QUALITATIVE BAROMETRY OF HIGH P/T ROCKS WITH FIELD BASED NIR SPECTROSCOPY OF WHITE MICA

White micas are widespread in rocks of high-pressure, low-temperature (high P/T) terranes. Pressure increase results in forward progress of the aluminoceladonite exchange (Al-Cel) in the muscovite-phengite series, producing high Si content in white micas in blueschists and eclogites. Visible to near-infrared spectroscopy (Vis–NIR, 350–2500 nm) provides an effective monitor of the Al-Cel exchange because the substitution of Fe and Mg for Al in the octahedral site shifts the frequency of the Al-OH absorption band near 2200 nm to longer wavelengths. Here we present results of a field-based Vis–NIR study into variation of white mica Al-OH wavelength values and the potential to perform qualitative field-based barometry in high P/T terranes. Field spectra and samples were collected in five regions in northwest Turkey in which previous studies documented metamorphism over a wide range of pressure at relatively constant temperature. To address possible bulk composition effects, different lithologies were evaluated. The field-based Al-OH wavelength values range from 2190 nm to 2235 nm. White mica Si contents range from 2.97 a.p.f.u. (on an 11‑oxygen basis) in paragonite in retrograded calc-schist to 3.66 a.p.f.u. in lawsonite blueschist. In each high P/T region the field-based Al-OH wavelength values and white mica compositions are highly variable at scales of 100 s of meters to thin section scale. We interpret this to record variable retrograde re-equilibration at pressures lower than the peak pressure. Despite the variability, the maximum Al-OH wavelength values in each region, taken to represent the highest Al-Cel content in white mica and the most pristine high-pressure assemblage, correlate well with estimated peak pressure for the region. The maximum wavelength increases from ~2202 nm at P ≈ 6.5 kbar to ~2228 nm at P ≈ 11 kbar to ~2235 nm at P ≈ 24 kbar. Moreover, different lithologies in each region exhibit similar Al-OH wavelength variability and maximum Al-OH wavelength values. This suggests that bulk composition effects are minor compared to pressure effects on Al-OH wavelength variation. This study shows that field-based Vis–NIR spectroscopy can provide valuable information regarding peak metamorphic pressures in high P/T terranes as well as the extent and distribution of post-peak re-equilibration.