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Origin and Evolution of the Continental Crust and Mantle

McCulloch has also made important contributions to fundamental questions in the Earth Sciences, such as:- What processes were responsible for the formation of continental crust? How and at what rate did the continental crust grow? What is the relationship between crustal growth, crustal recycling and mantle evolution? What imprint have these processes left in the ultimate source of continental crust, the Earth's mantle? A significant part of Malcolm McCulloch's research has focussed on these questions and his major contributions are outlined below.

Continental Crust formation ages

In a seminal paper, McCulloch and Wasserburg, (1978) showed that Sm-Nd isotopic system could be used to provide important constraints on crustal evolution. It was demonstrated that the rare earth elements Sm and Nd, was largely unaffected by processes such partial melting, metamorphism and weathering and therefore the decay of 147Sm (1.06 x 1011 year half-life) to 143Nd provides direct information on the time of formation of continental crust and the provenance of sediments. Tracing the provenance and determining 'crustal formation' ages of sedimentary rocks has continued to be a very powerful tool, not only in the general understanding of continental crustal evolution, but also in determining pathways of sediments (and particle reactive phosphorous) into sensitive environments such as rivers and near-shore coastal regions. Another important outcome of the McCulloch and Wasserburg's (1978) study, was the recognition from Sm-Nd model ages, of periods of high rates of crustal growth that occurred in the late Archean. This finding has been extended by the use of combined U-Pb zircon ages in detrital zircons in conjunction with Sm-Nd model ages (eg Liew et al., Maas and McCulloch, 1991, and Zhou et al., 1992) with further research showing of the importance of early Proterozoic crust, particularly in the formation of the Australia continent (McCulloch, 1987, Wyborn et al., 1988, Zhao et al, 1992, 1993).

McCulloch and Chappell (1982) also provided one of the first demonstrations that that the parent material from which granites were derived could be distinguished using Sm-Nd isotopic systematics. They showed that sedimentary (S-type) and igneous (I-type) sources have distinctive Nd and Sr isotopic characteristics. Combined Nd-Sr isotopic systematics showed that most I-type as well as the S-type granites were produced by melting of substantially older continental crust, with S-types being derived from more felsic sedimentary precursors. The distinctive isotopic signatures for S- and I- type granites from the Lachlan Fold Belt of SE Australia have also been identified in New England (Hensel et al., 1985) as well as Malaya (Liew and McCulloch, 1985). Based on Pb isotopic systematics, evidence has also been obtained for large-scale fluid circulation in the deep crust prior to or associated with crustal melting (McCulloch and Woodhead, 1993, McCulloch, 1995). This type of approach using combined Nd-Sr-Pb isotopic together with trace element systematics is now being widely utilised in the study of granitic rocks generally.

McCulloch has also undertaken research on the origin of early Archean high grade metamorphic gneisses from Enderby Land, Antarctica (McCulloch and Black, (1984), and in one of the first studies of its kind showed that in some circumstances the Sm-Nd whole rock system can be disturbed, in high grade metamorphic terranes. He has also pursued the origin of granulites in a study of the Cretaceous examples from Fiordland, New Zealand (McCulloch et al., 1987. In these studies the relationship between the protolith and metamorphic ages in granulites from Antarctica and Fiordland, New Zealand were determined (McCulloch and Black, 1984, McCulloch et al., 1987, Windrim and McCulloch, 1986, Black and McCulloch , 1987, and Black et al., 1987) which has important implications for the formation of granulites generally. He has undertaken complementary work relevant to understanding the deep continental crust using lower crustal xenoliths (McCulloch et al., 1982 and Rudnick et al., 1986).

Depleted mantle evolution in the early Archean

McCulloch and Compston (1981) identified in the Archean greenstones from Kambalda in Western Australia, the first evidence for the existence of early-Archean depleted mantle. This implies that the growth of early-Archean continental crust was an important process. An important follow-up finding was the identification of extremely depleted mantle in the very early-Archean, approximately 3700-3800 million years ago (Bennett, Nutman and McCulloch, 1993). A model (McCulloch and Bennett, 1994) has also been proposed that provides a coherent explanation for the isotopic as well as trace element evolution of the Earth's crust and mantle. An interesting aspect of the modelling (McCulloch, 1993) is an explanation for the long-standing Pb paradox as well U-Th systematics of the mantle, a consequence of a long-term recycling of U enriched oceanic crust, that commenced at the end of the Archean due to more oxidised regimes on the Earth's surface. McCulloch has also obtained a 'best estimate' for the Earth's initial Sr isotopic composition which enables limits to be placed on the timescale for the accretion of the Earth (McCulloch, 1994). The results imply that the accretion of the Earth occurred over an extended interval and was probably not complete until 4400 million years.

Geochemistry of subduction zone processes

From his work on subduction zone processes in island-arc systems, McCulloch has also made important contributions to our knowledge of how continental crust is produced. The study of McCulloch and Perfitt (1981) provided a good example of how trace element and isotopic systematics can together provide powerful constraints on the sources of magmatic rocks. It was shown that the incompatible elements are largely decoupled from isotopic variations, consistent with the role of fluids in transporting incompatible elements from the subducting slab. McCulloch and Gamble (1991) developed a geochemical and dynamical model for the operation of subduction zones which attempts to account for their generally enriched incompatible element but depleted HFSE (high field strength element) characteristics. He has also been involved in the study of high Mg boninite lavas (McCulloch and Cameron, 1983, Cameron et al., 1983, Nelson et al., 1984 and Fallon et al., 1989) and in the case of the controversial Troodos Ophiolite (McCulloch and Cameron, 1983) clearly showed that it was produced in a subduction related setting, now a generally accepted finding. In collaboration with Dr R.T. Gregory and coworkers, MCulloch also undertook one of the first detailed Nd-Sr and oxygen isotopic studies of obducted oceanic crust, the Samail Ophiolite complex (McCulloch et al., 1980,1981). These studies showed that the Samail Ophiolite has back-arc affinities rather than being representative of 'normal' oceanic crust and provided insights into the effects of sea-floor hydrothermal alteration.

Isotopic anomalies in meteorites

In the early stages of his career, McCulloch working with Professor G.J. Wasserburg from the California Institute of Technology, made some of the first discoveries of 'heavy element' isotopic anomalies in Ba, Nd and Sm in the Allende meteorite. This discovery (McCulloch and Wasserburg, 1978ab) was important, as prior to that time only oxygen anomalies had been identified whose nucleosynthetic origin was uncertain. The discovery of the anomalies in the high Z elements clearly identified an r-process late-stage spike probably associated with nucleosynthethesis in a supernovae.