Boron, lithium and nitrogen isotope geochemistry of NH4-illite clays in the fossil hydrothermal system of Harghita Bai, East Carpathians, Romania

Coarse (2.0-0.2 mu m) and fine (< 0.2 mu m) clay fractions of NH4-illite-smectite (I-S) mixed-layered and K-illite/ (NH4, K)-illite (I) mixed phases that vary in age and trace element composition were collected from the fossil hydrothermal system of Harghita Bai, East Carpathians/Boron and Li isotope ratios were measured by secondary ion mass spectrometry (SIMS), and N by isotope ratio mass spectrometry (IRMS) with the aim to characterize the isotope geochemistry and source of light elements fixed in authigenic NH4-illitic clays. Boron in NH4-I-S clays ranges from 513 to 1457 ppm and reached similar to 1000 ppm in the K-I/(NH4, K)-I/The delta B-11 (%) measured in NH4-I-S ranges from -12.6 to -22.4 (+/- 0.3%) and in K-I/(NH4, K)-I is consistently -5.5 to -5.1 (+/- 0.3%)/Boron isotopes systematically become lighter in the NH4-I-S series as temperature increased from 90 to 270 degrees C/Low Li content (1 to 8 ppm) was found in illitic clay fractions/The delta Li-7 (%) shows negative values ranging from -8.6 to -12.3 (+/- 0.8%) for the coarser (2.0-0.2 mu m) NH4-I-S clays and from + 4.3 to + 14.1 (+/- 1%) for the finer (< 0.2 mu m) fraction of NH4-I-S and K-I/(NH4, K)-I clays/The N (%) measured in the NH4-I-S clays ranges from 0.70 to 1.50 (+/- 0.2%), whereas in the K-I/(NH4, K)-I is about 0.70 (+/- 0.2%)/The delta N-15 (%) ranges from + 4.8 to + 7.4 (+/- 0.6) for most NH4-I-S and NH4, K-I clays, with one outlier for NH4-I-S of + 14.6 (+/- 0.6). The delta B-11 of K-I/(NH4, K)-I clays reflect a magmatic source, whereas the NH4-I-S series is consistent with the influx of isotopically light-B waters derived from hydrothermal leaching of continental evaporites and/or organic- rich sediments/The delta Li-7 signature measured on K-I/(NH4, K)-I clays also support a magmatic fluid, enriched in heavy Li, followed by precipitation of coarser NH4-I-S from more recent sedimentary contributions of isotopically light Li/This interpretation is also supported by the delta N-15, which reflect an influx of waters from an organic sediment origin/The delta N-15 of+ 14.6%(+/- 0.6) measured on NH4-I-S could be attributed to the presence of meteoric waters mixed with hydrothermal fluids/The isotopic data obtained trace the mobility of magmatic and organic-sedimentary components in the upper continental crust.