  We propose a new method to measure the metallicity of Type Ia supernova
progenitors using Mn and Cr lines in the X-ray spectra of young supernova
remnants. We show that the Mn to Cr mass ratio in Type Ia supernova ejecta is
tightly correlated with the initial metallicity of the progenitor, as
determined by the neutron excess of the white dwarf material before
thermonuclear runaway. We use this correlation, together with the flux of the
Cr and Mn Kalpha X-ray lines in the Tycho supernova remnant recently detected
by Suzaku (Tamagawa et al. 2008) to derive a metallicity of log(Z) = -1.32
(+0.67,-0.33) for the progenitor of this supernova, which corresponds to
log(Z/Zsun)= 0.60 (+0.31,-0.60) according to the latest determination of the
solar metallicity by Asplund et al. (2005). The uncertainty in the measurement
is large, but metallicities much smaller than the solar value can be
confidently discarded. We discuss the implications of this result for future
research on Type Ia supernova progenitors.
