Low δ30Si values in olivine-hosted melt inclusions trace sediment contributions to subduction zone melts

• MORB melt inclusions show homogeneous δ 30 Si similar to bulk rocks • Arc melt inclusions have lower δ 30 Si than MORB melt inclusions • Olivine-hosted melt inclusions from arcs samples show δ 30 Si- δ 18 O systematics • Sediment recycling drives δ 30 Si variations in arc magma sources This study reports the first high-precision silicon isotope (δ 30 Si) measurements of olivine-hosted melt inclusions (OHMIs) and their host olivine crystals using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). We analyzed samples from two contrasting tectonic settings: three mid-ocean ridge basalts (MORB) and four island arc volcanic rocks. Our results reveal systematic differences in silicon isotope compositions between these two environments. MORB samples show internally consistent δ 30 Si values in both melt inclusions and host olivine crystals, with isotopic fractionation between phases suggesting equilibrium crystallization processes in MORB magmas. In contrast, arc samples display greater heterogeneity and systematically lower δ 30 Si values that deviate from equilibrium fractionation factors between olivine and melt. The absence of a correlation with the degree of polymerization of the silicate melt (NBO/T) indicates that these isotopic signatures are not controlled by melt structure. The coupling of low δ 30 Si and high δ 18 O values suggests significant contributions from subducted sedimentary materials, particularly siliceous components, to the arc magma source. Our results demonstrate that OHMIs preserve small-scale Si isotope heterogeneities which are invisible at the bulk rock scale. This work highlights the potential of in situ silicon isotope analysis of melt inclusions and their host crystals to trace magmatic processes and source contributions in different tectonic environments, hence providing new insights into the geochemical evolution of arc magmas and the role of subducted materials in mantle heterogeneity.