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Volume 2 April 20, 2001 Paper number 2000GC000109
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society
ISSN: 1525-2027
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 11794-2100 (
[email protected])
[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 = 4100, V = 107, 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, 2001. McLennan, S. M., 2001. Relationships between the trace element composition of sedimentary rocks and upper continental crust, Geochem. Geophys. Geosyst., vol. 2, Paper number 2000GC000109 [8994 words, 10 figures, 5 tables]. Published April 20, 2001.
Theme: Geochemical Earth Reference Model (GERM)
1. Introduction The chemical composition of the upper continental crust is an important constraint on [2]
Copyright 2001 by the American Geophysical Union
Guest Editor: Hubert Staudigel
understanding the composition and chemical differentiation of the continental crust as a whole and the Earth in general [e.g., Taylor
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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. 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.
[3]
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
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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 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 [5]
Selected Trace Elements for Estimates of Various Shield Areas and the Average Upper Continental Crust
Element, ppm
East Chinac 15 3900 98 80 17 38 82 188 12 3.55 678 34.8 5.12 0.74 18 8.95
Sc Ti V Cr Co Ni Rb Zr Nb Cs Ba La Hf Ta Pb Th
7.0 3120 53 35 12 19 110 237 26
12 3180 59 76
1070 32.3 5.8 5.7 17 10.3
730 71
Area, 106km2
5.56
a
19 190
18 10.8
0.95
Scotlandd
New Mexicoe
2400
2700
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±10. 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 10% 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±10. [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 10±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
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number of analyses (n = 7±19, depending on element) results in 95% confidence limits on the means of 10±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. 4.1.1. 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).
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). [21]
[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 postArchean 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].
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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]. [24]
[25] 6. Average Phanerozoic cratonic shale is from Condie [1993] (n > 100). [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 = 10). [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].
4.1.2. Sand and sandstones (coarse grain) Coarser-grained sediment averages that were used are described below (see Table 3). In estimating the average coarse-grained sedi-
[29]
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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 > 100). [32] 3. Average Phanerozoic greywacke is from the mean of Paleozoic (n > 100) and MesozoicCenozoic (n > 100) averages [Condie, 1993].
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).
[33]
[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 = 10) margins [Bhatia, 1981, 1985b; Bhatia and Crook, 1986].
4.2. 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
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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 10%).
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5. Results 5.1. REE, Th, and Sc [39] On Figure 3, the REE patterns of the various averages and composites are plotted
100.0
ppm / ppm Chondrites
[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 ( 100). 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 = 10). 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 > 100). l Average Phanerozoic greywacke, taken from mean of Paleozoic (n > 100) and Mesozoic ± Cenozoic (n > 100) 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 = 10) [Bhatia, 1981, 1985b; Bhatia and Crook, 1986]. b
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a
18.3 5520 147 129 18.9 62 113 191 17.1 8.84 635 43.3 5.77 1.39 (73) 14.7
PAAS Shaled
f
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Sc Ti V Cr Co Ni Rb Zr Nb Cs Ba La Hf Ta Pb Th
c
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Table 4.
Regional Averages of Selected Trace Elements in Quaternary Loess
Element, New United Statesb Kaiserstuhlc Spitsbergend Argentinae United Franceg a f ppm Zealand Kingdom Sc Ti V Cr Co Ni Rb Zr Nb Cs Ba La Hf Ta Pb Th
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8.1 3480 65 31 9.6 13 83 366 12.7 3.8 572 35.4 10.4 1.12 13 10.2
5.3 4020 57 32 5.6 12 74 420 15.0 2.8 681 34.3 13.5 1.03 14 9.5
5.85 1860 40 42 6.5 24 47 255 9.8 1.9 195 25.0 6.8 0.76 4.2 5.6
4140 119 109 14 24 80 278 14.3 3.9 554 29.3
4860 105 36 14 16 79 234 10.1 5.0 578 23.9
0.93 17 9.0
0.80 18.3 8.7
3420 58 67 9 20 73 327 11 332 19.7
4320 69 61 8 17 61 357 10.5 2.9 298 24.9
15 7
13.2 7.9
Chinah Average Loessi 11.9 3770 99 69 18 34 89 181 11.7 6.5 457 30.4 4.8 0.87 18.0 10.9
7.8 3730 77 56 10.6 20 73 302 12.7 3.8 458 27.9 8.9 0.92 14.1 8.6
a
Average of five loess samples from Banks Penninsula, New Zealand [Taylor et al., 1983; Barth et al., 2000]. 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. b
Pb = 14±18 ppm. Of note is the exceptionally low La/Pb ratio of average suspended river sediment (Figure 10a). 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 10b), 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 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 = 10,000, and K/Rb = 250 [McLennan et al., 1980; TM85]. Since no [49]
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-
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ered in this study suggests an upper crustal Rb/ Cs ratio of 24. This ratio is likely to be a lower limit because the larger Cs ion would be expected to be held preferentially over Rb onto clay minerals [e.g., Dupre et al., 1996], leading to low Rb/Cs ratios in the finest-grained sediment, and excluded preferentially over Rb from Ca- and Na-bearing carbonate and evaporite minerals [e.g., Okumura and Kitano, 1986], leading to high Rb/Cs ratios in the nonclastic parts of the sedimentary record (see further discussion in section 3.1). [51] The high upper crustal Cs content estimated by Plank and Langmuir [1998] may in fact be an artifact of their approach, where Cs was estimated from a correlation between Rb and Cs in marine sediments and upper crustal Rb content was taken from TM85. From Table 3 it can be seen that the average subducting sediment (GLOSS in Table 2) in fact has a relatively low Cs content compared to any of the other fine-grained sediment averages and is rather similar to the TM85 upper crustal estimate. However, GLOSS also has a much lower Rb content than most of the other fine-grained sediment averages and more than a factor of 2 lower than the upper crustal Rb estimate of TM85. The overall effect is that the La/Rb ratio of GLOSS is the highest of any of the sediment averages, whereas the La/Cs ratio of GLOSS falls within the range of the other fine- and coarse-grained sediment averages (Figure 5). Accordingly, the marine sedimentary record studied by Plank and Langmuir [1998] appears to be characterized by anomalously low Rb content rather than high Cs content, and the high estimate of upper crustal Cs abundances is probably an artifact of this anomaly.
The values for Nb and Ta obtained in this study are essentially identical to those suggested by Plank and Langmuir [1998] and Barth et al. [2000]. The values of TM85 were based on the Canadian Shield Nb estimate of [52]
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Shaw et al. [1976] and an upper crustal Nb/Ta of 11.6 [from Wedepohl [1977]; however, all other shield areas that have been examined have lower Nb by a factor of >2 on average (Table 1), and accordingly, the lower estimates also appear consistent with the surface sampling of shields. [53] The proposed increases in the ferromagnesian trace elements Cr, V, Ni, and Co average about a factor of 2 greater than the TM85 values and are most similar to, although generally lower than, the upper crustal estimates of Condie [1993]. The TM85 values were essentially identical to the Canadian Shield estimates (see Table 1). In general, the various shield estimates are highly scattered for these elements (Table 1), but the ferromagnesian trace elements proposed here are slightly higher than the shield average.
6.2. Implications for the Bulk Continental Crust and Lower Crust In the model of crustal composition proposed by TM85 the lower crustal composition is derived by assuming the upper crust constitutes 25% of the total continental crust. Accordingly, any revision to upper crustal abundances has an effect on estimates of the lower continental crust. [54]
In the case of Cs, this leads to difficulties in that the elevated upper crustal Cs proposed here (Cs = 4.6 ppm) when combined with the bulk crustal estimate of TM85 (Cs = 1.0 ppm) creates mass balance difficulties for the lower crust, assuming it constitutes 75% of the total. Accordingly, the bulk crustal abundances of the most incompatible elements (Cs, Rb, K, Th, and U) require reevaluation [McLennan et al., 2001]. McLennan and Taylor [1996] suggested that the continental heat flow data were consistent with slightly higher bulk crustal abundances for K, Th, and U compared to [55]
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Table 5.
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Composition of the Continental Crust Revised From Taylor and McLennan [1985]
Element
Upper Crust
Bulk Crust
Lower Crust
Li, ppm Be, ppm B, ppm Na, wt % Mg, wt % Al, wt % Si, wt % P, ppm K, wt % Ca, wt % Sc, ppm Ti, wt % V, ppm Cr, ppm Mn, ppm Fe, wt % Co, ppm Ni, ppm Cu, ppm Zn, ppm Ga, ppm Ge, ppm As, ppm Se, ppm Rb, ppm Sr, ppm Y, ppm Zr, ppm Nb, ppm Mo, ppm Pd, ppb Ag, ppb Cd, ppb In, ppb Sn, ppm Sb, ppm Cs, ppm Ba, ppm La, ppm Ce, ppm Pr, ppm Nd, ppm Sm, ppm Eu, ppm Gd, ppm Tb, ppm Dy, ppm Ho, ppm Er, ppm Tm, ppm Yb, ppm Lu, ppm Hf, ppm Ta, ppm W, ppm
20 3.0 15 2.89 1.33 8.04 30.8 700 2.80 3.00 13.6b 0.41b 107b 83b 600 3.50 17b 44b 25 71 17 1.6 1.5 50 112 350 22 190 12b 1.5 0.5 50 98 50 5.5 0.2 4.6c 550 30 64 7.1 26 4.5 0.88 3.8 0.64 3.5 0.80 2.3 0.33 2.2 0.32 5.8 1.0b 2.0
13 1.5 10 2.30 3.20 8.41 26.8 1.1a 5.29 30 0.54 230 185 1400 7.07 29 128 75 80 18 1.6 1.0 50 37c 260 20 100 8.0b 1.0 1 80 98 50 2.5 0.2 1.5c 250 16 33 3.9 16 3.5 1.1 3.3 0.60 3.7 0.78 2.2 0.32 2.2 0.30 3.0 0.8b 1.0
11 1.0 8.3 2.08 3.80 8.52 25.4 0.53a 6.07 35b 0.58b 271b 219b 1700 8.24 33b 156b 90 83 18 1.6 0.8 50 12c 230 19 70 6.7b 0.8 1 90 98 50 1.5 0.2 0.47c 150 11 23 2.8 12.7 3.17 1.17 3.13 0.59 3.6 0.77 2.2 0.32 2.2 0.29 2.1 0.73b 0.7
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Table 5. Element Re, ppb Os, ppb Ir, ppb Au, ppb Tl, ppb Pb, ppm Bi, ppb Th, ppm U, ppm
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(continued) Upper Crust
Bulk Crust
Lower Crust
0.4 0.05 0.02 1.8 750 17b 127 10.7 2.8
0.4 0.05 0.1 3.0 360 8.0 60 4.2a 1.1a
0.4 0.05 0.13 3.4 230 5.0b 38 2.0a 0.53a
a
Revisions suggested by McLennan and Taylor [1996]. Revisions suggested in this study. c Revisions discussed in this study and by McLennan et al. [2001]. b
those suggested by TM85 and proposed K = 1.1%, Th = 4.2 ppm, and U = 1.1 ppm. Maintaining a bulk crustal K/Rb ratio of 300, this also suggests that the bulk crustal Rb content should be revised slightly upward to 37 ppm. The bulk crustal TM85 Cs estimate came from an assumed crustal Rb/Cs ratio of 30. The bulk crustal composition of TM85 is based to a large degree on Archean crustal compositions, but there are in fact very few high-quality Cs data for Archean rocks. For example, Cs data are absent from the most recent compilation of Archean igneous rock compositions [Condie, 1993]. If the average island arc volcanic rock Rb/Cs of 25 is adopted instead (from Taylor and McLennan [1985]; note McDonough et al. [1992] obtained the same bulk crust Rb/Cs ratio using a different approach), this leads to a revised bulk crustal Cs content of 1.5 ppm, a value adopted here. Note that this Rb/Cs ratio is also very close to the clastic sedimentary Rb/Cs ratio of 24, discussed above. These revisions result in lower crustal abundances of Cs = 0.47 ppm and Rb = 12 ppm. [56] In calculating the Nb and Ta abundances of the bulk crust, Taylor and McLennan [1985] relied on average Phanerozoic island arc volcanics because there were few relevant data
from Archean rocks. This situation has changed, and recently, Condie [1993] compiled Nb and Ta concentrations for the dominant Archean igneous lithologies. For the model proposed by TM85 these data suggest a slight downward revision of bulk crustal abundances for these elements, such that Nb = 8 ppm and Ta = 0.8 ppm, resulting in calculated lower crustal abundances of Nb = 6.7 ppm and Ta = 0.73 ppm. In Table 5, all of the revisions to the crustal abundances of TM85 that are suggested here as well as by McLennan and Taylor [1996] and McLennan et al. [2001] are summarized.
Acknowledgments I am grateful to G. Xiao for assistance in compiling Rb-Cs data in sedimentary environments and to Ross Taylor for providing some unpublished Cs data and for a continuing discussion on crustal composition. I also thank Kent Condie, Bill McDonough, Roberta Rudnick, Hubert Staudigel, Bill White, and an anonymous reviewer for comments.
[57]
References Barth, M. G., W. F. McDonough, and R. L. Rudnick, Tracking the budget of Nb and Ta in the continental crust, Chem. Geol., 165, 197 ± 213, 2000. Bhatia, M. R., Petrology, geochemistry and tectonic setting of some flysch deposits, Ph.D. dissertation, 382 pp., Aust. Natl. Univ., Canberra, ACT, 1981.
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