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Procedía Earth and Planetary Science 17 (2017) 448 - 451
15th Water-Rock Interaction International Symposium, WRI-15
REE and Sm-Nd clues of high-temperature fluid-rock interaction in the Riópar dolomitization (SE Spain)
Dídac Navarro-Ciurana^*, Esteve Cardellacha, Carmen Galindob, José Manuel Fuenlabradac, Albert Grieraa, David Gómez-Grasa, Elena Vindeld, Mercè Corbellaa
aDepartament de Geología, Facultat de Ciències, Universität Autónoma de Barcelona, Edifici Cs s/n, Bellaterra 08193, Spain bDepartamento de Petrología y Geoquímica e Instituto de Geociencias, Universidad Complutense de Madrid-CSIC, c/ José Antonio Novais 12,
Madrid 28040, Spain
cCAI de Geocronología y Geoquímica Isotópica, Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, c/ José Antonio Novais
2, Madrid 28040, Spain
dDepartamento de Cristalografía y Mineralogía, Facultat de Ciencias Geológicas, Universidad Complutense de Madrid, c/ José Antonio Novais
12, Madrid 28040, Spain
Abstract
REE geochemistry and Sm-Nd isotope data of Mesozoic stratabound and patchy dolostones of the Riopar area (Prebetic Zone, SE Spain) are presented. The results, combined with previously published data, suggest the dolomitizing fluid was a warm brine that interacted with siliciclastic rocks of Triassic age and with the host carbonates at low fluid-rock ratios. The positive Eu anomaly, negative eNd values and MREE patterns confirm that the dolostones were formed by interaction with warm acidic crustal fluids. C-O isotopic interaction models indicate that these fluids were characterized by S18O-enriched and S13C-depleted compositions, pointing to low ratios of fluid to rock volumes. The studied dolostones show two Sr sources: one Sr signature is close to the host carbonate values and the other one is more radiogenic, indicating that fluids became enriched in 87Sr after interacting with siliciclastic rocks. Furthermore, the Sr-Nd isotope data systematic depicts a positive correlation, thus probably the same rock sources are shared for both elements. Moreover, the warm fluids interacted with regional limestones achieving a negative Ce and positive La anomalies, low S O compositions and similar S C values than the host carbonates. © 2017 The Authors.Publishedby ElsevierB.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15
Keywords: geochemistry; rare earth elements; stable and radiogenic isotopes; water-rock interection; Riopar hydrothermal dolomites.
* Corresponding author. Tel.: +34-93-581-4773. E-mail address: didac.navarro.ciurana@gmail.com
1878-5220 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of WRI-15
doi:10.1016/j.proeps.2016.12.113
1. Introduction
The study of dolostones and dolomitizing processes is of a great interest as the resulting rocks may host economic base-metal (Zn-Pb-F) ore-deposits and host more than half of the world's hydrocarbon reserves1. Dolostone formation has been linked with different geological processes and settings, but after more than two hundred years of intensive research, their origin is still controversial1. Most of the existing dolostone case studies are based on detailed petrographic descriptions, geochemical data analyses of trace elements and of the isotopic compositions of oxygen, carbon, and strontium to gain insight into dolomitization processes2. Although not as widely applied as previous analyses, rare earth elements (REE) and the Sm-Nd isotopic system are increasingly used as proxies in order to understand the flow controls and the origin of fluids involved on the dolomite formation in diagenetic environments2.
In the Riopar area (Prebetic, SE Spain) dolomitized bodies replace Upper Jurassic to Lower Cretaceous carbonatic rocks. Detailed descriptions of the different dolostone geobodies, dolomite phases as well as geochemical data (major element composition, C/O and Sr isotopes) and fluid inclusions microthermometrical analyses were reported by3. The present study incorporates REE and Sm-Nd isotope geochemistry into the analysis of Riopar dolostones; to our knowledge, this constitutes the first report of this kind focused on carbonate rocks from Iberian Mesozoic basins. The combination of new and previously published data is here used to constrain the type of dolomitizing fluid and sheds light on the fluid-rock interaction during dolomitization.
2. Geological setting and dolomite distribution
Riopar is located at the limit between the External and Internal Prebetic Zones, in the outer portion of the NNWverging fold-and-thrust belt of the Betic Cordillera (SE Spain). The External Prebetic Zone consists in extensively exposed Triassic siliciclastic sediments and Jurassic carbonate rocks and scarcity of Cretaceous and Paleogene sediments. In contrast, the Internal Prebetic Zone is characterized by broad outcrops of Cretaceous and Paleogene sediments4. From a tectonic standpoint, the Prebetic Zones are separated by two major system faults: the Alto Gualdalquivir-San Jorge fault with NE-SW to E-W trending and SE- to S-dipping, and the Socovos-Calasparra dextral strike-slip fault with NW-SE to W-E trending.
In the Riopar area, stratabound and patchy dolostones are found in carbonate rocks from Upper Jurassic to Lower Cretaceous ages, bounded between San Jorge and Socovos faults. Two stratabound bodies respectively replace Kimmeridgian and Upper Berriasian to Valanginian grainstones, packstones and mudstones. They are predominantly constituted by microcrystalline replacive dolomite crystals (ReD). The patchy dolostones affect Kimmeridgian up to Aptian carbonates, also connecting the stratabound bodies. They consist of sucrosic dolomite cements (SuD) and fine to coarse saddle dolomites (SaD). These dolomites outcrop nearer the San Jorge than Socovos faults, suggesting a structural control for this dolomitization (see3).
3. REE geochemistry characterization
Highest REE concentrations are found in the stratabound dolostones constituted by matrix-replacive dolomite (ReD), which varies between 14.36 and 53.79 ppm. These dolomites are characterized by elevated concentrations of shale-normalized (SN) middle-REE, showing a "MREE bulge". The (La/La*)SN ratios range from 1.19 to 1.30, (Pr/Pr*)SN varies between 1.00 and 1.07, (Ce/Ce*)SN ranges from 0.80 to 0.87, and (Eu/Eu*)SN falls between 1.07 and 1.35. The average REESN pattern of ReD shows negative CeSN, and positive LaSN and EuSN anomalies (Fig. 1).
Sucrosic dolomite cements (SuD) have lower total SN REE contents than matrix-replacive dolomites (Fig. 1a). Their REE concentrations range from 4.56 to 19.44 ppm. These values are characterized by a light-REE enrichment, negative REE slope tendency and a slightly negative CeSN anomaly and positive LaSN and EuSN anomalies (Fig. 1). The (La/La*)SN ratios range from 0.97 to 2.23, (Pr/Pr*)SN falls between 0.99 and 1.03, (Ce/Ce*)SN varies between 0.67 and 0.97 and (Eu/Eu*)SN range from 1.11 to 1.45.
Saddle dolomite cements (SaD) contain total REE from 7.26 to 35.03 ppm. This dolomite also depicts an "MREE bulge" (Fig. 1a). The (La/La*)SN ratio is restricted between 1.19 and 1.55, (Pr/Pr*)SN ranges from 0.96 to 1.12,
(Ce/Ce*)SN ranges from 0.64 to 0.88 and (Eu/Eu*)SN varies from 1.20 to 1.44. The average SN REE pattern of SaD samples show overall negative CeSN and notably positive LaSN and EuSN anomalies (Fig. 1).
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Fig. 1. (a) Average shale-normalized REE patterns of the different Riôpar dolomite types against the standard Post-Archean Australian Shale (PAAS)5. (b) Cross plot of shale-normalized (Pr/Pr*)SN versus (Ce/Ce*)SN. ReD: matrix-replacive dolomite; SuD: sucrosic dolomite cement; SaD:
saddle dolomite cement.
4. Stable (C and O) and radiogenic (Sr, Sm and Nd) isotope data
The Upper Jurassic to Lower Cretaceous host limestones show 813C values between +0.5 and +3.2%o, 518O compositions from +27.6 to +30.9% and 87Sr/86Sr ratios of 0.70723 to 0.70731 (Figs. 2a and 2b)3. These values are compatible with carbonates precipitated from seawater of Jurassic to Cretaceous age6. According to3 stratabound (ReD) and patchy dolostones (SuD and SaD) show similar C and O isotope values (813C: -2.3 to +0.8%; 518O: +25.1 to +27.6%) (Fig. 2a) and a 87Sr/86Sr ratio that varies from 0.70736 to 0.70830 (Fig. 2b).
The studied dolomite types (ReD and SaD) show restricted 147Sm/143Nd (0.112-0.144) and 143Nd/144Nd (0.512160.51226) isotopic ratios (Fig. 2c). These compositions are not high enough and widespread enough to obtain a reliable isochron from the data. Nevertheless, initial ratios of Nd isotopes can be obtained with an assumption for the oldest age possible: Lower Albian. Recalculations for a geological age of 112 Ma does not alter the Sr isotopic signatures as the Rb concentration in these dolomites appear to be below the detection limit. With this age, the Riopar dolostones display initial 143Nd/144Nd(t=112 Ma) ratios of 0.51207 to 0.51215, and a calculated initial sNd(t=112 Ma) ranging from -6.7 to -8.2 (Fig. 2c). Although the Nd isotopic concentrations at 112 Ma are lower than the
isotopic ratios at actual times, the both geologic times (Fig. 2c).
Nd/ Nd- Sr/ Sr isotopic relationships show the same distribution pattern for
01îO%e(PDB)
(b) 51bO%o(PDB) (c) £Nd,„11JUal
-5.7 -4.7 -3.7 -2.7 -«.7 -6.7 -4.7
0.7085 ■ i ■■ I ■ ■ ■ ■ I ■ I i ■ I ■ ■ i i. 0.7085
Jurassic-Cretaceous •ine carbonate Signatur«
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180 °C »ReD
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25.0 26.0 27.0
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43Nd/144Nd-87Sr/86Sr-sNd(t -112 Ma) cross-plot of ReD and SaD dolomite types at actual times and at an initial age of 112 Ma.
5. Discussion
Fig. 2. (a) ô C vs ô O cross-plot of host limestone and ReD, SuD and SaD dolomite types. C-O isotope model curves were calculated in terms of
fluid-rock interaction for dolomite at temperatures of 180 and 230°C. (b) 5 O vs Sr/ Sr cross-plot of host limestone and different dolomites. (c)
The positive Eu anomalies (Fig. 1a) of stratabound dolostones could be attributed to hydrothermal alteration from high temperature solutions7. This is consistent with microthermometrical data reported by3 (Th=150-250°C). Moreover, the observed MREE enrichments in the studied matrix-replacive and saddle dolomites (Fig. 1a), suggests the contribution of acidic hydrothermal fluids8 in the dolostone formation. Using an average Th value of 205°C, the C and O isotopic distribution can be modeled as an interaction between the regional limestones and a hydrothermal fluid with an initial isotopic composition of ô13C ~ -8% and ô18O ~ +17%. The data best fits the interaction at low
fluid-rock ratios (Fig. 2a). This 818O-rich isotopic composition can be achieved by a low 818O fluid (i.e. evaporated seawater, residual brines) after isotopic equilibration with carbonates. Furthermore, the narrow 813C range of dolostones (Fig. 2a), which is close to the original signature of the host limestones, indicates that the 813C-depleted composition of the fluid was buffered by the host rocks.
The studied dolostones Sr isotopic signatures point to two sources of Sr, one close to the Jurassic and Cretaceous marine carbonates and another more radiogenic. The latter indicates that the dolomitizing fluid became enriched in 87Sr after circulating and interacting with siliciclastic rocks, most probably of Triassic age due to its abundance in the External Prebetic Zone. Moreover, the LREE enrichment of sucrosic dolomites (Fig. 1a), which is typical of marine limestones with detrital sediment contributions9, might reflex the interaction between hydrothermal fluids and host limestones and siliciclastic rocks. The Sr and O isotope systematic depicts a trend relating lower 818O with
87 86 87
higher Sr/ Sr values (Fig. 2b), pointing to an increase of Sr in the fluid as the temperature increases. Furthermore, dolostones display a remarkably small range of 147Sm/143Nd and 143Nd/144Nd ratios, showing negative sNd values, which is a typical of crustal signature. The Sr and Nd isotope systematics depicts a trend relating higher 87Sr/86Sr with higher 143Nd/144Nd values (Fig. 2c). This relationship points to the same siliciclastic and shallow marine carbonate rock sources for both radiogenic isotopes.
On the other hand, the Riópar dolostones commonly display light negative Ce and positive La anomalies (Fig. 1). These features are widely regarded as characteristic of marine carbonates formed from oxygen-rich shallow water solutions10. Recent studies11 suggested that unless there is a large amount of diagenetic fluid to flush the system several times through intense water-rock interaction, diagenesis has no significant effect on the composition and distribution of REEs in carbonates. Therefore, the Ce and La anomalies reflect such interaction.
6. Conclusions
Geochemical data (REE, 813C, 618O, 87Sr/86Sr, 147Sm/143Nd and 143Nd/144Nd) from stratabound dolostones connected by patchy dolomite geobodies in the Riópar area (Prebetic Zone, SE Spain) suggests that both dolostone types are formed by acidic crustal hydrothermal fluids (818O-enriched and 813C-depleted) that acquired the radiogenic signature by interaction with siliciclastic rocks. Given the geology of the area, this would be Triassic formations. Furthermore, the warm dolomitizing fluids must have interacted with regional limestones of Jurassic and
18 13 87 86
Cretaceous age at low fluid-rock ratios, consequently buffering the low 8 O and 8 C compositions and Sr/ Sr values to the original marine signature, together with the typical negative Ce and positive La anomalies of the host carbonates.
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