Note that "mass anomolies" are almost universal charateristics of lunar (and other) craters, as evidenced by this image from the GRAIL gravimetric survey:<p><a href="https://www.nasa.gov/images/content/711375main_grail20121205_4x3_946-710.jpg" rel="nofollow">https://www.nasa.gov/images/content/711375main_grail20121205...</a><p><a href="https://en.wikipedia.org/wiki/GRAIL" rel="nofollow">https://en.wikipedia.org/wiki/GRAIL</a><p>Specifics will vary by both impactor size and composition -- meteorites can be largely rocky, that is less dense, or metallic iron-nickel, extremely dense. The Aitken basin was likely formed by a both large and dense impactor.<p>Somewhat related, in the evolution of the Earth, the thought's occurred that the late-stage colision between proto-Earth and Theia might have resulted in a greater prevalence of heavy elements in Earth's crust and outer mantle than an otherwise uniform early accretion process might have generated. Speculation on my part, no literature of which I'm aware.