpx) is a major constituent of terrestrial planetary interiors, thus constraining its elasticity across key solid solutions is essential for interpreting geophysical observations. While the effects of coupled NaAl–CaMg substitution along the diopside (Di)–jadeite (Jd) join are well characterized, the influence of Fe–Mg substitution along the Di–hedenbergite (Hd) join remains less certain. We measured the single-crystal elasticity of three Fe-rich natural Cpx spanning the Di–Hd join using Brillouin spectroscopy. We found that the elastic moduli of Cpx vary linearly with Fe–Mg substitution: longitudinal and shear moduli decrease with Fe (or Hd) content, whereas most off-diagonal elastic moduli increase. Increasing Fe content has a negligible effect on the bulk modulus but produces a pronounced reduction in the shear modulus. Combining our results with previous studies, we developed an empirical compositional model for the single-crystal elastic moduli of Cpx at ambient conditions, which predicts that each 10 mol% increase in Hd decreases VP by ∼1.1% and VS by ∼1.8%. These results provide improved constraints on the seismic signatures of Fe-enriched terrestrial upper mantle lithologies.