Widespread reworking of Hadean-to-Eoarchean continents during Earth’s thermal peak

The nature and evolution of Earth’s crust during the Hadean and Eoarchean is largely unknown owing to a paucity of material preserved from this period. However, clues may be found in the chemical composition of refractory minerals that initially grew in primordial material but were subsequently incorporated into younger rocks and sediment during lithospheric reworking. Here we report Hf isotopic data in 3.9 to 1.8 billion year old detrital zircon from modern stream sediment samples from West Greenland, which document successive reworking of felsic Hadean-to-Eoarchean crust during subsequent periods of magmatism. Combined with global zircon Hf data, we show a planetary shift towards, on average, more juvenile Hf values 3.2 to 3.0 billion years ago. This crustal rejuvenation was coincident with peak mantle potential temperatures that imply greater degrees of mantle melting and injection of hot mafic-ultramafic magmas into older Hadean-to-Eoarchean felsic crust at this time. Given the repeated recognition of felsic Hadean-to-Eoarchean diluted signatures, ancient crust appears to have acted as buoyant life-rafts with enhanced preservation-potential that facilitated later rapid crustal growth during the Meso-and-Neoarchean.

Fort-Dauphin beach sands, south Madagascar: natural radionuclides and mineralogical studies

The Fort-Dauphin beach sand placer occurs as black sands on the East-South of Madagascar. The placer contributes 2/3 of the total heavy mineral resources of this country. The major minerals of the deposit are monazite, zircon, quartz, garnet, spinel, sillimanite as non-refractory minerals; ilmenite, anatase, rutile, titanite, leucoxene, pseudorutile and as a refractory one. The average concentration of the ilmenite, monazite, zircon and other minerals is 66.72%, 2.3%, 2.8%, and 28.18% respectively. Ilmenite contains 63 wt.% of TiO2, Zircon - 44 wt.% of ZrO2, Monazite contains 53 wt.% of oxide rare earth elements (REE) and up to 2 wt.% of UO2 and 9 wt.% of ThO2. The total REE in the studied samples was observed high concentration up to 16000ppm and a high ratio of Σ LREE/Σ HREE>31. The principal natural radionuclide in this placer is 232Th with the concentration of 232Th from 2710 to 6000 ppm, 3620 ppm on average while for the 238U from 124 to 340 ppm, 237 ppm on average which are higher than the average of their in Earth’s crust 360 and 70 times respectively.

Fort-Dauphin beach sands, south Madagascar: natural radionuclides and mineralogical studies

The Fort-Dauphin beach sand placer occurs as black sands on the East-South of Madagascar. The placer contributes 2/3 of the total heavy mineral resources of this country. The major minerals of the deposit are monazite, zircon, quartz, garnet, spinel, sillimanite as non-refractory minerals; ilmenite, anatase, rutile, titanite, leucoxene, pseudorutile and as a refractory one. The average concentration of the ilmenite, monazite, zircon and other minerals is 66.72%, 2.3%, 2.8%, and 28.18% respectively. Ilmenite contains 63 wt.% of TiO2, Zircon - 44 wt.% of ZrO2, Monazite contains 53 wt.% of oxide rare earth elements (REE) and up to 2 wt.% of UO2 and 9 wt.% of ThO2. The total REE in the studied samples was observed high concentration up to 16000ppm and a high ratio of Σ LREE/Σ HREE>31. The principal natural radionuclide in this placer is 232Th with the concentration of 232Th from 2710 to 6000 ppm, 3620 ppm on average while for the 238U from 124 to 340 ppm, 237 ppm on average which are higher than the average of their in Earth’s crust 360 and 70 times respectively.

Effect of base type on the dissolution of quartz with triisopropanolamine in minerals with silver occluded in quartz

The processing of refractory minerals of gold and / or silver occluded in quartz in sizes less than 1 μm is not viable, due to the chemical inertness of the silica. The treatment of these minerals is usually carried out by reduction of particle size by fine grinding that allows the release of the occluded particles. As an alternative to processing these minerals, has proposed the partial dissolution of the silica with diols in basic medium; in these studies partial dissolution of the matrix has been observed in 25% with triisopropanolamine and 3% mol KOH. The present work shows the effect of the type of base (organic-amines) in the dissolution of the matrix, observing a greater dissolution with the basic character of these and with the chelating effect of the same ones. Et3N was the weak base that showed the highest dissolution of the matrix, observing a dissolution rate (0.67 mmolmin-1) 2.5 times lower than that observed with KOH (1.66 mmolmin-1), similarly the observed effective diffusion coefficient was of an order of magnitude lower than that observed with KOH.