The measurement of δ15N in crustal rocks by static vacuum mass spectrometry: Application to the origin of the ammonium in the Cornubian batholith, southwest England

Abstract

The release of ammoniacal nitrogen from muscovite, biotite, orthoclase, microcline, and a whole-rock granite by vacuum pyrolysis has been investigated. Under the heating regimes employed, it was found that micas lose most of their nitrogen below the temperature at which the powders begin to sinter (ca. 1100°C), the peak release of N occurring in either the 900°C or 1000°C heating step. It is possible that the loss of ammonium is linked to the loss of H 2O and perhaps F. For the feldspars there was insignificant release of N below the melting point of the minerals (about 1150°C), suggesting that diffusion of ammonium or ammonia is the principal control. For both micas and feldspars there was significant kinetic fractionation of the nitrogen isotopes during stepped heating. From these results a technique has been devised for the routine determination of δ 15N and ammonium concentration, in whole-rock granites, by static vacuum mass spectrometry. The ammonium concentrations and δ 15N values have been determined in whole-rock samples from the Cornubian batholith, southwest England. The ammonium concentrations were in the range 8 to 187 ppm and the δ 15N values were in the range +5.1 to + 10.2‰. Most of the granite samples had δ 15N values between +8.4 and +10.2‰ and are within the range that have been obtained from amphibolite facies metasediments. These granites are also typified by high 87Sr 86Sr init (0.7130 to 0.7166) and high normative corundum (2.31 to 2.55%). Taken together, the data suggest that the ammonium present within these granites is of biological origin and that it was incorporated via the anatexis of metasediments which originally contained organic material. The samples were collected over a distance of 120 km and the limited range in δ 15N suggests either 1. (1) the δ 15N values within the metasedimentary protolith were rather homogeneous; or 2. (2) the nitrogen isotopes were homogenised during the incorporation of ammonium into the granite magmas. Two of the granites had lower δ 15N values of +5.1 and +7.0‰ These granites also have lower ammonium concentrations (8 and 15 ppm), lower 87Sr 86Sr init (0.7094 –0.7095), and lower normative corundum (1.44 and 1.63%). The preferred explanation is that they were derived from or have had a contribution from a second crustal source containing sedimentary (or metasedimentary) material with a lower proportion of pelitic to other constituents.