Calcium segregation at antiphase boundaries in pigeonite

Over the past 30 years the cooling rates of terrestrial and extraterrestria l basaltic rocks have commonly been inferred from the size of antiphase dom ains (APDs) in pigeonite. However, the coarsening rate of APDs has been obs erved to deviate substantially from an ideal rate. It is believed that this deviation is caused by Ca segregation to the antiphase boundaries (APBs) o f domains, since Ca is expected to substantially slow boundary migration ra tes due to solute drag. This letter presents direct experimental evidence of Ca segregation to APBs in pigeonite. The local atomic structure and chemistry of APBs in pigeonit e were examined using high-resolution and energy-filtered transmission elec tron microscopy. High-resolution images show that APBs in pigeonite have a structure similar to that of augite, and energy-filtered compositional imag es reveal that Ca segregates to APBs. These results have direct ramificatio ns for the use of antiphase domain (APD) size as a marker for the thermal h istory of the rocks in which pigeonite crystallizes.