Qualitative barometry of high P/T rocks with field based NIR spectroscopy of white mica

Abstract

White micas (muscovite, phengite, paragonite) are widespread in rocks of high-pressure, low-temperature (high P/T) terranes. Metamorphism in these settings occurs over a wide range in pressure and a relatively narrow range in temperature. Pressure increase results in forward progress of the aluminoceladonite exchange (Al-Cel, AlIV + AlVI=SiIV+[Fe2+,Mg]VI) in the muscovite-phengite series, ultimately producing very 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. Importantly, Vis–NIR measurements are readily acquired with field spectrometers and similar data can be acquired with airborne and spaceborne imaging spectrometers. 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. Spectra were collected from 186 samples and 20 samples were prepared for electron microprobe analysis (EMPA) and scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) to determine mineral assemblages and white mica compositions. To address possible bulk composition effects, different lithologies were evaluated, broadly categorized as metabasite, metacarbonate, and metapelite. The field-based Al-OH wavelength values range from 2190 nm to 2235 nm, which is equal to the full range of published values for metamorphic or hydrothermal white micas. 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. This can help identify the most pristine, peak-pressure samples as well as those that may yield important information regarding patterns of hydration and fluid flow along the retrograde P-T path at scales ranging from hundreds of meters to centimeters.