Preliminary phase equilibria of the nepheline-diopside system under variable pressures (up to 28 kb) and temperatures

The nepheline-diopside join defines the ultra-alkaline portion of the basalt tetrahedron and the bulk composition of nephelinitic rocks lie in this join. Schairer and others established that under atmospheric pressure, the join cuts through the primary phase volumes of olivine_ss, carnegieite_ss and nepheline_ss. Melilite coexists with nepheline_ss, olivine_ss and diopside_ss below 1160±10°C and olivine reacts out at low temperature. Experimental studies on seven compositions show the presence of a pseudoeutectic at Ne₇₀Di₃₀ and 1420°C, where diopside_ss, nepheline_ss and liquid are in equilibrium. Olivine and melilite do not appear in the system and the assemblage below 1225±20°C is diopside_ss+nepheline_ss.     

Anthropogenic and impact spherules: Morphological similarity and chemical distinction – A case study from India and its implications

This paper provides first report of silica-rich anthropogenic spherules of varying colour, shape, size, surface texture and chemical composition found in road-deposited sediments (RDS) of Allahabad city, Uttar Pradesh, India. Morphological details and lithophile elemental composition of the silica-rich spherules are compared to microtektites and impact spherules from India to demonstrate their striking morphological similarities and chemical variability. This study suggests the need to use spherule data carefully while assigning an impact origin to spherule-finds or spherule-bearing lithological horizons.     

The Dhala structure,Bundelkhand craton,Central India—Eroded remnant of a large Paleoproterozoic impact structure

Abstract— The newly discovered Dhala structure, Madhya Pradesh State, India, is the eroded remnant of an impact structure with an estimated present‐day apparent diameter of about 11 km. It is located in the northwestern part of the Archean Bundelkhand craton. The pre‐impact country rocks are predominantly granitoids of ?2.5 Ga age, with minor 2.0–2.15 Ga mafic intrusive rocks, and they are overlain by post‐impact sediments of the presumably >1.7 Ga Vindhyan Supergroup. Thus, the age for this impact event is currently bracketed by these two sequences. The Dhala structure is asymmetrically disposed with respect to a central elevated area (CEA) of Vindhyan sediments. The CEA is surrounded by two prominent morphological rings comprising pre‐Vindhyan arenaceous‐argillaceous and partially rudaceous metasediments and monomict granitoid breccia, respectively. There are also scattered outcrops of impact melt breccia exposed towards the inner edge of the monomict breccia zone, occurring over a nearly 6 km long trend and with a maximum outcrop width of ?170 m. Many lithic and mineral clasts within the melt breccia exhibit diagnostic shock metamorphic features, including multiple sets of planar deformation features (PDFs) in quartz and feldspar, ballen‐textured quartz, occurrences of coesite, and feldspar with checkerboard texture. In addition, various thermal alteration textures have been found in clasts of initially superheated impact melt. The impact melt breccia also contains numerous fragments composed of partially devitrified impact melt that is mixed with unshocked as well as shock deformed quartz and feldspar clasts. The chemical compositions of the impact melt rock and the regionally occurring granitoids are similar. The Ir contents of various impact melt breccia samples are close to the detection limit (1–1.5 ppb) and do not provide evidence for the presence of a meteoritic component in the melt breccia. The presence of diagnostic shock features in mineral and lithic clasts in impact melt breccia confirm Dhala as an impact structure. At 11 km, Dhala is the largest impact structure currently known in the region between the Mediterranean and southeast Asia.