Meteoric calcitization of magnesite in Miocene lacustrine deposits (Calatayud basin, NE Spain)

ELSEVIER Sedimentary Geology 119 (1998) 183–194

Meteoric calcitization of magnesite in Miocene lacustrine deposits(Calatayud basin, NE Spain)

J.C. Canaveras a,b,Ł, S. Sanchez-Moral a,c, E. Sanz-Rubio a, M. Hoyos a

a Departamento Geologıa, Museo Nacional de Ciencias Naturales, CSIC, Jose Gutierrez Abascal, 2, E-28006 Madrid, Spainb Departamento Ciencias de la Tierra y del Medio Ambiente, Fac. de Ciencias, Universidad de Alicante,

E-03080 San Vicente Raspeig, Alicante, Spainc Departamento Geologıa, Fac. de Ciencias, Universidad de Alcala de Henares, N-II km 33.6,

E-28871 Alcala de Henares, Madrid, Spain

Received 2 September 1997; accepted 24 March 1998

Abstract

Sedimentary magnesite deposits are commonly present in evaporite successions in the Tertiary Calatayud basin (NESpain). Outcropping evaporite sequences, mainly composed of secondary gypsum after anhydrite–glauberite, show cen-timetre-thick layers of magnesite passing laterally into calcite pseudospar. Petrographic and stable isotope data indicate thatcalcite pseudospar formed via late-stage calcitization of magnesite under near-surface conditions. Calcitization occurredby the interaction between magnesite and Ca-enriched waters derived from the dissolution of gypsum and=or glauberite.The geochemical simulation of this process, petrographic evidence, and the correlation between the magnesite=calcite ratioand the Ž18O values of both magnesite and calcite samples across the reaction front are in agreement with the existence ofneoformed 18O-depleted magnesite during the waning stages of the calcitization process. 1998 Elsevier Science B.V.All rights reserved.

Keywords: magnesite; calcitization; evaporites; diagenesis; stable isotopes; Miocene; Calatayud basin

1. Introduction

With the exception of large stratabound accu-mulations, magnesite and related Mg-rich carbonatedeposits are not volumetrically important in sedi-mentary sequences, although these carbonates arecommon in modern coastal and continental evapor-itic environments (Muller et al., 1972; Pueyo andIngles, 1987; Last, 1992; among others), as well asin ancient sedimentary sequences (Zachmann, 1989).

Ł Corresponding author. Tel.: C34 (1) 411-1328 (ext. 1178); Fax:C34 (1) 564-4740; E-mail: [email protected]

Studies of sedimentary magnesite were traditionallyfocused on genetic aspects and on the characteri-zation of its depositional environments. Likewise,studies on the diagenesis of magnesite focused onthe primary (syndiagenetic) versus secondary ori-gin of magnesite (Irion and Muller, 1966; Aharon,1988; Last, 1992), but only few workers studied indetail the later diagenetic evolution of sedimentarymagnesite.

Magnesite is a common constituent of evaporitedeposits in the Calatayud basin. Localized calcitiza-tion affecting magnesite-rich mudstones interbeddedwith shales and gypsum have been recognized in

0037-0738/98/$19.00 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 6 3 - 3

184 J.C. Canaveras et al. / Sedimentary Geology 119 (1998) 183–194

this continental basin. The present work aims todocument and interpret the mechanisms of late-stagemeteoric alteration of magnesite to diagenetic cal-cite.

2. Geological setting

The Calatayud basin (NE Spain) is an intermon-tane depression filled by a succession of lacustrineand alluvial Tertiary sediments reaching at least 1200m in thickness in the central part. The sedimentaryfilling of the basin displays a concentric facies ar-rangement where clastics in the margins are progres-sively passing to evaporites and carbonates towards

Fig. 1. (A) Schematic geological map of the Calatayud basin. (B) Palaeogeographic map showing the facies distribution in the Calatayudbasin at the end of the Lower Unit (after Hoyos et al., 1996). Magnesites are mainly present at the inner lake margin and transitionalfacies.

the central area of the basin (Fig. 1A). Saline facies,corresponding to the top of the Lower Miocene Unit(Hoyos and Lopez-Martınez, 1985), are mainly com-posed of anhydrite, gypsum, and glauberite, whilsthalite and anhydrite were described in deep bore-holes realized in central areas of the basin. Althoughno sodium–calcium-sulphate minerals are present atthe surface, they appear replaced by secondary gyp-sum under near-surface conditions. Efflorescencesof epsomite constitute a common feature associatedwith outcropping evaporite deposits. Thenardite andmirabilite efflorescences have been also identifiedbeing specially common in the studied outcrops.Likewise, magnesium–calcium-sulphate and, in mi-nor proportions, sodium-sulphate waters are rela-

J.C. Canaveras et al. / Sedimentary Geology 119 (1998) 183–194 185

tively abundant in springs in the Calatayud basin. Adense network of joints is well developed throughoutthe evaporite sequences, thus favouring the infil-tration and subsurficial circulation of meteoric wa-ters. Karstification processes affecting these mate-rials and Quaternary alluvial deposits have beenreported by Hoyos et al. (1996) and Gutierrez-Santo-lalla (1996).

The distribution pattern of magnesite at the Ca-latayud basin is closely related to the areal extensionof evaporitic lacustrine deposits of the Lower Unit,being more abundant towards the marginal facies(Fig. 1B). Magnesite is the main carbonate min-eral directly associated with the saline facies inthe Lower Unit, occurring as: (i) white to grey-coloured, thin (0.5–2 cm thick) continuous laminaealternating with clay and gypsum layers after anhy-drite and=or glauberite (Fig. 2); (ii) intercrystallinematerial in aggregates of euhedral–discoidal crys-tals of gypsum after glauberite; or (iii) forming thematrix of displacive–replacive gypsified anhydritenodules. Dark organic matter-rich magnesite layersoccur in association with sapropelic beds. Magnesitelayers are generally structureless, only discontinuouslamination formed by plant remains or clay accu-mulations are present. Sulphate phases associatedwith magnesite under surface conditions consist en-tirely of secondary gypsum, but the occurrence ofpseudomorphs of glauberite and the existence ofsodium-sulphate efflorescences provide evidence forthe existence of glauberite in the evaporitic forma-tions of the Lower Unit (Sanchez-Moral et al., 1993;Ortı and Rosell, 1994).

In many outcrops along the basin, magnesite lam-inae pass laterally into calcite micro-pseudosparite(Fig. 3). The magnesite laminae and the magnesite-rich part of the laminae are readily discriminatedfrom calcite-rich laminae because the former arewhite to grey in colour, show higher porosity and areless hard than the brown-coloured limestones. Like-wise, in each discrete lamina, the calcite-rich partis slightly thicker than the pure magnesite part. Thetransition is relatively abrupt, taking place withinonly a few centimetres. These magnesite–calcite lay-ers form part of evaporite sequences mainly com-posed of alternating clays and nodular to enterolithicgypsum (Figs. 2 and 3). Sampling was focused onmagnesite–calcite layers that outcrop at San Ramon

Fig. 2. Stratigraphic column of the studied outcrop (San Ramonsection).

quarry (Lower Unit), near Calatayud City and adja-cent to the alluvial plain of the Jalon River (Fig. 1A).Quaternary fluvial deposits, belonging to terraces ofthe Jalon River, are overlying these units.

3. Materials and methods

Mineralogical, petrographic and geochemicalstudies are mainly based on twelve samples collectedfrom two approximately 1 centimetre-thick discretelayers of magnesite-rich mudstones that pass later-ally into beige calcite-rich sparite (Figs. 3 and 4A).

X-ray diffraction analysis was used to determinethe mineralogical composition of powdered sam-


Fig. 3. Detail of whitish magnesite layer passing laterally from left to right into darker calcite. White arrow marks the transitional zonewithin the layer interbedded with laminated clays and nodular gypsum. Coin is 2 cm in diameter.

ples. MgCO3 content of calcite was estimated bymeasuring the position of the (104) reflection usingquartz as an internal standard (Goldsmith and Graf,1958). Petrographic observations are based on theexamination of polished thin sections by transmittedlight microscopy. Selected samples were studied us-ing a Philips scanning electron microscope (SEM)equipped with an energy dispersive X-ray analyticalsystem (EDS). Oxygen and carbon isotope com-positions of calcite and magnesite-rich mudstoneswere determined using standard procedures (Mc-Crea, 1950; Kralik et al., 1989; Brydie et al., 1993).Pure magnesite samples were reacted in vacuo withanhydrous (100%) phosphoric acid at 50ºC for 72–120 h. Mixed carbonates were pre-treated by reactionwith phosphoric acid at 25ºC for several hours in or-der to eliminate the calcite fraction, and the isotopiccomposition of the remaining magnesite was then de-termined. Magnesite Ž18O values were corrected ac-cording to Perry and Tan (1972); 103 lnÞCO2�magnesite

was 1.0116 at 50ºC. Mixed-carbonates samples werereacted with phosphoric acid at 25ºC for 2 h in orderto analyse the isotopic composition of the calcitefraction. Both oxygen and carbon isotope values are

reported in the delta-notation relative to the PDBstandard. Geochemical calculations were performedusing the computer program PHRQPITZ (version0.2, 1990; Plummer et al., 1988).

4. Results

4.1. Mineralogy

XRD analyses indicate that the mineralogy ofthe studied materials consists predominantly of car-bonates (70–98%) and clays (0–27%), with minoramounts of quartz (0–5%) and gypsum (0–2%) (Ta-ble 1). In addition, Fe(Mn)-oxide crystals (<2%)have been observed under the microscope and SEM.The clay fraction is mainly composed of illite,Mg-smectite and kaolinite. The carbonates are exclu-sively magnesite and calcite in variable proportions;dolomite has not been found. The two discrete layersanalysed show a lateral transition with respect tothe proportion of these carbonate minerals (Fig. 4A),from a pure magnesite composition towards a cal-cite-rich composition.


Fig. 4. (A) Plot showing the relative location, mineralogical and stable isotope compositions of samples belonging to the two analysedlaminae. (B) Ž18O vs. Ž13C plot of magnesite and calcite pseudosparite from the Calatayud basin.


Table 1Mineralogical and stable-isotope data from two discrete laminae (SR1 and SR2) at San Ramon section

Sample Mineralogy Magnesite=calcite MgCO3 (mole%) Magnesite Calcite

Mag Cal Phy Q Gyp Mag Cal Ž18O Ž13C Ž18O Ž13C

SR1-1 82 0 16 2 0 1 95 – 4.9 �7.3 – –SR1-2 88 0 10 2 0 1 94 – 4.4 �7.1 – –SR1-3 60 10 27 3 0 0.85 94 4.6 3.4 �6.5 �5.4 �6.9SR1-4 52 24 19 3 2 0.6 95 4.6 1.8 �6.6 �5.4 �6.9SR1-5 34 52 12 2 0 0.4 94 3.0 1.3 �6.8 �5.5 �6.7SR1-6 25 61 10 2 1 0.3 94 5.0 0.1 �6.8 �4.8 �5.3SR1-7 29 68 0 2 1 0.3 92 3.0 0.0 �6.8 �5.2 �5.9SR1-8 14 81 0 5 0 0.15 95 5.0 �1.6 �7.1 �4.6 �5.2SR2-1 93 0 5 2 0 1 93 – 3.6 �7.9 – –SR2-2 89 0 9 2 0 1 93 – 4.2 �8.1 – –SR2-3 40 48 9 3 0 0.45 92 4.6 1.0 �7.4 �5.3 �6.4SR2-4 28 53 15 4 0 0.35 93 4.6 0.9 �7.2 �4.7 �4.0

Mineralogical composition is given in %. Stable-isotope composition in ‰ vs. PDB. Mag D magnesite; Cal D calcite; Phy D clayminerals; Q D quartz; Gyp D gypsum. See sample location in Fig. 4A.

4.2. Petrography and chemical composition

Petrographically, magnesite appears as structure-less mudstones; only thin lamination defined byclays with some terrigenous grains are present.Some peloidal and=or intraclastic textures togetherwith desiccation cracks and root traces were recog-nized. SEM examination reveals that pure magne-site mudstones consist of a homogeneous idiotopic–hypidiotopic mosaic composed of subhedral to rhom-bohedral crystals (Fig. 5A). The size of the individ-ual crystals ranges from 1 to 3 µm. Magnesite inmixed-carbonate samples is revealed by BSE imag-ing as cloudy to sub-rhombic cores within the calcitespar crystals (Fig. 5B). Two types of magnesite crys-tals have been recognized in these cores by SEMexamination of freshly broken surfaces: (i) euhedralto subhedral crystals, which are similar in shape andsize to magnesite crystals in pure samples; and (ii)anhedral crystals with sizes ranging from 1 to 5 µm(Fig. 5C). High resolution BSE imaging (Fig. 5B)in conjunction with EDS analyses reveals that bothmagnesite belonging to pure magnesite samples andmagnesite belonging to mixed-carbonate samples arecompositionally homogeneous; compositional zon-ing in the crystals is not observed. The calcitesconsist of a dense micro-pseudosparitic mosaic com-posed of anhedral crystals (10 to 100 µm; Fig. 5D).These crystals are rich in inclusions, mainly Fe-

oxides. Silica rosettes and relicts and=or pseudo-morphs of gypsum are present in variable amounts.A porosity reduction is evidenced in the calcite-richmicro-pseudosparites with respect to the pure mag-nesite precursor.

By measuring the position of the (104) re-flection, MgCO3 content of magnesite and cal-cite average 93.6 mole% and 4.3 mole%, re-spectively (Table 1). The chemical compositionof pure magnesites, determined by EDS, averageMg0.91Ca0.06Mn0.01Fe0.02CO3 .n D 6/. The elemen-tal composition of euhedral magnesite in mixed-carbonate samples is similar to magnesite in puresamples, averaging Mg0.905Ca0.055Mn0.017Fe0.018CO3

(mean of n D 8 analyses); although anhedral magne-site crystals show slightly lower Ca, Fe and Mn values(Mg0.967Ca0.03Mn<0.002Fe<0.002CO3; n D 5).

4.3. Carbon and oxygen-isotope composition

The Ž18O values of calcites in the two layers aver-age �5.1‰, ranging from �4.6 to �5.5‰ (Fig. 4).The magnesites show higher and more variable oxy-gen-isotope values (4.9 to �1.6‰), averaging 2‰.A significant correlation .r D 0:95/ between Ž18Ovalues of magnesites and the magnesite=calcite ra-tio of the samples is observed (Fig. 4A), the higheroxygen-isotope values corresponding to the magne-site-rich samples. The Ž18O values of calcites and the

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Fig. 5. (A) Idiotopic mosaic of magnesite crystals in pure magnesite samples. SEM photomicrograph. Scale bar 2 µm. (B) Magnesite (dark grey) within calcite pseudospar(light grey). BSE image. Scale bar 20 µm. (C) Magnesite crystals in mixed-carbonate samples. Two types of magnesite crystals can be observed: anhedral crystals and,euhedral to subeuhedral crystals as in pure magnesite samples. SEM photomicrograph. Scale bar 5 µm. (D) Photomicrograph (plane-polarized light) of calcite pseudosparite.Scale bar 150 µm.


magnesite=calcite ratio also show a moderately goodcorrelation .r D 0:7/. The Ž13C values of magnesitesaverage �7.1‰, ranging from �6.5 to �7.9‰. Thecalcites show a wider range of Ž13C values, from �4to �6.9‰, averaging �5.9‰ (Fig. 4B).

5. Discussion

The occurrence of fine-grained magnesite lami-nae associated with saline facies in the Lower Unitof the Calatayud basin and the high Ž18O valuessuggest an early (primary or syndiagenetic) originof the magnesites under evaporitic conditions. TheŽ18O values of Calatayud magnesites are in concor-dance with other fine-grained evaporitic lacustrinemagnesites (Kralik et al., 1989). The low Ž13C valuessuggest a continental environment where the water isenriched in soil-derived organic carbon. On the otherhand, the petrographic features (habit and size ofcrystals, abundance of Fe-oxide inclusions, existenceof magnesite relicts) and the low isotopic valuessuggest that the calcite micro-pseudosparites are ofsecondary origin (replacement) and probably formedunder the influence of meteoric waters. Petrograph-ically, the fabric described here is quite similar tothat of some Tertiary pseudosparites, also interpretedas the result of calcitization processes under oxidiz-

Fig. 6. (A) Isotopic composition of water in equilibrium with magnesite. Ž18O values of waters were calculated using the equationof Spotl and Burns (1994). Enhanced (black) area corresponds to purely magnesite samples; adjacent shaded (grey) area representsmagnesite in mixed-carbonate samples (see text). Ž18O water values for ‘late magnesite’ are equivalent to the isotopic composition ofwater in equilibrium with calcite at 15ºC. (B) Isotopic composition of water in equilibrium with calcite (calculated using the equation ofFriedman and O’Neil, 1977) (see text).

ing near-surface meteoric conditions (Rossi, 1993;Canaveras et al., 1996), although in those cases,the precursor was peritidal or lacustrine fine-graineddolomites. In mixed-carbonate samples, two types ofmagnesite crystal have been recognized, subhedralto rhombohedral crystals as in the pure magnesitesamples, and euhedral (platy to rhombic) crystals;the latter showing slightly higher MgCO3 content.This fact seems to be related to the existence of asecond, probably late diagenetic, generation of mag-nesite.

Assuming that the magnesites were formed inisotopic equilibrium under evaporitic conditions andusing the magnesite–water fractionation curves ofSpotl and Burns (1994), the Ž18O of the parent wa-ter would range from C4 to �2.7‰ (SMOW) (at25ºC) (Fig. 6A). If the calcites formed coevally withthe magnesite, the isotopic composition of their par-ent fluid (at 25ºC) would be quite different, with aŽ18O value ranging from �2.6 to �3.4‰ (SMOW)(Fig. 6B). These calculations also favour a late di-agenetic origin of the calcite via calcitization bymeteoric-derived ground waters under near-surfaceconditions. This process is similar to the mechanismcommonly suggested for near-surface dedolomiti-zation, i.e. reaction of CaSO4-enriched fluids withdolomite (magnesite in this case) producing cal-cite and releasing dissolved magnesium and sulphate


(Folkman, 1969; Sanchez-Moral et al., 1993). Thedissolution of interbedded gypsum and=or glauberitedrives the replacement of magnesite by calcite byproviding abundant calcium. Calcitization of magne-site proceeds according to the following reactions:

CaSO4.aq/CMgCO3 ! CaCO3CMg2CCSO2�4 (1)

Na2Ca.SO4/2.aq/CMgCO3 ! CaCO3 C 2 NaC

CMg2C C 2SO2�4 (2)

An attempt to model these reactions has been car-ried out using the geochemical simulation computercode PHRQPITZ. Two different pathways are postu-lated according to the reactions (1) and (2) (Fig. 7):(1) meteoric water infiltration through gypsum rock

Fig. 7. Flow diagram used for the geochemical simulation of the calcitization process. See text for explanation.

and subsequent circulation through magnesite layers;(2) meteoric water flow through glauberite=gypsumand further flow through magnesite layers. In bothcases, the chemical composition of the starting so-lution corresponds to pure water which is in equi-librium with a pCO2 value of 10�3.5 atm, that is, towaters that equilibrated with the atmosphere. Thefinal point has been fixed at the moment of for-mation of the main saline efflorescences (epsomite,mirabilite=thenardite) as is shown in field observa-tions. Assuming that calcitization took place undermeteoric conditions, the temperature of the circulat-ing waters was fixed at 15ºC which is the present-day annual mean temperature in the study area. Thedissolution=precipitation reaction of any mineral isaccomplished at equilibrium of the solution with the


given mineral. Mass mineral transfer at each stepof water–rock interaction is expressed in cubic cen-timetres of precipitated and=or dissolved mineral perlitre of water.

5.1. Meteoric water infiltration through gypsum(Fig. 7)

Infiltrated water dissolving gypsum leads to a cal-cium-sulphate solution. Then, the calcium-rich solu-tion circulates through the magnesite layer drivingreaction (1) to the right side, resulting in the for-mation of calcite at the expense of magnesite (0.95moles of calcite per mol of dissolved magnesite).The volume of neoformed calcite is 25.5% largerthan the volume of dissolved magnesite, which isin accordance with the porosity reduction revealedby field and petrographic observations. When themagnesite–calcite equilibrium is attained, the inter-stitial solution stays at near-surface conditions whereevaporation processes would be effective, causingthe precipitation of magnesite and degassing of CO2.Evaporation continues with the precipitation of bothmagnesite and gypsum, and finally the magnesium-sulphate-enriched solution reaches saturation withrespect to epsomite. The crystallization of late-stagemagnesite would mean that about 4.5% of the mag-nesite dissolved during the calcitization process.

5.2. Meteoric water infiltration throughglauberite=gypsum (Fig. 7)

In this case, infiltrated waters dissolvingglauberite causes precipitation of secondary gypsum.Then, the sodium–calcium-sulphate solution reachesthe magnesite layer and calcitization takes place witha mass transfer of 0.81 moles of neoformed calciteper mol of dissolved magnesite. In this reaction, themass mineral transfer is slightly lower than in theprevious simulation. Likewise, the increase in vol-ume is smaller (6.8%). Evaporation of the remainingsolution also leads to crystallization of magnesite.The proportion of this late-stage magnesite (9.8%)is higher than that of reaction (1). Finally, the solu-tion reaches saturation with respect to mirabilite, andshows a sodium-sulphate composition that is similarto those of the springs close to the studied outcrop(Sanchez-Moral et al., 1993).

These results fit well with the field observations,isotopic data and petrographic evidences, indicat-ing that the hypothesis of meteoric water infiltrationthrough gypsum- and=or gypsum–glauberite-bearingsequences and the following interaction with mag-nesite could be an effective mechanism for its cal-citization. Likewise, the first reaction pathway couldexplain the magnesium-(calcium)-sulphate composi-tion of some springs in the Calatayud basin and theoccurrence of epsomite efflorescences; and the sec-ond reaction pathway is more consistent with the oc-currence of mirabilite–thenardite efflorescences andthe sodium-sulphate composition of spring waters inthe study area.

The inferred neoformation of late-stage magne-site related to the calcitization should be reflectedby their stable isotope composition. Variations inMgCO3 content do not seem to be responsible forthe variation in Ž18O of the magnesites (as is thecase with magnesian calcites; Tarutani et al., 1969),because the molar MgCO3 content of all magnesitesis nearly constant (Fig. 4A). Therefore, the rela-tionship between the oxygen-isotope composition ofmagnesites and the magnesite=calcite ratio (Fig. 4Aand Fig. 8A) is thought to be due to the alterationof the isotopic composition of magnesite due to re-crystallization processes and=or the formation of latediagenetic O18-depleted magnesite during calcitiza-tion. Recrystallization at near-surface temperatures(15–25ºC) seems to be unlikely because magnesiteonce formed is thermodynamically stable (Christ andHostetler, 1970), and no petrographic evidence of re-crystallization has been found in these samples.

The fluid from the diagenetic calcite precipitatedhad a Ž18Owater value of �4.8 to �5.7‰ (SMOW)at 15ºC (Fig. 6B), which is consistent with me-teoric water. Using this isotopic composition as asource water for the precipitation of secondary mag-nesite, the isotope composition of this ‘late-meteoric’magnesite could have ranged from �0.6 to �1.6‰(PDB) (Fig. 6A). In order to explain the oxygen-iso-tope composition of the magnesite and considering aŽ18O value of C4 to C3.6‰ for the pure early mag-nesite, the proportion of the late magnesite wouldrange from 0 to 30% of the total carbonate fraction(Fig. 8B). This proportion would be similar to thatobtained from modelling, if we consider a lowerŽ18O mean value for the early magnesite. It is widely


Fig. 8. (A) Diagram showing the relation between the magnesite=calcite ratio and the oxygen-isotope values of carbonates. (B)Mineralogical composition of the carbonate fraction as calculated based on oxygen-isotopic analysis (lm=em D late magnesite=earlymagnesite ratio).

reported that evaporitic lacustrine magnesites showa considerable spread in Ž18O values (Kralik et al.,1989).

6. Conclusions

Tertiary evaporite successions of the Calatayudbasin are characterized by the common presence offine-grained sedimentary magnesite that is confinedto particular and continuous stratigraphic layers, andhas sharp contacts with overlying and underlyingbeds. In addition to these stratigraphic observations,textural and geochemical data provide evidence fora syndepositional origin of this magnesite. In out-crops, magnesite is replaced locally by micro-pseu-dospar calcite. Stable isotope data suggest that cal-citization took place under near-surface conditionsby 18O-depleted meteoric waters. The interactionbetween magnesite and Ca-enriched fluids derivedfrom the dissolution of overlying gypsum and=orglauberite beds is regarded as the driving force ofthis process. By-products of the calcitization processare magnesium-sulphate and=or sodium-sulphate so-lutions and precipitation of epsomite or mirabilite–thenardite efflorescences, respectively. Geochemicalsimulations and oxygen-isotope values of magne-site indicate that waning stages of the calcitiza-

tion process are represented by the crystallizationof 18O-depleted late-stage magnesite. These stagesof calcitization obey to a decrease in the rate ofinfiltrated waters due to either porosity reduction in-volved by the replacement process itself or a changein the flow regime. The direct relation between theoxygen-isotope composition of magnesites and themagnesite=calcite ratio, as well as the different mor-phology and chemical composition of magnesite inpure samples vs. mixed-carbonate samples seemsto indicate that waning stages of the calcitizationprocess are represented by the crystallization of18O-depleted late-stage magnesite. These stages ofcalcitization obeying to a decrease in the rate ofinfiltrated waters due to either porosity reduction in-volved by the replacement process itself or a changein the flow regime. Likewise, geochemical simula-tions suggest that the existence of this late-stagemagnesite generation is thermodynamically feasible.

Acknowledgements

This research was supported by SpanishD.G.I.C.Y.T. projects PB92-0024 and PB95-0112.Mineralogical analyses were conducted with the as-sistance of R. Gonzalez (MNCN-CSIC). J. Bedoya(MNCN-CSIC) assisted with the SEM and EDS.


The authors are indebted to Drs. B. Sellwood and C.Spotl and an anonymous reviewer whose suggestionsconsiderably improved the paper.

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Meteoric calcitization of magnesite in Miocene lacustrine deposits (Calatayud basin, NE Spain)

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