Sial

Abstract: [1] 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.

Abstract: The composition of the present-day upper crust, inferred from the uniformity of sedimentary rock r.e.e. (rare earth element) patterns, is close to that of granodiorite. A revised ‘andesite’ model is used to obtain total crustal composition. The lower crust is the composition remaining, assuming that the upper crust, one-third of the total, is derived from intracrustal partial melting. The upper-crustal r.e.e. pattern has pronounced Eu depletion (Eu/Eu* = 0.64), the lower-crustal pattern has Eu enrichment (Eu/Eu* = 1.17) and the total crust has no Eu anomaly relative to chondritic abundances. The Eu depletion in the upper crust is attributed to retention of Eu in plagioclase in the lower crust. Because plagioclase is not stable below 40 km (> 10 kbar), the anomaly is intracrustal in origin. The Archaean upper crust has a different r.e.e. pattern to that of the present-day upper crust, being lower in total r.e.e., and La/Yb ratios, and lacking an Eu anomaly. These data are used to infer the Archaean upper-crustal composition, which resembles that of the present-day total crust, except that Ni and Cr contents are higher. The Archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites. The latter have steep light r.e.e.-enriched-heavy r.e.e.-depleted patterns, consistent with equilibration with garnet and hence probable mantle derivation. There is little reason to suppose that the Archaean lower crust was different in composition from the upper crust, except locally where partial melting episodes occurred. The r.e.e. evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early Archaean, a massive increase (to about 70% of the total crust) between about 3000 and 2500 Ma B.P. and a slow increase until the present day. The change from Archaean to post-Archaean r.e.e. patterns in the upper crust is not isochronous, but is reflected in the sedimentary rock r.e.e. patterns at differing times in different continents. On the basis of a model composition for the mantle, 36% of the potassium, 30% of uranium, 15% of lanthanum and 3 % of ytterbium are concentrated in the present continental crust. This enrichment is related to ionic size and valency differences from common mantle cations (e.g. Mg, Fe). Pre-3.9 Ga B.P. crusts were obliterated by meteorite bombardment. No geochemical evidence exists for primordial anorthositic, sialic or mafic crusts.

Abstract: R & F T & M R & F T & M Li 11 13 Ba 390 250 Be 1.5 La 18 16 B 10 Ce 42 33 Sc 22 30 Pr 5 3.9 V 151 230 Nd 20 16 Cr 119 185 Sm 3.9 3.5 Co 25 29 Eu 1.2 1.1 Ni 51 105 Gd 3.6 3.3 Cu 24 75 Tb 0.56 0.6 Zn 73 80 Dy 3.5 3.7 Ga 16 18 Ho 0.76 0.78 Ge 1.6 Er 2.2 2.2 As 1 Tm 0.32 Se 0.05 Yb 2 2.2 Rb 58 32 Lu 0.33 0.3 Sr 325 260 Hf 3.7 3 Y 20 20 Ta 1.1 1 Zr 123 100 W 1 Nb 12 11 Re, ppb 0.4 Mo 1 Os, ppb 0.005 Pd, ppb 1 Ir, ppb 0.1 Ag, ppb 80 Au, ppb 3 Cd, ppb 98 Tl, ppb 360 In, ppb 50 Pb 12.6 8 Sn 2.5 Bi, ppb 60 Sb 0.2 Th 5.6 3.5 Cs 2.6 1 U 1.42 0.91 MAJOR OXIDES (weight percent) R & F T & M SiO2 59.1 57.3 TiO2 0.7 0.9 Al2O3 15.8 15.9 FeO 6.6 9.1 MnO 0.1 0.18 MgO 4.4 5.3 CaO 6.4 7.4 Na2O 3.2 3.1 K2O 1.88 1.1 P2O5 0.2