15th International Congress of Speleology ...

Speleothems

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2009 ICS Proceedings

Uranium mapping in speleothems: occurrence of diagenesis, detrital contamination, and geochemical consequences RICHARD MAIRE1, GUILLAUME DEVES2, ANNE-SOPHIE PERROUX3, BENJAMIN LANS1, THOMAS BACQUART2, CYRIL PLAISIR2, RICHARD ORTEGA2 1 CNRS, Laboratoire ADES, UMR 5185, Maison des Suds, Univresité de Bordeaux 3, Esplanade des Antilles, 33605 Pessac cedex, France, [email protected] 2 CNRS, Laboratoire CNAB, UMR 5084, CENBG, Chemin du Solarium, 33175 Gradignan, France 3 CNRS, Laboratoire EDYTEM, UMR 5204, Université de Savoie, Campus scientifique, 73 376 Le Bourget du Lac, France The microscopic distribution of uranium and other chemical elements was studied with high spatial resolution geochemical imaging techniques in speleothems unusually rich in uranium. We combined three complementary methods: digital autoradiography of radioactive elements, X-ray fluorescence microspectroscopy (micro-XRF), and nuclear microprobe analysis using a particle accelerator. The application of these techniques to karstology opens perspectives for improving our understanding of diagenesis and crystallochemistry of speleothems. The first studied example is a calcite-aragonite corallite of Pierre Saint-Martin (France). Quantitative imaging of Sr (4909 to 18571 µg.g-1) and U (88.7 to 350.0 µg.g-1) reveals that both elements concentrate into the aragonite. In calcite Sr content is 439 to 1097 µg.g-1 and U 10.9 to 19.3 µg.g-1. The second example is a calcite-aragonite-opal corallite of Baïikal (Russia). Digital autoradiography and micro-XRF analyses indicated exceptionally high U contents in solution voids filled by neoformation opal (up to 1300 µg.g-1). The third example is a calcite stalagmite polluted by detrital minerals (Gironde, France). The digital autoradiography shows radioactive rich zones corresponding to detrital contamination. The XRF analysis shows that the origin of radioactivity is due to zircon minerals rich in Zr, Hf and Th. In conclusion the chemical imaging methods are powerful tools to reveal: (1) the evidence and distribution of U or other elements ; (2) particular diagenesis phenomena which can be observed by micropetrography on thin sections ; (3) the opening of geochemical systems which must be considered in U/Th datings and more generally for U geochemical stability in CaCO3.

1. Introduction

The geochemistry of uranium in calcium carbonate constitutes a large topic. The uranium content in CaCO3 is usually low: often 10,000). Uranium concentrations for all the samples are indicated in shades of grey, from white (0 µg.g-1 U) to black (highest measured concentration). The quantitative analyses corresponding to each shade of grey were determined by comparison with a set of CaCO3 standards of known U concentrations ranging from 10 to 1000 µg.g-1. The mean U content for the whole sample was 80 µg.g-1, a value that is unusually high for a speleothem. Moreover, the uranium distribution is very heterogeneous within the sample. Certain areas of sample Sib25a contained exceptionally high levels of U, ranging from 360 up to 950 µg.g-1. These areas were small, from 2 to 10 mm height, and mostly coincided with the aragonite (white) zones of the speleothem (Fig. 3).

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covered zone 2, with one of the highest U contents as shown by the autoradiography analysis. The elements detected in this zone were Ca, Sr, Si and U. Another studied area (4 x 1 mm) confirms the highest concentrations of U in opal, locally up to 1300 µg.g-1. On the maps, we observe an inverse correlation between Ca and U and between Sr and U, but a strong correlation between Si and U (Fig. 4).

3.3 Trou Noir Cave (Gironde, France)

Figure 3: Corallite speleothem in Mechta Cave, Tageran (Baikal, Russia). Top: polished section (left) and autoradigraphy (right) showing the radioactive areas (in black). The rectangle indicates the area analyzed by µ-XRF. Bottom: µ-XRF mapping of Ca, Sr, Si and U. There is a reverse correlation between Ca and Si (opal), and a positive correlation between Si (opal) and U. Superimposing the autoradiography image (exposure 66 hours) and the speleothem cross-section image shows that the three areas with the highest U contents (zones 2, 5 and 6) coincide with a group of laminas with the same age, although the central parts of these laminas are not affected. The other three uranium-rich areas have lower U contents and are grouped in the right-hand part of the sample; that is to say, near the apex of the speleothem. It is interesting to note that the heart of the concretion, which is the oldest part of the sample, gave much lower (< 10 µg.g-1) and very homogenous levels of radioactivity. Micro-XRF element mapping was carried out on two small parts of the sample. The first area chosen (5.12 mm x 10.24 mm)

The Trou-Noir is an active cave system (sinkhole-resurgence) situated in the Oligocene porous limestone of Entre Deux Mers plateau near Bordeaux (Gironde) (LANS et al., 2006). An active stalagmite (TN 05) has been sampled near a narrow passage located in the middle of the cave and responsible of high waters in upstream. This stalagmite, 12 cm high, shows a smooth corroded surface with fissures due to mechanical shocks (floods, floating wood). The polished section shows dark laminated calcite divided in twelve growth cycles. The distribution of radioactivity by digital autoradiography (exposure time 66 hours) is not homogeneous; it is concentrated particularly in the lower part (0-37 mm). On thin section, we observe that radioactive zones are related with levels rich in detrital material. The mineralogical determination, after dissolution by HCl, indicates quartz (80 %), zircon (> 10 %) along with smaller amounts of glass and heavy minerals, such as staurolite and rutile. A three-hour µ-XRF analysis with a carbon filter to stop the Si signal shows the Zr, Fe and Ti rays. A focus between Fe and Zr rays indicate the presence of Hf, Zn and Th. Thorium is responsible for the radioactvity in the detrital particles (Fig. 5).

Figure 5: Autoradiography of Trou Noir stalagmite (left) and µ-XRF spectrum of a zircon (right). The radioactivity is due to thorium. Figure 4: Photo of thin section in polarized light showing an aragonite bud (grey), 1 mm wide, replaced partially by opal (black). Residual aragonite needles are visible in opal because of dissolution. Mechta Cave (Baikal, Russia).

4. Discussion 4.1 Recrystallization of aragonite into calcite (Pierre Saint-Martin : consequences for the geochemical system

15th International Congress of Speleology

Speleothems U and Sr concentrations measured in the aragonitic speleothems of Aranzadi Gallery are one of the highest of those analysed in cave concretions of meteoric origin. The uranium amount in natural calcites is typically in the range 0.01-3 µg/g. The chemical fixation of uranium in the aragonite and calcite is a complex phenomenon very difficult to identify at the atomic level. When the U content is of the order of a few ng/g to 1 µg/g, it localizes in the structural defects of the crystals. But the abnormal abundance of uranium in calcite, in several speleothems from the Aranzadi Gallery (10.9 to 19.3 µg/g), poses an interesting geochemical problem. The pseudomorphosis of the uraniferous aragonite by calcite, by recrystallization is proved in the top by a series of remarkable occurrences: altered aragonite, residual needles of aragonite partially “consumed” by calcite, microfractures, neoformation mosaic calcite. The abnormal abundance of uranium, as well as Sr, in the calcite is a strong argument in favour of recrystallization. At the time of the recrystallization of aragonite to calcite, the large ionic dimension of the tricarbonated-uranyl anion UO2(CO3)34- trapped in the orthorhombic structure of the aragonite (with a noncoordinated site) results in both a loss of uranium in the neoformation calcite, and an abnormally strong residual uranium content (Ortega et al, 2005). We conclude as hypothesis the loss of uranium in neoformation calcite is due to U migration towards aragonite through the crystalline system, but we do not know if the the geochemical system is open because U/Th datings do not indicate radiochronologic anomalies in the middle of the sample on the same stratigraphic level: 43426±324 yr in aragonite and 42644±622 yr in neoformation calcite (lab. GEOTOP, Montreal). It is a very important perspective of research; indeed the phenomenon of recrystallization of aragonite into calcite and dolomite is well known in the marine environment for corals. During the recrystallization of a coral reef, U is expulsed resulting in a decreased concentration, from 3 µg/g into initial aragonite to 0.5 µg/g into secondary calcite (methods deLALOU, 1985). Similar to what is reported for marine calcium carbonates, recrystallization of speleothem aragonite into calcite, and subsequent uranium loss, may probably lead sometimes to erroneous dating.

4.2 Diagenesis of aragonite-calcite into opal (Baikal) and origin of U The map showing the distribution of Ca includes areas with very low Ca concentrations that appear as “voids” in the calcium carbonate matrix. These voids are filled with Si and U. Some of them appear black in crossed-polarized light, indicating that the Si is present in the form of opal (SiO2, nH2O) which is amorphous. This neoformed opal is particularly rich in U, with small areas showing concentrations of more


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than 1300 µg.g-1. There is a good agreement between the U concentrations obtained using the two analytical methods, as the U concentrations measured by autoradiography (up to 950 µg.g-1) are of the same order of magnitude as the point values measured using micro-XRF (up to 1300 µg.g-1). The differences between the two methods can be attributed to the sizes of the measurement areas: autoradiography gives mean concentrations for relatively large areas of the sample, whereas micro-XRF measures concentrations in very small areas (100-µm to 10-µm beam). Both methods demonstrate the presence of small areas that are enriched in U by several hundred µg.g-1. The process by which opal replaces calcite and aragonite in speleothems and wall crusts that formed in carbonate environments has been described only in a few cases. For example, a similar process - involving speleothems in basalt lava tubes on Jeju Island in Korea - has been observed. In this case, the source of the Ca and Si was the leaching of the basalt by percolation waters (CHOI et al., 2005). In Mammoth Cave (Kentucky), the opal crust is only in the cave walls, but also in gypsum and aragonite crusts, as well as in the bedrock, and some of it has replaced calcite (PALMER and PALMER, 1995). In hydrothermal caves of New Mexico (USA), opal is quite common because sulfuric acid attacks clay and releases dissolved SiO2 (as H4SiO4), so opal precipitates more rapidly. The acid also dissolves carbonate minerals, and there is a replacement of some of the carbonates by SiO2 (PALMER, 2007). In Siberia, for Mechta Cave, the most probable hypothesis is silicilic acidrich and uraniferous solutions coming from the leaching of volcanic ash. Indeed, the presence of uranium in volcanic glass is well known and the Baikal region presents many volcanic areas (ALIOUKA, 1999).

4.3 Role of detrital zircon in Trou Noir cave (Gironde) In mineralogy and geochemistry, we know the zircons with actinides are the main radioactive minerals in granitoids and syenites, with substitutions and concentrations of U3O8 and ThO2 (BARIAND et al., 2003). In this example, the radioactivity of zircon is due to Th as we can see on the XRF spectrum.

Acknowledgments

AIFIRA facility (Fr.), Climanthrope program (ANR, Fr.) and Arthur Palmer for his advice.

References

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