Oxygen and sulphur isotopes of gypsum from the Mogilno Salt Dome cap-rock (Central Poland)

Geo log i cal Quar terly, 2012, 56 (2): 249–260 DOI: http://dx.doi.org/10.7306/gq.1019

Ox y gen and sul phur iso topes of gyp sum from the Mogilno Salt Dome cap-rock (Cen tral Po land)

Joanna JAWORSKA and Pawe³ WILKOSZ

Jaworska J. and Wilkosz P. (2012) – Ox y gen and sul phur iso topes of gyp sum from the Mogilno Salt Dome cap-rock (Cen tral Po land).

Geol. Quart., 56 (2): 249–260, doi: 10.7306/gq.1019

The sta ble-iso tope com po si tions of ox y gen and sul phur from 30 gyp sum sam ples ob tained from three drill cores of the Mogilno Salt Dome cap-rock have been ana lysed; the Mogilno Salt Dome it self is com posed of Zechstein evaporites. d³⁴S val ues were mea sured for SO4 ions, whereas d¹⁸O val ues were mea sured for SO4 and H2O. For the anal y ses, bulk sam ples of gyp sum (CaSO4·2H2O) and its de hy - drated equiv a lents (CaSO4) were used. The d¹⁸O and d³⁴S val ues in SO4 were ana lysed us ing a di rect method, but the d¹⁸O value in H2O (wa ter of crystallisation) was de ter mined us ing an in di rect method, which in flu enced the re sults. The SO4 ap peared to be dis tinctly en - riched in heavy ox y gen (d¹⁸O = 11.0–14.6‰), which shows that the Zechstein sea was the source of the sul phate; the d³⁴S val ues (10.9–13.6‰) sup port this con clu sion. Vari able d¹⁸O val ues of the wa ter of crystallisation (–11.3 to 10.7‰) in di cate that diagenetic (hydration) pro cesses af fected the var i ous sam ples. Ac ces sory min er als oc cur ring in gyp sum prob a bly also in flu enced the d¹⁸O val ues.

Five groups of gyp sum are dis tin guished: (1) gyp sum in which the d¹⁸O val ues of the H2O are less than –10.2‰ (this gyp sum recrystallised in the pres ence of wa ter en riched in light ox y gen iso topes), (2) gyp sum in which the d¹⁸O val ues of H2O range be tween –6.6 and –4.4‰ (in ter preted as gyp sum in equi lib rium with wa ter from the cap-rock), (3) gyp sum in which the d¹⁸O value of the H2O amounts to 10.7‰ (anhydrite-bear ing sand stone near the salt mir ror), (4) gyp sum in which the d¹⁸O value of the H2O ranges be tween –3.4 and 1.8‰ (sam ples con tam i nated by anhydrite; the real d¹⁸O con tent is un known), and (5) gyp sum in which the d¹⁸O value of the H2O ranges be tween –8.7 and –6.9‰ (in ter preted as gyp sum with a mixed iso to pic com po si tion due to mix ing of wa ter from cap-rock and wa ter en riched in light iso topes of ox y gen: re cent me te oric wa ter or postglacial wa ter?, or as gyp sum formed in a warm in ter val when the ground wa ter was some what en riched in heavy iso topes of ox y gen if com pared with the ground wa ter at pres ent).

Joanna Jaworska, In sti tute of Ge ol ogy UAM, Maków Polnych 16, 61-606 Poznañ, Po land, e-mail: veronika@amu.edu.pl; Pawe³ Wilkosz, Investgas S.A., Al. Jana Paw³a II 70, 00-175 Warszawa, Po land, e: mail: pawel.wilkosz@investgas.pl (re ceived: April 13, 2011, ac cepted: Jan u ary 19, 2012; first pub lished on line: April 26, 2012).

Key words: gyp sum, ox y gen iso topes, sul phur iso topes, cap-rock, Zechstein, Mogilno Salt Dome.

INTRODUCTION

Salt diapirs have been the ob jects of salt min ing since the 19th cen tury, but only dur ing the past 30 years they have been in ves ti gated and ex plored in de tail as po ten tial hosts for a re - pos i tory for all kinds of ra dio ac tive waste as well as for un der - ground stor age of oil and gas by the pe tro leum in dus try (Jack - son and Seni, 1984; Kreitler et al., 1985; Posey and Kyle, 1988;

Werner et al., 1988; Bornemann and Bräuer, 1999; Klinge et al., 2002; Brewitz and Rothfuchs, 2007).

Cap-rocks cover the tops of salt domes; they are in te gral parts of the salt bod ies, be ing the prod ucts of a se quence of pro - cesses near the salt mir ror, on top of the salt diapirs. In this salt-dis so lu tion zone, salt rock co mes in con tact with fresh wa - ter or just an NaCl-undersaturated so lu tion, and the rock salt be comes dis solved. Rock salt al ways con tains some im pu ri ties:

anhydrite (in par tic u lar), and lo cally car bon ates (cal cite, do lo - mite, magnesite), clay min er als and fer rous com pounds. All this ma te rial, be ing less sol u ble, is left on top of the salt body; in the course of time, it be comes com pacted and trans formed into a rocky cover, the so-called cap-rock. The main diagenetic pro - cess is the hydration of anhydrite, trans form ing it into gyp sum, so that a salt body is of ten cov ered by a gypsiferous cap-rock.

Some of the cap-rocks have a long-lived diagenetic his tory, e.g., sur face ex po sure and ero sion, frac tur ing, in ter nal cir cu la - tion of me te oric wa ter or hy dro ther mal flu ids, metasomatism, and gla ci ation (Posey and Kyle, 1988; Prikryl et al., 1988;

Werner et al., 1988; Jaworska and Ratajczak, 2008). A key for un der stand ing this his tory might be a study of the iso to pic com - po si tion of the sulphates and wa ter of crystallisation that built the gyp sum (gypsiferous cap-rock). Isotopic anal y ses com - monly pro vide in for ma tion about the con di tions and the stages of hydration of the sulphates (Pi erre and Fontes, 1978; Ha³as

and Krouse, 1982; Kasprzyk and Jasiñska, 1998; Peryt et al., 2002) and may, in the case of cap-rock, help to re con struct the pro cesses that have oc curred near the salt mir ror, par tic u larly the wa ter/sulphates in ter ac tions. Ex am ples of such in ves ti ga - tions are the stud ies of the Gyp Hill and Oakwood Domes in Texas (Kreitler and Dutton, 1983) and the Wapno Salt Dome in Po land (Jaworska, 2010).

One of the great est haz ards in mines, in clud ing salt mines, are in flows of wa ter; cap-rock, be ing a cover of the salt body, tends to pro tect the salt quite well against ero sion and ground - wa ter, but not al ways. In flow may oc cur par tic u larly when the cap-rock is partly dam aged, by na ture or by hu man ac tiv ity (see Zuber et al., 2000). The re sults of cap-rock re search should be taken into con sid er ation when de cid ing about the po si tion ing in a salt struc ture of a salt mine, a re pos i tory for ra dio ac tive waste or a fa cil ity for un der ground stor age of oil or gas.

The pur pose of this re search was to ana lyse the d¹⁸O lay out of the crystalisation wa ter of a cap-rock gyp sum and as sess whether the d¹⁸O value the same over in the whole cap-rock sec tion or whether it shows dif fer ent val ues for dif fer ent lev - els. The an swer to this ques tion has prac ti cal means:

– sim i lar d¹⁸O val ues can in di cate in tense frac tur ing/karst for ma tion of the cap-rock, sub ter ra nean/ground/sub - surface wa ter pen e tra tion through it and iso to pic ex change be tween the gyp sum crys tal li sa tion wa ter and the wa ter from the frac tures, re sult ing in iso to pic equil i bra tion;

– vari able val ues of the d¹⁸O iso tope com po si tions of gyp sum crystallisation wa ter may in di cate multi-stage

cap-rock gen e sis. It also al lows determination of the type of wa ter with which sulphates form ing in di vid ual cap-rock parts had con tact with.

GEOLOGY

Salt domes be long to the most char ac ter is tic struc tural el e - ments of the Zechstein de pos its in the Pol ish Ba sin (Fig. 1). The ax ial zone of the ba sin has a NW–SE ori en ta tion and is called the Mid-Pol ish Trough (MPT). It is a part of the South ern Perm ian Ba sin of West ern and Cen tral Eu rope (Ziegler, 1990;

Dadlez et al., 2005; Doornenbal and Stevenson, 2010).

The MPT de vel oped along the NW–SE trending Teisseyre-Tornquist Zone dur ing the Perm ian to Cre ta ceous (Po¿aryski and Brochwicz-Lewiñski, 1978; Ziegler, 1990;

Dadlez, 1997; Guterch and Grad, 2006); it be came filled with a suc ces sion of siliciclastic deposoits, car bon ates and evaporites (in the Zechstein) sev eral kilo metres thick. The Zechstein de pos - its were bur ied by Tri as sic, Ju ras sic, Cre ta ceous and Ce no zoic strata, while a com plex sys tem of more than 90 salt struc tures evolved si mul ta neously in the Pol ish Ba sin (Dadlez et al., 1998;

Krzywiec, 2006). They are of var i ous size and oc cur in var i ous lev els of Me so zoic rocks in which the salt had in truded (Fig. 1).

Ten of the salt diapirs cut the up per de nu da tion or struc tural sur - face of the Me so zoic, and in trude into the Ce no zoic cover.

The Mogilno Salt Dome (MSD) is the only one that reaches the pres ent-day sur face. The de vel op ment of this struc ture oc -

Fig. 1. Lo ca tion map of the Mogilno Salt Dome and other salt struc tures in Po land (based on Dadlez, 1997) I – Wapno Salt Dome, II – Damas³awek Salt Dome, III – Inowroc³aw Salt Dome, IV – Góra Salt Dome,

V – Izbica Kujawska Salt Dome, VI – K³odawa Salt Dome, VII – Lubieñ Salt Dome, VIII – £aniêta Salt Dome, IX – RogoŸno Salt Dome

curred in three main phases (Wilkosz, 2005), form ing a pil low, a diapir and a post-diapir struc ture, re spec tively. Halokinesis started dur ing Mid to Late Tri as sic times. A thick salt pil low was formed dur ing the Late Tri as sic to Early Cre ta ceous. The diapir stage was reached in the Early Cre ta ceous and diapirism con tin ued dur ing the Paleocene and late Eocene. The post-diapir stage be gan in the late Eocene to early Oligocene and has con tin ued to the pres ent.

The MSD is lo cated in Cen tral Po land (Fig. 1). It is about 30 km long, up to 7 km wide, and up to 6 km high (Fig. 2); it stretches in a NW–SE di rec tion. At pres ent, this dome is used for gas stor age and salt min ing (Czapowski and Bukowski, 2010). It is a typ i cal diapir with a cen tral zone where a small salt crest rises up. This asym met ric form is about 5 km long, and up to 1 km wide; it in trudes the Ce no zoic cover. The salt crest con sists of a se quence of PZ2 (Stassfurt) and, on the flanks, PZ3 (Leine) and PZ4 (Aller) de pos its. Char ac ter is tic of this struc ture is the ver ti cal ar range ment of strata of the three Zechstein salt cy cles. The in ter nal struc ture of the salt body is com plex, in clud ing folds with ver ti cal, steeply in clined and over turned axes.

The salt crest is sur rounded by Tri as sic, Ju ras sic and Cre ta - ceous de pos its. The Me so zoic de pos its in the area of the MSD

show a dis tinct in crease in thick ness from the cen tre of the dome to its pe riph ery. Nu mer ous hi a tuses oc cur in the el e vated parts of the struc ture.

The salt crest has a man tle of cap-rock rest ing on the top of the salt core (Wilkosz, 2005; Wilkosz et al., 2006). This cap-rock var ies in thick ness, li thol ogy and mor phol ogy of the top sur face. The ir reg u lar to pog ra phy of the top re sults in a num ber of out crops of the salt crest. The cap-rock im me di ately un der the Ce no zoic cover is about 5 km long and var ies in width from 250 to 750 m. Its thick ness ranges from 77 to 190 m, most fre quently from 130 to 170 m. The up per sur face of the cap-rock is undulose, with height dif fer ences ex ceed ing 100 m, from 30.4 m be neath sea level (b.s.l) to 78 m above sea level (a.s.l.). Two ar eas, sit u ated in the NW and SE parts of the dome, con tain cap-rock above 70 m a.s.l.; these ar eas are sep a - rated by an area where the cen tral part of the cap has a max i mal height of 30.4 m b.s.l.

The li thol ogy and in ter nal struc ture are in ter preted mainly on the ba sis of cores from bore holes M III, M IX and M XIII (Fig. 3A, B). On this ba sis, the fol low ing five rock types and de pos its build ing the so-called “gypsiferous cap” cov er ing the salt crest of the MSD have been dis tin guished.

Oxygen and sulphur isotopes of gypsum from the Mogilno Salt Dome cap-rock (Central Poland) 251

Fig. 2. Geo log i cal sketch-map of the Mogilno Salt Dome struc ture with out Ce no zoic de pos its, with geo log i cal cross-sec tion (Wilkosz, 2005, mod i fied af ter Soko³owski, 1966) 1 – Zechstein rock salt, 2 – Up per Tri as sic, 3 – Lower Ju ras sic, 4 – Mid dle Ju ras sic, 5 – Up per

Ju ras sic, 6 – Lower Cre ta ceous, 8 – Ce no zoic, 9 – cap-rock

1. The main (al most ex clu sive) com po nent of the cap-rock is gyp sum, which oc curs in vari able shape and size. It forms auto-, hypidio- and xenomorphic crys tals, and shows fine-, me - dium and coarse-crys tal line tex tures. Two spe cific forms of gyp sum are pres ent; len tic u lar crys tals (Fig. 4) and sel e nite (Fig. 5). Auto- and hypidio-, coarse- to me dium-crys tal line gyp sum dom i nates near the bot tom of the cap-rock, which grad u ally be comes finer and more xenomorphic up wards.

2. Anhydrite is the main com po nent at the bot tom of the cap-rock: it con sti tutes a layer of anhydrite-bear ing sand stone about 20–40 cm thick (Figs. 3B and 4) in bore holes M III, M IX and M XIII, at a depth of 245–250 m be low ground level (b.g.l.), just above the salt mir ror.

3. Sul phate rocks (Zechstein an hyd rites – Fig. 6 – chem i - cally trans formed into gyp sum in the outer parts of the blocks) are lo cally in ten sively cor roded and frac tured (frac tures are fre - quently healed with gyp sum and car bon ates). This fa cies shows a wide ar ray of tex tures, rang ing from cloudy amaeboid-xenotopic with anhydrite rel ics to idiotopic with out anhydrite rel ics.

4. Sul phate-terrigenous rocks in the form of a brec cia of gypsiferous clay (Fig. 7) were formed by com plete chem i cal trans for ma tion of an hyd rites into gyp sum, their sec ond ary dis - so lu tion, frac tur ing and grav i ta tional and tec tonic redeposition.

5. Allochthonous fill ings of karst cav erns, tec tonic slits and col lapse forms com pris ing Oligocene sands and muds, Mio - cene and Oligocene sands and brown coal and Qua ter nary sands with gravel and small boul ders of crys tal line rocks.

MATERIAL

Thirty rep re sen ta tive cal cium-sul phate core sam ples (29 gyp sum and 1 anhydrite) were ana lysed for their d¹⁸O and d³⁴S val ues. They are from three bore holes (Mogilno III, Mogilno IX, Mogilno XIII); 9 sam ples from Mogilno III, 14 from Mogilno IX and 7 from Mogilno XIII. The sam ples rep re - sent all types of gyp sum and de rive from depths of the cap-rock from 150.4 m to 249.5 m b.g.l. Some sam ples in cluded car bon - ate (cal cite or magnesite, Fig. 8) and in clu sions of anhydrite in the gyp sum crys tals.

METHODS

The gyp sum sam ples were ana lysed for their d¹⁸O and d³⁴S val ues by Dr. S. Poulson, at the Uni ver sity of Ne vada (De part - ment of Geo log i cal Sci ence, Reno). The val ues were mea sured us ing an Eurovector model 3000 (el e men tal analyser to a Micromass IsoPrime sta ble-iso tope ra tio mass spec trom e ter) in a con tin u ous flow mode.

Two meth ods were ap plied. The first was used to de ter mine the d³⁴S value (cf. Giesemann et al., 1994); V2O5 was added to the sul phate sam ples as a com bus tion aid. The val ues found are ex pressed in per mil, rel a tive to the Vi enna-Can yon Diablo Troilite stan dard (V-CDT); the an a lyt i cal pre ci sion is ±0.2‰.

The sec ond method (cf. Kornexl et al., 1999) was used for the d¹⁸O anal y sis; nickelised graph ite was added to each sam ple to pro mote CO for ma tion. These val ues are also ex pressed in per

Fig. 3A – top o graphic sur face of the Mogilno Salt Dome cap-rock, with lo ca tion of the geo log i cal cross-sec tion I–I’ (Wilkosz, 2001); B – geo - log i cal pro files of the drill cores from the bore holes M III, M IX and M XIII (Wilkosz, 2005)

mil, rel a tive to the Vi enna-Stan dard Mean Ocean Wa ter (V-SMOW); the an a lyt i cal pre ci sion is ±0.4‰.

The d¹⁸O val ues were mea sured twice for both the bulk sam ple (CaSO4·2H2O) and for its de hy drated form (CaSO4 res - i due af ter de hy dra tion). The iso to pic com po si tion of the ox y - gen in the bulk sam ple (CaSO4·2H2O) is a mix ture of d¹⁸O from the SO4 and from the H2O (crys tal li sa tion wa ter). In or der to in - ves ti gate the dif fer ences of the d¹⁸O con tent in both com po - nents, the gyp sum pow der was there fore slowly heated (to elim i nate wa ter) un der vac uum to 450°C for 30 min utes (see:

Dowuona et al., 1992). The d¹⁸O val ues in the de hy drated form in SO4 were mea sured di rectly by mass spec trom e try, whereas the iso to pic com po si tion of the wa ter of crystallisation was de - ter mined in di rectly, by cal cu lat ing the mean d¹⁸O value of the bulk gyp sum sam ple (CaSO4·2H2O) and its SO4 com po nent (from the res i due af ter de hy dra tion). The re sults ob tained us ing the in di rect method have not been ver i fied by a di rect method.

RESULTS

SULPHUR

The d³⁴S val ues in the gyp sum cap-rock sam ples in ves ti - gated show a range from 10.9 to 13.6 ±0.2‰ and thus in di cate

Oxygen and sulphur isotopes of gypsum from the Mogilno Salt Dome cap-rock (Central Poland) 253

Fig. 4. Mi cro- and macrophotos of rocks from the con tact zone be tween the salt mir ror and the cap-rock A – len tic u lar crys tals of gyp sum (bore hole M III, 249.3 m b.g.l.); B and D – gyp sum with anhydrite sand rel ics

(bore hole M III, 249.2–249.5 m b.g.l.); C – anhydrite sand from the bot tom of the cap-rock di rectly on the salt mir ror (bore hole M III, 249.5 m b.g.l.); E – anhydrite sands level; crossed polars

Fig. 5. Sel e nite gyp sum gen er ally oc curs in tec tonic slits and karstic caves; sin gle crys tals have lengths to 10 cm (M XIII, 237.6 m b.g.l.)

Fig. 6. Zechstein anhydrite (M XIII, 192 m b.g.l.)

A – de tached block of Main Anhydrite in cor po rated in the cap-rock; B – the outer parts of the block are com pletely trans formed into gyp sum, C – in the cen tral parts of the block an hyd rites rel ics are pre served; crossed polars

Fig. 7. Brec cia of sul phate rocks (A) – M IX, 192.6–197.9 m b.g.l.;

B, C – blocks of gyp sum rock, clay and mud ma te rial; crossed polars

that these sulphates are dis tinctly en riched – with re spect to V-CDT – in the heavy iso tope; the mean d³⁴S value is 11.9‰

for the core sam ples from Mogilno III, 11.8‰ for the core sam - ples from Mogilno IX and 11.6‰ for the core sam ples from Mogilno XIII (all ±0.2‰; Ta ble 1).

OXYGEN

The d¹⁸O val ues for the SO4 from the gyp sum sam ples range from 11.0 to 14.6‰; the mean d¹⁸O value is 13.5‰ for the core sam ples from Mogilno III, 13.0‰ for the core sam ples from Mogilno IX and 13.1‰ for the core sam ples from Mogilno XIII.

The d¹⁸O val ues for the wa ter of crystallisation from the gyp sum sam ples are highly vari able and range from –11.3 to 10.7‰; their mean d¹⁸O value is –4.9‰ (–5.7‰ if the two gyp - sum sam ples en riched in anhydrite are ex cluded) for the core sam ples from Mogilno III, –4.8‰ (–6.0‰ if the sam ple of anhydrite sand stone is ex cluded) for the core sam ples from Mogilno IX and –8.3‰ for the core sam ples from Mogilno XIII (Ta ble 1).

DISCUSSION

SULPHUR

Sul phur iso topes are use ful in di ca tors of the or i gin of sulphates be cause they pre serve their pri mary iso to pic com po - si tion very well. Diagenesis (hydration/de hy dra tion), re duc tion and weath er ing do not lead to con sid er able frac tion ation of the sul phur iso topes; these fac tors can jointly make a dif fer ence only of about –1.3‰, which can com monly be ne glected (Holser et al., 1989; Worden et al., 1997; Hoefs, 2004).

The ³⁴S/³²S ra tio of pres ent-day sulphates (ma rine evaporites) in oce anic wa ter is fairly con stant (+20 ±0.5‰ with re spect to V-CDT; Pi erre, 1988). The frac tion ation dis tin guish - ing dis solved cal cium sulphates in oce anic wa ter from gyp sum or anhydrite is neg li gi ble (Thode and Mon ster, 1965; Raab and Spiro, 1991), so that re cently de pos ited sulphates in ma rine evaporite bas ins have the same d³⁴S value as the sea wa ter (Claypool et al., 1980). The d³⁴S val ues for cal cium sulphates in the geo log i cal past have changed, but the gen eral trends (S-iso tope age curve), in clud ing mi nor os cil la tions (Niel sen,

Oxygen and sulphur isotopes of gypsum from the Mogilno Salt Dome cap-rock (Central Poland) 255

Fig. 8. Microphotos from thin sec tions of gyp sum sam ples

A – len tic u lar gyp sum with cal cium (3–5% ), M IX, 153 m b.g.l.; B – len tic u lar gyp sum with out cal cium, M IX, 153.5 m b.g.l.; C – fine-crys tal line gyp sum with magnesite (10%), M XIII, 174.1 m b.g.l.; D – fine-crys tal line gyp sum with magnesite (1%), M XIII, 198.1 m b.g.l.; crossed polars

1989), are well-known (see Claypool et al., 1980; Holser et al., 1989; Strauss, 1999, 2004).

Zechstein evaporites are char ac ter ized by min i mum d³⁴S val ues for the Phanerozoic (close to 10–12‰; see Gar - cia-Veigas et al., 2011). The val ues found for the gyp sum sam - ples from in ves ti gated the Mogilno sam ples (10.9–13.6‰) are char ac ter is tic of late Perm ian sulphates; the iso to pic com po si - tion of the sul phur in these sam ples (1.65‰) re flects, tak ing into con sid er ation the frac tion ation fac tor (Thode and Mon ster, 1965; Pi erre, 1988), the iso to pic com po si tion of Zechstein brines. This re sult is con sis tent with the sul phur-iso tope data for Zechstein evaporites from North ern Ger many (9.7–12.6‰:

Kampschulte et al., 1998), north east ern Po land (polyhalite)

(10.0–12.1‰; Peryt et al., 2005), south west ern Po land (9.4–13.3‰; Vovnyuk and Czapowski, 2007) and West Po - land (9.6–12.6‰ in anhydrite; Peryt et al., 2010).

OXYGEN

In com par i son to the sul phur anal y sis, the ox y gen-iso tope com po si tion of gyp sum is com plex, be cause it con sists of two

“kinds” of ox y gen: from SO4 (sul phate ion) and H2O (wa ter of crys tal li sa tion). Both cal cium sulphates and their crys tal li sa tion wa ter are com monly formed in iso to pic equi lib rium with the mother brine (Sofer, 1978), but this may change dur ing

Sam ple Depth [m b.g.l.]

Com - ment

Gyp sum–

d¹⁸O V-SMOW [‰]

CaSO4– d¹⁸O V-SMOW

[‰]

H2O–d¹⁸O

[‰] d³⁴S V-CDT [‰]

Ma jor, mi nor, trace com po nents:cal cite (C),

magnesite (M), anhydrite (A)

H2O–d¹⁸O com ment

M IX

150.4 8.4 13.6 –2 12.4 no data

152.25 6.8 13.3 –6.2 11.7 no data

153 8,1 13.8 –3.3 12.2 C – 5% over stated value

153.5 7.3 13.3 –4.7 12.3 C <1% ac tual value

175.5 7.1 13.6 –5.9 12.1 C, A<1% ac tual value

180.7 7.3 13.8 –5.7 13.6 no data

183.1 7,1 14.6 –7.9 11.6 no data

189.6 5.9 12 –6.3 11.3 – ac tual value

191.1 7.7 14 –4.9 11.4 C, M – 10–15%, A – 3% prob lem atic

213.2 5.8 12.2 –7 11.5 A, C –1% ac tual value

229.9 6.1 12.6 –6.9 11.5 C <1% ac tual value

249.3 5.5 12.6 –8.7 11.3 A – 15% over stated value

249.35 4.5 11 –8.5 11 A – 15% over stated value

249.5 11.7 12.2 10.7 10.9 anhydrite-bear ing

sand stone

Av er age M IX 7.1 13 –4.8 11.8

M III

239.3 6.5 12.7 –5.9 11.5 – ac tual value

240.7 6.3 13.3 –7.7 11.6 – ac tual value

244 len tic u lar 7.9 14.6 –5.5 12.4 – ac tual value

244 mas sive 7 13.4 –5.8 11.9 A – 1% ac tual value

244.3 7.2 13.4 –5.2 11.9 – ac tual value

244.9 7.6 13.1 –3.4 11.9 C – 5% over stated value

244.9 6.9 13.4 –6.1 11.8 A – 30% over stated value

249.5 len tic u lar 7.8 14.6 –5.8 12.3 A – 10–15% over stated value

249.5 mas sive 9.3 13.1 1.7 11.6 A – 50% high over stated value

Av er age M III 7.4 13.5 –4.9 11.9

M XIII

174.1 5.1 13.3 –11.3 11.7 M – 10% prob a bly no

in flu ence

175.35 4.8 12.6 –10.8 11.5 M – 10% prob a bly no

in flu ence

186.2 5.8 12.9 –8.4 11.7 M – 10% prob a bly no

in flu ence

198 6.1 13.5 –8.7 11.6 C – 1% ac tual value

242.7 6.2 12.6 –6.6 11.3 A – 1% ac tual value

242.7 vein 6.9 13.6 –6.5 11.5 A – 1% ac tual value

244.8 6.7 13.1 –6.1 11.6 – ac tual value

Av er age M XIII 5.9 13.1 –8.3 11.6

A – anhydrite, C – cal cite, M – magnesite

T a b l e 1 Re sults of iso tope anal y sis with com ments

diagenesis. The rate of iso tope ex change be tween a sul phate (SO4) and sur round ing wa ter is ex tremely low (Lloyd, 1968;

Holt and Kumar, 1991), so that the iso to pic com po si tion in sul - phate ions is quite sta ble; it barely changes and re mains in equi - lib rium with the mother brine of the di rectly pre cip i tated cal - cium sul phate.

It is dif fer ent for the wa ter of hydration. Gyp sum eas ily loses its orig i nal wa ter of crystallisation dur ing con tact with sea wa ter or ground wa ter. Recrystallisation, de hy dra tion and hydration (some times re peated) af fect the pri mary iso to pic com po si tion of the gyp sum crystallisation wa ter due to dif fu sion of wa ter of what ever or i gin (me te oric, open-ma rine, in ter sti tial brine or a mix ture of these) into the crys tal. Changes in the d¹⁸O value of the wa ter of crystallisation of cal cium sulphates may thus orig i - nate from the ab sorp tion of “new” wa ter.

Mod ern ocean wa ter con tains sulphates of which the d¹⁸O val ues amount to 9.5 ±0.5‰ with re spect to V-SMOW (Longinelli and Craig, 1967). Dur ing recrystallisation of oce - anic sulphates, the d¹⁸O value rises by 3.5–3.6‰ (the frac tion - ation fac tor; Lloyd, 1968; Pi erre, 1988) with re spect to the mother wa ter; the gyp sum wa ter of hydration is then en riched by about 4.0‰ (Gonfiantini and Fontens, 1963). This im plies that the d¹⁸O value of evaporites that pre cip i tated from sea wa - ter should be 13.0 ±0.5‰ (13.5 ±0.5‰ in the crystallisation wa ter of gyp sum).

The d¹⁸O value of ma rine sulphates in the geo log i cal past has been more sta ble than that of sul phur. The ¹⁸O/¹⁶O ra tio in evaporites has changed slightly over geo log i cal time and the gen eral trend is known. The re sul tant O-iso tope age curve can be used to de ter mine the time of evaporite pre cip i ta tion, and Zechstein evaporites show, like the d³⁴S val ues, a min i mum d¹⁸O value for the Phanerozoic, close to 10‰ (see Claypool et al., 1980). This is con sis tent with the ox y gen-iso tope data for Zechstein evaporites from north east ern Po land (polyhalite) (10.0–13.5‰; Peryt et al., 2005), south west ern Po land (9.4–10.4‰; Vovnyuk and Czapowski, 2007) and West Po - land (9.4–15.5‰ in anhydrite; Peryt et al., 2010).

The data shown in Ta ble 1 in di cate that the ox y gen-iso tope com po si tion of sul phate ions (SO4) and wa ter of crystallisation (H2O) of gyp sum are very dif fer ent. The d¹⁸O value in SO4 from the Mogilno gyp sum sam ples ranges from 11.0 to 14.6‰

(Fig. 9). The mea sured ra tio cor re sponds to the value known for pre cip i tated cal cium sulphates in the iso lated evaporite ba sin of the Zechstein sea and dem on strates the ma rine or i gin of this salt.

The d¹⁸O val ues in the wa ter of crystallisation from the Mogilno gyp sum sam ples range from –11.3 to +1.7‰ (mostly be tween –8 and –5‰). This gyp sum is sec ond ary: it does not con sist of pri mary cal cium sulphates that pre cip i tated from so lu - tion but rather of hy drated an hyd rites; con se quently, their wa ter of crystallisation is not in iso to pic equi lib rium with the mother brine. The vari able d¹⁸O val ues of the H2O in di cate that the cal - cium sulphates have been in con tact with non-ma rine (me te oric and/or ground) wa ter that was en riched in light iso topes dur ing the var i ous stages of diagenesis that formed the cap-rock.

The Mogilno cap-rock formed from the Cre ta ceous on, when the salt in truded the Me so zoic over bur den, reach ing the zone of per co lat ing ground wa ter. The salt then started to go into so lu tion, and the res i due (anhydrite sand) started to trans -

form into gyp sum. The for ma tion of the in cip i ent cap-rock con - tin ued and karst pro cesses af fected the ma te rial dur ing that time-span; this en abled “new” wa ter, from the sur face or the sur round ing diapir, to pen e trate the cap-rock and the gyp sum un der went recrystallisation; the wa ter of hydration then reached a new equi lib rium. An as sump tion has been made that in the case of the gyp sum sam ples, the crys tal li sa tion wa ter d¹⁸O val ues cor re spond to the me te oric wa ter d¹⁸O val ues; the sulphates un der went hydration un der the in flu ence of me te oric wa ter, and there fore the crys tal li sa tion wa ter of gyp sum is in iso tope equi lib rium with the me te oric wa ter.

Anal y ses of the crystallisation wa ter of the gyp sum from the Mogilno cap-rock give re sults that are com pa ra ble to those of the anal y ses of wa ter of crystallisation from the cap-rock of the Gorleben Salt Dome (Her bert et al., 1990), where the d¹⁸O val ues in the H2O of the gyp sum sam ples range from –12.4 to –5.2‰, and of the Wapno Salt Dome (Jaworska, 2010), where the d¹⁸O val ues in the H2O of the gyp sum sam ples range from –13.1 to –5.6‰.

In the Pol ish Low land area, the mean iso to pic com po si tion of ox y gen in mod ern me te oric wa ter and in re cent in fil tra tion wa ter ranges, as a rule, from –10.2 to –9.2‰ (d’Obryn et al., 1997; Duliñski et al., 1997). Dur ing an cient times of low tem - per a tures such as the Pleis to cene, as well as im me di ately af ter cold in ter vals, the ground wa ter (de rived from melt wa ter) was en riched in the light ox y gen iso tope, so that its d¹⁸O val ues were more neg a tive than those of the wa ter from warm and hot times such as the Paleogene and Neo gene, when the ground wa - ter be came en riched in the heavy ox y gen iso tope (Duliñski et al., 1997). The sta ble-iso tope com po si tion of wa ter col lected from the Mogilno bore holes shows a sig nif i cantly larger en - rich ment in the heavy ox y gen iso tope than is to be ex pected for in fil tra tion wa ter in this part of Po land (Górski and Rasa³a,

Oxygen and sulphur isotopes of gypsum from the Mogilno Salt Dome cap-rock (Central Poland) 257

Fig. 9. Val ues of d³⁴S vs. d¹⁸O of re sid ual sul phate depth – 191.1 (m b.g.l.)

2008). The d¹⁸O val ues of wa ter from the Kg-1 and Kp-2 bore - holes were (in 2005) –4.3 and –6.6‰ with re spect to V-SMOW, ac cord ingly.

The d¹⁸O anal y ses (us ing the in di rect method) of wa ter of crystallisation in the Mogilno gyp sum sam ples sug gest the fol - low ing five groups of cal cium sulphates (Fig. 10):

1. Two sam ples in which the d¹⁸O val ues of the H2O are less than –10.2‰; this gyp sum was recrystallised in the pres ence of wa ter en riched, in com par i son to re cent me te oric wa ter, in the light iso topes of ox y gen. This im plies that these gyp sum sam - ples (i.e., this part of the cap-rock) came in con tact with wa ter sup plied dur ing cooler time-spans such as, pos si bly, the Pleis - to cene. The d¹⁸O val ues of this crystallisation wa ter prob a bly re flect the ¹⁸O/¹⁶O ra tio of postglacial melt wa ter;

2. A group of 15 sam ples in which the d¹⁸O val ues of H2O range be tween –6.6 and –4.4‰; this gyp sum was recrystallised in the pres ence of wa ter from the cap-rock and has reached iso to pic equi lib rium with this wa ter;

3. One sin gle anal y sis of the d¹⁸O value of the H2O amounts to +10.7‰; this value was not found for a true gyp sum sam ple but for a sam ple of gyp sum from the anhydrite sand stone that oc curs near the salt mir ror; this value there fore does not re flect the real ox y gen-iso tope ra tio of hydration wa ter, but rather the slightly mod i fied/lower iso tope ra tio from the sul - phate ion;

4. A group of 4 sam ples in which the d¹⁸O val ues of the H2O range be tween –3.4 and 1.7‰; these sam ples rep re sent gyp - sum with anhydrite, so that the val ues found do not re flect the real iso tope ra tio of the hydration wa ter. The d¹⁸O val ues in di cate rather a mix ture the ox y gen-iso tope ra tio in sul - phate ions and the ox y gen-iso tope ra tio in the wa ter of crystallisation of gyp sum; in short, the real d¹⁸O con tent is un known;

5. A group of 8 sam ples in which the d¹⁸O val ues of the H2O range be tween –8.7 and –6.9‰; these sam ples are in ter preted as ei ther gyp sum with an iso to pic com po si tion that rep re sents a mix ture of wa ter from the cap-rock and wa ter en riched in light iso topes of ox y gen (for ex am ple re cent me te oric wa ter or postglacial wa ter) or gyp sum which was formed in a warmer cli mate than the pres ent one, when the ground wa ter was slightly en riched in the heavy iso topes of ox y gen.

Com par ing the data of the cap-rock gyp sum iso tope com po - si tion of the Mogilno Salt Dome with anal o gous data from the Wapno Salt Dome cap-rock (Jaworska, 2010), it seems that:

– in the gyp sum of both cap-rocks the d¹⁸O val ues in di cate the in volve ment of wa ter: (a) mixed, (b) en riched with light ox y gen iso tope – in di cat ing the pres ence of wa ter from colder pe ri ods;

– nearly half of the gyp sum from the Mogilno cap-rock con tains crystallisation wa ter which rep re sents the d¹⁸O val ues cor re spond ing to wa ter in side the cap-rock (from –6.6 to –4.3‰), while there is prac ti cally no gyp sum with d¹⁸O cor re spond ing to the pres ent wa ter (class of –10.2 to –9.2‰); only two sam ples got close to this value, reach ing –8.7‰, while four of eight sam ples from Wapno con tained crystallisation wa ter with d¹⁸O val ues cor re spond ing to the pres ent wa ter; ad di tion ally, re gard ing the sam ples from Mogilno, the high per cent -

age of gyp sum shows the d¹⁸O value slightly en riched in the heavy ox y gen iso tope (be tween –8.7 and –6.8‰);

– gyp sum ma te rial in sam ples from Wapno were lack ing im pu ri ties such as car bon ates, so all anal y ses cor re - sponded to the ac tual state; in Mogilno, part of the sam - ples con tained car bon ate im pu ri ties, (cal cite) or anhydrite, which fal si fied the proper re sults.

Fig. 10. Val ues of d³⁴S vs. d¹⁸O of wa ter of crystallisation

SUMMARY

The gyp sum from the cap-rock of the Mogilno Salt Dome is not a pri mary pre cip i tate from so lu tion but hy drated anhydrite.

The ox y gen- and sul phur-iso tope com po si tions in the sul phate ion of the gyp sum clearly show that sea wa ter/brine is the source of these cal cium sulphates. The d¹⁸O and d³⁴S val ues mea sured in the SO4 point to the Zechstein evaporite ba sin as the pri mary source of these sulphates.

The pri mary iso to pic com po si tion of the SO4 was pre served de spite re peated recrystallisation (hydration and de hy dra tion) phases. The iso to pic com po si tion of the crystallisation wa ter of this gyp sum is a use ful di ag nos tic tool to un ravel the diagenetic his tory of this min eral. Two gyp sum sam ples in di cate prob a bly Pleis to cene or post-Pleis to cene diagenetic pro cesses, be cause the wa ter of hydration is in equi lib rium with wa ter en riched in the light ox y gen iso tope (–11.3 and –10.8‰), prob a bly postglacial wa ter. These two gyp sum sam ples come from bore hole M XIII, sit u ated near the cen tre of the cap-rock, at a depth of 174–175 m b.g.l. (near the sur face of the cap-rock) where the gyp - sum cover is prob a bly cut by fis sures that form path ways for cir - cu lat ing wa ter. The cap-rock is in this place quite eas ily pen e trated by wa ter; it is prob a bly “the weak est” point of this cap-rock.

A ma jor ity of the gyp sum sam ples crys tal lised in the pres ence of wa ter en riched in the heavy ox y gen iso tope. The d¹⁸O val ues of the crystallisation wa ter can be grouped into two main cat e go ries:

(1) a group where the val ues are slightly en riched in the heavy iso - topes of ox y gen (“mix ing” wa ter or wa ter from a warmer in ter val of time) and (2) a group with wa ter from the cap-rock.

The re sults of some sam ples are prob lem atic be cause these sam ples show val ues that are mix tures of real d¹⁸O val ues of

crystallisation wa ter and d¹⁸O val ues of im pu ri ties in the gyp - sum sam ples such as anhydrite and car bon ate min er als (see Ta - ble 1). The ac tual iso to pic com po si tion of the crystallisation wa ter of these cal cium sulphates re mains un known.

The pres ence of a small per cent age of anhydrite im pu ri ties in the gyp sum sam ples also sug gests that the me te oric wa ter which in flu enced the sulphates un der went hydration, could not have had free ac cess to the cap-rock, and in re al ity, the crys tal li - sa tion wa ter of gyp sum had not ob tained an equi lib rium state with the me te oric wa ter (Bath et al., 1987). Dif fi cult ac cess of the me te oric wa ter to the cap-rock can in di cate that the cap-rock in this area has re mained an im pen e tra ble body (with - out larger frac tures and joints) – with out wa ter cir cu la tion and mi gra tion routes; and has func tioned prop erly as ef fec tive pro - tec tion for the up per parts of the salt de posit.

One of the main ob jec tives of the study was to test whether the ox y gen-iso tope ra tio of gyp sum sam ples from cap-rock can be mea sured re li ably with the in di rect method. It was found that the anal y sis of pure gyp sum sam ples gives re li able val ues but that the an a lyt i cal re sults of other gyp sum sam ples (gyp sum with anhydrite and gyp sum with cal cite) yield prob lem atic val - ues, which needs to be taken into ac count when ana lys ing such gyp sum sam ples.

Ac knowl edg ments. The study was sup ported by grant no.

4T112B03729 from the Pol ish State Com mit tee for Sci en tific Re search (KBN). The Au thors wish to ac knowl edge Prof.

T. van Loon for re vi sion of the pre vi ous Eng lish draft of this pa per and two re view ers: Prof. J. Parafiniuk and Prof. T. M.

Peryt for help ful com ments and sug ges tions re gard ing the manu script.

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