ISO TOPIC MASS BAL ANCE OF META MOR PHIC FLU IDS
IN THE GOGO£ÓW–JOR DANÓW SER PEN TIN ITE MAS SIF,
LOWER SILE SIA, SW PO LAND
Adam MYD£OWSKIPol ish Geo logi cal In sti tute, Lower Sile sian Branch, Ja wor owa 19, 53- 122 Wroc³aw, Po land; e- mail: adam.myd lowski@pgi.gov.pl
Myd³owski, A., 2009. Iso topic mass bal ance of meta mor phic flu ids in the Gogo³ów–Jor danów ser pen tin ite mas sif, Lower Sile sia, SW Po land. An nales So cie ta tis Ge olo go rum Po lo niae, 79: 147–158.
Ab stract: This work aims at es ti ma tion of the amount of meta mor phic flu ids which led to the present- day vari abil ity of iso topic com po si tions, dD and d18O, in ser pen tinites from the Gogo³ów–Jor danów Mas sif. This goal was achieved by means of a nu meri cal model re pro duc ing se lected fea tures of geo logi cal en vi ron ment and a com puter ap pli ca tion sup port ing this model. The Gogo³ów–Jor danów Mas sif con sists of peri do tites trans formed to a dif fer ent de gree into ser pen tinites and sub ject to brit tle de for ma tion that pro duced a com plex frac ture sys tem. The po ten tial tec tonic con trol on the pat tern of the fluid mi gra tion paths was in ves ti gated us ing pa laeostress analy sis based on slick en side meas ure ments. Iso topic analy ses were car ried out for hy dro gen and oxy gen from ser pen tine, oxy gen from mag net ite, and hy dro gen and oxy gen from car bon ates. The vari abil ity of dD and d18O in the ser pen tinites along the mod elled mi gra tion paths dem on strates that ser pen ti ni za tion of peri do tites was caused by flu ids de rived from at least three sources re veal ing dif fer ent iso topic char ac ter is tics. Flu ids pro duced dur ing the magmatic- hydrothermal stage played a ma jor role in ser peti ni za tion, since they rep re sent ap proxi mately 95% of all flu ids in ter act ing with the rock. In con trast, oce anic wa ter rep re sents only 1% of flu ids in volved in ser pen ti -ni za tion. The cal cu lated mean amount of fluid re quired for ser pen ti -ni za tion of 1 m3 of peri do tite is equal to 98 × 104 kg.
Key words: ser pen ti ni za tion, sta ble iso tope, tec ton ics, nu meri cal mod el ling, Gogo³ów–Jo ra danów Mas sif, Lower Sile sia, SW Po land.
Manu script re ceived 30 September 2008, ac cepted 27 July 2009
IN TRO DUC TION
For the last thirty years the vast amount of work has been done to doc u ment the or i gin, evo lu tion and tec tonic po si tion of the Œlê¿a ophiolite com plex (Majerowicz, 2006 and ref er ences therein). Con se quently, there is no doubt to -day that the Œlê¿a com plex rep re sents a fos sil rem nant of ocean floor obducted on con ti nen tal crust dur ing the Varis-can orog eny. Nev er the less, some open ques tions re main con cern ing the tim ing and mech a nism of serpentinization that af fected the ultra mafic mem ber of the Œlê¿a ophiolite. Un cer tain is the source of serpentinizing flu ids and the paths of their in fil tra tion into the ultrabasic rocks. Some of these ques tions are ad dressed in the pres ent pa per that is fo cused on the mass bal ance of meta mor phic flu ids ac com pa -ny ing meta mor phism of pri mary ultra mafic rocks of the Go- go³ów–Jordanów ser pen tin ite mas sif. Con sid er able prog -ress in dig i tal pro cess ing of data al lows cal cu la tion of the mass of flu ids (Tay lor, 1977; Vollmer, 1976; Sverjensky, 1981; Zheng & Hoefs, 1993) us ing a channelized flow model (Fyfe et al., 1978; Thomp son, 1987). A key role of
the cana lised fluid trans fer in largescale meta mor phic pro -cesses, es pe cially dur ing serpentinization of ultra mafic rocks and dehydratation of serpentinites, is em pha sized in the ma jor ity of up-to-date pa pers deal ing with these is sues (e.g., Ol i ver, 1996; Ord & Ol i ver, 1997; Barnes et al., 2004; Mas ters & Ague, 2005). There fore, the model of fluid mi -gra tion paths pre sented herein does not take into ac count the pos si bil ity of pen e tra tive in fil tra tion of flu ids through out the ultra mafic rocks due to mas sive de vel op ment of mi crocracks. The growth of such dis crete frac tures may be a con -se quence of vol ume-chang ing chem i cal re ac tions, connected for in stance with hydratation, re sult ing in lo cal per tur ba -tions of the stress field (e.g., Watt et al., 2000; MaltheSÝrenssen et al., 2006). The com mon de vel op ment of dis -crete frac tures may pro duce lo cal per me abil ity changes that pro mote fluid flow. The mod el ling of fluid mi gra tion as sisted by pen e tra tive frac tur ing would re quire an ap pli ca -tion of the fi nite el e ment method. How ever, this ap proach can not be im ple mented to the pre sented nu mer i cal model.
Fur ther more, any ex am ples of fluid trans port mod els suc cess fully in te grat ing cana lised flow and pen e tra tive mi gra tion are hith erto miss ing from sci en tific lit er a ture. The sig nif i cance of the lat ter pro cess has not been so far re li ably es -ti mated else where and, thus, has been omit ted here.
The mod el ling of the po ten tial fluid paths was sup -ported by a pa laeostress analy sis based on slick en side data, which were proc essed us ing Tec ton ics FP soft ware. A sim -pli fied model of the Gogo³ów–Jor danów ser pen tin ite mas sif was cre ated to re con struct the mi gra tion of flu ids re spon si -ble for ser pen ti ni za tion. The model was im ple mented us ing a pro duced in- house com puter ap pli ca tion. The iso tope data em ployed in the model were de rived from labo ra tory analy sis of ori ented ser pen tin ite sam ples. Iso topic analy ses of hy dro gen and oxy gen from ser pen tin ite, oxy gen from mag net -ite, and car bon and oxy gen from car bon ates were ob tained for 173 sam ples.
GEO LOG I CAL SET TING
Œlê¿a Ophiolite Com plex
Mafic and ultra mafic rocks in the vi cin ity of the Œlê¿a Mt. form a nearly com plete ophiolite as so ci a tion in clud ing serpentinized peri dot ites, a nar row zone of meta mor phosed ultra mafic cumulates, metagabbros and metabasalt vol ca nic suite ac com pa nied by a sheeted dyke com plex (Majerowicz, 1979, 1981, 1984; Majerowicz & Pin, 1989, 1994; Pin et
al., 1988). The Œlê¿a ophiolite is in tec tonic con tact with the
low-grade meta mor phosed phyllites, cherts and radiolarites that ad join the ophiolite com plex from the north. In the south, the Œlê¿a ophiolite bor ders on the Góry Sowie gneiss mas sif be ing sep a rated from one an other by a Neo gene E–W trending fault sys tem. Ac cord ing to Majerowicz and Pin (1989), the geo chem i cal data sug gest (1) the co-magma- tic or i gin for all the ophiolite seg ments, (2) the oc cur rence of mafic cumulates, and (3) the re sem blance to mag mas gen er ated in a MOR or back-arc set ting. The es ti mates of dD and d18
O point to sea-floor meta mor phism of the ophiolite (Jêdrysek & Ha³as, 1989). Dur ing obduction of the ophiolite the metagabbros re corded the youn ger meta -mor phic over print un der greenschist fa cies con di tions (Majerowicz et al., 2000; Floyd et al., 2002). Floyd et al. (2002) sug gest, in con trast to the ear lier stud ies (e.g., Maje-rowicz & Pin, 1989, 1994), that the dif fer ences be tween vol ca nic and plutonic mem bers of the Œlê¿a ophiolite pre -clude their af fil i a tion to one co her ent ophiolitic suite.
The Œlê¿a ophio lite has been origi nally in ter preted as in di vid ual in tru sions of ma fic and ul tra ma fic mag mas in their present- day set ting (Finckh, 1928; Teis seyre et al., 1960; Oberc, 1972). Since the be gin ning of the 1980s, a con sen sus has been achieved that the Œlê¿a ophio lite rep re -sents a ves tige of oce anic crust (Ma jerowicz, 1979, 1981; Narêb ski et al., 1982; Ma jerowicz & Pin, 1986). Con se -quently, the tec tonic mod els pos tu lat ing the al lochthonous po si tion of the ophio lite be gan to pre domi nate (Zno sko,
Fig. 1. Geo log i cal sketch map of the Œlê¿a en vi rons (based on Majerowicz, 1995, 2006): 1 – bi o tite granodiorite, 2 – gran ite around the con tact zone of am phi bo lites, gab bro, and serpentinites, 3 – Wierzbice two-mica gran ite, 4 – alaskite metagranite, 5 – un di vided granitoids, cov ered, 6 – metagabbro, 7 – metagabbro, cov ered, 8 – am phi bo lites, 9 – am phi bo lites, cov ered, 10 – serpentinites of the Gogo³ów–Jordanów mas sif, 11 – serpentinites, cov ered, 12 – gneiss es of the Góry Sowie Mts., 13 – gneiss es of the east ern cover of the Œlê¿a Mas sif, 14 – phyllites, epimetamorphic greywacke and silliceous schists, 15 – phyllites, epimetamorphic greywacke and silliceous schists, cov ered, 16 – mylonites, 17 – meta mor phic schists
1981; Que nar del et al., 1988; Cymer man, 1987). The ma -jor ity of re cent tec tonic in ter pre ta tions are based on pseudo stratigraphy of the ophio li tic com plex with a young -ing di rec tion to wards the north. Bas -ing on the re sults of his struc tural analy sis, in clud ing joints and slick en sides, Jêdry sek (1985) sug gested that the en tire ophio lite is in an over turned po si tion with the se pen tinites oc cu py ing a struc tur -ally high est level. Ac cord ing to Ma jerowicz and Pin (1989, 1994), the ophio lite is thrust over to wards the north on the low- grade meta mor phic phyl litic base ment. Mier ze jewski and Abdel Wahed (2000) pos tu late two phases of de for ma -tion, both be ing as so ci ated with dis place ments along the low- angle dis lo ca tions dip ping to the east. In the tec tonic model re cently pre sented by Win ches ter et al. (2002) for the north ern part of the Bo he mian Mas sif, the Œlê¿a ophio lite rep re sents the up per most unit in a nappe pile over thrust to -wards the west dur ing the Varis can orog eny. The time of early ser pen ti ni za tion in ul tra ma fic rocks of the Œlê¿a ophio lite has been dated at 400±3 Ma bas ing on the zir con ages, which were ob tained for the rod in gite from Nas³awice (Du bi ñska et al., 2004). This age is con sis tent with the ear -lier dat ing of the Œlê¿a gab bro, the pro to lith of which was dated at ca. 420 Ma (Ol iver et al., 1993). The whole rock Sm- Nd age of 353±21 Ma, ob tained for the Œlê¿a ophio lite by Pin et al. (1988), pre suma bly cor re sponds to meta mor -phism or cool ing of the ophio lite com plex.
The Gogo³ów–Jor danów Mas sif con sists of serpen -inized ul tra ma fic rocks that crop out in the area of 90 km2. The mas sif be longs to a group of mafic- ultramafic ophio lite mem bers that sur round the Sowie Góry gneis sic mas sif. It in cludes peri do titic rocks, which un der went ser pen ti ni za -tion to a dif fer ent de gree. To the south, the ser pen tinites are in tec tonic con tact with the Sowie Góry gneis ses via a sys tem of Neo gene faults. To the north, they con tact with ul tra ma fic cu mu lates and gab bros, and to the NW with grani -toids of the Strze gom–Sobótka Mas sif. This bound ary is cov ered by Ce no zoic strata, like that of the Gogo³ów–Jor danów Mas sif with Pa laeo zoic meta mor phic se ries (phyl -lites) on the east.
Brightly- coloroured, fine- grained vein rocks pres ent in ser pen tinites in clude aplites and as so ci ated grani toids. Some of these rocks, like those ex posed at Czarna Góra Mt. (Span gen berg, 1943) or in a mag ne site mine at Wiry (Sa chan bi ñski, 1984), rep re sent pla gio gran ites. Chro mite de -pos its oc cur as lenses at Czarna Góra and near the T¹pad³a Pass, close to the bound ary with ul tra ma fic cu mu lates (Spangenberg, 1943; Bi recki, 1962). For ma tion of the chro mite bod ies was proba bly as so ci ated with man tle mag ma -tism in a re gime of trans fom faults (Gu nia, 1989).
METH ODS
Field and lab o ra tory work
The prin ci pal aim of the field work was mesostruc tural analy sis and col lec tion of sam ples for chemi cal, X- ray, and iso topic analy ses. Both chemi cal and iso topic analy ses were aimed at es ti mat ing mu tual re la tion ships be tween iso topic com po si tion of hy dro gen and oxy gen and the ori en ta tion of brit tle struc tures. Ori ented rock sam ples were col lected
manu ally from pre vi ously meas ured joint sur faces and slick en sided sur faces. Sam ples for iso topic analy ses were treated me chani cally to ob tain pow dered ma te rial.
Vac uum iso topic prepa ra tion is used to ob tain se lected ele ments de rived from the rock in a gase ous state. The com -mon fea ture of these pro ce dures is cryo genic clean ing of the gases that en ables meas ur ing iso topic com po si tion of the lat ter in a mass spec trome ter. For iso topic analy sis it is nec -es sary to ob tain hy dro gen and oxy gen from sili cat-es, and car bon di ox ide from car bon ates.
Analy ses of iso topic com po si tion of oxy gen and hy dro -gen were made in the De part ment of Iso topic Ge ol ogy and Geoe col ogy of the In sti tute of Geo logi cal Sci ences, Uni ver -sity of Wro claw, in the Labo ra tory for Mass Spec trome try of the Fac ulty of Phys ics and Ap plied In for mat ics of the AGH Uni ver sity of Sci ence and Tech nol ogy, as well as in the De part ment of Geo logi cal Sci ences, In di ana Uni ver sity, Bloom ing ton, USA. The Fin ni gan Mat delta E, Fin ni gan MAT 252, and Var ian Mat CH7 spec trome ters were used. De ter mi na tions of the car bon ate con tents and iso topic analy ses of car bon and oxy gen de rived from car bon ates were con ducted in the De part ment of Iso topic Ge ol ogy and Geoe col ogy, In sti tute of Geo logi cal Sci ences, Uni ver sity of Wroc³aw, us ing the Fin ni gan Mat delta E mass spec trome -ter.
Nu mer i cal mod el ling
The prin ci pal aim of this work was to con struct and im ple ment a dig i tal model por tray ing the in ten sity of meta mor phic fluid mi gra tion, with the use of iso to pic mass bal -ance. The model is mainly aimed at es ti mat ing the scale and di rec tions of mi gra tion of serpentinizing flu ids, as well as at ex plain ing the in ter ac tion of these flu ids with ultra mafic rocks us ing the pro cess of Ray leigh dis til la tion. The model is based on a ba sic as sump tion of channelized fluid mi gra tion in the rock. The pre sented model is a static ho mo mor -phic model. The in put data were discretised and sub di vided into cal cu la tion blocks of iden ti cal spa tial di men sions.
The main source of in put data are pieces of in for ma tion com prised in the data base, such as ori en ta tion of joints, slick en sides and tec tonic straie, as well as iso to pic anal y ses of ori ented samples.
Tak ing into ac count com pli cated geo log i cal struc ture and the lack of possibililties of de ter min ing prin ci pal tec -tonic di rec tions in the field, a method of clus ter ing pla nar struc tures was used (Myd³owski & Jêdrysek, 2004). Ori en -ta tion of a given frac ture set was de ter mined bas ing on the an gle com prised be tween the se lected pairs of read ings, i.e. the an gle be tween vec tors per pen dic u lar to the frac tures plot ted on the Schmidt’s net. The an gle was calculated in the following way:
For each pair of an gles: az i muth of the dip ai and dip bi ,
the co ef fi cients xi, yi, zi were de ter mined:
x y z i i i i i i i i = - × = - × = cos sin sin sin cos a b a b b (1) The an gle be tween the se lected pairs of mea sure ments is (Myd³owski & Jêdrysek, 2004):
g = × + × + × + × × + + arccos x x y y z z x y z x y z 1 2 1 2 1 2 1 2 1 2 1 2 2 2 2 2 2 2 (2)
De ter mi na tion of the av er age value of an gle g be tween the se lected mea sur ing point and the re main ing points pro vides a ba sis for dis tin guish ing in di vid ual groups of mea -sure ments, their num ber and spa tial ori en ta tion.
In or der to visu al ise the in put struc tural data and the ef fects of their clus ter ing, a tool was de vised to pro ject se lected struc tures on the Schmidt’s net, en abling the im me di -ate over view of read ings col lected from a given exposure.
Com pu ta tional pro ce dures used for each of the ex po sures made it pos si ble to col lect a set of data which em -braces both ori en ta tion of the struc tures and the re sults of iso to pic and chem i cal anal y ses. The data ob tained pro vided a ba sis for de fin ing sets of po ten tial paths of meta mor phic flu ids. The pres ence of such paths is based on an as sump -tion that flu ids mi grated through the rock mas sif uti lis ing frac tures, which, in gross part have been con tin u ous and open for fluid mi gra tion (Jêdrysek, 1989; Jêdrysek & Ha³as, 1989). Such frac tures could have rep re sented po ten tial paths of serpentinizing flu ids, and their spa tial dis tri bu tion was ana lysed qual i ta tively. To de ter mine priv i leged ori en ta -tions of tec tonic prin ci pal stresses in the Gogo³ów–Jorda-nów Mas sif, tech niques of palaeostress anal y sis were used. These ori en ta tions were as so ci ated with the tec tonic re gime that ex isted dur ing serpentinization of the rocks, and be -came a ref er ence point for the re sults of mod el ling of fluid mi gra tion paths based on iso to pic com po si tion of min er als. Visu ali sa tion of the re spec tive stress field was made for the NDA (Spang, 1972), in ver sion (Angelier & Goguel, 1979; Angelier, 1979), and right dihedra meth ods (Angelier & Mechler, 1977). To ob tain the ex pected stress pat tern at ev ery point of the study area, the axes of max i mum com pres -sion cal cu lated for ev ery ex po sure were in ter po lated for the entire study area. The same procedure was applied in rela-tion to the minimum and neutral stress axes.
For the sake of the model, a tem per a ture cal cu la tor was con structed bas ing on curves of a iso to pic frac tion ation, en abling fast con ver sion of the co ef fi cient of a iso to pic frac -tion a-tion into the D value and vice versa. It is also pos si ble to read tem per a tures at a given value of a, i.e. the co ef fi cient of iso to pic frac tion ation bas ing on a se lected curve for a known tem per a ture. In ter ac tive plot of a vs. t°C por trays the trend of se lected curves of iso to pic frac tion ation in the re quired tem per a ture range. More over, a pos si bil ity ex ists to se lect the re quired curve from the li brary of iso to pic frac -tion a-tion curves ob tained from the SIFC (Sta ble Iso tope
Frac tion ation Cal cu la tor; Beaudoin & Therrien, 2004) and
from other sources, what en hances the model flex i bil ity and makes the choice of a given curve much more re li able (e.g., Wenner & Tay lor, 1973; Sakai & Tsutsumi, 1978; Gra ham
et al., 1980, 1987; Satake & Matsuo, 1984; Saccocia et al.,
2001) at a stage of model con struc tion. The tem per a ture cal cu la tor is ac tive in other tasks of the model, be ing then ac ti -vated with out the graphic part pro vid ing only the re sults of cal cu la tions.
Ap pli ca tion of the Ray leigh dis til la tion model made it pos si ble to de ter mine the ex tent and vari abil ity of dD and d18
O of the rock and fluid in re spect to the tem per a ture and
ad vance ment (F) of the re ac tion (Ray leigh, 1896; Hoefs, 1996; Val ley & Cole, 2001). In or der to de ter mine the mo lar pro por tions of wa ter/rock in ter ac tions, the geo chem i cal and struc tural parts of the model were used. Both the ini tial iso -to pic com po si tion, as well as the tem per a ture and wa -ter-to-rock mo lar ra tio of the so lu tion en ter ing the mas sif rep re sent unkown val ues. The base of de ter mi na tion of the wa ter-to-rock ra tio is the set of val ues of dD and d18
O of the rock, ar ranged ac cord ing to a given fluid mi gra tion path, i.e. in the or der of the pro gress ing re ac tion. There is a link be -tween each pair of the dD – d18
O val ues of the rock and a given point on the sur face of the study area. Us ing an iterrative tech nique, the shape of cur va ture of the model vari abil ity of dD – d18O was ap plied to the real course of dD – d18
O from a given mi gra tion path through the rel e vant change of tem per a ture of re ac tion, the ini tial iso to pic com po si tion of the so lu tion, and the wa tertorock ra tio. The re -sult is given to gether with the de gree of fit ness (£omnicki, 1999) to the set of real val ues of dD and d18
O, show ing that frag ment of the mi gra tion path which meets the as sumed cri te ria of the good ness of fit. Visu ali sa tion marks the se -lected frag ment of the fluid mi gra tion path in a 3D view of the study area.
The pro posed model makes it pos si ble to trace re la tion -ships be tween in di vid ual ar eas of the stud ied ser pen tin ite mas sif, the tem per a ture of serpentinization, and mu tual share of the given wa ter types in shap ing the re cent iso to pic com po si tion of hy dro gen and ox y gen of the serpentinites. For each path oc cu py ing the known sur face of the study area, it is pos si ble to cal cu late the rock mass, which, to -gether with the known wa ter-to-rock mo lar ra tio makes it pos si ble to de ter mine the min i mal mass of serpentiniziting fluid en ter ing the mas sif. More over, know ing the re main ing frac tion of fluid, F, at each point of the study area, it is pos -si ble to cal cu late the in ten sity of serpentinization and probable sites of fluid vanishing.
An ap pli ca tion was con structed which en abled proper ac tiv ity of the model cal cu lat ing the iso to pic mass bal ance of serpentinizing flu ids. When con struct ing this ap pli ca tion, the ex treme pro gram ming was used (eX treme Pro gram -ming, XP), ex cept the pair pro gram ming. The pro gram in uncompiled ver sion includes 16,120 code lines.
RE SULTS AND DIS CUS SION
In this pa per, I used 511 mea sure ments of frac tures and slick en sides, as well as 102 ori ented sam ples col lected from 37 ex po sures. To gether with ad di tional anal y ses, 76 iso to -pic anal y ses of hy dro gen from the group OH, 52 ox y gen anal y ses of sil i cates, 11 ox y gen anal y ses from mag ne tite, and 34 iso to pic anal y ses of car bon and ox y gen from car bon -ates were made.
The re sults of hy dro gen iso to pic anal y ses (dD) of ser -pen tin ite rocks vary from –102.3 ‰ to –13.4 ‰, whereas those of the Œlê¿a gab bros bor der ing from the north the Gogo³ów–Jordanów Mas sif range be tween –49.6 ‰ and –2.9 ‰. The high est val ues of d18
O of ser pen tine at tains 11.15 ‰, the low est is 1.45 ‰. For the gab bros, the d18
val ues are 7.32 ‰ and 5.48 ‰, re spec tively. From some sam ples, it was pos si ble to ob tain mag ne tite, the d ox y gen val ues of which ranged be tween 0.44 ‰ and 5.85 ‰. Dif -fer ences of hy dro gen D (dmax – dmin) from ser pen tine be
-tween sam ples ori ented in the scale of in di vid ual ex po sures, are re mark able (Fig. 2).
The wa ter-to-rock mo lar ra tio in re la tion to the Ray leigh dis til la tion
Be fore in cor po rat ing hy dro gen par ti cles from the fluid into ultra mafic rock dur ing serpentinization, the wa -ter-to-rock mo lar ra tio (W/R) should be close to in fin ity for the hy dro gen, and af ter the re ac tion is fin ished it should ap -proach zero (all hy dro gen re acted with the rock). How ever, the hith erto used for mula for the W/R ra tio (Tay lor, 1977), di rectly ap plied to the Ray leigh dis til la tion model, im plies that this ra tio will never at tain the en tire range of val ues (¥:0).
At tempts at de ter min ing the W/R ra tio, us ing the rule pub lished by Tay lor (1977) in re la tion to the Ray leigh dis -til la tion, were un suc cess ful. Ac cord ing to the au thor, the W/R ra tio at the on set of re ac tion of serpentinizing flu ids is ap prox i mated best by the fol low ing formula (Myd³owski, 2006): W R rrz f rrz i w i rrz f = -( ) ( ) d d d d (3) where:
– drrzi, drrzf are the ini tial and fi nal, re spec tively, gen u
-ine iso to pic com po si tion of the rock mea sured in a sam ple, – dwi is the ini tial iso to pic com po si tion of the fluid = the
fi nal ex pected com po si tion of the rock (the fi nal iso to pic rock com po si tion re sult ing from the Ray leigh dis til la tion model).
The wa ter-to-rock mo lar ra tio will at tain val ues de rived from a set of real pos i tive val ues. Know ing W/R val ues be fore the iso tope ex change re ac tion starts is of cru cial im por -tance in es ti mat ing the amount of fluid re act ing with the rock and its fur ther in flu ence on metamorphosis.
A sum mary char ac ter iz ing po ten tial pos si bil i ties of channelized fluid mi gra tion (Fyfe et al., 1978; Thomp son, 1987) in the Gogo³ów–Jordanów Mas sif (Jêdrysek, 1989; Jêdrysek & Ha³as, 1989) is shown in Fig. 3.
A num ber of sce nar ios in clud ing dif fer ent or i gin, their amounts and tem per a tures of re ac tion with the rock, which could have led to the pres ent val ues of dD and d18O may be gen er ated. There fore, it was as sumed that if re cent iso to pic com po si tion of serpentinites of the Gogo³ów–Jordanów Mas sif can be achieved at the low est pos si ble num ber of flu -ids and at the great est pos si ble amounts of the lat ter. Hence, the mod el ling started with the most sim ple sce nario, in clud -ing one-stage serpentinization.
Fig. 2. Com par i son of D (dmax – dmin) hy dro gen and ox y gen
val ues in the ser pen tine be tween ori ented sam ples at the sin gle out crop scale
Fig. 3. 3D view of the mi gra tion path of serpentinized flu ids from se lected path’s group on the back ground of geo log i cal map. The squares ex plain the lo cal ori en ta tion of joint sur faces. The black curves shows pre dict able di rec tion of the mi gra tion paths through the Gogo³ów-Jordanów Mas sif. The se lected out crops are also marked
One-stage serpentinization
The aim of one-stage mod el ling of serpentinization of the Gogo³ów–Jordanów Mas sif is to ex plain iso to pic vari -abil ity of hy dro gen and ox y gen in the most sim ple, pos si ble way. Should this model be true, it would con tra dict the re sults ob tained by other au thors, who ap plied dif fer ent meth -ods (Jêdrysek & Sachanbiñski, 1994). One stage of serpen-tinization means that the re cent iso to pic com po si tion of the rock (dD and d18O) orig i nated in each mi gra tion path due to re ac tion with a de fined amount of fluid, ge net i cally as so ci -ated with one source and of known tem per a ture of re ac tion (Fig. 4).
Two-stage serpentinization
A two-stage model of serpentinization should en able the re ac tion of rock masses with two flu ids of any iso to pic com po si tion (dD, d18
O), tem per a ture, and the amount of
(W/R(H,O)). The fluid en ter ing the mas sif in the sec ond
stage re acts with the rock of iso to pic com po si tion mod i fied by a fluid of the first stage. The two-stage mod el ling should be pre ceded by ver i fi ca tion whether the pres ence of two flu ids is suf fi cient to change iso to pic com po si tion of ultra -mafic rock (dD, d18
O) to the pres ent-day state (re verse mod el ling), keep ing the dD and d18O val ues of de liv ery sources known from nat u ral en vi ron ment. This can be ver i -fied by us ing graphic con struc tion shown in Fig. 5.
Multi-stage serpentinization
A model of multi-stage serpentinization should en able one to ex am ine the vul ner a bil ity of iso to pic com po si tion of the rock (dD, d18O) to changes re sult ing from dD and d18
O of wa ter, tem per a ture of re ac tion, and mo lar ra tios W/R(H,O)
for sev eral con sec u tive stages. It is as so ci ated with the in -flu ence, on the same rock mass, of sev eral, con sec u tive serpentinizing flu ids of vari able iso to pic com po si tion (dD, d18
O), dif fer ent tem per a ture, and the amount of (W/R(H,O)).
Fluid of the fol low ing, youn ger stage mod i fies iso to pic
Fig. 4. Graphic in ter pre ta tion of wa ter-to-rock mo lar ra tio in the dD – d18O space with marked ini tial iso to pic com po si tion of the rock, vari abil ity dD and d18O of rock from se lected mi gra tion path, and ad justed to model dD and d18O of the rock. Ex pla na tions: dirrz, dfrrz –
ini tial and fi nal real iso to pic com po si tion of rock, re spec tively – iso to pic com po si tion from the rock-sam ple mea sured; diw – ini tial iso to
pic com po si tion of fluid = fi nal ex pected iso to pic com po si tion of rock (fi nal iso to pic com po si tion of rock ac cord ing to Ray leigh dis til la -tion); dir ocz – ini tial ex pected iso to pic com po si tion of rock; dfr ocz – fi nal ex pected iso to pic com po si tion of the rock
com po si tion of the rock, al ready al tered by flu ids ac tive dur ing pre vi ous stages of serpentinization. In this sense, the mo lar ra tio W/R(H,O) ac quires a new mean ing, con strain ing
mo lar pro por tions of wa ter from a given stage of serpentini-zation in re spect to the rock al tered by pre vi ous flu ids.
The ba sic task pre ced ing multi-stage mod el ling con sists in ver i fy ing whether the pres ence of at least three flu ids is suf fi cient for chang ing iso to pic com po si tion (dD, d18O) of ultra mafic rock to the pres ent-day state (re verse mod el ling), main tain ing at the same time the dD and d18O val ues of de
-liv ery sources found in nat u ral en vi ron ment. Fig. 6 shows graphic in ter pre ta tion of such a ver i fi ca tion.
Each of three iso to pic de liv ery sources A, B, C can be lo cated at a point (dD, d18O), whose de vi a tion (DD, D18
O) from the iso to pic com po si tion of the rock is in versely pro -por tional to the W/R mo lar ra tio (W/RA, W/RB, W/RC). The
DD/D18
O ra tio is de pend ent on mo lar pro por tions be tween hy dro gen and ox y gen within the fluid and the rock. In case of pure dis tilled wa ter, this pro por tion equals to 2. This fig -ure may in sig nif i cantly shift de pend ing on the so lu tion’s pH: ris ing at low pH, and fall ing at high pH. The hy dro gen/ ox y gen mo lar ra tio for peridotite ap proaches zero, while for ser pen tine it is close to 2/7 (Tay lor, 1974). More over, each of three iso to pic de liv ery sources A, B, C is lim ited by en vi ron men tal con straints. The pat tern pre sented in Fig. 6 im -plies that con tem po rary iso to pic com po si tion (d18O and dD) of ser pen tin ite, as a com bined prod uct of iso to pic de liv -ery sources, could have orig i nated due to the in flu ence of three sources: oce anic, mag matic, and me te oric. It was ac -cepted that d18O and dD val ues of me te oric wa ter (source
C) match those of the Wroc³aw wa ter (Duliñski et al., 2001). Wa ter of mag matic or i gin has the great est share in mod el ling vari abil ity of d18O and dD in ser pen tin ite, ow ing to high W/RB ra tio, while wa ters of oce anic and me te oric
or i gin are of mi nor im por tance. As sum ing that source B is com pat i ble with iso to pic com po si tion of meta mor phic or for ma tion wa ters of lower dD and higher d18O ranges, the W/R ra tio of source B would be lower, and frac tions of A and C sources should in crease, to ob tain the same joint prod uct of d18O and dD, typ i cal for ser pen tin ite. The model shown in Fig. 6 is a gen eral one, and does not take into ac -count all fac tors con trol ling the real iso to pic com po si tion of
Fig. 5. Ar range ment of two fluid sources with re spect to hypothetic fluid source de ter mined in one-stage serpentinization model
Fig. 6. Test of pos si bil ity to ob tain the pres ent val ues of ox y gen and hy dro gen iso to pic com po si tion in ser pen tin ite through mod i fi ca -tion with al ter a-tion of three sources of serpentinizing flu ids
the flu ids ac tive in serpentinization, like, for in stance, those of tem per a ture or vari abil ity of d18O and dD of the rock along fluid mi gra tion paths.
dD and d18
O val ues of the serpentinizing flu ids in a 3-stage model of serpentinization of a se lected mi gra tion path in the Gogo³ów–Jordanów Mas sif are plot ted in Fig. 7.
Tem per a tures of re ac tion and mo lar ra tios W/R cal cu lated with equa tion (3) are also given. The re sults are true for a mi gra tion path run ning from the south-east ern edge of the ser pen tin ite mas sif to wards its cen tre (Fig. 8).
Know ing mo lar re la tion ships of each stage of serpen-tinization, it is pos si ble to cal cu late the to tal mass of flu ids ac tive in meta mor phic al ter ing of the se lected frag ment of the Gogo³ów–Jordanów Massif.
IN FLU ENCE OF CO
2-BEAR ING FLU IDS
ON THE CON TENT OF CAR BON ATES
Nu mer ous au thors tried to build mod els ex press ing the mode of rock-CO2bear ing fluid in ter ac tion (Rye & Wil
-liams, 1981; Matsuhisa et al., 1985; Zheng, 1990). Some mod els as sume the bal anced re ac tions fluid-rock (Zheng & Hoefs, 1993), what is not the case in the Gogo³ów–Jorda-nów Mas sif. How ever, none of the so lu tions known to the au thor ac cept dif fer en ti a tion of the iso to pic d13C and d18O com po si tion of the rock and fluid bas ing on the Ray leigh dis til la tion. Dur ing an at tempt of cal cu la tion of the mass of CO2 and de ter min ing their source of or i gin, the same rules
were used which ac com pa nied mod el ling of the bulk flow of the fluid com posed of wa ter. Since the well-cal i brated curve for car bon for magnesite-CO2 is not known, the curve
of iso to pic frac tion ation, a, for of do lo mite-CO2 (Sheppard
& Schwarcz, 1970; Ohmoto & Rye, 1979) was used, due to close re sem blance of both curves (Weber-Weller, 2000; Gartzos, 2004). More over, the amount of car bon ates within the rock was taken into ac count. Com par ing the amounts of
Fig. 7. A – iso to pic fluid source in a 3-stage serpentinization model and model pro cess of dD and d18O of the rock af ter each serpentinization stage, with fixed val ues of tem per a ture and mo -lar-to-rock ra tio on the back ground of dD and d18O of ser pen tin ite. Mark ers e1, e2 i e3 de note one-, two- and three-stages of serpen-tinization. Dashed and solid lines shows the changes dD and d18O of rock and wa ter to wards Ray leigh dis til la tion; B – a part of Fig. 7a. Model pro cess of dD and d18O of rock (e1 + e2 + e3) with fixed val ues of tem per a ture and mo lartorock ra tio on the back -ground of dD and d18O of ser pen tin ite (cf. Fig. 8)
cal cite and magnesite dis persed in the ser pen tine, it is likely to see that magnesite con sti tutes the main car bon ate phase, par tic u larly when the amount of car bon ate min er als within ser pen tin ite in creases.
PAT TERN OF TEC TONIC STRESSES IN
THE GOGO£ÓW–JORDANÓW MAS SIF
The com pres sive stresses axis is ori ented east-west and shows shal low plunge. Its ori en ta tion im plies hor i zon tal com pres sion, ori ented E–W to NE–SW in the SW part of the mas sif. The tra jec to ries of s1 stress mark the pre ferred
strike ori en ta tion of extensional joints and faults, which are po ten tial paths of fluid mi gra tion. Com pres sion axes de ter -mined by the in ver sion method are, un like those ob tained from the NDA method, clearly de flected from the east-west ori en ta tion, and in the east ern part tend to par al lel the ser -pen tin ite-gab bro bound ary. On the other hand, the axis of ten sional stress is nearly ver ti cal, point ing to small thick -ness of the over bur den dur ing de for ma tion. High val ues of R co ef fi cients for the NDA method and F co ef fi cients for the in ver sion method in di cate con sid er able elon ga tion of stress el lip soids.
CON CLU SIONS
Iso to pic anal y ses of ori ented sam ples col lected from the Gogo³ów–Jordanów Mas sif, to gether with mea sure -ments of pla nar struc tures and a com puter-gen er ated model of the channelised iso to pic fluidrock in ter ac tion, al low cal cu la tion of the iso to pic mass bal ance. Bas ing on the vari -abil ity of dD and d18O val ues for serpentinites along the paths of fluid mi gra tion it was found that serpentinization of peri dot ites was driven by flu ids de rived from at least three sources of dif fer ent iso to pic char ac ter is tics. The three fol
-low ing stages of serpentinization were dis tin guished that were as sisted by oce anic, mag matichy dro ther mal and me -te or itic wa -ters.
1. The magmohy dro ther mal flu ids played the dom i -nant role in the serpentinization of peri dot ites since they con sti tuted ca. 95% of the to tal mass of flu ids, whereas the mass of oce anic wa ter ac counts to ca. 1% of the to tal mass of fluids.
2. To serpentinize 1 m3 of rock, ca. 98 × 104 kg of fluid was nec es sary. The in ten sity of iso to pic ex change be tween the fluid and the rock within a mi gra tion path, por trayed by the wa ter-to-rock mo lar ra tio, is strongly dif fer en ti ated, and re cal cu lated into the mass of fluid changes be tween 44 × 103
kg/m3 to 56 × 105 kg/m3 of the rock.
3. Strong dif fer en ti a tion in the in ten sity of iso to pic ex change within a sin gle mi gra tion path was ob served: the ac -tiv ity of flu ids di min ishes with fluid mi gra tion along the path and some times drops to zero, leav ing the rock unaltered.
4. Flu ids bear ing CO2 most prob a bly mi grated ver ti
-cally. The d13
C val ues of dis persed magnesites point to a close re la tion ship with endogenic sources. In the west ern part of the mas sif, d13
C of dis persed magnesites at tains a broader range of val ues com pared to that of the east ern part, sug gest ing the in flux of CO2-bear ing flu ids from sev eral
feed ing iso to pic sources.
The com pat i bil ity of mi gra tion paths withthe tra jec to -ries of com pres sive stresses in the Gogo³ów–Jordanów Mas sif points to a de ci sive role of frac tures and faults in the in fil tra tion of serpentinizing fluids.
The pre sented re sults do not di rectly im ply a tec tonic set ting for serpentinization. In stead, they al low es ti ma tion of a source of fluid that was ap par ently dom i nated by hy dro -ther mal wa ters. This cor ol lary does not nec es sar ily mean that serpentinization oc curred in a con ti nen tal set ting. Hy -dro ther mal wa ters could have po ten tially al tered the ear lier
Fig. 8. The mi gra tion path ar range ment of serpentinizing flu ids (Fig. 7a) in the study area. The black line shows the point of en ter ing fluid into the rock and range of in flu ence on the rock
iso to pic com po si tion of ser pen tin ite ac quired dur ing sea-floor meta mor phism. In such a case, the pres ent iso to pic com po si tion would rep re sent a later stage of hydratation that was ac tive during and/or after obduction of the Œlê¿a ophiolite.
Ac knowl edge ments
I would like to ex press my sin cere thanks to the su per vi sors of my Ph.D. the sis, the se lected por tions of which are pre sented in this pa per, namely to: Prof. Mariusz Orion Jêdrysek and Prof. Stanis³aw Mazur, for their in dis pens able help, sup port and knowl -edge shar ing. Prof. Stanis³aw Ha³as and Prof. Piotr Gunia are grate fully thanked for their de tailed re views.
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