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Geology; March 2009; v. 37; no. 3; p. 259-262; DOI: 10.1130/G25355A.1
© 2009 Geological Society of America
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Magmatic anhydrite-sulfide assemblages in the plumbing system of the Siberian Traps

Chusi Li1,*, Edward M. Ripley1, Anthony J. Naldrett2, Axel K. Schmitt3 and Craig H. Moore1

1Department of Geological Sciences, Indiana University, Bloomington, Indiana 47401, USA
2Department of Geology, University of Toronto, Ontario M5S 3B1, Canada
3Department of Earth and Space Sciences, University of California–Los Angeles, Los Angeles, California 90095, USA

Correspondence: *E-mail: cli{at}indiana.edu.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 GEOLOGY OF THE KHARAELAKH...
 MAGMATIC ANHYDRITE
 SULFUR ISOTOPES
 POTENTIAL VOLCANIC SO2FLUX
 REFERENCES CITED
 
We report the first discovery of magmatic anhydrite-sulfide assemblages in a subvolcanic intrusion associated with the Siberian Traps. The {delta}34S values of anhydrite and coexisting sulfide crystals analyzed by ion probe are 18{per thousand}–22{per thousand} and 9{per thousand}–11{per thousand}, respectively. More than 50% of the total sulfur in the intrusion is estimated to derive from marine evaporites in the footwall strata. The contaminated magma was highly oxidized and able to dissolve up to one order of magnitude more sulfur than pure mantle-derived basaltic magma. Such contaminated magma, if erupted, would have released far more SO2 into the atmosphere than is generally appreciated.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
GEOLOGY OF THE KHARAELAKH...
 MAGMATIC ANHYDRITE
 SULFUR ISOTOPES
 POTENTIAL VOLCANIC SO2FLUX
 REFERENCES CITED
 
Volcanic volatiles released from flood basalts are composed pre-dominantly of H2O, H2S, SO2, and CO2 with minor halogens (Cl, F, and perhaps Br) (Self et al., 2005). H2S is readily oxidized to SO2 at surface conditions, and total volatile sulfur is commonly expressed as SO2. Although the annual flux of CO2 released from flood basalts is much smaller than the current anthropogenic flux (Self et al., 2005), it is still significant because the mean lifetime of CO2 n the atmosphere is much longer (many thousands of years) than generally appreciated (Archer, 2005; Lenton et al., 2006). Thus, millennia-scale events such as flood basalts can cause atmospheric buildup of CO2 and affect global climate. Studies of modern basaltic eruptions (Dartevelle et al., 2002; Chenet et al., 2005) and theoretical modeling (Jones et al., 2005; Oman et al., 2006) have revealed that the release of large quantities of volcanic SO2 into the atmosphere may have had devastating climate and environmental effects.

The Permian-Triassic mass extinction was the most severe mass extinction on Earth (Wignall, 2001). It occurred ca. 251 Ma (Bowring et al., 1998; Mundil et al., 2004), coeval with the largest known continental flood basalt province on Earth, the Siberian Traps (Kamo et al., 2003). Campbell et al. (1992) suggested that the eruptions of the Siberian Traps led to biotic extinction by the injection of large amounts of SO2 into the atmosphere. They argued that a thick sequence (5–5.5 km) of evaporites through which the magmas passed could have provided crustal sulfur to the magmas, thereby producing far more volcanic SO2 than a pure mantle source. Here we report mineralogical and sulfur isotopic evidence for magma-evaporite interaction in the Kharaelakh subvolcanic intrusion that is coeval with the Siberian Traps. We use available experimental constraints to estimate sulfur concentration in the contaminated magma and the total flux of SO2 from such magma if erupted.


   GEOLOGY OF THE KHARAELAKH INTRUSION
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGY OF THE KHARAELAKH...
MAGMATIC ANHYDRITE
 SULFUR ISOTOPES
 POTENTIAL VOLCANIC SO2FLUX
 REFERENCES CITED
 
The Kharaelakh intrusion, also referred to as the northwest Talnakh intrusion in the literature, is one of several sill-like, sulfide ore-bearing gabbroic intrusions that are coeval with the overlying flood basalts in the Noril'sk region, Siberia (Fig. 1). The total sulfide content of the Kharaelakh intrusion is estimated as 7 wt% (Czamanske et al., 1995). It intruded Devonian sediments that contain abundant marine evaporites with {delta}34S values ranging from 18{per thousand} to 22{per thousand} (all {delta}34S are reported versus Vienna Canyon Diablo Troilite: Gorbachev and Grinenko, 1973; Grinenko, 1985). Like all ore-bearing intrusions in the region, the sulfide ores in the Kharaelakh intrusion are characterized by high Pt (~7 ppm) and Pd (~25 ppm) (Naldrett et al., 1996), and high {delta}34S values ranging from 10{per thousand} to 12{per thousand} (Li et al., 2003), significantly higher than typical mantle-derived values of 0{per thousand} ± 2{per thousand}. This feature, together with high 87Sr/86Sr ratios of the host rocks (up to 0.709), has been considered to be evidence for evaporite contamination (Arndt et al., 2003). Variations in mineral chemistry (Li et al., 2003), whole-rock trace element concentrations, and Sr-Nd isotopes (Arndt et al., 2003) suggest that the Kharaelakh intrusion formed by injection of multiple pulses of magma. The Kharaelakh intrusion and other ore-bearing intrusions in the region have been interpreted to be conduits of flood basalts in almost all petrogenetic models proposed to date, although detailed correlations between the intrusive and extrusive rocks are still debated (Naldrett et al., 1996; Lightfoot and Hawkesworth, 1997; Arndt et al., 2003; Lightfoot and Keays, 2005).


Figure 01
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  		Figure 1. Kharaelakh intrusion, Siberia. A: Geological map. B: Cross section.

 

   MAGMATIC ANHYDRITE
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGY OF THE KHARAELAKH...
 MAGMATIC ANHYDRITE
SULFUR ISOTOPES
 POTENTIAL VOLCANIC SO2FLUX
 REFERENCES CITED
 
Magmatic anhydrite crystals found in the olivine-bearing (picritic) gabbros with disseminated sulfides from the Kharaelakh intrusion are characterized by planar boundaries with olivine (Fig. 2A) and augite (Figs. 2B and 2C). Dihedral angles of ~120°, which are characteristic of simultaneous crystallization, are common in anhydrite-augite assemblages (Figs. 2B and 2C). Inclusions of anhydrite in augite and vice versa (Fig. 2B) are also observed in the samples. Rounded and subrounded sulfide inclusions composed of pyrrhotite, pentlandite, and chalcopyrite that crystallized from immiscible sulfide liquid droplets in the magma are commonly present within anhydrite crystals (Fig. 2C). Visual estimates based on 5 polished thin sections indicate that the ratio of anhydrite to sulfide in the samples varies from 0.05 to 0.1.


Figure 02
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  		Figure 2. Photomicrographs of magmatic anhydrite-sulfide assemblages in Kharaelakh intrusion.

 

   SULFUR ISOTOPES
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGY OF THE KHARAELAKH...
 MAGMATIC ANHYDRITE
 SULFUR ISOTOPES
POTENTIAL VOLCANIC SO2FLUX
 REFERENCES CITED
 
The sulfur isotopic compositions of magmatic anhydrite-sulfide assemblages in the Kharaelakh intrusion, as determined by ion probe, are listed in Table 1. The {delta}34S values of anhydrite and coexisting chalcopyrite vary between 18{per thousand} and 22{per thousand} and between 9{per thousand} and 12{per thousand}, respectively. The sulfate-sulfide {Delta} values vary from 9{per thousand} to 12{per thousand}, corresponding to equilibrium temperatures of 1400–600 °C based on extrapolated fractionation factors given by Ohmoto and Rye (1979). The range of the calculated temperatures is larger than the ranges for basaltic magma and sulfide liquid, suggesting S isotopic disequilibrium in the samples. Sulfate-sulfide isotopic equilibrium is not crucial here; only the bulk {delta}34S value, which is elevated and very anomalous. High {delta}34S values for the magmatic anhydrite-sulfide assemblages are consistent with assimilation of evaporite footwall strata that have high {delta}34S values between 18{per thousand} and 22{per thousand} (Gorbachev and Grinenko, 1973; Grinenko, 1985). The textural relationships between anhydrite and spatially associated silicates and sulfides in the igneous rocks (Fig. 2) suggest that anhydrite crystallized from contaminated magma, and is not a remnant of incomplete digestion of country-rock xenoliths. Because the melting temperature of anhydrite (>1500 °C) is significantly higher than the temperature of the magma (1200–1300 °C), it is unlikely that the magma of the Kharaelakh intrusion could have assimilated anhydrite by melting. Chemical dissolution of anhydrite by sulfate-unsaturated magma is a more plausible mechanism.


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  		TABLE 1. {delta}34S VALUES OF MAGMATIC ANHYDRITE-SULFIDE ASSEMBLAGES IN THE KHARAELAKH INTRUSION, SIBERIA

 
Degassing may affect S isotope compositions in volcanic and subvolcanic systems. Ripley et al. (2003) discussed the possible effects of degassing on the S isotope compositions of volcanic rocks in the Noril'sk area. During degassing the S isotope compositions of magma are critically dependent on the temperature, pressure, the mass of the vapor lost, the composition of the vapor, and the fO2 condition, which controls speciation in the melt and vapor. It can readily be shown that only under highly oxidizing conditions [> {Delta}FMQ (fayalite-magnetite-quartz) + 2], where SO42– is the predominant species in magma, can elevated {delta}34S values of magma be produced due to degassing. Ripley et al. (2003) showed that the vast majority of volcanic rocks in the Noril'sk region are characterized by {delta}34S values <5{per thousand}. Although S contents indicated extensive sulfur loss via degassing, fO2 conditions were significantly below those required to generate large positive values. In addition, even under highly oxidizing conditions, loss of ~80% of the S in a basaltic liquid is required to increase the {delta}34S of a mantle-derived melt with an initial {delta}34S value of near 0{per thousand}, to values near 10{per thousand}. The high S contents of the sulfide ore-bearing intrusions such as the Kharaelakh intrusion indicate that they have certainly not degassed to the extent of the coeval lavas. It is therefore extremely unlikely that the elevated {delta}34S values found in sulfide ore-bearing sills of the Noril'sk region are a result of degassing.


   POTENTIAL VOLCANIC SO2FLUX
TOP
 ABSTRACT
 INTRODUCTION
 GEOLOGY OF THE KHARAELAKH...
 MAGMATIC ANHYDRITE
 SULFUR ISOTOPES
 POTENTIAL VOLCANIC SO2FLUX
REFERENCES CITED
 
In addition to total pressure, temperature, and composition, the concentration of sulfur dissolved in silicate melts at S saturation depends on the oxidation state, because that controls sulfur speciation. Results from measured wavelength shifts of sulfur K{alpha} X-rays (Carrol and Rutherford, 1988; Wallace and Carmichael, 1994) indicate that under reducing conditions (<{Delta}FMQ + 1.5) sulfur is dissolved predominantly as S2– (>90% of total sulfur), under oxidizing conditions (>{Delta}FMQ + 2) sulfur is dissolved as S6+ (>90% of total sulfur), and under transitional oxidation states (between {Delta}FMQ + 1.5 and {Delta}FMQ + 2) both S2– and S6+ species are important in silicate melts (Fig. 3). The maximum concentrations of sulfur dissolved in a basaltic magma at 1300 °C and 1 GPa total pressure are 0.11–0.18 wt% under reducing conditions and 1.1–1.8 wt% under oxidizing conditions (Fig. 3; Jugo et al., 2004). These experimental results provide useful constraints for estimating the degree of contamination in the Kharaelakh intrusion. The content of sulfur in mantle-derived magmas in continental rift settings such as the Deccan magmas is estimated to be ~0.14 wt% (Self et al., 2008). Assuming that the content of mantle-derived sulfur in the parental magma of the Kharaelakh intrusion is also ~0.14 wt% ({delta}34S, 0{per thousand}), mixing calculations indicate that ~0.7 wt% assimilation of anhydrite ({delta}34S, 20{per thousand}) from the evaporite footwall strata is required to account for the elevated {delta}34S values (~11{per thousand}, weighted average for magmatic sulfide and anhydrite in the five samples studied) in the Kharaelakh intrusion. The total amount of sulfur in the contaminated magma would be ~0.3 wt%. The occurrence of sulfide droplets as inclusions in anhydrite crystals (Fig. 2) suggests that the contaminated magma became saturated with immiscible sulfide liquid prior to anhydrite saturation as assimilation progressed. On cooling, anhydrite crystallized from the contaminated magma in response to decreasing solubility with temperature.


Figure 03
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  		Figure 3. Relation between S content in basaltic melt at S saturation and oxidation state. FMQ—fayalite-magnetite-quartz.

 
The available experimental results from Jugo et al. (2004) (Fig. 3) allow us to assume that the concentration of sulfur dissolved in the Kharaelakh magma prior to possible eruption is ~0.22 wt%. This value is close to the upper limit for a basaltic magma at sulfide saturation, but is only 20% of the lower limit for the magma at sulfate saturation (Fig. 3). Using this value and the density of magma with a composition similar to the average composition of the Kharaelakh-type (i.e., ore bearing) sills given by Zen'ko and Czamanske (1994), the total mass of sulfur dissolved in the Kharaelakh magma prior to possible eruption is estimated to be ~5.5 Tg/km3 of magma. More than 70% of the coeval lavas in the Noril'sk region contain <440 ppm (Brügmann et al., 1993; Ripley et al., 2003), which is equal to 20% of the calculated sulfur dissolved in the Kharaelakh magma. If erupted, 80% degassing of the sulfur dissolved in the Kharaelakh magma translates to ~10 Tg of SO2/km3 of lava, which is twice the value estimated for the Deccan magmas (Self et al., 2008).

A more conservative estimate can be made using the sulfur isotopic values of lava samples from the Siberian Traps. The {delta}34S values of sulfide minerals from 34 drill-core lava samples from the Siberian Traps analyzed by Ripley et al. (2003) vary from –5{per thousand} to 9{per thousand}. The majority of the samples have {delta}34S values between 0{per thousand} and 5.5{per thousand}, similar to the values of relatively S-poor intrusions that occur in a large area (~100 x 200 km2) in the region (Grinenko, 1985). The vertical intervals between the lava samples that have been analyzed for sulfur isotopes to date vary from 20 to 200 m. More detailed sampling is required to investigate whether high {delta}34S values similar to that of the Kharaelakh-type sills are present in the lava sequence of the Siberian Traps. Using the same parameters as above, 0.09 wt% anhydrite assimilation is required to increase the {delta}34S value in the magma from 0{per thousand} to 3{per thousand}. The content of sulfur in such contaminated magma is ~0.16 wt%, which is close to the assumed saturation level described above. Degassing of 80% sulfur from this magma translates to 6 Tg of SO2/km 3 of lava, slightly higher than the value estimated for the Deccan magmas (Self et al., 2008).

The total volume of the Siberian Traps is estimated to be ~4 x 106 km3 (Reichow et al., 2002), twice the volume of the Deccan Traps (Jay and Widdowson, 2008). The potential yield of SO2 from the Siberian Traps is thus estimated to be between 2 x 107 and 3 x 107 Tg. These values are two to three times the total yield of SO2 from the Deccan Traps (Self et al., 2008). The eruptions of the Deccan flood basalts temporally coincide with the mass extinction at the end of the Cretaceous Period (ca. 66 Ma; Courtillot and Renne, 2003), the severity of which is second only to the Permian-Triassic mass extinction of ca. 251 Ma (Bowring et al., 1998; Mundil et al., 2004), when the eruptions of the Siberian flood basalts took place (Kamo et al., 2003). Is the severity of mass extinctions on Earth correlated to the total mass of coeval volcanic SO2 released into the atmosphere? This is an intriguing question we hope scientists studying extinctions will explore.


   ACKNOWLEDGMENTS
 
We are grateful to Viktor Radko of the Noril'sk Nickel Company for supplying one of the samples used in this study. Thorough and constructive reviews by Chris Hawkesworth, Andy Saunders and an anonymous reviewer are appreciated. Research at Indiana University on basaltic magmatism, metallogeny, and environmental impacts has been partially supported by grants from the National Natural Science Foundation of China (40573020), the Ministry of Education of China (Project 111-B07011) and the National Science Foundation of the United States (EAR-0710910).


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 GEOLOGY OF THE KHARAELAKH...
 MAGMATIC ANHYDRITE
 SULFUR ISOTOPES
 POTENTIAL VOLCANIC SO2FLUX
 REFERENCES CITED
 

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Received for publication 21 July 2008

Revised manuscript received 29 October 2008

Manuscript accepted 2 November 2008





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