We present a phenomenological framework in which microscopic irreversibility arises from constitutive phase fluctuations of an underlying quantum phase field (CQPF). The model introduces a stochastic field equation with local phase noise characterized by a single dissipation rate χ. We demonstrate that, in the conservative limit χ→0, the dynamics reduces exactly to the Schrödinger equation, with an effective particle mass emerging from the kinetic coefficient of the constitutive field. Averaging over phase fluctuations yields a completely positive Lindblad master equation describing intrinsic pure dephasing. The framework predicts a temperature-independent decoherence floor, yielding testable constraints from precision experiments such as optical lattice clocks (χ≲10−3 s−1), Ramsey interferometry, and macromolecular interferometers. The CQPF should therefore be regarded as an effective open-system description of vacuum-induced phase diffusion, providing a falsifiable alternative to purely statistical accounts of the quantum arrow of time.