Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons

Continental crust forms from, and thus chemically depletes, the Earth's mantle. Evidence that the Earth's mantle was already chemically depleted by melting before the formation of today's oldest surviving crust has been presented in the form of Sm-Nd isotope studies of 3.8-4.0 billion years old rocks from Greenland(1-5) and Canada(5-7). But this interpretation has been questioned because of the possibility that subsequent perturbations may have re-equilibrated the neodymium-isotope compositions of these rocks(8). Independent and more robust evidence for the origin of the earliest crust and depletion of the Archaean mantle can potentially be provided by hafnium-isotope compositions of zircon, a mineral whose age can be precisely determined by U-Pb dating, and which can survive metamorphisms(4). But the amounts of hafnium in single zircon grains are too small for the isotopic composition to be precisely analysed by conventional methods. Here we report hafnium-isotope data, obtained using the new technique of multiple-collector plasma-source mass spectrometry(9), for 37 individual grains of the oldest known terrestrial zircons (from the Narryer Gneiss Complex, Australia, with U-Pb ages of up to 4.14 Gyr (refs 10-13)). We find that none of the grains has a depleted mantle signature, but that many were derived from a source with a hafnium-isotope composition similar to that of chondritic meteorites. Furthermore, more than half of the analysed grains seem to have formed by remelting of significantly older crust, indicating that crustal preservation and subsequent reworking might have been important processes from earliest times.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62681/1/399252a0.pd

Three-dimensional cathodoluminescence imaging and electron backscatter diffraction: tools for studying the genetic nature of diamond inclusions

As a step towards resolving the genesis of inclusions in diamonds, a new technique is presented. This technique combines cathodoluminescence (CL) and electron backscatter diffraction (EBSD) using a focused ion beam-scanning electron microscope (FIB-SEM) instrument with the aim of determining, in detail, the three-dimensional diamond zonation adjacent to a diamond inclusion. EBSD reveals that mineral inclusions in a single diamond have similar crystallographic orientations to the host, within ±0. 4°. The chromite inclusions record a systematic change in Mg# and Cr# from core to the rim of the diamond that corresponds with a ~80°C decrease of their formation temperature as established by zinc thermometry. A chromite inclusion, positioned adjacent to a boundary between two major diamond growth zones, is multi-faceted with preferred octahedral and cubic faces. The chromite is surrounded by a volume of non-luminescent diamond (CL halo) that partially obscures any diamond growth structures. The CL halo has apparent crystallographic morphology with symmetrically oriented pointed features. The CL halo is enriched in ~200 ppm Cr and ~80 ppm Fe and is interpreted to have a secondary origin as it overprints a major primary diamond growth structure. The diamond zonation adjacent to the chromite is complex and records both syngenetic and protogenetic features based on current inclusion entrapment models. In this specific case, a syngenetic origin is favoured with the complex form of the inclusion and growth layers indicating changes of growth rates at the diamond-chromite interface. Combined EBSD and 3D-CL imaging appears an extremely useful tool in resolving the ongoing discussion about the timing of inclusion growth and the significance of diamond inclusion studies. © 2010 The Author(s)

U-Pb geochronology of the Fort Augustus granite gneiss: constraints on the timing of Neoproterozoic and Palaeozoic tectonothermal events in the NW Highlands of Scotland

The West Highland granite gneiss suite in Inverness-shire, Scotland, represents a series of S-type, anatectic granites formed by partial melting of host Neoproterozoic metasediments of the Moine Supergroup. U-Pb (SHRIMP) dating of zircons from a member of the suite, the Fort Augustus granite gneiss, indicates that the granitic protolith to the gneiss was intruded at 870 +/- 30 Ma. This is indistinguishable from the published age determined by the same method for the Ardgour granite gneiss at Glenfinnan, thus supporting the assumption that the various members of the West Highland granite gneiss are part of a single intrusive suite. The spread of ages from the zircon cores (1626-947 Ma) is interpreted to indicate a Proterozoic source terrain for the Moine sediments that were later melted to form the granitic protolith. A U-Pb age of 470+/-2 Ma obtained for titanite in the Fort Augustus granite gneiss is interpreted to date amphibolite-facies metamorphism during the early to mid-Ordovician Grampian Orogeny, The emerging similarity in the timing of this event either side of the Great Glen Fault implies that this structure does not juxtapose crustal blocks with significantly different histories with respect to the Grampian Orogeny

U-Pb zircon geochronology, geochemistry and Nd isotopic study of Neoproterozoic bimodal volcanic rocks in the Kangdian Rift of South China: implications for the initial rifting of Rodinia

SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neoproterozoic Suxiongvolcanic rocks in the Kangdian Rift, western South China. These volcanic rocks are bimodal, consisting mainly of mildly alkaline basalts and trachydacites to rhyolites. SHRIMP U-Pb zircon age determination indicates that theywere erupted at 80312 Ma. Most basaltic rocks are characterized by high positive Nd(T) values (+5 to +6),pronounced enrichment in Th, Ta, Nb, LREEs, Sr, P, Zr, Hf, Ti, smooth LREE-enriched patterns and generally 'humped' trace element spidergrams. They resemble the alkali basalts of the Hawaiian oceanic island basalts (OIB)and the Ethiopian continental flood basalts (CFB). These features suggest that the basaltic rocks were most probably derived from an OIB-like mantle source without appreciable crustal/lithospheric contamination. Differentiated basalt and trachyandesite samples show relatively low Nd(T) values (+1.7 to +2.4) and Nb-Ta depletion due to contamination by the mafic lithosphere and/or crustal materials. The rhyolite and dacite samples have small positive Nd(T) values (+1.1 to +2.6), general enrichment in most incompatible trace elements (K, Rb, Th, Zr, Hf and REEs) but significant depletion in Nb, Ta, Sr, P, Eu and Ti. They share geochemical characters of A2-type granites, and are likely generated by shallow (P4 kbar) dehydration melting of hornblende-bearing granitoids. Geochemical and Nd isotopic characters and high-volcanicity of the Suxiong bimodal volcanic successions are consistent with their formation in a continental rift environment, such as the Ethiopian rift. The Kangdian Rift is considered as part ofa wider continental rift system produced by a starting mantle plume beneath South China during the Neoproterozoic breakup of Rodinia

Geochronology of Neoproterozoic syn-rift magmatism in the Yangtze Craton, South China and correlations with other continents: evidence for a mantle superplume that broke up Rodinia

Neoproterozoic granitic intrusions in South China have traditionally been interpreted as related to orogenesis, marking thecratonisation of the Yangtze Block. However, a number of ca. 830-820 Ma granitoids and mafic-ultramafic intrusions unconformablyoverlain by Neoproterozoic rift successions have recently been reinterpreted as being related to a mantle plume duringthe breakup of the supercontinent Rodinia. In this paper, we report SHRIMP zircon U-Pb ages from granitoids and gabbros thatare closely related to the Neoproterozoic rifting event, and one age from a volcanic unit in the rift successions. We demonstrate that there were two major phases of widespread bimodal magmatism in South China during the Neoproterozoic. The first one,at ca. 830-795 Ma, started before the continental rift but continued into the first two stages of the rifting. The second one, ca.780-745 Ma, occurred during the later stages of the rifting. Some co-magmatic mafic dykes have rare-earth element and traceelement distribution characteristic of continental flood basalts. Similar age patterns of Neoproterozoic anorogenic magmatismare recorded in most other Rodinian continental blocks, such as Australia, India, Madagascar, Seychelles, southern Africa andLaurentia. The widespread occurrence and protracted duration (ca. 85 million years) of such anorogenic magmatism require alarge and sustained heat source. We interpret these magmatism as results of a mantle superplume beneath Rodinia, which wasresponsible for the breakup of the supercontinent during the Neoproterozoic