Vertical distribution characteristics and origin of radiogenic heat-producing elements (HPE) in the first 2000 m of the main hole of the CCSD Project

Abstract： How radiogenic heat-producing elements (HPE) are distributed as a function of depth in the Earth is critical to determine the thermal and rheological structure of continental crust and to constrain geochemical, petrological, and tectonic models of crustal evolution. Our knowledge of HPE distribution within the crust has been derived predominantly from studies of large-scale granitic batholiths. The HPE distribution within high-grade metamorphic terrains are not well studied and thus poorly understood. The Chinese Continental Scientific Drilling (CCSD) Project provides us an unparallel opportunity to characterize the vertical HPE distribution in a type example of high- to ultrahigh-pressure metamorphic terrains worldwide. U, Th, and K2O contents have been measured on 734 core samples taken at intervals of 2 to 5 m and used to determine the distribution pattern of radiogenic heat production. Preliminary results show that： (1) the granitic gneisses have the highest heat production with an average of 1665×10-11W/kg; (2) the paragneisses have the intermediate heat production values; (3) the rutile eclogites and garnet peridotites have the lowest and similar values at about 17~20×10-11W/kg；(4) radiogenic HPE concentrations and heat production change stepwise as lithology changes downward. These data reveal a sandwich-like structure for the HPE in the drilling core in which relatively high heat production sections are enclosed by relatively low heat production sections. Such a pattern is not consistent with the downward-decreasing exponential distribution predicted from modeling of surface heat flow data. However, it may represent a typical structure in UHP metamorphic terrains as a result of deformation during or after the exhumation which resulted in the juxtaposition of mafic to ultramafic blocks with felsic blocks.