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

Iuliu Bobos, Lynda Williams

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16 Scopus citations


Coarse (2.0–0.2 μm) and fine (< 0.2 μm) 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 Bãi, 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 ~ 1000 ppm in the K-I/(NH4, K)-I. The δ11B (‰) 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 °C. Low Li content (1 to 8 ppm) was found in illitic clay fractions. The δ7Li (‰) shows negative values ranging from − 8.6 to − 12.3 (± 0.8‰) for the coarser (2.0–0.2 μm) NH4-I-S clays and from + 4.3 to + 14.1 (± 1‰) for the finer (< 0.2 μm) 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 δ15N (‰) 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 δ11B 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 δ7Li 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 δ15N, which reflect an influx of waters from an organic sediment origin. The δ15N 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.

Original languageEnglish (US)
Pages (from-to)22-39
Number of pages18
JournalChemical Geology
StatePublished - Nov 30 2017


  • Ammonium illite clays
  • Calc-alkaline volcanism
  • East Carpathians
  • Hydrothermal system
  • Light element geochemistry
  • Volcano-basement interaction

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology


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