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Department of Geological Sciences, University of Canterbury, PB 4800, Christchurch 8020, New Zealand
Correspondence: * E-mail: cdd21{at}student.canterbury.ac.nz
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ABSTRACT |
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 Top Abstract IntroductionAnalytical techniquesDiscussionConcluding remarksAcknowledgmentsReferences cited |
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Ti-tschermakite tschermakite magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (to 70 °C), indicating that new, hotter magma periodically fluxed the crystal mush. (3) The dominant population of rhyolite amphibole is small and bladed (magnesiohornblende) and crystallized at low P-T conditions (P = 0.3 GPa, T = 765 °C), equivalent to the eruptive P-T conditions. Dacitic and low-silica rhyolitic amphibole (tschermakite-magnesiohornblende) form two distinct populations, which nucleated at two different T (high: 820 °C and low: 750 °C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive temperature. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals + liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed.
Key Words: Amphibole • crystal growth • crystal mush • cummingtonite • geothermometry • geobarometry
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INTRODUCTION |
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TopAbstractIntroductionAnalytical techniquesDiscussionConcluding remarksAcknowledgmentsReferences cited |
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Few amphibole-rich basalt-andesite eruptions are recorded globally (Cerro la Pilita basalt, Western Mexico, Luhr and Carmichael 1985; Montserrat andesite, Rutherford and Devine 2003; Huerto andesite, Parat et al. 2008). The importance of their role in generating large silicic magma bodies, as well as their role in providing a mechanism for the remobilization of more evolved magma via gas sparging, has recently been proposed (Sisson and Bacon 1999; Bachmann and Bergantz 2004). Therefore, crystal "mushes" presumably represent the root zone of a magma reservoir or a zone of accumulation and are rarely erupted to the surface as primary magmas.
Post-collapse deposits associated with eruption of the voluminous (~250 km3) Matahina ignimbrite are basaltic-rhyolitic in composition and represent the incorporation of exotic magma types in the late stages of the eruption (Deering et al. 2008). These deposits include a suite of crystal-rich (up to 50 vol%), hornblende-bearing mafic pumice and a felsic biotite-bearing pumice. The foundering of the overlying crust during this event is hypothesized to have induced chaotic mixing in the chamber (e.g., Kennedy et al. 2008), thereby assisting in the withdrawal of comparatively small volumes of these magmas.
This paper presents a detailed evaluation of the temperature and pressure evolution and interaction of a complex suite of magmas, some of which are thought to represent the active, and very productive, mushy accumulation and ascent zone beneath the Taupo Volcanic Zone where rhyolitic magmas are produced (Deering et al. 2008). Chemical variations along amphibole traverses provide clues to the complex relationship among the diverse magma compositions, both from the main rhyolitic magma body and the basalt-dacite exhumed in the late stages of the Matahina eruption (post-caldera collapse). Amphibole chemistry is well suited for evaluating the evolution of this magmatic system due to the early appearance of amphibole on the liquidus in calc-alkaline, arc-related magmas and the availability of appropriate geothermometers and geobarometers (e.g., Holland and Blundy 1994; Schmidt 1992; Anderson and Smith 1995; Féménias et al. 2006).
Geologic background
The TVZ in the North Island of New Zealand is a rifted arc (Wilson et al. 1995) and can be divided into segments of dominantly andesitic cone-forming volcanism in the north and south, and a central region characterized by rhyolitic caldera-forming and dome-building eruptions (Healy 1962; Wilson et al. 1984) (Fig. 1
). Since the inception of volcanism in this central region, 1.6 Ma, >6000 km3 of rhyolitic magma has erupted from eight major eruptive centers. The Okataina Volcanic Centre (OVC), located at the northern extent of the TVZ, is defined as a complex of coalescing collapse structures, which formed during numerous pyroclastic eruptions over >400 kyrs (Nairn 2002).
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) as a slightly compositionally zoned series of fall and flow deposits (Bailey and Carr 1994). A ca. 30 km3, plinian style eruption preceded the voluminous expulsion of >160 km3 of rhyolitic magma during the catastrophic, caldera-forming event. The ignimbrite lies mainly to the northeast, east, and southeast, extending radially up to 30 km from the caldera margin (Fig. 1; Bailey and Carr 1994).
Matahina petrology
The bulk composition of the Matahina ignimbrite (s.s.) varies from 72.2 to 77.8 wt% SiO2, but is predominantly from 75.2 to 77.8 wt% SiO2, consisting of ash and pumice lapilli (Bailey and Carr 1994). Juvenile lapilli are crystal poor (2–21 vol%) and contain phenocrysts of plagioclase, bipyramidal quartz, orthopyroxene, hornblende, Fe-Ti oxides, and rare biotite and clinopyroxene (Table 1). The early, plinian phase of the Matahina eruption is characterized by rhyolitic pumice with a mineral assemblage grading from plagioclase + quartz + orthopyroxene + Fe-Ti oxides to plagioclase + quartz + orthopyroxene + amphibole + Fe-Ti oxides (Table 1). Amphibole crystals are almost exclusively small and bladed (<1 mm; Fig. 2a), but a few larger tabular types occur. Only in later stages of the eruption does a slightly higher crystal content (up to 15 vol% compared to <5 vol%), amphibole (typically tabular or stubby) + orthopyroxene ± biotite ± sanidine, and a low-silica rhyolite pumice type appear (Table 1).
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; locality 2), consists of a range of pumice types (basalt-rhyolite), which include the dominant rhyolite type (orthopyroxene ± amphibole) from the Matahina ignimbrite. Andesite to dacite clasts are typically crystal rich (up to 30 vol%) and porphyritic, and some display obvious macroscopic signs of mingling between a mafic and felsic melt composition. Total crystal contents decrease with increasing bulk-rock SiO2 for clasts within this deposit (Table 1). Rare, basaltic clasts are fine grained and crystal rich (45 vol%; Table 1), consisting predominantly of amphibole with plagioclase microlites in the groundmass. Rhyolitic clasts display a diverse ferromagnesian mineralogy: (1) orthopyroxene-only; (2) orthopyroxene + amphibole; and (3) amphibole + orthopyroxene + biotite.
Along the western margin of the caldera, a lithic-rich, PDC deposit (Fig. 1; locality 1), containing crystal-rich basaltic-andesite to dacite and crystal-poor rhyodacite to rhyolite clasts, was also erupted following the Matahina (s.s.) caldera-collapse. The basaltic-andesite to dacite clasts are similar in composition and mineralogy to those deposited on the eastern margin (Deering et al. 2008), but have comparatively higher crystal contents, which also decrease with increasing bulk-rock SiO2 (Table 1). However, the distinctly different, biotite-bearing, co-magmatic, high-silica pumice type is only found at this locality. Other rhyolitic clasts containing biotite are found within exposures to the south of the caldera margin (Fig. 1; locality 3) and within the PDC deposit to the east (Fig. 1; locality 2), but are distinguished from the biotite-bearing, high-silica pumice by the presence of orthopyroxene and amphibole.
In general, the basalt to dacite clasts erupted from both sides of the caldera are characterized by a multi-modal mineral distribution of glomerocrysts (Fig. 2b) and individual crystals ranging in size up to ca. 4.0 mm. Although the mineral abundances vary, the clasts contain plagioclase + amphibole + orthopyroxene + magnetite + clinopyroxene + ilmenite ± biotite ± quartz (Table 1). Individual amphiboles are blocky (Fig. 2c), tabular, or stubby and range in size up to ca. 3.5 mm. Adcumulate textures, numerous glomerocrysts, and disequilibrium textures (e.g., resorbed and embayed plagioclase, crystal overgrowths, amphibole reaction rims, mingled mafic and felsic glasses) are ubiquitous (Figs. 2c and 2d).
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ANALYTICAL TECHNIQUES |
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TopAbstractIntroductionAnalytical techniquesDiscussionConcluding remarksAcknowledgmentsReferences cited |
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Amphibole compositions
Over 85 amphibole crystals from 29 samples were analyzed representing a diverse range of bulk-rock compositions (basalt-rhyolite) and stages of the Matahina eruptive sequence. Representative analyses are presented in Table 2 and the full geochemical database is available as a supplementary table.1 Cation distributions for structural formulae were recalculated using the procedure suggested by Leake et al. (1997), which estimates the proportion of ferric iron based on the maximum stoichiometric limits (13eCNK).
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All Matahina amphiboles can be broadly classified as calcic [CaB 
1.5; Ti < 0.50 atoms per formula unit (apfu)] and divided into two subgroups: (1) (Na + K)A 0.50 apfu, and (2) (Na + K)A < 0.50 apfu (Fig. 3a). Amphibole with (Na + K)A 0.50 and Si < 6.5 are classified as magnesiohastingsite, rather than pargasite based on the low Alvi content relative to estimated Fe3+ (Fig. 3a). Additionally, they all classify as magnesian amphibole ranging in XMg (=Mg/Mg + Fe2+) between 0.5 and 1.0 (Fig. 3b). Stubby, small amphibole (<0.5 mm) from the fine-grained basalt are magnesiohornblende and compositionally similar to the rhyolitic hornblende. Basaltic-andesite amphiboles range from magnesiohastingsite tschermakite magnesiohornblende, whereas andesite and dacite amphiboles are tschermakite and magnesiohornblende. Some amphibole core compositions in the basaltic-andesite and andesite also have Ti between 0.25 and 0.49 apfu and are classified as titanian-magnesiohastingsite and titanian-tschermakite progressing from tschermakite to magnesiohornblende or cummingtonite at the rims. Individual cummingtonite crystals (<250 µm) also occur in the groundmass of one mingled andesite sample (UC1239).
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). The composition of individual phenocrysts in the dacite and low-silica rhyolite are tschermakite to magnesiohornblende, with some displaying a range from tschermakite cores to magnesiohornblende rims. Low-silica rhyolite have a bimodal population of magnesiohastingsite/tschermakite, which are the largest amphibole (up to 1.5 mm), and smaller (<1.0 mm) magnesiohornblende. Nearly all the bladed, elongate (up to 1.0 mm; Fig. 2a) amphiboles from the rhyolite are defined as magnesiohornblende with rare tschermakite.
Geothermobarometry
Experimental studies that have evaluated the effects of pressure and temperature on amphibole composition (Johnson and Rutherford 1989; Thomas and Ernst 1990; Schmidt 1992; Spear 1981; Blundy and Holland 1990) have concluded that the Al-Tschermak substitution (SiT + MgM1-M3 = AlT + AlM1-M3) is sensitive to changes in pressure and the Ti-Tschermak (2SiT + MnM1-M3 = 2AlT + TiM1-M3) and edenite [SiT + 
A = AlT + (Na + K)A] exchanges in response to changes in temperature. Anderson and Smith (1995) determined that two additional intensive parameters might induce an Al-Tschermak exchange, leading to an overestimation of pressure: (1) decreasing 
O2 lowers Mg (relative to Fe2+) forcing a Tschermak substitution, or (2) low Fe3+ relative to total Fe induces a Fe3+AlM1-M3 exchange. However, calculated Fe3+/(Fe3+ + Fe2+) ratios for the Matahina hornblende are similar to those in barometric calibrations and oxygen fugacity calculated using Fe-Ti oxides are higher than the QFM buffer, which precludes a significant effect on hornblende compositions governed by changes in these conditions. Increasing H2O, independent of P and T, was shown by Scaillet and Evans (1999) to increase AlT; however, these changes were small (1.64 to 1.81 AlT; 5.7 to 7.0 wt% H2O, respectively) in relation to the AlT range for the Matahina amphiboles (0.91 to 1.89) and cannot account for the total variation within the population.
The Al-variations in the Matahina amphiboles can be assessed based on the above mentioned preferred substitutions governed by P and T. The (Na + K)A-, Ti-, and Ca-content display good correlations with AlT (Figs. 4a, 4b, and 4d) and indicate a strong compositional dependency on temperature. In contrast, the AlT and AlM1-M3 (Al-Tschermak exchange) are poorly correlated, but an increasing trend is recognized from rhyolite to basaltic-andesite (Fig. 4c), indicating that pressure also influenced this compositional variation in the amphibole.
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) observed among the entire data set and are consequently utilized as a relative pressure indicator. Therefore, the pressure estimates must be evaluated with caution as the geobarometer has not yet been calibrated to higher pressures and temperatures with the coexisting phases present in the mafic magmas.
The Anderson and Smith (1995) experimental calibration is temperature dependent and, at temperatures >800 °C, the correction increases and becomes unsuitable for estimating pressure. In fact, similar to the results of Féménias et al. (2006) at temperatures >800 °C, the calculations produce negative results. Independent T-estimates from Fe-Ti oxides and orthopyroxene-clinopyroxene (author unpublished data) yield temperatures consistently >800 °C (Table 3), indicating that this method is inappropriate for many of the Matahina hornblende grains. Estimation of P-T using the semi-quantitative, graphical method devised by Ernst and Liu (1998) is appropriate for the range of P, T, and compositions of the Matahina hornblende and is generally consistent with P estimates from other calibrations (Hollister et al. 1987; Hammarstrom and Zen 1986; Schmidt et al. 1992). To reduce the uncertainty inherent in graphical estimations, pressures were obtained using the experimental calibration of Schmidt (1992).
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) for a range of temperatures with well-constrained plagioclase compositions that co-exist with, or are included in, dacitic-rhyolitic hornblende. However, the intra- and inter-phenocryst compositional diversity of hornblende and plagioclase in the basaltic-andesite to dacite samples, revealed by detailed traverses (most <20 µm), limits the interpretation of these results for the full range of samples evaluated and, therefore, the Holland and Blundy (1994) estimates were only used on the rhyolites.
According to Helz (1973, 1976, 1979), Otten (1984), Ernst and Liu (1998), and Féménias et al. (2006), Ti solubility appears an acceptable geothermometer within the P-T envelope (P = 0.1–0.6 GPa; T = 700–950 °C) of the Matahina magmas. The Féménias et al. (2006) calculations are within error of the plagioclase-hornblende T estimates by the aforementioned independent geothermometers for well-constrained (equilibrated and homogenous) rhyolite samples (Table 2). Therefore, considering that the mafic Matahina magma is Ti-saturated (ilmenite present), it is suggested that, in contrast to the felsic compositions, Ti-hornblende is most appropriate for interpreting the thermal evolution of the basaltic to dacitic magmas.
Most of the amphiboles (Ti-magnesiohastingsite and Ti-tschermakite) in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (Figs. 5c and 5d; P to 0.5 ± 0.06 GPa and T to 950 °C), but also display the widest range and show a relative decrease in pressure and temperature from core to rim when compared to the dacitic to rhyolitic hornblende. The lowest rim/near-rim temperature estimates (<750 °C) are from the mingled andesite sample UC1239 and are considered tenuous as these are from tschermakite with cummingtonite overgrowths, and Ti along these diffuse boundaries are low. Pressure and temperature estimates from dacitic and low-silica rhyolitic amphiboles (tschermakite-magnesiohornblende) show a near bi-modal distribution (Fig. 5b; high: P = 0.35–0.55 GPa, T = 775–850 °C; and low: P = 0.2–0.3 GPa, T = 750–775 °C) indicative of two distinct crystallization sequences at pressures and temperatures similar to crystallization conditions estimated for the andesite and rhyolite (s.s.) (Figs. 5a and 5d). The dominant population of rhyolite amphibole (magnesiohornblende) crystallized under comparatively low P-T conditions (Fig. 5a; P = 0.2–0.3 GPa, T = 725–800 °C), but a subordinate population encompasses higher P-T (Fig. 5a; P = 0.2–0.45 GPa, T = 775–860 °C). This subordinate population represents the larger (up to 1.6 mm) phenocrysts (stubby or tabular), whereas the dominant population are small (<0.7 mm), bladed phenocrysts.
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In general, Al2O3 is positively correlated with TiO2, Na2O, K2O, and MgO, whereas the FeO and SiO2 are inversely correlated; MnO and CaO remain relatively constant (e.g., Fig. 6). As AlT and Ti appear to be good indicators of P-T changes, these elements were chosen to evaluate the core-rim variations among amphibole. The AlT and Ti contents of 23 of the amphibole traverses are shown in Figure 7, representative of the basalt to rhyolite bulk-rock compositions (Deering et al. 2008). Amphibole compositions have been divided into high- and low-AlT–Ti contents. The low-AlT–Ti (LAT) category corresponds with the eruptive P-T estimates (P = 0.3 GPa and T = 765 °C), respectively, for the Matahina outflow sheet (s.s.). The high-AlT–Ti (HAT) category encompasses a range in T from 850 to 950 °C, but the highest P-T estimates from the basaltic-andesite amphibole are used as a graphical reference (Fig. 7).
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). A Ti element map of the largest phenocyrst found in the basalt displays several distinct zones from a low-Ti core to high-Ti rim (Fig. 8a). The one small basaltic-andesite amphibole displays a clear increase from a LAT core toward a HAT rim (Fig. 7b). In contrast, the larger amphiboles from the basaltic-andesite and andesite have HAT core contents and display variable magnitude oscillations with steep decreases to LAT at the rim (Figs. 7b and 7c). The Ti element map of a large glomerocryst (3.0 mm) shows an anhedral orthopyroxene core with distinct bands in the hornblende overgrowth (Fig. 8b). Notably, a near rim decrease occurs over the last ~200–250 µm on each amphibole. The smallest andesite amphibole traverses with HAT cores are also ~200–250 µm in length and have steep decreases to a LAT rim, with the exception of one larger traverse (~625 µm) that shows little change from core to rim (Fig. 7d). Tschermakite with cummingtonite overgrowths also occur within one mingled andesite clast. Magnesium and Ti element maps clearly show the distinct compositional change along the tschermakite edge up to ~250 µm thick (Figs. 9a and 9b). Small amphiboles from dacite to rhyolite predominantly show two contrasting trends from core to rim: (1) an increase from the lowest AlT–Ti to the LAT, and (2) decrease from HAT to LAT, with two HAT amphibole that show little variation from core to rim (Figs. 7e and 7f).
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DISCUSSION |
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TopAbstractIntroductionAnalytical techniquesDiscussionConcluding remarksAcknowledgmentsReferences cited |
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; Fig. 1: localities 1 and 2).
Calcic-amphibole phenocrysts/antecrysts and/or groundmass microcrysts appear as the dominant ferromagnesian phase, over clinopyroxene/orthopyroxene, in the basaltic-andesite to dacite bulk-rock compositions, but vary in rhyolite bulk-rock compositions when orthopyroxene is present. Since amphibole is always present, it remained on the liquidus throughout the magmatic evolution, and compositional variations are hence interpreted to reflect a record of magmatic differentiation (Ti-magnesiohastingsite Ti-tschermakite tschermakite magnesiohornblende), which is correlated with the basaltic-andesite/andesite to rhyolite bulk-rock compositions. In particular, amphibole from the crystal-rich basaltic-andesite (UC816) and andesite (UC1100) and some individual phenocrysts (e.g., from samples: UC816, UC1100, UC1283) include the entire range of compositions of the magmatic differentiation trend consistent with in situ crystallization. The largest Ti-magnesiohastingsite, Ti-tschermakite, and tschermakite are mostly found in the basaltic-andesite to andesite, occur as glomerocrysts or large blocky phenocrysts (Fig. 2), and were equilibrated at the highest pressures (0.4 to 0.6 GPa) and temperatures (800–950 °C). Some of these amphibole grains display gradual decreases in temperature from core to rim (Fig. 10a), indicative of a progressive cooling at mid-crustal depths between 13–22 km, using an average continental crust density of 2.67 g/cm3.
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), but probably reacted with the melt early as either the temperature decreased or aH2O increased into the hornblende stability field. Several large (>2.5 mm) phenocryst traverses display low-frequency (200 to 800 µm apart), high-amplitude temperature increases (up to 70 °C) interpreted to reflect magma recharge events into the mid-crustal basaltic-andesite to andesite cumulate (Fig. 10b). However, temperatures appear to remain within the amphibole stability field. The frequency and amplitude of the temperature variations are relatively broad (Figs. 8 and 10) and can be constrained within the time required to grow millimeter-sized phenocrysts as the magma rehomogenizes and/or cools between recharge pulses (Kuo and Kirkpatrick 1982; Cashman 1992, 1993; Wörner 1996). In addition, the 3-D crystal network, as indicated by glomerocrysts and adcumulate textures (Fig. 2) and high crystallinity (~50%) indicates this magma was probably near the rheologial locking point, where convection would have ceased (Bachmann and Bergantz 2004). Differences within and among several crystals in the population indicate that crystallization probably occurred at several discrete depths prior to eruption. These steps in crystallization can be related to the continued evolution of the central, andesitic mush by recharge, prior to exhumation following chaotic mixing in the main magma reservoir.
Magma mingling—quenching
The general relationship between decreasing crystal contents and increasing SiO2 contents within both the andesite to low-silica rhyolite from the PDC deposits and the main Matahina outflow sheet (s.s.) (Table 1) are suggestive of a subtle zoning within the dominant portion of the magma reservoir. However, two important observations indicate that additional processes, other than crystal fractionation occurred just prior to or during the Matahina eruption: (1) high-silica, crystal-poor rhyolite pumice, which also erupted during the initial plinian phase, occur within the PDC deposits, and (2) the rhyodacite to low-silica rhyolite clasts within the PDC deposits have lower crystal contents than those of comparable bulk-rock composition from the main Matahina outflow sheet (Table 1; e.g., samples UC813 and UC498) (Fig. 1; localities 1 and 2). These features indicate that the evacuation of the chamber was not continuous and normal, but probably piece-meal, incorporating the basalt to dacite compositions only during the final stages of the eruption and that the rhyodacite to low-silica rhyolite from post-collapse deposits resided separately from the main Matahina rhyolitic reservoir.
Some of the basaltic-andesite to low-silica rhyolite clasts from the PDC deposits (Fig. 1; localities 1 and 2) show variable signs of macroscopic mingling of a mafic and felsic component. Amphibole grains are characterized by a near bi-modal distribution of large (glomerocrysts, tabular, blocky) and small (stubby and bladed) forms, which correspond to HAT and LAT types, respectively (Fig. 7). However, limitations in the pressure estimates based on AlT variation (as outlined earlier) restrict the interpretation and, therefore, discussion refers to intracrystalline T, not P changes. The large amphibole from basaltic-andesite to andesite (up to 3.5 mm) have high-T cores with steep decreases to low-T 200–250 µm rims that correspond in thickness and T to rims of the dacite to rhyolite and small fine-grained basalt crystals (Fig. 7). In particular, the small amphibole from the andesite with high-T cores also have steep T decreases, consistent with late nucleation and rapid, cooling induced crystallization (Fig. 7). Crystals that formed at low T show an increase from core to rim, indicating heating (Fig. 7), but this cannot be quantified as it is within the error of the estimates (±30 °C). Overall these trends indicate that two distinct crystal populations merged and grew, approaching a thermal equilibrium close to the eruptive temperature (~765 °C).
Perhaps the most convincing evidence of these distinct episodes of crystallization and mixing are the tschermakite with cummingtonite overgrowths and small individual cummingtonite phenocrysts from a mingled andesite clast (Fig. 9) and the abundant magnesiohornblende (up to 90% modal) in the fine-grained basalt (Table 1). The stability field for cummingtonite in a rhyolitic melt was determined for TVZ rhyolites by Nichols et al. (1992). Their experiments showed that cummingtonite was restricted to pressures <0.35 GPa and a temperature of ca. 750 °C. Cummingtonite rims on hornblende have also been observed in other silicic eruptions (e.g., Pinatubo 1991; Luhr and Melson 1996) and attributed to crystallization at low temperatures (<800 °C). Crystallization experiments on mafic inclusions from the Adamello Massif granitoids (Blundy and Sparks 1992) attributed the abundance of hornblende, plagioclase, and magnetite to overstepping of the olivine and clinopyroxene nucleation field by rapid quenching below 970 °C. Therefore, temperature estimates (~750 °C) for the overgrowths (<250 µm) of cummingtonite on tschermakite from the mingled andesite and the abundant, small (<250 µm) magnesiohornblende from the basalt, record a dramatic decrease in temperature. Together, these features provide further support for rapid quenching as a hot mafic magma mingled with a cool felsic magma upon ascent.
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CONCLUDING REMARKS |
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TopAbstractIntroductionAnalytical techniquesDiscussionConcluding remarksAcknowledgmentsReferences cited |
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Ti-tschermakite tschermakite magnesiohornblende in basaltic-andesite/andesite to rhyolite, a decrease in crystal contents with increasing bulk-rock SiO2, and gradual decreases in temperature within the largest individual basaltic-andesite/andesite crystals support a model whereby an andesitic progenitor cools and fractionates at mid-crustal levels, producing rhyolitic magmas. However, the distinct pre-eruptive storage conditions of the basaltic-andesite/andesite (~875 °C and 0.6 GPa) and Matahina rhyolite (~765 °C and 0.3 GPa) suggest development of discrete thermal zones, which were maintained prior to the catastrophic eruption. Second, the low-frequency, high-temperature fluxuations recorded within single hornblende crystals in the basaltic-andesite to andesite indicate that periodic fluxing by hot, mafic magma into the mid-crustal accumulation and ascent zone occurred. Third, the steep decreases in temperature observed along the rims of amphibole (qualitatively displayed as cummingtonite overgrowths) from the basaltic-andesite/andesite and slight increases in temperature along some dacite to rhyolite hornblende rims, both to eruptive temperatures (~765 °C), indicate that these magmas rapidly reached thermal equilibrium. In addition, crystallization within the amphibole + plagioclase + oxide stability field of the fine-grained basalt reflects a rapid temperature decrease, likely from temperatures >970 to ~740 °C. Hence, chaotic mixing, probably induced by caldera collapse, initiated the withdrawal of this mush from the base of the reservoir, quenching the hotter basalt to dacite to the cooler rhyolite; this "quenching" probably occurred during ascent and produced the distinct dacite to low-silica rhyolite types erupted at the final eruptive stage.
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ACKNOWLEDGMENTS |
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TopAbstractIntroductionAnalytical techniquesDiscussionConcluding remarksAcknowledgmentsReferences cited |
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Footnotes |
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1 Deposit item AM-09-038, Supplementary Table (the full geochemical database). Deposit items are available two ways: For a paper copy contact the Business Office of the Mineralogical Society of America (see inside front cover of recent issue) for price information. For an electronic copy visit the MSA web site at http://www.minsocam.org, go to the American Mineralogist Contents, find the table of contents for the specific volume/issue wanted, and then click on the deposit link there. 
MANUSCRIPT RECEIVED November 3, 2008; MANUSCRIPT ACCEPTED April 3, 2009
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