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

摘要: [1] This study examines constraints on the 232Th/238U ratio of continental crust (κC) imposed by heat flow, Th-U systematics and Pb isotope ratios. The depleted upper mantle (κUM) as sampled mid-ocean ridge basalt is well-constrained to be 2.5 ± 0.1. lower (κLM) can constrained follows. Oceanic island basalts (OIB) have average κ 3.78. If OIB are derived from plumes if representative mantle, then κLM ≤ Alternatively, “primitive”, similar chondritic κS = 4 0.2. Mass balance calculations assuming κUM 2.5, with production suggest that κC in range 5–6. Further support for substantially greater than comes isotopic composition crust. We estimate 208Pb/204Pb 37.36 37.84 208Pb*/206Pb* total 1.046 1.059, which corresponds a time-integrated (κC(Pb)) 4.25 − 4.30. A four-reservoir (upper crust, primitive mantle) forward transport model evolution reproduces difference κUM(Pb) values suggests achieve κC(Pb) 4.25, present must ≈5. Thus several lines evidence converge value around 5 or slightly higher.