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Journal of Petrology | Volume 45 | Number 4 | Pages 723-738 | 2004
Journal of Petrology 45(4) © Oxford University Press 2004; all rights reserved.
The Role of Open-System Processes in the Development of Silicic Magma Chambers: a Chemical and Isotopic Investigation of the Fogo A Trachyte Deposit, São Miguel, Azores
1 DEPARTMENT OF GEOLOGY, MIAMI UNIVERSITY, OXFORD, OH 45056-1300, USA
2 DEPARTMENT OF TERRESTRIAL MAGNETISM, CARNEGIE INSTITUTION OF WASHINGTON, 5241 BROAD BRANCH ROAD, NW, WASHINGTON, DC 20015, USA
RECEIVED SEPTEMBER 15, 2002; ACCEPTED AUGUST 18, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL...RESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The processes operating in the development of chemical zonation in silicic magma chambers have been addressed with a Sr–Nd–Pb–Hf–Th isotope study of the chemically zoned trachyte pumice deposit of the Fogo A eruption, Fogo volcano, Azores. Sr isotopic variation is observed in whole rocks, glass separates and sanidine phenocrysts (whole-rock 87Sr/86Sr: 0·7049–0·7061; glass 87Sr/86Sr: 0·7048–0·7052; sanidine 87Sr/86Sr: 0·7048–0·7062). Thorium isotopic variation is observed in glass separates, with (230Th/232Th)o ranging from 0·8737 to 0·8841, and exhibiting a negative correlation with Sr isotopes. The Nd, Pb and Hf isotopic compositions of the whole-rock trachytic pumices are invariant and indistinguishable from basalts flanking the volcano. The Sr isotope variations in the whole rocks are proposed to be the result of three distinct processes: contamination of the Fogo A magma by assimilation of radiogenic seawater-altered syenite wall rock, to explain the Sr and Th isotopic compositions of the glass separates; incorporation of xenocrysts into the trachytic magma, required to explain the range in feldspar Sr isotopic compositions; and post-eruptive surface alteration. This study emphasizes the importance of determining the isotopic composition of glass and mineral separates rather than whole rocks when pre-eruptive magmatic processes are being investigated.
KEY WORDS: Azores; open-system processes; Sr isotopes; trachytic pumices; zoned magma chambers
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL...RESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The products of many large explosive volcanic eruptions are chemically zoned. The process of zonation may generate highly evolved (e.g. trachytic or rhyolitic) and volatile-oversaturated magmas. Because volatile oversaturation is recognized as an important trigger of explosive volcanic eruptions (Blake, 1984
), understanding the processes that lead to zonation and volatile oversaturation and the timescales over which these processes occur is important.
Chemical, as well as isotopic, zonation has been recognized in many large silicic volcanic deposits (Hildreth, 1981; Johnson, 1989). Chemical zonation within silicic magma chambers has commonly been attributed to processes such as fractional crystallization (Michael, 1983; Cameron & Cameron, 1986) or convection-driven thermogravitational diffusion (Hildreth, 1981). Other processes are required to explain isotopic zonation within magma chambers. Processes that can contribute to the development of intra-magma chamber isotopic gradients include in situ radioactive decay (Christensen & DePaolo, 1993), wall rock assimilation, hydrothermal fluid interaction (Hildreth et al., 1984; Wörner et al., 1985; Bindeman & Valley, 2001), and magma recharge/mixing (Eichelberger et al., 2000).
Although variations in Nd (Tegtmeyer & Farmer, 1990) and Pb (Wolff & Palacz, 1989) isotopic compositions have been reported in silicic volcanic deposits, silicic deposits are most commonly zoned with respect to Sr isotopic composition. The low Sr contents of these highly evolved systems give enormous leverage to almost any potential contaminant, be it mafic magma, sediment, or ocean spray. In contrast, Pb and Nd contents of such liquids are usually similar to, or greater than, those of potential contaminants. Hence, open-system processes often have little effect on Pb and Nd isotopic compositions, but significant effects on Sr isotopic compositions. Additionally, many highly evolved, low-Sr magmas have extremely high Rb/Sr ratios, allowing closed-system development of relatively high 87Sr/86Sr ratios. Prolonged magma residence times following magmatic fractionation events could thus produce significant variation in Sr isotope signatures. Determining which of these processes is responsible for the Sr isotopic variation has important implications for timescales of magmatic systems.
Christensen & DePaolo (1993) called upon dominantly closed-system processes to explain both the Sr isotopic zonation and the Sr isotopic disequilibrium displayed between feldspar phenocrysts and glass from the chemically zoned Bishop Tuff. Their model suggests that the increasing 87Sr/86Sr ratios in the glasses are primarily due to radioactive decay of 87Rb in the high Rb/Sr liquids and requires a magma residence time between 300 and 500 kyr. However, magma residence time estimates based on Rb–Sr are dependent on the assumption of a closed system and are thus model dependent. More recent results based on U–Pb isotope analyses of zircons suggest that magmatic residence times for the early Bishop Tuff rhyolite are only a few tens of thousands of years (Reid & Coath, 2000).
This paper focuses on the isotopic characteristics of a chemically zoned trachytic pumice deposit in São Miguel, Azores, to assess the importance of open-system processes in the development of isotopic heterogeneities in zoned magma chambers. Sr, Nd, Pb, Hf and Th isotopic data, major and trace element data and field relationships allow for constraints to be placed on the pre-eruptive magma chamber processes. Our results show that whole-rock isotopic analyses combined with isotopic data on constituent materials (glass and phenocrysts) can provide fundamental information regarding the processes responsible for the isotopic zonation.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL...RESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The Azores lie approximately 1400 km west of Portugal, straddling the Mid-Atlantic Ridge (MAR) between 37° and 40°N. The island of São Miguel is located near the eastern extent of the Azores platform (Fig. 1). São Miguel has three active stratovolcanoes, Sete Cidades, Fogo and Furnas, that are separated by two fields of Quaternary alkali basalt cinder cones and lava flows (Booth et al., 1978
; Moore, 1990). The island is composed primarily of basaltic and trachytic rocks, although minor amounts of intermediate and mixed-magma hybrids have been identified (Storey et al., 1989; Moore, 1991).
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The Fogo volcano, located in the central portion of the island (Fig. 1), is composed of >90% trachytic pumice. The oldest dated unit is a 181 ka trachytic dome (Gandino et al., 1985
). The formation of the outer caldera took place between 46 and 26·5 ka, resulting in approximately 5 km3 of welded and nonwelded pumice (Moore, 1990). The inner caldera forming eruption took place at about 15·2 ka, resulting in pumice-fall deposits and locally welded pyroclastic flows (Moore, 1990). No major eruptions occurred within the outer caldera for 
10–30 kyr between the formation of the inner and outer calderas. An additional hiatus in explosive activity of 10 kyr took place between formation of the inner caldera and eruption of the Fogo A deposit 5 kyr ago (eleven 14C ages range from 4·44 to 5·38 ka; Shotton et al., 1968, 1969; Moore & Rubin, 1991). This paper focuses on the isotopic variations within the Fogo trachytic pumice and constrains the physical and chemical processes operating in the pre-eruptive magmatic system.
Fogo A deposit
The Fogo A deposit, first named and described by Walker & Croasdale (1970), was formed by a plinian eruption that produced 0·6–0·7 km3 (dense rock equivalent) of trachytic pumice lapilli and ash. Additional surge and flow deposits were identified by Widom et al. (1992). The samples used in this study were collected from the area around Ribeira Chã. Although summarized here, a detailed description of the Fogo A deposit from the area around Ribeira Chã has been given by Widom et al. (1992). At this location, the total thickness of the deposit is about 5 m, and it can be texturally divided into two subunits (Fig. 2). Subunit A is about 3 m thick and forms 70–80% of the total erupted volume (Walker & Croasdale, 1970). The dominant constituent is light colored pumice grading upwards into darker and denser pumice (Bursik et al., 1992). A maximum pumice size of 40 cm is observed about 250 cm above the base where the first syenite xenoliths and banded pumices occur (Widom et al., 1992). Subunit B is about 1·5 m thick and contains three major surge horizons (Widom et al., 1992). Large syenite xenoliths and banded pumices become abundant about 90 cm above the lowermost surge horizon. The petrogenesis of these xenoliths has been detailed by Widom et al. (1993). The syenites are miarolitic with up to 10% void space. Fresh nodules have subhedral granular textures with interlocking sanidine crystals, whereas plutonically altered syenites are subhedral to anhedral granular with sanidine and perthite crystals forming interdigitating boundaries (Widom et al., 1993).
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Widom et al. (1992)
provided a detailed model for the petrogenetic evolution of the Fogo A magmatic system through the use of chemical, textural, and U-series isotopic data. Major and trace element variations from throughout the stratigraphic extent of the Fogo A deposit indicate that the trachytes represent the inverted, extrusive equivalent of a chemically zoned magma chamber (Fig. 2). Fractional crystallization models using the observed phenocryst phases suggest that the observed chemical zonation was achieved via 70–75% crystallization. |
SAMPLES AND ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL...RESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Seven trachytic pumice samples were selected for whole-rock Sr, Nd, Pb and Hf isotopic analyses to best represent the chemical diversity of the Fogo A deposit. The trachytic pumices have 0·79–2·05 wt % CaO and 60·90–62·90 wt % SiO2, and have widely varying trace element concentrations (e.g. Zr 791–1417 ppm; Ba 9–514 ppm; Sr 3–478 ppm; Widom et al., 1992
). These samples were powdered using a Spex alumina shatterbox. Six glass separates, also chosen to best represent the chemical diversity of the deposit, were analyzed for Sr and Th isotopic compositions. The glass was separated from the whole rock by hand picking under a binocular microscope, avoiding any apparent impurities. No secondary alteration products were observed in any of the material during this procedure. Glass separates were rinsed in methanol then cleaned with 2N HCl for 10 min in an ultrasonic bath followed by a rinse in high-purity water to remove any adhering salts. Individual sanidine phenocrysts from uncrushed whole-rock pumices were hand picked using a binocular microscope. Sanidines were obtained from whole rocks spanning much of the observed range in Sr isotopic composition. The size and abundance of phenocrysts were controlling factors for which pumices were chosen for sanidine extraction. Petrographic examination of the pumices indicates that <5% of the feldspar phenocrysts are perthitic; all are subhedral to euhedral and appear fully disaggregated. Prior to digestion, the individual crystals were cleaned with 2·5N HCl in an ultrasonic bath for 45 s, then rinsed thoroughly in high-purity water to remove any salts from the surface of the crystals. Twelve whole-rock syenite xenoliths from the Fogo A deposit were rinsed in high-purity water, powdered using a Spex alumina shatterbox, and analyzed for Sr isotopic compositions.
All chemical separations and isotopic measurements were carried out at the Department of Terrestrial Magnetism. Whole-rock powders and sanidine separates were dissolved in Savillex vials in concentrated HF–HNO3, to eventually be taken up in HBr (for Pb) or HCl (for Sr only). Pb was first separated from the samples following the procedure described by Walker et al. (1989). All material eluted before the Pb cut was kept for further separation of Rb, Sr, Nd and Hf. Separation of these elements involved the column procedure described by Walker et al. (1989), with the exception that the sample was loaded in 1N HCl–0·1N HF and the Hf eluted immediately using 10 ml of this acid mixture. Hf was further purified using an anion column procedure similar to the second column procedure described by Blichert-Toft et al. (1997). Separation of Ti from Hf was accomplished by placing the Hf cut from the second column back on the first cation column in 2·5N HCl with 50 µl of H2O2, eluting the Ti with 24 ml of 2·5N HCl, then eluting the Hf with 2·5N HCl–0·3N HF. Sm–Nd separation for whole rocks was accomplished using hydrogen-diethyl-hexyl-phosphate (HDEHP) columns. Sr was separated from single sanidine crystals using Eichrom Sr resin following procedures similar to those of Deniel & Pin (2001). Sr, Th and U were separated from glass following the procedure described by Pietruszka et al. (2002), with the exception that the Th and U cuts for isotopic composition were passed through a second 50 µl column of Eichrom TRU (TransUranic) Resin to remove potential high-mass hydrocarbon interferences.
The isotopic compositions and concentrations of strontium by isotope dilution were measured by thermal ionization mass spectrometry (TIMS) on a VG-354 system. Strontium isotope ratios are corrected for fractionation using 86Sr/88Sr = 0·1194. All isotopic measurements assume an exponential mass dependence of the fractionation. All errors reported for isotopic measurements are 2 standard deviations of the mean. The reported values are normalized to a value of 87Sr/86Sr = 0·71025 for the NBS 987 standard. Measurements of this standard show a long-term reproducibility of ±0·000024 about a mean of 0·710281. Neodymium, Hf, Pb and Th isotopic compositions and Rb, Nd, Sm, Hf, Pb and Th concentrations by isotope dilution were measured by inductively coupled plasma mass spectrometry (ICP-MS) on a VG-P54 multicollector (MC) system. Neodymium and Hf were measured statically. Hafnium is fractionation corrected to 178Hf/177Hf = 0·7325 and reported relative to a value of 176Hf/177Hf = 0·282160 for the JMC475 standard. Nine measurements of this standard provide an average value of 176Hf/177Hf = 0·282152 ± 0·000011. Neodymium is fractionation corrected to 146Nd/144Nd = 0·7219 and normalized to the La Jolla value of 143Nd/144Nd = 0·51186. Fifteen measurements of this standard provided a mean value of 0·511855 ± 0·000118 during the course of this study. However, 4–6 repeat measurements of this standard during individual analytical sessions indicate a much better precision of ±0·000020. Therefore, the Nd data reported here are corrected to the average value determined for 4–6 repeat analyses of the La Jolla standard run on the same day as the samples. Lead isotopic compositions were measured statically on the VG-P54 system, using the thallium addition method for fractionation correction (e.g. Rehkamper & Halliday, 1998). Our overall precision for Tl-spiked Pb measurements as determined for 16 repeats of the NIST 981 standard value is 206Pb/204Pb = 0·14%, 207Pb/204Pb = 0·17% and 208Pb/204Pb = 0·30%, approximately the same as we achieve for thermal ionization analysis of Pb. Within one analytical session, however, the fractionation of Tl and Pb track one another much better, as indicated by precision of five repeat measurements of NIST 981 of 206Pb/204Pb = 0·06%, 207Pb/204Pb = 0·07% and 208Pb/204Pb = 0·15%. Consequently, we normalize the sample Pb data reported here to the average value determined for 2–5 repeat analyses of NIST 981 run on the same day as the samples. After this correction, the data are normalized to the NIST 981 values of 206Pb/204Pb = 16·9356, 207Pb/204Pb = 15·4891 and 208Pb/204Pb = 36·7006 as reported by Todt et al. (1996). The Th and U concentrations and isotope ratios were measured using the high-precision isotope dilution MC-ICP-MS technique described by Pietruszka et al. (2002). Th/U, (234U/238U) and (232Th/230Th) ratio measurements of the Pliocene rock standard TML (Jar #3) show a long-term reproducibility of ±0·06%, ±0·19% and ±0·23% 2
, respectively (Pietruszka et al., 2002).
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL...RESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Whole-rock trachytic pumices
The seven whole-rock trachytic pumices display a wide range of 87Sr/86Sr (0·7049–0·7062) but a narrow range of 143Nd/144Nd (0·51272–0·51274), 176Hf/177Hf (0·28280–0·28281) and 206Pb/204Pb (19·92–19·98) isotope ratios (Table 1). The Nd and Pb isotopic compositions of the whole rocks are within the range of those reported by Widom et al. (1997)
for basalts flanking Fogo volcano (143Nd/144Nd = 0·51273–0·51280 and 206Pb/204Pb = 19·83–19·91), whereas the range in Sr isotopic compositions of the whole rocks extends to more radiogenic values than those of the flanking basalts (87Sr/86Sr = 0·7046–0·7055) (Fig. 3). The 87Sr/86Sr ratios in the Fogo A whole rocks exhibit an inverse correlation with Sr concentration (Fig. 4), with the most differentiated (lowest Sr concentrations) samples exhibiting the highest 87Sr/86Sr ratios.
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) trachytic glasses (
) and individual sanidine crystals (open rectangles). Sr, behaving compatibly as a result of feldspar fractionation, shows a significant increase in concentration from the bottom to top of the sampled Fogo A deposit [data from Widom et al. (1992)Whole-rock syenite xenoliths strontium isotope compositions
Measured whole-rock Sr isotopic compositions and Sr concentrations for syenite xenoliths are reported in Table 1. The 87Sr/86Sr ratios of 12 whole-rock powders range from 0·7048 to 0·7067. The most radiogenic syenites represent the most radiogenic material associated with the Fogo A deposit.
Glass and sanidine strontium isotope compositions
Glass separates from six samples exhibit a range in 87Sr/86Sr ratios from 0·7048 to 0·7052 (Table 2), and, similar to the whole rocks, the 87Sr/86Sr ratios are inversely correlated with Sr concentrations. With the exception of the least radiogenic sample, the whole rocks are more radiogenic than the corresponding glass separate, and the overall range in 87Sr/86Sr ratios of the glasses is much smaller than that of the whole rocks (Fig. 4). The sanidine crystals exhibit a range in 87Sr/86Sr ratios both between and within samples, with the full range (0·7048–0·7062) found within a single sample (ASM-201) (Table 2). The most radiogenic sanidine has a higher 87Sr/86Sr ratio than the most radiogenic whole-rock trachytic pumice and glass separate, but is less radiogenic than the most radiogenic syenite. The Pb isotopic compositions of all of the sanidine crystals are indistinguishable from the whole-rock hosts.
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Glass thorium and neodymium isotope compositions
Six glass separates exhibit a range in measured Th activity ratios, shown as (230Th/232Th), from 0·8724 to 0·8795. This variation is only slightly outside analytical uncertainty (±0·0026). However, the (230Th/232Th)o values corrected for eruption age (denoted by the subscript) range from 0·8737 to 0·8841 (Table 2) and are inversely correlated with Th concentrations, with the most evolved samples having the lowest (230Th/232Th)o. The (230Th/232Th)o values were calculated based on the measured (230Th/238U) of each sample and an eruption age of 4600 years BP (Shotton et al., 1969
). The (230Th/232Th)o values of the Fogo A glasses are consistently lower than those reported for Azores basalts (0·89–1·4) (Turner et al., 1997; Widom et al., 1997; Claude-Ivanaj et al., 2001). The 143Nd/144Nd ratios of the six glass separates do not vary outside analytical uncertainty. Although the averaged 143Nd/144Nd ratio of the six glasses (0·512767) is higher than the averaged 143Nd/144Nd ratio of the seven whole-rock pumices (0·512737), this difference is within our analytical uncertainty.
Leached trachytic glass strontium isotope compositions
Leaching experiments were performed to investigate the source of the high 87Sr/86Sr component(s) in the Fogo A trachytic pumices. Because of the Sr isotope heterogeneity amongst the sanidine crystals within individual pumices, it was not possible to identify the source of the high 87Sr/86Sr component by leaching the whole-rock pumices in total. Therefore, the leaching experiments were conducted on two hand-picked glass separates including both the least radiogenic (ASM-218) and one of the most radiogenic (ASM-210) pumices. The hand-picked glasses were powdered by mortar and pestle. Four separate leaches were conducted on both samples: (1) 10 min in 5 ml high-purity water in an ultrasonic bath; (2) 10 min in 5 ml 1N HCl in an ultrasonic bath; (3) 10 min in 5 ml 4N HCl in an ultrasonic bath; (4) 15 min in 5 ml 4N HCl in an ultrasonic bath followed by addition of four drops of concentrated HF with continued sonication for 5 min. At the end of each leach step the leachate was removed and saved, and the samples were rinsed thoroughly with high-purity water, combining the rinse with the leachate. The leachates and final powder residue were then dried and spiked for isotope dilution analyses. The results are presented in Table 3 and Fig. 5. The percentage of Sr removed during each leach step is similar between both samples and 77% of the total Sr remained in the powder residues. The leachable radiogenic Sr appears to have been removed by the end of the second leach (1N HCl), as the two subsequent leachates and the powder residues have the same 87Sr/86Sr ratios for each sample. The two powder residues retain slightly different final Sr isotopic compositions (ASM-210, 0·70504; ASM-218, 0·70493) from one another (Fig. 5) and are the same as those of the respective glass separates cleaned with 2N HCl (Table 2). Thus, the variations in 87Sr/86Sr in the evolved glass separates are not signatures of post-eruptive alteration, but rather indicate isotopic variability within the pre-eruptive magma chamber. These results also suggest that at least part of the radiogenic signature exhibited by the whole-rock trachytic pumices is due to post-magmatic surface contamination, as this easily leachable radiogenic Sr component must be present in the whole rocks.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL...RESULTSDISCUSSIONCONCLUSIONREFERENCES |
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The observed isotopic signatures in the glass separates and single sanidine crystals are a key to elucidating the magmatic processes that preceded the Fogo A eruption. Significant observations that need to be addressed include the following: the whole-rock trachytic pumice Nd, Pb and Hf isotopic compositions are constant throughout the deposit; the glass separates exhibit a small range in 87Sr/86Sr ratios that varies outside analytical error; the glass separates exhibit a narrow range in (230Th/232Th)o; the sanidine phenocrysts exhibit a range in 87Sr/86Sr ratios both within and between samples, and many of these phenocrysts are more radiogenic than both the whole rock and glass from within a given sample. In the discussion that follows, we attempt to address all of these observations.
Pre- vs post-eruptive signatures
Several studies have identified post-magmatic alteration as a mechanism affecting the Sr isotopic compositions of evolved volcanic rocks (i.e. Halliday et al., 1984; Cousens et al., 1993; Kar et al., 1998). Halliday et al. (1984) suggested post-eruptive alteration of glass by high 87Sr/86Sr groundwater or fluids as a mechanism for introducing Sr isotopic disequilibrium between glasses and feldspars in aged glassy pyroclastic rocks from the Bishop Tuff. Similarly, Kar et al. (1998) proposed that the high 87Sr/86Sr in the trachytic and rhyolite flows and pumice from Ascension is generated by subsolidus interaction of the felsic rocks with geothermal fluids. This interpretation is based on whole-rock and feldspar separate Sr isotopic data. The 87Sr/86Sr ratios of the feldspars are similar to those of the mafic rocks on the island, and acid leaching of the whole rocks removed their radiogenic signature (Kar et al., 1998). As demonstrated in the leaching experiments of silicic ignimbrites by Cousens et al. (1993), Rb and Sr can be mobilized during subsolidus processes while leaving Nd and Pb isotope ratios undisturbed. Alteration of the 87Sr/86Sr ratios will be stronger in highly evolved rocks with low Sr concentrations.
In the Fogo A deposit, surface accumulation of salts derived from sea spray (87Sr/86Sr = 0·709) probably contributes to the leachable radiogenic Sr signature of the glass separates, but must be combined with an additional easily leachable source of Sr (possibly from meteoric water) to explain our leaching results (leachate 87Sr/86Sr 
0·7059). Despite the fact that low-Sr glass separates from Fogo A exhibit the highest 87Sr/86Sr ratios, there is no evidence in our leaching data to suggest that the observed range in 87Sr/86Sr ratios of the glass is due to post-eruptive alteration. The glass separates were cleaned with 2N HCl for 10 min, whereas results of our leaching experiments indicate that a 10 min leach in 1N HCl adequately removed all leachable radiogenic Sr. We have demonstrated that extensive post-eruptive alteration cannot explain the Sr isotope variations in the Fogo A glass separates. Therefore, we now consider possible pre-eruptive mechanisms to explain the observed Sr and Th isotope variations, including both closed-system and open-system processes.
Closed- vs open-system processes
Closed-system processes
Simple closed-system crystal fractionation does not appear to be a viable explanation for the Sr isotopic compositions of the Fogo A trachytic glasses. Although the glass separates exhibit a positive slope on a Rb–Sr isochron diagram, they do not display an isochronous relationship (r2 < 0·81). In addition, within a given sample, the sanidine phenocrysts have 87Sr/86Sr similar to or higher than the whole rocks and glass, yet have Rb/Sr either similar to (ASM-218) or lower than (ASM-201 and 205) the host glass. Consequently, the radiogenic 87Sr/86Sr ratios of the sanidines compared with the glasses cannot be the result of radiogenic ingrowth. The negative correlation of (230Th/232Th)o with (238U/232Th) in the glass separates is also inconsistent with closed-system crystal fractionation; radiogenic ingrowth of 230Th following closed-system crystal fractionation would result in the opposite correlation.
Open-system processes
Sr isotope data on phenocrysts and glass separates can provide important information regarding the relative timing of open-system processes. Isotopic disequilibrium between phenocrysts and more radiogenic host glass is indicative of assimilation or contamination of the magma subsequent to phenocryst formation, whereas glass and phenocrysts with similar isotopic compositions (in an isotopically zoned tephra) indicate that contamination of the magma occurred prior to the growth of the phenocrysts. Sr isotope disequilibrium between phenocrysts and matrix has been identified in numerous cases in continental and oceanic settings, but in all of these cases, the Sr isotopic composition of glass is more radiogenic than the phenocrysts (Halliday et al., 1984; Wörner et al., 1985; Palacz & Wolff, 1989; Kar et al., 1998).
Sr isotopic disequilibrium between Fogo A glasses and sanidines
Our data indicate significant isotopic disequilibrium between phenocrysts and glass in the Fogo A trachyte deposit, based on the analysis of individual sanidine crystals and unaltered glasses. However, unlike the cases of the Bishop Tuff, Laacher See and Grandilla pumice cited previously, the glass in the Fogo A trachyte has 87Sr/86Sr either the same as or lower than the sanidine phenocrysts.
One possible mechanism causing the high 87Sr/86Sr ratios in some of the sanidines could be in situ radioactive ingrowth of 87Sr, by which crystals initially in radioactive equilibrium with the liquid become more radiogenic as a function of their residence times in the magma chamber and their Rb/Sr ratios. The maximum amount of time over which this could occur can be assumed to be 200 kyr, based on the age of the oldest dated material associated with the Fogo volcano. Sr diffusion in sanidine at magmatic temperatures should have a negligible effect on the 87Sr/86Sr ratios of these crystals over 200 kyr, based on the low diffusion rates reported by Cherniak (1996). Using 200 kyr as the maximum allowable time, the measured 87Rb/86Sr ratios for each of the sanidines, and the 87Sr/86Sr ratio of the respective glass separates, the maximum 87Sr/86Sr ratios attainable by in situ radioactive decay were calculated (0·704896–0·705049). Because the measured 87Sr/86Sr ratios of many of the sanidines (up to 0·706208) are far higher than the calculated maximum 87Sr/86Sr ratios that could result from in situ radioactive ingrowth over the life span of the Fogo volcano, the Sr isotope compositions of the sanidine crystals cannot easily be explained by this mechanism alone.
A potential source of radiogenic Sr for the sanidines is syenite within the volcano's intrusive complex, as sampled in the form of xenolithic syenite nodules within the Fogo A deposit. Although the least radiogenic syenite in the Fogo A deposit has the same Sr isotopic composition as the least radiogenic Fogo A glass and sanidine (87Sr/86Sr = 0·70484), and is interpreted to represent the uncontaminated pre-eruptive Fogo A Sr isotope composition, other syenites range in 87Sr/86Sr up to 0·70697. The existence of quartz with highly saline (salt crystal-bearing) fluid inclusions has been interpreted as evidence for interaction of the syenites with high-salinity, aqueous fluids (Widom et al., 1993; E. Widom, unpublished data, 1990). Although Larson et al. (2001) have shown that meteoric water can dominate hydrothermal fluids in an ocean island setting, the hydrothermal system in an ocean island setting may represent a mixture of seawater and meteoric fluids (Kar et al., 1998), which would provide a potential source of radiogenic Sr.
Incorporation of high 87Sr/86Sr sanidine crystals (up to 200 ppm Sr) from radiogenic, hydrothermally altered syenite similar to those observed (or similar older plutonic rock) by relatively low Sr Fogo A liquid (1–2 ppm Sr) could account for the elevated Sr isotopic compositions of some of the whole-rock trachytic pumices. For example, one sanidine of typical size (20 mg) incorporated as a xenocryst from syenite 207 s (87Sr/86Sr = 0·70697) into 2 g of glass (liquid) with 87Sr/86Sr = 0·7051 could raise the 87Sr/86Sr ratio of the whole rock to 0·706.
Isotopic variation in trachytic glasses
Although the range in Sr isotopic composition of the trachytic glass (0·7048–0·7052) is much smaller than the range exhibited by the whole-rock trachytic pumices (0·7049–0·7062), the range in Sr isotopic compositions of the glasses is significantly outside analytical error. Potential sources for radiogenic Sr in a young (<200 ka) ocean island setting are limited owing to the lack of radiogenic continental crust. However, seawater, marine sediments, altered oceanic crust and aged and/or hydrothermally altered rocks (including syenites) making up the volcanic edifice are possible candidates.
Sr, Nd, Pb and Hf isotopic data do not allow the potential contaminants to be uniquely identified (Fig. 6). Mixing paths were calculated assuming that the range in Sr concentration of the glasses was present prior to the contamination event, and using the least radiogenic 87Sr/86Sr ratio of the glasses. Although these models allow for constraints to be placed on the amount of contamination involved, the mixing paths of altered oceanic crust, seawater and marine sediments are indistinguishable in the range of interest (Fig. 6).
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The range in (230Th/232Th)o exhibited by the trachytic glass separates (0·8737–0·8841) cannot be explained by contamination of the Fogo A magma chamber by seawater, hydrothermally altered mid-ocean ridge basalt (MORB), or hydrothermally altered basalt from the volcanic edifice. Unreasonably large amounts of seawater are required to modify the (230Th/232Th)o of the Fogo A magma because seawater Th concentrations are extremely low (
9 x 10-6 ppm) (Emsley, 1998). Constraints on assimilation of other contaminants may be made based on the correlation of (230Th/232Th)o with 87Sr/86Sr and elemental concentrations. The inverse correlation of (230Th/232Th)o with 87Sr/86Sr is suggestive of assimilation resulting in a decrease in (230Th/232Th)o and a corresponding increase in 87Sr/86Sr (Fig. 7a). Both (230Th/232Th)o and 87Sr/86Sr also correlate well with major elements. The correlation of (230Th/232Th)o with Al2O3 is consistent with assimilation–fractional crystallization (AFC) processes in which fractional crystallization of sanidine causes a decrease in Al2O3 and assimilation is responsible for the decrease in (230Th/232Th)o (Fig. 7b).
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Crustal lithologies characterized by low (230Th/232Th)o and high 87Sr/86Sr, relative to the most evolved trachytic glass, must be evaluated as potential contaminants. The (238U/232Th) ratios of altered oceanic crust can range from 1·7 to >20 (Staudigel et al., 1995
). Old altered oceanic crust (in radioactive equilibrium) thus should have (230Th/232Th) >1·7, significantly higher than that required of the Fogo A contaminant, which must be <0·8737. Thus, altered MORB cannot be considered a likely contaminant of the Fogo magma chamber. The (238U/232Th) values of some young basalts from the Azores (0·797–1·13) are similar to those measured on the trachytic glasses (Turner et al., 1997; Widom et al., 1997; Claude-Ivanaj et al., 2001), and thus with aging and growth towards radioactive equilibrium could develop (230Th/232Th) sufficiently low to be considered as potential contaminants. However, older submarine Fogo basalts would be expected to experience U enrichment during seawater alteration; thus they also would be expected to have (230Th/232Th) higher than that of the Fogo A contaminant. Subaerially altered edifice basalts may have experienced preferential U loss similar to subaerially altered basalts at Mauna Loa (Cohen et al., 1996). Assimilation of such material by the Fogo A magma could produce the observed decrease in (230Th/232Th), but should not result in the observed increase in 87Sr/86Sr ratios, as subaerial alteration is unlikely to produce substantially elevated 87Sr/86Sr ratios in basalts.
The (230Th/232Th) of aged wall rock syenite (0·5–1) (Widom et al., 1993) and marine sediments (0·3–1·3) (Ben Othman et al., 1989; McDermott & Hawkesworth, 1991) are sufficiently low to cause a decrease in the (230Th/232Th) of the Fogo A magma via contamination (Fig. 8a). Calculated AFC models (DePaolo, 1981) indicate that the observed variations in the Sr and Th concentrations in the Fogo A glasses could be produced by 70% crystallization combined with a moderate assimilation rate (r = 0·075) of either marine sediment or syenite wall rock (Fig. 8b, inset). However, combined fractional crystallization and assimilation of marine sediment produces rapid changes in the Sr isotopic signature (Fig. 8b), and cannot simultaneously account for both the observed Sr and Th concentration and isotopic variations. Unreasonably high (>1) Th/Sr ratios in the assimilating sediment over a wide range of assimilation rates (r = 0·02–0·1) would be required to reproduce the trachytic glass data. In contrast, the observed Sr and Th concentration and isotopic variations can be achieved via 69% crystallization combined with 5% bulk assimilation of the syenite wall rock (r = 0·075) (Fig. 8b). Although the temperature of the syenite is poorly constrained, these data can be reproduced using the energy-constrained AFC model of Spera & Bohrson (2001).
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Additional constraints on assimilation may be made based on
18O from two glass separates (18O = 6·2 and 6·3 ± 0·30
for ASM-204 and ASM-210, respectively), which indicate that there is no variation in 18O despite significant differences in (230Th/232Th)o and 87Sr/86Sr (0·7048 and 0·7051). The measured oxygen isotopic compositions of the glasses are an upper limit for the 18O of the magma, because post-eruptive exchange will result in elevated 18O even in petrographically pristine glass (Muehlenbachs, 1987; Cousens et al., 1993). However, even 5% assimilation of sediment should result in elevation of 18O of the more evolved trachytic pumices to at least 7·0, owing to the high 18O of marine sediments (15–25) (Eiler et al., 1997). Sediment assimilation thus cannot explain the radiogenic Sr and low (230Th/232Th)o of the evolved glasses. In contrast, 5% assimilation of syenite would not significantly affect the magmatic 18O, based on preliminary 18O in Fogo syenites (2·6 to 5·6 ± 0·30). These results therefore also suggest that syenite, rather than marine sediment, is probably responsible for the observed Sr and Th isotope variations.
Multi-stage open-system evolution model
We propose that three processes are responsible for the Sr isotope variations observed within the Fogo A trachytic pumice deposit. Two magmatic processes are proposed to explain the variations in Sr and Th isotopic compositions of the glasses and the radiogenic signature exhibited by the sanidines. These two processes are divided into Stage 1 and Stage 2 (Fig. 9). Stage 1 involves hydrothermal alteration by seawater-derived fluids of the syenite wall rock and contamination of the Fogo A magma by melts from this altered rock. Hydrothermal alteration of the wall rock results in elevated 87Sr/86Sr ratios (
0·707) while leaving the (230Th/232Th) unchanged. Assimilation of the wall rock is required to explain the Sr and Th isotope variability in the glass. Although the syenitic wall rock is thought to be the plutonic equivalent of the pre-eruptive Fogo A magma and older trachytic liquids, we suggest that the Fogo A magma chamber experienced a complex thermal history, which caused remelting of the syenites. Multiple episodes of basaltic volcanism occurred on the flanks of the Fogo volcano prior to the Fogo A eruption, providing a heat source for remelting and assimilation of the syenite wall rock into the Fogo A magma. This contamination is envisioned to have occurred concurrent with the development of the chemical zonation via crystal fractionation within the magma chamber. Stage 2 involves incorporation of xenocrystic sanidines from the previously altered wall rock, probably during the eruption, and accounts for the variable and highly radiogenic sanidine Sr isotopic compositions. The origin of the radiogenic sanidines described here may be analogous to those from the Bandelier Tuff described by Wolff et al. (2002). The third process is post-eruptive surface alteration, which results in the elevation of the whole-rock and glass 87Sr/86Sr ratios, without affecting phenocrysts.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGSAMPLES AND ANALYTICAL...RESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Whole-rock trachytic pumices show variation in Sr and Th isotopic compositions that can be attributed to melting and assimilation of hydrothermally altered wall rock, incorporation of radiogenic xenocrystic sanidine from the syenite wall rock, and post-eruptive alteration. Sr and Th isotopic compositions of the trachytic glasses are controlled by contamination of the magma by hydrothermally altered wall rock that had low (230Th/232Th) as a result of aging and whose Sr isotopic compositions have been influenced by seawater. The compositionally variable and radiogenic sanidines are explained by incorporation of xenocrystic sanidine from disaggregation of hydrothermally altered (high 87Sr/86Sr) syenitic wall rock. Because of the low Srglass/Srsanidine ratios, only one or two crystals of radiogenic sanidine (150–300 µg each) per 2–4 g of glass would elevate the 87Sr/86Sr ratios of the whole rocks.
Our results emphasize that magmatic systems whose chemical variations appear fully consistent with closed-system processes (e.g. Widom et al., 1993) may nevertheless have experienced complex open-system processes. Our results also emphasize that isotopic analyses of whole rocks can be misleading if interpreted as ‘liquid’ compositions. To gain a detailed understanding of petrogenetic processes, it is critical to obtain isotopic data on glass and mineral separates. Our results also highlight the fact that multiple open-system processes may operate in a given magmatic system, and different processes may be recorded in mineral vs glass compositions.
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ACKNOWLEDGEMENTS |
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M. Horan and T. Mock are thanked for assistance with all isotope analyses at the Department of Terrestrial Magnetism. Mark Reagan, John Wolff and Dennis Geist provided thorough and insightful comments on an earlier draft. Financial support for this project was provided by the Geological Society of America (student research awards to D.S. 2000 and 2001), Sigma Xi (Grants-in-Aid of research awards to D.S. 2000 and 2001) and the National Science Foundation (NSF EAR 0207529 and EAR 9902956 awarded to E.W.)
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FOOTNOTES |
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* Corresponding author. Telephone: (513) 529-3217. Fax: (513) 529-1542. E-mail: snyderdc{at}muohio.edu
Present address: Department of Geological Sciences, 5500 Campanile Drive, San Diego State University, San Diego, CA 92182, USA. 
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