Cent. Eur. J. Geosci. • 4(2) • 2012 • 246-260 DOI: 10.2478/s13533-011-0055-x Central European Journal of Geosciences Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history: gypsum pseudomorphs of the Tera Group, Tithonian-Berriasian, Cameros Basin Research Article Laura González-Acebrón1, Robert H. Goldstein2, Ramon Mas1, Jose Arribas3 1 Dpto. Estratigrafía, Facultad de Ciencias Geológicas, Universidad Complutense de Madrid (UMC) – Instituto de Geociencias (IGEO, CSIC-UCM), Madrid, Spain 2 Department of Geology, University of Kansas, Lawrence, Kansas, USA. 3 Dpto. Petrología y Geoquímica. Facultad de Ciencias Geológicas, Universidad Complutense de Madrid (UCM) – Instituto de Geociencias (IGEO, CSIC-UCM), Madrid, Spain Received 30 October 2011; accepted 30 January 2012 Abstract: This study reports a complex fluid and thermal history using petrography, electron microprobe, isotopic analysis and fluid inclusions in replacement minerals within gypsum pseudomorphs in Tithonian-Berriasian lacustrine de- posits in Northern Spain. Limestones and dolostones, formed in the alkaline lakes, contain lenticularly shaped gypsum pseudomorphs, considered to form in an evaporative lake. The gypsum was replaced by quartz and non-ferroan calcite (Ca-2), which partially replaces the quartz. Quartz contains solid inclusions of a preexisting non-ferroan calcite (Ca-1), anhydrite and celestine. High homogenization temperatures (Th) values and inconsistent thermometric behaviour within secondary fluid inclusion assemblages in quartz (147-351◦C) and calcite (108-352◦C) indicate high temperatures after precipita- tion and entrapment of lower temperature FIAs. Th are in the same range as other reequilibrated fluid inclusions from quartz veins in the same area that are related to Cretaceous hydrothermalism. Gypsum was replaced by anhydrite, likely during early burial. Later, anhydrite was partially replaced by Ca-1 associated with intermediate burial temperatures. Afterward, both anhydrite and Ca-1 were partially replaced by quartz and this by Ca-2. All were affected during higher temperature hydrothermalism and a CO2-H2O fluid. Progressive heating and hydrothermal pulses, involving a CO2-H2O fluid, produce the reequilibration of the FIAs, which was followed by uplift and cooling. Keywords: Fluid inclusions • gypsum pseudomorphs • Cameros Basin • thermal reequilibration © Versita sp. z o.o. 1. Introduction This study provides an example of the use of easily al-tered geologic materials (lenticular gypsum, calcite, fluidinclusions in calcite and quartz) to ascertain a complex diagenetic, fluid, and thermal history as well as events ofhydrothermal metamorphism. In such easily altered ma-terials, one might typically determine that no record ispreserved, but this study shows that easily replaced sol-uble minerals, such as gypsum, provide the medium for 246 L. González-Acebrón, R. H. Goldstein, R. Mas, J. Arribas partial replacement to preserve complexities of the fluidand thermal history, despite, or perhaps as result of, sig-nificant replacement and thermal alteration.Although macroscopic gypsum crystals occur in a varietyof habits, one of the most widespread in distribution andenvironmental significance is lenticular gypsum. Lentic-ular gypsum crystals occur in many parts of the worldin present-day sediments and in ancient rocks [1] and isusually considered to form early and displacively in asso-ciation with evaporative conditions.Replacement of gypsum by calcite is known in various di-agenetic environments. It can happen at low temperaturesoon after gypsum precipitation as well as during uplift;it can also happen in the deep burial environment, com-monly associated with hydrocarbon migration and ther-mochemical sulphate reduction [2]. The absence of com-pactional textures and strained crystal forms in gypsumpseudomorphs is commonly used to interpret them as earlydiagenetic replacements [3]. In our case we observe non-deformed well-preserved pseudomorphs that have had ahighly complex replacement history over an extended pe-riod of time.Petrography, fluid inclusion, electron microprobe, and sta-ble isotope studies in gypsum pseudomorphs and theirhost-rocks from a Tithonian-Berriasian lacustrine sed-imentary sequence Northern Spain provides evidencefor multiple episodes of lenticular gypsum replacementin different diagenetic environments, overprinted by hy-drothermal metamorphism. This work pinpoints the im-portance of combining petrography of replacement phasesand solid inclusions with electron microprobe, cold-cathodoluminescence, isotopic geochemistry and fluid in-clusion study in easily replaced minerals or thermally al-tered fluid inclusions. The partial alteration of the min-erals and the fluid inclusions, including those affected bynatural thermal reequilibration, preserve a record of fluidand thermal history that might not otherwise have beenpreserved in unaltered materials. As most workers shyaway from such materials as too easily altered, this studydemonstrates a contrasting finding that partial preserva-tion of easily altered materials may be the key in betterrefining the fluid and thermal history of a basin. 2. Geological setting 2.1. Basin formation The Cameros Basin in the northern Iberian Range (Fig. 1)is part of the Mesozoic Iberian Rift System [4–7]. In-traplate rifting was a consequence of a generalized ex-tensional regime, which separated Iberia from Europe. The subsidence and sedimentation rates in Cameros Basinwere very high, with vertical thicknesses up to 6 km,and up to 9 km of stratigraphic record in the direc-tion of the northward migration of depositional sequencesrecorded from the Tithonian to the early Albian. Thebasin has been interpreted as a hanging wall synclinalbasin (extensional-ramp basin). It formed over a roughlysouth-dipping ramp between two nearly horizontal sec-tions (flats) in a deep extensional fault (detachment) insidethe 7 to 11 km-deep Variscan Basement. The direction ofdisplacement for the hanging wall was S-SW, parallel tothe direction of the basin extension [4, 5, 7–9].Alternatively, Guiraud and Seguret [10], Casas-Sainz andGil-Imaz [11], Mata et al. [12], Villalaín et al. [13] andCasas-Sainz et al. [14] interpreted the Cameros Basin asa synclinal basin, with vertical superposition of Lower Cre-taceous sedimentary units rather than laterally juxtaposedbodies onlapping the prerift sequence. These authors con-sider a thrust fault to the north as a result of tectonic in-version of a normal fault generated at the beginning of theCretaceous, which reached the Keuper facies at depth. Ithas been suggested that this hypothesis has a mechani-cal flaw that makes it unlikely [4, 5, 7, 9]. The hypothesiswould require a slab of Jurassic rocks, only 500-800 mthick but more than 30 km wide and 100 km long, to bepulled from the South without suffering any break in con-tinuity and without forming a fault over the ramp nearthe northern basin margin. This would have had to occurunder subaerial conditions because of the continental na-ture of the rocks that filled the basin. Their model wouldrequire, an even more difficult to produce, later reversedisplacement of tens of kms to the North, still somehowmaintaining the continuity of the thin marine Jurassic slabafter the Cenozoic contraction. 2.2. Stratigraphy The basin record essentially consists of continental sed-imentary rocks corresponding to alluvial and lacustrinesystems, with rare marine incursions [4, 15]. Figure 1shows the location of the studied stratigraphic sections.The sedimentary infill of the Cameros Basin has been di-vided into eight depositional sequences [7, 9]. The TeraGroup represents the first stage of rifting sedimentationand is formed by two depositional sequences (DS 1 andDS 2), which are Tithonian-Berriasian in age [4, 16, 17].DS 1 is represented by siliciclastic alluvial fan faciesand lacustrine-palustrine carbonate facies. The thicknessof DS 1 is highly variable, with maximum thickness of255 m [18]. DS 2 is up to 1500 m-thick in the depocen-tre, and consists of siliciclastic fluvial facies, which gradeupwards and laterally to carbonate lacustrine facies. At 247 Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history Figure 1. Geologic map of the Cameros Basin showing the areas affected by metamorphism. The positions of the stratigraphic sections of the Tera Group are also marked: ARZA: Almarza. PRA: Pradillo. CSP: El Collado de San Pedro. MAG: Magaña. SAN: San Felices. Cited miliolid foraminifera by Gómez-Fernández and Meléndez [15]. Position of a stratigraphic cross section (Fig. 2) marked with purple line. the top of DS 2, Gómez-Fernández and Meléndez [15]proposed minor marine influence towards the SW of thestudied area (see blue dot in Fig. 1) based on the presenceof foraminifera.This study is focused on the lacustrine facies of DS 2.The analysis of these lacustrine facies reveals the follow-ing evolution: (1) shallow carbonate ramps in lakes; to (2)shallow alkaline ephemeral lakes and then (3) carbonatelakes rich in organic matter [19]. This facies evolution in-dicates that these lakes evolved from open to more closedhydrologic conditions, also experiencing variations in nu-trients and climate. The cause in part may be a localtectonic one, with progressive rift development and fault-ing leading to progressive isolation. All three types oflacustrine facies are found only in the southern part ofthe study area (Fig. 2, MAG, CSP, SAN), whereas in thenorthern part, lacustrine facies only appear in ARZA sec-tion and consist only of shallow carbonate ramp lacustrinefacies. 2.3. Metamorphic processes During the Late Albian to Coniacian, hydrothermal alter-ation affected the deposits of the Eastern sector of theCameros Basin, in SAN area (Fig. 1) [19–26]. The main features of this thermal alteration are: (1) metamorphicgrade is controlled by changes in rock composition andpermeability rather than by burial depth [20, 21, 27, 28];(2) thermal inversions across sections in the depocen-tre [24, 25, 28]; (3) post-rift age of alteration (107±5to 85±6 My dated by K-Ar on authigenic illites [19] or99±2 My by U-Pb SHRIMP on monazites [29]) after themaximum burial stage, reached during the Early Albian;and (4) metamorphic conditions from very low-grade (an-chizone) to low-grade (epizone), with temperatures of 350-370◦C at the metamorphic peak and maximum pressureof 1 Kbar [19, 20]. Lines of evidence 1 and 2 point toa hydrothermal process rather than the regional meta-morphic model suggested by Guiraud and Seguret [10],Casas-Sainz and Gil-Imaz [11] and Mata et al. [12].The Tera Group was buried to 5900 m from the Tithonianto Lower Paleogene at the more depocentral section (SAN,Fig. 1). Burial depth is based on partial restored cross-sections from Guimerá et al. [5] and Mas et al. [9]. 3. Sampling and analytical methods A total of 45 samples were collected systematically fromfive representative stratigraphic sections of the DS2 la- 248 L. González-Acebrón, R. H. Goldstein, R. Mas, J. Arribas Figure 2. Stratigraphic cross section of the lacustrine record in the southern part of the study area of the Cameros Basin. The location is marked in Fig. 1. For names of the stratigraphic section see caption of Fig. 1. Positions of samples are shown. Thin sections have been represented with a different pattern than those samples in which petrography has been combined with stable isotope study and microthermometry by using both thin and thick sections. 249 Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history custrine facies of the Tera Group (Figs. 1, 2), trying torepresent the vertical variation. For each sample, a pol-ished epoxy-mounted 30 µm thin section was prepared.In addition, ten doubly polished selected thick sectionswere prepared for fluid inclusion study without any heat-ing and glued to frosted glass with cyanoacrilate. Afteroptical petrographic analysis of these sections, selectedareas of 4 of these sections were cut and removed fromthe glass using acetone. The microthermometric studywas performed on these portions of samples in a LinkamTHMSG-600 heating and freezing stage. The stage wascalibrated with synthetic fluid inclusions, including triplepoint of CO2, melting point of H2O, and critical point ofH2O. The accuracy for low-temperature measurements isbetter than ±0.1◦C and for high temperature measure-ments are better than ±1.0◦C. Abbreviation for homogenization temperature is T h, for fi-nal melting temperature of ice is Tm(ice), for nucleationtemperature is T n, for clathrate nucleation temperatureis T n(cl) and for final melting temperature of clathrate is Tm(cl). The T h have been interpreted as minimum en-trapment temperatures. In this case, pressure correctionswere not applied because a pressure determination wouldinvolve too many error-prone assumptions without inde-pendent pressure estimation methods. Representing T has minimum entrapment temperatures is a typical proce-dure in interpreting T h data [29, 30]. To interpret salinityfrom Tm(ice), a NaCl-H2O model was used on the basisof the observed first melting temperatures from fluid in-clusions [32], and the NaCl-H2O-CO2 model system whenclathrates are observed. The calculations employed theICE software [33]. Sixteen epoxy-mounted 150 µm-thick sections were pre-pared on carbonate bearing pseudomorphs. CL examina-tion was carried out using a Technosyn® cold cathodolu-minescence unit operating at 14-17 KV with 350-450 µAbeam current. These sections were etched and stainedusing Alizarin Red S and potassium ferricyanide for car-bonate identification [34] after cathodoluminescence (CL)study. Carbonate samples were taken using a microdrilland analyzed for δ13C and δ18O. Carbonate minerals fromboth gypsum pseudomorphs and their host rock were takento obtain coupled values in the same sample (if there wasenough material for each one). This helped in distin-guishing between depositional, early, and late diageneticsignatures. Where abundant enough, samples were alsotaken for 87Sr/86Sr analysis (six samples). Analyses of δ13C and δ18O were performed at the Keck Paleoenvi-ronmental and Environmental Stable Isotope Laboratory(KPESIL) at the University of Kansas. Strontium isotopeswere performed in the laboratory of Isotopic Geochronol- ogy and Geochemistry of the Complutense University ofMadrid.For stable isotope analysis, all sample powders wereroasted in vacuum for one hour at 200◦C to remove volatileorganic contaminants, and afterward reacted at 73◦C in anautomated carbonate reaction system (Kiel-III) coupled di-rectly to the inlet of a Finnigan MAT 253 gas ratio massspectrometer. Isotopic ratios were collected for 17O con-tribution and are reported in per mil notation relative tothe VPDB standard. Values were calibrated using NBS-19 as the primary standard. The precision of the analysisis 0.1h for both Oxygen and Carbon.For 87Sr/86Sr, dolomite or calcite powder was dissolved in2 mL of a solution of 2.5N HCl, later evaporated to dry-ness at 80-100◦C. Samples were re-dissolved in 2.5N HClsolution and Sr was pre-concentrated by standard meth-ods of column chromatography. Following this, the Sr-concentrated samples were dissolved in 2 mL of phospho-ric acid and Sr isotopic ratios were then determined witha VG SECTOR 54 five-collector mass spectrometer. Iso-topic ratios were corrected for possible interferences from87Rb and normalized to the value of 87Sr/86Sr= 0.1194,to correct the isotopic fractionation effect. Analytical pre-cision was monitored by analysis of NBS-987 standardand measurement precision was maintained at better than ±5·10−5.A Jeol JXA-8900 M electron microprobe was used to char-acterize major and minor element composition of the sam-ples. Operating conditions were 15 kV, 20 nA and 5 µmbeam diameter. Measured oxide standards were CaO,Na2O, SrO, MgO, FeO and MnO, and mean detection lim-its were 130, 110, 140, 130, 310 and 330 ppm for eachelement respectively. 4. Results 4.1. Petrography of the lacustrine facies and the gypsum pseudomorphs The lowermost occurrences of carbonate deposits are as-sociated with meandering fluvial systems of DS 2 (ARZA,PRA: Figs. 1, 2). Laterally and towards the top of thestratigraphy (CSP, MAG, SAN: Figs. 1, 2) carbonate fa-cies are related to shallow carbonate ramp lacustrine fa-cies [18]. Strata are lime mudstone and dolostone withostracodes and characea, with very sparse gastropods andinterlayered with channelized fine-grained sandstones.Towards the eastern part of the study area (CSP, MAG,SAN: Fig. 1, 2) and at the top of the stratigraphic suc-cession, there is evidence of shallow alkaline ephemerallakes [18]. This facies association contains the gyp- 250 L. González-Acebrón, R. H. Goldstein, R. Mas, J. Arribas Figure 3. Field photographs of lacustrine deposits of DS 2. A. Abun- dant lenticular gypsum pseudomorphs in micritic lime- stone. B. Swallow-tail lenticular gypsum pseudomorphs. C. Domal stromatolites and autoclastic breccias in lacus- trine sequences. D. Tepee at the top of the sequence. sum pseudomorph-bearing layers (Fig. 3A, B). These lay-ers are made up by coarsening upward limestone and/ordolostone carbonate sequences with common stromato-lites (Fig. 3C), autoclastic breccias (Fig. 3C) and tepeestructures (Fig. 3D). Limestones and dolostones formedin these alkaline lakes have a characteristic fetid smellwhen breaking and contain abundant lenticularly shapedgypsum pseudomorphs.Gypsum pseudomorph size ranges among different layersfrom 1 mm to 20 cm long. The concentration of the pseu-domorphs in each level is also highly variable. They areirregularly distributed and some have swallow-tail twinmorphologies (Fig. 3A, B). Gypsum pseudomorphs are forthe most part replaced by quartz and non-ferroan calcite(Ca-2, Fig. 4A, B). The quartz contains solid inclusions ofnon-ferroan calcite (Ca-1), anhydrite, and less commonlycelestine (Figs. 5A, B, C). Solid inclusions are typicallyoriented in same direction, 40-100 µm-size, rectangularin shape (Fig. 5A). Ca-2 calcite also contains solid in-clusions of anhydrite, and in a few examples, celestine(Fig. 5C).Small framboidal pyrite crystals (10-100 µm) are com-monly observed in the limestone host rock and in the in-ner boundaries of the pseudomorphs (Fig. 5D). The solidinclusions in quartz and Ca-2 calcite are probably re-licts of earlier diagenetic processes. Quartz is corrodedby Ca-2 calcite supporting that Ca-2 calcite postdatesquartz (Fig. 4B). Ca-2 calcite is either very clean andnon-luminescent under CL, with composition of (Sr 0.103Mg 0.617 Na 0.068 Fe 0.128 Mn 0.192 Ca 98.893) (CO3)100 andMg/Ca ratio is 0.006 (n=9) or cloudy with dull orangezoned (in places) CL, with composition of (Sr 0.124 Mg 1.046 Figure 4. Photomicrographs of gypsum pseudomorphs. Crossed nichols A. Gypsum pseudomorph mainly replaced by quartz in a limestone micritic matrix with ostracode frag- ments. B. Non ferroan calcite (Ca-2) and quartz (Q) in gypsum pseudomorph. Notice the corrosion of the quartz by Ca-2. Figure 5. Photomicrographs of gypsum pseudomorphs. A. Detail of the solid inclusions of non ferroan calcite (Ca-1, arrows) in quartz (Q). Crossed nichols. B. Anhydrite and celestine in- clusions in quartz. BSE image. C. Anhydrite and celestine inclusions in Ca-2. Notice the corrosion of quartz by Ca- 2. BSE image. D. Pyrite (arrows) in the micritic limestone host rock and on the inner boundaries of a Ca-2-replaced pseudomorph. Na 0.019 Fe 0.321 Mn 0.251 Ca 98.238) (CO3)100 and Mg/Caratio is 0.011 (n=23). 4.2. Stable isotope geochemistry The microfacies of the above described carbonate stratahave been studied in all sections for the stable isotopestudy. Depositional matrix is either calcitic or dolomiticwith micritic texture, showing in some cases microspariticand pseudosparitic recrystallization fabrics. The mostfine-grained micritic matrix was sampled to be most rep-resentative of original depositional material for isotopicanalysis. 251 Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history Figure 6. Stable isotope data in relation to the different sedimentary environments. Figure 7. Stable isotope data of limestone and dolostone micritic matrix and Ca-2 in gypsum pseudomorphs. Notice the enrichment in δ18O of three dolomite matrix samples com- pared to other matrix samples. In the lowermost occurrences of carbonate deposits (ARZA,PRA: Figs. 1, 2) only dolomicritic matrix was abundantenough for sampling, and has isotopic values of δ13C=-6.01/-5.53h VPDB and δ18O = -9.54/-8.09h VPDB(n=2) (Table 1, Figs. 6, 7).Towards the top of the stratigraphy (CSP, MAG, SAN:Figs. 1, 2), the limestone has isotopic values of δ13C= -9.43/-7.44h VPDB and δ18O =-7.56/-4.67h VPDB(n=9, ARZA, MAG, SAN) (Table 1, Figs. 6, 7). At thetop of the stratigraphic succession (CSP, MAG, SAN:Fig. 1, 2), isotopic values in limestones are δ13C = -6.67/-4.14h VPDB and δ18O = -9.36/-4.87h VPDB (n=7,MAG, SAN) (Table 1, Figs. 6, 7), and in dolostones are δ13C =-6.59/-6.11h VPDB and δ18O = -1.48/-0.94hVPDB (n=3, MAG) (Table 1, Figs. 6, 7).In the gypsum pseudomorphs, stable isotopic values (n=8)in Ca-2 calcite are δ13C = -7.74/-4.88h VPDB (reach- Figure 8. Photomicrographs of Ca-1 and quartz in gypsum pseudo- morph. In the box appears the area magnified in B. Notice the corrosion of quartz by Ca-2 (A) and the lower Th of the fluid inclusion in the inclusion of Ca-1 compared to fluid in- clusion in quartz. ing -10.88h VPDB in MAG) and δ18O =-9.73/-6.14hVPDB, with extreme values of δ13C = -3.02h VPDB inMAG and δ18O = -15.76h VPDB in a slightly Fe-richsample of CSP (Table 1, Figs. 6, 7).87Sr/86Sr values are very homogeneous in both calcite anddolomite micritic matrix with a mean value of 0.7082 (n=6,MAG, CSP) (Table 2). 4.3. Fluid inclusion data Fluid inclusions in the minerals that form the gypsumpseudomorphs were characterized petrographically andfollowed by microthermometric analysis. In order to char-acterize the fluid composition and temperature history,Ca-1 calcite (early) quartz (later) and Ca-2 calcite (latest)were selected for analysis of fluid inclusions.Fluid inclusions are very scarce in the small crystals ofCa-1 (40-100 µm). For this reason, only one fluid inclu-sion could be studied in Ca-1. This inclusion has a largesize relative to the size of the crystal and appears iso-lated (Fig. 8); thus, it is most reasonably characterized asprimary. It yields T h of 51-52◦C.The quartz contains fluid inclusion assemblages (FIAs) ofbiphasic inclusions with sizes ranging between 2-14 µm(Fig. 9A, Table 3). These FIAs show no relationship togrowth patterns, and some are distributed along planararrays, or appear in different planes, indicating a prob-able secondary origin (Fig. 9A). In FIAs, inclusions havevariable liquid: vapor ratios: 15 to 40 vol% vapor and novapor-dominant inclusions were found (Table 3). In ad-dition, petrographically paired vapor-rich and vapor-poorinclusions were not observed. Homogenization tempera-tures are variable within each FIA, and range from 147-351◦C (n=27, Fig. 10), with higher Th in the deeper sam-ples (Table 3). Due to the small size of the fluid inclu-sions, very few final melting of ice measurements (Tm ice)have been recorded (n=7). They vary between -5.8 and-1.6◦C among different samples. Final melting tempera-tures of clathrate are typically between 5 and 10◦C (Ta- 252 L. González-Acebrón, R. H. Goldstein, R. Mas, J. Arribas Table 1. Stable isotope data of carbonate matrix and Ca-2 in gypsum pseudomorphs. δ 13C δ 18O Sample (hVPDB) (hVPDB) FormationPRA-03 -6.01 -8.09 Ferroan dolomitic matrix FluvialARZA-05 -4.67 -7.77 Calcite matrix Carbonate lakeARZA-05 -4.52 -7.44 Calcite matrix Carbonate lakeMAG-02 -5.53 -9.54 Dolomitic matrix FluvialMAG-06 -7.47 -9.43 Calcite matrix Carbonate lakeMAG-06 -6.95 -9.35 Calcite matrix Carbonate lakeMAG-06 -7.56 -9.90 Calcite matrix Carbonate lakeMAG-07 -4.14 -9.36 Calcite matrix Alkaline lakeMAG-07 -4.51 -9.26 Calcite matrix Alkaline lakeMAG-11 -6.11 -1.48 Dolomitic matrix Alkaline lakeMAG-12 -6.67 -7.30 Calcite matrix Alkaline lakeMAG-14 -6.59 -0.94 Dolomitic matrix Alkaline lakeMAG-14 -6.54 -1.39 Dolomitic matrix Alkaline lakeMAG-14 -10.88 -3.02 Pseudomorph Alkaline lakeSAN-18 -6.46 -8.90 Calcite matrix Carbonate lakeSAN-19 -6.32 -9.04 Calcite matrix Carbonate lakeSAN-19 -6.04 -8.28 Pseudomorph Carbonate lakeSAN-20 -6.87 -8.64 Calcite matrix Carbonate lakeSAN-25 -7.38 -11.19 Ferroan dolomitic matrix Carbonate lakeSAN-26 -6.93 -7.76 Dolomitic matrix Carbonate lakeSAM-33 -4.14 -7.25 Dolomitic matrix Alkaline lakeSAN-33 -4.88 -8.05 Pseudomorph Alkaline lakeSAN-35 -6.87 -7.62 Pseudomorph Alkaline lakeSAN-36 -6.11 -7.84 Pseudomorph Alkaline lakeSAN-36 -4.73 -7.64 Dolomitic matrix Alkaline lakeCSP-101 -4.68 -4.87 Dolomitic matrix Alkaline lakeCSP-101 -5.52 -15.76 Pseudomorph Alkaline lakeCSP-101 -7.69 -9.73 Pseudomorph Alkaline lakeCSP-103 -7.74 -6.14 Pseudomorph Alkaline lakeCSP-103 -5.60 -6.08 Dolomitic matrix Alkaline lake Table 2. 87Sr/86Sr data of carbonate matrix in pseudomorphs bearing samples. Sample 87Sr/86Sr MatrixMAG-12 0,708286 CalciticMAG-11 0,708123 DolomiticMAG-11-2 0,708095 DolomiticMAG-12-2 0,708287 CalciticCSP-101 0,708233 DolomiticCSP-103 0,708208 Dolomitic ble 3). Nucleation of clathrate was detected from -53 to-58◦C in some fluid inclusions.Ca-2 contains FIAs which appear in different planes thatare unrelated to growth patterns. Thus, we consider themof secondary origin. They have very irregular shapes and Figure 9. Photomicrographs of fluid inclusions in quartz and Ca-2. A. Secondary biphasic fluid inclusions in quartz distributed in different planes. The line separates quartz and Ca-2 in order to facilitate their distinction. B. Secondary fluid inclusions in Ca-2 distributed along planar arrays. variable sizes (1-16 µm). In addition to FIAs with all-liquid fluid inclusions, there are also FIAs with bipha-sic fluid inclusions of small size (typically 2-6 µm) and 253 Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history Table 3. Microthermometry data of secondary fluid inclusions in quartz. Samples SAN-33, SAN-35 and CSP-101. Units: size: µm; Th, Tn, Tn(cl), Tm(ice), Tm(cl): ◦C; Salinity1: following [32] and Salinity2: following [33], both measured in NaCl wt % eq. * Fluid inclusions with enough data for calculating the salinity using clathrate equation of state. Sample FI FIA Size L:V Th Tn Tmice Salinity1 Salinity2 Tn cl Tm clSAN-35 1 1 6 70:30 178.0-182.0 -44 -2.9/-3.0 4.80-4.96 - - -2 2 4 70:30 147.0-149.0 - - - - - -3 2 8 80:20 155.0-157.0 - - - - - -4 2 3 80:20 165.0-173.2 - - - - - -5 3 2 70:30 220.0-223.0 - - - - - 9.46 4 13 80:20 208.0-209.5 - - - - - -7 5 7 70:30 295.0-297.5 - - - - - -8 6 8 40:60 >350 - - - - -30 5.8-8.79 6 4 50:50 >350 - - - - -31 6.8-10.110 6 2 70:30 254.0-260.0 - - - - - -11 7 3 70:30 - - -1.6/-1.9 2.74-3.23 - - -* 12 8 11 80:20 - -38 -4.7/-4.9 7.45-7.73 4.9 -30 2.9-6.6SAN-33 1 1 6 80:20 252.8-260.8 - - - - -29 -2 1 5 80:20 260.4-266.4 - - - - -29 8.63 1 3 70:30 277.0-285.2 - - - - -29 8.84 2 3 60:40 322.0-329.0 - - - - - -5 3 - 70:30 280.0-286.7 - - - - - -6 4 12 80:20 210.0-220.4 - - - - - -7 5 3 85:15 299.0-301.6 - - - - -43 -8 5 4 80:20 340.4-351.0 - - - - -29 9.59 6 6 80:20 235.0-240.0 -30 - - - -51 8.6CSP-103 1 1 3 80:20 136.0-146.0 - - - - - -2 2 4 80:20 341.6-351.5 -33 -4.9/-4.7 7.45-7.73 - -58 -5.4/-4.9* 3 2 5 80:20 294.0-297.0 - -4.7? 7.45 5.1 -53 0.3-5.04 2 4 75:25 166.0-172.0 - -2.5?/-3.0? 4.18-4.96 - -54 -9.7/-13.65 3 8 80:20 245.0-251.0 - - - - - -6 3 8 75:25 165.0-170.0 -41 -5.6/-5.8 8.68-8.95 - - - variable liquid:vapor ratio: 10 to 40 vol% vapor (Fig. 9B,Table 4). Again, petrographic pairing or vapor-dominantinclusions were not found. T h measured in these FIAsrange from 108 to 352◦C (n=25, Fig. 10A and B) with Tm(ice) of -0.1◦C (n=5) (Table 4). As can be shown in the histograms, there is high variabil-ity of T h data among the same FIAs in all samples, forboth fluid inclusions in quartz and Ca-2 (Fig. 10). 5. Discussion On the basis of the paragenetic, fluid inclusion, and geo-chemical data we have established different stages in thefluid and thermal history that include: (1) deposition inlake; (2) early burial; (3) late burial; (4) hydrothermalism;and (5) unroofing and cooling (Figure 11). 5.1. Syndepositional through intermediate burial diagenesis Lenticular gypsum is considered to form early and synde-positionally with evaporative conditions or either marineor nonmarine origin. Although the succession has beenshown to be dominantly lacustrine, there is evidence fora possible marine incursion towards the SW of the studyarea [15]. The 87Sr/86Sr analyses from carbonate matrix ofgypsum pseudomorphs in two stratigraphic sections fromthe southern end of the study area (MAG, CSP) can helpto assess if matrix around the gypsum is marine or nonma-rine in origin (Table 1). The 87Sr/86Sr data measured is0.7082 (mean value, n=6) whereas Tithonian-Berriasianmarine data of Veizer et al. (1999) [35] range from 0.7065to 0.7075. This lends support for a nonmarine origin forthe matrix, although we cannot rule out diagenetic reset-ting. In addition, we have not observed any sedimentaryevidence for the marine influence in this area. Thus, we 254 L. González-Acebrón, R. H. Goldstein, R. Mas, J. Arribas Table 4. Microthermometry data of secondary fluid inclusions in calcite. Samples SAN-33 and SAN-35. Units: size: µm; Th, Tn, Tm(ice), Tm(cl):◦C; Salinity: NaCl wt % eq. following [32] Sample FI FIA Size L:V Th Tn Tmice Salinity Tn clSAN-35 1 1 - 85:15 174.5-176.5 - - - -2 1 - 80:20 150.0-156.0 - - - -3 1 - - 155.0-160.0 - - - -4 1 - 70:30 225.0-229.0 - - - -5 2 - 80:20 217.0-218.5 - - - -6 3 7 80:20 193.0-195.0 - - - -7 3 9 70:30 285.0-288.0 - - - -8 3 10 80:20 231.0-237.0 - - - -9 4 - 80:20 240.0-243.0 -39 -0.1 0.18 -10 5 - 80:20 229.0-231.0 - - - -11 6 6 85:15 211.0-214.0 -39 -0.1 0.18 -12 6 10 70:30 242.0-247.0 - -0.1 0.18 -13 6 4 80:20 242.0-247.0 - - - -14 6 5 80:20 233.0-236.0 -39 -0.1 0.18 -15 7 - 70:30 321.0-326.0 -40 -0.1 0.18 -16 8 4 70:30 180.0-182.7 - - - -17 9 2 80:20 198.0-204.0 - - - -18 9 3 80:20 190.0-194.2 - - - -19 9 3 80:20 195.2-200.1 - - - -20 9 4 70:30 229.3-232.8 - - - -21 10 6 75:25 - -46 - - -SAN-33 1 1 13 90:10 108.0-116.0 - - - -2 2 11 60:40 278.0-282.0 - - - -3 2 9 70:30 266.0-271.0 -39? - - -39?4 3 10 50:50 350.0-352.0 - - - - consider more plausible that lenticular gypsum and itssurrounding matrix formed in shallow alkaline ephemerallakes. On the basis of the fossil content and associated faciespresented, a lacustrine depositional environment is widelyaccepted for most of the section. No significant differ-ences are observed when comparing the stable isotopedata among carbonate facies or among different strati-graphic sections (Figs. 6, 7), with the exception of threedolomite samples at the top of MAG section (Fig. 1), with δ18O around -1 VPDBh. These samples are from theuppermost deposits of shallow alkaline ephemeral lakes,and they are clearly enriched in δ18O if compared to therest of the alkaline lake samples (Figs. 6, 7). As the ma-rine influence is unlikely on the basis of Sr isotope data,we interpret these less negative values of δ18O to be aproduct of strong evaporation. As early gypsum precipi-tation reduces sulphate and increases Mg/Ca in solution,dolomitization could be favoured, eg: [36]. The occurrenceof the inferred early dolomitization at the end of alkalinelacustrine sedimentation can be an indicator of an evapo- ration increase probably related to a climatic change dur-ing the Berriasian in the studied area.Anhydrite solid inclusions in quartz and Ca-2 are point-ing to gypsum anhydritization. This process probably oc-curred during shallow burial (Fig. 3), because if gypsumhad preserved until significant burial depths, them thereshould have been some plastic deformation and modifica-tion of crystal forms. Although primary surface and near-surface anhydrite has been described [37, 38] we can ruleout this hypothesis because of lenticular shape is charac-teristic of gypsum crystals. The gypsum must have subse-quently been transformed to anhydrite before further re-placement, and this dehydration is to be expected duringshallow burial and increasing temperature.The other very early replacement is Ca-1. It has a non-ferroan composition consistent with oxidized waters [39].Its position in the diagenetic sequence is before quartzand the only primary aqueous inclusion measureable hadlow T h (51-52◦C). Both facts are pointing to the earlyreplacement of anhydrite (Fig. 11). One could posit thatpreservation of such a low-temperature fluid inclusion, ina weak mineral such as calcite, might be surprising con- 255 Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history Figure 10. Histograms of Th of fluid inclusions in quartz and Ca-2. Each FIA marked with a different pattern. Vertical axis indicates number of fluid inclusions. Figure 11. Relative chronology of the different synsedimentary, dia- genetic and hydrothermal processes and products. De- position in lake corresponds to Tithonian- Berriasian and hydrothermalism with Late Albian to Coniacian. sidering its entrapment during prograde conditions. Itspreservation, however, might be the result of the fluidinclusion’s occurrence within a small calcite crystal sur-rounded by a single, stronger, crystal of quartz.The origin of celestine is more complex, because primarybioinduced precipitation of celestine in evaporitic environ-ments has been described [40–42], as well as a by-productof anhydrite hydration [43–46]. Furthermore, hydrother-mal celestine has been documented [47]. The anhydritehydration hypothesis is unlikely because there is no pet-rographic evidence of this and because anhydrite relictsare protected inside quartz and Ca-2. The hydrothermalorigin is also unlikely in our case, on the basis of crystalmorphology and paragenetic position. Thus, we considerit is more probable that celestine is a primary precipitateor at least has a very early diagenetic origin as a re-placement of gypsum. Celestine may have been preservedbecause its solubility is lower than that of gypsum [48].As no relationship has been observed between anhydrite,celestine and Ca-1 solid inclusions we are unable to es-tablish the chronology among them. Finally, pyrite crys-tals probably predate both quartz and Ca-2 (Fig. 11), be-cause are commonly observed in the inner boundaries ofthe pseudomorphs.The observations indicate early gypsum precipitation as-sociated with an evaporitic lake environment, and possiblyassociated with early dolomitization (Fig. 11). This wasfollowed by replacement with Ca-1, and anhydrite in ashallow burial environment, likely no more than tens ofdegrees C higher than depositional temperatures. 5.2. Diagenesis during late burial and hy- drothermal metamorphism In gypsum pseudomorphs replacement proceeded withquartz followed by Ca-2 calcite. Other studies in the areademonstrate syntaxial quartz precipitation during pro-gressive deep burial and in fracture fills associated withlater hydrothermal processes [26]. First quartz formationin this area (SAN, Fig. 1) proceeded at high temperaturesafter depths of approximately 3 km were reached, afterthe late Barremian. This timing consideration is based onprevious work of the authors [26], considering petrographyand fluid inclusion microthermometry in quartz in relationto burial history and geothermal gradients. On this ba-sis, quartz replacement in pseudomorphs is hypothesizedto be coeval with syntaxial quartz overgrowths in sand-stones or fracture fills, associated with deep burial andthe first stages of Cretaceous hydrothermalism.Later, Ca-2 calcite replacement likely also proceeded athigher temperature. Ca-2 stable isotopic data can be usedto further constrain the conditions of Ca-2 precipitation. 256 L. González-Acebrón, R. H. Goldstein, R. Mas, J. Arribas The light δ13C (-5.0 to -11.0h) values in Ca-2 (figs 6, 7)are supportive of breakdown of organic matter due to ther-mochemical reduction of sulphate [49, 50] or bacterial sul-phate reduction [3]. The presence of pyrite and the fetidsmell when breaking samples is consistent with sulphatereduction. Typical temperatures expected for thermochem-ical sulphate reduction are greater than 100◦C [50], andit is expected that Ca-2 formed at or above this temper-ature. The petrographic observations do not support ashallow depth for Ca-2, and thus, the low-temperaturebacterial sulphate reduction hypothesis is unlikely. In ad-dition, the most negative values of δ18O in Ca-2 (-15.76hVPDB) also indicate high temperatures. Both Ca-2 andmatrix data are highly scattered and suggest some degreeof isotopic reset at high temperature. Thus, replacementof quartz with Ca-2 probably took place well after quartzbegan precipitating; i.e. after burial deeper than 3 km.Secondary fluid inclusions in quartz and Ca-2 providea record of the temperatures and salinities of post-precipitation fluids. No FIAs with consistent T h have beenobserved. Neither petrographically paired vapor-rich andvapor-poor inclusions nor vapor-dominant inclusions wereobserved in quartz and Ca-2 cements, so there is no evi-dence for necking down after a phase change nor evidenceof heterogeneous entrapment (e.g. [30]).The high Th values and the inconsistency within FIAs(Fig. 5) measured in secondary fluid inclusions in quartz(147-351◦C) and calcite (108-352◦C) demonstrate thatboth minerals experienced high temperatures after theirformation and after entrapment of originally lower tem-perature FIAs. The temperatures are in the same rangeas other thermally reequilibrated fluid inclusions mea-sured in thick quartz veins in the same area, related toa Cretaceous hydrothermal processes that affected thispart of basin [26]. As the quartz precipitation postdatesdeep burial in the area, and some formed during hy-drothermal thermal metamorphism, we consider that thesecondary fluid inclusions were formed after deep burialand perhaps during hydrothermal thermal metamorphism.Clearly, thermal reequilibration of these inclusions indi-cates their alteration during one or more pulses of hy-drothermal metamorphism that postdated entrapment ofthe secondary fluid inclusions at a lower, yet still quitehigh, temperature.The salinities of the high-temperature fluid inclusionsin quartz, would be interpreted to be between 2.74 to7.73 NaCl wt % eq. (Table 3), without consideration ofclathrates [32]. As first melting temperatures are closeto the metastable eutectic for the NaCl-H2O (observedaround -29/-31◦C), the NaCl-H2O model system is inpart appropriate. Other studies in the region, however,demonstrated that fluids associated with the hydrothermal quartz [26], were rich in CO2. Our data of show clathratenucleation events between -53 and -58◦C and formation ofsignificant amounts of clathrate (Table 3) in high tempera-ture fluid inclusions offers support for significant amountsof CO2. The presence of clathrates would indicate thatbulk salinities would be lower than those calculated from Tm(ice) measurements using the NaCl-H2O model system.In inclusions in which the appropriate microthermometricmeasurements can be made, use of the NaCl-H2O-CO2model system indicates salinities around 5 wt % NaCl.The calculations employed the ICE software [33] (Table 3).The Tmice measurements in Ca-2 FIAs with high T h val-ues are commonly close to 0◦C (Table 4), indicating salin-ities significantly lower than those in quartz FIAs. Para-genetic position and stable isotopic composition of Ca-2,and high T h values in these inclusions support formationand reequilibration of fluid inclusions after deep burial.Fluid inclusion salinities that vary from those in quartzsupport the idea of variable salinities during pulses ofhydrothermal fluid flow. 5.3. Uplift and cooling The thermal and fluid record concludes with uplift andunroofing manifested by the presence of secondary all-liquid FIAs in Ca-2. 6. Conclusions 1. A complex geological history can be ascertainedfrom replacements of gypsum crystals that origi-nally formed in an alkaline lake system. It is thechemical malleability of the gypsum and subse-quent minerals and fluid inclusions that are advan-tageous in recording multiple events in the geologichistory through partial resetting during recrystal-lization or thermal reequilibration. 2. The history starts with deposition in a nonmarinelacustrine system that evolves to an evaporativesystem through time. This results in dolomitizationand lenticular gypsum precipitation syndeposition-ally with the lake. Shallow to intermediate burialincludes anhydritization of primary gypsum, celes-tine precipitation, and non-ferroan calcitization, allat relatively low temperatures. 3. During deep burial and hydrothermalism, pseudo-morphs are partially replaced by quartz and thencalcite Ca-2. On the basis of regional evidencequartz precipitates at high temperature after LateBerriasian, deeper than 3 km, from a highly saline 257 Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history fluid. It is followed by calcitization associated withthermochemical sulphate reduction at high temper-ature, on the basis of isotopic data. 4. The main replacement phases (quartz and Ca-2)contain secondary fluid inclusions reequilibratedduring later hydrothermal processes (T h = 108-352◦C) with intermediate and low salinities. In caseof quartz there was presence of dissolved CO2. 5. Fracturing and fluid inclusion entrapment continuesduring uplift at low temperature. 6. This extensive record of fluid and thermal history isgleaned from materials that are partially replacedand partially reequilibrated. 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Sedimentology, 2001, 48, 29-55[50] Machel H.G., Krouse R.H., Sassen R., Products anddistinguishing criteria of bacterial and thermochem-ical sulphate reduction. Appl. Geochem., 1995, 10,373-389 260 Introduction Geological setting Sampling and analytical methods Results Discussion Conclusions Acknowledgements References