  A spinor derivation is presented for quasilocal mean-curvature mass of
spacelike 2-surfaces in General Relativity. The derivation is based on the
Sen-Witten spinor identity and involves the introduction of novel nonlinear
boundary conditions related to the Dirac current of the spinor at the 2-surface
and the tangential flux of a boundary Dirac operator, as well the use of a spin
basis adapted to the mean curvature frame of the 2-surface normal space. This
setting may provide an alternative approach to a positivity proof for
mean-curvature mass based on showing that Witten's equation admits a spinor
solution satisfying the proposed nonlinear boundary conditions.
