Geochemical and isotopic characteristics of palaeo-hydrothermal fluids related to granite magmatism, S W England.
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 Sl3C characterisation of palreofluid CO2 (down to nanomole quantities) with an
accuracy approaching that of the corresponding analytical precision. Similar procedures were
successfully applied to the ol5N measurement of palreofluid nitrogen at the sub-nanomole
An investigation into the origin of fluids which characterised the earliest episodes of palreohydrothermal
activity associated with the granites of S W England indicates that the
abundance of trace carbon species (C02
, CH4 ) and nitrogen in the fluids was correlated with
the metasedimentary contribution to the respective granite source. Furthermore, SiSN and
ol3C data (obtained on fluid components and local Palreozoic metasediments, in conjunction
with published ol5N 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 Palreozoic 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 palreofluid solutes entered the early hydrothermal systems.
Hydrogen stable isotope data, measured on the extracted palreowaters, 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