The Kübler Index in Late Diagenetic to Low-Grade Metamorphic Pelites: A Critical Comparison of Data From 10 Å and 5 Å Peaks

Abstract

Abstract 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 (KI 10 Å ) with those obtained from the 5 Å reflection (KI 5 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, KI 5 Å values from fitted and decomposed profiles show much lower scattering in comparison with KI 10 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 × 0.965 + 0.023°2θ) 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.