Tibljas et al.indd 1. INTRODUCTION Turonian to Upper Santonian limestones rich in rudist remains crop out in the southernmost part of Istria, at the Mrlera and Premantura peninsulas (Fig. 1). Accord- ing to the latest stratigraphic and palaeoenvironmental investigations of the rudists from these rocks (MORO, 1997; MORO & ĆOSOVIĆ, 2000) specimens of the genera Durania, Praeradiolites and Radiolites thrived in the shallower parts of the subtidal zone in an inner shelf environment, while specimens of the genera Gor- janovicia and Vaccinites lived in the deeper subtidal environment with pelagic influxes (outer shelf). The main aim here was to determine the mineral (i.e. phase) and chemical composition of the valves of several rudist genera from pure carbonates without the Mineral and Chemical Composition of Rudist Valves from Upper Cretaceous Limestones of Southern Istria, Croatia Darko TIBLJAŠ1, Alan MORO2 and Željka OSTREŽ3 influence of terrigenous material, and to explain any observed differences. 2. MATERIALS AND METHODS During the study, 22 samples of rudist valves (five of the genus Durania, samples D1–D5; four of the genus Gorjanovicia, G1–G4; four of the genus Praeradiolites, P1–P4; five of the genus Radiolites, R1–R5, and four of the genus Vaccinites, V1–V4) were analysed. Accord- ing to the biostratigraphy, the rudist genera Durania, Radiolites and Praeradiolites lived during the Middle to Upper Turonian, Gorjanovicia lived in the time span from Upper Turonian to Santonian while Vaccinites specimens are of Santonian age (MORO, 1997; MORO & ĆOSOVIĆ, 2000). Zonation of shells was absent in observed hand specimens, so complete shell mate- rial, comprising the outer and inner layers, was taken for analysis. Micro- and macro-photographs of some analysed rudists from the investigated area are given in POLŠAK (1967b – plates 18–21, 69, 70) and MORO (1997 – plate 1: figs. 1, 2, 3, 5 & 6; plate 2: figs. 1 & 3). The mineral composition was determined by the X- ray powder diffraction method (XRD) using a Philips instrument equipped with vertical goniometer PW 1050, Cu tube, a graphite monochromator, and a proportional counter. Unit cell dimensions of calcite were calculated by the UnitCell software (HOLLAND & REDFERN, 1997) with the aim of determining the Mg-content of the calcite on the basis of diagrams constructed by MACKENZIE et al. (1983) that show the relationship between unit cell dimensions and magnesium content of Mg-calcites. Concentration of Fe, Mn, Mg and Na in the valves was determined by the commercial ACME Analytical Laboratories, Canada by the ICP-ES method. The 0.5 g of the sample was leached for one hour with 3 ml of hot (95˚C) aqua regia and diluted to 10 ml. Content of other trace elements (Cu, Nb, Pb, Rb, Sr, U, Zn, Zr, Y) was determined on an ARL 8410 X-ray fluorescence (XRF) spectrometer equipped with Rh-tube on pressed powder pellets, using a slightly modified procedure from NIS- BET et al. (1979). Mass absorption coefficients for the wavelengths of measured lines were calculated assum- ing, in accordance with the results of performed XRD analyses, that all of the samples have practically the Geologia Croatica 57/1 73–79 7 Figs. 3 Tabs. ZAGREB 2004 Key words: Rudist valves, Mineral composition, Tra- ce element content, Turonian to Upper Santonian, Istria, Croatia. 1 Institute for Mineralogy and Petrography, Faculty of Science, University of Zagreb, Horvatovac b.b., HR-10000 Zagreb, Croatia; e-mail: dtibljas@public.srce.hr 2 Institute for Geology and Palaeontology, Faculty of Science, University of Zagreb, Zvonimirova 8, HR-10000 Zagreb, Croatia; e-mail: alan.moro@public.srce.hr 3 Crosco, Vukovarska 18, HR-10000 Zagreb, Croatia; e-mail: ostrezz@net.hr Abstract Turonian to late Santonian limestones of southern Istria are rich in rudist remains. The main aim of this study was to determine the mineral and chemical composition of 22 samples of rudist valves belonging to the genera Durania, Praeradiolites, Radiolites, Gor- janovicia and Vaccinites, and to explain any observed differences. X-ray diffraction and chemical analyses showed that in all the ana- lysed samples, the primary constituents of rudist shells, aragonite and low-Mg calcite, were transformed into diagenetic low-Mg calcite. The strontium concentrations of all the analysed shells correspond to pelagic bulk rock samples altered by diagenesis and are significantly lower than those for biological calcite. Observed differences in the chemical composition of diagenetically altered rudist shells belong- ing to different rudist genera, namely a slightly higher content of Sr in specimens of Gorjanovicia and Durania and lower concentrations of Mg in shells of Vaccinites are most probably the result of different shell structures and original mineral constituents. 74 Geologia Croatica 57/1 75Tibljaš, Moro & Ostrež: Mineral and Chemical Composition of Rudist Valves... same composition that correspond to the CaCO3. Aver- age accuracy of the XRF measurements, as compared to recommended values (GOVINDARAJU, 1994) for USGS (GXR–1 to 6), GSJ (JB–2, JB–3, JF–2, JG–2, JG–3, JR–2) and IGGE (GSD–1 to 12, GSR–1 to 6) standard rocks was 4% for Sr, 20% for Pb, 12% for Ga, 8% for Zn, 14% for Cu and 11% for Ni. The precision of XRF and ICP-ES measurements was within several %, except when values were very close to the limit of detection (LOD). The results of chemical analysis have been statisti- cally processed by the Statistica computer software (STATSOFT, 1995). Besides descriptive statistical methods, discriminant analysis was used as it is a pow- erful tool for classifying individual cases (samples) into previously defined groups on the basis of multiple variables, with wide application in solving problems in geosciences (DAVIS, 1986). For statistical analysis Mn concentrations bellow LOD were replaced with 70% of LOD (MIESCH, 1976). 3. RESULTS Phase analysis has shown that all the samples consist of just one mineral phase, almost pure calcite. No arago- nite was registered. Comparison of calculated unit cell dimensions of calcite with diagrams of MACKENZIE et al. (1983) revealed a very low magnesium content (0 to 2 mol %) (Table 1). The results of chemical analyses of valves of selected rudist genera are given in Table 2 while in Table 3 relevant basic statistical parameters are pre- sented. Concentrations of niobium, zirconium, yttrium, uranium, and rubidium are not shown because in all analysed samples they were below the LOD (app. 1 ppm for all of these elements except U for which the LOD was 4 ppm). The results showed some differences in chemical composition of valves belonging to different rudist genera, although the content of most analysed trace elements is not influenced by rudist genus. The strontium content of valves of different genera varies significantly (Fig. 2). Praeradiolites, Radiolites and Vaccinites have very similar concentrations of stron- tium while the genera Durania and Gorjanovicia are differentiated by their remarkably higher concentration of strontium. Generally, the low content of magnesium, already inferred by X-ray investigations, is especially low in Vaccinites (Fig. 3). Minor differences were also observed in the sodium content, rudists of the genera Durania and Vaccinites are characterised by lower con- centrations of Na (Fig. 4). These differences in chemical composition were confirmed by discriminant function analysis. The results of this analysis, though limited due to the number of samples on which analysis has been based, are rep- resented by Fig. 5 and Tables 4 and 5. These results suggest that each rudist genus has a moderately distinct chemical composition, and samples of different rudist Fig. 1 Map with sampling locations (D1 – Durania sample 1 etc., G = Gorjanovicia, P = Praeradiolites, R= Radiolites, V = Vaccinites; abbrevi- ations are also used on Figs. 2–5). Simplified geology after POLŠAK (1967a), MORO (1997) and MORO & ĆOSOVIĆ (2000): 1 – Ceno- manian, 2 – Turonian, 3 – Coniacian–Santonian, 4 – Palaeogene. 74 Geologia Croatica 57/1 75Tibljaš, Moro & Ostrež: Mineral and Chemical Composition of Rudist Valves... a c c/a V Sample Å Å Å Å3 Mg mol.% D1 4.9788 17.0303 3.421 365.5904 1 D2 4.9861 17.046 3.419 367.0039 1 D3 4.9912 17.0459 3.415 367.7535 1 D4 4.9847 17.0357 3.418 366.5737 1 D5 4.9872 17.0248 3.414 366.7116 2 G1 4.9934 17.0473 3.414 368.1167 1 G2 4.9890 17.0695 3.421 367.9478 0 G3 4.9809 17.0424 3.422 366.1645 1 G4 4.9843 17.0279 3.416 366.3588 2 P1 4.9878 17.0509 3.419 367.3691 1 P2 4.9918 17.0564 3.417 368.0699 1 P3 4.9921 17.0426 3.414 367.8125 2 P4 4.9840 17.0441 3.420 366.6538 1 R1 4.9922 17.0452 3.414 367.8906 1 R2 4.9870 17.0497 3.419 367.2159 1 R3 4.9936 17.0687 3.418 368.6063 0 R4 4.9763 17.0387 3.424 365.4098 1 R5 4.9865 17.0344 3.416 366.8188 2 V1 4.9843 17.0611 3.423 367.0628 0 V2 4.9915 17.0709 3.420 368.3358 0 V3 4.9840 17.0487 3.421 366.7595 1 V4 4.9913 17.0467 3.415 367.7886 1 Table 1 Unit cell dimensions and inferred (after MACKENZIE et al., 1983) Mg- content of calcite in valves of different rudist genera. Legend: R = Radiolites, P = Praeradiolites, D = Durania, V = Vac- cinites, G = Gorjanovicia. Sample Cu Fe Ga Mg Mn Na Ni Pb Sr Zn ppm % ppm % ppm % ppm ppm ppm ppm D1 18 <0.01 3 0.20 <2 0.03 4 5 295 19 D2 2 <0.01 4 0.19 3 0.03 3 7 289 8 D3 9 <0.01 4 0.18 <2 0.04 5 4 256 12 D4 9 <0.01 4 0.22 9 0.03 4 6 442 13 D5 6 <0.01 4 0.23 <2 0.03 5 7 492 12 G1 9 <0.01 3 0.23 3 0.06 5 9 630 17 G2 11 <0.01 3 0.16 <2 0.07 3 6 540 16 G3 3 <0.01 4 0.05 7 0.01 4 5 220 10 G4 2 <0.01 4 0.12 11 0.07 3 4 465 10 P1 14 <0.01 3 0.16 <2 0.10 4 5 169 15 P2 12 <0.01 3 0.20 6 0.12 4 8 169 15 P3 2 <0.01 4 0.21 <2 0.04 4 4 239 9 P4 2 <0.01 4 0.17 <2 0.06 4 6 175 8 R1 14 <0.01 4 0.18 6 0.06 4 7 209 16 R2 17 <0.01 4 0.18 4 0.04 2 5 255 19 R3 9 <0.01 4 0.18 3 0.05 5 7 248 13 R4 7 <0.01 3 0.18 9 0.05 3 7 226 14 R5 1 <0.01 2 0.17 15 0.08 2 3 213 10 V1 2 <0.01 3 0.06 <2 0.02 4 7 158 9 V2 23 0.03 4 0.08 8 0.01 4 10 164 20 V3 12 0.02 4 0.07 3 0.01 7 6 182 15 V4 16 <0.01 4 0.06 5 0.02 5 5 258 16 LOD 0.7 0.01 1.1 0.01 2 0.01 0.7 1.6 0.9 0.9 Table 2 Elemental concentrations in valves of different rudist genera. Legend: R = Radiolites, P = Praeradiolites, D = Durania, V = Vac- cinites, G = Gorjanovicia, LOD = limit of detection. 76 Geologia Croatica 57/1 77Tibljaš, Moro & Ostrež: Mineral and Chemical Composition of Rudist Valves... genera can be distinguished in discriminant space fairly well (Fig. 5). 4. DISCUSSION According to XRD analyses, no remnants of bimineralic valves, which are otherwise characteristic for rudist shells (STEUBER, 2002) were found. This indicates diagenetic alteration of the studied valves and the trans- formation of aragonite to diagenetic low-Mg calcite (dLMC). The grade of diagenetic alteration of calcareous fossils is often inferred from their chemical composi- tion. As a result of meteoritic diagenesis Mn and Fe concentrations typically increase while Sr and Na con- centrations decrease (BRAND & VEZIER, 1980; AL- AASM & VEZIER, 1986). Measured concentrations of Mn in the investigated rudist valves is quite low (Table 2, Fig. 6), well within the range of concentrations of Rudist Cu Ga Mg Mn Na Ni Pb Sr Zn genera ppm ppm % ppm % ppm ppm ppm ppm Durania Min. 2 3 0.18 <2 0.03 3 4 256 8 Max. 18 4 0.23 9 0.04 5 7 492 19 Mean 9 (6) 4 (0) 0.20 (0.02) 3 (3) 0.03 (0) 4 (1) 6 (1) 355 (105) 13 (4) Gorjanovicia Min. 2 3 0.05 <2 0.01 3 4 220 10 Max. 11 4 0.23 11 0.07 5 9 630 17 Mean 6 (4) 4 (1) 0.14 (0.08) 6 (4) 0.05 (0.03) 4 (1) 6 (2) 464 (176) 13 (4) Praeradiolites Min. 2 3 0.16 <2 0.04 4 4 169 8 Max. 14 4 0.21 6 0.12 4 8 239 15 Mean 8(6) 3 (1) 0.19 (0.02) 3 (2) 0.08 (0.04) 4 (0) 6 (2) 188 (34) 12 (4) Radiolites Min. 1 2 0.17 3 0.04 2 3 209 10 Max. 17 4 0.18 15 0.08 5 7 255 19 Mean 10 (6) 3 (1) 0.18 (0) 7 (5) 0.06 (0.02) 3 (1) 6 (2) 230(21) 14 (3) Vaccinites Min. 2 3 0.06 <2 0.01 4 5 158 9 Max. 23 4 0.08 8 0.02 7 10 258 20 Mean 13 (9) 4 (1) 0.07 (0.01) 4 (3) 0.02 (0.01) 5 (1) 7 (2) 190 (46) 15 (5) All samples 9 (6) 4 (1) 0.16 (0.06) 5 (4) 0.05 (0.03) 4 (1) 6 (2) 286 (136) 13 (4) Table 3 The minimum, maximum and mean concentrations of trace and minor elements in valves of different rudist genera. Standard devia- tions calculated from measured concentrations for samples of each genus are given in parentheses. Data for Fe are not given because most of the measured values are below LOD. For calculation purposes concentrations of Mn bellow LOD were replaced with 70% of LOD (MIESCH, 1976). Fig. 2 Strontium concentration in shells of different rudist genera. Fig. 3 Magnesium concentration in shells of different rudist genera. 76 Geologia Croatica 57/1 77Tibljaš, Moro & Ostrež: Mineral and Chemical Composition of Rudist Valves... recent marine bivalve calcite shells (STEUBER, 1999 and references therein), and diagenetic alteration is indicated only by concentrations of Sr which are much lower than in recent shells. Observed Sr concentrations are significantly lower than those presented by several authors (McARTHUR et al., 1994, 2000; PODLAHA et al., 1998; STEUBER, 2002) for biological calcite, but in good agreement with data for diagenetically altered pelagic carbonate rocks (RENARD, 1986). This is also visible in Fig. 7, where the measured Mg and Sr concen- trations cluster close to the regression line showing com- positions of abiotic calcite. Similar diagenetic trends, with Mn and Fe that are not elevated and low Sr, were already observed by STEUBER (2002) in some rudist shells. Therefore, the same author considered that Mg/Sr ratios were the best indicator for diagenetic alteration of shells. The chemical pattern indicates that diagenesis ended with transformation of both primary constituents of the shells, aragonite and low-Mg calcite (LMC), into dLMC, despite the fact that LMC is considered to be relatively stable in a meteoric environment. According to the measured low Mn and Fe contents there was no precipitation of late ferroan calcitic cement. Transfor- mation processes were most probably accomplished in the vadose or phreatic–meteoric zone (AL-AASM & VEZIER, 1986). The results of chemical and discriminant analyses point out that the chemical content of shells of different rudist genera is rather distinct. Due to the fact that ana- lysed shells are diagenetically altered, chemical compo- sition of the shells should be primarily the result of dia- genetic processes. These processes are controlled by a Fig. 4 Sodium concentration in shells of different rudist genera. Fig. 5 Plot of discriminant scores along Function 1 vs. Function 2, to discriminate rudist valves of different genera. Discriminant function 1 2 3 4 log Sr -1.48068 -0.568105 0.373568 0.082307 Mg 1.53415 -0.542829 -0.489472 -0.036865 Mn 0.02954 -0.266035 -0.307657 -0.992138 Na 0.10066 0.209920 1.068870 -0.010185 Eigenvalue 7.36468 1.586070 0.949597 0.285667 % of variance – relative 72.30 15.57 9.33 2.80 % of variance – cumulative 72.30 87.87 97.20 100.00 Table 4 Standardised dis- criminant function coeffi- cients and related stati- stics. Percent correct Durania Gorjanovicia Praeradiolites Radiolites Vaccinites Durania 100.00 5 0 0 0 0 Gorjanovicia 75.00 0 3 0 0 1 Praeradiolites 75.00 0 0 3 1 0 Radiolites 100.00 0 0 0 5 0 Vaccinites 100.00 0 0 0 0 4 Total 90.91 5 3 3 6 5 Table 5 Classification matrix; rows contain number of samples in each observed group while in columns the number of samples in a predicted group are given. 78 Geologia Croatica 57/1 79Tibljaš, Moro & Ostrež: Mineral and Chemical Composition of Rudist Valves... variety of factors, such as their original mineralogy and chemistry and shell structure, and the physicochemical properties of diagenetic fluids and their position in the sedimentary column. It can be reasonably presumed that the latter factors were practically the same for all the analysed samples, since their sampling positions were very close, and Cretaceous limestones are only slightly tectonically deformed (POLŠAK, 1967a). Strontium concentrations measured in valves of dif- ferent rudist genera was slightly variable. A possible explanation for different Sr contents lies in the fact that some analysed shell material most probably contained a considerable portion of diagenetic cement (AL-AASM & VEZIER, 1986; STEUBER, 1999), which frequently has lower Sr concentrations than a biogenic one, so consequently the measured concentrations were lower. The presence of diagenetic cements is due to the cel- lular structure of the outer shell of most radiolitids which allows entrance of diagenetic fluids and cement formation (AL-AASM & VEZIER, 1986; STEUBER, 1999; REGIDOR-HIGUERA et al., 2002). The highest measured Sr contents were observed in Gorjanovicia, a radiolitid rudist with a compact, fibrous prismatic structure, similar to that of most hippuritids, in which the proportion of diagenetic cements was probably the lowest. Relatively high Sr concentrations were also observed in Durania. Although it has an open cellular structure, the wall structures (funnel plates and muri) are relatively thick which increases the proportion of biogenic calcite. Low Sr concentrations measured in Radiolites and Praeradiolites could be explained by a higher amount of diagenetic calcite in the analysed material due to their very thin shells with typical radi- olitid microstructure. However, the low Sr concentration in the hippuritid cannot be explained by shell structure alone, because its compact structure inhibits access of diagenetic fluids, and it is probably the result of the primary mineral composition. Hippuritid shells have a greater proportion of aragonite (STEUBER, 2002) which is less stable then LMC in the diagenetic environ- ment, and consequently they will be more diagenetically altered. The observed low Mg content of Vaccinites is most probably also influenced by their structure, prima- ry mineral and chemical composition. Unaltered shells of Vaccinites contained less Mg because a significant part of them was composed of aragonite, and the more compact structure inhibited sub-marine cementation by high-Mg cements which could result in the lower Mg content of altered shells. 5. CONCLUSIONS Mineral composition and trace element contents were obtained in order to find possible differences in rudist genera from specimens that were taken from pure car- bonates without the influence of terrigenous material. In all analysed samples, the primary constituents of rudist shells, aragonite and low-Mg calcite, are trans- formed into diagenetic low-Mg calcite. Measured stron- tium concentrations of all analysed shells correspond to pelagic bulk rock samples altered by diagenesis and are significantly lower than those for biological calcite. Observed differences in the chemical composition of diagenetically altered rudist shells belonging to different rudist genera, namely the higher strontium content in Gorjanovicia and Durania, and lower magnesium con- tent in Vaccinites, are most probably the result of dif- Fig. 6 Ranges and means of Mn and Sr concentrations in rudist valves from southern Istria compared with concentrations of modern bivalve calcite shells represented by the shaded area (STEUBER, 1999 and references therein). Symbols: ◊ = Durania, ■ = Gorjanovicia, ○ = Praeradiolites, □ = Radiolites, ● = Vaccinites. Fig. 7 Ranges and means of Mg and Sr concentrations in rudist valves from southern Istria compared with mean compositions of modern biotically (solid line) and abiotically (broken line) pre- cipitated calcite (CARPENTER & LOHMANN, 1992). 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Revised manuscript accepted May 04, 2004. 80 Geologia Croatica 57/1