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Reconciling Heterogeneity in Crystal Zoning Data: an Application of Shared Characteristic Diagrams at Chaos Crags, Lassen Volcanic Center, California

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Reconciling Heterogeneity in Crystal Zoning Data: an Application of Shared Characteristic Diagrams at Chaos Crags, Lassen Volcanic Center, California Contrib Mineral Petrol (2005) 149: 98–112 DOI 10.1007/s00410-004-0639-2 ORIGINAL PAPER Glen S. Wallace Æ George W. Bergantz Reconciling heterogeneity in crystal zoning data: An application of shared characteristic diagrams at Chaos Crags, Lassen Volcanic Center, California Received: 29 March 2004 / Accepted: 8 November 2004 / Published online: 4 December 2004 Ó Springer-Verlag 2004 Abstract Isotope, trace element, and textural crystal cesses in upper crustal reservoirs (Vance 1962). Crystals zoning patterns record heterogeneity in magmatic sys- are particularly useful as indicators of magmatic pro- tems not resolved by whole rock analyses. These zoning cesses because they both record chemical conditions and data are used to infer crystal residence times, magma act as physical objects (Fig. 1). This is an important mixing, and other magmatic processes in many magmatic distinction from the melt phase, which cannot preserve a systems. We present the shared characteristic diagram structured record of previous environmental conditions. (SCD) as an organizational framework for crystal zoning Additionally, the ‘‘stratigraphy’’ preserved in zoning data that compares information from different phases profiles provides chronological ordering for processes and chemical tracers in a common framework. An recorded in crystal textures and chemical variations. example from Chaos Crags in the Cascade arc, produces Identification of crystal populations with similar zoning three main results. (1) Anorthite zoning profiles in pla- characteristics thus provides the opportunity to mutually gioclase have fewer shared characteristics in mafic constrain the physical and chemical evolution of mag- inclusions than in the host rhyodacite. (2) Single-crystal matic systems (Izbekov et al. 2002; Wallace and Bergantz 87Sr/86Sr data from previous studies (Tepley et al. 1999) 2002). Early study of plagioclase anorthite profiles from are consistent with more shared history between crystals plutonic and volcanic rocks showed variable degrees of than in anorthite profiles. This difference reflects a more similarity, interpreted as differences in the heterogeneity homogeneous distribution of 87Sr/86Sr than the intensive of the crystal growth environments or transient changes parameters controlling plagioclase composition. (3) The in magmatic environments (Greenwood and McTaggert Chaos Crags system exhibits a layer of heterogeneity in 1957; Anderson 1983). Micro-analytical advances have crystal populations that is not represented in whole-rock revealed a scale of heterogeneity in which REE and iso- analyses that indicate only simple binary mixing. The topic characteristics vary between and within individual inconsistency between 87Sr/86Sr and anorthite zoning crystals (Davidson and Tepley 1997; Tepley et al. 1999, data highlights decoupling between compositionally 2000; Davidson et al. 2001; Vazquez and Reid 2001, controlled and temperature/water-controlled zoning in 2002). plagioclase from Chaos Crags. Many studies exploit the information content of crystal populations to infer magma mixing and transient changes in intensive variables in magmatic systems. For example, glomerocrysts from the Bandelier Tuff with Introduction high 87Sr/86Sr record variable degrees of contamination from radiogenic country rocks dependent on distance Compositional zoning profiles in crystals have long been from the country rock contact (Wolff et al. 1999). Dif- recognized as a key to interpreting physiochemical pro- ferences in the Sr concentration of plagioclase from the eruption of Mt Mazama record gathering of crystals from high- and low-Sr magma reservoirs (Bacon and Editorial Responsibility: K. Hodges Druitt 1988; Druitt and Bacon 1989). In the Pleasant Bay G. S. Wallace Æ G. W. Bergantz (&) intrusion, plagioclase phenocrysts are in isotopic dis- Department of Earth and Space Sciences, equilibria with enveloping mafic magma, but in equilib- University of Washington, Box 351310, rium with the bulk granite indicating crystal transfer Seattle, WA 98195-1310, USA E-mail: [email protected] during intrusion of the mafic magma (Waight et al. 2001). Tel.: +1-206-6854972 Plagioclase in andesites from Soufriere Hills, Montserrat, Fax: +1-206-5430489 reflect mixing of crystal populations in a thermally het- 99 lines to profile alignment, substantially reduce selective A 3. bias, and return information on the spatial distribution 1. shared characteristics. The distribution of shared char- acteristics in zoning profiles provides a proxy for shared physiochemical history in crystal populations. We selected Chaos Crags for this study as a well- documented type locality for magma mixing (Fig. 2) (Williams 1931; Eichelberger 1980; Heiken and Eichel- 2. berger 1980; Davidson and Tepley 1997; Tepley et al. 1999). Results from SCD analyses revealed greater het- erogeneity in major element zoning patterns than ex- pected and, along with a difference in heterogeneity 87 86 B between anorthite and Sr/ Sr, prompted an evalua- tion of differences in boundary conditions for each chemical tracer. This line of research leads us to inter- pret a transiently decoupled thermal and compositional evolution of the Chaos Crags magma system. Crystal histories as recorded in zoning profiles Compositional profiles in individual crystals reflect the often complicated interplay between crystal-melt kinet- ics, transient changes in the system, and subsequent effects of intracrystalline diffusion (Fig. 1) (Izbekov et al. Fig. 1 The culmination of populations of crystals with individually 2002; Troll and Schmincke 2002; Costa et al. 2003). The complex histories produces diverse crystal populations. a An variability of chemical tracers is controlled by changes in individual crystal may experience changing thermal and composi- environmental conditions, distribution coefficients for tional environments during crystallization. These changes produce REE, and solid solution for major elements (Blundy and sequential features in zoning profiles and a crystal-stratigraphic record of the transport path of the crystal. b Zoned crystal Wood 2003). Different phase-tracer pairs produce un- populations record the details of the integrated characteristics ique zoning patterns when they respond differently to averaged in whole rock analyses changes in the environment or they have contrasting histories. However, if a transient change occurs over a shorter timescale than the crystal can grow a new zone, erogeneous magma chamber (Murphy et al. 2000). or if a crystal is resorbed, events may not be recorded. However, while analytical methods for individual crys- Shared history is inferred from the similarity of tals provide detailed chemical information, profile zoning profiles assuming that crystals in the same phase– alignment and data organization has remained largely tracer pairing growing in the same environment preserve subjective. The problem of profile alignment has pre- the same zoning pattern. Minerals with solid solution vented the use of statistically constrained profile com- exhibit two types of compositional zoning reflecting parison. both kinetics and environmental changes. Feedback We present the shared characteristic diagram (SCD) between crystal growth rate and diffusion in a boundary as an organizational framework for categorizing data layer of melt around the crystal produces small com- from zoning profiles. The SCD framework provides positional variations in zoning profiles referred to as statistically constrained correlations between profiles in oscillatory zoning (Bottinga et al. 1966; Allegre and a time-stratigraphic format. SCD for geochemical trac- Jaupart 1981; Pearce 1994; Lheureux and Fowler 1996; ers in specific phases are compared to infer the amount Fowler et al. 2002). Changes in the local environment of shared history in populations of crystals. The SCD can produce larger, more distinct spikes and plateaus in increases the utility of existing data and provide a zoning profiles—the features of interest for most SCD framework to guide sample selection for expensive or applications. The expression of these zoning types is time-consuming analyses. dependent on the sensitivity of each phase to changing Empirical profile alignment techniques compensate local conditions. Oscillatory, or Type I, zoning in pla- for geometric distortions in crystal zoning profiles gioclase is characterized by 1–3% changes in anorthite measured from thin sections. Direct comparison of content over lengths scales of 1–100 lm (Pearce and aligned zoning profiles permits quantitative comparison Kolisnik 1990). Type II zoning in plagioclase reflects between crystals and diverse chemical and zoning data changes in the local environment of the crystals and is types (Wallace and Bergantz 2002, 2004). In our ap- characterized by variations >4% anorthite over widths proach, computer algorithms apply parametric guide- >50 lm. The identification of phase–tracer populations 100 allows evaluation of the relative heterogeneity of or crystal sources. For example, changes in temperature chemical tracers in magmatic systems. will affect major element compositions and crystal growth Isotopic variations in crystal populations provide a rates, but will not affect the isotopic characteristics of unique constraint on crystal histories because isotopes of growing crystals. Additionally, environmental changes the same element do not fractionate during crystal may produce different zoning profiles in other mineral growth. Isotopic values provide an index of crystal his- phases due to differences in
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