PII S0016-7037(99)00365-8 Change of Sm-Nd isotope composition during weathering of till BJ ¨ ORN O ¨ HLANDER, 1, *JOHAN INGRI, 1,² MAGNUS LAND, 1,‡ and HANS SCH ¨ OBERG 2 1 Division of Applied Geology, Luleå University of Technology, SE-97187 Luleå, Sweden 2 Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden (Received November 13, 1998; accepted in revised form September 23, 1999) Abstract—Weathering of till in northern Sweden results in the formation of well-developed spodosols. The till is dominated by 1.9 –1.8 Ga granitic material. The REE are among the elements most strongly depleted during weathering, and the loss of REE from the E-horizon decreases as the atomic number increases. To study if weathering leads to a change of the Nd isotope composition, we have analysed the Nd isotopic composition of the various horizons including living plants and humus of two profiles of weathered till (typic haplocryods) in northern Sweden. As much as between 65.6 and 75.3% of the Sm and Nd in the 0.2 mm fraction has been lost from the E-horizon, and between 32.5 and 54.7% from the B-horizon. Nd has been lost to a slightly greater extent than Sm. The two C-horizon samples have  Nd(0) values of -22.1 and -23.2. Corresponding E-horizon values are -18.1 and -20.2. The B-horizon values are intermediate between the values of the E and C horizons. It is concluded that the weathering leads to a change in the Sm/Nd ratio resulting in a change of the Sm-Nd isotope composition. The plant and humus samples deviate even more from the unweathered till. For one station the results could be interpreted as if the Sm and Nd taken up by the plants had similar isotope characteristics as the amounts of these elements released by weathering in the E-horizon. For the other station it is probable that the Nd isotope composition of the organic samples is dominated by Nd released by till weathering which, however, is mixed with another Nd-source, possibly an airborne component. The expla- nation to the change of isotope compostion in the till is that a larger proportion of the Nd released by weathering is released from minerals with a lower Sm/Nd ratio than the bulk soil, compared with the amount released from minerals with a higher Sm/Nd ratio. Although the various REE-carrying minerals had the same initial Nd isotopic composition, 1.8 –1.9 Ga of decay of 147 Sm to 143 Nd has resulted in a higher present 143 Nd/ 144 Nd ratio in the minerals with a higher Sm/Nd ratio. Copyright © 2000 Elsevier Science Ltd 1. INTRODUCTION Neodymium is not a conservative element in water, and the Nd isotopic composition of seawater varies between the major oceans. Variations also occur between different areas of indi- vidual oceans (Piepgras et al., 1979; Piepgras and Wasserburg, 1980; Piepgras and Wasserburg, 1982; Piepgras and Wasser- burg, 1983; Piepgras and Jacobsen, 1988; Bertram and Elder- field, 1993). There is a broad correlation between the age of the terranes that supply the Nd to the oceans, and the Nd isotopic characteristics of seawater (Goldstein and Jacobsen, 1988; Stordal and Wasserburg, 1986). Nd isotopic composition of the dissolved load of rivers correlates with the mean age of the bedrock in their drainage basins (Goldstein and Jacobsen, 1988), but although river input is an important source of Nd in seawater, the origin of the dissolved Nd isotopic composition of seawater is not understood in detail (e.g., Albare ´de et al., 1997). Jeandel et al. (1995) found that atmospheric input is the major source of Nd in the Sargasso Sea, indicating another a type of continental source other than rivers. A large part of the Nd in soils on Hawaii is probably derived from Asian dust (Chadwick et al. 1999), illustrating that atmospheric transport may be very important. Andersson et al. (1992) found a general agreement between the Nd isotopic composition in waters from the Baltic Sea, an intracontinental estuarine-like system with brackish water, and the age of the surrounding terranes (ranging in age from Archaean to Phanerozoic). It has in most cases been assumed that the continentally derived Nd has the same mean Nd isotopic composition as the mean value of the bedrock. However, several studies, starting with Balashov et al. (1964), have shown that the rare earth elements (REE) are mobile during weathering in warm climate (Banfield and Eggleton, 1989; Price et al., 1991; Mongelli, 1993; Braun et al., 1993; Nesbitt and Markovics, 1997). Studies of palaeoweathering recorded in sedimentary rocks and fossile weathering profiles suggest that Sm/Nd ratios and Nd isotopic composition may also change during weathering (McDaniel et al., 1994; Macfarlane et al., 1994; Bock et al., 1994). Floss and Crozaz (1991) showed by detailed studies of an Anatarctic eucrite that the REE may also be mobile during weathering in cold climate. The mobility of REE in recent soil processes was shown by O ¨ hlander et al. (1991) in a regional reconnaissance study of weathering of till in the Kalix River watershed in northern Sweden. Lanthanum was even one of the elements most strongly depleted in the E-horizon (the uppermost leached horizon of the till wich in these areas often is ca. 5 cm thick and has a characteristic light grey colour). When assuming that the total Zr content of the till occurs in zircon, which may be a reasonable approximation in these till samples dominated by granitic material (Watson and Harrison, 1983), and assuming that the zircon is resistant to weathering (Nickel, 1973), the *Author to whom correspondence should be addressed (Bjorn. Ohlander@sb.luth.se). ² Present address: Department of Geology and Geochemistry, Stock- holm University, SE-10691 Stockholm, Sweden. ‡ Present address: Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA 91125, USA. Pergamon Geochimica et Cosmochimica Acta, Vol. 64, No. 5, pp. 813– 820, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 + .00 813