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Geochemistry of red residua underlying dolomites in karst terrains of Yunnan-Guizhou Plateau II


Chemical Geology 203 (2004) 29 – 50 www.elsevier.com/locate/chemgeo

Geochemistry of red residua underlying dolomites in karst terrains of Yunnan-Guizhou Plateau II. The mobility of rare earth elements during weathering
Hongbing Ji a,b,c,*, Shijie Wang a, Ziyuan Ouyang a, Shen Zhang c, Chenxing Sun a, Xiuming Liu a, Dequan Zhou a
a b

The State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China Resource, Environment and GIS Key Laboratory of Beijing City, Department of Geography, Capital Normal University, Beijing 100037, China c Key Laboratory of Environmental Biogeochemistry, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China Received 19 August 2002; received in revised form 27 August 2003; accepted 28 August 2003

Abstract The aim of this study is to characterize the evolution of the rare earth elements (REE) in the Pingba red residua on karst terrain of Yunnan-Guizhou Plateau. The in-situ weathering and the two-stage development of the profile had been inferred from REE criterions. The REE were significantly fractionated, and Ce was less mobilized and separated from the other REEs at the highly enriched top of the profile. This is consistent with the increase of oxidation degree in the regolith. And it is also suggested that the wet/dry climate change during chemical weathering caused Ce alternative change between enrichment and invariance in the upper regolith. Chondrite-normalized REE distribution patterns for samples from dolomites and the lower regolith are characteristic of MREE enrichment and remarkable negative Ce-anomalies patterns (similar to the convex-up REE patterns). The following processes are interpreted for the patterns in this study: (1) the accumulation of MRRE-rich minerals in dolomite dissolution, (2) water – rock interaction in the weathering front, and (3) more leaching MREE from the upper part of the profile. The latter two explanations are considered as the dominant process for the formation of the REE patterns. Samples from the soil horizon exhibit typical REE distribution patterns of the upper crust, i.e., LaN/YbN = 10 and Eu/Eu* = 0.65. All data indicate that the leaching process is very important for pedogenesis in this region. The experiments demonstrating that abnormal enrichment of REE at the upper regolith – bedrock interface is caused by a combination of volume change, accumulation of REE-bearing minerals, leaching of REE from the upper regolith, and water – rock interaction during rock – soil alteration processes. Our results support the conclusion that the weathering profile represents a large, continental elemental storage reservoir, whereas REE enrichment occurs under favorable conditions in terms of stable tectonics, low erosion and rapid weathering over sufficiently long time. D 2003 Elsevier B.V. All rights reserved.
Keywords: REE, Fractionation; Abnormal enrichment; The convex-up patterns; Red residua; Karst terrain

* Corresponding author. Resource, Environment and GIS Key Laboratory of Beijing City, Department of Geography, Capital Normal University, Beijing 100037, China. Tel.: +86-10-68907073. E-mail address: hbji@sina.com (H. Ji). 0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.08.013

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H. Ji et al. / Chemical Geology 203 (2004) 29–50

1. Introduction Chemical weathering of rocks is a major geological process, which modifies the earth’s surface and controls geochemical cycling of elements, thereby, controlling rare earth elements (REEs) distribution and fractionation in various naturally settings as well. At present, two kinds of approaches are available in the study of the geochemical mobility of elements during rock weathering, the first approach is to study the geochemistry of aqueous fluids (Aiuppa et al., 2000 and reference therein; Phillips and Rojstaczer, 2001); the second approach is to study the mineralogy and geochemistry of weathering profiles (Nesbitt, 1979; Carr et al., 1980; Chesworth et al., 1981; White, 1983; Nesbitt and Wilson, 1992). Trace elements, especially REEs are widely used to investigate the weathering process. In the past, it prevailed that the REEs could not be mobilized during weathering. Later, increasing evidences shown that the REEs were mobilized in the process (e.g., Balashov et al., 1964; Ronov et al., 1967; Nesbitt, 1979; Boulange et al., 1990; Braun et al., 1990). Moreover, the REE fractionation and cerium anomalies had been studied extensively in the weathering environments (e.g., Nesbitt, 1979; Nesbitt et al., 1980; Gouveia et al., 1993; Marsh, 1991; Prudencio et al., 1993; Boulange and Colin, 1994; Sawka et al., 1986; Banfield and Eggleton, 1989; Braun et al., 1993; ¨ hlander et al., 1996). Some thought the mineral O control would be the most important factor for the REE fractionation process (Nesbitt, 1979; Duddy, 1980; Banfield and Eggleton, 1989); and some stressed heavy REE (HREE) capacity to form more solubility of complexes than light REE (LREE) (e.g., Cantrell and Byrne, 1987), and at the same time some suggested that the climate change would be an important reason for their fractionation (e.g., Balashov et al., 1964). Therefore, from all those studies it becomes evident that the REE mobility is mainly controlled by two factors, the climatic weathering conditions and the stability of the primary REE-carrying minerals (Aubert et al., 2001 and references therein). Meanwhile, a positive Ce anomaly has been found in some weathering profiles developing on various parent rocks, and the oxidizing conditions which lead to the formation of cerianite are generally interpreted the result (Braun et al., 1990 and references therein; Marsh, 1991; Mongelli, 1997). Laterites in tropical and subtropical areas are of partic-

ular significance in this aspect because great variations in chemical composition would occur during their formation. Few studies on REE distribution and fractionation have been conducted for red residua in karst terrains, but the distribution of elements including the rare earth elements has been extensively carried on for the karstbauxite deposits under the Mediterranean climatic conditions (Maksimovic and Roaldset, 1976; Maksimovic and Panto, 1980, 1985, 1991; Mongelli, 1997), and some authigenic-REE minerals have also been determined in the boehmite-rich bauxite profiles, i.e., La-rich bastnasite, Nd-rich synchysite and Nd-rich goyazite. However, at present there are still many open questions about REE mobilization and/or precipitation in red residua developed on carbonate rocks with very small amounts of REEs, such as how they are transported and enriched and what are major controlling factors for their fractionation. The purpose of this paper is to investigate REE in the red residua underlying dolomite rocks under warm – humid subtropical climatic conditions in upland of karst terrains, Guizhou Province. Emphasis is given particularly to the REE geochemical behaviors during chemical weathering/soil formation, and fractionation and enrichment mechanism in the process of the formation of the profile.

2. Geologic setting and methodolgies 2.1. Geological setting and sampling The study area is on Yunnan-Guizhou Plateau, which is located along the southwestern margin of the Yangtze Platform and adjacent to the suture zone between the Eurasian and Indian plates, i.e., the eastern Tethys (insert in Fig. 1). Guizhou Province is part of the Yangtze Platform. It contains mainly Proterozoic clastic sedimentary rocks and Palaeozoic to Upper– Middle Triassic marine carbonate rocks; Post Triassic rocks are mainly fluvial (Guizhou Province Geological Survey Team, 1995). The Pingba profile (26j24VN, 106j30VE), which is located in the upland of karst terrain of central Guizhou Province, developing on gently dipping Early – Middle Triassic dolomites with about 1% acid-insoluble residues (Anshun formation, T1a,

H. Ji et al. / Chemical Geology 203 (2004) 29–50

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Fig. 1. The simplified sketch of the Pingba profile showing the location of the sampling and sample numbers, those not listing the number samples are a space of 10 cm between each two sampling. (Above right) Geologic setting of the studies area (after Yu et al., 1997) and refers to in the text.

712 m in total thickness) (Fig. 1). The major mineral constituents of acid-insoluble residues include illite (70.8 – 73.3%), quartz (8.2 –9.3%), plagioclase (3.8 – 5.4%), potassium feldspar (4.4 – 4.5%), pyrite (3.9%) and anatase (2.7 –3.6%), etc. The main mineral assemblages in the profile are: illite + kaolinite + gibbsite + smectite + hydroxy-interlayered minerals of vermiculite or chlorite +

quartz + feldspar + iron oxides + anatase, and both hematite and goethite are regarded as iron oxides. The profile is thick, with four horizons recognized in the field, i.e., the soil horizon (A-horizon, top for cultivated layer), the regolith horizon (B-horizon), which can be subdivided into the red soil layer, ferruginous crust layer, reddish soil layer, yellow soil layer and chocolate layer (so named as the soil

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H. Ji et al. / Chemical Geology 203 (2004) 29–50

Table 1 The Zr, Nb, Th and rare earth element contents of bulk samples from the Pingba dolomite profile Samplesa HY Y1 Y2-1 Y2-2 Y3-1 Y3-2 YT1 YT2 YT3 T1 T2 T3 T4 T5 28.69 265.37 26.58 34.45 115.36 292.06 27.50 97.35 18.43 3.13 12.66 1.49 7.80 1.50 4.07 0.66 4.26 0.64 586.91 18.30 3.94 2.41 1.21 0.63 T6 29.95 235.59 24.08 33.17 115.89 203.25 28.52 100.98 18.76 3.20 12.38 1.62 7.67 1.50 3.87 0.61 4.18 0.63 503.06 18.74 3.89 2.40 0.83 0.64

Y 3.06 3.13 3.41 4.21 5.50 9.35 42.04 27.66 22.88 564.34 217.57 91.97 36.38 Zr 2.14 2.28 1.79 1.76 3.19 2.81 388.93 390.95 261.28 272.65 213 247.36 285.76 Nb 0.28 0.31 0.15 0.18 0.40 0.34 40.61 41.77 24.14 27.45 23.23 25.00 26.66 Th 0.24 0.27 0.13 0.13 0.23 0.24 14.73 12.20 10.46 35.24 29.18 34.22 34.38 La 2.78 3.02 2.98 3.33 8.08 15.99 61.30 48.85 41.30 4791.17 1146.88 246.63 173.21 Ce 1.91 2.02 1.36 1.10 2.26 2.14 88.16 72.23 51.43 194.91 206.30 343.43 450.56 Pr 0.73 0.78 0.86 1.03 2.77 5.73 9.31 8.09 6.84 1307.64 324.14 114.84 54.39 Nd 4.09 4.32 3.91 6.77 17.01 35.02 28.94 25.76 26.09 6206.9 1401.01 534.91 227.65 Sm 1.61 1.84 2.51 3.73 7.25 16.88 5.51 5.19 6.59 1362.38 296.00 199.91 63.60 Eu 0.62 0.65 0.85 1.29 2.41 5.59 1.33 1.28 1.61 302.72 62.89 41.91 11.89 Gd 2.75 3.02 5.90 7.72 13.78 25.86 10.26 8.71 7.00 1583.15 308.67 164.75 45.27 Tb 0.43 0.48 0.88 1.11 2.03 3.86 2.03 1.60 1.32 182.87 36.37 23.37 6.60 Dy 1.58 1.68 3.37 4.70 7.51 12.61 15.30 10.01 6.65 625.79 163.28 92.97 29.89 Ho 0.21 0.22 0.39 0.53 0.77 1.36 2.97 1.91 1.09 73.78 19.66 11.90 4.20 Er 0.43 0.48 0.71 1.01 1.51 2.36 7.64 4.68 3.29 150.71 41.22 24.15 10.07 Tm 0.07 0.07 0.09 0.11 0.20 0.37 1.09 0.78 0.53 17.58 4.91 3.71 1.76 Yb 0.39 0.43 0.52 0.65 1.16 1.80 6.98 5.18 3.65 117.6 26.45 24.21 13.27 Lu 0.06 0.07 0.06 0.09 0.15 0.26 1.03 0.76 0.50 14.8 3.72 3.38 1.86 AREEb 17.66 19.03 24.39 33.17 66.89 129.82 241.85 195.03 157.89 16932.00 4041.50 1830.07 1094.22 LaN/YbN 4.85 4.73 3.87 3.48 4.71 5.99 5.93 6.37 7.65 27.53 29.30 6.88 8.82 LaN/SmN 1.09 1.04 0.75 0.56 0.70 0.60 7.00 5.92 3.94 2.21 2.44 0.78 1.71 GdN/YbN 5.74 5.67 9.20 9.69 9.63 11.62 1.19 1.36 1.55 10.91 9.46 5.52 2.76 Ce/Ce* 0.31 0.31 0.20 0.14 0.11 0.05 0.86 0.85 0.72 0.02 0.08 0.48 1.09 Eu/Eu* 0.90 0.84 0.67 0.73 0.74 0.82 0.54 0.58 0.72 0.63 0.64 0.71 0.68
a

Sample YT1, YT2 and YT3 stand for the acid-insoluble residues from samples Y1, Y2 – 2 and Y3 – 2, respectively, others are the same as Fig. 1. Trace elements values are in ppm. b AREE = the sum of La~Lu, Ce/Ce*= CeN/(LaN ? PrN)0.5, Eu/Eu*= EuN/(SmN ? GdN)0.5, and LaN/YbN, LaN/SmN, GdN/YbN, where N refers to a chondrite-normalized value (see Taylor and McLennan, 1985).

layer’s color is similar to chocolate), the weathering bedrock horizon (C-horizon), which can be subdivide into the flour dolomite layer, cracked dolomite layer and primary dolomite layer) and fresh bedrock horizon (R-horizon). Samples were collected from an artificial cell well of a natural profile (Fig. 1). 2.2. Methodologies The method to extract insoluble residues from dolomites for the samples was described in detail by Wang et al. (1999) and the detail procedure of the analyses was given in the previous paper (Ji et al., 2004). The air-dry bulk samples were finely ground in agate mortar into the size (200 mesh) before analysis and dissolution. The trace elements were analyzed using inductive-coupled plasma-mass spectroscopy (ICP-MS) techniques (ELEMENT, Finnigan MAT). The open vessel acid digestions method was

used for the pretreatment of the samples (Jarvis, 1992). The analyses monitored with standard samples and the analytical uncertainties involved in measurements are 10% for rare earth elements (see Appendix A). The procedures for leaching experiment for of samples from the lower regolith in the Pingba profile was: The accurate 5 g powder samples were weighed and placed into beakers, soaked for 30 min with 1.0 N hydrochloric acid at room temperature, then use the filter paper to separate acid-soluble (AS) fractions from acid-insoluble (AI) fractions. The AI was rinsed twice with de-ion water, and was heat-dried at about 100 jC, cooled down and weighed. The AS concentrations, which not measured directly, were the difference of the total (‘‘untreated’’) and AI concentrations (see Awwiller, 1994). The above analyses and measurements were all accomplished at the Institute of Geochemistry, Chinese Academy of Sciences.

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T7

T8

T9

T10

T11

T12

T13

T14

T15

T16

T17

T18

T19

T20

T21

T22

T23

28.11 28.41 32.24 29.44 28.39 34.93 30.32 28.18 27.76 256.09 254.88 272.32 252.66 252.32 257.08 264.31 260.29 258.66 27.45 26.87 28.56 26.99 26.45 27.40 28.68 28.72 28.96 35.18 36.56 38.10 36.29 35.42 37.69 36.97 36.62 37.65 94.92 93.54 95.67 93.19 90.05 94.98 88.90 84.21 85.64 186.38 173.59 279.01 150.50 144.26 161.17 161.65 171.38 486.50 21.70 20.86 21.30 19.99 19.67 21.18 18.53 17.65 17.86 69.06 64.58 68.56 61.08 59.96 68.39 59.28 55.29 54.46 11.24 10.13 10.83 9.34 8.89 10.71 9.09 8.49 8.01 1.92 1.70 1.91 1.58 1.57 1.91 1.69 1.42 1.42 7.82 7.07 8.34 7.15 6.89 8.66 7.68 6.99 6.87 1.08 1.04 1.16 1.03 0.95 1.28 1.08 1.05 0.91 6.14 6.18 7.03 6.03 5.87 7.522 6.81 6.41 5.61 1.33 1.34 1.51 1.34 1.36 1.63 1.47 1.36 1.20 3.63 3.61 4.12 3.56 3.52 4.32 4.04 3.62 3.50 0.57 0.59 0.64 0.55 0.57 0.70 0.59 0.61 0.56 4.01 3.87 4.30 3.68 3.67 4.44 4.09 3.99 3.64 0.56 0.60 0.64 0.58 0.55 0.68 0.63 0.61 0.56 410.36 388.70 505.02 359.60 347.78 387.57 365.53 363.08 676.74 16.00 16.33 15.03 17.11 16.58 14.46 14.69 14.26 15.90 5.32 5.81 5.56 6.28 6.38 5.58 6.16 6.24 6.73 1.58 1.48 1.57 1.57 1.52 1.58 1.52 1.42 1.53 0.96 0.92 1.45 0.82 0.80 0.84 0.93 1.04 2.91 0.63 0.61 0.61 0.59 0.61 0.61 0.62 0.56 0.58

27.59 29.58 23.82 25.76 21.48 25.91 24.66 24.033 274.06 269.40 251.24 258.48 247.72 259.94 263.09 254.77 29.15 29.74 27.26 28.04 28.40 28.94 29.52 26.87 238.46 39.10 34.73 37.54 34.20 38.22 37.42 35.33 86.66 90.91 77.85 81.82 70.33 80.12 78.40 80.75 182.20 223.71 164.28 227.11 198.70 215.91 184.08 131.54 17.60 18.53 15.68 16.67 14.29 15.91 15.22 15.59 53.94 57.67 46.30 49.36 41.76 47.90 45.38 45.25 7.95 8.89 6.61 7.31 6.23 7.07 6.78 6.21 1.42 1.61 1.16 1.29 1.10 1.23 1.09 1.13 6.13 7.11 5.13 5.80 4.97 5.75 5.24 4.92 0.95 0.93 0.74 0.86 0.70 0.80 0.83 0.77 5.38 5.96 4.88 5.29 4.45 5.23 4.76 5.11 1.29 1.32 1.11 1.19 1.03 1.16 1.14 1.07 3.54 3.65 3.00 3.32 2.79 3.40 3.37 3.26 0.57 0.62 0.50 0.50 0.45 0.53 0.54 0.52 3.60 3.82 3.23 3.40 3.02 3.84 3.47 3.36 0.60 0.60 0.56 0.50 0.44 0.58 0.55 0.51 371.83 425.33 331.03 404.42 350.26 389.43 350.85 299.99 16.27 16.08 16.29 16.26 15.74 14.10 15.27 16.24 6.86 6.44 7.41 7.05 7.11 7.13 7.28 8.18 1.38 1.53 1.29 1.38 1.33 1.21 1.22 1.19 1.09 1.28 1.10 1.44 1.47 1.42 1.25 0.87 0.62 0.62 0.61 0.61 0.60 0.59 0.56 0.62 (continued on next page)

According to Brimhall and Dietrich (1987), for in situ weathering, the ratio of the concentration of element j in the weathering profile (Cj?w) to that in the primary rock (Cj?p) can be defined as Cj?w =Cj?p ? ?qp =qw ? ? ?1=?ej?w ? 1?? ? ?1 ? sj?w ? ? 1? Where qp/qw refers to the bulk density ratio of the primary rock to the weathering horizons, and 1/ (ej + 1) is a strain factor in which ej is called the volume strain or volume change and ej = Vw/Vp ? 1. sj is the mass transport coefficient, defined as the mass of element j transported in unit volume in the weathering profile. When sj = 0, element j is immobile, affected only by the internal closed chemical system processes, specifically, residual and strain effects. On the other hand, when sj = ? 1, it means that the mass of element j is completely removed

from the system during weathering. From Eq. (1), we obtain: sj?w ? ?qw ? Cj?w ? ? ?ej?w ? 1?=?qp ? Cj?p ? ? 1 ?2?

For an immobile element I(si?w = 0), the volume strain ei in Eq. (1) can be calculated from the ratios of density data and concentration of element I in the regolith and primary rocks: ei?w ? ?qp =qw ? ? Ci:p =Ci?w ? 1 ?3?

Substituting Eq. (3) into Eq. (2), we have (see White et al., 1998): sj?w ? ?Cj?w =Cj?p ?=?Ci?w =Ci?p ? ? 1 ?4?

It should be kept in mind that two prerequisites must be satisfied in using Eq. (4), i.e. firstly, the

34 Table 1 (continued) T24 T25 T26 T27 T28

H. Ji et al. / Chemical Geology 203 (2004) 29–50

T29

T30

T31

T32

T33

T34

T35

T36

T37

T38

T39

T40

22.27 23.40 24.02 22.53 22.90 23.03 24.65 25.75 25.28 25.30 25.93 9.18 21.51 24.47 22.79 31.00 31.13 250.38 265.94 273.12 258.49 242.14 285.94 253.87 264.68 261.46 250.27 233.53 87.64 234.58 272.98 345.38 337.47 371.26 28.56 29.30 30.90 29.92 26.71 32.61 26.64 28.38 27.67 27.45 24.76 9.09 24.53 29.50 38.74 41.18 35.22 6.93 35.99 35.54 38.48 35.32 32.12 35.27 36.44 35.81 35.91 33.47 12.97 33.27 36.21 26.45 29.73 30.37 71.23 76.39 74.74 75.04 76.70 74.64 83.52 91.64 84.57 82.19 89.01 37.42 73.56 74.79 42.61 54.44 58.46 174.61 148.52 155.80 172.32 206.61 345.67 269.44 482.32 241.30 565.84 236.33 393.35 357.35 140.45 75.55 115.35 148.89 14.25 15.11 14.81 14.69 14.97 13.93 17.15 18.29 16.98 17.14 18.82 7.75 15.22 15.30 8.83 11.26 11.73 41.20 44.68 42.58 43.40 42.56 41.76 49.52 52.64 48.03 50.16 54.26 22.79 43.88 44.80 27.00 33.64 38.59 6.09 6.32 6.04 6.17 6.28 6.27 6.98 7.31 7.13 6.86 7.67 3.26 6.16 6.39 4.40 5.59 6.56 0.97 1.08 1.10 1.09 1.00 1.12 1.27 1.30 1.20 1.26 1.30 0.52 1.07 1.13 0.71 0.94 1.13 4.71 4.77 4.99 4.67 4.86 5.68 5.94 6.50 5.53 6.24 5.91 2.99 5.02 5.00 3.89 4.95 5.28 0.68 0.76 0.78 0.78 0.81 0.85 0.86 0.99 0.86 0.76 0.81 0.35 0.70 0.79 0.69 0.81 0.83 4.54 4.92 4.52 4.51 4.85 5.24 5.18 5.52 5.56 5.22 5.25 2.01 4.12 4.65 4.43 5.74 5.85 1.02 1.05 1.02 0.99 1.09 1.07 1.20 1.28 1.17 1.11 1.15 0.42 0.98 1.07 0.98 1.28 1.19 2.97 3.01 2.86 2.84 2.80 3.36 3.39 3.45 3.46 3.25 3.24 1.14 2.74 3.14 2.87 3.79 3.94 0.47 0.50 0.51 0.47 0.48 0.56 0.55 0.54 0.50 0.50 0.52 0.17 0.41 0.54 0.43 0.61 0.54 3.13 3.40 3.41 3.34 3.19 3.66 3.45 3.97 3.70 3.23 3.39 1.29 2.96 3.37 3.08 3.77 4.18 0.50 0.52 0.47 0.49 0.53 0.60 0.57 0.61 0.55 0.53 0.50 0.16 0.46 0.52 0.51 0.54 0.60 326.37 311.03 313.63 330.80 366.73 504.41 449.02 676.36 420.54 744.29 428.16 473.62 514.63 301.94 175.98 242.71 287.76 15.38 15.18 14.81 15.18 16.25 13.78 16.36 15.60 15.45 17.19 17.74 19.60 16.79 15.00 9.35 9.76 9.46 7.36 7.61 7.79 7.66 7.69 7.49 7.53 7.89 7.47 7.54 7.30 7.22 7.52 7.37 6.10 6.13 5.61 1.22 1.14 1.19 1.13 1.23 1.26 1.40 1.33 1.21 1.57 1.41 1.88 1.37 1.20 1.02 1.06 1.03 1.28 1.02 1.10 1.22 1.43 2.51 1.67 2.76 1.49 3.53 1.35 5.41 2.50 0.97 0.91 1.09 1.33 0.55 0.60 0.61 0.62 0.55 0.57 0.60 0.58 0.58 0.59 0.59 0.51 0.59 0.61 0.52 0.55 0.59

weathering rock must be basically similar to the underlying bedrock and, secondly, the immobile element (i.e. Ci) considered must be conservative in its real sense. This approach allows calculating of the mass flux of each selected element independent of the gains and losses of other elements. We calculate the depthintegrated change in each element j by means of weight sj?w for nth soil horizon according to the product of horizon thickness h and dry bulk density q (Chadwick et al., 1990; Kurtz et al., 2000): d ? Rsn ? qn ? hn =m ?n ? 1 to d ? ? 5?

When d < 0, a net loss of element j from the soil layer due to leaching was indicated, and when d>0, a net gain of an element relative to the soil’s immobile element stock was suggested.

3. Results 3.1. REE concentrations and patterns The REE data and relevant parameters are listed in Table 1. The total REE contents of samples from Bhorizon greatly vary (Table 1), with the exception of the dolomite samples and their ‘‘insoluble residues’’, The other samples have total REE abundances more than 300 ppm, exceeding those of shales (e.g. PAAS, NASC and ES) and the average upper continental crust (UCC) (Taylor and McLennan, 1985), and Chinese loess and paleosols (Gallet et al., 1996). However, the total REE abundances of samples from A-horizon are

Where m is the mass per unit area of the soil column to a depth d, calculated by summing the product of each horizon’s density and thickness: m ? Rqn ? hn ?n ? 1 to d ? ? 6?

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obviously lower than those of the samples from Bhorizon (Table 1). The REE patterns of dolomite samples from Chorizon in the Pingba profile show remarkable negative Ce-anomalies (Ce/Ce* = 0.05 – 0.31), moderate to weak Eu-anomalies (Eu/Eu* = 0.67 –0.90), and unusual enrichments of the middle REE (LaN/SmN = 0.56– 1.09, GdN/YbN = 5.67 – 11.62). Meanwhile, the MREE tends to become more rich (LaN/SmN and GdN/YbN ratios tend to decrease and increase, respectively) and the negative Ce-anomalies become more remarkable in the weathering process from the primary dolomite through the cracked dolomite to the flour dolomite (Fig. 2A). Samples T1 –T4 are enriched in the LREE (LaN/YbN = 6.88 –29.30) and MREE (LaN/ SmN = 0.78– 2.44, GdN/YbN = 2.76 – 10.91) and their REE patterns are similar to those of the underlying dolomites, showing the distinctive characteristics of inheritance. At the same time, remarkable negative Ce-anomalies are also noticeable (Ce/Ce* = 0.02 – 0.48) in Samples T1 – T3. Sample T1 from chocolate layer close to the modern weathering interface has the highest total REE (abnormal enrichment, total REE = 1.7%, and repeated sampling and measurement showed that the total REE abundances reached up to 3.1%; Ji et al., 1999) and displays the most remarkable Ce-anomaly (Ce/Ce* = 0.02) (Fig. 2A), but samples T5 –T10 slightly away from the weathering interface are characterized by a decrease in total REE. The LRRE and MREE enrichment patterns are replaced by the LRRE enrichment patterns (LaN/SmN = 3.89 – 6.28, GdN/YbN = 1.48 – 2.41), and the negative Ce-anomalies decrease or disappear, even instead weak positive Ce-anomalies appear (Ce/Ce* = 0.83 –1.45) (Fig. 2A). The other samples from B-horizon all display LRRE enrichment (LaN/ YbN = 13.78 – 17.74, sample T35 from the ferruginous crust is an exception), weak to remarkable positive Ce-anomalies (Ce/Ce* = 0.80 – 3.53), and moderate Eu-anomalies (Eu/Eu* = 0.55 – 0.62). Sample T35 from the ferruginous crust is characterized by a remarkable positive Ce-anomaly (Ce/Ce* = 5.41) and slight HREE depletion (Fig. 2B). Samples from Ahorizon at the top of the profile feature LRRE enrichment (LaN/YbN = 9.35– 9.76), but they have smooth REE patterns characterized by slight depletion in LREE as compared with those from B-horizon (Fig. 2C). Samples from A-horizon exhibit the typical REE

Fig. 2. Chondrite-normalized REE patterns of samples from the Pingba profile. Normalized to the concentrations in Chondrite (Taylor and McLennan, 1985). Samples from the bedrock horizon and the lower regolith horizon, the upper regolith horizon, the soil horizon and the insoluble residues from dolomites are shown in (A), (B), (C) and (D) respectively. Shaded fields outlining the ranges for the regolith (sample numbers T5 – T10) are shown in (A) and for the regolith (sample numbers T11 – T34 and T36 – T37) in (B).

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distribution patterns of the upper crust, i.e., LaN/ YbN = 10 and Eu/Eu* = 0.65. The insoluble residues extracted from primary dolomite, cracked dolomite and flour dolomite possess strikingly similar REE distribution patterns, characterized by LREE enrichment (LaN/YbN = 5.93 – 7.65, but more depleted in LREE than most of the samples from B-horizon and A-horizon), weak negative Ce-anomaly and moderate negative Eu-anomaly. Moreover, the insoluble residues tend to gradually become enriched in LREEs and depleted in HREEs in the order of primary dolomite through cracked dolomite to flour dolomite (i.e., LaN/SmN ratios decrease while GdN/YbN ratios increase) (Fig. 2D). 3.2. Non-isovolumetric weathering and REE mobility For a weathering profile developing on homogenous parent materials, its mass balance approach

was based on the application of chemically immobile elements as reference elements to quantify the mass fluxes of other elements. Such an approach allows to quantity the mass flux loss and gain of each of elements relative to the other elements. Using Eq. (3) given in this paper, the volume strain ( e) of the elements Zr, Nb, and Th that are generally considered as the immobile elements can be worked out as shown in Fig. 3A. A notable feature of this figure is that great changes have experienced in volume from the bedrock to the regolith horizon in the profile, which is obviously different from the features of the weathering processes of other kinds of rocks, namely non-isovolumetric weathering. The increases of the volume strain of the elements Zr, Nb and Th took place in the middle part of the weathered bedrock and the regolith –bedrock interface. From the interface to the top of the profile the volume strain is ei c ? 1,

Fig. 3. Weathering characteristics as a function of depth in the Pingba profile as defined by Eq. (1) (after White et al., 1998). (A) the regolith volume change ej?w (zero is equivalent to no volume change), two processes, namely dilation and collapse, were found at the middle weathering bedrock and the regolith – bedrock interface, respectively. (B), REE mass transfer sj?w ( ? 1.0 and 0 refer to total loss and no loss, respectively).

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Fig. 4. Integrated the regolith net losses and gains of REE for the Pingba profile as defined by Eq. (5) in this paper.

no obvious variation occurs. The former is caused by a dilation process of the profile in which cracks increase from primary dolomite to cracked dolomite and the porosity is enhanced (see Brimhall and

Dietrich, 1987; Chadwick et al., 1990); the latter caused by a collapse process of the weathered dolomite because the insoluble residues account for 1% of the total mass of the dolomites and the

Table 2 REEs concentrations and related parameters resulted from the leaching experiment for samples from the lower Pingba profile (unit: ppm) Samples Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu AREE LaN/YbN LaN/SmN GdN/YbN Ce/Ce* Eu/Eu* Y-3 5.50 8.08 2.26 2.77 17.01 7.25 2.41 13.78 2.03 7.51 0.77 1.51 0.20 1.16 0.15 66.89 4.71 0.70 9.63 0.11 0.74 T-1 617.70 11834.5 197.4 2925.3 12929.0 2546.4 508.2 2304.7 263.52 829.50 85.94 171.13 20.88 124.79 16.31 34757.6 64.08 2.93 14.97 0.01 0.64 T-2 260.71 946.9 192.9 276.5 1297.5 266.8 55.1 325.4 39.78 164.01 20.80 45.93 5.59 36.59 4.79 3678.59 17.49 2.23 7.21 0.09 0.57 T-3 107.52 183.1 281.3 107.4 541.9 183.02 40.73 162.12 22.02 82.37 9.76 21.65 2.98 19.12 2.64 1660.11 6.47 0.63 6.87 0.47 0.72 Y-3(AI) 1.94 1.85 2.06 0.59 3.12 1.29 0.41 2.10 0.35 1.44 0.17 0.38 0.06 0.33 0.04 14.19 3.79 0.90 5.16 0.46 0.76 T-1(AI) 136.70 1178.2 161.1 320.6 1409.4 293.89 65.19 304.15 40.90 160.17 19.08 41.51 5.90 38.04 5.28 4042.41 20.93 2.52 6.48 0.06 0.67 T-2(AI) 86.09 262.9 184.1 74.7 340.1 68.64 15.03 84.05 11.06 50.38 6.55 16.07 2.16 14.37 2.15 1132.26 12.36 2.41 4.74 0.31 0.60 T-3(AI) 45.51 115.5 273.7 37.0 167.5 52.55 11.83 47.54 6.88 28.15 3.75 9.04 1.33 9.08 1.29 765.14 8.60 6.15 4.24 0.98 0.72 Y-3(AS) 3.56 6.23 0.2 2.18 13.89 5.96 2 11.68 1.68 6.07 0.6 1.13 0.14 0.83 0.11 52.70 5.07 0.66 11.40 0.01 0.76 T-1(AS) 481 10656.3 36.3 2604.7 11519.6 2252.51 443.01 2000.55 222.62 669.33 66.86 129.62 14.98 86.75 11.03 30714.16 83.01 2.98 18.69 0.002 0.64 T-2(AS) 174.62 684 8.8 201.8 957.4 198.16 40.07 241.35 28.72 113.63 14.25 29.86 3.43 22.22 2.64 2546.33 20.80 2.17 8.80 0.01 0.56 T-3(AS) 62.01 67.6 7.6 70.4 374.4 130.47 28.9 114.58 15.14 54.22 6.01 12.61 1.65 10.04 1.35 894.97 4.55 0.33 9.25 0.03 0.72

Sample numbers Y-3, T-1, T-2 and T-3 in the table and sample numbers Y3-2, T1, T2 and T3 in Table 1 from the same locality are shown in Fig. 1. The AI and AS stand for acid-soluble (AS) and acid-insoluble (AI) fractions of the samples, respectively. For other parameters, see Table 1.

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leaching-accumulation of the dolomites may lead to more than 90% volumetric change. The currently known equations of mass balance calculations are usually based on the isovolumetric weathering, but ¨ hlander et al. (2000) and Kurtz et al. according to O (2000) the mass balance calculation in chemical weathering would still be true if variations in volume and density are not taken into consideration. We can take Zr as an indicator element (see Harden, 1987; Chadwick et al., 1990; Brimhall et al., 1992) and use Eq. (4) to characterize the variation trend of mass transport coefficient (sj) of REE in the Pingba profile (Fig. 3B). A significant fractionation of REE in the weathering front has been observed. The element Ce has been separated

from the other REEs, and remained largely enriched at the upper regolith. From Eqs. (4) and (5), the net variation of the elements in the horizons A and B can be also calculated (Fig. 4) if the underlying dolomite and the less mobile Zr are taken as the precursor material and the reference element, respectively. Samples from both horizons A and B have almost similar characteristics of element variation. The net gains and losses of Ce are presented relative to Zr stock in B-horizon and A-horizon in the profile, respectively. Other REE have experienced more or less leaching-loss relative to Zr stock in the weathering process, however, there exists difference among them. The extent of the net loss of HREE

Fig. 5. Shale-normalized REE patterns of samples from the lower regolith in the Pingba profile. The shale values are from Sholkovitz (1988).

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is greater than that of LRRE, and that of MREE is the largest (Fig. 4). 3.3. Mineral control on REE distribution The selective chemical leaching experiments aim at understanding the distribution of trace elements in different phases of sediments and soils so as to reveal the geochemical characteristics of provenance and geochemical behaviors of trace elements during

sedimentary diagenesis and supergene weathering (Sholkovitz, 1989; Awwiller, 1994; Quade et al., 1995). The leaching results of samples from the lower part of the Pingba profile are shown in Table 2 and Fig. 5. Shale-normalized REE patterns of the primary samples are similar to those of soluble constituents (AS) and they are of the ‘‘hat’’ type patterns with Gd as the center, which may reflect the existence of biogenic apatite or apatite nodules in the primary samples (Wright et al., 1987; Grand-

Fig. 6. (A) LaN/SmN ratio vs. GdN/YbN ratio and (B) total REE abundance vs. loss on ignition (LOIs) content for bulk samples from the Pingba profile. The UCC values are from Taylor and McLennan (1985). In the diagram (A) the solid arrow hyperbola after Savoy et al. (2000). The legends of bedrock, regolith, soil and residues represent samples in the bedrock horizon (sample numbers: HY f Y3 – 2), the regolith horizon (sample numbers: T1 f T37), the soil horizon (sample numbers: T38 f T40) and the insoluble residues (sample numbers: YT1 f YT3 ) from dolomite, respectively.

40 H. Ji et al. / Chemical Geology 203 (2004) 29–50 Fig. 7. SEM obsevation of rhabdophane-La crystals coupled with EDS. Backscattered electron imaging is also used to identify the primary and secondary P REE-bearing minerals in samples for the chocolate layer (The large of Cu peak in the EDS map is due to the influence copper net for loading the sample).

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jean and Albarede, 1989; Reynard et al., 1999). The residual constituents (AI) are characterized by the decrease of LaN/YbN ratios and negative Ce anomaly, near to the values of samples above the weathering front in the lower B-horizon (Fig. 5B). The total amount of leached REE reached 65– 88%, particularly large in the chocolate layer. The highly solubility of REE as what has been observed here may be the result of using pH-low leaching solution rather than natural water, preventing REE from being re-adsorbed on residual phases (Sholkovitz, 1989). Among the several compositional variables of samples from the profile, LaN/SmN –GdN/YbN ratios and LOIs (loss on ignition)-REE contents show a correlation in the lower parts of the regolith horizon, respectively (Fig. 6). Most of the examined samples exhibit pretty good logarithmic relations in Fig. 6A, which can be attributed to the occurrence of bio-apatite with enrichment of GdN/YbN in the samples (see Savoy et al., 2000). The abundance of bio-apatite tend to increase in the weathering process from primary dolomite to flour dolomite (leaching-accumulating process), and to decrease in samples from the lower to upper B-horizon (residue-weathering process). Moreover, SEM-EDS investigations have demonstrated that sample T1 from the chocolate layer contains LREE-rich secondary phosphate mineral (The apatite grain has been found in the sample, Sun personal correspondence), with La, Nd and P being dominant. The grain size of the mineral is very small (less than 1 Am in length) and it usually occurs as acicular or fibrous aggregates. It is inferred that the mineral is rhabdophane-La (Fig. 7) and similar to Ce-deficient rhabdophane described by Adams (1968) and Hildebrand et al. (1957), who attributed the mineral to the partial fractionation of the REE through the oxidation of Ce3 + to Ce4 + at a certain stage prior to the formation of rhabdophane. Moreover, the LOIs shows a positive correlation with total REE (R2 = 0.72) for samples from the lower regolith, the dolomites and their insoluble residues (R2 = 0.49), respectively (Fig. 6B). This characteristic suggests that compositional control on REE chemistry is partial due to the LOI contents. It has been known the LOI contents of the samples mainly reflect the influence of high waterbearing phases (i.e., clay minerals) (Ji et al., submitted

for publication). Therefore, the apatite, phosphate and clay minerals all control the REE chemistry in the lower profile.

4. Discussion 4.1. REE criteria for in-situ weathering As viewed from Table 1, the REE contents of samples from the Pingba profile are far higher than those of potential material sources in this Plateau, for instance, modern sediments or loess. The REE distribution patterns in samples from both the bedrock and the lower B-horizon are characterized by MREE and HREE enrichments (Fig. 2A), demonstrating that there exists a pronounced inheritance between bedrock and residual soil. The above REE features strongly support the cognate relation between the dolomite and its overlying regolith. It is known that Y and Ho have identical ionic radii and coordinating numbers and show much similarity in most geologic and geochemical processes. However, there would occur remarkable fractionation between Y and Ho does occur in weathering, riverine transport and in the marine environment (through biogeochemical processes) (Nozaki et al., 2000). In the process of weathering, for example, the complex ability of Ho to coordi? nate with organic material or HCO3 is greater than that of Y, and Y shows a stronger tendency to adsorb on solid particles in water – rock interaction (Kawabe et al., 1991). Ho would have more easily be lost due to leaching, resulting in an increase in the Y/Ho ratio in weathered profile. In Fig. 8A, the Y/Ho ratios of all samples in the Pingba profile are lower than those of the UCC. A positive correlation (R2 = 0.68) has been found among the dolomite samples, with Y/Ho and Er/Ho decreasing in the processes from primary dolomite to flour dolomite. It is a weathering trend in this study because the process from primary dolomite to flour dolomite is a known chemical weathering process of the dolomites. Meanwhile, the other positive correlation (R2 = 0.75) has been observed among samples from the B-horizon, and with both Y/Ho and Er/Ho being precisely increased upward the regolith. The samples from the insoluble residues and A-horizon

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Fig. 8. (A) Er/Ho ratio vs. Y/Ho ratio, (B) Eu/Eu* value vs. GdN/YbN ratio and (C) Ce/Ce* value vs. LaN/YbN ratio for bulk samples from the Pingba profile. Solid and dotted arrow lines indicate the weathering trend for samples from the lower regolith and bedrock horizon in this study, respectively. And in the map C lines Eu/Eu*= 0.85 and GdN/YbN = 2.0 after McLennan et al. (1993). For symbols see Fig. 6.

are also distributed on this trend (Fig. 8A). An inherited relationship has been noticed between the two correlation lines (i.e., the ratios Y/Ho and Er/

Ho from samples in the lower B-horizon are similar to those from flour dolomite samples), and the rise of Y/Ho in the Pingba profile is consistent with the

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above-described feature of weathering process accompanying water/rock interaction. Therefore, the line in Fig. 8A composed by B-horizon samples also precisely represents the weathering trend. The GdN/YbN vs. Eu/Eu* diagram has been widely applied in constraining the origin of sediments with different ages (McLennan, 1989). In Fig. 8B only dolomite samples and those from the lower part of the profile have GdN/YbN ratio more than 2.0 and the other samples have values around the UCC. Samples from the lower part of the B-horizon constitute a line with GdN/YbN ratio continuously decreasing but Eu/Eu* remaining unchanged. However, in the weathering process of dolomites from primary dolomite to flour dolomite GdN/YbN ratios tend to increase steadily and Eu/ Eu* ratios tend to reduce. The decrease of Eu/Eu* ratios in the process of weathering may be ascribed to the destruction of plagioclase (see Condie et al., 1995). Increasing GdN/YbN ratios or MREE abundances are consistent with an increase of some minerals in the samples, for example, the increase in apatite (Savoy et al., 2000). The above characteristics of GdN/YbN ratio demonstrate that there is a pronounced inherited relationship between flour dolomite and the chocolate layer. Meanwhile, the weathering process from primary dolomite to flour dolomite and the evolution of samples from the lower part of the B-horizon are not the same and belong to two different processes. Some researchers suggested that the heavy REEs presented as stable elements while the LREEs were preferentially adsorbed on clay minerals in weathering process, thus leading to an increase in La/Yb ratio in weathered profile (Nesbitt, 1979; Duddy, 1980). But it is not supported for the paleo-weathered profile investigated by Condie et al. (1995). The major samples in the middle B-horizon fall in the range of Ce/Ce* = 0.8 – 1.67 and La N / YbN = 14.10 – 18.74 (Fig. 8C). Which suggests that the REE-bearing minerals in the B-horizon may not mainly influence the variable Ce anomalies. However, some dolomite samples are characterized by the increase of LaN/YbN ratios and the decrease of negative Ce anomalies (Fig. 8C), and combine with samples from the ferruginization and rubification processes constitute an actual leaching-accumulating trend (Ji et al., submitted for publication), which is

similar to the above-described phenomenon of the increase in La/Yb ratio in the weathered profile. But the case is not true for the other dolomite samples and samples from the lower B-horizon (Fig. 8C), reflecting considerable REE fractionation happening during the initial period of chemical weathering. The LaN/YbN ratios markedly decreased from samples T1 – T4 and the total amount of REE is also decreased (Fig. 2A). The decrease of LaN/YbN ratios described above may be related mainly to the weathering of REE minerals. In general, the above REE characteristics prove that the Pingba profile has experienced an in-situ weathering process and the two-stage nature of weathering can be distinguished precisely. 4.2. Mechanism for REE fractionation in red residua The fractionation of REE in weathering profiles is constrained mainly by weathering conditions and stable primary REE-bearing minerals, even secondary minerals such as apatite and sphene (Braun et al., 1998; Aubert et al., 2001). The subtropical climate and stable karst terrains are leading to the formation of the Pingba profile in the YunnanGuizhou Plateau. As Banfield and Eggleton (1989) pointed out that pronounced LREE fractionation would occur in the initial period of weathering in the study of the Bemboka granodiorite massif of Australia. It is also true for the Pingba profile that the REE fractionation has been also observed at the beginning of the weathering process (e.g., Figs. 3B and 8C). The LREEs are gained more than the HREE at the weathering front in the profile (Fig. 3B), one of the reasons that are related to the enrichment of LREE-bearing minerals during the dolomite dissolution, i.e. rhabdophane-La appear in the front (Fig. 7). It is well known that three fractions in carbonate rocks containing REE: (1) the detritus fraction, chiefly composed of stable REE independent minerals; (2) the absorbed fraction, with the ion form adsorbed on clay minerals; and (3) the authigenic carbonate and phosphate, which formed in the sedimentary and diagenetic processes. The latter two forms are mainly fractions for the hosted REEs (Balashov and Girin, 1969; Parekh et al., 1977). The large and uneven gains of REEs have been

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noticed during weathering process from primary dolomite to flour dolomite (Fig. 3B). These gains (positive mass fluxes) of REE imply that the primary dolomites were initially heterogeneous (such as authigenic REE-hosted minerals) and/or the water –rock interaction led to addition of contrasting material during the process (such as the ion form adsorbed on clay minerals). The latter explanation is also likely. The uneven gains of REE, especially HREE in the weathering bedrock are due to the HREE compounds in the solution that are relatively stable and preferentially transported downward. The replacement of primary apatite by secondary LREE-bearing phosphate (e.g. fluorencite and rhabdophane) is one of the important factors leading to negative Ce-anomalies. Meanwhile, the weathered profile would display both positive and negative Ceanomalies due to the influence of oxidation – reduction. There would appear highly variable Ceanomalies in lateritic profiles and remarkable positive Ce-anomalies in upper parts of the profiles. The latter has been caused due to the formation of REE-bearing secondary minerals (cerianite) during weathering (Braun et al., 1990, 1998). Ce separation from the other REE and its gain in the upper part of the profile and repeated alternation of gain and immobility (Fig. 3B) are mainly due to: (1) immobility of Ce and its separation from the other REE in the supergenic oxidizing environment (e.g., Vlasov, 1966; Marsh, 1991); (2) enrichment of Ce due to the enhancement of oxidation in the upper part of the profile (e.g., Braun et al., 1998); and (3) the alternation of the redox (reduction – oxidation) state in the upper part of the profile. On the basis of pedologic research (Xi, 1990), if the climate becomes arid, solution is deprived from the soil and the oxidation increase, and if the climate changes moist, element export intensifies following the increase of moisture contents, and the oxidation state turns to the reduction one. Therefore, we propose that the redox state in the profile was created by the alternative occurrence of dry and wet climates, which caused the alternation of gain and immobility of Ce in the upper profile. Although REE may be locally remobilized in a weathering profile, there are no selective losses of any REE during weathering (Nesbitt, 1979; Condie,

1991; Condie et al., 1995). However, the uneven transport of REE have been found in the A- and B-horizon of the Pingba profile (Fig. 4), for example, the larger loss of HRRE (i.e., yYb = ? 0.95) than LREE (i.e., yLa = ? 0.89) in the profile development. It is similar to the result gained from the till weathering in northern Sweden, which was interpreted to be due to the release of REE from ¨ hlander et al., 1996). The different minerals (O largest net loss of MREE (i.e., yGd = ? 0.99) is also similar to the depletion in eolian component of North Pacific sediments, and the reason for this phenomenon is attributed to the labile fine grain phosphate rich fraction in loess (Weber et al., 1998). Therefore, the uneven transport of REE in the profile is mainly due to: (1) HREE preferentially transporting downward in the regolith, because HREE are present as stable compounds in the solution while LREE should be preferentially adsorbed on clay minerals (e.g., Nesbitt, 1979; Duddy, 1980; Braun et al., 1990); and (2) MREE preferentially dissolving since phosphate-rich minerals distribute in the profile. Moreover, the net gain of element Ce (yCe = 0.56) in the B-horizon is closely related to the net loss of Ce (yCe = ? 0.68) in A-horizon (Fig. 4), which is ascribed to the lateritic origin (the regolith reduction) by the residual surficial accumulation of insoluble oxides of Ce. 4.3. REE evolution processes in red residua The REE distribution patterns of dolomite and lower B-horizon in Pingba profile are marked by the notable negative Ce anomaly and MREE enrichment (Fig. 2A), which are similar to the ‘‘hat’’-type REE patterns of the typical conodont and other apatite minerals (Wright-Clark and Holser, 1981; Wright et al., 1987; Shields and Stille, 2001), or the convex-up REE patterns in acidic waters (Johannesson and Zhou, 1997; Serrano et al., 2000 and reference therein). At present, four hypotheses have been put forward to explain the latter (Serrano et al., 2000 and reference therein): (1) aqueous complex effect; (2) inheritance of original patterns of the weathered primary REE-bearing minerals; (3) fractionation overprinted by secondary mineral formation during

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weathering; or (4) some consistent combinations of the above. The REE patterns in dolomite remain regular changes during weathering from primary dolomite to flour dolomite, and characterized by the increase of total REE, GdN/YbN ratio and negative anomaly (Table 1 and Fig. 2). The explanations for the increased total REE and Gd N /Yb N ratio are: (1) the accumulation of MREE-rich minerals is accompanied with the progressive accumulation of insoluble residues in the leaching-accumulating process; (2) the supergenic water – rock interaction has occurred at the weathering front of the profile, i.e., secondary REE-rich phases formation during dolomite weathering (Fig. 7 and (3) more leaching MREE from the upper part of the profile. However, the notable increase in the P2O5 abundances has not been noticed during the dolomite weathering from the major elements data (Ji et al., submitted for publication), which may bring into some problems on the first interpretation. Meanwhile, the negative Ce anomaly increased precisely during the weathering, indicating that Ce lost as a result of supergene oxidation, or it may be related to the replacement of primarily accumulated minerals by apatite and phosphate minerals (Banfield and Eggleton, 1989), or it may be related to the less leaching Ce from the upper part of the profile (Figs. 3B and 4). The REE patterns of insoluble residues extracted from the dolomites are characterized by LREE enrichment with weak negative Ce anomaly and moderate Eu anomaly (Fig. 2D). REE enrichment during dolomite weathering from primary dolomite through cracked dolomite to flour dolomite is recorded in Table 1 and Fig. 2A. It may have shown an increase in total REE in insoluble residues from the weathering process. However, a decrease in total REE has been found in insoluble residues in the progressive weathering process (Table 1 and Fig. 2D). Evidently, the leaching experiment by 1 N hydrochloric acid is a main factor leading to this difference. The same REE distribution patterns between the dolomites and their insoluble residues have been found from Fig. 2A and D, for example, LaN/SmN ratios tend to decrease and GdN/YbN ratios increase during the dolomite weathering. It is similar to the result that the leaching

does not apparently affect the relative REE patterns (Cullers, 1988). Therefore, the amount of REEs in dolomites is different from that of their insoluble residues, due to the increase of the amount of soluble REE during the dolomite weathering. Meanwhile, the HREE abundances in the insoluble residues tend to decrease steadily, which is probably due to intensifying the extent of REE releasing and transport, especially HREE in response to water/rock interaction, or the leaching processes during chemical weathering (Shields and Stille, 2001). From the weathered bedrock (flour dolomite) to the chocolate layer there exist drastic increase of AREE, especially LREE and MREE, as well as the maximization of negative Ce anomaly (Ce/ Ce* = 0.02) (Table 1). The possible explanation is the large volumetric change between dolomite and residual soil, accumulation of REE-rich minerals or secondary minerals (see Fig. 7), and REE enrichment in response to water/rock interaction. Remarkable negative anomaly may be consistent with the weathering characteristics of P-rich minerals (Banfield and Eggleton, 1989). From samples T1– T4 in the weathering front the abundances of REE and ratios of LaN/YbN and GdN/YbN tend to decrease obviously. This may be related with the leaching loss of LREE- and MREE-rich minerals in the development of the profile. Large amount of REE, especially HREE, lost in from samples T4– T5, in consistency with the preferential transport of HREE compounds during weatheringleaching process (Nesbitt, 1979; Duddy, 1980). From sample T5 to those in the lower part of B-horizon, LaN/YbN tends to increase while GdN/ YbN decreases slightly (Fig. 6A), indicating the leaching loss of MREEs. Compared with other regolith samples, the ferruginous crust samples from the upper part of B-horizon have HREEs depleted (Fig. 2B), which can be interpreted in terms of the leaching. So, this paper supports such a conclusion that the ferruginous crust is the product of in-situ leaching – weathering of the profile (see Nahon, 1986; Tardy et al., 1991). The total REEs and LaN/YbN and LaN/SmN ratios all decreased and REE distribution patterns steadily approach to those of loess and UCC in samples from the A-horizon at the top of the

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profile (Fig. 2C). This is the result from leaching loss of clay minerals and the relative increase in quartz minerals in the profile reported by Ji et al. (submitted for publication). 4.4. Mechanism for abnormal enrichment of REEs in red residua The presence of REE enrichment has been reported in recent years in continental weathering profile but the proposed positions of the REE-rich layer vary and the mechanism of enrichment has been explained in various ways (Banfield and Eggleton, 1989; Morey and Setterholm, 1997; Nesbitt and Markovics, 1997; Braun et al., 1998). Meanwhile, people have also found the REE-rich layers at the basement of karst-bauxite deposits, which is attributed to the concentrated REEs at alkaline barrier of the carbonate footwall during the bauxitization (e.g., Maksimovic and Panto, 1991) and at the above soil – rock interface of weathering crust underlying carbonate rocks (e.g., Ji et al., 1999). The amount of REEs in carbonate rocks is extremely lower than those of other sedimentary rocks, even less than 1 –10 Ag/ g. As the aforementioned in the introduction of this paper, can the parent pure carbonate rocks provide REE enough for REE enrichment in its weathering profiles, and/or can significant amounts of REE from weathering profiles be stored in continents? From the weathered bedrock (flour dolomite) to the chocolate layer the amount of REE increased drastically (Table 1 and Fig. 2). If Ce is considered to be less mobile during weathering process from flour dolomite (total REE = 129.82 ppm) to the chocolate layer (Fig. 3B), a simple mass balance calculation would indicate that the enrichment of REEs due to volume change in this transformation process may reach such an extent in which its total REE amount comes up to 11823.00 ppm. Obviously, the dolomite may be a main source that can provide sufficient amount of REEs for the chocolate layer in the Pingba profile. But an approximate 30% difference is recognized from the actual REE amount in chocolate layer (AREE = 16932 ppm, Table 1). It is deduced that other minerals or processes would be

responsible for the difference of REE enrichment in this layer. In the leaching experiment the solubility of REE, especially LREE, is very high (Fig. 5A), probably demonstrating that apatite is dissolved in the hydrochloric acid-bearing leaching solution. But it is known that the apatite is also enriched in Sr, which shows a feature of extremely high solubility. The experimental result shows that a small amount of Sr is contained in the leaching solution (Sr leached out of the chocolate layer accounts for about 14%, unpublished data). This phenomenon may reflect not only the existence of dissolved apatite, but also the enrichment of REE in clay minerals and other secondary phases (see Banfield and Eggleton, 1989; Martin and McCulloch, 1999). The secondary rhabdophane-La has been found in the chocolate layer (Fig. 7), but more than 88% soluble REE in the chocolate layer (Table 2) are not derived mainly from the mineral because it has very low solubility in nature (e.g., Jonasson et al., 1985). As suggested by Balashov and Girin (1969), the soluble REEs in carbonate rocks were up to 43 – 95% and mainly adsorbed on clay minerals. It can be seen from the above discussion that REEs adsorbed on clay minerals have been found on the above regolith – bedrock interface (Fig. 6B). And the leached REE from samples from the upper profile have also been discovered in Fig. 3B. So, they may constitute be another important factor constraining the enrichment of REE at the regolith – bedrock interface. Therefore, it is proved that the strong enrichment of REE at the weathering front of the Pingba profile may dominate due to volumetric changes in the process of chemical weathering from dolomite to residual soil, and the combined result of the accumulation of REE-rich minerals, water/rock interaction at the weathering front and leaching and permeating in the upper part of the profile. The above explanation may help us to understand the mechanism of REE enrichment in the soils and weathering profiles underlying carbonate rock in southern China. This work leads greatly support that the mature weathering profile on the continents may became large, long-term storage for REEs, where erosion is sufficiently slow and chemical

H. Ji et al. / Chemical Geology 203 (2004) 29–50

47

weathering sufficiently rapid (Nesbitt and Markovics, 1997).

5. Conclusions This work, combining with the bulk density of the Pingba profile and the distribution of the immobile Zr, Nb, and Th implies that the chemical weathering of dolomites is a typical non-isovolumetric weathering process. The volumetric strain tends to increase in the middle part of the weathered bedrocks, i.e., the formation of cracked dolomites is a dilation process of the profile. An enormous change of volumetric strain is recognized at the regolith –bedrock interface, i.e., the collapse process of the profile. The pronounced REE fractionation has been indicated by mass balance calculation, and highly enriched REE at the weathering front has been observed in this study. Ce is separated from the other REE and highly enriched at the top of the profile. It is consistent with the enhancement of oxidation degree in the regolith. The climate change (i.e., alternative wet and dry climates) during the profile formation has been suggested as the cause of change of redox state in soil profile and for the explanation for Ce alternation of gain and immobility in the upper regolith. The reasons of uneven transportation of REE in the profile have been ascribed to: (1) the HREE preferentially transported downward in the regolith; (2) phosphate rich minerals distributing in the profile (preferential dissolution of MREE); and (3) the residual accumulation of insoluble oxides in the regolith reduction (e.g., Ce gain in B-horizon). REE patterns of samples from bedrock and the lower regolith in the profile are similar to those of conodont or apatites in the geologic records or acidic waters in natural environment, which are characterized by the MREE enrichment and negative Ce anomalies (namely the ‘‘hat’’-type or convex-up patterns). The explanations for the REE patterns are: (1) the accumulation of MRRE-rich minerals in dolomite dissolution; (2) the water – rock interaction during the chemical weathering process at the weathering front; and (3) more leaching MREE from the upper part of the profile. The latter two reasons may be a dominant mechanism

in this study. The REE patterns of samples from Ahorizon at the top of the profile steadily approach to those of loess and UCC, i.e., LaN/YbN = 10 and Eu/Eu* = 0.65. This is the result from leaching and loss of clay minerals and relative increase of quartz minerals in the A-horizon. Mass balance calculation also shows that the great volumetric change between dolomite and the overlying residual soil may account for 70% of the highly enriched REEs in the chocolate layer. And the leaching REEs from the upper part of the profile are also been found. Leaching experiments have shown not only the existence of dissolved apatite in the samples (less than 14%), but also the enrichment of REE in clay minerals and other secondary phosphate phases. SEM-EDS studies have indicated that samples from the chocolate layer contain a secondary LREE-enriched phosphate mineral-rhabdophane-La. Therefore, the volumetric change and the combined result of the accumulation of REE-rich minerals, water –rock interaction at the weathering front and leaching in the upper part of the profile are the main reasons for abnormal enrichment of REEs at the weathering front of the Pingba profile. This work leads more support to the viewpoint that the mature weathering profile may become large, long-term storage for REEs, where erosion is sufficiently slow and chemical weathering sufficiently rapid.

Acknowledgements The authors are most grateful to Profs. A.F. White, L.M. Walter and an anonymous referee for their helpful insights, which contributed to an obvious improvement of the manuscript. Especial thanks to Prof. A.F. White, Ms. L. Zhang (IGSNRR, CAS), Prof. J. Liu (Jilin Univ.) and Dr. C. Zhou are express here for correcting English writing. This work was jointly supported by the Knowledge-renovation Project of Chinese Academy of Sciences (KZCX2-105), the National Natural Science Foundation of China (NSFC) grants (No. 49833002 and 40243020), the Postdoctoral Foundation of China, the CAS K.C. Wong Post-doctoral Research Award Fund, and the ‘‘Western Light’’ Program sponsored by the Chinese Academy of Sciences. [LW]

48

H. Ji et al. / Chemical Geology 203 (2004) 29–50

Appendix A
Table A1 ICP-MS analyses of the blanks and international reference standards and those of repeatedly determined samples both at home and abroad (unit: ppm) NBS1633a La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 81.81 170.60 20.66 76.78 16.50 3.41 16.11 2.45 14.80 3.13 8.28 1.18 7.30 1.08 Recommended value 79.1 168.3 19.3 77 16.7 3.58 16 2.53 15.2 2.97 8.45 1.2 7.6 1.08 RSD, % 3.4 1.4 7.1 ? 0.3 ? 1.2 ? 4.8 0.7 ? 3.2 ? 2.6 5.4 ? 2.0 ? 1.7 ? 4.0 0.0 JB-1a Recommended value 38.1 66.1 7.3 25.5 5.07 1.47 4.54 0.69 4.19 0.64 2.18 0.31 2.1 0.32 RSD, % 7.9 8.0 7.0 ? 4.0 4.1 ? 1.4 8.2 10.1 3.1 40.6 4.1 6.5 ? 3.3 6.3 C4CH C4CH Reference value* 58.0 F 0.6 111 F 2 47 F 3 8.32 F 0.12 1.60 F 0.02 0.80 F 0.02 RSD, % 12.3 8.7 8.3 5.9 8.8 11.3 Blank, ppb 0.035 0.005 0.001 0.021 0.009 0.004 0.008 0.005 0.004 0.001 0.007 0.001 0.006 0.001 Detection limit, ppb 0.018 0.028 0.005 0.076 0.009 0.002 0.021 0.002 0.009 0.003 0.005 0.002 0.003 0.0005

41.09 71.37 7.81 24.48 5.28 1.45 4.91 0.76 4.32 0.90 2.27 0.33 2.03 0.34

65.11 120.70 13.91 50.88 8.81 1.74 6.82 0.89 5.13 1.06 2.76 0.37 2.37 0.35

2.21 F 0.03 0.313 F 0.06

7.2 11.8

Sample number NBS-1633a and JB-1a represent coal fly ash and basalt samples from NIST (National Institute of Science and Technology) and GSJ (Geological Survey of Japan), respectively; C4CH is a sample collected from Chuanlinggou shale in the Yuxian section, Hebei province, China, the reference values were measured by Prof. Randy Korotev of Washington University in St. Louis (the sample and reference values are provided by Prof. Yuzhuo Qiu).

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publication at: https://www.researchgate.net/publication/230143482 Correlating specific conductivity with total hardness in limestone and dolomite karst waters....
...AND RECHARGE OF A GYPSUM - DOLOMITE KARST AQUIFER IN ....unkown
//www.researchgate.net/publication/242297727 HYDROGEOLOGY AND RECHARGE OF A GYPSUM - DOLOMITE KARST AQUIFER IN SOUTHWESTERN OKLAHOMA, U.S.A ARTICLE ...
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www.elsevier.com/locate/geomorph Development of a deep karst system within a transpressional structure of the Dolomites in north-east Italy Francesco ...
...within a transpressional structure of the Dolomites in ....unkown
www.elsevier.com/locate/geomorph Development of a deep karst system within a transpressional structure of the Dolomites in north-east Italy Francesco ...
...within a transpressional structure of the Dolomites in ....unkown
www.elsevier.com/locate/geomorph Development of a deep karst system within a transpressional structure of the Dolomites in north-east Italy Francesco ...
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