  In this paper we present an extension of the \emph{Recent Fluid Deformation
(RFD)} closure introduced by Chevillard and Meneveau (2006) which was developed
for modeling the time evolution of Lagrangian fluctuations in incompressible
Navier-Stokes turbulence. We apply the RFD closure to study the evolution of
magnetic and passive scalar fluctuations. This comparison is especially
interesting since the stretching term for the magnetic field and for the
gradient of the passive scalar are similar but differ by a sign such that the
effect of stretching and compression by the turbulent velocity field is
reversed. Probability density functions (PDFs) of magnetic fluctuations and
fluctuations of the gradient of the passive scalar obtained from the RFD
closure are compared against PDFs obtained from direct numerical simulations.
