Geochemical and isotopic characteristics of palaeo-hydrothermal fluids related to granite magmatism, S W England

Miller, Martin Fitzhardinge (1994). Geochemical and isotopic characteristics of palaeo-hydrothermal fluids related to granite magmatism, S W England. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000e063

Abstract

An assessment of stepped heating procedures for the extraction and isolation of carbonaceous species from fluid inclusions resulted in the development of low-blank procedures which permitted δ¹³C characterisation of palӕofluid CO₂ (down to nanomole quantities) with an accuracy approaching that of the corresponding analytical precision. Similar procedures were successfully applied to the δ¹⁵N measurement of palӕofluid nitrogen at the sub-nanomole level.

An investigation into the origin of fluids which characterised the earliest episodes of palӕohydrothermal activity associated with the granites of S W England indicates that the abundance of trace carbon species (CO₂ , CH₄ ) and nitrogen in the fluids was correlated with the metasedimentary contribution to the respective granite source. Furthermore, δ¹⁵N and δ¹³C data (obtained on fluid components and local Palӕozoic metasediments, in conjunction with published δ¹⁵N values of Cornubian granites), indicate that carbon and nitrogen in the hydrothermal systems were derived from the granite magmas.

The chemical composition of the early hydrothermal fluids, together with geochemical and isotopic constraints from the characterisation of Palӕozoic metasedimentary country rocks, support the view that the fluids were genetically associated with the granites. Fluid interaction with the local metasedimentary rocks at a high level crustal appears to have been very limited. The incorporation of sedimentary matter into granitic protoliths during anatexis, with subsequent transfer to an exsolved hydrous phase during pluton cooling, is the most probable route by which palӕofluid solutes entered the early hydrothermal systems.

Hydrogen stable isotope data, measured on the extracted palӕowaters, indicate that meteoric water was not a significant component of early hydrothermal systems associated with either the Dartmoor granite or the nearby Hemerdon Ball intrusive, if sub-solidus isotopic exchange was significant. In contrast, comparable data from early fluids associated with other component intrusives of the batholith (as characterised by W ± Sn oxide paragenesis) are consistent with the progressive dilution of a magmatic-hydrothermal component by local groundwaters.