The pressure dependence of melting temperatures for boron carbide and B4C-carbon eutectic has been studied up to 8 GPa, and it was found that in both cases the melting curves exhibit negative slope (-13\pm6 K/GPa), that is indicative of higher density of the melt as compared to the solid phase.
@article{arxiv.1206.0538,
title = {On melting of B4C boron carbide under pressure},
author = {Vladimir A. Mukhanov and Petr S. Sokolov and Vladimir L. Solozhenko},
journal= {arXiv preprint arXiv:1206.0538},
year = {2012}
}