If it is one large homogeneous unit, then it is possible that heavier/densier minerals would be concentrated toward the middle, as the melt cools from the top and bottom leaving the interior a melt longer. The melt in the middle would become more "concentrated" in incompatiable elements until they are saturated to the point they start crystallizing.
However, if it is a stratified unit, in this case numerous volcanic flows. Then it is possible that there would be multiple zones of higher concentration through each flow (cooling) unit, as each volcanic flow would cool/crystallize in the manner I decribed above.
In addition, you have to factor in what happen at this interval. Did the upper volcanic unit get eroded, either by weathering processes, or even by subsequent eruptions. This would change how they make their interpretations of the drill core and possible indirect evidence of elevated REEs at depth.
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