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THE KÜBLER INDEX IN LATE DIAGENETIC TO LOW-GRADE METAMORPHIC PELITES: A CRITICAL COMPARISON OF DATA FROM 10 Å AND 5 Å PEAKS

¹ Istituto di Geoscienze e Georisorse, (Consiglio Nazionale delle Ricerche), Area S. Cataldo, Via Moruzzi n.1, Pisa 56124, Italy
² Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria n. 53, Pisa 56126, Italy

^* E-mail address of corresponding author: leoni{at}dst.unipi.it

A set of 99 samples covering the whole range of low- and very low-grade metamorphic conditions has been used to compare Kübler index (KI) values measured on the illite 10 Å reflection (KI₁₀ Å) with those obtained from the 5 Å reflection (KI₅ Å). Evaluation of peak widths have been carried out both graphically on the recorded peak profiles from chart-strip X-ray diffraction (XRD) patterns and with the WINFIT computer program (Krumm, 1996) on fitted and decomposed profiles. All the measurements were performed both on air-dried and glycolated preparations. The data collected show that in the rock samples where illite is associated with significant amounts of I-S interstratified minerals, and/or K/Na intermediate micas, paragonite and other interfering phases, full width at half-maximum (FWHM) measurements on the 5 Å peak in fitted and decomposed XRD profiles from glycolated mounts give more reliable KI values than those obtained from the 10 Å peak. This is because of the easier and more complete de-summation of the illite second reflection from the contributions of all the interfering phases. In each sample population in which the rocks have the same metamorphic grade, KI₅ Å values from fitted and decomposed profiles show much lower scattering in comparison with KI₁₀ Å values and appear consistent with the values measured in lithologies without interfering phases.

The relationship between KI_{10 Å} and KI_{5 Å} appears very close to a 1:1 linear relationship; nevertheless, the conversion from KI_{5 Å} to KI_{10 Å} through the appropriate equation (KI_{10 Å} = KI_{5 Å} x 0.965 + 0.023°2) is recommended in order to avoid a small, but systematic, error.

In the range from late diagenesis to middle anchizone, where several phases may give diffraction effects interfering with illite peaks, the proposed procedure (i.e. FWHM measurements on the 5 Å peak in fitted and decomposed XRD profiles from glycolated mounts) seems to allow better estimates of the metamorphic grade than those obtained through traditional KI measurements.

Key Words: 10 Å and 5 Å Peaks • Air-dried and EG-solvated Preparations • Fitted or Fitted and Decomposed Peaks • Illite • Kübler Index • Late Diagenesis • Low-grade Metamorphism • Metapelites

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