If you start with the assumption that the initial activity of U234 was approximately the same as U238, (since this is what is observed in all natural waters and magmas today)
This is often not the case. Waters are known in which the U-234 is highly enriched. The decays leading to U-234 can leave it in a quite different chemical state from the parent U-238, through what are called hot-atom effects. Further, the intermediate nuclides have quite different chemistries. For example, thorium is often insoluble where uranium is soluble. If you are actually thinking of U-235, which is primordial, your comments are correct.
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