THE MULTIPLE PERSONALITIES OF ZONED CRYSTALS: COMPLEX GROWTH HISTORIES, THE MIXING PHYLOGENY, AND GEOCHRONOLOGY, WITH APPLICATIONS TO THE LASSEN VOLCANIC SYSTEM

The use of crystal phylogeny provides an organizational framework for the interpretation of microanalytical data from zoned crystals. Data is organized from crystal rim to core and spatially correlated between grains using adaptive normalization routines. The pseudo-stratigraphy preserved in the composition of progressive zones allows zoned crystals to provide information on the temporal evolution of magmatic systems in a manner that is not accessible by whole rock or single-crystal analyses. Moving from the rim to core, the positions in profiles at which the data are no longer in agreement are identified as decorrelation points. A decorrelation point is a location in the zoning profiles that indicates where the chemical environment of the crystals is no longer similar within analytical error, or a user-specified threshold.

The use of multiple crystals in geochronology can be complicated by complex histories in light of recent evidence that individual crystals may have residence times ranging from 10¹ to as much as 10⁶ years. The incremental growth of crystals is particularly problematic for isotopic systems in which the characteristic decay time is less than the residence time of the crystal, and analyses of whole crystals can be strongly biased by a small amount of growth just prior to eruption (e.g. Charlier et al, 2002). Additionally, if crystals have been mixed from multiple environments such that individual grains have unique growth histories, the use of populations of grains and commonly applied statistical measures can produce spurious results. The phylogeny provides a framework for evaluation of outliers in geochronology, integration of microanalytical data sets and the determination of a schematic kinematic history based on the co-distribution of decorrelation points.

Charlier, B, and Zellmer, G, 2000, Some remarks on U-Th mineral ages from igneous rocks with prolonged crystallisation histories: EPSL, 183, 457-469.