Journal of Petrology Advance Access published online on April 4, 2008
Journal of Petrology, doi:10.1093/petrology/egn015
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Crystal–Melt Separation and the Development of Isotopic Heterogeneities in Hybrid Magmas
Virginia Museum of Natural History, 21 Starling Avenue, Martinsville, VA 24112, USA
Received February 21, 2007; Revised typescript accepted February 22, 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMIXING OF ISOTOPICALLY DISTINCT,...DISCUSSIONCONCLUSIONSREFERENCES |
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If a magma is a hybrid of two (or more) isotopically distinct end-members, at least one of which is partially crystalline, separation of melt and crystals after hybridization will lead to the development of isotopic heterogeneities in the magma as long as some of the pre-existing crystalline material (antecrysts) retains any of its original isotopic composition. This holds true whether the hybridization event is magma mixing as traditionally construed, bulk assimilation, or melt assimilation. Once a magma-scale isotopic heterogeneity is formed by crystal–melt separation, it is essentially permanent, persisting regardless of subsequent crystallization, mixing, or equilibration events. The magnitude of the isotopic variability resulting from crystal–melt separation can be as large as that resulting from differential contamination, multiple isotopically distinct sources, or in situ isotopic evolution. In one model, a redistribution of one-third of the antecryst cargo yielded a crystal-enriched sample with 87Sr/86Sr of 0·7058, whereas the complementary crystal-poor sample has 87Sr/86Sr of 0·7068. In other models, crystal-rich samples are enriched in radiogenic Sr. Isotopic heterogeneities can be either continuous (controlled by the modal distribution of crystals and melt) or discontinuous (when there is complete separation of crystals and liquid). The first case may be exemplified by some isotopically zoned large-volume rhyolites, formed by the eruptive inversion of a modally zoned magma chamber. In the latter case, the isotopic composition of any (for example) interstitial liquid will be distinct from the isotopic composition of the bulk crystal fraction. The separation of such an interstitial liquid may explain the presence of isotopically distinct late-stage aplites in plutons. Crystal–melt separation provides an additional option for the interpretation of isotopically zoned or heterogeneous magmas. This option is particularly attractive for systems whose chemical variation is otherwise explicable by fractionation-dominated processes. Non-isotopic chemical heterogeneities can also develop in this fashion.
KEY WORDS: isotopic heterogeneity; zoning; hybrid magma; crystal separation; Sr isotopes; aplite; rhyolite
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMIXING OF ISOTOPICALLY DISTINCT,...DISCUSSIONCONCLUSIONSREFERENCES |
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Most large-volume, continental magmas are complex hybrids of mantle-derived basaltic melts and crustal rocks (e.g. Lipman, 1984
; DePaolo et al., 1992; Davidson et al., 2005). Zoning and other heterogeneities both chemical and isotopic, are common in, if not characteristic of, large bodies of hybrid magma, both volcanic and plutonic (Noble & Hedge, 1969; Moll, 1981; Halliday et al., 1984; Kistler et al., 1986; Johnson, 1989; Johnson et al., 1990; Hildreth et al., 1991; Verplanck et al., 1995; Reiners et al., 1996; Chesner, 1998; Hildreth & Fierstein, 2000; Barbey et al., 2001; Tsuboi & Suzukiba, 2003; Mikoshiba et al., 2004; Dreher et al., 2005; Wilson et al., 2006). The most common interpretations of isotopic variations in magma bodies are differential contamination or magma mixing, source variability, or isotopic evolution in long-lived magma chambers with high Rb/Sr (Noble & Hedge, 1969; Moll, 1981; Johnson, 1989; Davies & Halliday, 1998; Hildreth & Fierstein, 2000; Mikoshiba et al., 2004). However, there is another mechanism by which isotopic variability can develop in magmas. Given a hybrid magma in which the end-members are isotopically distinct and one or more of the end-members is partially crystalline, isotopic variability can result from the simple separation of solids from liquids. The end-members can be either multiple magmas (hybridization mechanism = magma mixing) or a magma that entrains partially molten xenoliths (hybridization mechanism = contamination/bulk assimilation). The isotopic variability can be large and may be a major component of overall isotopic heterogeneity in hybrid systems. It should be noted at the outset that this is not isotope ‘fractionation’. It is rather, the preservation, propagation, and re-expression of extant isotopic and chemical variability.
The principal assumption for the models proposed here is that the solid components of the mixed system retain their isotopic and chemical identity. In other words, crystals present at the time of mixing, including both phenocrysts present in magmas prior to mixing and any xenocrystic crystalline material derived from an assimilant [referred to together as antecrysts, an expansion of the term defined by Bacon & Lowenstern (2005) to include both xenocrystic and cognate solids], do not equilibrate with a thoroughly mixed melt. At the outset it is acknowledged that this is unlikely to be strictly true. However, the salient features modeled below will persist to a greater or lesser degree as long as some crystals inherited from the pre-mixing end-members retain any of their isotopic identity once the end-members are mixed. This is clearly not an unreasonable assumption, as both disparities in the isotopic compositions of phenocrysts and melt and isotopic zoning within phenocrysts are well-documented in both volcanic and plutonic rocks (Johnson et al., 1990; Davidson & Tepley, 1997; Knesel et al., 1999; Wolff et al., 1999; Baker et al., 2000; Waight et al., 2000, 2001; Halama et al., 2002; Tepley & Davidson, 2003; Wolff & Ramos, 2003; Gagnevin et al., 2005; Ramos & Reid, 2005; Wilson et al., 2006). In fact, it has been argued that most arc magmas are mixtures of melts and an unrelated, pre-existing ‘crystal cargo’ (Davidson et al., 2005). A second assumption, important for several of the models, is that, during partial melting of xenoliths, the isotopic composition of residual minerals and melt may be decoupled. Specifically, preferential melting or retention of minerals of unlike isotopic composition during (dehydration) melting will result in partial melts having different isotopic composition from the coexisting restite (e.g. Watson & Harrison, 1984; Hogan, 1995). This decoupling has been verified by observation and experiment for Sr isotopes and observed in migmatites and inferred from granite compositions for Nd (Hammouda et al., 1996; Tommasini & Davies, 1997; Ayres & Harris, 1997; Knesel & Davidson, 1999; Zeng et al., 2005a, 2005b).
A note of clarification is in order here. Obviously, hybridization is an open-system process. However, for the purposes of this paper, it will be generally assumed that once hybridization has occurred, the system is closed to further material input. It is in this context that ‘closed-system’ behavior is discussed from time to time.
This paper focuses on Sr isotope behavior, with a few exemplars using Nd isotopes. However, the principles outlined here are, within the limits discussed above, applicable to any isotopic or, for that matter, chemical system.
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MIXING OF ISOTOPICALLY DISTINCT, PARTIALLY CRYSTALLINE MAGMAS |
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TOPABSTRACTINTRODUCTIONMIXING OF ISOTOPICALLY DISTINCT,...DISCUSSIONCONCLUSIONSREFERENCES |
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The system chosen for modeling here is bulk assimilation of a partially molten xenolith by a partially crystalline basaltic magma. This system was chosen because it permits construction of models applicable to hybridization by either magma mixing or bulk assimilation. Models 1 and 2 (Figs 1 and 2), in which both end-members are in chemical and isotopic equilibrium prior to hybridization, are equally applicable to bulk assimilation (‘contamination’) or to magma mixing as traditionally construed (e.g. Eichelberger, 1975
). Model 3 (Fig. 3), in which one partially molten end-member is not isotopically or chemically equilibrated, will apply, for the most part, to hybridization by bulk assimilation. All percentages used in this paper are weight percentages unless otherwise noted.
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Mixing calculations
Two-component mixing lines are calculated using standard formulations (e.g. Faure, 1986
). For components A and B with isotopically distinct Sr,
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| (1) |
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| (1a) |
In these models, the bulk mixture of host magma and xenolith lies along the bulk mixing line in Figs 1–3
. The bulk mixing line is the commonly used means for relating the isotopic composition of a mixture to its end-member components. However, in the models presented here (in Fig. 1b, for example), four chemically and/or isotopically distinct components are recognized: (1) the liquid in the host magma; (2) the phenocrysts in the host magma; (3) the liquid in the partially molten xenolith; (4) the residual crystals in the partially molten xenolith. Thus, in addition to the bulk mixing line, mixing lines can be calculated between the liquid and solid components of the xenolith (xenolith solid–liquid mix), and the liquid and solid components of the host (host solid–liquid mix), the liquid components of host and xenolith (mixed liquids), and the solid components of host and xenolith (mixed solids) (Figs 1–3). When the host and assimilant are thoroughly mixed, the liquid in the mixed system will lie along the ‘mixed liquid’ line, the solids along the ‘mixed solids’ line.
Another set of solid–liquid mixing lines can be calculated for any binary mixture of xenolith and host. This requires several sets of mixing calculations. First, for a given mixture one must determine the fraction of solids and liquids derived from the end-members. For components A and B the weight fraction of solids derived from component A in a given mixture (SA,m) is given by
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| (2) |
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| (3) |
Looking again at the 80% host mixing line in Fig. 1b, one can now calculate the chemical and isotopic variation along this line related to the relative proportions of crystals and liquids. If the liquid is extracted entirely from the solids, it will have the chemistry and isotopic composition of the end-point on the ‘mixed liquids’ line whereas the solid remainder will have the chemical and isotopic composition of the end-point on the ‘mixed solids’ line. Alternatively, if no separation occurs, the mixture will lie on the bulk mixing line and have the isotopic and chemical composition of the bulk mixture. However, the general case will involve incomplete separation of liquids and crystals. The chemical and isotopic compositions of these partially separated mixtures lie along the 80% host mixing line.
Models for mixing of two solid–liquid mixtures
The two end-members chosen for the mixing models are a 600 Ma biotite gneiss (assimilant or xenolith) which is assimilated by a mantle-derived basalt (host). For all models, the basalt host is taken to be 50% crystalline with liquid and crystals in chemical and isotopic equilibrium. The isotopic and appropriate chemical and mineralogical compositions of the two end-members are given in Table 1.
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Sr is strongly partitioned into plagioclase in comparison with other solid phases considered in the models below. Because of this, the behavior of plagioclase during melting will control the distribution of Sr between the solid and liquid fractions.
The six models for Sr isotopes in Figs 1–3 represent three different behaviors of plagioclase during xenolith melting (models 1–3) and two different bulk Kd values for Sr in the host magma (a and b). In models 1a, 2a, and 3a, the bulk Kd for Sr in the host basalt is assumed to equal unity, hence the solid and liquid in the host have identical Sr concentration. In models 1b, 2b, and 3b the bulk Kd for Sr (modeled for Rayleigh fractionation) in the host is taken to be two, hence the host solids are enriched in Sr relative to the host liquid.
For model 1 (Fig. 1) plagioclase and melt in the xenolith are in chemical and isotopic equilibrium prior to mixing. The melting reaction,
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| (4) |
) for the melting of a biotite gneiss at 950°C and 5 kbar (Table 2). This is an end-member case in which it is assumed that all plagioclase in the system re-equilibrates during partial melting (bulk Kd for Sr = 2) and takes on the Sr isotopic composition of the bulk xenolith (87Sr/86Sr = 0·71136). Most Sr resides in plagioclase (and, thus, the solids) and the 87Sr/86Sr is equal for solids and liquids in both the host and xenolith, Hence, as is clear from inspection of Fig. 1, the solid component of the both the 60–40 and 80–20 mixtures is enriched in both total Sr and radiogenic Sr relative to the liquids in those mixtures (Fig. 1a and b). This assimilation model is akin to a magma mixing model where plagioclase is an abundant phenocryst phase in a magma that is then mixed into the basalt host.
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For model 2 (Fig 2a and b) it is assumed that all of the plagioclase is consumed during melting of the xenolith via the reaction
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| (5) |
For model 3 (Fig. 3a and b), plagioclase is indifferent to melting in the xenolith, neither reacting with, contributing to, consuming, nor equilibrating with the melt. The melting reaction
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| (6) |
) for the melting of a biotite gneiss at 950°C and 5 kbar (Table 2). In this model, all of the Sr in the partial melt of the xenolith is derived from the biotite, whereas essentially all of the Sr in the xenolith residua resides in plagioclase. Thus the liquid and solid share, respectively, the isotopic characteristics of biotite and plagioclase. The striking feature of Fig. 3a and b is the extremely radiogenic character of the xenolith melt (87Sr/86Sr = 0·9145), a consequence of high Rb/Sr in the biotite (Table 1; Knesel & Davidson, 1999). Thus, even though this melt contains only 36 ppm Sr (as opposed to 400 ppm in the host magma) it has a strong influence on the isotopic composition of the system. The xenolith solid, in contrast, is nearly as non-radiogenic as the host magma. In fact, it was necessary to omit the ‘mixed solids’ line in Fig. 3b for clarity. It should be noted that if the initial 87Sr/86Sr of the xenolith is elevated, 87Sr/86Sr in the plagioclase will be as well. In such cases, 87Sr/86Sr of a mixed solid in a mixed system could conceivably be as high as or higher than the mixed liquid (analogous to the behavior seen in Fig. 1). The range of xenolith solid and liquid compositions that can be produced by combinations of the three models lies within the areas outlined by the mixing lines in Fig. 4a (for liquid compositions) and Fig. 4b (for solid compositions). For the equilibrium models (1 and 2), the isotopic composition of the solids and liquids does not vary between end-member models; only the Sr content of the solids and liquids is affected. However, even partial disequilibrium during xenolith melting will result in deviation from isotopic and chemical equilibrium. In particular, even small amounts of disequilibrium (e.g. 10%) during melting can have large effects on liquid compositions in a crystal-rich system and solid compositions in a melt-rich system (Fig. 4a and b).
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For model 3, if plagioclase is actually formed as a consequence of the melting reaction (e.g. amph = plag + melt), the composition of the solids will vary along the line in Fig. 4c. The consequences of neoblastic plagioclase formation on mixing relations are seen in Fig. 4d. It should be noted, in particular, that the solids in this case will have a higher Sr content and 87Sr/86Sr than the liquids. These diagrams are shown for demonstration purposes only. Large amounts of new plagioclase will not generally form during dehydration melting of biotite gneiss (e.g. Patiño Douce & Beard, 1995
).
The two models for Nd isotopes (Table 3; Fig. 5a and b) are essentially equivalent to models 1 and 3 for Sr isotopes (Table 2). For both models, a bulk Kd for Nd of 0·3 is assumed for the host magma. In the first model, based on melting reaction (4) (Fig. 5a), the liquid and solid phases in the partially melted xenolith are in chemical and isotopic equilibrium. For this model, 143Nd/144Nd is lower in mixed solids than in the coexisting mixed liquids. In the second model, based on melting reaction (6) (Fig. 5b), all Nd in the partial melt of the xenolith is derived from biotite, which is modeled as having Sm/Nd of 0·16 (Yang et al., 1999). REE in the restite (Sm/Nd = 0·27) are largely contained in trace phases such as apatite and sphene (Condie et al., 1995; Ayres & Harris, 1997; Bea & Montero, 1999). For this model, 143Nd/144Nd is lower in the mixed liquids than in the coexisting mixed solids.
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Mixing of liquids with a partially crystalline host magma
In general, assimilation of a liquid by a partially crystalline magma will increase isotopic inequalities in the system. In the simplest case, where the crystals and liquid in the host magma have the same chemical and isotopic composition, the mixture appears to devolve to a simple mixing line (Fig. 6a). However, there will be significant differences in the chemical and isotopic compositions of the bulk mixture on the one hand and the mixed liquid on the other (Fig. 6a). Mixing relations become more apparent in Fig. 6b and c. These models are equivalent, the only difference being in the composition of the assimilant liquid. A noteworthy feature is the large potential isotopic heterogeneities manifest in Fig. 6c and d, where the melt modeled in Fig. 3a [derived from reaction (6)] is mixed directly with the host magma.
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Development of isotopic heterogeneity in homogenized binary mixtures of assimilant and host
Of particular interest for all models are the 60–40 and 80–20 host–assimilant mixing lines (labeled ‘60% host mixing line’ and ‘80% host mixing line’ in Figs 1–4
). These binary mixtures can be used to represent crystal–liquid separation in physically homogenized mixed magmas in which the antecrysts retain their isotopic identity. As an example, Fig. 7 shows an annotated expansion of part of Fig. 3a. The 60% host mixing line connects a point on the host–assimilant liquid mixing line (A) to a point on the host–assimilant solid mixing line (B). These two points are the compositions of, respectively, the liquid and the solid fractions of a bulk mixture consisting of 60% host magma and 40% assimilated xenolith. The intersection (C) of the 60% host liquid–solid mixing line with the bulk mixing line gives the relative proportions of solid and liquid in the mixed system, in this case (coincidently) 60% solid and 40% liquid. Thus for this model, a homogenized bulk mixture of 60% host and 40% assimilant will be 60% crystalline and have 87Sr/86Sr = 0·70625. If we now allow solids and liquids in this homogenized system to separate without further crystallization, separation of the magma into solid- and liquid-rich regions will yield crystal-rich regions with relatively low 87Sr/86Sr and complementary melt-rich regions with higher 87Sr/86Sr (Fig. 7). Such separation occurring on the scale of a magma chamber will produce an isotopically zoned body. As an example, let us start with the homogenized bulk mixture with 60% crystals 87Sr/86Sr = 0·70625. If one-third of the crystals are removed from one region of the magma [leaving it with 50% crystals (60–20 = 40 mass units crystal, 40 mass units melt)] and added to another region of magma [which would now have 67% crystals (60 + 20 = 80 mass units crystal, 40 mass units melt)] an isotopic heterogeneity will be created. The volume of magma containing 50% crystals will have 87Sr/86Sr = 0·7068, and the complementary volume containing 67% crystals will have 87Sr/86Sr = 0·7058. If the modal variation in antecryst content is continuous, zoning will be continuous as well.
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Crystallization of the homogenized hybrid magma
The mixing behaviors shown in Fig. 8 reflect the assumption that pre-existing crystals (antecrysts) in the end-members retain their isotopic characteristics after hybridization (see the Introduction). However, once the system is mixed, any new solids that crystallize from the hybrid magma are assumed to be in chemical equilibrium with the hybrid melt phase, either in their entirety (equilibrium crystallization) or instantaneously (fractional crystallization). In either case, isotopic equilibrium between new solids and extant melt is to be expected. This will lead to isotopic zoning in the crystals, with a relict, disequilibrium antecrystic core and an isotopically equilibrated, neoblastic rim.
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Both fractional (Rayleigh) and equilibrium (Bertholot–Nernst) crystallization paths were calculated using
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| (7) |
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| (8) |
Now, let us close the system to further material input. During closed-system crystallization of the homogenized hybrid magma, the isotopic composition of the bulk solid evolves on curved paths towards the bulk isotopic composition of the system (point C, Fig. 8a) as it incorporates radiogenic Sr from the liquid. The isotopic composition of the liquid will not change during crystallization unless there is diffusive exchange with the antecrysts. However, the very act of crystallization will further isolate the antecrystic cores and help prevent isotopic exchange between the liquid and relict antecrysts sequestered in the cores of crystals. In Fig. 8, the enrichment in Sr in the solid seen during the early stages of crystallization reflects a high bulk Kd for Sr (2) used in the crystallization models. If the bulk Kd = 1, the solid evolution line will follow the mixing line toward the bulk composition, whereas the liquid composition will remain constant.
Figure 8b shows tie-lines [actually, they are calculated as solid–liquid mixing lines using equation (1)] connecting coexisting solid and liquid compositions at various points during equilibrium crystallization of the homogenized hybrid magma. This emphasizes the continuing isotopic inequality (a consequence of the presence of antecrystic cores in the growing crystals) between solid and liquid in the system even as it crystallizes. In short, even though the neoblastic crystal rims are in isotopic equilibrium with the liquid, the bulk solid and bulk liquid will not be in equilibrium. This is shown diagrammatically in Fig. 9.
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It should be noted that at any point along the crystallization path, separation of crystals from liquid will, perforce, yield an isotopic heterogeneity in the magma whose magnitude can be calculated (as for Fig. 7) by reference to the 10% tick marks in Fig. 8b. This holds for both equilibrium and fractional crystallization paths.
Partial equilibration of antecrysts during post-mixing crystallization will move the liquid composition toward the bulk composition along the tie-lines in Fig. 8b. In a completely equilibrated system, of course, the final solid and liquid will be isotopically identical. In many or most hybrid systems, however, it is likely that some sort of isotopic heterogeneity will be preserved, even if it is not as extreme as that posited by Fig. 8.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMIXING OF ISOTOPICALLY DISTINCT,...DISCUSSIONCONCLUSIONSREFERENCES |
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Once isotopic inequality between the solid and liquid components of a magma is established, it tends to persist regardless of subsequent fractionation, crystallization, or mixing events (Fig. 8). This is because isotopic homogenization between the liquid and crystal fractions of the mixed magma can occur only via diffusion or recrystallization and, especially, only if isotopic homogenization occurs before crystal-enriched volumes of magma form. Sr in feldspar, in particular, preserves records of isotopic heterogeneity during the lifetime of many magma chambers (e.g. Davidson & Tepley, 1997
), and, indeed for millions and even billions of years beyond that time in felsic and mafic plutons (Waight et al., 2000; Halama et al., 2002; Tepley & Davidson, 2003). If antecrysts do not fully equilibrate, the mixing behavior of isotopically dissimilar, partially crystalline magmas combined with normal (gravitational separation, sidewall accumulation, filter-pressing, etc.) crystal–liquid separation processes will result, perforce, in the development of a magma-scale isotopic heterogeneity. Although diffusive or other equilibration early in the mixing history can mitigate the development of large-scale isotopic heterogeneity in hybrid magmas, other factors, especially separation of melt from xenolith (Fig. 6), will tend to exacerbate it. Once magma-scale heterogeneities form, they will be unaffected by any subsequent crystallization, mixing, or equilibration event. The heterogeneity can be mitigated only by diffusion at the scale of the magma chamber and, hence, is essentially permanent.
Aplites and pegmatites
From a mechanistic point of view, a logical interpretation of late, differentiated intrusions (e.g. aplites and pegmatites) that cut across many plutons is that of interstitial melt expressed during the last stages of crystallization. However, the isotopic composition of many late aplitic or pegmatitic intrusions differs from the bulk isotopic composition of the host pluton (e.g. Kistler et al., 1986; Johnson et al., 1990; Barbey et al., 2001; Ernst et al., 2003), leading to their interpretation, in many cases, as local injections of unrelated magma. The behavior of isotopes in partially crystalline mixtures provides a means whereby, in some cases, the chemical and mechanical interpretations can be reconciled (Figs 8 and 9).
Of particular importance here is the idea that once crystal–liquid isotopic heterogeneities are established in a mixed magma, they persist. Let us take, for example, a liquid separated from solids after 90% crystallization (point Z, Fig. 8b). This liquid will have an isotopic composition inherited and unchanged from the original mixture. The bulk solid composition will have evolved along the line labeled ‘solids’ to point Y (Fig. 8b). Separation of liquid from solid at that point will yield a chemically differentiated liquid in apparent isotopic disequilibrium with its host as shown diagrammatically in Fig. 9. However, it is clear that the isotopic heterogeneity is, in this case, inherited from the original mixing event. There is no need to call upon a separate and unrelated aplite magma.
Isotopic zonation in high-silica rhyolites
The behavior of isotopes in mixed systems may have implications for a much more significant (volumetrically, at least) geological problem; the origin of large-volume rhyolite tuffs. Recent studies and syntheses across a variety of disciplines are now concluding [and confirming; see, for example, Buddington (1959) or Lipman (1984)] that caldera-forming rhyolites are the surface manifestation of the same large-volume magmatic events that produce granitic batholiths. Rhyolites represent the evolved, melt-rich part of the system, and granitic plutons represent the complementary crystal-rich portion (Halliday et al., 1991; Bachmann & Bergantz, 2004; Lipman, 2008). Implicit in this interpretation is that crystal–melt separation is important in the petrogenesis of large-volume rhyolites. Given that these rocks are exemplars of hybrid magmas, they would seem to provide an important test of the relationship between crystal–melt separation and the development of isotopic zoning in hybrid magmas
Many zoned ash or ignimbrite eruptions are characterized by the early eruption of chemically and isotopically evolved, crystal-poor magma, followed by magma that is increasingly crystal-rich and less evolved. The process typically envisioned for this is the ‘inversion’ of a zoned magma chamber (Smith & Bailey, 1966; Hildreth, 1979; Smith, 1979; Duffield et al. 1995; Brown et al., 1998; Hildreth & Fierstein, 2000; Dreher et al., 2005; Bindeman et al., 2006). Figure 10 is a diagrammatic illustration of the development of such a magma chamber by crystal–melt separation in a hybrid magma and its subsequent eruption as a zoned tuff. In magmas where recognizable, isotopically distinct antecryst populations are present and/or isotopic contrast is correlative with apparent fractionation relationships amongst similar (e.g. dacitic to rhyolitic) magmas, crystal–melt separation must be considered as a strong candidate for the source of isotopic variation. On the other hand, if isotopic zonation is manifest in glass separates, at least some of the zoning must be related to differential contamination or mixing.
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Isotopic zoning in plutons
An isotopically zoned magma chamber could, of course, freeze in place without erupting, yielding an isotopically zoned pluton (e.g. Fig. 10e and f). A note of caution, however, is in order. Large-volume rhyolites, however complex their petrogenesis, represent short-lived eruptive events rooted in a single, contemporaneously active magma system, perhaps even a single magma chamber. Large plutons on the other hand, may represent longer-lived composites that, furthermore, are easily remobilized and reworked by subsequent magmatic events (Bindeman & Valley, 2003
; Glazner et al., 2004; Bacon & Lowenstern, 2005; Bindeman et al., 2006; Lipman, 2008). The character of the zoning might provide a clue to its origin. If the modal abundance of isotopically distinct antecrysts correlates with the zoning pattern, not only might this explain the origin of the zoning, but it may be an indication that the pluton represents a single-stage magma chamber. Obviously, if the zoning correlates with other features, such as chilled intrusive contacts, a composite origin may be indicated. |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMIXING OF ISOTOPICALLY DISTINCT,...DISCUSSIONCONCLUSIONSREFERENCES |
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The solid and liquid components produced by the mixing of isotopically distinct, partially crystalline end-members will themselves be isotopically distinct. Thus crystal separation in hybrid magmas can be responsible for a significant component of the overall isotopic heterogeneity of the magma. The magnitude of the isotopic variability resulting from crystal–melt separation subsequent to hybridization may be as large as that resulting from differential contamination or multiple isotopically distinct sources. In effect, crystal–melt separation provides another option for generating isotopic heterogeneities in magmas. This option is particularly attractive for systems (e.g. Hildreth & Fierstein, 2000
; Wilson et al., 2006) whose chemical variation is otherwise explicable by fractionation-dominated processes. |
ACKNOWLEDGEMENTS |
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I would like to thank John Hogan, Kurt Knesel, Frank Ramos, and Associate Editor Wendy Bohrson for insightful and thorough reviews. This work is an outgrowth of pluton studies supported by NSF grant EAR0000719 with continuing support provided by the Virginia Museum of Natural History. This contribution is dedicated to the memory of Paul Ragland.
*Corresponding author. E-mail: Jim.Beard{at}vmnh.virginia.gov
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