The role of element distribution in production and release of radiogenic helium: the Carnmenellis Granite, southwest England

Abstract

The abundance and isotopic composition of He has been measured in whole-rock samples and mineral separates from the Carnmenellis Granite and compared with circulation fluids from the experimental hot dry rock (HDR) project. These fluids are not now in He isotopic equilibrium with the rock through which they flow. Their 3 He 4 He ratios ( R) relative to an atmospheric standard ( R a) average R R a =0.025±0.003 , whereas the whole-rock and mineral separate values are R R a =0.0007–0.0033 , a factor of 8–36 times lower. These differences may be explained by dissimilar formation mechanisms for the two isotopes, and the consequent difference in release kinetics. 4He contents of whole-rock samples show that ⩾60% of He produced over the entire age of the granite is still retained. The bulk of U (∼95%) and Th (∼77–91%) and therefore 4He production is concentrated in uraninite and monazite. Analysis of a single grain of uraninite (1.6 μg) showed 4He retention of 71±9%. 3He on the other hand is formed by neutron-induced fission of Li, mainly in biotite. 3He escapes from this mineral relatively easily. In a rock where U and Th are not uniformly distributed and concentrated into accessory phases, the neutron production may be lower (by up to a factor of ∼5) than predicted by simple calculations based on the homogeneous situation. A model which allows for this geometry-dependent neutron production has been constructed which accounts for the low measured values of 3 He 4 He in the Carnmenellis Granite ( 0.0015⩽ R R a ⩽0.025 ) despite its high Li concentration. These results have two important implications for fluid tracing using He isotopes and for crustal degassing: (1) the difference in release mechanism of 3He and 4He may lead to a 3 He 4 He ratio in scavenging fluids either higher or lower than the production ratio; and (2) 4He accumulation in the crust may occur where U and Th are concentrated in accessory phases, but probably only at relatively low temperatures in the upper part of the continental crust. This may in some cases cause crustal degassing to be episodic.