Experimental evidence for the preservation of U-Pb isotope ratios in mantle-recycled crustal zircon grains

Zircon of crustal origin found in mantle-derived rocks is of great interest because of the information it may provide about crust recycling and mantle dynamics. Consideration of this requires understanding of how mantle temperatures, notably higher than zircon crystallization temperatures, affected the recycled zircon grains, particularly their isotopic clocks. Since Pb2+ diffuses faster than U4+ and Th+4, it is generally believed that recycled zircon grains lose all radiogenic Pb after a few million years, thus limiting the time range over which they can be detected. Nonetheless, this might not be the case for zircon included in mantle minerals with low Pb2+ diffusivity and partitioning such as olivine and orthopyroxene because these may act as zircon sealants. Annealing experiments with natural zircon embedded in cristobalite (an effective zircon sealant) show that zircon grains do not lose Pb to their surroundings, although they may lose some Pb to molten inclusions. Diffusion tends to homogenize the Pb concentration in each grain changing the U-Pb and Th-Pb isotope ratios proportionally to the initial 206Pb, 207Pb and 208Pb concentration gradients (no gradient-no change) but in most cases the original age is still recognizable. It seems, therefore, that recycled crustal zircon grains can be detected, and even accurately dated, no matter how long they have dwelled in the mantle.This paper has been financed by the Spanish Grants CGL2013-40785-P and CGL2017-84469-P

Tailing dumps of the tyrnyauz tungsten–molybdenum mining and processing complex: Current state and outlooks

The Tyrnyauz W–Mo deposit was developed by opencast and underground mines until 2003. The assets of the Tyrnyauz Tungsten–Molybdenum Mining and Processing Complex (TTMC) include two tailing dumps: Tailing 2 (housed on the left-hand side of the Baksan River valley, 2 km south of the settlement of Bylym) and Supertailing 2 (a superdump housed in the valley of the Gizhgit River, a left-hand tributary of the Baksan River).The height of the rock-filling dam of Supertailing 1 reaches 160 m. A pond on its top protects the ecosystems from the wind erosion of the dumped industrial wastes. The protecting pond is equipped with a tunnel drainage system, which is used to discharge excess water to the Baksan River, to a certain technological water level in the pond.Shallow-focus earthquakes (with M = 5–7) and/or debris and mud flows are able to destroy the dam, and this will result in the transfer of toxic compounds to the Baksan River and water-bearing Quaternary alluvial rocks in the foredeep, where the river flows into the plain.The following soil contamination sources were identified: (1) Winds continuously blowing along the Baksan valley erode fines where the recultivation layer of Tailing 2 is disturbed and in the beach parts of Supertailing 1; the extent of this contamination varies from hundreds of meters to a few kilometers; (2) Massive blasting operations at the opencast mines before 2003 resulted in atmospheric emissions of dust clouds with ore minerals; this pollutions extends for dozens of kilometers (along the valleys of the Baksan River and its tributaries).The most ecologically hazardous emissions are those of quartz dust and dust with heavy-metal minerals, including sulfides. A method for utilizing TTMC wastes was engineered and patented. The results provide a basis for designing measures aimed at decreasing the adverse load on the ecosystems in the Elbrus area, which is highly attractive to tourists.</p