Experimental and textural constraints on mafic enclave formation in volcanic rocks

Journal of Volcanology and Geothermal Research 119 (2002) 125^144 www.elsevier.com/locate/jvolgeores

Experimental and textural constraints on ma¢c enclave formation in volcanic rocks Michelle L. Coombs a; , John C. Eichelberger a , Malcolm J. Rutherford b a

Alaska Volcano Observatory, Geophysical Institute/Department of Geology and Geophysics, University of Alaska, Fairbanks, AK 99775, USA b Department of Geological Sciences, Brown University, Providence, RI 02912, USA Received 1 September 2001; received in revised form 11 February 2002; accepted 11 February 2002

Abstract We have used experiments and textural analysis to investigate the process of enclave formation during magma mixing at Southwest Trident volcano, Alaska. Andesite enclaves are present throughout the four dacite lava flows produced by the eruption, and resemble mafic enclaves commonly found in other volcanic rocks. Our experiments replicate the pressure^temperature^time path taken by enclave-forming andesite magma as it is engulfed in dacite during magma mixing. Pressure and temperature information for the andesite and dacite are from [Coombs et al., Contrib. Mineral. Petrol. 140 (2000) 99^118]. The andesite was annealed at 1000‡C, and then cooled to 890‡C at rates of 110‡C h31 , 10‡C h31 , and 2‡C h31 . Once cooled to 890‡C, andesite was held at this lower temperature from times ranging from 1 to 40 h. The andesite that was cooled at the slower rates of 2‡C h31 and 10‡C h31 most resembles enclave groundmass texturally and compositionally. Based on simple thermal calculations, these rates are more consistent with cooling of the andesite groundmass below an andesite^dacite interface than with cooling of enclavesized spheres. If enclaves do crystallize as spheres, post-crystallization disaggregation must occur. Calculations using the MELTS algorithm [Ghiorso and Sack, Contrib. Mineral. Petrol. 119 (1995) 197^212] show that for incoming andesite to become less dense than the dacite V34 volume % of its groundmass must crystallize to undergo V18 volume % vesiculation; these values are similar to those determined for Southwest Trident enclaves. Thus such crystallization may lead to ‘flotation’ of enclaves and be a viable mechanism for enclave formation and dispersal. The residual melt in the cooling experiments did not evolve to rhyolitic compositions such as seen in natural enclaves due to a lack of a decompression step in the experiments. Decompression experiments on Southwest Trident dacite suggest an average ascent rate for the eruption of V2^3 MPa h31 . An andesite experiment that was cooled and then decompressed at this rate contains melt that matches that of the natural enclaves. It is apparent that decompression (ascent)-induced crystallization occurs in enclaves, but not in the form of microlites as happens in the dacite host, due either to insufficient residence time at chamber temperatures or to the pre-existing microphenocrysts which act as sites for new growth. A 2002 Elsevier Science B.V. All rights reserved. Keywords: magma mixing; ma¢c enclaves; experiments; crystallization; textural analysis

* Corresponding author. Present address: US Geological Survey, MS 910, 345 Middle¢eld Road, Menlo Park, CA 94025, USA. Tel.: +1-650-329-5251; Fax: +1-650-329-5203. E-mail address: [email protected] (M.L. Coombs).

0377-0273 / 02 / $ ^ see front matter A 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 3 0 9 - 8

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1. Introduction The presence of ma¢c magmatic inclusions, or enclaves, in intermediate to silicic composition volcanic rocks has been recognized as evidence of mixing between two at least partially molten magmas (e.g., Eichelberger, 1978, 1980; Bacon and Metz, 1984; Bacon, 1986; Stimac et al., 1990; Clynne, 1999; Murphy et al., 2000). Chilled margins with ¢ner grain size, cuspate margins, ellipsoidal shapes, and groundmass textures with acicular mineral phases suggest that enclaves are parcels of ma¢c magma that rapidly crystallized upon contact with cooler silicic melt (Bacon, 1986). Magma mixing may initiate eruption (e.g., Sparks et al., 1977; Huppert et al., 1982; Eichelberger et al., 2000); several cases have been documented where mixing is believed to have been followed by eruption within days or weeks (e.g., Nakamura, 1995; Pallister et al., 1992; Coombs et al., 2000). Ma¢c enclaves in lavas from arc volcanoes follow a complex pressure^temperature^time path prior to eruption. This path may begin with ascent-induced decompression of the ma¢c magma after generation in the mid to lower crust. When the magma encounters upper crustal silicic magma, it will cool. Sometime after mixing, the enclave magma experiences eruptive decompression while in the host magma. The groundmass texture commonly seen in ma¢c enclaves ^ a network of acicular plagioclase, orthopyroxene and/or hornblende microphenocrysts and clear, microlite-free glass ^ is distinct from that of the host lavas in which the enclaves are found, and must result from the path described above. Enclave groundmass microphenocrysts are distinct from larger phenocrysts and smaller microlites found in the silicic host. Elongate, sometimes skeletal enclave microphenocrysts that comprise the groundmass are similar in morphology to crystals formed during rapid experimental cooling of silicate melts (Lofgren, 1980). The enclave groundmass assemblage is commonly interpreted to form during cooling from the original magmatic temperature to that of the silicic host. Interstitial glass found in enclaves is often compositionally indistinguishable from that found in lava groundmass (e.g.,

Miller et al., 1999 ; this study). Di¡usional homogenization of melt in a 5-cm-diameter dacite enclave with surrounding rhyolite melt in the dacite host would take V103 years (Baker, 1991). Alternatively, the similarity of host and enclave melt suggests that thermal equilibrium between the two has been reached and the enclave groundmass has crystallized to a greater degree than the host groundmass. This process would happen on much shorter timescales than di¡usion, as the rate of melt evolution would be controlled by thermal di¡usion and crystallization only. Based on these considerations, it is possible that enclave textures may form relatively quickly, much shorter than years. Crystallization-induced vesiculation in ma¢c magma has been proposed as the process that enables the £otation of ma¢c magma in overlying silicic magma, and thus the formation of ma¢c enclaves within the silicic host (Eichelberger, 1980; Huppert et al., 1982). Extensive crystallization drives the volatile concentration in remaining melt above saturation, thus causing exsolution and vesiculation. Fluid dynamical experiments (Thomas et al., 1993) and enclave size distribution studies (Thomas and Tait, 1997) support this theory. Alternatively, the presence of chilled, ¢ner-grained rinds (Bacon, 1986; Murphy et al., 2000; Miller et al., 1999) as well as positive correlation between enclave and crystal size within individual enclaves from some eruptions (Stimac et al., 1990) suggest that enclaves undergo signi¢cant crystallization after being entrained by a silicic host. Whether the majority of crystallization and vesiculation occurs before or after entrainment has implications for the mechanisms of magma mixing. In order to further constrain the processes responsible for enclave formation, we quantitatively describe the compositions and textures of andesite enclaves from Southwest Trident volcano, Alaska. The eruptions at Southwest Trident produced four distinct lava £ows in 1953, 1957, 1958, and 1959. This temporal information allows us to look for changes in enclave characteristics as a function of time as well as enclave size. We have also attempted to experimentally replicate their P^T^t path, and to compare experimentally produced

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textures and compositions to those of the natural enclaves. Finally, we use these results, as well as density and crystallinity calculations for enclave and host magmas to assess models of enclave formation.

2. Samples and techniques Representative whole rock compositions of enclaves and host lava from the four Southwest Trident lava £ows are given in Table 1 ; further analyses are in Coombs (2001). Constraints on temperatures, pressures, and water contents for the andesite and dacite were obtained from Fe^ Ti oxide and pyroxene geothermometry, hydrothermal phase equilibria experiments, and meltinclusion analyses, respectively (Coombs et al., 2000). Starting material for the andesite experiments is enclave 97NT20 (57 wt% SiO2 ); for the decompression experiments, dacite lava 97NT31 (63 wt% SiO2 ; Table 1). 2.1. Experimental techniques All experiments were conducted in the Experimental Petrology Laboratory at Brown University. Su⁄cient water was added to all experiments to ensure that all experiments were water-saturated (PH2O = Ptotal ). Andesite experiments were run in Ar-pressurized Ti^Zr^Mo (TZM) pressure vessels in a Deltech furnace. Oxygen fugacity was maintained at the Ni^NiO (NNO) oxygen bu¡er by the addition of CH4 to the Ar pressurizing medium, and monitored by the presence of Ni

metal and NiO powder in an unsealed Pt tube within the sealed Ag70 Pd30 tube that held the experimental sample. Experiments were quenched by directly immersing the bomb in cold water. Dacite experiments were run in Ag70 Pd30 capsules in cold-seal pressure vessels; oxygen fugacity was bu¡ered near the Ni^NiO (NNO) oxygen bu¡er curve by the reaction between the Ni-alloy vessel, Ni ¢ller rod, and water used as the pressurizing £uid (Geschwind and Rutherford, 1992). Coldseal experiments were quenched by a stream of compressed air followed by immersion in cold water. Experimental run conditions are shown in Table 2. The andesite was lightly crushed (to preserve original phenocrysts that might later serve as nucleation sites during groundmass crystallization) and annealed for V48 h at 1000‡C and PH2O of 90 MPa, the pre-eruptive storage conditions of the magma as determined by Fe^Ti oxide geothermometry and phase equilibrium experiments (Coombs et al., 2000). This served as starting material for cooling experiments. Pre-annealed aliquots of the andesite were then re-annealed at 1000‡C for 1^3 h, cooled to 890‡C, held at the lower temperature for time t, and then quenched. These conditions are designed to simulate the path taken by enclave-forming magma as it was engulfed in cooler silicic magma, and held at the cooler temperature for a certain residence time, t. A series of experiments were run in which cooling rate and t were varied (Table 2). Three cooling rates were used: 110‡C h31 , 10‡C h31 , and 2‡C h31 . Once cooled to 890‡C, andesite was held at this lower temperature for time t ranging from

Table 1 Whole rock compositions of starting materials and selected other Southwest Trident enclaves 97-NT-20 enclave SiO2 Al2 O3 TiO2 FeO (tot) MnO CaO MgO K2 O Na2 O P2 O5

57.14 17.25 0.80 7.39 0.14 8.23 4.51 1.02 3.37 0.15

97-NT-31 lava 63.76 16.14 0.68 5.65 0.12 5.54 2.71 1.54 4.25 0.14

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97-NT-18 enclave 56.87 17.77 0.81 6.99 0.15 8.50 4.33 0.95 3.49 0.14

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97-NT-25 enclave 57.18 17.26 0.80 7.39 0.14 8.23 4.51 1.02 3.37 0.15

97-NT-30 enclave 56.79 17.64 0.79 7.31 0.14 8.47 4.40 0.94 3.39 0.14

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Table 2 Experimental conditions Typea

Run

NT-2010a NT-2019 NT-2028 NT-2029 NT-2060 NT-2058 NT-2062 NT-2063 NT-2030 NT-2023 NT-2044 NT-2038 NT-2039 NT-2061 NT-2031 NT-2018 NT-2040 NT-2037 NT-2017 NT-3115 NT-3116 NT-3117 NT-3118 NT-3119 NT-3120 NT-2059 a

a a a a a c c c c c c c c c c c c c c a d d d d d c+d

Starting material 97NT20 97NT20 97NT20 97NT20 97NT20 NT-2028 NT-2060 NT-2060 NT-2028 NT-2019 97NT20 NT-2029 NT-2029 NT-2060 NT-2029 NT-2010a NT-2029 NT-2029 NT-2010a 97NT31 NT-3115 NT-3115 NT-3115 NT-3115 NT-3115 NT-2058a

T

P

(‡C)

(MPa)

Total time (h)

1000 1000 1000 1000 1000 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 1000^890 890 890 890 890 890 890 1000^890

90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90^3 90^3 90^3 90^3 90^3 90^93

48 48 48 48 48 59 61 96 15 16 17 24 34 61 5 6 14 24 43 96 10 81 161 33 55 45

Time at 1000‡C (h)

Cooling rate (‡C/h)

Time at 890‡C (h)

3 2 1 3 2 2 3 3 2 3 2 3 3 2

2 2 2 10 10 10 10 10 10 110 110 110 110 110

1 4 40 1 3 5 10 20 48 1 3 10 20 40

2

10

1

Decomp. time (h)

8 80 160 32 54 30

a = annealing, c = cooling, d = decompression, c+d = cooling followed by decompression.

1 to 48 h. Decompression experiments were run on Southwest Trident dacite in order to constrain an average ascent rate for the eruption. The dacite was annealed at 890‡C and 90 MPa (storage conditions for the dacite as determined by Coombs et al. (2000)) for 96 h. Aliquots of this starting material were re-annealed at the same conditions for 1^2 h, and then isothermally decompressed at rates of 0.6^11 MPa h31 (Table 2). These conditions correspond to magma ascent rates of 0.005^ 0.1 m s31 , the published range for e¡usive eruptions (Rutherford and Gardner, 1999). Decompressions were conducted by manually lowering pressure in 1.2^2.8 MPa steps at intervals of 11 min to 4 h, depending on total decompression time. Final pressures for all decompressions were V2 MPa. Finally, one andesite experiment was decompressed at 3 MPa h31 after initial cooling from

1000‡C to 890‡C at 2‡C h31 and a residence time of 1 h. This experiment was performed to replicate the complete path of enclave-forming andesite during magma mixing and eruptive ascent. 2.2. Analytical techniques Mineral and glass analyses of polished thin sections and grain mounts were conducted on the University of Alaska Fairbanks Cameca SX-50 electron microprobe. Mineral analyses used a focused beam, accelerating voltage of 15 keV, and a beam current of 15 nA. Glasses were analyzed with a 10Wm defocused beam, accelerating voltage of 15 keV, and a beam current of 10 nA. Hydrous glasses were analyzed using an automated Na-loss routine described by Devine et al. (1995). Whole rock analyses of natural enclaves and lavas were performed at Washington State Uni-

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versity using X-ray £uorescence (see Nye et al., 1994 for a full description of analytical procedure). Mineral modes for enclaves were determined by counting 500^1000 points using a petrographic microscope. Textural data were obtained by acquiring backscattered electron (BSE) images of enclave and experiment groundmass using the Cameca SX-50 microprobe and analyzing the images for crystal shape and size information. All images were acquired at 1600U magni¢cation at a resolution of 1.5 pixels/Wm. Plagioclase is the most abundant phase in the enclaves and was chosen for textural analysis. Crystal measurements were made by hand-tracing crystals within the program NIH Image, which then recorded width, length, and area for each crystal. For enclaves, between 72 and 453 crystals were measured for each sample (fewer measurements were made on small enclaves with large crystals). Less material was present for experiments and thus between 63 and 269 crystals were measured for each. For the calculation of crystal size distributions (CSDs) and crystal number densities, counting at least 80 crystals per sample results in repeatable CSD calculations (Peterson, 1996). As our measurements are done on a two-dimensional image of each sample, we used the method of Higgins (1994) to estimate the true shape of crystals based on their two-dimensional intersections. For each sample, a short:intermediate:long ratio was determined. The mode of the two-dimensional width/length ratio equals the short/intermediate ratio in three dimensions, and the intermediate/long ratio equals the skewness of the width/length ratio+0.5 (Higgins, 1994). Using this estimate of true crystal shape, we then calculated CSDs for each sample, using the program CSDCorrections (Higgins, 2000). The fabric of the samples is in all cases isotropic, and the crystals unrounded. We use ¢ve logarithmic length bins per decade. The method was applied alternatively to width and length measurements ; most samples showed good agreement between the two, but in general the width measurements resulted in nV with smaller errors and thus are presented here. Crystal number density (NV ), the total number of crystals per unit volume, was cal-

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culated by summing the number of crystals for each size interval for a given sample. It has been shown that the areal proportion of a phase in any section of a rock is equal to the volume proportion (see Higgins, 2002). Therefore we used the raw two-dimensional data to calculate crystal volume fraction (P, equal to 4crystal area/reference area). The total area of each sample used to calculate P is the groundmass area; vesicle and phenocryst areas were subtracted from the image prior to calculation of P.

3. Results 3.1. Southwest Trident enclaves Southwest Trident enclaves are andesitic, ranging in composition from 55.8 to 58.9 wt% SiO2 (Coombs et al., 2000; Table 1). The enclaves comprise from 6 2% to 10% of the rock at a given outcrop. Enclave abundance does not appear to vary systematically with the age of lava £ow or the position within a single £ow. They are spherical to ellipsoidal to rarely angular in shape, with a ¢nely crystalline and vesicular texture in hand specimen, similar in appearance to ma¢c enclaves described elsewhere (e.g., Bacon, 1986; Eichelberger, 1978; Clynne, 1999; Miller et al., 2000). Enclaves range in size from several millimeters to a meter in diameter. Based on point counts of eight enclaves they contain 43 V 6 volume % phenocrysts, where phenocryst is characterized as being greater than 0.4 mm in diameter (Table 3). Mineral assemblages of both phenocrysts and groundmass include, in decreasing abundance: plagioclase, orthopyroxene, clinopyroxene, titanomagnetite, and rare olivine; ilmenite is also occasionally present. Enclave groundmasses consist of typically elongate microphenocrysts of plagioclase and pyroxene and equant titanomagnetite, which form a loose network with abundant void space. Groundmass crystal laths span a range in lengths up to 0.4 mm. The interstices between crystals are ¢lled with glass and, in the case of the larger interstices, rounded vesicles. Enclaves analyzed by point count contain 18 V 3 volume % vesicles (Table 3). The glass in the enclave groundmass is

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Table 3 Modal data for Southwest Trident enclaves Whole rocka Phenocrysts 97NT16 97NT17 97NT18 97NT20 97NT21 97NT23 97NT25 97NT30 a b c

41.0 34.6 40.0 25.4 37.2 37.2 37.8 34.6

(2.9)c (2.6) (2.8) (2.3) (2.7) (2.7) (2.7) (2.6)

Groundmass onlyb Vesicles

Groundmass

Glass

Plagioclase

Pyroxene

Vesicles

19.4 18.8 19.4 22.0 16.4 18.4 13.8 15.2

39.6 46.6 40.6 52.6 46.4 44.4 48.4 50.2

44.7 37.0 30.7 35.7 26.4 43.6 47.9 43.7

15.3 16.5 25.0 25.7 33.8 21.7 21.9 20.2

7.1 (1.6) 17.7 (2.3) 12.0 (2.0) 9.1 (1.6) 13.7 (2.1) 5.4 (1.3) 8.0 (1.6) 12.8 (2.0)

32.9 28.7 32.3 29.5 26.1 29.3 22.2 23.2

(2.0) (1.9) (2.0) (2.1) (1.8) (1.9) (1.7) (1.7)

(2.8) (3.1) (2.8) (3.2) (3.0) (3.0) (3.1) (3.2)

(3.9) (3.4) (3.2) (3.1) (2.9) (3.7) (3.9) (3.7)

(2.3) (2.2) (2.9) (2.6) (3.3) (2.6) (2.7) (2.5)

(3.3) (3.0) (3.3) (2.8) (2.9) (3.1) (2.7) (2.7)

Whole rock components (phenocrysts, vesicles, and groundmass) sum to 100. Groundmass components (glass, plagioclase and pyroxene microphenocrysts, and vesicles) sum to 100. Volume percent errors are one standard deviation based on counting statistics.

often clear or brown and microlite-free, even when the host lava is completely charged with microlites (Fig. 1). 3.1.1. Groundmass compositions Interstitial glass in the enclaves is rhyolite, ranging in SiO2 content from 74.3 to 77.2 wt% (Table 4). Groundmass plagioclase microphenocrysts are normally zoned (Fig. 2) from An72 to An32 . In the outer 5^10 Wm the composition typically undergoes a change from VAn50 to An30 35 . The compositions of these outermost rims are similar and in some cases more sodic than plagioclase microlites found in Southwest Trident dacite host lavas, the most sodic of which are An38 . Orthopyroxene is also normally zoned from En71 to En64 (Table 5). Less common clinopyroxene is En41 and Wo44 . Microphenocryst

Fig. 1. Photomicrographs of andesite enclave (A) and dacite lava (B) groundmasses. Enclave groundmass consists of microphenocrysts, glass, and vesicles. Dacite groundmass is charged with microlites, which are absent in enclave. Scale bars are 250 microns in length.

Fig. 2. Portion of the anorthite^albite^orthoclase ternary diagram showing enclave groundmass and experimental plagioclase rims (circles) and cores (diamonds).

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Table 4 Groundmass glass compositionsa of selected natural enclaves SiO2 Al2 O3 TiO2 FeO (tot) MnO CaO MgO K2 O Na2 O Cl Total nc a b c

97NT20

97NT25

97NT26

96NT9a

97NT30

74.32 (0.40) 12.63 (0.15) 0.50 (0.06) 2.04 (0.17) 0.07 (0.04) 0.50 (0.18) 0.13 (0.04) 5.67 (0.13) 4.15 (0.32) n.d.b 100.01 (0.31) 5

74.86 (1.35) 13.02 (0.29) 0.52 (0.16) 1.62 (0.35) 0.06 (0.04) 0.78 (0.49) 0.09 (0.02) 4.83 (1.63) 3.98 (0.61) 0.24 (0.08) 99.08 (0.35) 5

77.12 (0.66) 12.34 (0.19) 0.49 (0.25) 1.49 (0.14) 0.05 (0.06) 1.12 (0.05) 0.23 (0.03) 3.17 (0.19) 3.81 (0.67) 0.18 (0.07) 100.76 (0.29) 9

77.21 (0.29) 13.27 (0.39) 0.40 (0.17) 1.77 (0.21) 0.07 (0.04) 1.33 (0.13) 0.30 (0.13) 2.60 (0.04) 2.83 (0.23) 0.20 (0.06) 97.17 (0.48) 5

74.33 (0.62) 12.97 (0.58) 0.73 (0.19) 2.36 (0.17) 0.08 (0.06) 1.42 (0.14) 0.11 (0.02) 2.76 (0.13) 5.20 (0.31) n.d. 99.94 (1.02) 8

All values (except totals) recalculated on an anhydrous basis. n.d.: not determined. n: number of analyses; standard deviation shown in parentheses.

compositions generally coincide with those of rims of phenocrysts within the enclaves. 3.1.2. Groundmass texture Detailed textural analysis was performed on seven enclaves; representative images are shown in Fig. 3. Volume percent plagioclase varies from 19.4 to 49.2, and does not correlate with size or age of enclave (Table 6). Crystal number density (NV ) ranges from 2.2U102 to 1.3U104 , also showing no trend with enclave size or age of host £ow. Four of the enclaves have NV values that fall between 1.8U103 and 2.8U103 . Enclave 96NT9a

has a much lower NV of 2.2U102 ; enclaves 97NT18 and 97NT25 have NV values of V1.3U104 . CSD curves (Fig. 4A) show the same grouping. The curve for 96NT9a has a distinctly di¡erent shape, with fewer small crystals and more crystals larger than 0.35 mm. 97NT18 and 97NT25 both have overall shapes similar to the other enclaves, with the addition of sharp peaks at small crystal sizes. The small crystals in 97NT18 have very low width/length ratios (Fig. 5B) compared to the larger crystals in it and other enclaves. 97NT25 shows a similar pattern. Excepting the three enclaves mentioned above,

Table 5 Representative pyroxene analyses from enclave and experiment groundmass Enclaves SiO2 Al2 O3 TiO2 FeO (tot) MnO CaO MgO NiO Na2 O Total En Wo

Experiments

97NT30-core

97NT30-rim

97NT20

NT-2058-core

NT-2058-rim

52.70 1.48 0.27 18.06 0.60 1.63 24.78 0.07 0.01 99.59 68.0 3.2

52.14 1.58 0.34 19.18 0.58 1.80 24.25 0.03 0.01 99.92 66.2 3.5

52.25 1.57 0.41 9.13 0.38 21.54 14.34 0.09 0.34 100.06 40.8 44.0

52.53 1.93 0.30 16.52 0.27 1.52 25.46 0.00 0.05 98.59 70.8 3.0

52.44 1.02 0.24 19.00 0.64 1.14 23.75 0.00 0.03 98.35 66.7 2.3

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NT-2059 50.55 1.90 0.50 9.32 0.52 20.62 14.48 0.02 0.29 98.32 41.6 42.6

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Fig. 3. BSE images of Southwest Trident enclave groundmasses. All scale bars are 200 Wm. (A) 97NT26 (enclave diameter (d) = 1 cm). (B) 97NT25 (d = 7 cm). (C) 97NT30 (d = 30 cm). (D) 97NT18 (d = 100 cm). Note lack of correlation between crystal size and d.

the others have uniform NV values and similarly shaped CSDs. Their CSDs are slightly curved, intersect at a size of V0.1 mm, and form a fan at larger crystal sizes (Fig. 4A). On a plot of

length versus width/length, their crystals de¢ne a broad fan, only excluding large, highly elongate (low width/length) and small, equant (width/ height near one) crystals.

Table 6 Textural data for andesite enclaves and experiments

Enclave 97NT25 97NT26 97NT18 97NT30 97NT20 96NT9a 96NT7a Run No. NT-2058 NT-2062 NT-2063 NT-2030 NT-2038 NT-2061 NT-2031 NT-2017 NT-2059

Year 1953 1953 1957 1957 1957 1959 1959 Exp. Cond. 2‡C h31 +1 2‡C h31 +4 2‡C h31 +40 10‡C h31 +1 10‡C h31 +10 10‡C h31 +48 110‡C h31 +1 110‡C h31 +40 Decomp.

NV (mm33 )

Enclave diameter (cm)

No. of crystals

Reference area (mm2 )

P

7 1 100 20 14 0.5 0.5

453 159 322 226 137 72 208

0.8963 1.0688 1.0253 0.9860 0.9360 1.8269 1.0662

19.4 32.9 26.0 37.4 28.0 49.2 46.7

13000 1900 1300 2700 1800 220 2300

63 153 211 67 120 269 97 83 160

0.5831 0.5231 0.5201 0.1337 1.0173 0.2815 0.1280 0.4410 0.0916

4.8 25.6 24.6 21.4 15.3 17.4 9.6 4.1 26.9

2600 1500 6700 16000 1300 25000 39000 4900 99000

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Fig. 4. Plagioclase CSDs. (A) Southwest Trident enclaves. Note spike at small crystal size for enclave 97NT18, and lower slope for enclave 96NT9a. (B) 10‡C h31 and 2‡C h31 (main panel) and 110‡C h31 and 10‡C h31 +decompression (subpanel) experiments. In both, gray ¢eld denotes range of enclave CSDs, excluding 97NT18 and 96NT9a.

3.2. Andesite cooling experiments 3.2.1. Compositional results Cooling experiments on the crushed andesite produced newly crystallized phenocrysts of plagioclase, orthopyroxene, clinopyroxene, and titanomagnetite, in descending order of abundance. Compositions of phases produced experimentally overlap with those in enclaves (Table 5). In addition, amphibole and olivine, which are not present in enclave groundmass, are present in small amounts in experiments NT-2058 and NT-2059. The melt of the annealed-only starting material contains 58.6 wt% SiO2 (Table 7; Fig. 6A). The melt composition becomes more evolved for all cooling experiments as crystallization progresses. In general, the greatest compositional changes in the melt are seen in the ¢rst 10 h of residence time after cooling for all cooling rates (Fig. 6A^F). After V10 h, little compositional change occurs. SiO2 and K2 O both increase with increasing residence time for all cooling rates, whereas Al2 O3 , CaO, and FeO(t) decrease. Na2 O content of the melt increases with t for the ¢rst 4 h, after which time no trend is discernable. For t = 1 h, SiO2 , Al2 O, and K2 O show no discernable di¡erence between cooling rates. For Na2 O, and especially FeO(t), the melt composition of the 2‡C h31 experiment has undergone the greatest change after 1 h. The 2‡C h31 series has reached the lowest CaO composition by t = 4 h.

For most major elements, with the exception of Na2 O, decreasing cooling rate and increasing residence time act to drive the residual melt towards the melt composition of the natural enclave start-

Fig. 5. Width/length versus width for plagioclase microphenocrysts from Southwest Trident enclaves and andesite cooling experiments. (A) Enclaves 96NT7a, 97NT20, 97NT26, and 97NT30. (B) Enclave 97NT18. (C) Experiments NT-2017 and NT-2031. (D) Experiments NT-2030, NT-2038, and NT2061. (E) Experiments NT-2058, NT-2062, and NT-2063. Note varying scales for y axes in C through E.

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Fig. 6. Weight % oxides in residual melt versus residence time for andesite cooling and cooling+decompression experiments. Error bars show one standard deviation for experimental points. Average enclave groundmass composition, V one standard deviation, is shown as gray bars.

ing material, 97NT20 (Fig. 6). For most oxides, however, no cooling experiment produced compositions that match the natural melt. K2 O, in particular, shows a large gap between the most evolved experiments and the natural glass composition. The K2 O content of the 97NT20 melt is, however, the most evolved of all the enclaves analyzed; the rest have K2 O contents closer to the experimental values, between 2.6 and 4.8 wt% (Table 4). FeO(t) compositions of the melts from the two slowest cooling rates (2‡C h31 and 10‡C h31 ) do closely approach the composition of the natural enclave melt, and Na2 O compositions of several cooling experiments overlap with the Na2 O of the natural melt. Plagioclase crystals that grew in the cooling experiments are normally zoned from VAn67 ^An46

(Fig. 2). Zoning pro¢les in experiment plagioclase resemble those of natural enclave groundmass plagioclase, with the exception of the outer 5^10 Wm, which are strongly normally zoned down to An30 35 in natural enclaves. Orthopyroxenes resemble those seen in enclaves and are normally zoned from En71 64 (Table 5). Augite is, again, similar to compositions seen in enclave groundmass at En40 (Table 5). 3.2.2. Textural results Qualitatively, the crystals within the andesite experiments cooled at the slowest rates (2‡C h31 ) most resemble those from natural enclave groundmass (Fig. 7A,B). These contain euhedral, slightly elongate crystals of plagioclase, two pyroxenes, and oxides surrounded by brown glass.

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Table 7 Glass compositionsa of selected andesite experiments NT-2010a

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a b c

NT-2018

NT-2040

NT-2017

NT-2030

NT-2038

NT-2039

NT-2062

NT-2063

NT-2059

anneal only 110

NT-2031

110

110

110

10

10

10

2

2

10

0 58.57 (0.52) 16.39 (0.20) 0.77 (0.19) 7.10 (0.55) 0.10 (0.06) 5.87 (0.11) 5.74 (0.15) 1.34 (0.10) 4.08 (0.13) 97.87 (0.62) 11

3 67.25 (0.94) 15.53 (0.46) 0.65 (0.13) 4.79 (0.24) 0.12 (0.10) 2.99 (0.16) 1.52 (0.17) 2.54 (0.10) 4.48 (0.13) 96.59 (0.32) 6

10 68.72 (1.43) 15.68 (1.44) 0.40 (0.12) 3.69 (0.54) 0.07 (0.06) 3.12 (0.67) 1.02 (0.14) 2.40 (0.29) 4.83 (0.28) 96.43 (0.92) 6

40 68.22 (0.45) 15.26 (0.19) 0.48 (0.10) 4.41 (0.31) 0.08 (0.05) 2.77 (0.09) 1.38 (0.11) 2.79 (0.12) 4.51 (0.19) 96.39 (0.61) 5

1 65.71 (0.91) 15.36 (0.21) 0.90 (0.15) 6.01 (0.47) 0.12 (0.06) 3.43 (0.22) 1.59 (0.09) 2.13 (0.16) 4.66 (0.11) 95.66 (0.82) 7

10 66.40 (0.64) 15.19 (0.59) 0.76 (0.14) 5.48 (0.64) 0.09 (0.07) 3.58 (0.30) 1.80 (0.17) 2.20 (0.15) 4.45 (0.15) 96.13 (0.45) 9

20 67.65 (0.31) 15.08 (0.14) 0.59 (0.15) 4.97 (0.27) 0.10 (0.06) 2.97 (0.06) 1.55 (0.08) 2.21 (0.11) 4.83 (0.16) 95.34 (0.65) 11

4 70.60 (0.39) 14.82 (0.09) 1.06 (0.37) 2.70 (0.30) 0.09 (0.06) 2.60 (0.10) 0.32 (0.03) 2.44 (0.03) 5.36 (0.12) 95.10 (0.65) 5

40 71.38 (0.47) 14.53 (0.17) 1.09 (0.24) 2.35 (0.28) 0.11 (0.03) 2.56 (0.06) 0.31 (0.01) 2.51 (0.09) 5.15 (0.04) 95.14 (0.51) 6

1, +decomp. 73.39 (2.01) 12.82 (0.69) 0.21 (0.13) 2.64 (0.11) n.d.b 2.06 (0.10) 0.87 (0.05) 3.19 (0.04) 4.12 (1.10) 99.41 (0.89) 3

1 65.63 (0.67) 15.30 (0.24) 0.67 (0.19) 5.89 (0.40) 0.14 (0.06) 4.02 (0.36) 1.73 (0.09) 2.26 (0.18) 4.24 (0.08) 94.83 (1.33) 5

All values (except totals) recalculated on an anhydrous basis. n.d.: not determined. n: number of analyses.

Table 8 Glass compositionsa from dacite decompression experiments SiO2 Al2 O3 TiO2 FeO (tot) MnO CaO MgO K2 O Na2 O Cl Total nb decomp.c a b c

97NT31 groundmass glass

NT-3115

NT-3116

NT-3119

NT-3120

NT-3117

NT-3118

75.93 (0.25) 12.18 (0.33) 0.48 (0.16) 2.03 (0.17) 0.21 (0.10) 1.62 (0.18) 0.20 (0.01) 3.01 (0.21) 4.23 (0.13) 0.13 (0.02) 100.47 (0.24) 6

73.30 (0.30) 13.47 (0.21) 0.44 (0.26) 2.35 (0.16) 0.22 (0.17) 2.61 (0.10) 0.44 (0.04) 2.71 (0.13) 4.34 (0.15) 0.11 (0.02) 97.11 (0.46) 11 0

74.56 (0.79) 13.38 (0.32) 0.38 (0.10) 1.70 (0.17) 0.20 (0.17) 2.03 (0.20) 0.32 (0.03) 3.07 (0.16) 4.26 (0.17) 0.10 (0.03) 100.08 (0.95) 10 8

75.82 (0.90) 12.65 (0.46) 0.32 (0.28) 1.84 (0.39) 0.16 (0.12) 1.58 (0.27) 0.17 (0.07) 3.26 (0.15) 4.16 (0.15) 0.04 (0.03) 99.72 (0.75) 9 32

77.21 (0.75) 11.82 (0.37) 0.48 (0.33) 1.54 (0.28) 0.09 (0.09) 1.04 (0.13) 0.19 (0.14) 3.46 (0.11) 4.13 (0.10) 0.04 (0.02) 100.01 (0.43) 8 54

76.64 (0.67) 12.55 (0.42) 0.38 (0.19) 1.23 (0.16) 0.07 (0.08) 1.16 (0.13) 0.17 (0.03) 3.46 (0.23) 4.28 (0.27) 0.05 (0.03) 99.89 (0.63) 7 80

77.04 (0.48) 12.14 (0.30) 0.36 (0.09) 1.46 (0.05) 0.10 (0.14) 0.97 (0.08) 0.14 (0.03) 3.61 (0.24) 4.16 (0.14) 0.02 (0.02) 99.79 (0.24) 5 160

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Cooling rate (‡C h31 ) Time at Tf (h) SiO2 Al2 O3 TiO2 FeO (tot) MnO CaO MgO K2 O Na2 O Total nc

All values (except totals) recalculated on an anhydrous basis. n: number of analyses; standard deviation shown in parentheses. Duration of decompression (h). 135

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Andesite cooled at the fastest rate of 110‡C h31 contains smaller microphenocrysts that are needle-shaped, and in some cases appear dendritic (Fig. 7E,F). The 10‡C h31 experiments are transi-

tional between these (Fig. 7C,D). Vesicles are present in all cooling experiments. Quantitative textural analysis was performed on nine experiments (Table 6, Figs. 4, 5 and 8).

Fig. 7. BSE images of andesite cooling experiments. All scale bars are 200 Wm. (A) NT-2031(110‡C h31 , t = 1 h). (B) NT-2017 (110‡C h31 , t = 40 h). (C) NT-2038 (10‡C h31 , t = 10 h). (D) NT-2061 (10‡C h31 , t = 48 h). (E) NT-2062 (2‡C h31 , t = 4h). (F) NT-2063 (2‡C h31 , t = 40 h). (G) NT-2059 (10‡C h31 , t = 1 h, plus decompression).

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An increase in plagioclase crystal volume fraction (P) from 6 10% to V25% roughly correlates with decreasing cooling rate, with the exception of the 2‡C h31 +1 h run (Table 6). The 2‡C h31 experiments with longer t most closely approach the average for all natural enclaves. If only starting material enclave 97NT20 is considered (P = 28%), the longer 2‡C h31 runs are almost identical in terms of plagioclase content (P = 25^26%). CSDs of the experimal plagioclase vary greatly with cooling rate (Fig. 4). The 2‡C h31 and 10‡C h31 experiments have CSDs that are convex upward and overlap with those of the enclaves. The 110‡C h31 experiments have CSDs that do not contain crystals above 0.10 mm in size and have straight to slightly convex-up shapes. The 2‡C h31 experiments have total crystal number densities (NV ) between 1.5U103 and 6.7U103 (Fig. 8, Table 6), which are on the order of those calculated for the enclaves. NV values for the 10‡C h31 and 110‡C h31 experiments range from 1.3U103 to 2.5U104 and 4.9U103 to 3.9U104 , respectively. NV does not vary systematically with residence time for any of the experiments. In general, the NV of the experiments span a greater range than that seen in the enclaves. There is a systematic increase in crystal width and width/length ratio with decreasing cooling rate (Fig. 5C^E). Even though the width and width/length values shown have not been converted to true three-dimensional size and shape, the plots give a sense for how the size and shape of individual crystals vary from sample to sample. The 110‡C h31 experiments produced mostly small and elongate plagioclase crystals. The 10‡C h31 runs contain crystals that are slightly bigger with a range of width/length ratios, and the 2‡C h31 experiments have crystals that extend to similar sizes as those seen in enclaves over a range of width/length values. 3.3. Dacite decompression Decompression of Southwest Trident dacite from 90 to V2 MPa resulted in the growth of microlites within its groundmass. The amount of crystallization within the experiments varied with the rate of decompression. By comparing the de-

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Fig. 8. Residence time (h) versus total plagioclase number density (NV , mm33 ) for andesite experiments. Average and standard deviation of NV for enclaves (97NT20, 97NT30, 97NT7a, and 97NT26) shown by horizontal line and gray bar. The experiments show a much greater variation in NV than the typical enclaves.

gree of groundmass crystallization within the experiments to that of natural dacite groundmass it is possible to estimate the decompression history of the natural dacite (Geschwind and Rutherford, 1995). The experimental samples were typically extremely vesicular, and often completely fragmented, after decompression. This, combined with the small sizes of the decompression microlites, made it di⁄cult to analyze microlites within the experiments. Instead, we here use residual groundmass glass composition to track the changes in crystallinity, as this has been shown to vary regularly with crystallinity for other decompression experiments (Geschwind and Rutherford, 1995; Hammer and Rutherford, 2002). Groundmass glass from the decompression experiments ranges in composition from 74.6 to 77.2 wt% SiO2 (Table 8); SiO2 content initially increases with length of the decompression (Fig. 9A). The resulting curve crosses the natural groundmass glass composition (75.9 wt% SiO2 ; Table 8) at approximately 30 h, or very close to experiment NT-3119 (Fig. 9A). Al2 O3 , K2 O, and CaO also change systematically with decompression time (Fig. 9B^D). Within analytical uncertainties, the composition of residual glass (melt)

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in the 32 h experiment matches the starting material composition. This suggests an ascent time for the dacite magma of V30 h, equivalent to a decompression rate of V2^3 MPa h31 , assuming a constant rate of decompression. 3.4. Andesite cooling plus decompression The phases present in the 10‡C h31 plus decompression experiment NT-2059 are the same as in the cooling-only experiments: plagioclase, orthpyroxene, clinopyroxene, and titanomagnetite. The residual melt in this experiment is the most evolved (and thus closest to the enclave melt composition) of all the experiments for all major oxides except FeO(t) (Table 7 ; Fig. 6). This melt composition overlaps the average natural enclave melt composition for several of the major element oxides; for those which do not overlap, residual melt from this experiment is still the closest of all the melts to matching the natural glass (Tables 4 and 7). The results of this experiment, however, do not texturally resemble enclaves. NT-2059 contains some euhedral crystals surrounded by many smaller irregularly shaped microlites (Fig. 7G). This experiment yielded a higher NV than any of the cooling experiments or enclaves (Table 6 and Fig. 8). The CSD from this experiment extends to higher Ln (population density) at low crystal size than any of the other experiments or enclaves (Fig. 4).

4. Discussion 4.1. Cooling and crystallization of enclave groundmass Fig. 9. Decompression time versus groundmass glass wt% oxides for dacite decompression experiments. Error bars show one standard deviation. Composition of groundmass glass for natural sample 97NT31 is shown by horizontal dashed lines; gray bars represent one standard deviation. Hatched area represents the decompression time for which all oxides coincide between experimental and natural values.

Within the framework of cooling-induced crystallization, many factors play a role in the resulting texture, most fundamentally crystal nucleation and growth rates. The number of crystals per unit volume, or crystal number density (NV ), is typically regarded as a function of nucleation rate and time. For melts which begin the crystallization process below their liquidus, as these do, heterogeneous nucleation will be the dominant mode of

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nucleation (Lofgren, 1983). Density of pre-existing nuclei in the form of crystals, crystal fragments, or submicroscopic crystal remnants will likely dominate over cooling rate in driving nucleation and thus determining NV . Our cooling experiments used a natural andesite enclave starting material that was lightly crushed and then annealed for 48 h at 1000‡C, V80‡C below the magma’s liquidus, as calculated using melts (Ghiorso and Sack, 1995). Therefore, even though most visible crystals in the size range of interest (i.e., non-phenocrysts) were dissolved during the anneal, varying amounts of nuclei substrate existed in the starting materials. This likely led to the relatively wide range of NV values in the experiments, even for a given cooling rate. The relatively uniform NV values seen in the enclaves (excepting those with unusually large numbers of small crystals) are consistent with similar thermal histories that probably include a period of slow cooling followed by mixing and subsequent rapid cooling to V890‡C. Unlike NV , which re£ects the nature of nucleation within the system, crystal size and shape may re£ect crystal growth within the enclaves. For the enclave groundmasses, none of the textural parameters show a statistically signi¢cant trend with age of the host £ow. It can be inferred from this that undercooling, cooling rate, and residence time of the andesite did not change systematically throughout the eruption. Di¡usion pro¢les in olivine and magnetite phenocrysts from the eruption suggest that a single mixing event may have preceded the initial eruption in 1953 at Southwest Trident by V1 month (Coombs et al., 2000). It is important to note that the enclaves from the youngest £ow are also the smallest. Because of this, textural changes could re£ect either enclave age or enclave size. Future work that compares enclaves of similar sizes from di¡erent aged £ows, as well as sampling of many, variously sized enclaves from a single £ow, would be useful in better accessing crystallization as a function of enclave size and age. By comparing the results of our cooling experiments to the natural enclaves that we have examined, it is possible to estimate cooling rates that

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are reasonable for enclave groundmass crystallization. Texturally, 110‡C h31 runs do not resemble the enclaves. The 110‡C h31 experiments have crystals that are smaller than are found in any of the enclaves. In addition, these experiments have lower crystal volume fractions and crystals with higher aspect ratios. There are some similarities between the 10‡C h31 experiments and enclaves texturally: there are higher crystal volume fractions and bigger crystals than in the 110‡C h31 runs. The experiments at the slowest cooling rate, 2‡C h31 , produce textures that are consistently closest to those of the enclaves. These have the biggest crystals (with the exception of the 10‡C h31 +1 h run), higher width/length ratios, and highest crystal volume fractions. The melt compositions also show that for a given t, the 2‡C h31 runs most closely approach the composition of residual glass within the starting material enclave, 97NT20 (Fig. 6). The compositional trends with time are similar for the three cooling rates, with the exception of FeO(t) and Na2 O (Fig. 6D,F). Whereas the experiments cooled at 2‡C h31 and 10‡C h31 approach the enclave composition of 2.0 wt% FeO(t) by t = 40 and 48 h, respectively, the FeO(t) content of the melt in the 110‡C h31 runs plateaus at V4 wt% after 10 h. Possibly the nucleation of the Fe-bearing phases orthopyroxene and magnetite are inhibited at the higher cooling rate. The gap in melt composition between the 110‡C h31 runs and the enclave groundmass would not be closed by decompression crystallization, which would be dominated by non-Fe-bearing phases, as suggested by the similarity between the FeO content of the 10‡C h31 +48 h and 10‡C h31 +decompression runs. Na2 O is the only oxide for which the fastest cooling rate produces melt compositions that most closely approach the melt of the natural enclave. This may be due to di¡erences in the anorthite content of crystallizing plagioclase. Here, however, decompression drives crystallization of Na-rich plagioclase, thus lowering melt Na2 O content to that of the natural enclave (Fig. 6D). The textural and compositional similarities between natural enclaves and the slowest-cooled experiments suggest that enclaves underwent cooling

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at rates signi¢cantly slower than 110‡C h31 , and instead on the order of 2^10‡C h31 . Cooling rates slower than 2‡C h31 may produce textures that resemble enclaves even more closely; 2^10‡C h31 may be at the high end of cooling rates experienced by enclaves. 4.2. Enclave crystallization: pre- or post-dispersal? Enclave groundmass crystallization is driven by the temperature decrease that occurs when andesite magma comes in contact with cooler dacite. Once thermal equilibration is reached, the andesite will have essentially cooled to the temperature of the dacite. The rate at which the andesite cools, and thus the crystallization history of the enclave groundmass, will be a¡ected by the geometry of the andesite body. Whether enclaves crystallize

Fig. 10. Cartoon showing two possible mixing scenarios between resident dacite (light gray) and incoming andesite (dark gray). (A) Discrete enclaves form, lose heat to surrounding dacite (1), and then undergo inwards crystallization (2). The result is enclave groundmass crystallinity that is a function of enclave size. (B) Andesite in£ux occurs, and a ma¢c^silicic interface forms. Heat is lost along this boundary (1). Crystallization and vapor exsolution proceed within the andesite layer (2). ‘Flotation’ of enclaves occurs due to decreased density of the andesite (3). This results in a wide range of crystallinities with no correlation to enclave size.

after andesite becomes dispersed within the host, or before when the andesite is still a single body, has implications for how enclaves form. Fig. 10shows these two possibilities. In one scenario (Fig. 10A), andesite magma (phenocrysts plus melt) breaks up into discrete enclaves upon entering the dacite chamber. This may occur if convection is present in the dacite, or if injection of the andesite is forceful enough. Here these bodies lose heat to the dacite as spheres, and crystallize inwards. This would result in crystallinity parameters (NV , P, width/length) that are functions of enclave size. Smaller enclaves would have cooled faster than their larger counterparts, leading to higher crystal number densities, and smaller, more acicular crystals. Larger enclaves would presumably have interiors that cooled more slowly and thus would contain fewer, larger crystals. Larger enclaves would also exhibit gradations from a quickly cooled exterior to the slowly cooled interior, as seen for enclaves elsewhere (Bacon, 1986; Miller et al., 1999; Murphy et al., 2000). If andesite magma does not quickly disaggregate into discrete bodies immediately upon entering the dacite, then it may pond at the base of the less dense dacite (Fig. 10B). Such relationships have been observed at the bases of eroded plutons containing evidence for ma¢c in£uxes (e.g. Wiebe, 1994; Robinson and Miller, 1999) and are predicted by £uid dynamical considerations, if the rate of in£ux is not too high (Snyder and Tait, 1995). In this case, a ma¢c^silicic interface will be established, and the andesite will cool at di¡erent rates depending on its position below the interface. If enclaves form after cooling and crystallization below this interface, their crystallinity would be a function of depth below the interface, and would be independent of eventual enclave size. We have modelled the ¢rst scenario, crystallization as a function of depth below a ma¢c^silicic interface, using a Stefan cooling solution to model (Fig. 11). Several assumptions must be made to use this model for cooling of the andesite : that latent heat of fusion, or crystallization, can be linearly approximated for crystal^melt mixtures, that conduction, not convection, is the primary

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Fig. 11. The Stefan solution for cooling of the andesite from 1000‡C to 890‡C. The following values were used: latent heat of fusion, 400 kJ kg31 K31 ; thermal di¡usivity, 1.0 mm2 s31 . Curves represent volume fraction crystallized as a function of depth, for various times elapsed and thus cooling rates. Because natural enclave groundmass most closely matches experimental cooling rates of 2‡C h31 and 10‡C h31 , these curves most closely represent the crystallinity^ depth relationship of the enclaves. The gray box indicates the range of volume fraction crystals found in natural enclave groundmasses, as determined by point count.

cooling mechanism, and that crystallization of a multi-component melt behaves similarly to a single component melt. Each time curve in Fig. 11 corresponds to a cooling rate used in the experiments. If the natural enclaves cooled at rates on the order of 2^ 10‡C h31 , as discussed above, this suggests that enclaves formed at least 20 cm below a silicic^ ma¢c interface. If enclaves cooled as spherical bodies, the cooling rate at a given distance below the surface would be faster than what is shown in Fig. 11, as a sphere will cool faster than a body below a planar interface, especially as the size of the sphere decreases. This means that if originally spherical, enclaves would have had radii signi¢cantly greater than 20 cm to produce the crystal textures seen. Since most enclaves are 6 20 cm in diameter, disaggregation would have had to occur

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after formation of such large enclaves. In either case, some andesite would have cooled near the surface of the body(ies) at faster cooling rates; ideally the range of crystal textures should re£ect crystallization at a range of depths. Enclaves seen at Southwest Trident and elsewhere, many of which are 6 20 cm in diameter, probably did not crystallize directly from melt blobs at their current size. This is consistent with a model involving a ma¢c foam layer at the boundary between silicic host magma and intruding ma¢c magma. If the andesite did indeed cool as a discrete layer which resided below the resident dacite, then crystallization-induced vesiculation within the andesite may have played a role in subsequent enclave formation (Eichelberger, 1980). Vesicles were present in all of our cooling experiments, as is the case for natural enclaves (Figs. 3 and 7). As the melt within the andesite crystallizes, it becomes super-saturated with respect to water and other volatiles, driving exsolution and forming vesicles. As this exsolution process proceeds, vesicularity of the andesite increases and its bulk density decreases. If crystallization proceeds far enough, the density of the andesite may become less than that of the overlying dacite, and ‘£otation’ of andesite may be possible. Thomas and Tait (1997) ¢nd characteristic length scales of enclave that are consistent with formation due to dynamic interaction of rising gas bubbles from the ma¢c magma with the ma¢c^silicic interface. Using the MELTS algorithm (Ghiorso and Sack, 1995), we evaluate the changes in crystal content, vesicularity, and density for the andesite groundmass as it cools isobarically from 1000‡C to 890‡C at 90 MPa. The initial densities of the dacite and andesite are 2.45 and 2.58 g cm33 , respectively. Each is the weighted average of the densities of their phenocryst and groundmass melt volume fraction. As the andesite cools, its bulk density (groundmass plus phenocrysts) decreases as its groundmass crystallizes and becomes vesicular (Fig. 12A). Its density falls below that of the dacite after V34% of the groundmass has crystallized. Plagioclase would comprise 59 vol% of the new crystals, meaning that the overall groundmass would contain roughly 20 vol% plagioclase

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Trident enclaves of 18 vol% (Table 3). Thus crystallization-induced vesiculation is a valid mechanism for lowering andesite density below that of overlying dacite, possibly resulting in ‘£otation’ of enclaves. 4.3. Decompression crystallization within enclaves

Fig. 12. (A) Density versus groundmass crystallinity for Southwest Trident andesite cooling from 1000‡C to 850‡C. Calculated using MELTS (Ghiorso and Sack, 1995). Andesite density is determined for phenocrysts plus groundmass. Bulk density of the dacite is assumed to remain constant. Di¡erence between bulk andesite and andesite groundmass densities is due to phenocrysts and thus remains constant. Densities of the bulk magmas cross over at V34 crystal volume % in the andesite groundmass, indicated by gray bar. (B) Volume fraction plagioclase and vapor (or vesicles) versus groundmass crystallinity for Southwest Trident andesite as calculated using MELTS.

(Fig. 12B). This plagioclase content is similar to that seen in the natural enclaves (Table 3), suggesting that the natural enclaves reached this level of crystallinity and thus could have undergone such a density inversion. The vesicularity of the groundmass (V16 vol%; Fig. 12B) also corresponds to an average vesicularity of Southwest

Despite their compositional similarities, enclave glass and host groundmass glass di¡er signi¢cantly in the amount of microlites they contain (Fig. 1). Host lava groundmasses contain abundant microlites less than 50 Wm in length, while enclave groundmass contains few microlites. Microlite-charged lavas often contain enclaves with completely microlite-free groundmasses. As mentioned above, some enclaves contain what appear to be two populations of groundmass crystals. One of these, 97NT18, was chosen for detailed textural analysis. While one population of crystals resembles other enclave groundmass crystals, a second population does not (Fig. 5B). These smaller crystals could have grown during post-emplacement processes, as enclaves that contain these secondary populations are found in deep portions of lava £ows, and contain exsolved Fe^ Ti oxides, probably the result of slow post-emplacement cooling (Coombs et al., 2000). Some enclaves contain microlites, but it appears that they did not grow during decompression. The growth of microlites in magmas is thought to occur either during ascent of magmas to the surface, or during storage in a shallow, subsurface chamber (Cashman, 1992; Geschwind and Rutherford, 1995). It is necessary for nuclei to exist in the magma prior to ascent and depressurization for this process to result in microlite growth. Classic kinetic theory, as summarized by Dowty (1980), states that an incubation period is necessary for nucleation to occur at a given temperature after any amount of undercooling has occurred in the magma. Experimentally, it has been shown that an increase in initial temperature will inhibit nucleation of plagioclase, presumably due to the long relaxation time of silicate melts in the nucleation of plagioclase (Tsuchiyama, 1983; Sato, 1995). Absence of microlite-producing nucleation may result from eruptive quenching that

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took place too soon after mixing-induced cooling for nucleation to have occurred. For ma¢c magma that has recently cooled to thermal equilibrium several hundred degrees below its liquidus, growth of nuclei in the remaining melt may not occur until su⁄cient residence time has passed. Instead, pre-existing cooling-induced microphenocrysts are sites for additional growth. Of all the experiments, the melt and plagioclase compositions from the cooling plus decompression experiment most closely resemble the compositions of enclave melt and plagioclase. This suggests that the overall temperature^pressure path taken by this experiment may be similar to that of the enclaves. Texturally, however, this experiment has a higher NV and greater number of smaller crystals than the enclaves, suggesting that a new population of crystals may have grown upon decompression in the experiment, something that did not occur in the natural enclaves studied. It is not yet clear whether they were the result of a shorter residence time or some other experimental aspect that di¡ered from natural enclaves.

5. Conclusions (1) Crystallinity and composition of enclave groundmasses are consistent with cooling of andesite upon introduction to cooler dacite. Both andesite and dacite were at magmatic temperatures when they met. Signi¢cant crystallization occurs in times as short as 1 h for andesite undercooled to dacitic temperatures. This implies that andesite and dacite cannot be in prolonged contact without crystallization. (2) Cooling experiments on the andesite show that enclave groundmass textures are most consistent with cooling and crystallization s 20 cm below a ma¢c^silicic interface. The crystal content of enclave groundmasses, and subsequent vesicularity, are high enough to lower the density of the andesite below that of the dacite. Together these suggest a two-part process of enclave formation : crystallization within a ma¢c layer and subsequent ‘£otation’ of andesite as discrete enclaves. (3) Although enclaves were carried to the surface during eruption at an ascent rate of V2^3

143

MPa h31 , they did not undergo microlite crystallization as seen in the dacite host. Decompression crystallization in enclaves instead occurs as rim growth on pre-existing microphenocrysts and phenocrysts. This di¡erence is perhaps owed to insuf¢cient time between mixing, when undercooling of enclave melt occurred, and eruption so that separate nuclei for decompression crystallization did not develop.

Acknowledgements This work was supported by the Volcano Hazards Program of the U.S. Geological Survey through the Alaska Volcano Observatory. We would like to thank Ken Severin for microprobe assistance. Jim Gardner, Michael Higgins, Rainer Newberry, and Michael Zieg provided helpful reviews of the manuscript.

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