Let q1 and q2 be two binary
quartic forms. We consider the diophantine equation q1(x,y) = q2(z,w) from the
geometric view point. Under a mild condition we prove that the K3 surface defined
by the above equation admits an elliptic fibration whose Mordell-Weil group over
C(t) has rank at least 12. Next, we choose suitable q1 and q2 such that the
Mordell-Weil group contains a subgroup of rank 12 defined over ℚ(i) and a subgroup
of rank 8 defined over ℚ.