Subduction is fundamental for maintaining plate tectonics. Magmatic
record of subduction initiation offers pivotal clues for understanding
the nucleation and propagation of a new subduction zone. However, how
subduction is elicited to induce the first-order magmatic observations
is yet to be understood. Here, using numerical models, we demonstrate
that magmatic ignitor of mantle melting triggered-upwelling along wide
and weak fracture zones renders lithospheric mantle sinking and
subduction initiation. Progressive decompression melting is sequentially
followed by melting of oceanic crust, hydrated and depleted mantle, and
sediment of the retreating slab, respectively. This scenario is
compatible with the in-situ observations of petrogenesis in the western
Pacific, which also explains the enigmatic deferred sediment involvement
in the early magmatism. Our self-consistent model indicates that a
continuous magmatic ignitor beneath a transform fault resulting in
subduction initiation could be a prevalent process in Earth's evolution.