Journal of Petrology

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Journal of Petrology | Volume 45 | Number 1 | Pages 203-219 | 2004
© Oxford University Press 2004; all rights reserved

Chemical versus Temporal Controls on the Evolution of Tholeiitic and Calc-alkaline Magmas at Two Volcanoes in the Alaska–Aleutian Arc

RHIANNON GEORGE^1,*,, SIMON TURNER^1,, CHRIS HAWKESWORTH¹, CHARLES R. BACON², CHRIS NYE³, PETE STELLING² and SCOTT DREHER⁴

¹ DEPARTMENT OF EARTH SCIENCES, WILLS MEMORIAL BUILDING, UNIVERSITY OF BRISTOL, BRISTOL BS8 1RJ, UK
² US GEOLOGICAL SURVEY, MS 910, 345 MIDDLEFIELD ROAD, MENLO PARK, CA 94025, USA
³ ALASKA VOLCANO OBSERVATORY, ALASKA DIVISION OF GEOLOGY AND GEOPHYSICAL SURVEYS, 794 UNIVERSITY AVENUE SUITE 200, FAIRBANKS, AK 99709, USA
⁴ DEPARTMENT OF GEOGRAPHY, GEOLOGY AND ANTHROPOLOGY, INDIANA STATE UNIVERSITY, TERRE HAUTE, IN 47809, USA

^* Corresponding author. E-mail: rgeorge{at}els.mq.edu.au

RECEIVED SEPTEMBER 18, 2002; ACCEPTED JULY 22, 2003

	ABSTRACT

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
The Alaska–Aleutian island arc is well known for erupting both tholeiitic and calc-alkaline magmas. To investigate the relative roles of chemical and temporal controls in generating these contrasting liquid lines of descent we have undertaken a detailed study of tholeiitic lavas from Akutan volcano in the oceanic Aleutian arc and calc-alkaline products from Aniakchak volcano on the continental Alaskan Peninsula. The differences do not appear to be linked to parental magma composition. The Akutan lavas can be explained by closed-system magmatic evolution, whereas curvilinear trace element trends and a large range in ⁸⁷Sr/⁸⁶Sr isotope ratios in the Aniakchak data appear to require the combined effects of fractional crystallization, assimilation and magma mixing. Both magmatic suites preserve a similar range in ²²⁶Ra–²³⁰Th disequilibria, which suggests that the time scale of crustal residence of magmas beneath both these volcanoes was similar, and of the order of several thousand years. This is consistent with numerical estimates of the time scales for crystallization caused by cooling in convecting crustal magma chambers. During that time interval the tholeiitic Akutan magmas underwent restricted, closed-system, compositional evolution. In contrast, the calc-alkaline magmas beneath Aniakchak volcano underwent significant open-system compositional evolution. Combining these results with data from other studies we suggest that differentiation is faster in calc-alkaline and potassic magma series than in tholeiitic series, owing to a combination of greater extents of assimilation, magma mixing and cooling.

KEY WORDS: uranium-series; Aleutian arc; magma differentiation; time scales

	INTRODUCTION

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
The existence of the tholeiitic and calc-alkaline lines of liquid descent in arc magma suites has long been recognized (e.g. Miyashiro, 1974); many studies have investigated the role of parental magma composition and/or the effects of crustal assimilation in determining which evolutionary path is followed (e.g. Grove & Baker, 1984, and references therein). Fewer studies have sought to constrain the time scales involved in magmatic differentiation and how those might vary with magma composition and differentiation path, largely because this has only become possible recently through the application of U-series isotopes. Such investigations remain in their infancy and most to date have concentrated on within-plate lavas (e.g. Reagan et al., 1992; Bourdon et al., 1994; Condomines et al., 1995; Thomas, 1999; Vigier et al., 1999; Cooper et al., 2001).

Several studies of volcanic arc rocks have sought to use mineral–whole-rock isochrons to obtain crystallization ages. However, complexity arises from the potential for the ages obtained to reflect the entrainment of old cumulate crystals, and mixing between crystal aliquots of different age (Volpe & Hammond, 1991; Volpe, 1992; Schaefer et al., 1993; Heath et al., 1998). Thus, it is often more straightforward to look for correlations between U-series disequilibria and indices of differentiation in whole rocks [see Hawkesworth et al. (2000) for a recent discussion]. Trends of decreasing (²²⁶Ra/²³⁰Th) with increasing SiO₂ in several along-arc suites of rocks have suggested time scales ranging from hundreds to several thousand years for differentiation from basaltic to andesitic and dacitic magmas, respectively (Turner et al., 2000, 2001a). However, such interpretations implicitly assume that the primary disequilibria were similar for all volcanoes. This is clearly an oversimplification, and so the next step is to investigate how (²²⁶Ra/²³⁰Th) varies with indices of differentiation within individual volcanoes whose erupted products span a range of compositions. Interpretation in terms of differentiation time scales also requires the assumption that parental magmas for a particular volcano have similar initial disequilibria.

Here we present the results of a combined geochemical, radiogenic isotope and U–Th–Ra isotope disequilibria study of tholeiitic rocks from Akutan volcano in the oceanic Aleutian arc and calc-alkaline eruptive products of Aniakchak volcano on the continental Alaskan Peninsula. The aim was to investigate the relative roles of primary magma composition, crustal interaction and magma residence times in producing these two different liquid lines of descent. These data are then compared with the results from a study of the evolution of potassic magmas from a rear-arc volcano (Sangeang Api) in the Sunda arc (Turner et al., 2003b) allowing comparison between the three major liquid lines of descent (tholeiitic, calc-alkaline and potassic) commonly observed in arc magmas. Our evaluation of the results leads us to hypothesize that magma residence times beneath arc volcanoes are broadly similar, and that it is the rate of cooling and extent of assimilation and magma mixing that exert the primary controls on the efficiency of differentiation and which liquid line of descent is followed.

	THE ALASKA–ALEUTIAN ISLAND ARC

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
The Alaska–Aleutian island arc forms the northernmost segment of the circum-Pacific subduction system and transgresses both continental (Alaska) and oceanic (Aleutian) crust (Fig. 1a). Formed by the NE-directed subduction of the Pacific plate, the arc comprises some 40 historically active volcanoes, which erupt a diverse range of magmas (Myers et al., 1985; Fournelle et al., 1994; Kay & Kay, 1994). A recent along-arc U-series study by George et al. (2003) investigated the relative contributions of components from the subducting plate and the time scales of their transfer and provides the background to the present work.

View larger version (35K): [in this window] [in a new window]   Fig. 1. (a) Map of the Alaska–Aleutian arc showing locations of Akutan and Aniakchak. (b) Map of Akutan island showing the main Holocene eruptions. (c) Map of Aniakchak caldera showing the extent of the 1931 flows. [Note change of scale between (b) and (c).]

 
A key feature of the Alaska–Aleutian arc is that it is characterized by islands that frequently contain adjacent tholeiitic and calc-alkaline volcanoes and many individual volcanoes have erupted both tholeiitic and calc-alkaline products at different times in their history (e.g. Myers et al., 1985; Nye, 2000). Not surprisingly, there has been much interest in and debate about the origin of this juxtaposition and magmatic diversity. In one group of models, Myers et al. (1985) proposed that calc-alkaline centres form in an early, immature stage of conduit development where significant lithospheric debris is incorporated into the magma during transit. They argued that these centres subsequently evolve into tholeiitic ones as the conduit becomes increasingly thermally and chemically preconditioned, reducing the extent and effects of assimilation. On the other hand, Kay & Kay (1994) have long maintained that there is a tectonic control, whereby the magmas that feed the high-volume tholeiitic centres pass quickly through crust which is under extension at the ends of segments. These magmas enter shallow-level magma chambers where crystallization occurs at low pressures and minimal crustal interaction takes place. Conversely, calc-alkaline centres develop where there is little extension and small volumes of magma pass slowly through the crust and undergo both crystallization and more extensive assimilation at greater depths.

	AKUTAN VOLCANO

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
Akutan volcano is a composite stratovolcano (Romick et al., 1990) formed on oceanic crust at the eastern end of the Aleutian chain (Fig. 1a). The volcano is highly active, and has an active intra-caldera cinder cone (Fig. 1b) and a history extending back to the Pleistocene, including a <10 km³ caldera-forming eruption that occurred 1611 years BP (Richter et al., 1998; weighted mean of six calibrated radiocarbon age determinations). Young andesitic lava flows, some of which were erupted in 1978 and 1929 [see Richter et al. (1998) for age constraints], cover the 2 km diameter caldera floor south and north of the cinder cone and extend several hundred metres downslope through a gap in the crater rim. Flows extruded in 1947 blanket the central portion of the NW end of the island at Lava Point, where about 4 km² of jagged aa basalt occurs adjacent to several cinder cones. The majority of the erupted products range from basalt to andesite in composition. The pre-eruptive H₂O contents of some of the older magmas are estimated to be around 5% on the basis of the presence of pargasitic amphibole (Romick et al., 1990) but this is likely to be a maximum. A minimum H₂O content would be 2% and this is the estimate we will use in viscosity calculations below. A trachytic dyke (68% SiO₂), related to a late cinder cone, demonstrates that more evolved magmas are also occasionally produced in this magmatic system and a high-silica pyroclastic flow, of unknown age and apparent offshore origin, containing rhyolite obsidian clasts (72% SiO₂), is exposed along the southern shore of the island (Richter et al., 1998). The compositional affiliation of the magmas is inferred to have changed through time from tholeiitic to transitional tholeiitic, which Romick et al. (1990) suggested may reflect fractionation at increasingly lower pressures, possibly beginning at a depth of 25 km and becoming shallower over time. There is little field or petrographic evidence for mixing and within-suite variations can be explained by closed-system fractionation (Romick et al., 1990). Pyroxene geothermometry studies indicate temperatures around 1120 and 950°C for basaltic andesites and dacites, respectively (Romick et al., 1990), and this range is consistent with our own estimates based on calculations performed using the MELTS algorithm (Ghiorso & Sack, 1995). The samples analysed in this study are basaltic andesites from the post 1400 years BP period and include samples from the historical 1910, 1929 and 1978 eruptions, which were of the order of 0·1 km³ (Simkin & Siebert, 1994). They typically contain 1–3 mm sized phenocrysts in the proportions 10–20% plagioclase, 2–5% clinopyroxene and 1–4% orthopyroxene, with trace amounts of olivine and opaques set in a cryptocrystalline groundmass (Richter et al., 1998). A xenolith (found in lavas scattered on ridges) and a rhyolitic obsidian clast (denoted by the ‘glass’ in Table 1) from the pyroclastic flow were also analysed to expand the compositional range of the lavas investigated.

View this table: [in this window] [in a new window]   Table 1: Geochemical and isotopic data for Akutan lavas

 

	ANIAKCHAK VOLCANO

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
Aniakchak volcano lies to the east of Akutan on the Alaskan Peninsula (Fig. 1a), where numerous domes, flows and cones occupy the interior of a 10 km diameter caldera (Fig. 1c). This volcano is predominantly calc-alkaline but erupted tholeiitic lavas between 450 and 240 ka (Nye et al., 1993). The flows and tuffs of the pre-caldera volcano consist mainly of basaltic andesite, two-pyroxene andesite and dacite. Ash flows from the caldera-forming eruption at 3430 ± 10 years BP (Miller & Smith, 1987) may total >50 km³ (Miller & Smith, 1977, 1987), and range in composition from andesite to rhyodacite with whole-rock SiO₂ contents as high as 70·4% (Dreher, 2002). The post-caldera volcanic rocks include andesite and basaltic andesite but are predominantly crystal-poor (20%), plagioclase–pyroxene dacites erupted from intra-caldera cones. The largest cone is Vent Mountain, which is 2·5 km in diameter and has erupted mainly dacitic magmas with an SiO₂ content ranging from 61 to 67% (Neal et al., 1992). For these rocks, Fe–Ti oxide thermometry indicates pre-eruptive temperatures of around 855–860°C (Bacon et al., 1997). Melt inclusion studies of basaltic andesite from Blocky Cone, the dacite ‘pink pumice’ from Half Cone, and the 1931 dacite indicate that these magmas had pre-eruptive H₂O contents in the range 3–4% at depths of 3–5 km (Bacon, 2002). Macroscopic evidence for mingling between andesite and dacite magmas is common in the caldera-forming ignimbrite, and magma mixing is inferred from petrographic and chemical data to have been a major process throughout the post-caldera history of the volcano (Neal et al., 1992; Bacon et al., 1997; Bacon, 2002). For the purposes of this study, we analysed samples ranging from basaltic andesite to rhyodacite in composition. Two are from the zoned 3430 years BP caldera-forming eruption, four are post 3430 lavas, one a Vent Mountain lava and eight samples belong to the 1931 eruptions. The 1931 event produced 0·3–0·5 km³ of tephra zoned from dacite to andesite, and minor andesite and dacite lava flows (Neal et al., 2001).

	ANALYTICAL TECHNIQUES

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
Major element compositions were determined by X-ray fluorescence spectroscopy at the GeoAnalytical Laboratory of Washington State University (Johnson et al., 1999) and are reported here normalized to 100% on a volatile-free basis. Trace elements were measured by inductively coupled plasma mass spectrometry (ICP-MS) at Durham University, where powders were digested using standard HF–HNO₃ treatments, ensuring that no fluoride residues formed. Internal drift was monitored by spiking with Rh, In and Bi before dilution to 3·5% HNO₃. Solutions were analysed on a Perkin–Elmer–SCIEX Elan 600 system using a cross-flow nebulizer. Oxide interferences for most analyses were much less than 2·5% of the total signal. Corrections were made using oxide/metal ratios measured on matrix-matched standard solutions, and calibration was achieved using matrix-matched international (USGS W2) and in-house reference materials. Total procedural blanks for all elements were negligible for all analyses, based on digestion of 0·1 g of powder.

Sr, Nd and Pb were separated using cation and anion exchange separation techniques following standard HF–HNO₃ dissolutions at the Open University (OU), the NERC Isotope Geosciences Laboratory (NIGL) or at Adelaide University (AU). Sr and Nd isotopes were analysed by thermal ionization mass spectrometry (TIMS) at OU, NIGL and AU and corrected for within-run mass bias to ⁸⁶Sr/⁸⁸Sr = 0·1194 and ¹⁴⁴Nd/¹⁴⁶Nd = 0·7219. All Sr and Nd data are reported relative to values of 0·71025 for NBS 987 and 0·51184 for La Jolla. Uncertainties, as determined from the 2 reproducibility of the NBS 987 (OU and NIGL), La Jolla (AU) and J&M (OU) standards during the course of analysis, were 28 ppm for Sr and 21–25 ppm for Nd. Pb isotopes were analysed either at the OU, on a Nu-Instruments multi-collector ICP-MS system using Tl for internal mass bias correction (Belshaw et al., 1998), or at AU by TIMS where ratios were corrected for 1 per atomic mass unit mass fractionation using the recommended values of NBS 981 (Todt et al., 1996). Reproducibility is estimated at 0·2% 2. Total procedural blanks were less than 1 ng, 500 pg and 500 pg for Sr, Nd and Pb, respectively.

U–Th separation was carried out on standard HF–HCl–HNO₃ dissolutions to which a mixed ²²⁹Th–²³⁶U tracer had been added. Samples were treated with HCl and H₃BO₄ to ensure sample-spike equilibration and to eliminate fluorides. U and Th were isolated using anionic exchange resin, with HNO₃, HCl and HBr as elutants, and then loaded onto degassed Re filaments along with colloidal graphite and an HNO₃–H₃PO₄ solution, respectively. Th and U concentrations and (²³⁴U/²³⁸U) ratios were determined to ±0·5% at the OU by TIMS system fitted with an RPQ II energy filter for high abundance sensitivity (van Calsteren & Schwieters, 1995). The (²³⁰Th/²³²Th) measurements were made on the Nu-Instruments multi-collector ICP-MS system at the OU using techniques and reproducibility reported by Turner et al. (2001b). Decay constants used in the calculation of activity ratios (denoted by parentheses) were ²³⁰Th = 9·1952 x 10^-6, ²³²Th = 4·948 x 10^-11 and ²³⁸U = 1·551 x 10^-10. (²³⁴U/²³⁸U) ratios in all samples are within error of unity, suggesting that sub-solidus (seawater) alteration has not modified the primary compositions of the samples.

Ra was separated from samples with known historical eruption ages using techniques identical to those described by Turner et al. (2000). Powders were weighed to yield 50 fg of Ra, and spiked with ²²⁸Ra to achieve a ²²⁸Ra/²²⁶Ra ratio of 1. Ra was pre-concentrated using a double pass through cation exchange resin, using HCl, H₂O and HNO₃ as elutants. Ra and Ba were then separated by chromatographic separation using ElChrom Sr-spec resin^TM and HNO₃ as the elutant (Chabaux et al., 1994). Samples were loaded onto degassed Re filaments with a Ta–HF–H₃PO₄ activator solution (Birck, 1986). Samples were analysed dynamically by TIMS at the OU. Analytical precision was better than 1% (2). Repeat analyses of a sample from Mt. Lassen and an in-house standard (ThITS) were used to assess the accuracy and reproducibility of the analyses, which is estimated to be 1·3% for (²²⁶Ra/²³⁰Th) ratios. Total procedural blanks were below detection limits (<0·1 fg/g). The decay constant used to calculate ²²⁶Ra activities was ²²⁶Ra = 4·332 x 10^-4.

	RESULTS

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
The new analytical data obtained as part of this study are presented in Tables 1 and 2. These and existing major element analyses are plotted in Figs 2 and 3 with fields of regional data from the Aleutian islands and Alaskan Peninsula included for reference [Nye & Turner, 1990; Johnson et al., 1996; George et al., 2003; J. Myers & T. McElfrsh, unpublished data, 2003 (available at http://www.gg.uwyo.edu/aleutians/index.htm)]. The Akutan samples have been subdivided into historical and prehistoric subsets and the Aniakchak samples into pre-1931 and 1931 eruptive products. It should be noted that, to maintain an expanded scale, the xenolith (42·2% SiO₂) and rhyolitic obsidian (72·5% SiO₂) from Akutan have not been plotted in all of the diagrams. Excluding these two samples, the Akutan lavas that were young enough for a short-lived isotope study are all basaltic andesites and andesites with a compositional range between 52·8 and 59·7% SiO₂ and between 4·0 and 2·5% MgO. Trace element abundances are similarly restricted in the basaltic andesites, with Th varying between 1·2 and 2·8 ppm and Sr having a range from 426 to 345 ppm. In contrast, the Aniakchak samples exhibit a much greater compositional range, with SiO₂ ranging from 52·5 to 70·4% and MgO from 4·4 to 0·6%. Th contents in these rocks extend from 1·9 to 7·1 ppm and Sr from 483 to 226 ppm.

View this table: [in this window] [in a new window]   Table 2: Geochemical and isotopic data for Aniakchak volcanic rocks

 

View larger version (25K): [in this window] [in a new window]   Fig. 2. (a) K2O vs SiO2 variation diagram for the Akutan and Aniakchak samples. Xenoliths and prehistoric Akutan lavas () are compiled from this study and Romick et al. (1990). High- and low-K boundaries from LeMaitre et al. (1989). (b) FeO*/MgO vs SiO2 diagram with the tholeiitic–calc-alkaline dividing line from Miyashiro (1974). Akutan samples with SiO2 <65% plot on the tholeiitic side of the line and define a tholeiitic trend directed towards the trachytic dyke [SiO2 67·8%, FeO*/MgO = 5·53 from Richter et al. (1998)], whereas those from Aniakchak form a calc-alkaline trend, which is shallower than the divide. Fields for the Aleutian (pale grey) and Alaskan (dark grey) volcanics are shown (Nye & Turner, 1990; Johnson et al., 1996; George et al., 2003; J. Myers & T. McElfrsh, unpublished data, 2003).

 

View larger version (22K): [in this window] [in a new window]   Fig. 3. (a) 143Nd/144Nd vs 87Sr/86Sr, (b) 207Pb/204Pb vs 206Pb/204Pb, and (c) (230Th/232Th) vs (238U/232Th) diagrams showing the compositional range of Akutan and Aniakchak compared with Aleutian and Alaskan data (references as in Fig. 2b). Deep Sea Drilling Project (DSDP) Site 183 sediment average from Plank & Langmuir (1998), NE Pacific MORB from Langmuir et al. (1992), Northern Hemisphere Reference Line (NHRL) from Hart (1984).

 
On a K₂O vs SiO₂ diagram (Fig. 2a), the historical Akutan lavas form an array at low K₂O relative to the Aniakchak samples, whereas the rest of the two datasets overlap. According to the definition of Miyashiro (1974), tholeiitic and calc-alkaline suites converge at low silica, but tholeiitic suites show FeO/MgO vs SiO₂ trends that are as steep as or steeper than the line in Fig. 2b. At intermediate silica contents, the distinction refers to which side of the line a suite of lavas plot. At high silica, MgO decreases rapidly with differentiation and so FeO^*/MgO increases rapidly, leading to curved trends, which can, again, cross the line (Miyashiro, 1974). When the regional Aleutian–Alaska dataset is plotted on an FeO/MgO vs SiO₂ diagram (Fig. 2b), both data fields cross the tholeiitic–calc-alkaline dividing line at low silica but the oceanic (Aleutian) field is clearly steeper than the continental (Alaskan) one. The same general distinction is observed between Akutan and Aniakchak volcanoes when data in addition to those of this study are considered. If only lavas with <65% SiO₂ are considered, then even though some of the Aniakchak samples cross to the tholeiitic side of the Miyashiro line at low silica, they straddle it and overall form a tight array, which is clearly much shallower than the tholeiitic–calc-alkaline dividing line. The Akutan lavas analysed in our study scatter but suggest a trend that is distinctly steeper than the Aniakchak field and that is roughly parallel to, and lies on the tholeiitic side of the line. Although the Aniakchak data re-cross the line at higher silica, the high FeO/MgO trachyte dyke (and rhyolitic obsidian) suggests that Akutan products have higher FeO^*, Al₂O₃ and FeO/MgO at a given silica content (see arrows in Fig. 2b). Thus, according to the definition of Miyashiro (1974), and irrespective of the origins of the within-suite compositional variations that will be discussed further below, the young Aniakchak samples are classified as calc-alkaline, whereas the young Akutan lavas are tholeiitic (see also Romick et al., 1990).

Compatible trace element concentrations are similarly low in both analysed suites, but incompatible trace element concentrations serve to distinguish between the products of the two volcanoes. Briefly, Aniakchak samples are significantly more enriched in incompatible trace elements such as Rb and Zr, whereas Sr concentrations are similar. The Aniakchak rocks are also more enriched in the light rare earth elements, with La/Yb_N ranging from 1·4 to 2·1 in the Akutan lavas compared with 3·0 to 4·0 at Aniakchak.

The Sr and Nd isotope data are shown in Fig. 3a. Whereas the data broadly encompass a similar range in ¹⁴³Nd/¹⁴⁴Nd (0·51320–0·51296), in detail the Aniakchak samples have lower average ¹⁴³Nd/¹⁴⁴Nd and exhibit a much larger range in ⁸⁷Sr/⁸⁶Sr (0·70324–0·70351) compared with the range in ⁸⁷Sr/⁸⁶Sr in the Akutan rocks (0·70346–0·70358). In Pb isotope space, the Akutan samples typically have higher ²⁰⁶Pb/²⁰⁴Pb (18·93–18·98, excluding the obsidian) than the Aniakchak rocks (18·84–18·91) but span a similar range in ²⁰⁷Pb/²⁰⁴Pb (15·56–15·64 and 15·55–15·62, respectively) and ²⁰⁸Pb/²⁰⁴Pb (38·44–38·67 and 38·33–38·59, respectively, excluding the obsidian). In Fig. 3b, the Akutan and Aniakchak data form two sub-parallel arrays that are steeper than, and displaced above the Northern Hemisphere Reference Line. The data from these two volcanoes generally fall within the fields of the regional data from George et al. (2003) but with the Aniakchak analyses showing more isotopic diversity than those from Akutan.

The U-series data are also given in Tables 1 and 2, and the U–Th isotope data are displayed on the equiline diagram in Fig. 3c. The Akutan lavas have lower (²³⁰Th/²³²Th) and higher (²³⁸U/²³²Th) activity ratios (1·35 and 1·5–1·9, respectively) than the Aniakchak products and, overall, form a flattish array to the right of the equiline, similar to the other oceanic sector (Aleutian) data, with the ²³⁸U-excesses reaching 30%. In comparison, the Aniakchak samples have (²³⁰Th/²³²Th) = 1·35–1·57 and (²³⁸U/²³²Th) = 1·34–1·60 and have (²³⁸U/²³⁰Th) ratios ranging from 0·85 to 1·13, thus straddling the equiline (Fig. 3c) along with the other continental sector (Alaskan) rocks. There is no correlation between (²³⁸U/²³⁰Th) and SiO₂ (not shown) in the samples from either volcano. The rocks from both volcanoes preserve a very similar range of moderate Ra-excesses (1–18%) and are, therefore, largely distinguished by their (²³⁸U/²³⁰Th) ratios. The preservation of ²²⁶Ra-excesses suggests that the U–Th isotope variations are likely to be primary signatures, unaltered by post-eruptive ageing.

	INTERPRETATION

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
Key aspects to be drawn from the preceding section are that the lavas erupted from Akutan volcano belong to the tholeiitic magmatic series and have a restricted compositional range, excepting rare highly differentiated compositions such as the trachyte dyke. In contrast, eruptive products at Aniakchak commonly are calc-alkaline and exhibit a much more complete compositional range. At the low-SiO₂ end of the compositional range both lava suites converge in extent of ²²⁶Ra-excess. There is a clear distinction between the two volcanoes in U–Th isotope systematics. A striking observation from Fig. 3c is that this is mirrored regionally in that the oceanic Aleutian magmas have low (²³⁰Th/²³²Th) and sizeable ²³⁸U-excesses whereas the continental Alaska Peninsula magmas can have either ²³⁸U-excesses or ²³⁰Th-excesses and tend generally to have higher (²³⁰Th/²³²Th) (George et al., 2003). The Akutan lavas exhibit the strongest slab fluid signal identified with ²³⁸U-excesses and would accordingly be expected to be the most oxidized; nevertheless, it is the Ankiakchak rocks that show greater iron depletion relative to silica increase (Fig. 2b), a calc-alkaline characteristic that typically reflects early magnetite fractionation under oxidizing conditions. The explanation of this apparent paradox probably lies in the fact that none of the analysed samples is compositionally primitive (i.e. all are differentiated magmas) and that the Aniakchak magmas, at least, appear to have interacted with continental crust.

Relative roles of crystal fractionation and mixing
The large range in radiogenic isotope ratios observed in the Aniakchak products is suggestive of crustal assimilation. This is explored further on a plot of ⁸⁷Sr/⁸⁶Sr vs SiO₂ in Fig. 4. In this diagram the Akutan samples form no clear co-variation between ⁸⁷Sr/⁸⁶Sr and SiO₂; we note in passing that ⁸⁷Sr/⁸⁶Sr is no higher in the rhyolitic obsidian than in the basaltic andesites. In contrast, the pre-1931 Aniakchak products show a broad positive correlation between ⁸⁷Sr/⁸⁶Sr and SiO₂ (Fig. 4) indicative of mixing of at least two isotopic components, one from relatively non-radiogenic mantle and the other from comparatively radiogenic crust. This is important because Grove & Baker (1984) have argued that crustal assimilation may play a crucial role in leading a magma batch to follow a calc-alkaline evolutionary path. This trend is most clearly apparent when the full Aniakchak data set is considered; ⁸⁷Sr/⁸⁶Sr ratios of the 1931 samples span a larger range than those of the Akutan rocks, yet show no correlation between ⁸⁷Sr/⁸⁶Sr and SiO₂ (although there is a weak positive correlation if the two low-⁸⁷Sr/⁸⁶Sr, small-volume Slag Heap and Doublet Crater dacite lavas are excluded) (Fig. 4). Although it is not possible to determine unambiguously the extent to which assimilation was involved in the evolution of these magmas on the basis of the ⁸⁷Sr/⁸⁶Sr data alone (but see below), assimilation and mixing are inferred from petrographic and field data to have been important (Neal et al., 1992; Bacon et al., 1997; Bacon, 2002). In an associated regional study, George et al. (2003) concluded that much of the variation in radiogenic isotopes in primitive lavas along the Aleutian–Alaska arc is determined by the relative contributions from the subducting slab to the mantle wedge source region. However, we suggest that this source component balance is unlikely to change significantly beneath an individual volcano on the time scale over which the analysed Aniakchak products were erupted (3400 years). Therefore, it is suggested that the 1931 magmas probably evolved from parental liquids similar to those of earlier post-caldera magmas (Dreher, 2002).

View larger version (20K): [in this window] [in a new window]   Fig. 4. Variation of 87Sr/86Sr with SiO2 for the Akutan and Aniakchak samples, contrasting the range in 87Sr/86Sr and their correlation with SiO2 in the Aniakchak rocks with the limited isotopic and compositional range in the Akutan lavas.

 
Given the evidence that mixing was an important process in the compositional evolution of the Aniakchak magmas, it is important to distinguish whether this could be entirely due to two end-member mixing or whether crystal fractionation was also important. On many binary diagrams the 1931 Aniakchak data form linear arrays that could be consistent with two end-member mixing between andesite and dacite magmas (Neal et al., 1992; Bacon et al., 1997; Bacon, 2002). However, Dreher (2002) has shown that diagrams of V, Eu, TiO₂, FeO and P₂O₅ vs SiO₂ in pre-, syn- and post-caldera Aniakchak rocks all show inflected trends. These provide evidence that two end-member assimilation or magma mixing alone cannot explain all of the data and that crystal fractionation must also have played a role in producing the compositional range, at least below 62% SiO₂. Figure 5 shows that there are also inflections on plots of P₂O₅ and Sr vs SiO₂ (and also TiO₂ vs SiO₂, which is not plotted) for the young Aniakchak products analysed in this study. Because ⁸⁷Sr/⁸⁶Sr ratios also increase across this compositional range, it appears that assimilation of material having elevated ⁸⁷Sr/⁸⁶Sr, possibly siliceous partial melts of crustal wall rocks, by the magma in the storage reservoirs or conduits, must also have been involved.

View larger version (17K): [in this window] [in a new window]   Fig. 5. P2O5 and Sr vs SiO2 variation diagrams, showing the inflections that argue against purely two end-member assimilation or magma mixing in the Aniakchak suite.

 
To place more precise constraints on the compositional evolution of the magmas we have modelled the change in Ba/Th ratio with Eu/Eu^* in Fig. 6a. In gabbroic assemblages, such as those that characterize the phenocryst assemblages in the Ankiakchak and Akutan samples, plagioclase is the principal phase that incorporates Ba and Eu²⁺ (and Ra, see below) and thus controls the evolution of Ba/Th and Eu/Eu^* ratios. The partition coefficient of these trace elements can be accurately determined for a given temperature and bulk composition (Blundy & Wood, 1994). Figure 6a shows that crystal fractionation vectors using appropriate partition coefficients cannot reproduce the rate of decrease of Ba/Th with decreasing Eu/Eu^* observed in the Aniakchak data; a fractionation model forced through the data requires D_Ba = 3, which is several times higher than any Ba partition coefficients yet measured or predicted for plagioclase. The implication is that an additional process must be controlling the compositional evolution of the Aniakchak magmas and, given the discussion above, we infer that this is the effect of magma mixing, and possibly assimilation combined with crystal fractionation. It should be noted that this result from Fig. 6a is just as applicable to the 1931 suite as to the older Aniakchak rocks, even though the ⁸⁷Sr/⁸⁶Sr evidence for assimilation was less clear in the 1931 lavas. In contrast, we have found no requirement in our data for assimilation or magma mixing in the young Akutan lavas and the closed-system fractionation vector in Fig. 6a could easily simulate the majority of the Akutan array, so long as an appropriately different parental magma starting composition were chosen. A similar conclusion was reached by Romick et al. (1990) for within-suite variations in the Akutan lavas, although their data indicate a need for temporal changes in parental magma compositions between the older (1–2 Ma) and younger Akutan lava sequences.

View larger version (21K): [in this window] [in a new window]   Fig. 6. (a) Ba/Th vs Eu/Eu* compared with a calculated fractionation vector (dotted line), which shows that the decrease in Ba/Th in the Aniakchak samples cannot be accounted for by crystal fractionation alone. The partition coefficient for Ba into plagioclase at the temperature of the Aniakchak 1931 dacite magma (855°C; Bacon et al., 1997) was calculated using the model of Blundy & Wood (1994). The best-fit line (dashed) requires DBa = 3, which is impossible for a gabbroic fractionating assemblage with currently accepted KD values. (b) (226Ra/230Th) vs Eu/Eu* with an equivalent model fractionation vector, which, again, cannot simulate the Aniakchak data, implying that both mixing and the time taken for fractional crystallization are important factors in controlling the decreases in (226Ra/230Th) observed in the data. The Aniakchak data lie close to a mixing hyperbola calculated assuming two-component mixing between andesitic and dacitic end-members (continuous line). The Akutan lavas span a similar range in (226Ra/230Th) to those from Aniakchak, suggesting that a comparable storage time was spent in high-level magma chambers if initial (226Ra/230Th) ratios were similar. Modelling assumes an assemblage composed of 30% clinopyroxene (with DRa = 1·7 x 10-6, DTh = 0·013, DBa = 4 x 10-5, DEu = 0·7, DSm = 0·75 and DGd = 0·58) and 70% plagioclase (with DRa = 0·09, DTh = 1 x 10-9, DBa = 0·7, DEu = 3·8, DSm = 0·11 and DGd = 0·05). The parental composition was that of sample 97ANB 27 in both diagrams (open circle with black dot).

 
²²⁶Ra–²³⁰Th constraints on magmatic time scales
Current models suggest that ²²⁶Ra-excesses in island arc volcanic rocks reflect additions from the subducting slab (e.g. Gill & Williams, 1990; Turner et al., 2001a, 2003a; Sigmarsson et al., 2002), and the lack of within-suite variations of source component indicators, such as Zr/Nb ratio, suggests that there is no reason to assume that the parental magmas for the young lavas from each volcano varied significantly in the last few thousand years. Thus, much of the variation in the preserved (²²⁶Ra/²³⁰Th) ratios is likely to record the effects of ageing of the magmas prior to eruption such that they can be used to constrain the time scales of differentiation (e.g. Vigier et al., 1999; Cooper et al., 2001; Turner et al., 2001a). Ba provides a close, though not exact (see below), chemical analogue for Ra and so the plot of (²²⁶Ra/²³⁰Th) vs Eu/Eu^* in Fig. 6b is essentially equivalent to the Ba/Th vs Eu/Eu^* plot of Fig. 6a with the crucial difference that (²²⁶Ra/²³⁰Th) will decrease toward secular equilibrium, (²²⁶Ra/²³⁰Th) = 1, as a function of time and at a rate proportional to the half-life of ²²⁶Ra (1600 years). In practice, because Ra has an ionic radius 5% larger than that of Ba it will be more incompatible than Ba (Wood et al., 1999). This is illustrated in Fig. 6b, where the fractionation vector from Fig. 6a has been transcribed, taking into account the greater incompatibility of Ra relative to Ba. As can be seen, (²²⁶Ra/²³⁰Th) ratios change little with fractionation because Th is also very incompatible in gabbroic assemblages (Fig. 6b). Consequently, the large range in (²²⁶Ra/²³⁰Th) ratios observed in both the Akutan and Aniakchak datasets cannot be ascribed to crystal fractionation and must, additionally, reflect radioactive decay.

Several important results emerge from Fig. 6b. First, the historical products of Akutan and Aniakchak encompass almost identical ranges of (²²⁶Ra/²³⁰Th), even though the extent of primary ²²⁶Ra–²³⁰Th disequilibria generated in the mantle wedge is not well constrained. Second, the most mafic lavas analysed for ²²⁶Ra from each suite have similar (²²⁶Ra/²³⁰Th) ratios and major and compatible trace element compositions. In the simplest interpretation this suggests that the pre-eruption residence times of magmas beneath these two volcanoes were similar and the range in (²²⁶Ra/²³⁰Th) ratios can be used to calibrate the actual time scales involved. It should be noted that if the Ra–Th disequilibria in the primary magmas were significantly greater than that observed in the most primitive lavas (1·2) then the overall transfer time of the U-series signal from the subducting plate could be longer. As shown in Fig. 6b, the implied range of magma residence times is of the order of several thousand years. Such estimates are similar to numerical estimates of the time scales for crystallization as a result of cooling in crustal magma chambers (e.g. Marsh, 1989), rather than crystallization caused by decompression and degassing, which can result in much more rapid crystallization time scales approaching those of eruptive periodicity (Blundy & Cashman, 2001).

The Ra–Th isotope data permit that the time scale for fractionation below 62% SiO₂ might have been similar at both Akutan and Aniakchak. Above 62% SiO₂, the Aniakchak data appear to be linear on many plots and it has proved hard to distinguish unambiguously whether fractionation or mixing played the dominant role in producing this part of the compositional array. For example, there is a good correlation between indices of differentiation, such as Eu/Eu^*, and (²²⁶Ra/²³⁰Th) within the Aniakchak samples in Fig. 6b whereas the Akutan samples show a much more restricted compositional variation over the same range in (²²⁶Ra/²³⁰Th). However, both fractionation accompanied by ageing and mixing curves can reproduce these trends (see Fig. 6b). If mixing involves large volumes of a siliceous end-member that was in ²²⁶Ra–²³⁰Th equilibrium, then the decreases in (²²⁶Ra/²³⁰Th) with increasing differentiation will reflect a combination of assimilation or magma mixing and the time taken for differentiation. Thus, we consider the time scale implications of two end-member models for the Aniakchak data:

1. If fractionation was the dominant process, then it is striking that similar durations for magma evolution are inferred for magma suites representing very different amounts of differentiation beneath these two volcanoes.
   
2. However, if mixing was responsible for the Aniakchak data beyond 62% SiO₂, and the evolved end-member was in, or close to, ²²⁶Ra–²³⁰Th equilibrium, then the formation of this part of the compositional range could have been much faster than the few thousand years indicated in Fig. 6b. In this latter model, the andesite is formed largely by fractional crystallization of basalt and its ²²⁶Ra-excess reflects that of the parental basalt and the time taken for this fractionation. In principle, the dacitic end-member that mixes with the andesite may be either a differentiation product of a similar andesite or a partial or wholesale melt of upper-crustal rocks or a previous dacite. However, it would take 8000 years for a magma to reach ²²⁶Ra–²³⁰Th equilibrium. It seems unlikely that any magma could have remained liquid in the inferred shallow-level reservoir for this period of time or to have survived the 3430 years BP caldera-forming eruption. Therefore, partial melting of reservoir wall rocks seems the most likely model for this dacitic end-member.
   

Whichever model ultimately proves to be correct, it would appear that the greater compositional range observed at Aniakchak volcano was produced in a similar or shorter time than the more restricted compositional range observed at Akutan. Magma mixing and assimilation are inferred to have played a key role in the differences between the two volcanoes, the primary result being that differentiation was faster in the magma system beneath Aniakchak volcano.

	DISCUSSION AND COMPARISON WITH OTHER SYSTEMS

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
As the inferred time scales for both volcanic systems appear to be appropriate for crystallization driven by cooling, the principal controls on the extent of differentiation will be the rate of cooling and the efficiency of crystal–liquid separation ± mixing. The rate of cooling will be a function of magma volume and temperature relative to the country rock temperature and is likely to be increasingly slow at greater depths within the crust. The magma volumes of both the recent eruptive products and caldera sizes are greater at Aniakchak than at Akutan (Simkin & Siebert, 1994; Neal et al., 2001), even though overall Akutan is a larger volcano (considering the submarine part of the edifice). Country rock temperatures are poorly constrained and the larger chamber might be expected to cool more slowly. Additionally, greater heat loss may be implicit at Aniakchak if melting of wall rocks was necessary to produce the increases in ⁸⁷Sr/⁸⁶Sr ratios during differentiation and given that hydrothermal fluid circulation would have been promoted by fractures related to caldera collapse.

A simple crystal settling model provides a maximum likely time for production of evolved magmas. If differentiation occurs by crystal settling in a convecting magma chamber, the rate of crystal settling (t_settle) will be controlled by the radius of the crystals (r), their density contrast with the magma () and the magma viscosity (µ) (Martin & Nokes, 1988):

where H is the height of the magma chamber and g is the acceleration due to gravity. Taking two andesites with similar compositions from Akutan (96PS30) and Aniakchak (97ANB-44) for illustrative purposes, and magmatic temperatures and water contents of 970°C and 2 wt % and 855°C and 4 wt %, respectively (Romick et al., 1990, and discussion above; Bacon et al., 1997; Bacon, 2002), we calculate densities (Bottinga & Weill, 1970) and viscosities (Shaw, 1972) of 2490 kg/m³ and 1·23 x 10⁴ Pa s for the Akutan magma and 2516 kg/m³ and 1·03 x 10⁵ Pa s for the Aniakchak magma. Assuming a gabbroic fractionating assemblage with a density of 3000 kg/m³ and crystals with a radius of 10^-3 m, this leads to predicted crystal settling times of the order of 850 and 6900 years for Akutan and Aniakchak, respectively, in magma chambers of 10 km³ size, which is the order of magnitude of the volumes of the recent eruptives and caldera-forming eruptions of these two volcanoes. Although these calculations are clearly subject to large uncertainties, because of the sensitivity of the density and viscosity estimates to the magmatic temperatures and water contents (which are poorly constrained, especially at Akutan), it nevertheless seems likely that crystal–liquid separation by settling would require time scales that are significant relative to the residence times inferred from the ²²⁶Ra–²³⁰Th disequilibria data. Moreover, it would appear that crystal–liquid separation is predicted to be at least as fast at Aktuan yet these lavas show the more restricted compositional range. Either the greater compositional variation observed at Aniakchak reflects some other, more efficient, mechanism of crystal–liquid separation, such as gas-driven filter-pressing (Sisson & Bacon, 1999) or the extent of assimilation and magma mixing evident in the Aniakchak samples exerted a primary control on the difference in degrees of differentiation experienced beneath the two volcanoes. Although the former hypothesis cannot be tested using our data, we favour the latter given the good evidence for mixing at Aniakchak.

As outlined in the Introduction, many different models have been proposed to account for the distinction between the tholeiitic and calc-alkaline magmatic series, although we believe this is the first to investigate possible temporal differences. It seems unlikely that the liquid line of descent reflects parental magma compositions, as these appear to be similar beneath Akutan and Aniakchak and the Akutan system is, if anything, the most influenced by addition of oxidizing fluids from the subducting slab. Comparison of recent erupted volumes and caldera size suggests that the Akutan system currently has a smaller volume than that at Aniakchak, which is the opposite of the Kay & Kay (1994) model. Equally, there is little evidence that Akutan is a longer-lived volcano than Aniakchak, and the temporal changes from calc-alkaline to tholeiitic and back to calc-alkaline at Aniakchak (Nye et al., 1993) and from tholeiitic to transitional tholeiitic at Akutan are not predicted by the Myers et al. (1985) model. Our results suggest, in fact, that the time scales of magmatic evolution were probably similar in the two systems but that differentiation was more efficient in the calc-alkaline system, arguably because of a combination of greater extents of assimilation and cooling. The role of assimilation–magma mixing in the calc-alkaline system is a common feature of all of the Grove & Baker (1984), Myers et al. (1985) and Kay & Kay (1994) models.

Finally, in Fig. 7 we compare the Akutan and Aniakchak (²²⁶Ra/²³⁰Th) arrays with the results of two other recent studies. The rate of differentiation is proportional to the slope of the arrays on the (²²⁶Ra/²³⁰Th) vs Th diagram; the shallower the slope the more differentiation achieved per unit of ²²⁶Ra decay, or time. Binary mixing could produce similar arrays in this diagram or curves if the mixing end-members had notably different Th contents. A suite of tholeiitic lavas from the 1978 eruption of Ardoukoba in the Asal rift (Vigier et al., 1999) define an array with a slope that is indistinguishable from that formed by the Akutan transitional tholeiites. Thus, despite the difference in tectonic setting, it would appear that differentiation was restricted in both tholeiitic systems despite the magmas apparently residing in crustal magma chambers for several thousand years. Also depicted in Fig. 7 is the slope for a suite of potassic lavas from Sangeang Api volcano in the rear of the east Sunda arc (Turner et al., 2003b). The slope of this array is intermediate between those of the tholeiitic suites and the calc-alkaline lavas from Aniakchak, suggesting that differentiation, as measured by Th concentration, was more rapid in the potassic suite than in the tholeiitic ones, although less rapid than in the calc-alkaline suite. Turner et al. (2003b) have argued that assimilation (but not binary mixing) was also involved in the evolution of the Sangeang Api lavas.

View larger version (20K): [in this window] [in a new window]   Fig. 7. Diagram of (226Ra/230Th) vs Th (used as an index of differentiation) comparing the slopes of the Akutan lavas () with tholeiitic lavas from the Asal rift from Vigier et al. (1999), and the curvature of the 1931 Aniakchak samples (•) with that of high-K calc-alkaline lavas from Sangeang Api in the rear east Sunda arc (Turner et al., 2003b). The calc-alkaline and potassic suites both have shallower slopes, indicating more rapid differentiation than the two tholeiitic suites (see text for discussion).

 

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION THE ALASKA-ALEUTIAN ISLAND ARC AKUTAN VOLCANO ANIAKCHAK VOLCANO ANALYTICAL TECHNIQUES RESULTS INTERPRETATION DISCUSSION AND COMPARISON WITH... CONCLUSIONS REFERENCES

 
The recent magmas erupted from Akutan and Aniakchak volcanoes belong to the tholeiitic and calc-alkaline magma series, respectively. The Akutan lavas exhibit little within-suite variation in SiO₂ or ⁸⁷Sr/⁸⁶Sr and can be explained by closed-system magmatic evolution. They are characterized by ²³⁸U-excesses whereas those samples from Aniakchak straddle the U–Th isotope equiline. Because the size of ²³⁸U-excess will reflect both the oxygen fugacity and the Th content of the mantle wedge, this may provide evidence for more oxidizing conditions beneath Akutan (Turner et al., 2003a). However, the rate of iron depletion is less pronounced here than that at Aniakchak, suggesting that this may not reflect a difference in primary magma oxidation state. Curvilinear trace element trends and a large range in ⁸⁷Sr/⁸⁶Sr isotope ratios in the Aniakchak data appear to require the combined effects of fractional crystallization, assimilation and magma mixing. The products of both volcanoes preserve a range in ²²⁶Ra–²³⁰Th disequilibria, which suggests that the time scale of crustal residence of magmas beneath these volcanoes was similar, and of the order of several thousand years. During that time interval the tholeiitic Akutan magmas underwent restricted, closed-system, compositional evolution; a similar observation has been made for within-plate tholeiitic lavas from the Asal rift (Vigier et al., 1999). In contrast, calc-alkaline magmas beneath Aniakchak volcano underwent significant compositional evolution over a similar range in ²²⁶Ra–²³⁰Th disequilibria. Potassic lavas from Sangeang Api appear to be intermediate between Aniakchak and Akutan in their differentiation rate. Thus, it appears that differentiation may be more rapid in the calc-alkaline and potassic systems as a result of a combination of greater extents of assimilation and cooling.

	ACKNOWLEDGEMENTS

 
David Bruce, Julian Pearce, Louise Thomas, Mabs Gilmour and Geoff Nowell are all thanked for their assistance with the analytical work. We would like to thank Jake Lowenstern and an anonymous reviewer for internal US Geological Survey reviews, and we gratefully acknowledge the formal reviews by John Gamble, James Myers and Nathalie Vigier, as well as editorial assistance from Marjorie Wilson. During the course of this work R.G. was supported by NERC grant GR3/11701 to S.T. and C.J.H., and S.T. was funded by a Royal Society University Research Fellowship.

	FOOTNOTES

 
Present address: GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, N.S.W. 2109, Australia.

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