A proposed new approach and unified solution to old Pb paradoxes

• Radiogenic Pb isotopes of oceanic basalts and Pb paradoxes are related.
• Ce/Pb and Nb/U are only near constant.
• Pb paradoxes define a system of equations that needs to be solved altogether.
• Mixing between DMM and enriched melt can explain Pb paradoxes in MORB.
• Mixing between FOZO and enriched end-members can explain Pb paradoxes in OIB.

One of the most remarkable features of many and, perhaps, all oceanic basalts is that their Pb isotopic ratios (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) are too radiogenic to be coming from the undifferentiated mantle or bulk silicate Earth. This has created three major concerns in the behavior of U, Th and Pb in the Earth's mantle that have been termed the Pb paradoxes. These are the unexpectedly long time-integrated high U/Pb (1st paradox), long time-integrated low Th/U (2nd paradox) and constant Ce/Pb and Nb/U (3rd paradox) in the mantle sources of oceanic basalts. The origins of such unexpected ratios have been the object of intense studies that produced several highly significant, but generally individualized results during the last four decades. Detailed analysis of available data shows that the paradoxes are closely interrelated as they all pertain to the mantle and have many common characteristic features. Thus, the Pb paradoxes constitute a system of equations that must be solved all together as each solution must satisfy every equation in the system. For example, compositional data for the voluminous mid-ocean ridge basalts (MORB) show that the 1st and 2nd paradoxes exhibit a long time-integrated enrichment of U and the Th/U and Nb/Th ratios are also constant. A single solution to simultaneously explain the paradoxes in MORB is possible if recycled materials with variable enrichments in incompatible trace elements, particularly U and its daughter Pb* plus Nb, Ce, and Th are added to or mixed with the depleted upper mantle. Significantly, a similar binary mixing solution has been proposed for the Pb paradoxes in ocean island basalts.