Robert P. Rapp

Growth of early continental crust by partial melting of eclogite

The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic (‘grey gneiss’) and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of ‘average’ Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, ‘arc-like’ crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.

Garnet fractionation in a hydrous magma ocean and the origin of Al-depleted komatiites: melting experiments of hydrous pyrolite with REEs at high pressure

Abstract Melting experiments of a representative mantle composition doped with some key trace elements were conducted under hydrous conditions at pressures to 20 GPa. The results demonstrate that only ∼5–25% of garnet fractionation and relatively low pressures ( 3.5 Ga) in the solidifying hydrous magma ocean. This suggests that the Archean mantle was much cooler than so far believed from the origin of the komatiites based on anhydrous experiments.

Partitioning of rare earth elements between CaSiO3 perovskite and coexisting phases: constraints on the formation of CaSiO3 inclusions in diamonds

Minerals with CaSiO3 composition were found as inclusions in diamonds, and are considered to be originally of perovskite structure. To constrain their genesis and consequently the extent of circulation of mantle material, rare earth element (REE) partitioning between CaSiO3 perovskite and coexisting majoritic garnet (20 GPa, 1520°C) or MgSiO3 perovskite (25 GPa, 1600°C) was determined by combining the technologies of high-pressure synthesis and trace-element analysis using ion probe. It is consistent with previous experiments and confirms that CaSiO3 perovskite is the main REE depository, especially of the light-REE. KCe(CaPv/Gt) is ∼1900, and decreases gradually to 18.5 of KYb(CaPv/Gt). For CaPv/MgPv, it decreases gradually from 57.7 of KCe to about 10 of KYb. Estimated REE concentrations in the source lithology of the CaSiO3 inclusions according to these partitioning coefficients, either peridotitic or eclogitic paragenesis, show strong enrichment in light-REE (e.g. Cen 18–163), very different from normal mantle peridotite and subducted oceanic crust. It is proposed that interaction with carbonatic melt in the deep mantle may have played an important role in the formation of these CaSiO3 inclusions in diamonds, as well as in their ascending transportation.

Reconnaissance-style EPMA chemical U–Th–Pb dating of uraninite

EPMA chemical U-Th-Pb uraninite analysis has been used to constrain the age of the granite-related, Rössing South uranium prospect in Namibia and the Kintyre unconformity-related uranium deposit in Western Australia. Uraninite from the Rössing South prospect has an age of 496.1 ± 4.1 Ma, which is similar to the age of other uranium deposits in the region at Rössing and Goanikontes. Uraninite grains analysed from the Kintyre deposit have an age of 837 +35/-31 Ma suggesting that the uranium mineralisation occurred during or after the latest period of sedimentation in the Yeneena Basin during the ca 850 to ca 800 Ma Miles Orogeny.

Discussion: “Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust” by Wang et al. 2016 (Contributions to Mineralogy and Petrology) 171:62

Wang et al. (Contrib Mineral Petrol 171:62, 2016a) present data on composition of xenolith from Southern Tibet and conclude that ulrapotassic melts from the region formed by melting mantle, and complex interaction with a crustal component. In this discussion we demonstrate that numerous observations presented by Wang et al. (2016a) can be explained by partial melting of crust followed by interaction between that melt and the mantle. We show that this model can explain the variability of magmas in such suits without evoking occurrence of coincidental, unrelated events. Moreover we demonstrate that our model of a crustal origin of the proto-shoshonite melts is now supported by independent lines of evidence such as geochemistry of restites after high- and ultrahigh- pressure melting and melt inclusion studies.