Sophie Rihs

Geochemistry of Quaternary travertines in the region north of Rome (Italy): structural, hydrologic and paleoclimatic implications

In the Tyrrhenian region of central Italy, late Quaternary fossil travertines are widespread along two major regional structures: the Tiber Valley and the Ancona–Anzio line. The origin and transport of spring waters from which travertines precipitate are elucidated by chemical and isotopic studies of the travertines and associated thermal springs and gas vents. There are consistent differences in the geochemical and isotopic signatures of thermal spring waters, gas vents and present and fossil travertines between east and west of the Tiber Valley. West of the Tiber Valley, δ13C of CO2 discharged from gas vents and δ13C of fossil travertines are higher than those to the east. To the west the travertines have higher strontium contents, and gases emitted from vents have higher 3He/4He ratios and lower N2 contents, than to the east. Fossil travertines to the west have characteristics typical of thermogene (thermal spring) origin, whereas those to the east have meteogene (low-temperature) characteristics (including abundant plant casts and organic impurities). The regional geochemical differences in travertines and fluid compositions across the Tiber Valley are interpreted with a model of regional fluid flow. The regional Mesozoic limestone aquifer is recharged in the main axis of the Apennine chain, and the groundwater flows westward and is discharged at springs. The travertine-precipitating waters east of the Tiber Valley have shallower flow paths than those to the west. Because of the comparatively short fluid flow paths and low (normal) heat flow, the groundwaters to the east of the Tiber Valley are cold and have CO2 isotopic signatures, indicating a significant biogenic contribution acquired from soils in the recharge area and limited deeply derived CO2. In contrast, spring waters west of the Tiber Valley have been conductively heated during transit in these high heat-flow areas and have incorporated a comparatively large quantity of CO2 derived from decarbonation of limestone. The elevated strontium content of the thermal spring water west of the Tiber Valley is attributed to deep circulation and dissolution of a Triassic evaporite unit that is stratigraphically beneath the Mesozoic limestone. U-series age dates of fossil travertines indicate three main periods of travertine formation (ka): 220–240, 120–140 and 60–70. Based on the regional flow model correlating travertine deposition at thermal springs and precipitation in the recharge area, we suggest that pluvial activity was enhanced during these periods. Our study suggests that travertines preserve a valuable record of paleofluid composition and paleoprecipitation and are thus useful for reconstructing paleohydrology and paleoclimate.

Long-term behaviour of continental hydrothermal systems:: U-series study of hydrothermal carbonates from the French Massif Central (Allier Valley)

U-series dating of hydrothermal carbonates, deposited by CO2-rich thermo-mineral waters of the French Massif Central, provides new insights on the long-term behaviour of a continental hydrothermal system. Dating of aragonite veins and travertines impregnating old terrace levels of the Allier river allowed identification of the main episodes of surface deposition, and thus factors likely to influence these circulations. Fifteen layers from three veins and four travertines from two separate areas were analysed. Sr isotope compositions were also measured on vein samples. The results show that this region was subjected to at least 3 main episodes of surface or near surface hydrothermal deposition: 253 to 208 ky, 135 to 100 ky and less than 8 ky. Comparison of these ages with a global climatic curve indicates that preferential deposition of carbonates occurs during warm periods, suggesting a strong influence of climatic conditions on the hydrothermal system. It is suggested that this climatic influence does not necessarily imply the absence of carbonate deposition during cold and dry periods, but rather that carbonate precipitation might occur at depth before the geothermal fluids reach the surface. In addition, the isotope compositions of fluids recorded by the 87Sr/86Sr and (234U/238U) initial ratios in the aragonite veins from Coudes remained remarkably constant over 250 ky, ranging from 0.71360 to 0.71371 and from 3.10 to 3.39 respectively. The two samples coming from Saladis show a slightly higher (234U/238U) initial ratio around 3.95. The constancy of these ratios over such a long period suggests a hydrothermal system in a near steady state with respect to water-rock interaction. We thus propose a possible model allowing a conservative steady state despite variations in the water recharge rates, in response to the climatic variations. The difference between (234U/238U) initial ratios measured in the Coudes and Saladis systems suggests the existence of two separate reservoirs and constrains their lateral extension to a few km at most.

An improved method for Ra isotope (226Ra, 228Ra, 224Ra) measurements by gamma spectrometry in natural waters: application to CO2-rich thermal waters from the French Massif Central

Abstract An improved method was developed to measure 226 Ra, 228 Ra and 224 Ra in freshwaters by gamma spectrometry. Radium was selectively extracted from acidified samples using specific filters (3M EMPORE™ Radium Rad disks). The latter was subsequently analysed by gamma spectrometry. Simultaneous and direct determination of the activities of the three isotopes was performed by comparison of gamma rays of the Radium Rad disks with those of a calibrated standard disk. This efficient and reliable method allowed a reduction of sample processing to a few hours. This technique was applied to analyse the Ra isotope compositions of several CO 2 -rich hydrothermal springs of the western border of the Limagne graben (French Massif Central). The studied springs emerge from a succession of granitic outcrops lined up along a major fault. Their chemical compositions evolve from calcic and magnesian chloro-bicarbonated to sodic bicarbonated. All the springs display high Ra activities, probably linked to high CO 2 content and/or high cation content of these waters, with various Ra isotope ratios. 226 Ra activity ranges from 588 to 2287 mBq/l and 228 Ra activity from 260 to 1590 mBq/l, whereas 224 Ra displays an activity between 245 and 1808 mBq/l. Four of the six analysed springs have ( 228 Ra/ 226 Ra) activity ratios lower than 0.7, thus, significantly lower than the ratio expected from an interaction with a calc-alkaline granitoid (typically having ( 232 Th/ 238 U) activity ratio between 1 and 2). Low ( 228 Ra/ 226 Ra) ratio (0.27) of the northern water (Montpensier) suggests the existence in this area of a zone of U concentration, possibly resulting from U mobilization and accumulation induced by previous hydrothermal events. The ( 224 Ra/ 228 Ra) ratios display smaller variations. They suggest short transit times from the zone of Ra leaching to the surface (a few days) or a very shallow addition of 224 Ra (e.g., from a localised zone where 228 Th could be preferentially adsorbed on the mineral surfaces). In some cases, these ratios might be used to infer differences in transit times of waters between neighboring springs.

U, Ra and Ba incorporation during precipitation of hydrothermal carbonates: implications for 226Ra-Ba dating of impure travertines

Abstract We studied U, Ra and Ba incorporation in calcite in a natural CO 2 -rich hydrothermal area from the French Massif Central. Along the western border of the Limagne graben, several springs are exploited for the petrifaction of various artifacts with calcite. These sites offer the opportunity to sample the water and the calcite layers downflow from the spring, and thus to follow the evolution of their U, Ra and Ba contents as precipitation proceeds. Our results show that the apparent partition coefficients of U, Ra and Ba between water and calcite decrease during precipitation for the three elements. We found no direct relation between this variation and the main factors able to influence the partition coefficient, such as precipitation rate, which suggests that the incorporation of these trace elements could result from a composite process of adsorption and coprecipitation. Ra and Ba have a similar behaviour, with an apparent partition coefficient decreasing from 0.80 to 0.47 for Ra and 0.96 to 0.68 for Ba, resulting in a small (≤10%) variation of the Ra/Ba ratio. The apparent partition coefficient of U decreases from 0.38 to 0.20. These apparent coefficients are much higher than equilibrium values but might be applicable to natural systems with high precipitation rates. We also investigated the possibility of using the decay of the 226 Ra-excess, or the decrease of the ( 226 Ra)/Ba ratio to date older deposits. Whereas the 226 Ra initial activity at the time of deposition has not remained constant, and can not be used for dating, the ( 226 Ra)/Ba method gives better results, when appropriate corrections for detrital contamination in Ba are made. Mixing diagrams using Th as an indicator of contamination allow calculation of the ( 226 Ra)/Ba ratio of the pure carbonate component. The calculated ages of five travertine layers range from 330 to 800 years, suggesting a mean deposition rate of about 1 cm/yr. The relatively good agreement of the calculated ages and stratigraphic positions of the samples suggests that this method could be successfully applied to date impure hydrothermal carbonates in the range 0–10 ky.

Determination of the four natural Ra isotopes in thermal waters by gamma-ray spectrometry

Our method for the simultaneous determination of the four natural Ra isotopes ((226)Ra, (228)Ra, (224)Ra and (223)Ra) in thermal waters involves a separation of Ra on a selective filter (3M EMPORE Radium Rad disk), and a single counting using a broad-energy HPGe detector (BE Ge manufactured by CANBERRA). The calculation of (223)Ra and (228)Ra activities requires interference and cascade summing corrections. The (226)Ra activities in CO(2)-rich thermal waters of the Lodève Basin (South of France) range from 530 to 2240mBq/l. The low ((228)Ra/(226)Ra) activity ratios (0.19-0.29) suggest that Ra is mostly derived from the aquifer carbonates. The short-lived (224)Ra and (223)Ra are probably added to the water through recoil or desorption processes from Th-enriched coatings on the fracture walls.

Le radium et ses descendants dans quelques carbonates hydrothermaux d'auvergne: origine et utilisation pour la datation

Abstract Calcium carbonate (travertine) deposits precipitated from CO 2 -rich thermo-mineral waters of the Auvergne region (French Massif Central) have high 226 Ra (up to 2 Bq·g −1 ) and 228 Ra activities. 210 Pb and 228 Th, produced by decay of 226 Ra and 228 Ra respectively, are absent in the carbonate at the time of precipitation. It is thus possible to use these nuclides to date deposits in the range 0.1–100 years. In some cases, the decay of 226 Ra could be used for dating older travertines (up to 10 000 yr). 226 Ra in most thermal waters appears to be correlated with Mg, and both elements can be leached during water flow through formerly hydrothermally altered zones of the crystalline basement, which might have concentrated U in secondary minerals in a reducing environment.