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1. The Magmatic Architecture of Continental Flood Basalts: 2. A New Conceptual Model

   https://doi.org/10.1029/2021JB021807  

   Mittal, Tushar ; Richards, Mark A. ( December 2021 , Journal of Geophysical Research: Solid Earth)

   Continental flood basalts intruded and erupted millions of km³of magma over ∼1–5 Ma. Previous work proposed the presence of large (–10⁶ km³) crustal magma reservoirs to feed these eruptions. However, in Paper I, we illustrated that this model is inconsistent with observations, by combining eruptive rate constraints with geochemical and geophysical observations from the Deccan Traps and other Continental flood basalt provinces (CFBs). Here, we use a new mechanical magma reservoir model to calculate the variation of eruptive fluxes (km³/year) and volumes for different magmatic architectures. We find that a single magma reservoir cannot explain the eruptive rate and duration constraints for CFBs. Using a 1D thermal model and characteristic timescales for magma reservoirs, we conclude that CFB eruptions were likely fed by a number of interconnected small‐medium (∼10²–10³ km³) magma reservoirs. It is unlikely that each individual magma reservoir participated in every eruption, thus permitting the occasional formation of large xenocrysts (e.g., megacrystic plagioclase). This magmatic architecture permits (a) large volume eruptive episodes with 10–100s of years duration, and (b) relatively short time‐periods separating eruptive episodes (1000s of years) since multiple mechanisms can trigger eruptions (via magma recharge or volatile exsolution, as opposed to long term (10⁵–10⁶ year) accumulation of buoyancy overpressure), and (c) lack of large upper‐crustal intrusive bodies in various geophysical datasets. Our new proposed magmatic architecture has significant implications for the tempo of CFB volatile release (CO₂and SO₂), potentially helping explain the pre‐K‐Pg warming associated with Deccan Traps.

    

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2. Reconciling early Deccan Traps CO 2 outgassing and pre-KPB global climate

   https://doi.org/10.1073/pnas.2007797118  

   Hernandez Nava, Andres ; Black, Benjamin A. ; Gibson, Sally A. ; Bodnar, Robert J. ; Renne, Paul R. ; Vanderkluysen, Loÿc ( March 2021 , Proceedings of the National Academy of Sciences)

   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.

    

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3. The Magmatic Architecture of Continental Flood Basalts I: Observations From the Deccan Traps

   https://doi.org/10.1029/2021JB021808  

   Mittal, Tushar ; Richards, Mark A. ; Fendley, Isabel M. ( December 2021 , Journal of Geophysical Research: Solid Earth)

   Flood basalts are some of the largest magmatic events in Earth history, with intrusion and eruption of millions of km³of basaltic magma over a short time period (∼1–5 Ma). A typical continental flood basalt (CFB) is emplaced in hundreds of individual eruptive episodes lasting decades to centuries with lava flow volumes of 10³–10⁴ km³. These large volumes have logically led to CFB models invoking large magma reservoirs (–10⁵ km³) within the crust or at Moho depth. Since there are currently no active CFB provinces, we must rely on observations of past CFBs with varying degrees of surface exposure to develop and test models. In the last few decades, significant improvements in geochronological, geochemical, paleomagnetic, volcanological, and paleo‐proxy measurements have provided high‐resolution constraints on CFB eruptive tempo ‐ the volume, duration, and frequency of individual eruptive episodes. Using the well‐studied Deccan Traps as an archetype for CFB systems, we compile multiple lines of evidence–geochronology, eruption tempo, dike spatial distribution, intrusive‐extrusive ratio, geochemical variations, and volcanological observations–to assess the viability of previous models. We find that the presence of just a few large crustal magma reservoirs is inconsistent with these constraints. Although observations from the Deccan Traps primarily motivate our model, we discuss constraints from other CFBs to illustrate that this conclusion may be broadly applicable for CFB magmatic systems in general.

    

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4. The age of Naskal, the type locality of India’s first known Cretaceous mammal.

   https://doi.org/10.1130/abs/2021AM-366525  

   Tobin, T. S. ; Wilson Mantilla, G. P. ; Renne, P. R. ; Samant, B. ; Mohabey, D. ; Dhobale, A. ; Tholt, A. ; Widdowson, M. ; Wilson Mantilla, J. A. ( January 2021 , Geological Society of America Abstracts with Programs)

   null (Ed.)

   The first Cretaceous mammals described from India were recovered from the Naskal locality, located near the village of Naskal in the state of Telangana. The Naskal locality is located on the eastern edge of the Deccan Traps Volcanic Province (DTVP), where it is preserved between two basalt flows. Naskal and similarly preserved sites are “intertrappean” in position and are distinguished from “infratrappean” sedimentary exposures, which are positionally below the locally lowest basalt flow. Historically, this field-based designation has been used as a proxy for relative age assignments, with intertrappean sites generally considered to be of similar age to each other, but collectively younger than infratrappean (Lameta Formation) sites. However, the DTVP flow stratigraphy is complex, so this age proxy can be incomplete and misleading. Moreover, because the DTVP eruptions spanned the Cretaceous-Paleogene boundary (KPB), it is often unknown whether intertrappean sites, including Naskal, are Cretaceous or Paleogene in age. Naskal accounts for nearly half of published mammal records from DVTP-associated sediments, as well as a host of other microfossils. The age of the Naskal locality takes on singular importance in the context of mammalian evolution in India and the effects of the end-Cretaceous mass extinction and subsequent evolutionary radiation. Here we present evidence from 40Ar/39Ar geochronology, magnetostratigraphy, and chemostratigraphy of the over- and underlying basalt flows to narrow the permissible age of the sediments at the Naskal locality. In conjunction with palynostratigraphy and vertebrate biostratigraphy, this site can be confidently restricted to a <100 ka interval spanning the KPB. The most probable 40Ar/39Ar age is latest Cretaceous, but an earliest Paleogene age cannot be ruled out. We explore the implications of this age assignment, and additionally describe two new mammal species from the same genus from Naskal. 

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5. Expedition 391 summary

   Sager, W. ; Hoernle, K. ; Höfig, T.W. ; Avery, A.J. ; Bhutani, R. ; Buchs, D.M. ; Carvallo, C.A. ; Class, C. ; Dai, Y. ; Dalla Valle, G. ; et al ( October 2023 , Proceedings of the International Ocean Discovery Program Expedition reports)

   Hotspot tracks (chains of seamounts, ridges, and other volcanic structures) provide important records of plate motions, as well as mantle geodynamics, magma flux, and mantle source compositions. The Tristan-Gough-Walvis Ridge (TGW) hotspot track, extending from the active volcanic islands of Tristan da Cunha and Gough through a province of guyots and then along Walvis Ridge to the Etendeka flood basalt province, forms one of the most prominent and complex global hotspot tracks. The TGW hotspot track displays a tight linear age progression in which ages increase from the islands to the flood basalts (covering ~135 My). Unlike Pacific tracks, which are often simple, nearly linear chains of seamounts, the TGW track is alternately a steep-sided narrow ridge, an oceanic plateau, subparallel linear ridges and chains of seamounts (most are flat-topped guyots). The track displays isotopic zonation over the last ~70 My. The zonation appears near the middle of the track just before it splits into two to three chains of ridge- and guyot-type seamounts. Walvis Ridge, forming the older part of the track, is also overprinted with age-progressive late-stage volcanism, which was emplaced ~30–40 My after the initial eruptions and has a distinct isotopic composition. The plan for Expedition 391 was to drill at six sites, three along Walvis Ridge and three in the seamounts of the Guyot Province, to collect igneous rocks to better understand the formation of volcanic edifices, the temporal and geochemical evolution of the hotspot, and the variation in paleolatitudes at which the volcanic edifices formed. After a delay of 18 days to address a shipboard Coronavirus (COVID-19) outbreak, Expedition 391 proceeded to drill at four of the proposed sites: three sites on Walvis Ridge around Valdivia Bank, an ocean plateau within the ridge, and one site on the lower flank of a guyot in the Center track of the Guyot Province, a ridge located between the Tristan subtrack (which extends from the end of Walvis Ridge to the islands of Tristan da Cunha) and the Gough subtrack (which extends from Walvis Ridge to Gough Island). The first hole was drilled at Site U1575, located on a low portion of the northeastern Walvis Ridge just north of Valdivia Bank. At this location, 209.9 m of sediments and 122.4 m of igneous basement were cored. The sediments ranged in age from Late Pleistocene (~0.43–1.24 Ma) to Late Cretaceous (Campanian; 72–78 Ma). The igneous basement comprised 10 submarine lava units consisting of pillow, lobate, sheet, and massive lava flows, the thickest of which was ~21 m. Most lavas are tholeiitic, but some alkalic basalts were recovered. A portion of the igneous succession consists of low-Ti basalts, which are unusual because they appear in the Etendeka flood basalts but have not been previously found on Walvis Ridge. Two holes were drilled at Site U1576 on the west flank of Valdivia Bank. The first of these holes was terminated because a bit jammed shortly after entering the igneous basement. Hole U1576A recovered a remarkable ~380 m thick sedimentary section consisting mostly of chalk covering a nearly complete sequence from Late Pleistocene (~0.43–1.24 Ma) to Late Cretaceous (Campanian; ~79–81.38 Ma). These sediments display short and long cyclic color changes that imply astronomically forced and longer term paleoenvironmental changes. The igneous basement recovered in Hole U1576B yielded 11 submarine lava units (total thickness = ~65 m). The flows range from pillows to massive flows with compositions varying from tholeiitic basalt to basaltic andesite, only the second occurrence of the latter composition recovered from the TGW track thus far. These units are separated by seven sedimentary chalk units that range 0.1–11.6 m in thickness, implying a long-term interplay of sedimentation and lava eruptions. These intercalated sediments revealed Upper Cretaceous (Campanian) ages of ~77–79 Ma for the upper two interbeds and ~79–81.38 Ma for the lower beds. Coring at Site U1577, on the extreme eastern flank of Valdivia Bank, penetrated a 154.8 m thick sedimentary section ranging from the Paleocene (Thanetian; ~58.8 Ma) to Upper Cretaceous (Campanian; ~81.43–83.20 Ma). Igneous basement coring progressed only 39.1 m below the sediment/basalt contact, recovering three massive submarine tholeiitic basalt lava flows that are 4.1, 15.5, and >19.1 m thick, respectively. Paleomagnetic data from Sites U1577 and U1576 indicate that the former volcanic basement formed just before the end of the Cretaceous Normal Superchron and the latter during Chron 33r, shortly afterward. Biostratigraphic and paleomagnetic data suggest that Valdivia Bank becomes younger from east to west. Site U1578, located on a Center track guyot, provided a long and varied igneous section. After coring through 184.3 m of pelagic carbonate sediments mainly consisting of Eocene and Paleocene chalk (~55.64–63.5 Ma), Hole U1578A cored 302.1 m of igneous basement. Basement lavas are largely pillows but are interspersed with sheet and massive flows. Lava compositions are mostly alkalic basalts with some hawaiite. Several intervals contain abundant olivine (some fresh), and some of the pillow stacks consist of basalt with remarkably high Ti content. The igneous sequence is interrupted by 10 sedimentary interbeds consisting of chalk and volcaniclastics and ranging 0.46–10.19 m in thickness. Investigations of toothpick samples from the intercalated sediments were examined, each revealing the same age range of ~63.5–64.81 Ma (lower Paleocene; Danian). Paleomagnetic data display a change in basement magnetic polarity ~100 m above the base of the hole. Combining magnetic stratigraphy with biostratigraphic data, the igneous section is inferred to span >1 My. Nearly 7 months after Expedition 391, JOIDES Resolution transited from Cape Town to the north Atlantic. During this transit (Expedition 397T), 7.9 days of ship time were used to drill two holes (U1584A and U1585A) at sites on the Gough and Tristan tracks that had been omitted because of COVID-19–related time loss on the earlier cruise. For both, coring was begun only a short distance above the igneous basement to save time. The 75.2 m thick section drilled in Hole U1584A contains two sedimentary units: clay-rich carbonate sediments overlie a pumice-dominated volcaniclastic deposit containing basalt fragments. Because the goal was to core basalt and the base of the volcaniclastic deposit was not imaged in the seismic profile, the hole was terminated early to save operation time for the next site. In Hole U1585A, coring penetrated a 273.5 m thick sediment section overlying an 81.2 m thick pile of massive basalt flows. The sediment section is divided into four units: The uppermost unit consists of nannofossil chalk; The two intermediate units contain alternating chalk and volcaniclastic sediments containing several breccia units; and The lowermost unit consists of volcanic breccia containing juvenile blocks, bombs, and accretionary lapilli. This thick sedimentary section documents a transition from shallow-water volcanism to open-ocean sedimentation as the seamount subsided. The thick underlying basalt section is made up of four sparsely to highly phyric massive flows, the thickest of which is >43 m thick. Samples of these units are mostly basalt with a few trachybasalts and one trachyandesite. Although the igneous penetration was less than planned, coring during Expeditions 391 and 397T obtained samples that clearly will lead to an improved understanding of the evolution of the TGW hotspot and its track. Reasonable recovery of fresh basalt in some holes provides ample samples for geochemical, geochronologic, and paleomagnetic studies. Good recovery of Late Cretaceous and early Cenozoic chalk successions provides samples for paleoenvironmental study. 

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