DETAILED ANALYSIS OF X-RAY DIFFRACTION DATA TO RESOLVE MULTIPLE POPULATIONS OF A SINGLE MINERAL: APPLICATIONS IN INTERPRETING THERMOCHRONOLOGIC, STRUCTURAL, AND SEDIMENTOLOGICAL DATA

Powder X-ray diffraction (XRD) is a common technique used to identify minerals and quantify their abundances. However, rarely are the subtleties introduced to XRD patterns by the presence of multiple generations of a single mineral phase recognized, investigated, and used to aid in the interpretation other data sets. Here we present multiple studies from a variety of geologic settings that use XRD data to quantify multiple populations of the same mineral and decipher complicated thermochronologic, structural, and sedimentological datasets.

Deviations from ideal XRD peak shapes for any mineral can result from overlapping peaks of chemically distinct populations, and/or broadening due to intrinsic features caused by defects or small coherent scattering domains. Rietveld refinement models allow for the sources of imperfect peak shapes to be determined and quantified, yielding new data that allow for improved interpretation of other data sets. For example, these data have been used in ⁴⁰Ar/³⁹Ar studies of muscovite from metamorphic rocks where results are often complicated by the fact that muscovite can crystallize both above and below its Ar closure temperature. One study from the Bronson Hill terrane, NH, USA used XRD data to quantify 2 populations of chemically distinct muscovite and differentiate the timing of exhumation and low-temperature deformation. A second study from the Lishan fault, Taiwan used the microstrain peak broadening parameter to constrain relative abundances of detrital, authigenic, and metamorphic muscovite populations. These data aided in interpreting ⁴⁰Ar/³⁹Ar ages in and out of the fault zone. A third study used XRD data to assess relative abundances of authigenic and detrital quartz in Permian shales. Here XRD data showed variability in quartz peak breadths which was best modeled by the crystallite size broadening equation. Results strongly correlate to Si/Zr bulk chemistry data indicating that detrital and authigenic quartz ratios can be determined using this method.

In summary, peak shape analysis of XRD patterns can be used to quantify multiple populations of a single mineral phase. When combined with other data types, these data bring quantitative insight to complicated crustal processes.