Heat-producing elements in the first 2000 meters CCSD drilling core: Implications for heat production distribution in the high to ultrahigh pressure metamorphic terrains

How radiogenic heat production elements (HPE) distribute as a function of depth in the Earth is critical to determine crustal thermal and rheological structure and to constrain geochemical, petrological, and tectonic models of crustal evolution. Our knowledge on HPE distributions within the crust have been derived predominantly from studies on large-scale granitic batholiths. The HPE distributions 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 with a sampling interval of 1 to 5 meters and used to determine the distribution pattern of radiogenic heat production. Petrographic studies have demonstrated that the uppermost 2000 meters core can be subdivided into seven lithologic sections which consist of (I) quartz eclogite (100-248 m); (II) quartz eclogite alternate with rutile eclogite (248-530 m); (III) rutile eclogite (530-600 m); (IV) garnet peridotite with interlayers of eclogites (600-695 m); (V) paragneiss with intercalated eclogite layers (695-1160m); (VI) granitic gneiss with intercalated eclogite layers (1160-1600 m); and (VII) eclogite (1600-2050 m), respectively. Preliminary results show that: (1) the granitic gneisses have the highest heat production with an average of 1800¡A10^-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 4~5¡A10^-11W/kg; (4) radiogenic HPE concentrations and heat production change stepwisly as lithology changes downward. This data reveals 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.