The sul phur and ox y gen iso to pic com po si tion of anhydrite from the Up per Pechora Ba sin (Rus sia): new data in the con text of the evo lu tion of the sul phur iso to pic
re cord of Perm ian evaporites
Anatoliy R. GALAMAY1, Fanwei MENG2, Krzysztof BUKOWSKI3, *, Pei NI4, Svetlana N. SHANINA5 and Oleg O. IGNATOVICH6
1 Na tional Acad emy of Sci ences of Ukraine, In sti tute of Ge ol ogy and Geo chem is try of Com bus ti ble Min er als, Naukova, 3a, 79060 Lviv, Ukraine
2 Chi nese Acad emy of Sci ences, Nanjing In sti tute of Ge ol ogy and Pa le on tol ogy, State Key Lab o ra tory of Paleobiology and Stra tig ra phy, Nanjing 210008, China
3 AGH Uni ver sity of Sci ence and Tech nol ogy, Fac ulty of Ge ol ogy, Geo phys ics and En vi ron ment Pro tec tion, Al. A.
Mickiewicza 30, 30-059 Kraków, Po land
4 Nanjing Uni ver sity, State Key Lab o ra tory for Min eral De posit Re search, School of Earth Sci ences and En gi neer ing, Nanjing 210093, China
5 In sti tute of Ge ol ogy of the Komi Sci ence Cen ter, Urals Branch of RAS, Pervomayskaya 54, 167610 Syktyvkar, Rus sia
6 “MIREKO” State Com pany, Gromova 75, 167983 Syktyvkar, Rus sia
Galamay, A.R., Meng, F., Bukowski, K., Ni, P., Shanina, S.N., Ignatovich, O.O., 2016. The sul phur and ox y gen iso to pic com - po si tion of anhydrite from the Up per Pechora Ba sin (Rus sia): new data in the con text of the evo lu tion of the sul phur iso to pic re cord of Perm ian evaporites. Geo log i cal Quar terly, 60 (4): 990–999, doi: 10.7306/gq.1309
This study de scribes a new de ter mi na tion of the S and O iso tope com po si tion of Lower Perm ian (Kungurian) an hyd rites from the Up per Pechora Ba sin, Cis-Ural re gion, Rus sia. d34S val ues in sul phate fa cies vary from +13.7 to +15.1‰; and d18O val - ues range from +9.3 to +10.4‰. The val ues of d34S and d18O of anhydrite from ha lite fa cies var ies from +12.6 to +15.0‰ and +7.5 to +10.9‰ re spec tively. The quan ti ta tive ra tio of py rite con tent from the wa ter-in sol u ble res i due (silty-sand frac tion) is char ac ter ized by ex tremely low (<<1%) to high (4–5%) steep gra da tion val ues. The in creased pres ence of py rite in di cates the in flu ence of bac te rial sul phate re duc tion. The sul phate re duc tion pro cess was more in tense, es pe cially when evaporites were formed in mud. The nar row fluc tu a tion range of sul phur and ox y gen iso topes val ues of the mea sured anhydrite in di - cates low lev els of frac tion ation. It was es tab lished that dur ing the Perm ian, evo lu tion ary changes in the con tent of sul phate ions in sea wa ter cor re late with the sul phur iso to pic com po si tion of ma rine evaporites.
Key words: Up per Pechora Ba sin, Perm ian, iso to pic com po si tion, anhydrite, py rite, sul phate re duc tion.
INTRODUCTION
The sul phur and ox y gen iso tope com po si tion in min er als is the tool that al lows for the iden ti fi ca tion of the chem i cal, bio - chem i cal and phys i cal pro cesses that were oc cur ring in the sed i men tary ba sin. Vari a tions of the sul phur iso to pic com po si - tion of dis solved sulphates in an cient oceans have been es tab - lished based on the anal y sis of an cient ma rine evaporites (e.g., Holser and Kaplan, 1966; Niel sen, 1972; Holser, 1979; Pan - kina, 1978; Claypool et al., 1980; Strauss, 1997). The main fea - tures of the sul phur iso tope aged curve are a pro nounced max i -
mum with d34S-val ues around +30‰ dur ing Cam brian times, a de crease to a Perm ian min i mum at +10‰, and an in crease to - wards the value for mod ern, oce anic sul phate at +21‰
(Strauss, 1997 with ref er ences within). The ob served vari a tions are re lated to changes in ocean chem is try that are based, in prin ci ple, on vari a tions of the sul phur re dox cy cle (Ronov, 1980;
Yanshin, 1988; Kovalevich, 1990; Strauss, 1997; Hol land, 2003). The sul phur iso to pic con tent of the Perm ian evaporites has al ready been stud ied for many salt bas ins around the world (Ta ble 1; Cortecci et al., 1981; Pankina et al., 1985; Makhnach et al., 2000; Vysotskiy et al., 2004; Longinelli and Flora, 2007;
Shekhunova and Stadnichenko, 2010; GarcÍa-Veigas et al., 2011). De tailed stud ies of the sul phur (d34S) iso to pic com po si - tion of evaporites of Kungurian age were con ducted mainly for the Cas pian Ba sin (Makhnach et al., 2000); sin gle data from the Up per Kama Ba sin (Solikamsk) have also been pre sented (Dzinoridze et al., 2000; Kovalevych et al., 2002).
The fol low ing study de ter mines the sul phur (d34S) and ox y - gen (d34O) iso tope com po si tion of the Lower Perm ian (Kun -
* Corresponding author, e-mail: buk@agh.edu.pl
Received: May 15, 2016; accepted: July 19, 2016; first published online: September 5, 2016
gurian) anhydrite of the Up per Pechora Ba sin, Rus sia (Fig. 1).
The aim of the re search was to pro vide new data from the not pre vi ously in ves ti gated area and de ter mine the po ten tial im pact of re duc tion pro cesses on the iso to pic com po si tion of sulphates. In ad di tion, in or der to de ter mine the in ten sity of bac - te rial sul phate re duc tion in a near-bot tom depositional en vi ron - ment, the py rite con tent was stud ied. Py rite is re garded as a fi - nal prod uct of the sul phate re duc tion pro cess (e.g., Vinogradov, 1980) which is re corded in di rectly by changes in the iso to pic com po si tion of an hyd rites. The abun dance and size of crys tals of py rite have been de ter mined from the silty-sand frac tion (0.01–1 mm) of the in sol u ble salt res i due.
GEOLOGICAL SETTING
In Early Perm ian (Kungurian) times, evaporites (clastic-sul - phate de pos its, rock salts, and pot ash salts) were de pos ited in the Ural Foredeep. The larg est ba sin – the Up per Kama salt ba -
sin – lo cated more to the south part of the Cis-Ural De pres sion (Fig. 1), oc cu pies the Cherdyn-Solikamsk De pres sion where pot ash salt de pos its were sub se quently ex ploited (Solikamsk and Berezniki mines). The smaller ba sin, lo cated to the north – the Up per Pechora Sub-ba sin – is our study area. Both salt bas - ins are sep a rated by a trans verse ridge that con trolled wa ter ex - change (Ivanov and Voronova, 1972).
The Up per Pechora salt ba sin was dis cov ered dur ing gas-oil ex plo ra tion in the 1950s and 1960s, when sev eral widely sep a rated salt de pos its were found in the north ern part of the Ural Foredeep (Cis-Ural De pres sion). The length of the Up per Pechora Ba sin is 125–130 km and the width is 50 km (Fig. 1);
the area of the ba sin that has un der gone evaporite sed i men ta - tion is approx. 6,000 km2.
The to tal thick ness of the evaporite suc ces sion that ac cu - mu lated in the Up per Pechora Ba sin ranges from 600–700 m (Strakhov, 1967). This evaporite se quence is part of the Lower Perm ian (Kungurian) suc ces sion known as the Irensk ho ri zon or suite (e.g., Chuvashov, 1995) and in cludes three thick salt units: lower rock salt, po tas sium-mag ne sium salt, and up per The sul phur and ox y gen iso to pic com po si tion of anhydrite from the Up per Pechora Ba sin (Rus sia)... 991
T a b l e 1 The d34S and d18O val ues of Perm ian ma rine evaporites (num ber of anal y ses given in pa ren the ses)
Age Ba sin lo ca tion Fa cies d34S, ‰CDT Ref er ences
Range Mean
Lopingian
Del a ware, USA sul phate 11.8–13.8 12.2 (6) Claypool et al. (1980);
Holser and Kaplan (1966)
ha lite 11.3–11.3 11.3 (4) Claypool et al. (1980)
Zechstein, Po land
sul phate 9.6–12.6 11.4 (52) Peryt et al. (2010)
ha lite 8.84–13.29 10.9 (95) Kovalevych et al. (2002); Vovnyuk et al. (2004) Peryt et al. (2005); GarcÍa-Veigas et al. (2011)
Zechstein, Peri-Bal tic, Rus sia
sul phate 12.41–12.88 12.7 (4) Kovalevych et al. (2002); Vovnyuk et al. (2004) ha lite 10.26–13.23 11.9 (13) Kovalevych et al. (2002); Vovnyuk et al. (2004) sul phate
(Strassfurt) 10.1–11.8 10.9 (11) Niel sen and Ricke (1964); Holser and Kaplan (1966);
Kramm and Wedepohl (1991); Kampschulte et al. (1998) sul phate
(Werra) 10.5–12.6 11.5 (34) Niel sen and Ricke (1964); Claypool et al. (1980); Kramm and Wedepohl (1991); Kampschulte et al. (1998)
ha lite 9.4–12.4 10.7 (15) Holser and Kaplan (1966)
Den ver, USA sul phate
(Satanka) 12.6 12.6 (1) Holser and Kaplan (1966)
Av er age val ues 11.6 (235)
Roadian
Palo Duro, west ern USA
Pricaspian, Up per salt
sul phate 10.5–16.8 12.8 (12) Claypool et al. (1980); Makhnach et al. (2000)
ha lite 11.5–13.4 12.6 (8) Claypool et al. (1980); Makhnach et al. (2000)
Av er age val ues 12.7 (20)
Kungurian
Up per Pechora sul phate 13.7–15.1 14.6 (3) our data
ha lite 12.6–15.0 13.8 (21) our data
Pricaspian, Lower salt
sul phate 11.0–16.4 14.1 (18) Makhnach et al. (2000)
ha lite 10.5–15.1 12.7 (29) Makhnach et al. (2000); Vovnyuk et al. (2004)
Av er age val ues 13.8 (71)
Artinskian Spitsbergen,
Nor way sul phate 13.0–13.9 13.2 (4) Claypool et al. (1980)
Asselian- Sakmarian
Dnipro-Donets, Ukraine
sul phate 13.1–13.2 13.2 (2) Shekhunova and Stadnichenko (2010)
ha lite 8.4–11.6 10.4 (11) Shekhunova and Stadnichenko (2010)
Dvina-Sukhona, Belarus
sul phate 13.12 ± 0.07 13.1 (20) Vysotskiy et al. (2004)
ha lite 11.77–13.76 12.7 (7) Kovalevych et al. (2002)
Den ver, USA sul phate,
(Wolfcampian) 12.7–12.8 12.8 (2) Holser and Kaplan (1966)
Av er age val ues 12.4 (42)
rock salt (Foigt, 1965; Ivanov and Voronova, 1968; Vysotskiy et al., 1988). The lower rock salt thick ness var ies from 125–130 m in the west to 400 m in the east ern part of ba sin. The pot ash salt ho ri zon has a to tal thick ness in the range of 8–63 m. There are 4–7 beds of po tas sium-mag ne sium salt (0.5–0.75 m each), mainly sylvite and car nal lite. The max i mum thick ness of the up - per rock salt is 80 m (Fig. 2). The char ac ter is tic fea ture of the salt for ma tion in the Up per Pechora Ba sin is a high con tent of car bon ate-terrigenous clay ma te rial in the form of lay ers and ad mix ture, par tic u larly in the ini tial stage of evaporitic de po si - tion (Ivanov and Voronova, 1972). The evaporite for ma tion is un der lain by terrigenous de pos its with lime stones (Artinskian stage) and over lain by marly mudstones and sand stones (Ufimian) (Figs. 2 and 3).
The Kungurian sa line for ma tion in the Up per Pechora Ba sin as a whole is highly dis tinc tive. First, the ha lite fa cies is ex - tremely well-de vel oped, in area and in to tal mass, among other de pos its. At the same time, the dis tri bu tion of de posit types shows a very clear asym me try in the fa cies zone, both from east to west and from south to north (Fig. 1). This com plex asym met ri cal dis tri bu tion of evaporites is as so ci ated with sub - strate tec ton ics. The do lo mite-anhydrite, sul phate fa cies, for - med on a rel a tively sta ble, only slightly-in clined floor to the cen - tre of the ba sin and beds are there fore thin ner. The ha lite fa cies
de vel oped over tec toni cally more mo bile struc tures of the Ural Fore deep and the de pos its are con se quently much thicker (Stra khov, 1967). The pot ash salt ac cu mu lated in the most mo - bile and most in tense sub si dence zones (Fig. 3).
MATERIALS AND METHODS
For this study, the anhydrite from the sul phate fa cies was sam pled from bore holes #1 and 7, whilst an hyd rites from the lower rock salt were sam pled from bore holes # 3, 12, 14, 16, 17, 19. All these bore holes are lo cated about 230 km north of Solikamsk (Fig. 1).
PYRITE SAMPLE COLLECTION AND OBSERVATIONS
Frag ments of ha lite from lower rock salt for ma tions were sam pled (25–35 g each) from a bore hole and were sub se - quently dis solved in dis tilled wa ter. Pre vi ous stud ies of the in - sol u ble frac tion (Dzinoridze et al., 2000) showed that iron sul - phide prev a lence char ac ter izes the pelitic frac tion (0.01 mm) Fig. 1. Sche matic map of the Up per Pechora salt-bear ing ba sin
(af ter Foigt, 1965; Rayevskii et al., 1973; Vysotskiy et al., 1988; Shanina et al., 2012)
The sul phur and ox y gen iso to pic com po si tion of anhydrite from the Up per Pechora Ba sin (Rus sia)... 993
and larger size frac tion (Fig. 4). In our study py rite was par tic u - larly ob served in the silty-sand frac tion (0.01 to 1 mm). Crys tal microanalysis was com pleted us ing a SEM mi cro scope (LEO 1530 VP) with X-Ray spec trom e ter fol low ing La ser Mi cro Spec tro graphic Meth od ol ogy. Quan ti ta tive anal y sis of the py - rite con tent was com pleted by vi sual es ti ma tion of the crys tals.
ANHYDRITE SAMPLE COLLECTION AND ANALYTICAL PROCEDURE
Mas sive anhydrite sam ples were col lected from thick anhy - drite beds, from cores of two bore holes lo cated in the mar ginal
part of the ba sin (sul phate fa cies), whilst nod u lar anhydrite was col lected from crys tals of ha lite, ob tained af ter salt dis so lu tion in dis tilled wa ter. Nod u lar anhydrite oc curs mainly among sed i - men tary lay ers and at the junc tions of the in di vid ual ha lite crys - tals (Fig. 5).
Sta ble iso to pic anal y ses (d34S and d18O) of an hyd rites were con ducted by two lab o ra to ries: ALS (Aus tra lian Lab o ra tory Ser - vices) Lab o ra tory Group and Oxy-An ion Sta ble Iso tope Con - sor tium (OASIC) in Lou i si ana State Uni ver sity. The ma jor ity of iso to pic anal y ses for the 21 sam ples were per formed by ALS Lab o ra tory Group; three anal y ses (sam ples 16/12, 3/19, 9/7) were per formed by OASIC Lab o ra tory.
The an a lyt i cal workflows ALS lab o ra tory used for sul phur iso topes are as fol lows: sam ples were weighted into tin cap - Fig. 2. Lithostratigraphic po si tion of the Kungurian salts in the Up per Pechora Ba sin
(af ter Foigt et al., 1965)
Fig. 3. Sche matic geo log i cal cross-sec tion of the Up per Pechora salt ba sin (af ter Ivanov and Voronova, 1968)
sules and the sul phur iso to pic com po si tion was mea sured us - ing MAT 253 Sta ble Iso tope Ra tio Mass Spec trom e ter cou pled to a Costech ECS 4010 El e men tal An a lyzer. Sul phur iso to pic anal y ses were nor mal ized to the VCDT stan dard.
Ox y gen iso topes: sam ples were dis solved in 2N HCl and reprecipitated as BaSO4 by us ing a sat u rated so lu tion of BaCl2. Pre cip i tates were cen tri fuged, dried, and weighed into a sil ver cap sule. The ox y gen iso to pic com po si tion is mea sured us ing a MAT 253 Sta ble Iso tope Ra tio Mass Spec trom e ter cou pled to a Thermo Sci en tific TC/EA High Tem per a ture Con ver sion El e - men tal An a lyzer. The d18O val ues were cal cu lated by nor mal iz - ing the 18O/16O ra tios in the sam ple to that in the Vi enna Stan -
dard Mean Ocean Wa ter (VSMOW) in ter na tional stan dard. All val ues were re ported us ing the delta (d) no ta tion in units of permil (‰) and re pro duc ible to 0.2‰.
The an a lyt i cal workflows used in the OASIC lab o ra tory: the re sid ual of acid treat ment or ex tracted BaSO4 and Ag2S pre cip i - tate were com bined with ex cess V2O5 and ana lysed for con cen - tra tion and iso tope com po si tion of sul phur us ing an Isoprime 100 (Isoprime 100, Cheadle, UK) gas source mass spec trom e - ter fit ted with a pe riph eral el e men tal analyser (Mi cro Vario Cube, Isoprime Ltd., Cheadle, UK) for on-line sam ple com bus - tion. The an a lyt i cal er ror is <0.2‰, cal cu lated from du pli cate sam ple anal y ses and lab o ra tory stan dards.
Fig. 4. Mor phol ogy of the py rite grains, photo SEM mi cro scope, bore hole 16, sam ple 2/16
The sul phur and ox y gen iso to pic com po si tion of anhydrite from the Up per Pechora Ba sin (Rus sia)... 995
RESULTS
PYRITE OF THE ROCK SALT SILTSTONE-SANDY FRACTION
Py rite oc curs in nu mer ous shapes and hab its, for ex am ple as: iso met ric cu bic crys tals, oc ta he dral, oc ta he dral cubes, or ag gre gate con cen tra tions with in dis tinct hab its (Fig. 4). The ob -
served yel low-brown sin gle crys tals have sizes not <50 mm and rarely >200 mm. Min eral ag gre gates with py rite crys tals were ob served as large as 500 mm, where be sides py rite, plagio - clases (al bite), K-feld spars, quartz, and sin gle grains of wolla - stonite were found.
The abun dance of py rite in the sam ples was char ac ter ized by a strong con trast from low (<<1%) to high (4–5%) con tent of iso lated crys tals (Ta ble 2).
Fig. 5. Nod u lar anhydrite (an) within terrigenous ma te rial in chev ron ha lite (A) and at the junc tions of the in di vid ual ha lite crys tals (B), bore hole 5, sam ple 5/3
T a b l e 2 The d34S and d18O val ues for anhydrite of the Up per Pechora Ba sin, thick anhydrite of sul phate fa cies,
and in sol u ble res i due of lower rock salt (ha lite fa cies)
Bore hole Sam ple In ter val [m]
Con tent of py rite within sandy-silt frac tion of the rock salt [%]
Sam ple
char ac ter is tics d34S
‰CDT
d18O
‰SMOW
3
1/3 420.6–420.7 1
nod u lar anhydrite dis persed in ha lite,
lower rock salt
13.8 8.9
2/3 428.3–428.5 2–3 14.8 9.8
4/3 446.7–446.8 1 13.1 8.6
5/3 450.25–450.35 1 13.3 8.7
6/3 454.9–455.0 <1 13.1 8.9
12 16/12 282.4-282.6 2–3 15.0 10.9
14
2/14 429.5–430.1 <1 13.4 8.1
3/14 435.8–435.9 2–3 14.1 9.1
6/14 441.0–441.1 2–3 13.8 9.5
8/14 465.7–465.9 1–2 14.2 10.0
16
2/16 468.4–468.5 4–5 13.6 8.6
3/16 474.3–474.4 2–3 13.4 8.4
4/16 481.4–481.6 <1 12.6 8.6
5/16 493.2–493.4 1–2 14.6 9.4
17
2/17 373.7–373.9 1–2 13.0 7.5
3/17 377.6–377.7 1–2 14.0 8.4
9/17 409.1–409.2 1 13.9 8.7
19
2/19 454.5–454.7 <1 13.5 7.8
3/19 456.9 2–3 14.4 9.4
7/19 483.6–483.7 4–5 14.7 9.5
8c/19 492.3–492.4 <1 14.0 9.0
15 22/15 395.7–395.8 4–5
(py rite and pyrrhotite)
nod u lar anhydrite dis persed in ha lite,
K-Mg salt 13.2 8.7
4 1/04 220.85–222.35 – mas sive anhydrite
from the terrigenous
for ma tion 14.9 9.3
7
10/7 230.2–233 – mas sive anhydrite
from the subsalt anhydrite layer
13.7 9.4
9/7 230.1–230.2 – 15.1 10.4
THE SULPHUR AND OXYGEN ISOTOPIC COMPOSITION OF ANHYDRITE
The ox y gen and sul phur iso tope anal y ses were de ter mined on 24 anhydrite sam ples from the sul phate and ha lite fa cies.
The mea sure ment of the iso to pic com po si tion of sul phur for three sam ples of mas sive anhydrite from sul phate fa cies var ies from +13.7 to +15.1‰ (mean of 14.6 ± 0.8‰); the iso to pic com - po si tion of ox y gen for the same sam ples var ies from +9.3 to +10.4‰ (mean of 9.7 ± 0.6‰). The val ues of iso to pic com po si - tion of sul phur and ox y gen of anhydrite from ha lite fa cies (21 sam ples of nod u lar anhydrite) var ies from +12.6 to +15.0‰
(mean of 13.8 ± 0.6‰) and +7.5 to +10.9‰ (mean of 8.9 ± 0.8‰) re spec tively. All re sults are pre sented in Ta ble 2. The mean d34S value for all sam ples ana lysed was +13.9‰ with stan dard de vi a tion of ±0.7‰; the mean d18O value for all sam - ples was +9‰ with stan dard de vi a tion of ±0.8‰. The lin ear cor - re la tion co ef fi cient for iso to pic data of d34S and d18O in di cates
quite a strong re la tion ship: 0.77 in both fa cies (Figs. 7 and 8).
The change of d34S and d18O in the anhydrite ha lite fa cies is 2.4 and 3.4‰ (Fig. 6), whereas in the sul phate anhydrite fa cies the change is be tween 1.4 and 1.1‰.
INTERPRETATION AND DISCUSSION
The fluc tu a tion in the ob served iso tope val ues of anhydrite can be ex plained by chem i cal and bi o log i cal pro cesses dur ing sed i men ta tion and early diagenesis in the evaporite ba sin (Pankina et al., 1985; Shekhunova and Stadnichenko, 2010;
Galamay et al., 2014). There fore, frac tion ation in d34S and d18O of dis solved sul phate oc curs due to brine evap o ra tion and crystallisation of gyp sum (Thode and Mon ster, 1965; Holser and Kaplan, 1966; Niel sen, 1972; Pankina et al., 1985; Cendón et al., 2004). An im por tant fac tor in the iso to pic com po si tion of
Fig. 6. The fluc tu a tion val ues of d34S and d18O of the anhydrite in bore hole data of the Up per Pechora Ba sin
anhydrite is bac te rial sul phate re duc tion (e.g., Vinogradov, 1980; Vysotskiy et al., 2004; Peryt et al., 2007), con ti nen tal in - flux to a ma rine evaporite ba sin, and ero sion of pre vi ously pre - cip i tated sulphates (e.g., Taberner et al., 2000).
Con sid er ing that evaporites of the Up per Pechora Ba sin have formed in a ma rine depositional en vi ron ment (e.g., Strakhov, 1967; Galamay et al., 2013), the ob tained d34S and d18O re sults char ac ter ize the ox y gen and sul phur iso to pic com po si tion of ma rine sulphates (gyp sum and anhydrite) of Kun gurian age. The pres ence of py rite in in sol u ble rock salt res i due of the Up per Pechora Ba sin (see Ta ble 2) in di cates
bac te rial sul phate re duc tion pro cesses. Iso tope frac tion ation dur ing sul phate re duc tion by bac te ria in or ganic-rich sed i - ments leads to en rich ment in the light sta ble iso tope (32S) of hy dro gen sul phide (as the ear li est sul phate re duc tion prod uct) and py rite (as the fi nal prod uct of re duc tion). Vinogradov (1980) sug gested that nearly all py ritic sul phur formed in the ear li est pe ri ods of sed i ment diagenesis, and that the para - genesis of anhydrite and py rite in di cates a sim i lar time of crystallisation for both min er als.
As shown in the di a gram (Fig. 7), in ten si fi ca tion of sul phate re duc tion (in crease of py rite con tent) does not in all cases lead to the en rich ment of the heavy iso tope (d34S) of anhydrite. This may in di cate the si mul ta neous (syn chro nous) in flu ence of other fac tors, such as crys tal frac tion ation and the in flux of con ti nen tal wa ters. How ever, the sul phate re duc tion pro cess was a de ter - min ing fac tor of the frac tion ation of sul phur iso topes, es pe cially when dif fi cul ties with intercrystalline re place ment (in mud) with pre-bot tom brine have con trib uted to this pro cess. This hy poth - e sis was sup ported in sam ples 2/3, 16/12, 8/14, 5/16, 3/19, and 7/19 by the pres ence of silty-sand frac tion in dense lumps of clay and fine spongy anhydrite nod ules, filled with clay, which has not been ob served in other sam ples. The ex ten sive de vel - op ment of bac te rial sul phate re duc tion con trib uted to a high al - ka lin ity of brine (Galamay et al., 2013) and a high con tent of or - ganic mat ter bur ied in sed i ments. For ex am ple, the de fi ciency of such fac tors was the cause of the poor de vel op ment of bac te - rial sul phate re duc tion in the Badenian sa line bas ins (Peryt et al., 2002; Galamay et al., 2014).
As in the other for ma tions, a clear de crease of the d34S value in anhydrite was ob served in the up per part of lower rock salt unit, be fore de po si tion of po tas sium-mag ne sium salt in the Up per Pechora Ba sin (Ta ble 2 and Fig. 6). This de crease of d34S value was in the range from 0.7 to 1‰, and d18O in the range from 0.9 to 1.6‰. As shown by Raab and Spiro (1991), the d34S val ues of se quences re sult ing from evap o ra tion in a closed ba sin de crease to wards the end of the ha lite fa cies.
In pre vi ous stud ies (Holser and Kaplan, 1966; Claypool et al., 1980; Pankina et al., 1985), the d34S value of Perm ian anhydrite The sul phur and ox y gen iso to pic com po si tion of anhydrite from the Up per Pechora Ba sin (Rus sia)... 997
Fig. 7. Cor re la tion be tween sul phur and ox y gen iso to pic com po si tion of nod u lar anhydrite (ha lite fa cies)
Fig. 8. Cor re la tion be tween the con cen tra tion of sul phate ions in the brines of Perm ian salt bas ins and value d34S in an hyd rites
A – the value curve of the sul phate ions con tent (Kovalevich and Vovnyuk, 2010); for cal cu lat ing the con tent of sul phate ion, we used data from the fol low ing pa pers: Moskovskyi (1983), Petrychenko (1988), Horita et al. (1991), Kovalevych et al. (2002), Horita et al. (2002), Lowenstein et al. (2005), Galamay et al. (2013); B – the value d34S curve of an hyd rites (ac cord ing to Ta ble 2)
was es tab lished as an av er age value. Ta ble 2 pres ents d34S and d18O data of anhydrite char ac ter is tic for Perm ian ma rine evaporite de pos its, which were formed with the mi nor in flu ence of con ti nen tal clastic in put and iso tope frac tion ation pro cesses.
The ta ble data show that d34S of the anhydrite of Kungurian evaporites show a slightly dif fer ent 34S con cen tra tion com pared with the an hyd rites of the Asselian-Artinskian and Ufa-Ta tar bas - ins. In con trast, sea wa ter of the Kungurian in ter val has the low - est level of sul phate ions com pared to other parts of the Perm ian.
There fore we sug gest an in verse cor re la tion be tween d34S anhydrite and the con tent of sul phate ions in brine from fluid in - clu sions of evaporite bas ins (Fig. 8). The trends of sul phur iso to - pic com po si tion of sul phate anhydrite and ha lite fa cies are iden ti - cal. It re quires clar i fi ca tion of whether such a cor re la tion is of a co in ci den tal char ac ter or if the evo lu tion of the iso to pic com po si - tion of the dis solved ma rine sul phur was tak ing place dur ing the en tire Perm ian. Var i ous re search ers iden ti fied mul ti di rec tional (both di rect and in verse) cor re la tion be tween the con tent of sea wa ter sul phate ions and sul phur evaporite iso to pic com po si tion, for the larger in ter vals (dif fer ent pe ri ods) dur ing Phanerozoic time (Kovalevych and Vityk, 1995; Algeo et al., 2015). It was in - duced by global change of the depositional ra tio and ero sion of sulphates or sul fides (Pankina, 1978; Niel sen, 1973; Claypool et al., 1980) and/or in ten sity of a bi o log i cal sul phate re duc tion and other fac tors (Pankina, 1978).
CONCLUSIONS
New d34S and d18O val ues for anhydrite from the Up per Pechora Ba sin are char ac ter is tic of ma rine evaporites of
Kungurian age and they dif fer slightly from Kungurian anhydrite from the Cas pian Ba sin. The nar row fluc tu a tion range of sul - phur and ox y gen iso tope val ues of the mea sured anhydrite in di - cates their small frac tion ation. The frac tion ation was in flu enced by a va ri ety of fac tors, mainly bac te rial sul phate re duc tion pro - cesses. This is in di cated by the pres ence of syngenetic py rite (up to 5%), in the wa ter-in sol u ble res i due (silty-sand frac tion) from the ha lite. The Kungurian an hyd rites are char ac ter ized by a higher d34S value com pared to the an hyd rites of the Asselian-Artinskian and Ufimian-Tatarian se quences.
Sulphates dis solved in sea wa ter have var ied through geo - logic time, which con firms the evo lu tion ary changes in the Earth’s his tory. A com par i son with other Perm ian ep ochs shows that sea wa ter of Kungurian age had the low est con tent of sul phate ions cal cu lated from the chem i cal com po si tion of Xuid in clu sions in ha lite. The re sults of the sul phur iso to pic com po si tion for all known Perm ian ma rine evaporites in di cate that the high est val - ues of d34S were re corded for evaporites of Kungurian age.
Ac knowl edge ments. This re search was sup ported by the Na tional Nat u ral Sci ence Foun da tion of China (Nos: 41473039 and 4151101015) and the Bu reau of In ter na tional Co-op er a - tion, Chi nese Acad emy of Sci ences. Sul phur iso to pic anal y ses were done at ALS (Aus tra lian Lab o ra tory Ser vices) Lab o ra tory Group and at Lou i si ana State Uni ver sity by Dr. P. Yongbo.
Microanalyses of py rite were done at Nanjing In sti tute of Ge ol - ogy and Pa le on tol ogy CAS by us ing SEM (LEO 1530 VP) with En ergy-Dispersive X-Ray Spec trom e ter by en gi neer Fang Yan.
We give our thanks to all of them for their help. Geo log i cal Quar terly re view ers Prof. A.A. Makhnach and Prof. J.
Parafiniuk, and the ed i tor Prof. T.M. Peryt are thanked for their help ful com ments and sug ges tions.
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The sul phur and ox y gen iso to pic com po si tion of anhydrite from the Up per Pechora Ba sin (Rus sia)... 999
