Search for: All records

Deccan Traps flood basalt volcanism affected ecosystems spanning the end‐Cretaceous mass extinction, with the most significant environmental effects hypothesized to be a consequence of the largest eruptions. The Rajahmundry Traps are the farthest exposures (~1,000 km) of Deccan basalt from the putative eruptive centers in the Western Ghats and hence represent some of the largest volume Deccan eruptions. Although the three subaerial Rajahmundry lava flows have been geochemically correlated to the Wai Subgroup of the Deccan Traps, poor precision associated with previous radioisotopic age constraints has prevented detailed comparison with potential climate effects. In this study, we use new⁴⁰Ar/³⁹Ar dates, paleomagnetic and volcanological analyses, and biostratigraphic constraints for the Rajahmundry lava flows to ascertain the timing and style of their emplacement. We find that the lower and middle flows (65.92 ± 0.25 and 65.67 ± 0.08 Ma, ±1σsystematic uncertainty) were erupted within magnetochron C29r and were a part of the Ambenali Formation of the Deccan Traps. By contrast, the uppermost flow (65.27 ± 0.08 Ma) was erupted in C29n as part of the Mahabaleshwar Formation. Given these age constraints, the Rajahmundry flows were not involved in the end‐Cretaceous extinction as previously hypothesized. To determine whether the emplacement of the Rajahmundry flows could have affected global climate, we estimated their eruptive CO₂release and corresponding climate change using scalings from the LOSCAR carbon cycle model. We find that the eruptive gas emissions of these flows were insufficient to directly cause multi‐degree warming; hence, a causal relationship with significant climate warming requires additional Earth system feedbacks.

 

Refinements of the geological timescale driven by the increasing precision and accuracy of radiometric dating have revealed an apparent correlation between large igneous provinces (LIPs) and intervals of Phanerozoic faunal turnover that has been much discussed at a qualitative level. However, the extent to which such correlations are likely to occur by chance has yet to be quantitatively tested, and other kill mechanisms have been suggested for many mass extinctions. Here, we show that the degree of temporal correlation between continental LIPs and faunal turnover in the Phanerozoic is unlikely to occur by chance, suggesting a causal relationship linking extinctions and continental flood basalts. The relationship is stronger for LIPs with higher estimated eruptive rates and for stage boundaries with higher extinction magnitudes. This suggests LIP magma degassing as a primary kill mechanism for mass extinctions and other intervals of faunal turnover, which may be related to CO 2 ,   SO 2 , Cl, and F release. Our results suggest continental LIPs as a major, direct driver of extinctions throughout the Phanerozoic. 

Abstract We investigate the shallow plumbing system of the Deccan Traps Large Igneous Province using rock and mineral data from Giant Plagioclase Basalt (GPB) lava flows from around the entire province, but with a focus on the Saurashtra Peninsula, the Malwa Plateau, and the base and top of the Western Ghats (WG) lava pile. GPB lavas in the WG typically occur at the transition between chemically distinct basalt formations. Most GPB samples are evolved basalts, with high Fe and Ti contents, and show major and trace elements and Sr-Nd-Pb isotopic compositions generally similar to those of previously studied Deccan basalts. Major element modeling suggests that high-Fe, evolved melts typical of GPB basalts may derive from less evolved Deccan basalts by low-pressure fractional crystallization in a generally dry magmatic plumbing system. The basalts are strongly porphyritic, with 6–25% of mm- to cm-sized plagioclase megacrysts, frequently occurring as crystal clots, plus relatively rare olivine and clinopyroxene. The plagioclase crystals are mostly labradoritic, but some show bytownitic cores (general range of anorthite mol%: 78–55). A common feature is a strong Fe enrichment at the plagioclase rims, indicating interaction with an Fe-rich melt similar to that represented by the matrix compositions (FeOt up to 16–17 wt%). Plagioclase minor and trace elements and Sr isotopic compositions analyzed by laser ablation inductively coupled plasma mass spectrometry show evidence of a hybrid and magma mixing origin. In particular, several plagioclase crystals show variable 87Sr/86Sri, which only partially overlaps with the 87Sr/86Sri of the surrounding matrix. Diffusion modeling suggests residence times of decades to centuries for most plagioclase megacrysts. Notably, some plagioclase crystal clots show textural evidence of deformation as recorded by electron back-scatter diffraction analyses and chemical maps, which suggest that the plagioclase megacrysts were deformed in a crystal-rich environment in the presence of melt. We interpret the plagioclase megacrysts as remnants of a crystal mush originally formed in the shallow plumbing system of the Deccan basalts. In this environment, plagioclase acquired a zoned composition due to the arrival of chemically distinct basaltic magmas. Prior to eruption, a rapidly rising but dense Fe-rich magma was capable of disrupting the shallow level crystal mush, remobilizing part of it and carrying a cargo of buoyant plagioclase megacrysts. Our findings suggest that basaltic magmas from the Deccan Traps, and possibly from LIPs in general, are produced within complex transcrustal magmatic plumbing systems with widespread crystal mushes developed in the shallow crust. 

A 2 to 4 °C warming episode, known as the Latest Maastrichtian warming event (LMWE), preceded the Cretaceous–Paleogene boundary (KPB) mass extinction at 66.05 ± 0.08 Ma and has been linked with the onset of voluminous Deccan Traps volcanism. Here, we use direct measurements of melt-inclusion CO₂concentrations and trace-element proxies for CO₂to test the hypothesis that early Deccan magmatism triggered this warming interval. We report CO₂concentrations from NanoSIMS and Raman spectroscopic analyses of melt-inclusion glass and vapor bubbles hosted in magnesian olivines from pre-KPB Deccan primitive basalts. Reconstructed melt-inclusion CO₂concentrations range up to 0.23 to 1.2 wt% CO₂for lavas from the Saurashtra Peninsula and the Thakurvadi Formation in the Western Ghats region. Trace-element proxies for CO₂concentration (Ba and Nb) yield estimates of initial melt concentrations of 0.4 to 1.3 wt% CO₂prior to degassing. Our data imply carbon saturation and degassing of Deccan magmas initiated at high pressures near the Moho or in the lower crust. Furthermore, we find that the earliest Deccan magmas were more CO₂rich, which we hypothesize facilitated more efficient flushing and outgassing from intrusive magmas. Based on carbon cycle modeling and estimates of preserved lava volumes for pre-KPB lavas, we find that volcanic CO₂outgassing alone remains insufficient to account for the magnitude of the observed latest Maastrichtian warming. However, accounting for intrusive outgassing can reconcile early carbon-rich Deccan Traps outgassing with observed changes in climate and atmospheric pCO₂.

 

Late Cretaceous records of environmental change suggest that Deccan Traps (DT) volcanism contributed to the Cretaceous-Paleogene boundary (KPB) ecosystem crisis. However, testing this hypothesis requires identification of the KPB in the DT. We constrain the location of the KPB with high-precision argon-40/argon-39 data to be coincident with changes in the magmatic plumbing system. We also found that the DT did not erupt in three discrete large pulses and that >90% of DT volume erupted in <1 million years, with ~75% emplaced post-KPB. Late Cretaceous records of climate change coincide temporally with the eruption of the smallest DT phases, suggesting that either the release of climate-modifying gases is not directly related to eruptive volume or DT volcanism was not the source of Late Cretaceous climate change.