Constraints on the 232Th/238U ratio (κ) of the continental crust

This study examines constraints on the 232Th/238U ratio of the continental crust (κC) imposed by heat flow, Th‐U systematics and Pb isotope ratios. The 232Th/238U ratio of the depleted upper mantle (κUM) as sampled by mid‐ocean ridge basalt is well‐constrained to be 2.5 ± 0.1. The 232Th/238U ratio of the lower mantle (κLM) can be constrained as follows. Oceanic island basalts (OIB) have average κ of 3.78. If OIB are derived from mantle plumes and if plumes are representative of the lower mantle, then κLM ≤ 3.78. Alternatively, if the lower mantle is “primitive”, then κLM is similar to the chondritic 232Th/238U ratio of κS = 4 ± 0.2. Mass balance calculations assuming κUM = 2.5, and κLM ≤ 4 with heat production constraints suggest that κC is in the range 5–6. Further support for κC substantially greater than 4 comes from the Pb isotopic composition of the continental crust. We estimate the 208Pb/204Pb of the lower continental crust to be 37.36 to 37.84 and the 208Pb*/206Pb* ratio of the total continental crust to be 1.046 to 1.059, which corresponds to a time‐integrated 232Th/238U ratio (κC(Pb)) of 4.25 − 4.30. A four‐reservoir (upper and lower continental crust, depleted upper mantle, and primitive lower mantle) forward transport model of Pb isotope evolution that reproduces the difference in κUM and κUM(Pb) values in the depleted mantle suggests that to achieve a κC(Pb) of 4.25, the present 232Th/238U ratio in the crust (κC) must be ≈5. Thus several lines of evidence converge on a value κC of around 5 or slightly higher.