Fractionation of uranium, thorium and rare earth elements in a vertically zoned granodiorite: Implications for heat production distributions in the Sierra Nevada batholith, California, U.S.A.
The abundances of heat-producing elements increase and of rare-earth elements decrease upward in a 1 km vertical section through a pluton in the eastern Sierra Nevada batholith, California. This distribution results mainly from magmatic processes and not from alteration. For samples containing between 47.7% and 65.5% SiO 2, sphene is the predominant whole rock site for U, Th and REE. The variation in vertical heat production enables the characteristic depth scale ( D) to be calculated from the linear heat flow-heat production relationship. The calculated value for the pluton, D = 2.2 km, is significantly lower than that of the batholith, D = 10.1 km, calculated from combined heat flow and heat generation. This result implies that plutons within a heat flow province may not follow the same distribution law for heat-producing elements. In the Sierra Nevada batholith two fundamentally different heat-producing element fractionation trends are evident. The western granitoids have inherently low heat production (0.2-1.5μW m -3) for compositions between 57% and 77% SiO 2. The central-eastern granitoids have inherently high heat production (1.5-5.0 μW m -3) for compositions between 57% and 77% SiO 2. We conclude that there is no geochemical evidence to support an exponential vertical HPE distribution of heat-producing elements as the cause of a linear heat flow-heat production relationship in the Sierra Nevada batholith.