G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Characterization Volume 2 April 20, 2001 Paper number ISSN: Relationships between the trace element composition of sedimentary rocks and upper continental crust Scott M. McLennan Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York (Scott.McLennan@sunysb.edu) [1] Abstract: Estimates of the average composition of various Precambrian shields and a variety of estimates of the average composition of upper continental crust show considerable disagreement for a number of trace elements, including Ti, Nb, Ta, Cs, Cr, Ni, V, and Co. For these elements and others that are carried predominantly in terrigenous sediment, rather than in solution (and ultimately into chemical sediment), during the erosion of continents the La/element ratio is relatively uniform in clastic sediments. Since the average rare earth element (REE) pattern of terrigenous sediment is widely accepted to reflect the upper continental crust, such correlations provide robust estimates of upper crustal abundances for these trace elements directly from the sedimentary data. Suggested revisions to the upper crustal abundances of Taylor and McLennan [1985] are as follows (all in parts per million): Sc = 13.6, Ti = 40, V = 7, Cr = 83, Co = 17, Ni = 44, Nb = 12, Cs = 4.6, Ta = 1.0, and Pb = 17. The upper crustal abundances of Rb, Zr, Ba, Hf, and Th were also directly reevaluated and K, U, and Rb indirectly evaluated (by assuming Th/U, K/U, and K/Rb ratios), and no revisions are warranted for these elements. In the models of crustal composition proposed by Taylor and McLennan [1985] the lower continental crust (75% of the entire crust) is determined by subtraction of the upper crust (25%) from a model composition for the bulk crust, and accordingly, these changes also necessitate revisions to lower crustal abundances for these elements. Keywords: Geochemistry; composition of the crust; trace elements. Index terms: Crustal evolution; composition of the crust; trace elements. Received September 8, 2000; Revised December 3, 2000; Accepted December 11, 2000; Published April 20, McLennan, S. M., Relationships between the trace element composition of sedimentary rocks and upper continental crust, Geochem. Geophys. Geosyst., vol. 2, Paper number [8994 words, figures, 5 tables]. Published April 20, Theme: Geochemical Earth Reference Model (GERM) Guest Editor: Hubert Staudigel 1. Introduction [2] The chemical composition of the upper continental crust is an important constraint on understanding the composition and chemical differentiation of the continental crust as a whole and the Earth in general [e.g., Taylor Copyright 2001 by the American Geophysical Union

2 and McLennan, 1985, 1995; Rudnick and Fountain, 1995]. There have been a variety of estimates of upper crustal composition mostly based on large-scale sampling programs, largely in Precambrian shield areas, geochemical compilations of upper crustal lithologies, and sedimentary rock compositions (mainly shales). If the average chemical composition of the upper crust can be estimated from sedimentary rocks, then an especially powerful insight may be gained into the chemical evolution of the crust (and Earth) over geological time because of the relatively continuous record of sedimentary rocks, dating from 4 Ga to the present. [3] For the most part, estimates of upper crustal abundances from sedimentary data have been restricted intentionally to trace elements that are least fractionated by various sedimentary processes, such as chemical and physical weathering, mineral sorting during transport, and diagenesis [McLennan et al., 1980]. Included are the rare earth elements (REE), Th, and Sc as well as other elements (K, U, and Rb) that can be estimated indirectly using various so-called canonical ratios (Th/U, K/U, and K/Rb). Recently, however, this general approach has been applied to other trace elements, notably Nb, Ta, and Cs that at least potentially, may be more affected by various sedimentary processes [e.g., McDonough et al., 1992; Plank and Langmuir, 1998; Barth et al., 2000]. In this paper the relationships between the trace element composition of the sedimentary mass and the upper continental crust are evaluated for a variety of trace elements and new estimates of upper crustal trace element abundances, based on the sedimentary rock record, are presented. 2. Comparison of Upper Crustal Estimates [4] The most commonly cited estimates of upper crustal abundances are those of Taylor and McLennan [1985] (hereinafter referred to as TM85), which are based on a variety of approaches for different elements, including large-scale sampling programs (e.g., major elements, Sr, and Nb), average igneous compositions (e.g., Pb), compilations from Wedepohl [1969±1978] (e.g., Ba and Zr), sedimentary compositions (e.g., REE, Th, and Sc), and various canonical or assumed ratios, such as Zr/Hf, Th/U, K/U, K/Rb, Rb/Cs, and Nb/Ta (e.g., Hf, U, Rb, Cs, and Ta). Although there is widespread agreement that the upper crust approximates to a composition equivalent to the igneous rock type granodiorite, there is in fact considerable disagreement regarding the precise values of a variety of trace elements. In Table 1, estimates of selected trace elements are tabulated for various shield surfaces. Some of these compositions are compared to the upper crustal estimate of TM85 in Figure 1, where it can be seen that discrepancies by nearly a factor 2 or more are common and that in some cases, estimates differ by more than a factor of 3 (Nb, Cr, and Co). These differences are likely due to some combination of inadequate sampling, analytical difficulties, and real regional variations in upper crustal abundances. In Table 2, various other recent estimates of the upper crust (see Table 2 for methods of estimates) are also compared to TM85, and again, some significant differences can be seen. 3. Sedimentary Rocks and Upper Crustal Compositions [5] The notion that sediments could be used to estimate average igneous compositions at the Earth's surface was first suggested by V. M. Goldschmidt (see discussion by Goldschmidt [1954, pp. 53±56]), and using sedimentary data to derive upper crustal REE abundances was pioneered by S. R. Taylor [e.g., Taylor, 1964, 1977; Jakes and Taylor, 1974; Nance and Taylor, 1976, 1977; McLennan et al., 1980; Taylor and McLennan, 1981, 1985]. Goldschmidt used glacial sediments to estimate the

3 Table 1. Element, ppm Selected Trace Elements for Estimates of Various Shield Areas and the Average Upper Continental Crust Canadian Shield: Shaw a Canadian Shield: Eade and Fahrig b East Scotland d New Colorado f Baltic China c Mexico e Shield g Ukranian Shield h Anabar Shield i Average Shield j Average Shield (Area) k Sc Ti V Cr < Co Ni Rb Zr Nb (23) 25 Cs Ba La (43) 30 Hf Ta Pb Th Area, 6 km a Average Canadian Shield values from Shaw et al. [1967, 1976, 1986]. b Average Canadian Shield values from Fahrig and Eade [1968] and Eade and Fahrig [1971, 1973]. c Average central East China calculated on carbonate-free basis [Gao et al., 1998]. d Average of crystalline basement NW Scotland Highlands [Bowes, 1972]. e Average Precambrian surface terrane, New Mexico [Condie and Brookins, 1980]. f Average of Colorado Plateau upper crust derived from equal proportions of northwest and southeast sections [Condie and Selverstone, 1999]. g Average Baltic Shield (reported by Borodin [1999]). h Average Ukranian Shield (reported by Borodin [1999]). i Average Anabar Shield (reported by Borodin [1999]). j Average shield derived from simple average of the Canadian Shield to the Anbar Shield columns. k Average shield derived from weighted average by area of the Canadian Shield (average of both Shaw and Eade and Fahrig) to the Anbar Shield. Areas are taken from Goodwin [1991]. Note that La = 31 ppm if Canadian Shield values from Eade and Fahrig are excluded, Nb = 11 ppm if Canadian Shield from Shaw and Scotland are excluded, and Ta is not calculated owing to highly variable values. l Average upper continental crust [Taylor and McLennan, 1985]. Upper Crust l Geochemistry Geosystems G 3 G 3 mclennan: trace element composition and upper continental crust

4 Shield Estimate Shield Estimate Th Canadian Shield (Shaw) Canadian Shield (Eade & Fahrig) Ni Co La Pb Cr Nb 0 00 China Scotland New Mexico Colorado Co Th Pb Ni La Nb V Cr V Rb Zr Rb Ba Ba 0 00 Taylor & McLennan Upper Crust Figure 1. Comparison plots for selected trace elements in two independent estimates of the Canadian Shield surface and various other shields with the estimate of the average upper continental crust from Taylor and McLennan [1985]. Thick solid line represents equal compositions, and dashed lines represent difference by a factor of 2. Data are from Table 1. Zr (a) (b) major element composition of average igneous rocks because such sediment is dominated by mechanical rather than chemical processes. However, modern studies have used shale compositions to estimate upper crustal trace element abundances (TM85). This is because shales completely dominate the sedimentary record [Garrels and Mackenzie, 1971], constituting up to 70% of the stratigraphic record (depending on the method of estimating), and because most trace elements are enriched in shales compared to most other sediment types. The result is that shales dominate the sedimentary mass balance for all but a few trace elements. [6] Most studies also have been restricted to a few trace elements that are least affected by sedimentary processes and are transferred dominantly into the clastic sedimentary record during continental erosion, notably REE, Y, Sc, and Th. However, there are numerous other trace elements that are transferred from upper crust primarily into the clastic sedimentary mass, including Zr, Hf, Nb, Ta, Rb, Cs, Pb, Cr, V, Ni, and Co. Until recently, these elements have been largely neglected (see discussion by TM85) because of perceived problems of fractionation during mineral sorting, such that shales may not dominate the sedimentary mass balance (e.g., Zr, Hf, Nb, Ta, and Pb), and possible redistribution during weathering and/ or diagenesis (e.g., Rb, Cs, Pb, Cr, V, Ni, and Co). Given the large variability among the various upper crustal and shield estimates for these elements (Tables 1 and 2), such processes may well add relatively minor uncertainty to upper crustal estimates derived from the clastic sedimentary record Cs in the Upper Crust [7] The Cs content of the upper crust is given as 3.7 ppm by TM85 based on a Rb content of 112 ppm and a Rb/Cs ratio of 30. McDonough et al. [1992] argued that there was no fractionation of Rb from Cs during sedimentary processes and determined the average Rb/Cs of 140 sediments and sedimentary rocks to be 19 (standard deviation of 11), which he took to be equivalent to the upper crust and leading to an upper crustal Cs content of 6 ppm (using the Shaw et al. [1986] Canadian Shield average of Rb = 1 ppm). Rudnick and Fountain

5 Table 2. Element (ppm) Selected Estimates of the Average Composition of the Upper Continental Crust Condie ``Map'' a Condie Gaillardet et al. c Togashi et Wedepohl e Plank & Langmuir f Borodin g Taylor and ``Restoration'' b al. d McLennan h Sc Ti V Cr Co Ni Rb (112) Zr (190) Nb Cs Ba (550) La (30) Hf (5.8) Ta Pb Th (.7) a Average upper continental crust from ``map model'' of major upper crustal lithologies [Condie, 1993]. b Average upper continental crust from ``resoration model,'' where eroded crust is accounted for [Condie, 1993]. c Average upper crust in Central African Shield derived from ``corrected'' river suspended sediment in Congo River system [Gaillardet et al., 1995]. d Average upper crust for the Japan Arc based on mapped lithological balances [Togashi et al., 2000]. e Average upper crust for elements not taken from Shaw et al. [1986] [Wedepohl, 1995]. Cs derived from Rb/Cs = 19 [McDonough et al., 1992] and Shaw et al. [1986] Rb value; Ta derived from Nb/Ta = 17.5 and Shaw et al. [1986] Nb value. f Upper continental crust derived from marine sedimentary record [Plank and Langmuir, 1998]. g Upper continental crust estimated by weighting average shields (70%) and average granitoid rock (30%) [Borodin, 1999]. h Average upper crust [Taylor and McLennan, 1985]. i Average upper continental crust derived from sedimentary record for this study. Elements that are unchanged from Taylor and McLennan [1985] are shown in parentheses. Upper Crust i Geochemistry Geosystems G 3 G 3 mclennan: trace element composition and upper continental crust

6 [1995] adopted an upper crustal Rb/Cs ratio of 20 and reported a Cs content of 5.6 ppm (using the TM85 upper crustal value of Rb = 112 ppm). Recently, the TM85 estimate has also been questioned by Plank and Langmuir [1998] on the basis of young marine sedimentary data. They noted a correlation between Cs and Rb in modern deep-sea sediments from a variety of tectonic and sedimentological regimes. Using this correlation and accepting a Rb upper crustal abundance of 112 ppm, they derived a new Cs estimate of 7.3 ppm (implying an upper crustal Rb/Cs of 15.3). [8] The behavior of Cs in the sedimentary environment, in fact, is not well documented. On the basis of the data available at the time, McDonough et al. [1992] argued that the Rb/Cs ratio does not change during sedimentary processes. However, this conclusion does not seem to be consistent with the observations that seawater Rb/Cs is 400, typical river water Rb/Cs is 50 [e.g., TM85; Lisitzin, 1996], and some tropical river waters have ratios in excess of 00 [Dupre etal., 1996], whereas all workers seem to agree that the upper crustal Rb/Cs is <40 [TM85; McDonough et al., 1992; Gao et al., 1998; Wedepohl, 1995; Rudnick and Fountain, 1995; Plank and Langmuir, 1998]. [9] Rb/Cs ratios of weathering profiles appear to change systematically as a function of Rb content in both basaltic and granitic terranes (Figure 2), suggesting at least the potential for fractionation between these elements during surficial processes. Dupre etal.[1996] found Congo River suspended sediment, bed load sands, and dissolved load (including colloids) to have the following Rb/Cs ratios (average 95% confidence interval): 17 4 (n = 8), 47 8(n = 15), and (n = 8), respectively, and Gaillardet et al. [1997] found Rb/Cs ratios as low as 4 in suspended sediment from the Amazon River. Thus interaction of natural waters with typical upper crust appears to Rb/Cs Rb/Cs (a) (b) Unweathered Granodiorite Granodiorite Weathering Profile (Nesbitt & Markovics 1997) Unweathered Basalt Basalt Weathering Profile (Price et al., 1991; S. R. Taylor, 1997) Rb (ppm) Figure 2. Plots of Rb/Cs versus Rb for weathering profiles developed on granodiorite [Nesbitt and Markovics, 1997] and basalt ([Price et al., 1991] Cs data from S. R. Taylor (personal communication, 1997)) in Australia, suggesting Rb/Cs ratios may be strongly fractionated within weathering profiles. In spite of any fractionation within soil profiles both of these elements are carried from weathering sites predominantly in the particulate load. lower the Rb/Cs ratio in the resulting finegrained clastic sediments, likely due to the preferential exchange of the larger Cs ion onto clay minerals. [] There are few reliable data for Cs in carbonates, evaporites, and siliceous sediments;

7 however, from simple crystal chemical arguments the larger Cs ion would be expected to be preferentially excluded compared to Rb in most carbonate and evaporite minerals, leading to relatively high Rb/Cs ratios compared to the upper crust (see discussion regarding carbonates by Okumura and Kitano [1986]). Thus, although Rb and Cs are carried dominantly in clastic sediments, it is not obvious that the Rb/ Cs ratio of marine sediment studied by Plank and Langmuir [1998], where the terrigenous fraction is dominated by very fine grained clays, is fully representative of the upper crust Nb-Ta-Ti in the Upper Crust [11] The Ti and Nb contents of the upper continental crust are given as 3000 and 25 ppm, respectively, by TM85 on the basis of the large-scale sampling program in the Canadian Shield by D. M. Shaw [Shaw et al., 1967, 1976, 1986], and the Ta estimate of 2.2 ppm is based on a crustal Nb/Ta ratio of 11.6 (taken from Wedepohl [1977]). Recently, these estimates also have been questioned by Plank and Langmuir [1998] on the basis of sedimentary data. Plank and Langmuir [1998] noted correlations between Nb and Al 2 O 3, between Ti and Al 2 O 3, and between Nb and Ta in modern deep-sea sediments from a variety of tectonic and sedimentological regimes. From these relationships and by accepting the Al 2 O 3 upper crustal estimate of TM85 they estimated TiO 2 at 0.76%, Nb at 13.7 ppm, and Ta at 0.96 ppm. Barth et al. [2000] suggested estimates of Nb = 11.5 ppm and Ta = 0.92 ppm on the basis of the abundances of these elements in Australian post-archean shales (PAAS) and loess. [12] It has long been known that elements concentrated in heavy mineral suites (notably, Zr and Hf but also Sn, Th, LREE, etc.) may be strongly fractionated during mineral sorting of clastic sediments [McLennan et al., 1993]. Although the geochemistry of Ti, Nb, and Ta is likely to be less affected by such processes, these elements may be concentrated in heavy mineral suites (e.g., rutile, ilmenite, anatase, etc.), and like zircon, rutile and anatase are ``ultrastable'' heavy minerals [Pettijohn et al., 1972]. Accordingly, some care must be taken in interpreting the Ti, Nb, and Ta content of shales. On the other hand, the discrepancy between estimates of Plank and Langmuir [1998; Barth et al., 2000] and for the Canadian Shield [Shaw et al., 1986] is nearly a factor of 2, much greater than might be expected from any of these sedimentological considerations Cr-Ni-V-Co in the Upper Crust [13] Upper crustal ferromagnesian trace element abundances reported by TM85, based largely on the Canadian Shield estimates of Shaw et al. [1967, 1976] and Eade and Fahrig [1971, 1973; Fahrig and Eade, 1968], are relatively low (e.g., Cr = 35 ppm and Ni = 20 ppm) compared to a number of other shield estimates (Table 1 and Figure 1) and various other upper crustal estimates (Table 2). In contrast, the abundances of ferromagnesian trace elements in shales are typically a factor of 2 greater than these values (TM85). This discrepancy has rarely been discussed in any detail, although Condie [1993] has proposed significantly higher upper crustal abundances of ferromagnesian trace element abundances (see Table 2). [14] Relatively low upper crustal abundances of these elements were effectively a requirement of the once popular ``andesite model'' for crustal growth because average andesite has very low abundances for these elements [e.g., Taylor, 1967, 1977; Gill, 1981; Gill et al., 1994]. For example, Taylor [1977] estimated average andesite to have Cr = 55 ppm and Ni = 30 ppm. During intracrustal partial melting and differentiation, enrichments of such elements in the residual lower crust would be expected, but for the andesite model, high ferromagnesian trace element abundances in the upper crust (e.g., Cr > 55 ppm) would have predicted the opposite

8 and thus created mass balance difficulties. Accordingly, the low levels of ferromagnesian trace elements found in the Canadian Shield by Shaw et al. [1967, 1976] seemed consistent. [15] However, it is now understood that low abundances of these elements in typical orogenic andesites are a reflection of the fractionated nature of most andesites and that unfractionated mantle-derived arc magmas typically have much higher levels of ferromagnesian trace elements [e.g., Gill, 1981]. In addition, it is now widely accepted that much of the continental crust formed during the Archean and higher ferromagnesian trace element levels are characteristic of Archean orogenic igneous rocks [e.g., Condie, 1993]. Most models of bulk crustal abundances now reflect these higher levels [Taylor and McLennan, 1985, 1995; Rudnick and Fountain, 1995], but upper crustal abundances of the ferromagnesian trace elements have received little comment. 4. Methods 4.1. Database [16] The database consists of a variety of compilations based on large-scale averages or composites of several sedimentary rock types of different grain sizes and from a variety of tectonic and sedimentological settings. Where possible, old sedimentary rocks, especially of Archean through early Proterozoic age, were neglected in order to avoid any issues of secular change in upper crustal composition. In fact, even with this sampling strategy, it is impossible to entirely avoid issues of secular variations in composition because most sedimentary rocks are recycled over long periods of geological time [Veizer and Jansen, 1979, 1985]. [17] The Russian Shale average is based on a remarkable number of samples (n 40,000). Apart from this, >1200 samples have gone into the various other averages and composites. Table 3 lists the trace element analyses and data sources used in Figures 3±. There is a small amount of redundancy in some of these averages in that the same samples may be included in more than one of the averages. For example, modern turbidites analyzed by McLennan et al. [1990] are subdivided by lithology and tectonic setting in Table 3. However, these samples (n = 63) represent % of the analyses considered by Plank and Langmuir [1998] in estimating global subducting sediment (GLOSS). Loess is considered to be a sediment type that perhaps best reflects the upper crustal provenance for many elements because of the relatively minor effects of weathering [Taylor et al., 1983]. Accordingly, several regional loess averages are given in Table 4, and these are also plotted individually on Figures 4±. [18] It is not possible to fully evaluate formal statistical uncertainties for some of these averages because the primary sources do not provide sufficient information on variance. However, the large number of samples used to estimate many of the averages coupled with the fact that confidence in an average improves as a function of the square root of the number of samples results in relatively small uncertainties in the averages (at 95% confidence level). For example, Plank and Langmuir [1998] reported standard deviations for the GLOSS data that were typically ±20% of the average for most trace elements. Because of the very large number of samples used to formulate the average (>500), this results in 95% confidence levels on the means of 1±2%. At the other extreme, the average river suspended sediment data have relatively large standard deviations (25±50% of the average values), probably a result of the fact that these rivers sample upper crust of widely varying tectonic settings and climatic regimes. This coupled with the relatively small

9 number of analyses (n = 7±19, depending on element) results in 95% confidence limits on the means of ±30%. In the case of North American shale composite (NASC), the data represent a single analysis of a composite sample, and analytical error likely dominates the uncertainty Shales, muds, and loess (fine grain) [19] Fine-grained sediment averages and composites that are used are described below (see Table 3). In estimating the average fine-grained sediment, equal weight was given to each of the various sediment composites and averages. [20] 1. For the river suspended value, average suspended sediment is from near the terminus of 19 major rivers of the world that together drain 13% of the exposed land surface [Martin and Meybeck, 1979; Gaillardet et al., 1999]. Not all elements are reported for all rivers with the most extreme case being Sc (n =7). [21] 2. Average loess is determined from the mean of eight regional loess averages from New Zealand, central North America, Kaiserstuhl region, Spitsbergen, Argentina, United Kingdom, France, and China (see Table 4 for sources; n = 52). [22] 3. NASC is a composite of 40 sediments (mainly shales), mostly from North America [Gromet et al., 1984]. [23] 4. Post-Archean average Australian shale is an average of 23 Australian shales of post- Archean age [Nance and Taylor, 1976; McLennan, 1981, 1989; Barth et al., 2000]. The original PAAS [Nance and Taylor, 1976] reported REE data only; however, the remaining elements were compiled by McLennan [1981], and REE data were updated by McLennan [1989]. Ta values used here were recently reported by Barth et al. [2000]. [24] 5. Average Russian shale is an average of 1.6±0.55 Ga shales (4883 samples and 4 composites from 1257 samples) and 0.55± 0.0 Ga shales (6552 samples and 1674 composites from 28,288 samples). Samples are mainly from Russia and the former Soviet Union but also include representative samples from North America, Australia, South Africa, Brazil, India, and Antarctica [Ronov et al., 1988]. [25] 6. Average Phanerozoic cratonic shale is from Condie [1993] (n > 0). [26] 7. GLOSS is an estimate of the average composition of marine sediment reaching subduction zones, based on 577 marine sediments [Plank and Langmuir, 1998]. This average differs from the other fine-grained averages in that it includes a significant component of nonterrigenous material, including chemical sediment, pelagic sediment, and coarser-grained turbidites. This leads to some anomalies that are discussed below. [27] 8. Average passive margin turbidite mud is an average of modern turbidite muds from trailing edges and the Ganges cone [McLennan et al., 1990] (n = 9) and Paleozoic passive margin mudstones from Australia [Bhatia, 1981, 1985a, 1985b] (n = ). [28] 9. Average active margin turbidite mud is an average of modern turbidite muds from active margins [McLennan et al., 1990] (n = 18) and average Australian Paleozoic turbidite mudstones from oceanic island arcs (n = 9), continental arcs (n = 12), and Andean-type margins (n =2)[Bhatia, 1981, 1985a, 1985b] Sand and sandstones (coarse grain) [29] Coarser-grained sediment averages that were used are described below (see Table 3). In estimating the average coarse-grained sedi-

10 ment, equal weight was given to each of the various sediment composites and averages. [30] 1. Average tillite is derived from the average of Pleistocene till from Saskatchewan [Yan et al., 2000] (n = 33) and late Proterozoic tillite matrix (texturally a sandstone) from Scotland [Panahi and Young, 1997] (n = 21). A coarse-grained glacial sediment average was included to be comparable to the fine-grained loess deposits. [31] 2. Average Phanerozoic cratonic sandstone is from Condie [1993] (n > 0). [32] 3. Average Phanerozoic greywacke is from the mean of Paleozoic (n > 0) and Mesozoic- Cenozoic (n > 0) averages [Condie, 1993]. [33] 4. Average passive margin sand is an average of modern turbidite sands from trailing edges and the Ganges cone [McLennan et al., 1990] (n = 11) and Paleozoic passive margin sandstones from Australia [Bhatia, 1981, 1985b; Bhatia and Crook, 1986] (n = 15). [34] 5. Average active margin sand is an average of modern turbidite sands from active continental margins [McLennan et al., 1990] (n = 25, with aberrantly high Cr and Ni from one sample excluded) and average Australian Paleozoic turbidite sandstones from oceanic island arcs (n = 11), continental arcs (n = 32), and Andean-type (n = ) margins [Bhatia, 1981, 1985b; Bhatia and Crook, 1986] Approach [35] The approach adopted in this paper for estimating upper continental crustal abundances of certain trace elements makes two basic assumptions: (1) REE content of clastic sedimentary rocks best reflects upper crustal abundances and the upper crustal REE estimates of TM85 are adopted (e.g., La = 30 ppm), and (2) the sedimentary mass balance of the elements under consideration are dominated entirely by clastic sedimentary rocks such that they have low or negligible abundances in other sediments, such as pure carbonates, evaporites, or siliceous sediments. In practice, this assumption is more robust for some elements than others (see section 6). Accordingly, by examining the relationship between a variety of trace elements and REE (using the most incompatible REE, La) in clastic sediments and sedimentary rocks it is possible to evaluate upper crustal La/element ratios. This approach is similar to that used by McLennan et al. [1980] to estimate upper crustal Th abundances from the sedimentary record [also see McLennan and Xiao, 1998]. [36] Clastic sedimentary data are divided into ``fine-grained'' lithologies, including shales, muds, and silts (e.g., loess), and ``coarsegrained'' lithologies, including sands, sandstones, and tillites, as described above. The average composition of each lithology was determined by giving equal weight to each of the individual averages tabulated in Table 3. The upper crustal La/element ratios were calculated from the overall weighted average composition, using the relative proportions of shales (fine grained) to sandstones (coarse grained) found in the geological record (shale/ sandstone ratio of 6), and thus taken to be representative of average terrigenous sediment. Finally, the upper crustal abundances were determined from these La/element ratios, assuming an upper crustal La content of 30 ppm (TM85). [37] The uncertainties in this approach are likely to be dominated by issues such as weighting factors and representativeness of samples rather than the statistical uncertainty in the various sediment averages. As noted above, the 95% confidence intervals for the various sediment averages listed in Table 3 are generally fairly small (mostly less than %).

11 On the other hand, some of these averages are based on only a few sedimentary sequences. For example, the till average is from samples taken from only two sedimentary sequences, and the active margin sand and mud averages are largely based on relatively few sedimentary sequences in Australia. Whether or not these averages are representative of the various sedimentary settings cannot be evaluated and is the subject of further work. [38] An additional potential source of uncertainty is in the weighting factors used to determine the fine-grained and coarse-grained averages and overall averages. In calculating the fine-grained and coarse-grained averages an arbitrary weighting factor of 1 was given to each analysis listed in Table 3. In calculating the overall average, the fine-grained and coarse-grained averages were weighted to the ratio of shale to sandstone in the geological record. Although there is some uncertainty in this ratio (e.g., see recent discussion by Lisitzin [1996]), here I adopt the shale to sandstone mass ratio of 6:1, which is approximately midway between the average value measured by a variety of workers (4.3:1; see Garrels and Mackenzie [1971] for summary) and the theoretical value (7.1:1) calculated by Garrels and Mackenzie [1971]. Because trace element abundances in sandstones on average are significantly less than those in shales and the La/ element ratios are generally similar (the greatest difference, for La/Cs, is 50%), changing the proportion of coarse-grained sediment by as much as a factor of 2 has only a slight effect (<5%) on the final upper crustal concentrations. 5. Results 5.1. REE, Th, andsc [39] On Figure 3, the REE patterns of the various averages and composites are plotted ppm / ppm Chondrites ppm / ppm Chondrites Upper Continental Crust (TM85) Fine-grained Coarse-grained Upper Continental Crust (TM85) Average Clastic Sediment La Ce Pr Nd SmEuGd Tb Dy Ho Er Yb Figure 3. (a) Chondrite-normalized REE patterns for various fine-grained and coarse-grained sediment averages and composites listed in Table 3 compared to upper crustal REE pattern from Taylor and McLennan [1985]. (b) Comparison of weighted average clastic sediment and upper crustal REE patterns. and compared with TM85 estimate of the upper continental crust. The long-standing observation that post-archean sedimentary REE patterns are remarkably uniform is apparent. Although there is considerable variability in (a) (b)

12 absolute abundances, mainly related to grain size and dilution effects (e.g., quartz and carbonate), the general shapes of the patterns are similar. The weighted average REE pattern has somewhat elevated REE abundances compared to the upper crustal estimate (±15% on 0 0 La/Th=2.8 Coarse-grain Fine-grain Loess (a) 0 Th (ppm) La/Sc=2.7 Sc=11ppm La/Sc=2.2 Sc=13.6ppm (b) 0 Sc (ppm) Figure 4. Plots of (a) La versus Th and (b) La versus Sc for sediment averages and composites. Circled cross and thick solid line represent upper crustal abundances and La/element ratio of Taylor and McLennan [1985]. Sediment data fall close to the TM85 estimate of the upper crustal La/Th ratio. Dashed line represents revised upper crustal La/Sc ratio suggested by sedimentary data. average), and this is expected because low REE-bearing sedimentary lithologies, such as carbonates and evaporites, have not been considered. The average sediment pattern is also slightly less fractionated in light rare earth elements (LREE) compared to the upper crustal estimate, but the difference is not considered large enough to warrant any revisions to the upper crustal REE estimates. [40] The upper crustal abundances of Th and Sc by TM85 were estimated from the sedimentary data available at the time. Plots of La versus Th and La versus Sc are shown in Figure 4. The sedimentary data scatter around the TM85 upper crustal La/Th ratio of 2.8. Although the La/Sc ratios of the sediments are quite variable, they also plot close to the TM85 upper crustal ratio of 2.7. However, 11 of the 14 sediment averages plot at lower La/Sc ratio, and the weighted average of the sediment data has a lower La/Sc ratio, averaging 2.2. This lower La/Sc ratio leads to a revised upper crustal Sc abundance of 13.6 ppm, which is 24% higher than that suggested by TM85 and essentially identical to the estimate of Condie [1993]. This revision also leads to a decrease in the upper crustal Th/Sc ratio from 1.0 to Rb andcs [41] TM85 estimated upper crustal Rb indirectly from the sedimentary data by adopting standard values for a series of canonical ratios (Th/U, K/U, and K/Rb [also see McLennan et al., 1980]). The weighted average La/Rb ratio of clastic sediments, at 0.26 (Figure 5a), is essentially identical to the TM85 upper crustal La/Rb of Although coarse-grained averages plot at systematically lower La/Rb compared to TM85 upper crust, the fine-grained averages plot at both lower and higher values. Accordingly, the sediment data appear to provide direct confirmation of the upper crustal Rb content estimated by TM85.

13 0 0 Coarse-grain Fine-grain Loess GLOSS 0 Rb (ppm) GLOSS La/Rb=0.27 La/Cs=8.1 Cs=3.7 ppm La/Cs=6.5 Cs=4.6 ppm (a) (b) 1 Cs (ppm) Figure 5. Plots of (a) La versus Rb and (b) La versus Cs for sediment averages and composites. Circled cross and thick solid line represent upper crustal abundances and La/element ratio of Taylor and McLennan [1985]. Sediment data fall close to the TM85 estimate of the upper crustal La/Rb ratio. Dashed line represents revised upper crustal La/Cs ratio suggested by sedimentary data. Note that the sediment average global subducting sediment (GLOSS) plots at relatively low La/Rb and La/Cs ratios. See text for discussion. [42] On the plot of La versus Cs (Figure 5b) the sediment data exhibit substantial scatter, but with only one exception (GLOSS), the fine grained sediment averages uniformly show lower La/Cs ratios than the TM85 upper crustal value of 8.1. Coarse-grained sediment averages and some of the regional loess averages have La/Cs that are higher than the TM85 upper crustal value. Overall, 7 of the sediment averages plot at higher La/Cs, and the weighted average of the sediment data has La/Cs of 6.5 leading to an new upper crustal estimate of 4.6 ppm, which is 25% higher than the TM85 estimate Nb, Ta, andti [43] Plots of La versus Nb, Ta, and Ti are shown in Figure 6. As has been pointed out by several authors, the sedimentary data have uniformly lower Nb and Ta contents (i.e., higher La/Nb and La/Ta ratios) than the TM85 upper crustal estimate [Plank and Langmuir, 1998; Barth et al., 2000]. The sediment data evaluated here suggest an upper crustal La/Nb = 2.5 and La/Ta = 30.5, leading to upper crustal estimates of Nb = 12 ppm and Ta = 1.0 ppm, a reduction by half of those values suggested by TM85, and further confirming the suggestions of Condie [1993], Plank and Langmuir [1998], and Barth et al. [2000]. [44] In contrast, La/Ti ratios are systematically lower in clastic sediments than in the TM85 upper crustal estimate and suggest that the average upper crustal Ti concentration should be revised upward by 35% to 40 ppm or TiO 2 = 0.68%. This estimate is somewhat less than that suggested by Plank and Langmuir [1998] Ferromagnesian Trace Elements (Cr, Ni, V, andco) [45] The data considered here confirm the observation that fine-grained clastic sediments typically have about twice the levels of ferromagnesian trace elements compared to the TM85 upper crustal estimates. Figure 7

14 0 0 0 La/Nb=2.5 Nb=12 ppm La/Nb=1.20 Nb=25 ppm Coarse-grain Fine-grain Loess 0 Nb (ppm) La/Ta=30.5 Ta=1.0 ppm La/Ta=13.6 Ta=2.2 ppm 1 Ta (ppm) La/Ti=0.00 Ti=3,000 ppm La/Ti= Ti=4,0 ppm (a) (b) (c) Ti (ppm) plots La versus Cr and V as representative of this group of elements. All sediment averages plot at lower La/Cr and La/Ni (not shown) ratios, and all but two (coarse grained) sediment averages plot at lower La/V and La/Co (not shown) ratios. The weighted average of the clastic sediment data suggests the following upper crustal ratios: La/Cr = 0.36, La/V = 0.28, La/Ni = 0.68, and La/Co = 1.8. These ratios lead to proposed revisions to the TM85 upper crustal values as follows: Cr = 83 ppm, V = 7 ppm, Ni = 44 ppm, and Co = 17 ppm, approximately a factor of two higher. Although absolute values differ in detail (by 6±30%), these results confirm the suggestion of generally higher ferromagnesian trace elements in the upper crust by Condie [1993] Other Elements (Zr, Hf, Ba, andpb) [46] No revisions to the TM85 upper crustal estimates are indicated by the sedimentary data for either Zr or Hf (Figure 8). The sedimentary data scatter about the TM85 upper crustal La/ Zr and La/Hf ratios, and the weighted sedimentary means predict concentrations within % of those of TM85. On average, coarsegrained sediments plot at lower La/Zr and La/ Hf ratios than do fine-grained sediments, a predictable consequence of heavy mineral enrichments. It is of interest, however, that the loess data also plot with, by far, the lowest La/Zr and La/Hf ratios of the fine-grained sediments. This is consistent with the observation of Taylor et al. [1983] that the aeolian sedimentary transport processes involved with Figure 6. Plots of (a) La versus Nb, (b) La versus Ta, and (c) La versus Ti for sediment averages and composites. Circled cross and thick solid line represent upper crustal abundances and La/element ratio of Taylor and McLennan [1985]. Dashed line represents revised upper crustal La/element ratios suggested by sedimentary data.

15 0 Fine-grain Coarse-grain Loess La/Cr=0.86 Cr=35 ppm La/Cr=0.36 Cr=83 ppm 0 Cr (ppm) (a) tematics of clastic sediments (Figure 9), and most data plot at the TM85 upper crustal La/Ba ratio of It is noteworthy, however, that all of the fine-grained marine sediment averages (GLOSS and turbidite muds from both active and passive settings) plot at systemati- 0 Fine-grain Coarse-grain Loess La/Zr=0.16 Avg. Loess 0 La/V=0.50 V=60 ppm (a) 0 00 Zr (ppm) 0 La/V=0.28 V=7 ppm 0 V (ppm) (b) Figure 7. Plots of (a) La versus Cr and (b) La versus V for sediment averages and composites. Circled cross and thick solid line represent upper crustal abundances and La/element ratio of Taylor and McLennan [1985]. Dashed line represents revised upper crustal La/element ratios suggested by sedimentary data. Similar discrepancies are seen for Ni and Co. loess formation leads to significant heavy mineral enrichments. [47] Similarly, no revision to upper crustal Ba concentration is suggested by the La-Ba sys- La/Hf=5.2 Avg. Loess 1 Hf (ppm) Figure 8. Plots of (a) La versus Zr and (b) La versus Hf for sediment averages and composites. Circled cross and thick solid line represent upper crustal abundances and La/element ratio of Taylor and McLennan [1985]. Sediment data fall close to the TM85 estimate of the upper crustal La/Zr and La/Hf ratios. Note that loess falls at much lower La/ Zr and La/Hf ratios than do other fine-grained averages. See text for further discussion. (b)

16 0 Fine-grain Coarse-grain Loess La/Ba=0.055 Marine muds value is also consistent with the various shield estimates (Table 1) that are all in the range of 0 La/Pb=1.8 Pb=17 ppm River Suspended 0 00 Ba (ppm) Figure 9. Plot of La versus Ba for sediment averages and composites. Circled cross and thick solid line represent upper crustal abundances and La/Ba ratio of Taylor and McLennan [1985]. Sediment data fall close to the TM85 estimate of the upper crustal La/Ba ratio. Note that the finegrained marine muds have the lowest La/Ba ratios. See text for further discussion. cally lower La/Ba ratio than the other sediment averages. This is interpreted to result from anomalously high Ba contents related to marine productivity and enrichments in pelagic sediment [e.g., Francois et al., 1995]. Such sediment constitutes 1±2% of the sedimentary record [e.g., Garrels and Mackenzie, 1971; Lisitzin, 1996] and, accordingly, is likely to have a relatively minor effect on the upper crustal Ba estimate from the clastic sedimentary record. [48] The data for Pb do, however, indicate that a modest decrease in upper crustal Pb abundances is warranted (Figure a), and nearly all terrigenous sediment averages and composites plot at a higher La/Pb ratio than the TM85 upper crustal estimate. These relationships suggest that the upper crustal Pb concentration is 17 ppm, or a 15% decrease, a value also proposed by Condie [1993]. This slightly lower 0 La/Pb=1.5 Pb=20 ppm (a) 0 Pb (ppm) River Suspended Sediment (Martin & Meybeck, 1979; Gaillardet et al., 1999) La/Pb=1.5 Da Seine 0 Pb (ppm) Hong Da Danube Garonne Figure. (a) Plot of La versus Pb for sediment averages and composites. Circled cross and thick solid line represent upper crustal abundances and La/Pb ratio of Taylor and McLennan [1985]. Dashed line represents revised upper crustal La/Pb ratios suggested by sedimentary data. Note that average suspended river sediment falls at much lower La/Pb ratio and higher Pb content than do other fine-grained averages. (b) Plot of La versus Pb for suspended sediment from individual rivers [Martin and Meybeck, 1979; Gaillardet et al., 1999]. Note that some rivers with the highest Pb content are those with a long history of industrialization. See text for further discussion. (b)

17 Table 3. Selected Trace Elements for Estimates of Various Sediment Composites and Averages Elements, ppm River Loess b NASC c PAAS Suspended a Shale d Fine-Grained Clastic Sediment Averages Russian Condie f GLOSS g PM Shale e Mud h Coarse-Grained Clastic Sediment Averages AM Tillite j Condie Mud i Sandstone k Condie Greywacke l Sc Ti V Cr Co Ni Rb Zr Nb Cs Ba La Hf Ta Pb (73) Th a Average river suspended load from 19 major rivers of the world [Martin and Meybeck, 1979; Gaillardet et al., 1999]. High Pb is likely due to pollution in several rivers. b Average loess based on eight regional averages (total n = 52); see Table 4 for sources. c North American shale composite based on composite of 40 shales of mainly Phanerozoic age [Gromet et al., 1984]. d Post-Archean average Australian shale based on average of 23 Australian shales of post-archean age [Nance and Taylor, 1976; McLennan, 1981, 1989; Taylor and McLennan, 1985; Barth et al., 2000]. e Average of Phanerozoic (n = 34,840) and late Proterozoic (n = 5940) shales mainly from Russia and former Soviet Union [Ronov et al., 1988]. f Average post-archean cratonic shale [Condie, 1993] (n > 0). g Average global subducting sediment [Plank and Langmuir, 1998]. h Average passive margin turbidite mud derived from trailing edge and continental collision basins [McLennan et al., 1990] (n = 9) and average Paleozoic passive margin mud [Bhatia, 1981, 1985a, 1985b] (n = ). i Average of active margin turbidite mud derived from average modern active margins[mclennan et al., 1990] (n = 18) and average Paleozoic oceanic island arc (n = 9), continental arc (n = 12) and Andean-type margins (n = 2)[Bhatia, 1981, 1985a, 1985b]. j Average tillite derived from late Proterozoic Port Askaig tillite matrix [Panahi and Young, 1997] (n = 21) and Pleistocene till from central Canada [Yan et al., 2000] (n = 33). k Average Phanerozoic cratonic sandstone [Condie, 1993] (n > 0). l Average Phanerozoic greywacke, taken from mean of Paleozoic (n > 0) and Mesozoic±Cenozoic (n > 0) averages [Condie, 1993]. m Average post-archean modern Trailing Edge and Continental Collision basins [McLennan et al., 1990] (n = 11) and Paleozoic passive margin sandstone from Australia [Bhatia, 1981, 1985b; Bhatia and Crook, 1986] (n = 15). n Average of active margin turbidite sand derived from modern active margin basins [McLennan et al., 1993] (n = 25) and average Paleozoic oceanic island arc (n = 11), continental arc (n = 32), and Andean-type margins (n = ) [Bhatia, 1981, 1985b; Bhatia and Crook, 1986]. PM Sand m AM Sand n Geochemistry Geosystems G 3 G 3 mclennan: trace element composition and upper continental crust

18 Table 4. Regional Averages of Selected Trace Elements in Quaternary Loess Element, ppm New United States b Kaiserstuhl c Spitsbergen d Argentina e United France g China h Average Zealand a Kingdom f Loess i Sc Ti V Cr Co Ni Rb Zr Nb Cs Ba La Hf Ta Pb Th a Average of five loess samples from Banks Penninsula, New Zealand [Taylor et al., 1983; Barth et al., 2000]. b Average of four loess samples from Iowa and Kansas [Taylor et al., 1983; Barth et al., 2000]. c Average of two loess samples from Kaiserstuhl region, Germany [Taylor et al., 1983; Barth et al., 2000]. d Average of six loess samples from Spitsbergen [Gallet et al., 1998]. e Average of seven loess samples from Argentina [Gallet et al., 1998]. f Loess sample from United Kingdom [Gallet et al., 1998]. g Average of seven loess samples from France [Gallet et al., 1998]. h Average of 20 loess samples from China [Taylor et al., 1983; Gallet et al., 1996; Jahn et al., 2001]. i Average Loess estimated from average of New Zealand to China values. Pb = 14±18 ppm. Of note is the exceptionally low La/Pb ratio of average suspended river sediment (Figure a). This is due mainly to exceptionally high Pb concentrations in sediment from several rivers that in some cases are draining regions with a long history of industrialization (Figure b), suggesting that pollution may have a significant effect on the Pb content of suspended sediment in at least some modern rivers. 6. Discussion 6.1. Some Comparisons [49] The upper crustal abundances of K, U, and Rb have been estimated indirectly on the basis of the clastic sedimentary record, using ratios such as Th/U = 3.8, K/U =,000, and K/Rb = 250 [McLennan et al., 1980; TM85]. Since no change to Th is indicated from the analysis of sedimentary data presented in this paper, likewise no revisions to K, U, and Rb are suggested. It is of note that in this study Rb also is estimated directly from the sedimentary record, using La/Rb ratios, and the Rb value obtained agrees with the indirect estimate to within 4% and agrees with the average shield surface to within 12%. [50] The upper crustal TM85 Cs content of 3.7 ppm was determined indirectly from Rb = 112 ppm and an assumed upper crustal Rb/Cs ratio of 30. On the basis of sedimentary rocks, McDonough et al. [1992] suggested an upper crustal Rb/Cs of 19 and Cs content of 6 ppm (both with large suggested uncertainties), and Plank and Langmuir [1998] implied a ratio as low as 15.3 from a Cs content of 7.3 ppm. The large amount of sedimentary data consid-