ALBERT

Description: The Deccan large igneous province in India was emplaced temporally close to the Cretaceous–Palaeogene (K–Pg) boundary and is formed by tholeiitic flood basalts and less abundant alkaline rocks. Definition of the origin of Deccan magmatism and of its environmental impact relies on precise and accurate geochronological analyses. We present new 40 Ar/ 39 Ar ages from the northern sector of the province. In this area, tholeiitic and alkaline rocks were contemporaneously emplaced at 66.60 ± 0.35 to 65.25 ± 0.29 Ma in the Phenai Mata area, whereas rocks from Rajpipla and Mount Pavagadh yielded ages ranging from 66.40 ± 2.80 to 64.90 ± 0.80 Ma. The indistinguishable ages for alkaline and tholeiitic magmatism suggest that distinct mantle sources were synchronously active. The new ages are compared with previous ages, which were carefully screened and filtered and then recalculated to be comparable. The entire dataset of geochronological data does not support a time-related migration of the magmatism related to the northward Indian plate movement relative to the Reunion mantle plume. The main phase of magmatism, including the newly dated rocks from the northern Deccan, occurred at the K–Pg boundary. This suggests a causal link between the emplacement of the province and the K–Pg mass extinction. Supplementary material: Whole-rock and mineral compositions and the complete 40 Ar/ 39 Ar dataset are available at https://doi.org/10.6084/m9.figshare.c.2674441 .

Description: Lying atop the relatively motionless Antarctic plate (velocity ~6·46 mm a –1 ), the Crozet archipelago, midway between Madagascar and Antarctica, is a region of unusually shallow (1543–1756 m) and thickened oceanic crust (10–16·5 km), high geoid height, and deep low-velocity zone, which may represent the surface expression of a mantle plume. Here, new major and trace element data are presented for Quaternary alkali basalts of the subaerial eruptive stage of East Island, the most easterly and oldest island (~9 Ma) of the Crozet archipelago. Crystallization at uppermost mantle depths and phenocryst accumulation have strongly affected the parental magma compositions. Trace element patterns show a large negative K anomaly relative to Ta–La, moderate depletions in Rb and Ba with respect to Th–U, and heavy rare earth element depletions relative to light rare earth elements. These characteristics allow limits to be placed upon the composition and mineralogy of the mantle source of the magmas. The average trace element pattern of the East Island basalts can be matched by ~1·7% melting of a garnet–phlogopite-bearing peridotite source. The stability field of phlogopite restricts melting depths to lithospheric levels. The modelled source composition requires a multistage evolution in which the mantle has been depleted by melt extraction before being metasomatized by alkali-rich plume-derived melts. The depleted mantle component is inferred to be sourced from residual mantle plume remnants that stagnated at the melting locus owing to a weak lateral flow velocity inside the melting region, whose accumulation progressively forms a depleted lithospheric root above the plume core. Low-degree, alkali-rich melts are probably derived from the plume source. Such a mantle source evolution may be general to both terrestrial and extraterrestrial environments in which the lateral component velocity of the mantle flow field is extremely slow.

Description: The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, with an areal extent exceeding 10 7 km 2 . Here we document the geochemical characteristics of CAMP basalts from Triassic–Jurassic basins in northeastern USA and Nova Scotia (Canada). The CAMP rocks occur as lava flows, sills and dykes. All of our analysed samples show chemical characteristics typical of CAMP basalts with low titanium content, which include enrichment in the most incompatible elements and negative Nb anomalies. All the basalts also show enriched Sr–Nd–Pb initial ( t = 201 Ma) isotopic compositions ( 206 Pb/ 204 Pb ini. = 18·155–18·691, 207 Pb/ 204 Pb ini. = 15·616–15·668, 208 Pb/ 204 Pb ini. = 38·160–38·616, 143 Nd/ 144 Nd ini. = 0·512169–0·512499). On the basis of stratigraphy, rare earth element (REE) chemistry and Sr–Nd–Pb isotope composition, three chemical groups are defined. The Hook Mountain group, with the lowest La/Yb ratios, initial 206 Pb/ 204 Pb ini. 〉18·5 and 143 Nd/ 144 Nd ini. 〉 0·51238, comprises all the lastest and upper stratigraphic units. The Preakness group, with intermediate La/Yb ratios, 206 Pb/ 204 Pb ini. 〉 18·5 and 0·51233 〉 143 Nd/ 144 Nd ini. 〉 0·51225, comprises the intermediate units. The Orange Mountain group has the highest La/Yb ratios and 143 Nd/ 144 Nd ini. 〈 0·51235 and involves all the earliest and stratigraphically lowest units, including the entire North Mountain basalts from Nova Scotia. In this last group, three sub-groups may be distinguished: the Rapidan sill, which has 206 Pb/ 204 Pb ini. higher than 18·5, the Shelburne sub-group, which has 143 Nd/ 144 Nd ini. 〈 0·51225, and the remaining Orange Mt samples. With the exception of one sample, the Eastern North America (ENA) CAMP basalts display initial 187 Os/ 188 Os ratios in the range of mantle-derived magmas (〈0·15). Simple modelling shows that the composition of the ENA CAMP basalts cannot plausibly be explained solely by crustal contamination of oceanic island basalt (OIB), mid-ocean ridge basalt (MORB) or oceanic plateau basalt (OPB) magmas. Mixing of such magma compositions with sub-continental lithospheric mantle (SCLM)-derived melts followed by crustal contamination, by either assimilation–fractional crystallization (AFC) or assimilation through turbulent ascent (ATA) processes is somewhat more successful. However, this latter scenario does not reproduce the REE and isotopic composition of the ENA CAMP in a fully satisfactory manner. Alternatively, we propose a model in which asthenospheric mantle overlying a subducted slab (i.e. mantle wedge) was enriched during Cambrian to Devonian subduction by sedimentary material, isotopically equivalent to Proterozoic–Lower Paleozoic crustal rocks. Subsequently, after subduction ceased, the isotopic composition of this mantle evolved by radioactive decay for another 170 Myr until the CAMP magmatic event. Varying amounts and compositions of the incorporated sedimentary component coupled with radiogenic ingrowth over time can account for the main geochemical characteristics of the ENA CAMP (enriched incompatible element patterns, negative Nb anomalies, enriched Sr–Nd–Pb isotopic composition) and the differences between the three chemical groups.