THE PALEOENVIRONMENT OF EAST KIRKTON, WEST-LOTHIAN, SCOTLAND - STABLE-ISOTOPE EVIDENCE FROM SILICATES AND SULFIDES

Stable isotope data from the East Kirkton succession are used to eluci date the extent of hot-spring influence in the palaeoenvironment by co nstraining conditions of deposition of the silica and the formation of sulphides. Petrographically silica occurs as chert laminae thought to be primary, and as patchy chert considered as replacive. No evidence for biogenic silica was observed. For 20 silica samples deltaO-18 was measured for structural oxygen and deltaD for bound water. deltaO-18(S MOW) varied between +21 and +27 part per thousand with no sample group ings related to petrography. The range in deltaD(SMOW) was from -50 to -90 parts per thousand with lower values characterising replacive or altered silica; water contents of both petrographic groups were simila r. A plot of deltaO-18 versus deltaD for the laminated primary silica defines a grouping about the line defined by Scottish agates (Fallick et al. 1985). This suggests for the unaltered silica a formation tempe rature of about 60-degrees-C and a fluid containing a strong component of meteoric water. The data imply a Lower Carboniferous meteoric wate r deltaO-16 composition of -3 parts per thousand and deltaD of -15 par ts per thousand, consistent with the known palaeolatitude. The only su lphide observed was pyrite; 34 samples were selected from a wide varie ty of lithological and textural occurrences. deltaS-34(CDT) ranges wid ely and continuously between +8 and -34 parts per thousand with no str ong mode. The sulphur appears to be derived from several sources, and pyrite formation from a variety of conditions as indicated by such wid e ranging data, but for the samples with the lowest deltaS-34 the invo lvement of bacteria in sulphate reduction is inferred.