This critical review is intended to attract the interest of organic chemists and researchers on green and sustainable chemistry on the catalytic activity of supported gold nanoparticles in organic transformations. In the general part of this critical review, emphasis is given to the different procedures to form supported gold nanoparticles and to the importance of the support cooperating in the catalysis. Also the convergence of homogeneous and heterogeneous catalysis in the study of gold nanoparticles has been discussed. The core part of this review is constituted by sections in which the reactions catalyzed by supported gold nanoparticles are described. Special emphasis is made on the unique ability of gold catalysts to promote additions to multiple C–C bonds, benzannulations and alcohol oxidation by oxygen (282 references).

Supported gold nanoparticles as catalysts for organic reactions

High Sr/Y and La/Yb ratios: The meaning of the “adakitic signature”

The deep carbon cycle and melting in Earth's interior

Based on a compilation of published sources, rocks referred to as adakites show the following geochemical and isotopic characteristics: SiO2 ≥56 wt percent, Al2O3 ≥15 wt percent, MgO normally <3 wt percent, Mg number ≈0.5, Sr ≥400 ppm, Y ≤18 ppm, Yb ≤1.9 ppm, Ni ≥20 ppm, Cr ≥30 ppm, Sr/Y ≥20, La/Yb ≥20, and 87Sr/86Sr ≤0.7045. Rocks with such compositions have been interpreted to be the products of hybridization of felsic partial melts from subducting oceanic crust with the peridotitic mantle wedge during ascent and are not primary magmas. High Mg andesites have been interpreted to be related to adakites by partial melting of asthenospheric peridotite contaminated by slab melts. The case for these petrogenetic models for adakites and high Mg andesites is best made in the Archean, when higher mantle geotherms resulted in subducting slabs potentially reaching partial melting temperatures at shallow depths before dehydration rendered the slab infusible. In the Phanerozoic these conditions were likely only met under certain special tectonic conditions, such as subduction of young (≤25-m.y.-old) oceanic crust.

Key adakitic geochemical signatures, such as low Y and Yb concentrations and high Sr/Y and La/Yb ratios, can be generated in normal asthenosphere-derived tholeiitic to calc-alkaline arc magmas by common upper plate crustal interaction and crystal fractionation processes and do not require slab melting. An assessment of several arc volcanic suites from around the world shows that most adakite-like compositions are generated in this way and do not reflect source processes. Similarly, rare adakite-like intrusive rocks associated with some porphyry Cu deposits are the evolved products of extensive crustal-level processing of calc-alkaline basalt-andesite-dacite-rhyolite series magmas. If slab melts contribute to such magmas, their geochemical signatures would have been obliterated or rendered ambiguous by subsequent extensive open-system processes. In Archean terranes, where adakitic and high Al tonalite-trondhjemite-granodiorite (TTG) magma series rocks are more common, porphyry Cu deposits are rare and, where found, are associated with normal calc-alkaline suites rather than adakites. The two different magma series are compositionally distinct in terms of several major and trace element parameters.

Common upper plate magmatic processes such as melting-assimilation-storage-homogenization (MASH) and assimilation-fractional-crystallization (AFC) affecting normal arc magmas can be demonstrated to explain the distinctive compositions of most adakite-like arc rocks, including high Mg andesites and especially those rare examples associated with porphyry Cu deposits. In contrast, slab melting can in most cases neither be proved nor disproved and is therefore unsatisfactory as a unique factor in porphyry Cu genesis.

Special Paper: Adakite-Like Rocks: Their Diverse Origins and Questionable Role in Metallogenesis

About 30% of the Earth is covered by continents, but only about 10 small kernels of these continentsae—known as Archaean cratonsae—are continental fragments formed before 2.5 Gyr ago. The Kaapvaal craton of South Africa, which formed and stabilized between 3.7 and 2.7 Gyr ago, is one of the oldest reasonably sized examples of these continental fragments. It consists of a mosaic of subdomains that have been welded together by processes similar to those of modern-day plate tectonics. The earliest subdomains may have owed their origin to the onset of efficient recycling from the Earth's hydrosphere into the mantle.

Formation of an Archaean continent

What controls chemical variation in granitic magmas

We conducted fluid-absent partial melting experiments, at 0.5 and 1.0 GPa in the temperature range 750 to 1000 °C, to investigate the influence of bulk rock Mg ? [100Mg/(Mg+Fe)] and the effects of additional TiO2 on the granulite-grade anatectic evolution of relatively magnesian metapelites and metagreywackes. In these experiments, melting began between 780 and 830 °C by the incongruent breakdown of biotite to produce quartz-saturated, granulite-facies residual mineral assemblages in equilibrium with H2O-undersaturated granitic melt. The glass (quenched melt) compositions produced in this study vary little. Generally, the glasses have compositions similar to those of many natural strongly peraluminous leucogranites. The solidus temperatures in both rock types increase with increasing Mg ?, but are unaffected by the presence or absence of a TiO2 component. At 0.5 GPa the metapelites melted at temperatures up to 50 °C lower than the equivalent metagreywackes, but at 1 GPa there was no discernible difference. This study suggests that the fluid-absent solidus has a steep positive dP/dT slope in metapelites and steep negative dP/dT slope in metagreywackes. The pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability. In TiO2-free compositions this was found to increase by 15 to 20 °C in the metapelites and by 30 to 40 °C in the metagreywackes, as a function of increasing Mg ? from 49 to 81. The presence of a TiO2 component increases the upper limit of biotite stability by ∼50 °C in the metapelites and by ∼80 °C in the metagreywackes, over that observed in the equivalent TiO2-free compositions. In consequence, in the TiO2-free samples large pulses of melt (up to 35 wt%) are produced over narrow temperature ranges (as little as 15 °C in these experiments) between 830 and 875 °C. In the TiO2-bearing samples the major pulse of melt production occurs more gradually between 830 and >900 °C.

Melt production during granulite-facies anatexis: experimental data from “primitive” metasedimentary protoliths

Experimental melt (glass) compositions indicate that most of the S-type granites of the Cape Granite Suite in South Africa have ferromagnesian contents too high to represent melts. Consequently, the composition of the more mafi c granites demands the addition of an Fe- and Mg-rich component to the magma. The compositions of the granites evolve along well-defi ned trends away from the likely melt composition for many components plotted against Mg + Fe. An increase in A/CNK, Mg#, Ca, and HREEs, as well as a decrease in K and Si, as a function of increasing Mg + Fe appears to limit the contaminant to garnet (up to 20 wt%). The rate of Ti increase, as a function of Mg + Fe increase in the granites, matches that defi ned by the stoichiometry of high-temperature biotite, but cannot be the product of accumulation of biotite (± other phases) in the magma because the chemical trends are inconsistent with this, particularly those portrayed by K and Ti as a function of Mg + Fe. This, in conjunction with the fact that no large, counterbalancing population of very leucocratic material exists in the Cape Granite Suite, suggests that the relatively mafi c granites are not the products of garnet fractional crystallization. Rather, these appear to be the result of selective entrainment of peritectic garnet and ilmenite. Thus, this work indicates that much of the compositional variation in the granites is primary, refl ecting the magma composition that ascended from the source, and is controlled by the proportion of peritectic products entrained into the melt. There is no indication of entrainment of a mineralogically diverse residuum (restite).

Selective peritectic garnet entrainment as the origin of geochemical diversity in S-type granites

Archean TTGs (tonalite-trondhjemite-granodiorite) are sodic granitoids that represent the bulk of the Archean continental crust. They are formed by fluid-absent partial melting of amphibolites. A compilation of the published data on experimental melting of amphibolites allows a mineralogical model for amphibolite melting to be derived for three different starting lithologies. A major and trace element model for melt compositions is produced using the mineralogical model. This model suggests that TTGs formed at P > 15 kbar and T between 900°C and 1100°C, corresponding to low (15°C/km) geothermal gradients that are likely to be attained only in subduction zones. Furthermore, it appears that Nb/Ta, La/Yb, Eu/Eu*, Sr, and HREE contents are intercorrelated in TTGs and are indicators of the pressure of melting. TTGs were generated over a large range of depths, from at least 10 to 25 kbar, and this is reflected in TTG compositions.

Gary Stevens

Papers

What controls chemical variation in granitic magmas

Melt production during granulite-facies anatexis: experimental data from “primitive” metasedimentary protoliths

Selective peritectic garnet entrainment as the origin of geochemical diversity in S-type granites

Experimental Constraints on TTG Petrogenesis: Implications for Archean Geodynamics

The enigmatic sources of I-type granites: The peritectic connexion