The origin of several geochemical parameters in continental arc lavas (e.g., La/Yb, Ba/Th) is controversial as to whether it is imparted by the slab or acquired in the crust. In Ecuador, where volcanoes are built over a thick crust (~50 km), this problem is grounded in the lack of primitive rocks. Here, we use melt inclusions hosted in Fo80–90 olivines to decipher the slab component signatures that metasomatises the sub-arc mantle. We report major, trace, and volatile elements analyzed in experimentally heated melt inclusions, which are from primitive rocks of Cotacachi, Cubilche, Cono de la Virgen, Conos de Licto, and Sangay volcanoes located in the north and south of the Ecuadorian arc. Based on trace element data and geochemical modeling, we recognize two types of slab components: one is indicative of aqueous fluids (e.g., high Ba/La, Pb/Ce, B/Nb), and the other is indicative of hydrous siliceous melts (e.g., high La/Nb, Th/Nb). The aqueous fluid signature is recognized in all volcanoes (except for Cono de la Virgen), and their F/Cl are distributed around 0.43 ± 0.07, independent of the distance to the Benioff zone. We propose a model where hydrous siliceous melts result from the subduction of a young oceanic crust north of the Grijalva fracture zone. Additionally, we show that the mantle under Licto and Sangay volcanoes is enriched prior to metasomatism by slab fluids. This study shows that melt inclusions give valuable insights into the composition of primitive melts that are rarely accessible as whole rocks.