Crustal differentiation

Vielzeuf D. and Holloway J. R. (1988) Experimental determination of the fluid-absent melting relations in the politic system. Consequences for crustal differentiation. Contrib. Mineral. Petrol. 98, 751—21f>. 

This chapter will briefly review historical aspects of the development of radiogenic isotope geology as applied to continents. Some details, references and cross-references to other chapters in this volume will be provided for most major radiogenic isotopic methods, and for applications of these. However, this chapter will ultimately concentrate on two major approaches that dominate the research field today (i) crustal tectonic and magmatic ages from U-Pb dating of accessory minerals like zircon and (ii) crustal differentiation and growth from neodymium isotopic determinations on total rocks. 

To date, Re-Os isotope studies of peridotite xenolith suites, taken to be representative of the lithospheric mantle beneath continents, reveal that although some lithospheric mantle began to form beneath continents in Palaeo- to Mesoarch-aean time, major crustal differentiation events continued until Neoarchaean time when, at least in southern Africa, significant volumes of lithospheric mantle formed and stabilized the newly accreted continent. On the Kaapvaal Craton, major magmatic events such as that which formed the Bushveld intrusion, substantially modified or introduced new material into the lithospheric mantle (Carlson et al. 1999) and the lower crust (Schmitz Bowring 2000), but the craton retained its integrity for another 2 Ga. 

In the following section, we first review some of the outstanding issues that need to be resolved to better understand intraplate magmatism. We then discuss some of the specific advances that were possible using U-series. Specific features related to magma differentiation and crustal processes are dealt with in the chapter in this volume by Condomines et al. (2003). 

Sigmarsson O, Condomines M, Fomcade S (1992) A detailed Th, Sr and O isotope study of Hekla differentiation processes in an Icelandic Volcano. Contrib Mineral Petrol 112 20-34 Sigmarsson O, Condomines M, Fourcade S (1992) Mantle and crustal contribution in the genesis of recent basalts from off-rift zones in Iceland constraints from Th, Sr and O isotopes. Earth Planet Sci Lett 110 149-162... 

Figure 11,21 Age spectra of hornblendes of a Paleozoic gabbro (367 Ma) intruded by a granitic body during the Cretaceous (114 Ma). Samples underwent permeation of " Ar from lower crustal portions and differential losses of radiogenic argon (32, 57, and 78% respectively), proportional to distance from contact (0.3, 1, and 2.5 km). Reprinted from T. M. Harrison and I. McDougall, Geochimica et Cosmochimica Acta, 44, 2005-2020, copyright 1980, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

Low Nb and Ti in the feisic rocks are due to fractionation of Ti-bearing phases during calc-alkaline differentiation or during crustal partial melting. 

Sr/ Sr ratios that correlate with an increase in Si02 and decrease in Sr content. In contrast, a mantle melt, which evolves only through differentiation unaccompanied by interaction with crustal material, will have an 0-isotope composition that mainly reflects that of its source region, independent of variations in chemical composition. In this latter case, correlated stable and radiogenic isotope variations would be an indication of variable crustal contamination of the source region (i.e., crustal material that has been recycled into the mantle via subduction). 

Mesosiderites are stony irons in which the rocky material is a polymict breccia of crustal rock from a differentiated body— basalt, gabbro, pyroxenite, dunite, and anorthosite. The silicates are very similar to the HED suite meteorites. Thus, mesosiderite silicates are discussed here, but the metallic phase is not. 

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