Journal of Petrology Advance Access originally published online on March 28, 2006
Journal of Petrology 2006 47(7):1413-1437; doi:10.1093/petrology/egl016
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Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions
1 INSTITUT DES SCIENCES DE LA TERRE D'ORLÉANS, UMR 6113 CNRS-UO 1A RUE DE LA FÉROLLERIE, 45071 ORLÉANS, FRANCE
2 ENVIRONMENT CENTRE, LANCASTER UNIVERSITY LANCASTER LA1 4YQ, UK
RECEIVED NOVEMBER 9, 2004; ACCEPTED MARCH 6, 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL AND ANALYTICAL...RESULTSSULPHUR CONTENT OF CRUSTALLY...DISCUSSIONCONCLUSIONSREFERENCES |
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Many basaltic flood provinces are characterized by the existence of voluminous amounts of silicic magmas, yet the role of the silicic component in sulphur emissions associated with trap activity remains poorly known. We have performed experiments and theoretical calculations to address this issue. The melt sulphur content and fluid/melt partitioning at saturation with either sulphide or sulphate or both have been experimentally determined in three peralkaline rhyolites, which are a major component of some flood provinces. Experiments were performed at 150 MPa, 800–900°C, fO2 in the range NNO – 2 to NNO + 3 and under water-rich conditions. The sulphur content is strongly dependent on the peralkalinity of the melt, in addition to fO2, and reaches 1000 ppm at NNO + 1 in the most strongly peralkaline composition at 800°C. At all values of fO2, peralkaline melts can carry 5–20 times more sulphur than their metaluminous equivalents. Mildly peralkaline compositions show little variation in fluid/melt sulphur partitioning with changing fO2 (DS
270). In the most peralkaline melt, DS rises sharply at fO2 > NNO + 1 to values of >500. The partition coefficient increases steadily for Sbulk between 1 and 6 wt % but remains about constant for Sbulk between 0·5 and 1 wt %. At bulk sulphur contents lower than 4 wt %, a temperature increase from 800 to 900°C decreases DS by 
10%. These results, along with (1) thermodynamic calculations on the behaviour of sulphur during the crystallization of basalt and partial melting of the crust and (2) recent experimental constraints on sulphur solubility in metaluminous rhyolites, show that basalt fractionation can produce rhyolitic magmas having much more sulphur than rhyolites derived from crustal anatexis. In particular, hot and dry metaluminous silicic magmas produced by melting of dehydrated lower crust are virtually devoid of sulphur. In contrast, peralkaline rhyolites formed by crystal fractionation of alkali basalt can concentrate up to 90% of the original sulphur content of the parental magmas, especially when the basalt is CO2-rich. On this basis, we estimate the amounts of sulphur potentially released to the atmosphere by the silicic component of flood eruptive sequences. The peralkaline Ethiopian and Deccan rhyolites could have produced 1017 and 1018 g of S, respectively, which are comparable amounts to published estimates for the basaltic activity of each province. In contrast, despite similar erupted volumes, the metaluminous Paraná–Etendeka silicic eruptives could have injected only 4·6 x 1015 g of S in the atmosphere. Peralkaline flood sequences may thus have greater environmental effects than those of metaluminous affinity, in agreement with evidence available from mass extinctions and oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...RESULTSSULPHUR CONTENT OF CRUSTALLY...DISCUSSIONCONCLUSIONSREFERENCES |
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A detailed understanding of the solubility of sulphur in silicate melts is important for many geological processes (O'Neill & Mavrogenes, 2002
): the origin of magmatic sulphide ores, the geochemical behaviour of the chalcophile trace elements, including the platinum group elements and the Re–Os isotopic system and their use as tracers of core–mantle and crust–mantle differentiation in the Earth and other planets or asteroids (e.g. Kracher & Wasson, 1982; Jana & Walker, 1997; Chabot, 2004). Sulphur solubility along with melt–vapour partitioning also determines sulphur degassing during volcanic activity and the potential effects on climate variability.
Most studies of sulphur solubility, cited by Carroll & Webster (1994), O'Neill & Mavrogenes (2002) and Jugo et al. (2005), have focused on mafic and intermediate melt compositions. Our understanding of S behaviour in silicic magmas is more restricted and comes mainly from work on Fe-poor, metaluminous compositions [mol. (CaO + Na2O + K2O) > Al2O3 > (Na2O + K2O)], of the type commonly associated with volcanic arcs (Carroll & Rutherford, 1985; Luhr, 1990; Scaillet et al., 1998; Keppler, 1999; Scaillet & Evans, 1999; Clemente et al., 2004; Costa et al., 2004). Such compositions are not directly applicable to the magmatism of continental extensional zones, where the rhyolites tend to be alkali- and Fe-rich and either metaluminous or peralkaline [mol. (Na2O + K2O) > Al2O3] (Macdonald et al., 1992; King et al., 1997). The alkali/alumina balance of silicate melts is known to profoundly affect their geochemical and physical behaviour (Mysen, 1988). Most notable among those effects are the increase in Fe3+/Fe2+ redox ratio (Gwinn & Hess, 1989; Moore et al., 1995; Gaillard et al., 2001) and water solubility (Dingwell et al., 1997), and the decrease of viscosity (Scarfe, 1977; Baker & Vaillancourt, 1995; Dingwell et al., 1998) and liquidus temperatures (Bailey & Schairer, 1966) as peralkalinity increases. These important differences imply that insight gained from the study of sulphur in metaluminous silicate melts is of little help in anticipating its behaviour in peralkaline rhyolites.
Although many flood basalt provinces seem to be dominated by basaltic lavas, several recent studies have emphasized that some have significant volumes of associated silicic rocks (Bellieni et al., 1986; Harris & Erlank, 1992; Ewart et al., 1998, 2004; Ayalew et al., 2002; Bryan et al., 2002; Peccerillo et al., 2003). The objective of the present study is to evaluate the sulphur yield potentially delivered to the atmosphere by silicic flood eruptions. Given that peralkaline rhyolites can largely dominate over metaluminous types in some silicic provinces (Ayalew et al., 2002), we currently lack the fundamental information on which to base any such evaluation. We have, therefore, performed melt solubility and fluid/melt partitioning experiments for sulphur in peralkaline rhyolites from the Kenya Rift Valley, which broadly typify the felsic end-member of bimodal associations in rift-related settings (Bellieni et al., 1986; Harris & Erlank, 1992; Ewart et al., 1998, 2004; Ayalew et al., 2002; Bryan et al., 2002; Peccerillo et al., 2003). We combine these data with thermodynamic and mass-balance calculations to evaluate the sulphur contents of potential sources of felsic magmas associated with flood basalts, considering two end-member cases for the origin of the silicic end-member: fractional crystallization of flood basalt and partial melting of dehydrated lower crust. We show that the attainment of peralkaline conditions dramatically increases the sulphur-carrying capacity of rhyolite magmas. We then estimate the sulphur yields of some silicic flood sequences. The results appear to be significant for the current debate on the volcanic origin of some of the main mass extinctions (e.g. Wignall, 2001; Morgan et al., 2004).
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EXPERIMENTAL AND ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...RESULTSSULPHUR CONTENT OF CRUSTALLY...DISCUSSIONCONCLUSIONSREFERENCES |
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We have performed sulphur solubility and fluid/melt partitioning experiments on three well-characterized, peralkaline rhyolitic obsidians from the Naivasha area of the Kenya Rift Valley (Table 1). Two (ND, SMN) are mildly peralkaline comendites [(Na2O + K2O)/Al2O3 = 1·05 and 1·31, respectively from the Greater Olkaria Volcanic Complex and were previously used in phase equilibrium studies (Scaillet & Macdonald, 2001
, 2003). The third, EBU, is a pantelleritic obsidian from a welded fall deposit of the Eburru Volcanic Complex, immediately north of Olkaria. It is more strongly peralkaline than the comendites (NK/A = 1·88) and represents a composition that could have been generated by 50% crystallization of Olkaria-type comendites similar to SMN (Scaillet & Macdonald, 2003). These starting melt compositions (Table 1) encompass the entire peralkalinity range displayed by rhyolites associated with flood basalts (Trua et al., 1999; Ayalew et al., 2002; Peccerillo et al., 2003). We restricted our investigations to low-pressure conditions (150 MPa), thought to be relevant to the production of alkali rhyolites (Mahood, 1984; Scaillet & Macdonald, 2001) and we focused on the role of oxygen fugacity (fO2), which exerts a strong control on sulphur behaviour in silicic magmas (e.g. Carroll & Webster, 1994; Scaillet et al., 1998; Keppler, 1999).
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The experimental procedures are similar to those used in previous studies performed at the ISTO experimental petrology laboratory, such as that by Scaillet et al. (1998)
and Clemente et al. (2004). For each composition, batches of starting glass + sulphur powders with fixed sulphur contents were prepared by weighing 200 mg of glass powder with appropriate amounts of elemental sulphur (1–12 mg added sulphur). The resulting mixture was thoroughly mixed in an agate mortar for several minutes. Most experiments used ND, SMN and EBU batches with bulk sulphur contents of c. 1 wt % (1·01, 1·19 and 1·08 wt %, respectively; see Table 1). However, three series with EBU composition were performed with additional bulk sulphur contents of 0·5, 2·01, 3·85 and 6·14 wt % S, so as to explore the effect of sulphur fugacity at fixed P and T and fH2. We used Au capsules, which were loaded with the glass + sulphur powder (c. 20–30 mg, Table 1) plus weighed amounts of distilled water (c. 10 wt %, Table 1), and welded shut with a graphite arc welder. Weighing before and after welding, as well as after run completion, was used as a monitor for any volatile loss that occurred at P and T. For all charges used in this study, capsule weights remained constant to within 0·2 mg. We used either cold seal pressure vessels (CSPV) fitted with an H2 membrane (runs at 800°C) or an internally heated pressure vessel (IHPV) with a drop quench setting (900°C). Errors in quoted temperatures and pressures are ±8°C, and 2 MPa, respectively. The fO2 was varied using various Ar–H2 mixtures as the pressurizing gas. The H2 fugacity was either read with an H2-membrane (800°C) or known from previous experiments that used the same Ar–H2 ratio and that were run with an H2 membrane (900°C). Errors on H2 fugacities are ±0·01 MPa (800°C), or ±0·5 MPa (900°C), the latter determined by repeat experiments performed with the IHPV while fitted with a H2 membrane. The fO2 of each charge was computed using the dissociation reaction of water (Robie et al., 1979), the fH2 and the water fugacity (fH2O). The fH2O of each charge (Table 1) was computed assuming ideal behaviour in the fluid [i.e. aH2O = XH2Ofl, XH2Ofl being the mole fraction of H2O in the fluid and aH2O the activity of water defined as the ratio fH2O/fH2O°, fH2O° being the fugacity of pure water at P and T as given by Burnham et al. (1969)] and using the calculated fluid composition as determined from mass-balance constraints. The fluid composition was calculated knowing the amounts of dissolved water and sulphur, the amount of sulphur locked into solid phases (sulphide and sulphate) and the bulk content of water and sulphur loaded into the capsules. The water content of quenched melts (glasses) was determined using the by-difference method employing appropriate sets of hydrous glass standards of known H2O contents (Scaillet & Macdonald, 2001). For charges with c. 1 wt % bulk sulphur, the calculated XH2Ofl of the equilibrium fluid (Table 2) is close to, or higher than, 0·9, with the result that the calculated fO2 is only marginally lower than that corresponding to pure H2O (the difference is <0·05 log units). For those charges we estimate that the uncertainty on fO2 is <0·1 log units. In contrast, charges having bulk sulphur contents in excess of 1 wt % have calculated fluid compositions that are significantly poorer in H2O, with XH2Ofl going down to 0·68 (Table 2). As a result, the value of fO2 for the sulphur-rich charges is lower than that corresponding to the pure H2O case. However, for a fixed set of P–T–fH2 conditions (i.e. series EBU-9 in Table 1), the difference in calculated fO2 between the sulphur-poor and sulphur-rich charges does not exceed 0·3 log units (Table 1). The main source of uncertainty in S-rich charges comes from the estimate of the amount of H2O dissolved in the melt. An error of 1 wt % absolute (i.e. 4·5 wt % instead of 5·5 wt %) produces a change in calculated XH2Ofl of about ±0·07, which in turn induces a change in the calculated log fO2 of about ±0·07 units. Overall, for the sulphur-rich charges we estimate that the fO2 is known to within 0·3 log units.
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Because we use elemental sulphur as a source of sulphur, and not either sulphide or anhydrite (e.g. Luhr, 1990
), the initial redox state of the charges is grossly out of equilibrium, as the sulphur in the fluid must be complexed either with O2 to form SO2 or with H2 to produce H2S, which are the two dominant fluid species in the fluid at P and T, depending on fO2 (see Clemente et al., 2004). As long as H2 from the vessel does not diffuse across the capsule walls (i.e. below 600°C), the redox state of the charges will be internally controlled to some value that will depend on factors such as the ratio of H2O to S loaded, the amount of atmospheric O2 present in the capsule, and the rates of H2O and S dissolution into glass/melt. At 800°C, however, the kinetics of H2 diffusion even within Au is so fast that osmotic equilibrium in H2 is attained within a few tens of seconds across the capsule walls (Scaillet et al., 1992), and maintained at a fixed value owing to the large H2-buffering capacity of the vessel volume (Schmidt et al., 1995). Both the CSPV and IHPV attain the target temperature in about 20 min, so that the period elapsed at low-temperature conditions, during which transient redox state may occur, is short relative to the total run duration (minimum of 96 h, Table 1). Redox equilibrium will be reached as soon as the dissolution process of volatiles into melt is achieved. Using appropriate diffusivities for H2O and S [taken at 800°C as 10–7 cm2/s and 10–9 cm2/s, respectively (Watson, 1994)], it can be calculated that volatile dissolution occurs in a matter of minutes (H2O) to a few hours (S) owing to the finely powdered and well-mixed nature of our starting material, which minimizes the diffusion length scale of volatiles to a few tens of microns. None of the charges produced in this study displayed textural evidence of redox disequilibrium, such as sulphide rimmed by sulphate or vice versa, suggesting that either the charge has no time to record redox states radically different from that imposed at run conditions, or any evidence of such variations in fO2 during the heating-up period has been erased upon run completion.
Run products were characterized first by optical inspection with a metallographic microscope and then by electron microprobe analysis (EMPA). Observation under reflected light prior to carbon-coating allowed the oxide to be distinguished easily from sulphide owing to their contrasted colours (grey for oxide vs yellowish for sulphide), as well as shape (rectangular to equant shape for oxide vs pentagonal to rounded shape for smaller individual sulphide grains). Investigation under transmitted light and crossed polars allowed straightforward identification of anhydrite in oxidized charges because of its high birefringence. In anhydrite-bearing charges, anhydrite was fully enclosed by glass, suggesting that it grew from the melt and is not the result of back-reaction of fluid upon cooling, in which case it should have partially filled open cavities representing former gas bubbles. In all cases, optical identification of minerals was confirmed by subsequent EMPA, although owing to their generally small size, most analyses of minerals were contaminated by glass. We used the following conditions to determine the composition of glasses. For major elements, the conditions were: accelerating voltage 15 kV, sample current 6 nA, counting time 10 s on peak for all elements, and a beam defocused to 5 µm. Na and K were analysed first and a ZAF correction procedure applied. Correction factors for Na loss were based on analyses of synthetic peralkaline rhyolitic compositions and a set of metaluminous rhyolitic and dacitic glasses, all of known water content as determined by Karl Fischer titration. Between six and 10 analyses were performed for each glass. The water content of quenched glasses varied between 5 and 6 wt %, except for the two charges with c. 6 wt % sulphur, which displayed slightly lower values (between 4 and 5 wt %). Owing to the large uncertainties of the by-difference technique in determining the H2O content of quenched glasses (e.g. Devine et al., 1995), in this work we use a fixed melt water content of 5·5 wt % for all charges. The Na+K/Al ratios of quenched glasses do not show any significant departure from that of the starting materials (Table 2). The concentration of total sulphur in glasses was determined by EMPA using three synthetic hydrous dacitic glasses containing 750, 1400 and 1900 ppm sulphur (determined by wet chemistry) as standards (Clemente et al., 2004). The EMPA was run with the following conditions: accelerating voltage 15 kV, sample current 50 nA, beam diameter 10 µm, and a counting time of 60 s. The background was determined by analysing a dry rhyolitic glass without sulphur, using the above analytical procedure. The detection limit under these analytical conditions is about 80 ppm. The sulphur data given in Table 2 are averages of 5–16 analyses.
Apart from oxide and sulphur-bearing phases, no other mineral was detected. Two S-bearing phases crystallized: pyrrhotite at NNO <1 and anhydrite at NNO >1. At high fO2, an oxide co-precipitated with anhydrite. Knowing the amount of sulphur-bearing phases crystallized, and the amount of sulphur dissolved in the melt, the sulphur content of the coexisting fluid is calculated by difference with the known bulk sulphur content, using stoichiometric FeS and CaSO4 as sulphide or sulphate minerals (e.g. Scaillet et al., 1998). Because, depending on fO2, sulphide or sulphate + oxides were the sole phases crystallizing (Table 1), the maximum proportions of sulphur-bearing minerals can be determined by the changes in either FeOtot (fO2 < NNO) or CaO (fO2 > NNO + 1) of the quenched glass. This obviously assumes that neither iron or calcium is transported into the fluid (or lost to the capsule in the case of iron) but, under our experimental conditions, sulphur was never detected in Au capsules. If either of these elements is partitioned into the fluid, the amounts of sulphide or sulphate calculated from variations in FeO and CaO abundances in glass will be overestimated, with the consequence that there will be too much sulphur locked in solid phases. As a result, the calculated partition coefficient will be lower than its real value. Conversely, as stated above, we assume that pyrrhotite is end-member FeS. Pyrrhotite departure from FeS stoichiometry can be up to Fe5S6. An Fe5S6 stoichiometry would decrease our calculated partition coefficients, as it maximizes the amount of sulphur tied up with iron in pyrrhotite and this sulphur is, therefore, no longer available to the fluid. For charges saturated in anhydrite, a problem also arises from the low bulk CaO content of the starting rocks. Generally those charges have melts with CaO contents close to the detection limit. The fact that this extreme depletion in CaO goes along with a decrease in melt Cl content relative to the starting value (see Table 2) suggests that not all the Ca complexes with sulphur to form anhydrite, but that some goes into the fluid, possibly as CaCl2 species. The concentration of CaO in charges run at low fO2 remains close to the starting value, although some depletion does occur, possibly also as a result of Ca complexing with Cl. We recognize that partition coefficients of sulphur between fluid and melt determined in this way (by default) can be affected by a number of errors, as illustrated below. However, there is, unfortunately, no straightforward way of assessing this parameter in hydrothermal experiments. In particular, measuring the H–O–S fluid compositions upon quench is unlikely to retrieve the correct numbers, as back-reactions within fluid can alter both its speciation and composition (L. Baker, personal communication, 1996 20XX).
As an example we consider charge ND1 whose partition coefficient, DS, calculated using the above assumptions, is 307 (Table 1). This charge consisted of 20·2 mg of silicate powder with 1·01 wt % S (0·2 mg), and 1·9 mg of H2O. The bulk FeO is 1·81 wt % (0·37 mg) and after the run the hydrous glass (5·5 wt % or 1·11 mg H2O) has 0·39 wt % FeOtot (0·08 mg). The difference in FeO content in glass before and after the run (0·2867 mg) implies that 0·00399 millimoles of Fe is sequestered in pyrrhotite (0·2867/71·85). Assuming stoichiometric pyrrhotite this implies in turn that 0·128 mg (32 x 0·004) of sulphur is locked up in pyrrhotite. Knowing that the glass has 268 ppm dissolved sulphur, corresponding to 0·0054 mg sulphur, this leaves 0·07 mg sulphur for the fluid (or 0·074 mg H2S). The amount of water in the fluid is 0·79 mg (1·9–1·11), which implies that the mole fraction of H2O in the fluid, XH2O, is 0·95, and that the amount of fluid at P and T is 0·864 mg (0·79 + 0·074), which corresponds to 3·9 wt % fluid in the system [0·864/(20·2 + 1·9)], the fluid having 8·2 wt % sulphur. The amount of pyrrhotite is 1·65 wt %, calculated on the basis of condensed phases only (hydrous glass + pyrrhotite).
We now explore the individual effect of the main parameters that affect the calculated DS, namely glass iron content, pyrrhotite stoichiometry, iron loss toward the capsule, and glass water content. If, instead of 0·39 wt % FeO, the glass contains 0·49 wt % (i.e. the amount of iron is allowed to increase by 1
of EMPA), this increases DS from 307 to 342. Similarly, if instead of FeS a stoichiometry of Fe5S6 is taken (1·2 mole of S for 1 mole of Fe), this means that 0·15 g of sulphur is locked into pyrrhotite, which decreases DS to 202, or by 30%. Alternatively, if we assume that the Au capsule has dissolved 100 ppm Fe [corresponding to a loss of 8 wt % of iron relative to bulk content, as observed in supra-liquidus charges by Scaillet & Macdonald (2004)], then this will increase DS to 394. Finally, if the amount of H2O dissolved is 4 wt % instead of 5·5 wt % (which would correspond to the possible melt H2O content of the charges with 6 wt % sulphur), DS decreases to 227. There is clearly a considerable uncertainty on our fluid/melt partition coefficients, yet each parameter taken in isolation affects DS by less than 40% when allowed to vary within a reasonable range. Our assumptions of stoichiometric pyrrhotite and fixed melt water content, if incorrect, lead to an overestimation of the calculated DS. Conversely, other assumptions (in particular no Fe or Ca loss toward the fluid), if properly evaluated, would yield an underestimation of partition coefficients (i.e. real values are higher than the values listed in Table 2). This is, in fact, the main source of uncertainty in the present work, as we have no control on the amount of dissolved fluid species other than H2O and S. However, the work of Scaillet et al. (1998), using the same procedure, yielded partition coefficients for sulphur in silicic arc magmas that agree within a factor of two with those derived independently from remote sensing of volcanic plumes. We note in passing that this study explored the effect of CO2 and concluded that this volatile species has no detectable effect on the partition behaviour of sulphur between fluid and melt in silicic magmas in the fO2 range explored. We, thus, conclude that the partition coefficients reported here are accurate to within 50%, and the proportions of sulphide/sulphate are known to within 15%.
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RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...RESULTSSULPHUR CONTENT OF CRUSTALLY...DISCUSSIONCONCLUSIONSREFERENCES |
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The experimental conditions together with run products and phase proportions are listed in Table 1. The melt compositions together with the calculated partition coefficients are listed in Table 2. Variations in melt sulphur content (Smelt) with fO2 (here expressed in log units notation relative to the NiNiO solid buffer, such that NNO – 1 means one log unit below NNO) are shown in Fig. 1a. Also plotted is the Smelt of synthetic metaluminous silicic melts held under similar P and T and bulk S content (Sbulk) conditions (Luhr, 1990
; Scaillet et al., 1998). The sulphur solubility of rhyolite melts is strongly dependent on their NK/A ratio, in addition to fO2. At 800°C, under reduced conditions (<NNO), Smelt rises from c. 50 ppm in metaluminous rhyolite to nearly 1000 ppm in strongly peralkaline rhyolite (Fig. 1a). In the two less peralkaline compositions (ND and SMN), an increase in fO2 produces a smooth increase in Smelt, which broadly conforms with observed behaviour in other silicate melt compositions (e.g. Carroll & Webster, 1994). In contrast, in the most peralkaline composition EBU, the increase in Smelt peaks at around NNO + 1 and then sharply decreases, so that at higher fO2 the relative order of sulphur enrichment for the three peralkaline rhyolites is opposite to that observed below NNO. The enhanced sulphur solubility of peralkaline rhyolites at low fO2 is in part related to the higher iron content (Table 2), which complexes with S (Carroll & Webster, 1994), but may also be due to the greater proportion of oxygen anions in peralkaline melts relative to metaluminous varieties, as oxygen anions are believed to substitute for S (Carroll & Webster, 1994) via the following reaction:
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), this could be one explanation for their enhanced sulphur solubility at low fO2. At high fO2, the opposite, and thus peculiar, trend displayed by EBU warrants further discussion. The above reaction predicts that an increase in fO2 decreases the sulphur solubility, if again it is assumed that fS2 does not change across the interval of fO2 shown in Fig. 1. However, rigorously evaluating the reason for such a trend would require us to know the exact values of intensive parameters other than fO2; namely, fS2 and the activities of oxygen and sulphur anions in the melt, all of which is unknown in our experiments (and in almost all experimental work so far done at high P on aluminosilicate melts of geological interest). One possibility would be that our oxidized experiments with the EBU composition did not reach anhydrite saturation, and, thus, that the anhydrite in these charges represents a quench phase and that the drop in solubility observed at high fO2 simply reflects the fact that sulphur is being increasingly partitioned toward the fluid as fO2 increases. We do not agree with such an interpretation because of the textural evidence given above. In addition, the experimental conditions adopted in our study are similar to those of other experimental work aimed at defining the stability of anhydrite in silicate melts, in particular hydrous melts (e.g. Carroll & Rutherford, 1985; Luhr, 1990; Scaillet et al., 1998; Scaillet & Evans, 1999; Clemente et al., 2004), as are the criteria used to define anhydrite stability domains in magmas. In other words, if anhydrite is a quench product in our experiments, so it is in other experimental studies, in which case conclusions derived from those experiments about the stability field of anhydrite in magmas should be revised. Rather, we suggest that the trend shown by the EBU melt composition is a real feature that reflects the fundamental structural changes that occur in strongly peralkaline silicic liquids relative to metaluminous types, as noted in the Introduction, and the control that fO2 exerts upon them. By analogy with what we observe at low fO2, we can only speculate that the increase in fO2 dampens the role of oxygen anions in peralkaline melts, which results in a decreased Smelt (at fixed fS2).
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More generally, if sulphur solubility in hydrous silicic melts is a simple linear function of their iron content (at low fO2), as conventional wisdom would predict, then there would be no reason to have the huge difference between metaluminous and peralkaline compositions that we document here (Fig. 1a). The very fact that Smelt increases by a factor of c. 5 between standard metaluminous rhyolitic and slightly peralkaline melts with similar iron contents indicates that the FeO content is not the sole parameter affecting sulphur behaviour in the studied compositions, unlike in dry basaltic systems (O'Neill & Mavrogenes, 2002
). This is also shown by the three series of experiments carried out with various amounts of sulphur, in which the melt iron content decreases whilst the amount of sulphur dissolved in the melt increases (Table 2, series EBU 7, 8 and 9). For instance, at 900°C, the melt with 969 ppm dissolved sulphur has an FeO content of 6·32 wt %, whereas that with 2254 ppm of sulphur has an FeO content of 1·52 wt %. This trend is clearly due to our procedure of adding elemental sulphur, which forces the system to crystallize FeS, which in turn removes FeO from the melt. It shows, however, that the FeO content and dissolved sulphur in hydrous silicic melts are not simply correlated. In this respect, the experimental study of Clemente et al. (2004) has clearly shown that in metaluminous rhyolites, the iron control on sulphur solubility is not straightforward, and that variations in fS2, fH2S and fSO2 affect predominantly Smelt. This study has shown that in hydrous silicate melts, the Smelt can be modelled as the result of two dissolution reactions, one involving H2S and the other SO2, as proposed by Burnham (1979). Given the hydrous character of our experiments, it can be anticipated that the fugacities of sulphur-bearing species will exert a role, possibly a predominant one, on Smelt in peralkaline rhyolites as well. Direct comparison of our results with experimental studies bearing on the sulphur solubility of anydrous melts is, therefore, to a large extent meaningless (or of any anhydrous vs hydrous studies), because in the latter the dissolution reactions of S-bearing species cannot account for the role of H-bearing ones (i.e. H2S), leaving aside the profound structural changes produced by the incorporation of water into silicate melts. In particular, the presence of hydrogen opens the possibility that Fe is not the element with which S preferentially complexes in silicate melts, but instead that H plays that role, as proposed by Burnham (1979), based on theoretical and experimental considerations, a hypothesis that is in agreement with the findings of Clemente et al. (2004). In other words, because both the nature and abundances of dissolving species (H2S vs S2 or SO2) and the sulphur complexes in the melt (H2S vs FeS) are likely to differ between hydrous and anhydrous melts, there is no reason to expect that results gathered from the study of sulphur in anhydrous melts can be directly applicable to hydrous melts. If this statement could be rigorously demonstrated, there would be no reasons for performing the present experimental work. Whatever the mechanism of sulphur dissolution, our results show that at all values of fO2, the sulphur-carrying capacity of peralkaline rhyolites is 5–20 times greater than their metaluminous equivalents. Increasing the temperature to 900°C increases the Smelt by a factor of two, other parameters being kept equal (Fig. 2). Similarly, varying Sbulk from 0·5 to 6 wt % increases Smelt by 2–2·5 times, as a result of increasing sulphur fugacity (Fig. 2). The increase may also reflect the fact that the melt composition is changing when sulphur is added, as it removes part of the iron in solution to produce sulphide.
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Variations with fO2 in the sulphur content of fluid (Sfluid) over Smelt, or the partition coefficient DS, are shown in Fig. 1b for rhyolite melts having 1 wt % of Sbulk. Here again, the two less peralkaline compositions (ND, SMN) show little variation in DS with fO2, DS being
270, except at around NNO + 1 when anhydrite and pyrrhotite crystallize together, such as in charge ND5, which has a slightly lower DS (Table 1). In the most peralkaline melt (EBU), however, the partition coefficient rises abruptly at fO2 > NNO + 1, behaviour similar to that found in silicic magmas from volcanic arcs (Scaillet et al., 1998). The reason for the limited variation in DS in the two less peralkaline melts is that the increase in sulphur solubility with fO2 is compensated by crystallization of lower proportions of anhydrite at high fO2 relative to pyrrhotite at low fO2 (Table 1). In contrast, the increase in DS in the most peralkaline melt is because it crystallizes slightly higher proportions of sulphide at low fO2, whereas at high fO2 this composition does not crystallize anhydrite in higher modal amounts than ND and SMN. Because its solubility decreases beyond NNO + 1, this results in an overall increase in the partition coefficient as fO2 rises.
A comparison with experimental results obtained on a natural metaluminous dacite (Scaillet et al., 1998) shows that, under the same T–fO2 conditions, a peralkaline rhyolite (SMN) with bulk iron and sulphur contents similar to those in the dacite crystallizes 30–40 % less sulphide (Fig. 1c). Even the most peralkaline rhyolite, with 7 wt % FeOtot (EBU, Table 1), crystallizes less sulphide than the metaluminous dacite, which has 4·4 wt % FeOtot. We interpret this as resulting from the fact that, when held at the same fO2, the Fe2+/Fe3+ ratio is lower in peralkaline than in metaluminous melts (Gwinn & Hess, 1989; Gaillard et al., 2001), such that there is less Fe2+ available for sulphur complexation in the melt and consequently for sulphide crystallization. The lower modal proportion of sulphide in peralkaline rhyolites results in elevated fluid/melt partition coefficients, in particular for moderately peralkaline melts, even at low fO2, differing in this respect from metaluminous melts (Scaillet et al., 1998). The partition coefficient steadily increases for Sbulk between 1 and 6 wt %, but remains broadly constant for Sbulk between 0·5 and 1 wt % (Fig. 3). As for the solubility trend, this reflects an increase in fS2 perhaps coupled to a change in melt chemistry (which becomes iron-poor as the bulk sulphur content increases). At bulk sulphur contents lower than 4 wt %, a rise in temperature from 800 to 900°C decreases DS by only 10%, as a consequence of the modest increase of sulphur solubility with temperature (Fig. 3). Our results show, therefore, that any petrogenetic process favouring the formation of peralkaline over metaluminous silicic melts will minimize the possibility of sulphur loss via sulphide fractionation.
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The increased solubility of sulphur in peralkaline melts is consistent with the apparent scarcity of modal sulphides in peralkaline rhyolites. We are aware of three occurrences. Crisp & Spera (1987)
recorded scarce pyrrhotite microphenocrysts in comendites and pantellerites of the Tejeda volcano, Gran Canaria. Mungall & Martin (1996) noted rare pyrite phenocrysts in pantellerites of the Pico Alto volcano on Terceira Island, Azores. Lowenstern et al. (1993) found phenocrysts of pyrrhotite and molybdenite in pantellerites and pantelleritic trachytes from Pantelleria, Italy, the occurrence of molybdenite being apparently unique in a silicate magma. |
SULPHUR CONTENT OF CRUSTALLY AND MANTLE-DERIVED RHYOLITES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...RESULTSSULPHUR CONTENT OF CRUSTALLY...DISCUSSIONCONCLUSIONSREFERENCES |
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General
Before using the above experimental results to constrain the atmospheric sulphur yields of rhyolites associated with flood basalts, we need to fix possible limits on the bulk sulphur contents of rhyolites, which will depend on their source region. Two end-member petrogenetic models have been proposed bearing on sulphur enrichment in magmas: crystal fractionation of basaltic magmas, e.g. the Ethiopian ignimbrites (Ayalew et al., 2002
), and partial melting of the lower–middle continental crust (albeit with variable mantle input), e.g. the Paraná–Etendeka deposits (Bellieni et al., 1986; Garland et al., 1995; Harris & Milner, 1997; Ewart et al., 1998, 2004). The Ethiopian rhyolites are peralkaline, whereas the felsic magmas of the Paraná–Etendeka province are predominantly metaluminous, being in addition slightly less silicic and more K2O-rich than those in Ethiopia. Both types of magma have high iron contents, however, which is usually attributed to low fO2 during magma genesis (Ewart et al., 1998, 2004). Whatever their mode of origin, the sulphur content of silicic magmas is controlled by the source composition during partial melting and by subsequent evolutionary processes. Sulphur in a magma can reside in fluid, melt and solids, the last being mostly sulphides under the low fO2 thought to prevail in basalts and in the lower crust. In the following discussion we first consider a mantle origin (i.e. basalt fractionation) and then a crustal origin for rhyolites associated with trap sequences.
Mantle origin
Method
In basalts, sulphide stability largely dictates the possibility of generating a sulphur-rich silicic derivative because, given their high density, sulphides can easily settle out from the host magma. Thus, to produce a sulphur-rich rhyolite supposes that during fractionation the basalt does not crystallize significant amounts of sulphide. Generation of peralkaline rhyolites by fractionation of alkali basalt requires 80–90% crystallization (e.g. Barberi et al., 1975; Ayalew et al., 2002). Alkali basalts typically have bulk H2O contents of at least 1 wt %, CO2 contents sometimes higher than 1 wt %, and sulphur contents that can exceed 1000 ppm (Clocchiatti et al., 1992; Dixon et al., 1997; Bureau et al., 1999; Wallace, 2002). They evolve at fO2 
NNO (Dixon et al., 1997). We have, therefore, calculated the conditions under which alkali basalts become saturated in sulphide during crystallization, for bulk H2O–CO2 contents of 1–2 wt % as inferred from studies of modern analogues (Dixon et al., 1997; Gerlach et al., 2002; Lange, 2002), using a Sbulk of 0·1 wt %, and over a range of plausible redox conditions from NNO – 2 to NNO + 1 (Fig. 4). We also have calculated the corresponding proportions of bulk sulphur partitioned into sulphide and fluid phases (Fig. 5).
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Melt compositions used. We used the basalt–rhyolite sequence of the Boina centre in the Afar rift (Barberi et al., 1975
) as a proxy for the evolution of liquid compositions in a fractionating, mildly alkaline basalt series. Although we could have used a thermodynamic model to simulate this evolution, available models still fail to reproduce the transition from metaluminous to peralkaline melts, a feature that is critical for the understanding of sulphur behaviour, as shown below. The petrological and geochemical study of the Boina volcano concluded that the silicic magmas were derived from fractional crystallization of transitional basalt (Barberi et al., 1975). Five representative compositions were used here: G485, S52, D237, D210, D224B (Barberi et al., 1975) (see Table 2). The degree of crystallization was determined using the K2O content, assuming perfectly incompatible K2O behaviour during crystallization, a reasonable assumption except for composition D224B in which K-feldspar crystallizes, and thus for which the extent of crystallization relative to the parent basalt G485 is a minimum. The following proportions of residual liquids were calculated: S52, 73%; D237, 37%; D210, 24%; D224B, 18%. This natural liquid line of descent shows that peralkaline rhyolites can be produced from basalt crystallization after c. 80 wt % crystallization (Table 3, composition D224B). Temperatures were calculated assuming a linear relationship between melt fraction and T, and using a liquidus of 1200°C for basalt G485 and 890°C for the rhyodacite D210 (Table 3). Clearly, such an approximation is unlikely to be valid in detail. We are, however, interested in fixing the general behaviour of C–H–O–S volatiles in a crystallizing basalt, for which we need to know the approximate amount of residual melt available at each temperature in order to calculate the partitioning of each volatile species between melt and fluid using mass-balance constraints and solubility laws (see below). Any departure from a linear trend will either promote or delay the attainment of volatile and sulphide saturation. This will not affect, however, the general conclusion derived from our analysis, which concerns the importance of the role of CO2. In addition, we note that our assumption of linearity predicts a temperature of 861°C for the peralkaline rhyolite D224B, which is in reasonable agreement with available estimates for such compositions (Bizouard et al., 1980; Ayalew et al., 2002; Nekvasil et al., 2004). All calculations descibed below were performed for a pressure of 150 MPa, as our experimental results were collected at this pressure.
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Solubility and activity models. For a given initial volatile content and residual melt fraction, the amount of volatiles dissolved in the melt was calculated using the solubility models for H2O and CO2 of Dixon et al. (1995)
. In the absence of appropriate solution models for both H2O and CO2 in intermediate melt compositions, those for basalt were used. For sulphur we use the solubility model of Scaillet & Pichavant (2005). The latter is an empirical model that allows us to relate the melt sulphur content to the sulphur fugacity, for a variety of melt compositions, including hydrous basalts. This model was derived in an attempt to evaluate the behaviour of sulphur in hydrous mafic magmas, for which there are few experimental data [apart from those of Luhr (1990)]. The model has the form
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NNO and FFS are the referenced fO2 and fS2 against the Ni–NiO and Fe–FeS solid buffers respectively, Wi represents the weight % of oxide i, and a, b, c, d, e, f and gi are fitted parameters (Table 4) that were obtained by linear regression of the experimental databases of Luhr (1990), O'Neill & Mavrogenes (2002) and Clemente et al. (2004). The summation is carried out over all major oxides, including FeO, Fe2O3, OH– and H2O. This third-order polynomial function is necessary to reproduce the inverted bell-shaped pattern of sulphur solubility in silicate melts (e.g. Carroll & Webster, 1994; Clemente et al., 2004), and the crossed fO2–fS2 term is needed to take into account the effect of varying fS2 on the relationship between fO2 and S in melt (Clemente et al., 2004). This model encompasses a large SiO2 range (35–80 wt %), and reproduces measured fS2 within an average of 0·65 log unit, over more than 15 log units when normalized to the FFS solid buffer. Finally, it should be noted that the model of Scaillet & Pichavant (2005) is calibrated only for metaluminous compositions, which implies that sulphur solubilities of intermediate or silicic peralkaline compositions corresponding to any given fS2 are underestimated by this model and thus that sulphide activities calculated here for those melts are overestimated (see below).
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For any fO2 and fS2, the activity of FeS in the melt was calculated from the following equilibrium, using thermodynamic data from O'Neill & Mavrogenes (2002)
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method and an activity coefficient for FeO of 1·4 (O'Neill & Mavrogenes, (2002). For any melt in which the calculated FeS activity was found to exceed unity, the melt sulphur content was fixed to that corresponding to an aFeS = 1, and the excess sulphur was converted to sulphide (we thus neglect the role of additional elements such as Ni or Cu that could promote early saturation in sulphide).
The fluid phase composition was calculated using a Modified Redlich–Kwong type equation of state (MRK), using as input parameters fH2O, fCO2 and fS2. It must be stressed that, regardless of our current knowledge or body of experimental constraints on the sulphur content of fluids in magmas, the sulphur content of a fluid coexisting with a silicate melt can be calculated using a thermodynamic approach, provided that appropriate solubility laws exist for the main volatile species. This stems from the fact that in the C–O–H–S system, once fH2O, fCO2 and fS2 are fixed (at P and T), the fugacities of all other volatile species are fixed as well (Holloway, 1987; Scaillet & Pichavant, 2003, 2004). In other words, the composition of the fluid is uniquely defined, including its sulphur content. This allows us to compute the partition coefficients of sulphur between fluid and melt, as the melt sulphur content is fixed by fS2 (and fO2). The accuracy of such an approach relies, among other things, on our knowledge of the thermodynamic properties of C–O–H–S fluids, which appear to be reasonably well known (e.g. Shi, 1992), at least in the low-pressure range (<1000 MPa). Using this approach, Scaillet & Pichavant (2003) showed that there was a good overall agreement between calculated and measured fluid compositions for silicic arc magmas. The same method has been applied to active basaltic volcanoes (Scaillet & Pichavant, 2005), for which remote sensing of volcanic gases can be used to constrain the gas chemistry at depth. In this case too, generally good agreement is observed (Scaillet & Pichavant, 2005). Therefore, although we recognize that the current experimental database on the sulphur content of fluids of mafic magmas is almost non-existent, this gap can be partly circumvented by using a thermodynamic approach, which, when compared with independent estimates, appears to retrieve the correct order of magnitude in terms of the sulphur content of magmatic fluids.
Procedure. For any given initial H2O, CO2 and S contents, the equilibrium distribution of those volatiles between melt, fluid and sulphide was calculated via an iterative procedure by finding the fugacities that satisfy the following two sets of conditions: first, the condition of chemical equilibrium:
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). In this case, the sulphur was partitioned between melt and sulphide only.
Let us consider, as an example of the calculation procedure, the case of a basalt carrying only H2O and sulphur crystallizing at an fO2 of NNO – 2. At 890°C, the basalt has 24·46 wt % residual liquid with a composition of D210 (Table 3). If sulphur had a perfectly compatible behaviour (bulk S of 1000 ppm), then the melt composition D210 would have 4088 ppm of dissolved sulphur (1000/0·2446). Using the model of Scaillet & Pichavant (2005), this amount of dissolved sulphur would correspond to an fS2 of 0·035 MPa, or to an aFeS = 18 when calculated using equilibrium (2) and the assumptions given above. Clearly, there is too much sulphur in solution and some of it must be withdrawn to decrease the aFeS to unity, because by definition the activity of FeS cannot increase beyond unity (for appropriate standard states for solids and liquids at P and T). The next step is thus to remove the excess sulphur in solution in the melt until the calculated fS2 corresponds to an aFeS = 1, and convert that excess sulphur into immiscible/solid sulphide [considering also the constraints set out in equations (3)–(8)]. For that specific case, using equation (1), we calculate that saturation of melt composition D210 with sulphide (aFeS = 1, because we perform an equilibrium calculation, aFeS in the silicate liquid is equal to that in the sulphide) is achieved when fS2 = 10–4 MPa at 890°C and NNO – 2, which corresponds to a melt sulphur content of 850 ppm. In other words, this means that of the 1000 ppm sulphur dissolved in the molten basalt at 1200°C, 208 ppm (850 x 0·2446) are dissolved in the residual melt at 890°C, the remainder (792 ppm = 1000 – 208) being locked up in sulphide, as under these conditions no fluid is present [Table 3; the melt water content is below the saturation value of an andesitic melt at 150 MPa (Burnham, 1979)].The introduction of CO2 into the system promotes early fluid saturation and thus the sulphur content of melt at 890°C (i.e. when there is only 24·46 wt % residual melt) must be lower than when only H2O is present. For instance, implementing the example above with addition of CO2 (i.e. a basalt with 1 wt % H2O and 1 wt % CO2 at NNO – 2), we calculate that at 890°C the sulphur content of the residual melt is 635 ppm. Finally, as noted above, peralkaline melts dissolve more sulphur than metaluminous types, so our calculated sulphide proportions should be considered maxima for the most fractionated melt (D224B, Table 3). Assuming, on the basis of our experimental results, that the peralkaline melt D224B dissolves five times more sulphur than that calculated by the model of Scaillet & Pichavant (2004), or 4250 ppm, then it follows that such compositions would be barely saturated at NNO – 2, the proportion of sulphide being low. This implies that the amounts of CO2 needed to scavenge the sulphur toward the fluid, which under our P–T conditions is calculated to be 2 wt % (see below), must be considered maximum values where peralkaline derivatives are produced.
Results
The results of our calculations are shown on Figs 4–6. We have considered three cases corresponding to different bulk volatile contents: case (1), 1 wt % H2O and 1000 ppm sulphur; case (2), 1 wt % H2O, 1 wt % CO2 and 1000 ppm sulphur; case (3), 2 wt % H2O, 2 wt % CO2 and 1000 ppm sulphur. Figure 4a shows the evolution of aFeS of a basalt with degree of crystallization, calculated at four fO2 values. An H2O-bearing (1 wt %) but CO2-free alkali basalt crystallizing at or above NNO – 1 is sulphide-saturated after c. 60% of crystallization. At NNO – 2, saturation in sulphide is slightly delayed, to 70% crystallization. Figure 5a shows the evolution of the proportion of bulk sulphur sequestered in sulphide with degree of crystallization, corresponding to the calculations shown in Fig. 4a. By the time the residual melt is peralkaline (20–30% liquid; see Table 2), it can be seen that 90 wt % of Sbulk is locked up in sulphide (Fig. 5a). Further crystallization results in massive sulphide precipitation; after 82% crystallization more than 80 wt % of Sbulk is locked up in sulphide. The calculations corresponding to case (2) illustrate the role of CO2. When this volatile is introduced, the sulphur behaviour is dramatically altered because fluid saturation occurs at an early stage, owing to the low solubility of CO2 in silicate melts (e.g. Dixon et al., 1995) and chemical equilibrium demands that sulphur is also partitioned into the fluid, thus lowering Smelt and the activity of sulphide. With a bulk CO2 content of 1 wt % and 1 wt % H2O [case (2)], similar to the bulk content inferred for Kilauean or Etnean basalts (Clocchiatti et al., 1992; Gerlach et al., 2002), the calculations show that sulphide saturation is slightly delayed compared with the H2O-only case (Fig. 4). However, the main difference relative to the CO2-free situation [case (1)] is that after 80% crystallization, at least 60% of Sbulk is in the fluid phase, whatever the prevailing redox conditions, rising to 90% for fO2 = NNO + 1 (Fig. 5b). Finally, case (3) shows that at fO2 = NNO – 2, for initial CO2 and H2O contents of 2 wt % each, a crystallizing alkali basalt remains below sulphide saturation, even after 80% crystallization when derivative liquids are rhyolitic (Fig. 6). Under these conditions 95% of Sbulk is hosted by the fluid, even after 80 wt % of crystallization (Fig. 5b).
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We stress that such elevated bulk volatile contents have been inferred for some alkali basalts (Dixon et al., 1997
), including those in flood sequences (Lange, 2002). In addition, recent experimental data on the generation of silica-saturated alkalic suites from alkali basalts suggest that the latter contain at least 0·5 wt % H2O, and possibly up to 2 wt % (Nekvasil et al., 2004), which reinforces the view that trap basalts may be considerably richer in fluids than those at mid-ocean ridges. The above calculations show that, provided they are CO2- and H2O-rich, basaltic magmas can readily produce sulphur-rich derivatives, regardless of the imposed redox conditions. The amount of CO2 appears to be the key parameter, as it triggers early volatile exsolution, which prevents sulphide precipitation. Without CO2, fO2 exerts a prime control on the amount of sulphide crystallization in basalt with a certain initial sulphur content. A reduced CO2-poor basaltic magma will yield silicic derivatives that are sulphur-poor owing to extensive sulphide precipitation, whereas a basalt crystallizing at or above NNO + 1, as in arc settings, may give rise to sulphur-rich silicic magmas (Scaillet & Pichavant, 2003; Scaillet et al., 2003). The presence of CO2 in large amounts significantly reduces this fO2 control. If alkali basalts are CO2-rich, they can release >90% of their Sbulk during crystallization. After the stage of rhyolite production, the ultimate fate of sulphur will be dictated by the composition of the melt, particularly its NK/A ratio (in addition to fO2; Scaillet et al., 1998). As our experimental data show, metaluminous felsic magmas can carry less sulphur in solution than peralkaline types, especially at low fO2. We conclude that if alkali basalt fractionation produces peralkaline rhyolites, these may well be sulphur-rich, concentrating a significant proportion of the original sulphur content of the parent magma.
Crustal origin
Method
To constrain the sulphur content of silicic melts produced by partial melting of the lower crust, we have calculated the sulphur contents of (1) anatectic melts and (2) metamorphic fluids in the amphibolite–granulite facies. The sulphur content of crustal melts can be calculated using the solubility model for sulphur of metaluminous rhyolites derived by Clemente et al. (2004), as dehydration melting of mid- to lower crustal lithologies produces dominantly metaluminous or peraluminous silicic melts (Clemens & Vielzeuf, 1987). The ambient fS2 conditions during crustal melting are likely to be controlled by sulphide equilibria, such as pyrite–pyrrhotite (Poulson & Ohmoto, 1989). Under lower crustal conditions, this equilibrium fixes fS2 at c. 0·1 MPa (Shi, 1992). Therefore, the calculations were performed by fixing fO2, fH2O and fS2, at fO2 ranging from NNO – 2 to NNO + 2, which are likely to encompass redox conditions in the lower crust. We explored specifically the sulphur solubility for three melt H2O contents, 1, 2 and 3 wt %, as we are interested in anatectic melts that are relatively dry (see below). At any fO2, fixing the melt water content fixes fH2O [using the thermodynamic model of H2O solubility of Zhang (1999)] and thus fH2. Given that we set fS2 as constant, this allows us to calculate fSO2 and fH2S, which are then used as input parameters in the thermodynamic model of Clemente et al. (2004), from which the amount of sulphur dissolved in the rhyolitic melt arising from the additive contributions of H2S and SO2 species is derived. For metamorphic fluids, we have calculated the sulphur content of a C–O–H–S fluid phase likely to be present in the mid- to lower crust, assuming again an fS2 controlled by the pyrrhotite–pyrite equilibrium (i.e. fS2 = 0·1 MPa). To this end we have used the MRK equation of state introduced above and explored various fO2 conditions likely to encompass crustal redox states as well as various water activities (expressed as XH2O in Fig. 7b). As detailed for the basalt above, we fix fO2, fH2O and fS2 as input parameters to calculate the fluid phase composition. The temperature at which we calculate the composition of metamorphic fluids, 800°C, is lower than that considered for anatexis (1000°C), as we are interested in the amount of sulphur that can be transported in metamorphic fluids prior to widespread partial melting. In summary, as for the crystallization of basalt detailed above, we have calculated the equilibrium sulphur contents of a fluid and a rhyolitic melt, under P–T–fO2–fS2 conditions considered likely to prevail in the lower crust. By comparing the sulphur contents of crustally derived metaluminous melts with those of metamorphic fluids, we aim to constrain the overall behaviour of sulphur in hot mid- to lower crust flushed by melts or fluids.
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Results
Figure 7a shows that the sulphur content of silicic melts for melt water contents
3 wt % and fO2 in the range NNO – 2 to NNO + 1, all calculated for an fS2 of 0·1 MPa, does not exceed 200 ppm. In contrast, the sulphur content of metamorphic fluids under similar P–T–fO2 conditions is strongly dependent on fO2 and water fugacity, and it reaches several wt %, except for nearly dry conditions (Fig. 7b). From Fig. 7a, we conclude that silicic melts produced in the mid- to lower crust are unlikely to reach values of Smelt, and thus Sbulk if extracted from their source, much higher than 200 ppm. This value should be considered as a maximum for the following reasons. First, the sulphur content of the lowercrust is estimated to be c. 400 ppm (Wedepohl, 1995), which indicates a rather small sulphur buffering capacity during melting. Second, crustal melts associated with flood basalts record temperatures in the range 900°C to >1100°C (Bellieni et al., 1986; Harris & Erlank, 1992; Garland et al., 1995; Harris & Milner, 1997; Ewart et al., 1998, 2004), which has led to the suggestion that such felsic magmas were produced from previously dehydrated crust (Kilpatrick & Ellis, 1992). The dehydration event could have involved fluids and/or melts but in any case, in view of the sulphur content of such melts/fluids (Fig. 7), such a dehydration event is likely to have removed most of the sulphur budget of the crustal protolith. Subsequent partial melting of dehydrated lower crust is, thus, unlikely to produce sulphur-rich magmas.
Summary
We thus envisage basalt-derived rhyolites as magmas that can potentially carry several thousands of ppm of sulphur, either dissolved in the melt or in a separate fluid phase, with only minor amounts trapped in coexisting sulphide, especially when they are peralkaline. In contrast, the sulphur content of crustally derived melts is calculated to be less than 200 ppm, especially if formed from dehydrated lower crust.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...RESULTSSULPHUR CONTENT OF CRUSTALLY...DISCUSSIONCONCLUSIONSREFERENCES |
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Sulphur yields of Ethiopian, Deccan and Paraná–Etendeka silicic rocks
We first address the Ethiopian sequences for which the pre-eruptive sulphur contents are reasonably well known, melt inclusion analyses having yielded Smelt in the range 400–500 ppm (Ayalew et al., 2002
). The NK/A ratios of Ethiopian rhyolites range from c. 1 to 1·4, which corresponds to the two less peralkaline samples used in our experiments. Pre-eruptive temperatures have been estimated at 740–900°C. The pre-eruptive water content is not well known but similar rhyolites from the Ethiopian rift had melt H2O contents of 4–8 wt % (Webster et al., 1993). Redox states are poorly constrained, yet such rhyolites are generally inferred to evolve at fO2 < NNO (e.g. Mahood, 1984; Scaillet & Macdonald, 2001; White et al., 2005). High water contents are consistent with 80–90% crystallization of parent alkali basalts (Ayalew et al., 2002) with an initial bulk H2O content of 1 wt %. If fractionation occurs at shallow levels (<200 MPa), then H2O saturation of the liquid will occur after 80% crystallization if no CO2 is present, or earlier with CO2. At 150 MPa, a basalt with 1 wt % H2O initially can yield a rhyolite magma with 5–6 wt % dissolved H2O coexisting with 4–6 wt % fluid, provided that both melt and fluid have left their source. The amount of free fluid might well be larger, however, as this estimate does not include CO2. Using a conservative bulk initial S content of 1000 ppm for the parental basalt, then rhyolite produced after 90% crystallization will potentially carry up to 1 wt % sulphur. With this Sbulk and at fO2 < NNO, moderately peralkaline rhyolites dissolve 200–300 ppm sulphur at 800°C, or 400–500 ppm at 900°C (Fig. 1a). The sulphur content of the coexisting fluid phase ranges between 6 and 10 wt % (Table 1). The fact that the sulphur contents of both natural and experimental melts compare closely indicates that the chosen experimental conditions broadly correspond to those of magma storage before eruption.
We can now evaluate the potential sulphur delivery to the atmosphere by the Ethiopian rhyolites, whose estimated volume is 60 000 km3 (dense rock equivalent or DRE, Ayalew et al., 2002). Assuming that such magmas coexisted with 5 wt % fluid, the bulk sulphur erupted ranges from 4·3 x 1017 g for an Sfluid of 6 wt % to 7·3 x 1017g for Sfluid of 10 wt %. These estimates are one order of magnitude higher than previous figures (Ayalew et al., 2002), basically because the fluid contribution, in which 90% of the sulphur in melt + fluid is stored, has been taken into account.
Evaluation of the sulphur yield of the Paraná–Etendeka silicic eruptives is somewhat simpler. The magmas have been inferred to be nearly dry, on the basis of elevated temperature estimates and lack of hydrous phenocrysts (Ewart et al., 1998, 2004), indicating that they were probably not fluid-saturated at depth. This essentially restricts their atmospheric sulphur yield from melt degassing during ascent to the surface. Using an estimated volume of 20 000 km3 (Harris & Milner, 1997) and a conservative pre-eruptive Smelt content of 100 ppm, we calculate that these silicic magmas were capable of delivering no more than 4·6 x 1015 g of sulphur to the atmosphere when fully degassed, or almost two orders of magnitude less than the Ethiopian contribution, for comparable volumes of erupted magmas.
The Deccan flood basalts are associated with minor amounts of silicic rocks, estimated to reach 500 km3 (Lightfoot et al., 1987). Although a partial melting origin from basalt protoliths has been advocated (Lightfoot et al., 1987), the Deccan rhyolites share most of the major element characteristics of the Ethiopian sequences. In particular, they are strongly peralkaline, which is marked, inter alia, by very high Zr concentrations compared with metaluminous rhyolites. The high solubility of Zr in peralkaline rhyolites is a well-established feature (e.g. Watson, 1979; Linnen & Keppler, 2002; Scaillet & Macdonald, 2003).
Assuming that the sulphur yield of Deccan rhyolites scales with that of the Ethiopian rhyolites, between 3·6 x 1015 and 6·1 x 1015 g of sulphur could have been emitted. However, pre-erosional volumes of silicic magmas may have been much larger than the current outcrop (Bryan et al., 2002). Javoy & Courtillot (1989), for example, have suggested that a short-term variation in the seawater 87Sr/86Sr ratio at the Cretaceous–Tertiary boundary could have been due to intense silicic volcanism related to the Deccan basalts, and estimated their volume to about 50 000 km3, which is comparable with that of the Ethiopian rhyolites. If it is assumed that Deccan basalts were somehow able to produce peralkaline derivatives at the same yield as in Ethiopia (thereby producing roughly twice the Ethiopian silicic volumes), the Deccan rhyolites could have produced 0·9 x 1018 to 1·5 x 1018g of S, comparable with the estimate for the basalt emissions alone (Wignall, 2001).
Environmental consequences
The most common explanation for the formation of Large Igneous Provinces, including continental flood basalt sequences (Coffin & Eldholm, 1994), is that they result from periodic mantle–core instability that generates hot mantle plumes, which, via adiabatic melting, lead to a widespread but short-lived melting episode when the plume reaches shallow mantle levels. Much recent interest in flood basalt activity has stemmed from the recognition that there is a strong correlation between periods of major basalt eruption and worldwide environmental crises such as mass extinctions and oceanic anoxic events (e.g. Courtillot & Renne, 2003). A causal link between those two events has, therefore, been claimed (e.g. Vogt, 1972; Morgan, 1981; Courtillot, 1999; Rampino & Self, 2000; Courtillot & Renne, 2003). Establishing whether such a causal link indeed exists rests primarily on (1) obtaining precise timing constraints for the geological events and (2) evaluating the volatile contributions of flood events to the atmosphere, as it is thought that massive emissions of climate-sensitive volatile species are a major contributing factor to global climate change. In particular, the volcanic hypothesis of mass extinctions hinges critically on the injection of massive amounts of volcanic sulphur into the atmosphere, which, in addition to CO2, is one of the controlling factors of climate variability (Robock, 2000).
Whereas decisive progress has been made in recent years with respect to the effects of CO2 (see Courtillot & Renne, 2003), our ability to analyse the role of S emissions has been less successful. Sulphur emissions of past flood events have been largely evaluated using simple scaling arguments, which assume that present-day magmas have a sulphur content comparable with those erupted during flood eruptions, basically because, in most instances, it is the only approach that can be applied, especially when the events considered are old, in which case critical data needed to assess sulphur yield, in particular glassy melt inclusions, are lacking. For instance, the 1783–1784 Laki fissure eruption on Iceland and its climate aftermath (Métrich et al., 1991; Thordarson et al., 1996; Thordarson & Self, 2003) is often taken as a representative, yet smaller-scale, scenario for the environmental consequences of flood basalt emissions (e.g. Eldholm & Thomas, 1993; Stone, 2004). For silicic compositions, it is well established, however, that such an approach can lead to dramatic under- or over-estimations of the sulphur content of the original magmas (Scaillet et al., 1998). As shown for andesite and rhyolite in arc settings, the sulphur content at the time of eruption can differ by orders of magnitude between magmas having similar compositions but different redox states (Scaillet et al., 1998). The present study illustrates an additional factor to be considered when assessing the sulphur yield of silicic magmas; that is, their Na + K/Al ratio. For mafic magmas, given the vagaries of melt generation, storage and evolution in the mantle, there is no a priori reason to ascribe to flood basalts a common value for their sulphur content, and it remains to be established whether the scaling-up procedure yields correct numbers, although in most cases this is perhaps the only approach that we can use. Our analysis shows that the ultimate fate of sulphur in basalt will be heavily dependent on the bulk CO2 content of the system. It shows, though, that under near-liquidus conditions, most of the sulphur remains dissolved in the melt, even when the basalt coexists with considerable amounts of H2O and CO2. For instance, in Fig. 5b the case of a basalt at NNO – 2 with 2 wt % H2O + CO2 shows that at near-liquidus conditions (i.e. more than 80% melt) the magma has less than 10% of its sulphur hosted by the fluid, the remaining sulphur being dissolved in the melt, as no sulphide is present under such conditions (i.e. aFeS <1, Fig. 6). This implies that, for eruptions involving crystal-poor basaltic magmas, estimates of atmospheric sulphur yields relying on the difference in sulphur contents between melt inclusions and matrix glasses (i.e. estimates that ignore the fluid contribution) will obtain figures which are close to real values, as illustrated for recent basaltic eruptions for which independent estimates of sulphur yields using remote sensing methods are available (Sharma et al., 2004).
In addition to these evaluation problems, the causal link between volcanoes and climate has been criticized on several grounds (Wignall, 2001), and in particular on the fact that volatiles emitted during effusive basaltic eruptions are unable to reach the upper atmosphere. Although theoretical studies have shown that flood activity can lead to stratospheric loading of magmatic volatiles (Woods, 1993), the effusive nature of basalt is still considered as a major shortcoming of the flood-basalt driven mass extinction hypothesis (Wignall, 2001). This is unlike the case for their silicic counterparts, which, owing to their explosive nature, are able to result in major stratospheric sulphur loading with regional to hemispheric dispersion (Rampino & Self, 1992). However, all studies aimed at evaluating such a hypothesis have exclusively considered flood volcanic activity and its attendant environmental impact to be due to basalt emissions and have ignored the role of associated silicic magmas. In fact, the generation of copious amounts of silicic magmas by melting of continental crust should be an expected consequence of plume impingement at the base of the continental crust (Javoy & Courtillot, 1989). Recent detailed thermal modelling studies have indeed confirmed that widespread production of felsic magmas, either via crystal fractionation of basalt magma or via partial melting of the surrounding crust, is an inescapable by-product of basalt intrusion in the lower crust (Annen & Sparks, 2002; Annen et al., 2006). Thus, there exists the possibility that the overall volcanic events had a global, long-lasting, impact on climate through their silicic emissions. In this paper, we have attempted to evaluate the contribution of the silicic end-members of flood activity to atmospheric sulphur release and have shown that, in some cases at least, it may have been volumetrically as large as the associated basalts.
The Ethiopian Traps flood sequences coincided with a worldwide oceanic cooling event (Rochette et al., 1998; Touchard et al., 2003), whereas the Paraná–Etendeka eruptives are the only flood sequence not synchronous with a documented major environmental change or mass extinction event (Courtillot & Renne, 2003). Our results offer one explanation, namely the contrasted sulphur yields of the silicic activity. Besides this parameter, factors controlling eruption dynamics may also have played an important role. Water is critical in producing the high and sustained eruptive columns (Sparks et al., 1997) that are required for global dispersion of volcanic volatiles via injection of magmatic volatiles into the stratosphere. The Ethiopian rhyolitic magmas were possibly cooler and richer in water than the Paraná–Etendeka rhyolites. We, thus, speculate that in the former case, the conditions of explosive eruption were easily achieved, and the recent discovery of ash layers at several places in the Indian Ocean having the correct age and composition (Touchard et al., 2003) is circumstantial evidence that at least part of the Ethiopian rhyolites were emitted during highly energetic eruptions. In contrast, the high to very high pre-eruption temperatures inferred for the Paraná–Etendeka silicic deposits point to rather low magma viscosity which, together with their anhydrous character, could have favoured a mostly effusive regime (Kirstein et al., 2001), with much less potential for hemispheric stratospheric loading of its volatiles. Lastly, the recent thermal simulation results of Annen et al. (2006) of basalt intrusion at the base of the crust show that rhyolite production via fractionation of basalt occurs before that due to partial melting of the crust. In detail, the time interval between the onset of basalt intrusion (and emission) and the widespread production of silicic magmas is dependent on a number of factors (depth of intrusion, fertility of the crust, emplacement rate of basalt, ambient geotherm) and varies between 0·02 Myr and several million years in the case of slowly intruded mafic magmas. Yet, results show that rhyolite floods originated via partial melting will be erupted later than those produced by basalt fractionation. In addition to this incubation time, modelling shows clearly that the basalt fractionation mechanism for rhyolite generation has a higher productivity than partial melting of the crust. For instance, intrusion at 30 km depth of basalts with 2·5 wt % H2O, with 50 m thick sills injected every 10 kyr, is able to produce c. 18% of silicic melts via fractionation (18% relative to the total volume of intruded basalt) 500 kyr after the onset of the process, whereas, within the same time interval, partial melting of overlying amphibolite crust is able to yield only 3% of silicic melt (Annen et al., 2006, fig. 10). The above lines of evidence (higher sulphur yield and explosivity, shorter incubation time and greater productivity) all combine to suggest that flood rhyolites formed via basalt fractionation are more likely to have a greater environmental impact than those that result from partial melting of the crust.
The foregoing discussion suggests that magmatic traps associated with silicic emissions can be tentatively classified into two main groups: those having produced dominantly metaluminous magmas and those associated with peralkaline rhyolites sensu lato. Given the generally poor preservation of silicic flood sequences (Bryan et al., 2002), we can only speculate at this stage as to their classification. Silicic traps associated with the Karoo basalts are similar to those of Paraná–Etendeka (Harris & Erlank, 1992) and are associated with an extinction event that is minor considering the volume of erupted basalt (Wignall, 2001). On the other hand, the Emeishan Trap basalts, which are broadly coeval with the massive Permo-Triassic mass extinction, are associated with scarce silicic eruptives of possible peralkaline affinity (Xu et al., 2001). Given that most provinces display both metaluminous and peralkaline felsic rocks (Bryan et al., 2002), it is also important to evaluate the proportions of each magma type.
Finally, we note that melting of crust in back-arc settings may be a particularly favourable locus for the generation of voluminous amounts of silicic magmas with only minor basalts, e.g. the Sierra Madre Occidental in western Mexico and Chon Aike in South America (Bryan et al., 2002). Such rhyolites may have formed by crustal melting following protracted subduction zone magmatism, the latter having extensively hydrated the lower crust. Such a process could conceivably increase the sulphur content of the lower crust, given the high sulphur contents of arc magmas (Scaillet et al., 2003). Melting of such protoliths may produce silicic melts rich in water and perhaps in sulphur.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...RESULTSSULPHUR CONTENT OF CRUSTALLY...DISCUSSIONCONCLUSIONSREFERENCES |
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Our study shows that rhyolite magmas may act as an important conveyor of mantle sulphur toward the atmosphere, especially when their parental basalts are rich in volatiles, particularly CO2. We do not dismiss the important role played by the basaltic component in terms of volatile output. Our study simply suggests that, in addition to volumes of erupted basalt and the duration of peak activity, the environmental consequences of flood magmatism may or may not be greatly enhanced by their silicic component. When voluminous peralkaline derivatives are produced, their explosive emission may give the coup de grâce to the worldwide changes triggered by early basaltic emissions. We note that an alternative scenario to the conventional core–mantle origin of flood sequences is that trap basalts are rooted in an anomalously volatile-rich sublithospheric mantle (Anderson, 1994
). Although still scanty, available evidence suggests that trap basalts are indeed richer in volatiles than those erupted at oceanic ridges (e.g. Lange, 2002; Rodriguez Durand & Sen, 2004). Given the generally poor preservation of silicic members, in particular in old sequences, we suggest that future studies aimed at evaluating the environmental consequences of flood activity focus on the evaluation of the volatile contents of basalts. In addition, a better petrogenetic understanding of the processes leading to peralkaline derivatives in bimodal suites is highly desirable.
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ACKNOWLEDGEMENTS |
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Continuing discussions with Michel Pichavant and John Mavrogenes helped our understanding of important features related to the behaviour of sulphur in magmas. Thorough and helpful reviews were provided by Jim Webster, Malcolm Rutherford and an anonymous reviewer. The careful editorial handling of Marjorie Wilson and Alastair Lumsden is gratefully acknowledged.
*Corresponding author. E-mail: bscaille{at}cnrs-orleans.fr
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REFERENCES |
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