On the relative abundances of Cavansite and Pentagonite

Cavansite is a visually stunning blue vanadosilicate mineral with limited occurrences worldwide, whereas Pentagonite is a closely related dimorph with similar physical and chemical properties, yet is extremely rare. The reasons behind Pentagonite's exceptional rarity remain largely unknown. In this study, we utilize density functional theory (DFT) to investigate the electronic structures of Cavansite and Pentagonite at ground state and finite pressures. We then employ the Boltzmann probability model to construct a comprehensive phase diagram that reveals the abundance of each species across a wide range of pressure and temperature conditions. Our analysis reveals the key factors that contribute to the relative scarcity of Pentagonite, including differences in structural arrangement and electronic configurations between the two minerals. Specifically, because of the peculiar arrangements of SiO4 polyhedra, Cavansite forms a compact structure (about 2.7% less in volume) resulting in lower energy. We also show that at a temperature of about 600K only about 1% Pentagonite can be formed. This probability is only slightly enhanced within a pressure range of up to about 3GPa. We also find that vanadium induces a highly localized state in both of these otherwise large band gap insulators resulting in extremely weak magnetic phase that is unlikely to be observed at any reasonable finite temperature. Finally, our dehydration studies reveal that water molecules are loosely bound inside the microporous crystals of Cavansite and Pentagonite, suggesting potential applications of these minerals in various technological fields

Morphology of rubbly pahoehoe (simple) flows from the Deccan Volcanic Province: implications for style of emplacement

Lava flows with preserved bases and brecciated upper crusts constitute a morphological type that differs in character from typical pahoehoe and a'a: such flows have been reported from many provinces around the world. Previous studies had referred to these flows informally as ‘pahoehoe flows with rubbly tops’, ‘broken-top pahoehoe’ and ‘rubbly pahoehoe’. Recent studies have formally applied the latter term to describe parts of the well-studied Laki flow in Iceland as well as flows from the Columbia River Basalt province. Rubbly pahoehoe flows are abundant in the upper stratigraphic formations of the Deccan Volcanic Province (DVP), and are more commonly known as simple flows. This study presents detailed observations of such flows from various parts of the DVP and discusses their implications for understanding flow emplacement. These flows, which appear to be single units at the outcrop-scale, are generally much thicker and significantly more extensive than individual pahoehoe lobes that dominate the lower formations of the Deccan stratigraphy. They are characterised by preserved, gently undulating tachylitic bases but variably disrupted crustal zones that grade into flow-top breccias. The breccias are constituted of highly vesicular and oxidised fragments of varying sizes that appear to have been derived from previously formed pahoehoe crusts. Previous work has indicated that the morphology of these flows might be related to initial inflation, accompanied by rapid volatile exsolution and an increase in effusion rate and/or viscosity with time. This agrees reasonably well with the qualitative and quantitative models of emplacement developed for the Laki flow. The abundance of such flows in the upper formations of the Deccan stratigraphy clearly hints at a significant shift in the nature of the Deccan eruptions; this could be indicative of higher eruption rates during this period. This, in turn, raises the possibility of hazardous impact on the climate during the eruption of these flows, which is also discussed in the pape

Geodynamic evolution of the Tethyan lithosphere as recorded in the Spontang Ophiolite, South Ladakh ophiolites (NW Himalaya, India)

The authors thank the Head, Department of Geology and Inter-disciplinary School of Science (IDSS) , SPPU for providing necessary facilities. MKJ acknowledges the financial support received from Science Education and Research Board (SERB) and Department of Science and Technology (DST) by way of its Young Scientist Scheme (Ref. No. SR/FTP/ES-2/2014) Women's scientist scheme (Ref No. SR/WOS-A/EA-14/2017) . S. Mounic and A. Marquet respec-tively from Toulouse TIMS and ICPMS facilities. RT acknowledges a "Juan de la Cierva-formacion" Fellowship (FJC2018-036729) granted by the Spanish Ministry of Science and Innovation and co-funded by the European Development Fund and the European Social Fund."The Spontang Ophiolite complex represents the most complete ophiolite sequence amongst the South Ladakh ophiolites and comprises mantle rocks (depleted harzburgites, dunites and minor lherzolites) as well as crustal rocks (basalt, isotropic gabbros, layered gabbros etc.). In the present study, detailed geochemistry (whole rock as well as mineral chemistry) and Sr-Nd isotopic analyses of thirty-six ultramaficmafic samples have been attempted to constraint the evolution and petrogenetic history of the Tethyan oceanic crust. Major, trace-element and REE patterns of the peridotites and their minerals indicate that the lherzolites experienced lower degrees of partial melting resembling abyssal peridotites (at higher temperatures, TREE = 1216 C) than the harzburgites (6%–8% versus 15%–17%). Elevated eNd(t) and variable 87Sr/86Sr(t) ratios along with REE patterns suggest that the Spontang mafic rocks display N-MORB affinity with negligible participation of oceanic sediments in their genesis are originated from a depleted upper mantle with little contribution from subduction-related fluids. MORB-type Neotethyan oceanic crust is associated with the earliest phase of subduction (of older Jurassic age) through which a younger intra-oceanic island arc (Spong arc) subsequently developed. Harzburgites REE display typical U-shaped patterns, suggesting that these rocks have been metasomatized by LREE-enriched fluids. On the other side, mafic rocks are characterized by heterogeneous (Nb/La)PM and (Hf/Sm)PM and relatively homogeneous eNd(t), indicating interaction of subduction-related melts with the upper mantle during the initiation of subduction, in Early Cretaceous times.Science Education and Research Board (SERB)Department of Science & Technology (India) SR/FTP/ES-2/2014Women's scientist scheme SR/WOS-A/EA-14/2017"Juan de la Cierva-formacion" Fellowship FJC2018-036729Spanish GovernmentEuropean Social Fund (ESF)European Development Fun

Basaltic thermal and material inputs in rhyolite petrogenesis: the Chhapariyali rhyolite dyke, Saurashtra, Deccan Traps

The Saurashtra peninsula in the northwestern Deccan Traps continental flood basalt province, India, contains notable concentrations of rhyolitic rocks. The Chhapariyali rhyolite dyke, part of the compositionally diverse Southeastern Saurashtra dyke swarm, intrudes basaltic lava flows. It shows vitrophyric portions, basaltic magma enclaves, gabbroic mineral assemblages (plagioclase + clinopyroxene ± Fe-Ti oxide aggregates), spectacular quench textures, and intense spherulitisation. Thermobarometric calculations on equilibrium mineral–wholerock (or glass) pairs indicate clinopyroxene crystallisation at 1120–890 ◦C overlapping with plagioclase crystallisation at 948–932 ◦C, and a pressure range of 3.6–0.1 kbar, indicating crystallisation during magma storage or ascent in the upper crust. Petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd isotopic) data suggest advanced fractional crystallisation of a mafic magma with considerable assimilation of ancient granitic crust, or anatexis of such crust, due to heating by a basaltic magma chamber. In either scenario, abundant gabbroic cumulates were left by the crystallising basaltic magmas. A new basalt magma recharging the chamber entrained the gabbroic crystal mush and formed enclaves by mingling with the rhyolite magma stored above. Continued basaltic recharge pushed the enclave-bearing, gabbroic cargo-laden rhyolite magma out of the chamber and into the basaltic lava flows via a dyke. Strong supercooling within cold basaltic rock, possibly aided by meteoric water ingress, led to the development of spectacular quench textures in the crystallising rhyolite, whereas extensive low-temperature alteration produced intense spherulitisation. This case study of the Chhapariyali dyke underscores significant thermal and material inputs from basalt in rhyolite petrogenesis

Cooling history and emplacement dynamics within rubbly lava flows, southern Deccan Traps: insights from textural variations and crystal size distributions

We analyse two representative rubbly pāhoehoe lavas (F3 and F5) from drill cores at Tural-Rajwadi, southwest of Koyna, in the southern Deccan Traps. Low vesicle deformation (0.1 to 0.4) indicates that both lavas ultimately cooled under a low-stress regime. The crystal size distributions (CSDs) of most samples from F5 (especially those from within the core) are not linear but instead show kinks. These kinks are attributed to a rise in plagioclase nucleation due to degassing following the brecciation of the crust. Since it is difficult to constrain cooling time for ancient lava flows, we used the products of nucleation rates (Jt, 1.64 × 10–8 to 1.45 × 10–5 μm−3) and growth rates (Gt, 2.1 to 156 μm) with time. When compared with natural analogues as well as experimental results for basalt crystallisation, these values suggest a much faster lava cooling rate (~ 1 to 7℃/hr) than a conductive cooling model (≤ 0.1 ℃/hr). The CSDs for F3 fan with depth suggesting that the lava flow might represent local accumulation (ponding?) in a transitional lava flow field. CSDs for F5 show little variation with depth, with the exception of kinks for samples from the lower crust and core. The relatively higher number density of plagioclase microcrysts in our rubbly pāhoehoe (F5) and their CSD patterns are similar to those measured for transitional lavas from Hawaii. The vesicle data and CSDs indicate that brittle deformation was the primary mode of transition within these lavas. Identifying occurrence of thick ponded lavas within vertical stacks of rubbly pāhoehoe flows in the upper stratigraphic levels of the Deccan Traps are critically important as they demonstrate complex cooling styles, crystallisation histories, and emplacement dynamics. Transitional lavas such as rubbly pāhoehoe are important components of large CFB provinces such as the Deccan Traps and constitute nearly 46 to 85% of all lava types. Modelling of continental flood basalt provinces should therefore account for these diversities within lavas, and any oversimplified version using end-member morphotypes is unrealistic and untenable

Mid-Cretaceous lamproite from the Kutch region, Gujarat, India : genesis and tectonic implications

The Phanerozoic tectonic evolution of the Kutch region (NW India) has been attributed to rifting processes. In situ U-Pb perovskite dates (all data = 124 ± 4 Ma, MSWD = 42) obtained on three samples of a newly discovered lamproite dyke emplaced in the Kutch rift setting identify a previously unknown and relatively young (Mid-Cretaceous) magmatic event in this part of India. The dyke was emplaced into the Katrol Formation of Upper Jurassic to very early Cretaceous age. The presence of xenocrystic garnet and fractionated REE distribution patterns (low HREE/LREE) in the lamproite, along with high contents of transition metals and low Al₂O₃, suggest that it was derived from the garnet stability field in the mantle. Extreme impoverishment in HREE and HFSE suggest a mantle source region that has experienced both depletion and later enrichment. In situ analysis of Nd isotopes in perovskite of three samples gives a mean εNd = 0.4 ± 1 and a TDM model age of 598 ± 64 Ma, while in situ Sr-isotope analysis gives a mean ⁸⁷Sr/⁸⁶Sr = 0.70388 ± 2 (2SE), corresponding to εSr = 8.6 ± 03; both datasets suggest mixing between lithospheric-mantle and depleted-mantle components. The fault-controlled emplacement of this body highlights the role of deep-penetrating fault systems in providing a pathway and initiating decompressional melting of the mantle source rocks. The lamproite represents an earlier phase of rift related magmatism, different from the emplacement of the melanephelinite-basanites and basalts that dominate the igneous activity related to the Deccan volcanism in the Kutch mainland.15 page(s