Crustal growth versus recycling models and previous constraints

2. Crustal growth versus recycling models and previous constraints

2 . 1 . Growth and recycling Both crustal growth and recycling models need to account for the formation of new material, in the form of subduction-related igneous rocks, added to the continental crust throughout the geological record. Where the models differ funda- mentally, is the volume of the continental crust in the early Archaean and the magnitude of the return flux of material from the continental crust back into the mantle. Characteristics of early to middle Archaean crustal recycling and crustal growth models are illustrated in Fig. 1, and fol- lows the representation used by McCulloch and Bennett 1998. Crustal evolution is charted every 200 Ma. Recycling is expressed as R, the vol- ume relative to present day volume of continen- tal crust recycled into the mantle every 100 Ma, and growth is expressed as G, the volume of the continental crust formed every 100 Ma. Cainozoic sediment accumulation has been cal- culated at 2.4 9 1.2 km 3 per annum Davies et al., 1977. At this rate of accumulation the entire mass of the continental granitic sensu lato crust could be converted into sediment in 3600 Ma Armstrong, 1991, equivalent to R = 2.8. How- ever, processes such as tectonic erosion in subduc- tion zones are needed to remove into the mantle the sediments that were ultimately produced from erosion of granitic rocks. Tectonic erosion in sub- duction zones has been estimated at 1.1 km 3 per year Scholl et al., 1990, equivalent to R = 1.9. Allowing for a much more vigorous early Ar- chaean Earth by multiplying this value fivefold approximating to the greater heat production of those times; e.g. Dickinson and Luth, 1971, R : 10 could be postulated for the early Archaean. To be especially permissive to a recycling model, this value is doubled here to R = 20. With R = 20 and constant crustal volume equal to that of the present day, approximately a third of the 3500 Ma continental crust should have consisted of \ 3900 Ma material Fig. 1. At this recycling rate, \ 3900 Ma detrital zircons should be globally abundant in 3500 Ma sediments of broad provenance. By 3100 Ma, they should still form : 5 of global detrital zircon populations Fig. 1. Hence, unless both recycling and growth are small which does not fit the geological record, then by the middle Archaean 3100 Ma the ob- served proportion of ] 3900 continental crust will be small. This illustrates the difficulty of using a younger 3100 Ma detrital zircon record to provide robust constraints on the relative impor- tance of growth versus recycling in the earliest Archaean. Therefore, when using detrital zircon age spectra, the oldest parts of the geological record ] 3500 Ma here are needed to help assess the relative importance of crustal growth versus recycling. 2 . 2 . Whole rock isotopic constraints on crustal e6olution Nd, Sr and Pb isotope ratios measured on Archaean rocks have been the main constraints used in erecting models of Archaean crust-mantle evolution, with the importance of additional of juvenile low initial 87 Sr 86 Sr, elevated initial 143 Nd 144 Nd material to the Archaean crust long being recognised e.g. Moorbath, 1978; Moorbath et al., 1977, 1997; Miller and O’Nions, 1985; McCulloch and Bennett, 1993, 1994; Bennett et al., 1994; Bowring and Housh, 1995. In these Fig. 1. Examples of crustal growth and recycling models, and estimated maximum contribution of ] 3900 Ma detrital zir- cons to ancient sediments. isotopic systems, there is marked fractionation of the parent – daughter isotopes between the mantle, oceanic crust and continental crust, allowing in principle the evolution of different reservoirs to be charted through time. The closer chemical similarity of 147 Sm – 143 Nd compared with 238,235 U – 206,207 Pb, 232 Th – 208 Pb and 87 Rb – 87 Sr parent – daughter pairs means that Nd isotopic evolution should generally be less prone to disturbance by parent – daughter fractionation in metamorphismmetasomatism after the rocks formed. Therefore subsequent discussion concen- trates on Nd isotopic signatures. Even so, care should be taken in the choice of samples, because it is still possible to have secondary fractionation effects, as clearly demonstrated when ‘worst case scenario’ samples are studied e.g. Bridgwater and Rosing, 1995. 143 Nd 144 Nd isotope ratios are nor- mally expressed in o Nd notation, where one o Nd unit is a deviation of one part in 10 000 from the 143 Nd 144 Nd of a chondritic uniform reservoir at the time of interest. The depleted mantle and MORB have higher SmNd than CHUR, thus their o Nd values become more positive with time. Continental crust and most arc rocks have lower SmNd than CHUR, thus their o Nd values become increasingly negative with time. The evolution of the Archaean mantle has to a large degree been inferred from the Nd isotopic compositions of preserved Archaean mafic rocks derived directly from the mantle 9 crustal contamination and in- termediate rocks generally interpreted to be arc- related and ultimately incorporated into continental crust. Initial Nd isotopic values for early Archaean rocks e.g. compilations in Bennett et al. 1993, Bowring and Housh 1995 show considerable deviations from CHUR analytical uncertainties on individual determinations are B 1 o Nd , which shows that the Earth had fractionated into en- riched low SmNd and depleted high SmNd reservoirs, by 3800 – 4000 Ma when the geological record starts in earnest e.g. Hamilton et al., 1983; Jacobsen and Dymek, 1986; Collerson et al., 1991; Bennett et al., 1993; Bowring and Housh, 1995. Despite disagreement over the degree of fractiona- tions Collerson et al., 1991; Bennett et al., 1993; Vervoort et al., 1996; Moorbath et al., 1997; Bennett and Nutman, 1998; Kamber et al., 1998 there is general acceptance that by 3900 Ma there were already several fractionated crust and mantle reservoirs, with evidence of repeated additions of juvenile material to the crust in the early Ar- chaean. Nd isotopic studies of ancient detrital sediments e.g. Miller and O’Nions, 1985; Jacob- sen and Dymek, 1987 and Hf isotopic studies of detrital zircons in Archaean sediments Stevenson and Patchett, 1990 suggest juvenile components are dominant over recycled crustal components. However, it is shown by several workers that in isolation the Nd whole rock isotopic record does not provide a unique solution for the variables Archaean continental mass, mass of depleted mantle and degree of elemental fractionation of mantle e.g. Bowring and Housh, 1995; McCul- loch and Bennett, 1997. It is for this reason that a different set of data, the age spectra of detrital zircons in early Archaean sediments, is examined in this paper in order to provide a new perspective on early Archaean crustal evolution. 2 . 3 . Use of detrital zircon population age spectra to assess crustal e6olution The validity of using detrital zircon population age spectra in order to place constraints on growth and recycling rates of quartzofeldspathic continental crust must first be assessed. Zircons are mostly grown in granitic sensu lato magmas Poldervaart, 1956. Zircons may also grow within mafic magmas as magmatic phases e.g. Paces and Miller, 1993, but the amount of zircon formed this way, relative to that grown from granitic magmas, is very small. Furthermore, those formed in mafic magmas tend to but not always have distinctive morphologies, such as centres rich in other silicate inclusions sometimes to the ex- tent of the zircons being hollow tubesprismatic grains, and also angular-anhedral forms that are the result of co-precipitation with other phases. They also tend to have high ThU typically 0.8 – 2.0 and high but not always Th + U absolute abundances. Zircons can also grow in a wide variety of rocks during metamorphism and meta- somatism e.g. ultramafic rocks, Nutman et al., 1992; metasediments, Williams and Claesson, 1987; granitoids and gneisses, Kinny, 1986. Again, the amount of zircon grown this way is small relative to that grown from granitic magmas. Suites of tonalite – trondhjemite – diorite – gran- odiorite are the most abundant rocks in the Ar- chaean geological record. They are most likely dominated by partial melting products of hy- drated mafic crust, which was heated when de- pressed into the mantle at Archaean convergent plate boundaries e.g. Martin, 1986. Sr and Nd isotopic tracers show that these rocks are domi- nated by juvenile additions to the crust e.g. Moorbath et al., 1977; Bennett et al., 1993; Moor- bath et al., 1997. Extensive experimental petrol- ogy studies of tonalite and diorite compositions Rapp, 1997; Wyllie et al., 1997 for reviews, suggest that the observed whole rock composi- tions in Archaean tonalite – trondhjemite – diorite – granodiorite suites are close to that of precursor magmas, rather than migmatites of magma + abundant coexisting restite. Experimental studies e.g. Rapp, 1997; Wyllie et al., 1997 also showed that these magmas are hot, with temperatures of ] 850°C. Archaean tonalitic and dioritic rocks typically have Zr abundances of ] 200 ppm e.g. Wedepohl et al., 1991. Experimental studies of Zr solubility of granitic, sensu lato melts using a wide range of compositions, pressures and tempera- tures have been reported Watson and Harrison, 1983. These studies show that if the Archaean tonalitic and dioritic rocks were close to melting compositions, then at ] 850°C solubility of Zr in those melts would be ] 400 – 500 ppm. This is considerably higher than the B 200 ppm Zr present in these rocks. Thus the precursor melts were probably strongly undersaturated in Zr, and any zircon entrained in them would have had a strong potential to dissolve. Thus, it can be ar- gued that zircons from these rocks are predomi- nantly magmatic in origin. Zircons from these sources probably dominate early Archaean detri- tal suites, and consequently their detrital age spec- tra can be interpreted as predominantly but not exclusively controlled by the production of mag- matic zircon in granitoids that are juvenile addi- tions to the crust, rather than remelted older crust.

3. SHRIMP U – Pb zircon geochronology