Abstract Hydrothermal systems likely played an essential role in the origin of life, both on Earth and potentially on other planets. They form anywhere that heat and aqueous fluids interact, including within cooling hypervelocity impact craters. Longer periods of hydrothermal activity will generate extended windows of opportunity for prebiotic chemical reactions to occur, life to develop, and micro-organisms to thrive and propagate beyond their point of origin. Here, we present radioisotopic age constraints and numerical simulations for the duration of post-impact hydrothermal activity in and around the peak ring of the ~200 km diameter 66 Ma Chicxulub impact structure. We find that hydrothermal activity persisted for at least 8 million years (Myr), which is approximately four times longer than previously estimated by numerical simulations, palaeomagnetic records, and petrographic interpretations at Chicxulub, making it the longest-lived impact generated hydrothermal system documented on Earth.

The age of mineralization in the North Pennine orefield is controversial, with age estimates ranging from Permian to Paleocene times. Here we present Nd-isotope data from a single large crystal of fluorite from the Boltsburn vein in Weardale that define an errochron with an apparent age of 155 +/- 21 Ma, indicating formation in the Upper Jurassic. Nd-isotope growth curves based on these data and published fluorite data from several other Alston block mineral veins intersect growth curves for the basement rocks (Skiddaw Group mudstones and Weardale granite) at the same age, confirming that the date is robust. Our proposed date coincides with a major phase of extension in the North Sea and with the time at which global sea-level reached its highest point in the Phanerozoic, possibly submerging the Alston block. Upper Jurassic extension may have reactivated older fractures of Variscan origin on the Alston block and allowed seawater to penetrate deep into the basement rocks to feed the hydrothermal circulation responsible for the mineral deposits. Mineralisation followed a long period of tectonic inactivity when global sea-level was generally low, and these two factors combined to inhibit hydrothermal circulation through the basement rocks. The temperature of the Weardale granite would have increased by radioactive decay during this period of inactivity.

The Miocene basalts of Vestfir & eth;ir, northwest Iceland, provide a record of the Iceland mantle plume shortly after its arrival beneath the mid-Atlantic ridge. We present new trace element and Sr-Nd-Pb-He isotope data from a comprehensive collection of 16-8 Ma basalts to investigate the temporal and compositional evolution of the plume. Basalts erupted before a volcanic hiatus at similar to 14.9 Ma include both depleted, negative Delta Nb lavas that resemble N-MORB and enriched, plume-derived basalts with positive Delta Nb that resemble E-MORB. Basalts erupted after the hiatus all have positive Delta Nb, they are manifestly more enriched than the pre-hiatus lavas and have high He-3/He-4 (up to 42 R-a), consistent with the presence of plume mantle beneath the relocated rift. The plume-derived basalts record contributions from two distinct enriched components (NWE1 and NWE2) and a depleted component (NWD1), which show geochemical signatures similar to those recorded in modern Icelandic basalts. Both the enriched mantle components have high He-3/He-4 ratios, and the depleted high-He-3/He-4 mantle component that dominates the proto-Iceland plume sampled by Baffin Island and West Greenland lavas is not evident in the Miocene sequence. The absence of He-3/He-4 correlations with trace elements or radiogenic isotopes suggests the high-He-3/He-4 source does not have a unique geochemical composition.

Eastern Australia and the neighbouring Tasman and Coral Seas are home to extensive age-progressive volcanism spanning from ~55 Ma in the north to ~6 Ma in the south. This volcanism forms two offshore seamount trails, the Lord Howe and the Tasmantid Chains, as well as the onshore central volcanoes and leucitites of the East Australian Chain. The three volcanic chains are an average of just 500 km apart, erupted contemporaneously from 35-6 Ma, and share a common age-distance relationship, strongly suggesting a common source, most likely a deep-origin plume. However, they have erupted through lithosphere ranging from oceanic with well-developed seafloor spreading to drowned continental fragments to mainland Australia. How do these diverse settings influence the chemical and physical properties of the resulting mafic volcanism? The East Australian Chain has more fractionated mafic samples, reflecting more complex magmatic plumbing and longer magma residence times in the thick continental lithosphere. However, the most striking result is that the trace element and isotopic ratios remain remarkably similar across the three suites, showing little evidence of crustal or lithospheric assimilation affecting the mafic magmas.

Magma-rich continental rifting and breakup commonly show complex distributions of volcanism, potentially due to both plume and rifting controls. As such, interpreting the main controls on the spatial evolution of magmatism is debated, as is the point at which continental rifting transitions to oceanic spreading. Here we present new argon-argon dating of 16 lava flows from the Stratoid and Gulf series of the Afar rift. We reconstruct the spatio-temporal evolution of the rift from widely distributed to localised along narrow magmatic segments (i.e., rift localisation). Our results rule out an ocean spreading-like style of rifting. We show that over 2-2.5 million years since the early Pleistocene, the rift progressively narrowed by asymmetric in-rift localisation and propagated along-rift. Concurrently, the mantle partial melting and the crustal magmatic system shallowed. While mantle plume-related elevated temperatures influence overall melt volumes, our results suggest that the rapid and localised changes in plate thinning caused by rifting are the primary control on the spatio-temporal distribution of volcanism.

Regional tephrostratigraphic frameworks connect palaeoclimate, archaeological and volcanological records preserved in soils or lake sediments via shared volcanic ash (tephra) layers. In eastern Africa, tracing of tephra isochrons between geoarchaeological sequences is an established chronostratigraphic approach. However, to date, few long tephra records exist from sites with continuous depositional sequences, such as lake sediments, which offer the potential to connect local and discontinuous sequences at the regional scale. Long lake sediment sequences may also capture more complete eruptive histories of understudied volcanic centres. Here, we present and date the tephrostratigraphic record of a >250,000-year (>250-kyr) continuous sediment sequence extracted from Lake Chala, a crater lake on the Kenya-Tanzania border near Mt Kilimanjaro. Single-grain glass major and minor element analyses of visible and six cryptotephra layers reveal compositions ranging from mafic foidites and basanites to more evolved tephri-phonolites, phonolites, trachytes and a single rhyolite. Of these, nine are correlated to scoria cone eruptions of neighbouring Mt Kilimanjaro or the Chyulu volcanic field similar to 60 km to the north; seven are correlated to phonolitic eruptions of Mt Meru, similar to 100 km to the west; and four to voluminous trachytic eruptions of Central Kenyan Rift (CKR) volcanoes located similar to 350 km to the north. The only rhyolitic tephra layer, a cryptotephra, correlates to the 73.7-ka BP (before present, taken as 1950 CE) Younger Toba Tuff (YTT) from Sumatra. Two of the CKR tephra layers provide direct ties with terrestrial sequences relevant to Middle Stone Age archaeology of the eastern Lake Victoria basin in Kenya. Absolute age estimates obtained by direct Ar-40/Ar-39 dating of 10 tephra layers are combined with six Pb-210 and 162 C-14 dates covering the last 25-kyr and the well-constrained known age of the YTT to build a first absolute chronology for the full Lake Chala sediment sequence. The uninterrupted >250-kyr Lake Chala sedimentary archive represents a unique tephrostratotype sequence for eastern Africa, optimising the chronological value of tephra correlations in regional palaeoenvironmental, archaeological and volcanological research. Further study of cryptotephra in the Lake Chala sequence and additional geochemical characterisation and dating of ancient volcanic eruptions from nearby and further afield may eventually produce a regionally connected and detailed tephrostratigraphic framework for eastern equatorial Africa.

The latest generation of Antarctic paleo-ice sheet models, incorporating bedrock paleo-topography, reconstruct the development of substantial ice-sheet embayments during the Mid-Miocene Climate Optimum, a warm period between similar to 17-15 Ma ago. Here, we test these models using ice-rafted debris (IRD) recovered from the central Weddell Sea (Ocean Drilling Program Site 113-694). These IRD are entrained by the ice-sheet expanding into the embayment during the cooling associated with the subsequent Mid-Miocene Climate Transition and ultimately delivered to the deep marine sink by iceberg armadas during higher-frequency instability events. Due to the near-absence of heavy minerals in these volumetrically small samples, we utilise the novel in-situ K-feldspar Rb-87/Sr-87 provenance technique by laser-ablation inductively coupled mass spectrometer equipped with a mass-filtered reaction cell. We combine these single-grain ages with in-situ Pb-isotope analysis, as well as Ar-40/Ar-39 dating of a subset of grains. Results identify the Antarctic Peninsula and southern Dronning Maud Land as key sediment sources, including grains sourced from within the Recovery subglacial basin. These findings support ice sheet instability consistent with reconstructions of major ice sheet embayments formed by climatic conditions analogous to those predicted by modern anthropogenic warming trajectories. Additionally, in-situ Rb-87/Sr-87 and Pb-isotope analysis of detrital K-feldspar provide a high-throughput alternative to conventional single grain U-Pb or Ar-40/Ar-39 analysis, especially in small samples where the heavy mineral fraction is limited or absent.

The CM carbonaceous chondrites are key archives for understanding the earliest history of the solar system. Their C-complex asteroid parent body(ies) underwent aqueous alteration, among the products of which are carbonate minerals that can faithfully record the conditions of their formation. In this study we report carbon, triple oxygen and clumped isotope compositions of carbonates in six CM chondrites which span a range in degrees of aqueous alteration (Allan Hills 83100, Cold Bokkeveld, LaPaz Icefield 031166, Lonewolf Nunataks 94101, Murchison, Scott Glacier 06043). Δ17O values range from -1 to -2.6‰ (±0.1), and where calcite and dolomite co-exist their Δ17O differ by 0.6 permil, suggesting precipitation from distinct fluids. Calculated crystallization temperatures range from 5 to 51⁰C for calcite (typically ±10⁰C) and 75 to 101(±15)⁰C for dolomite. The δ18OVSMOW of the aqueous fluids from which they formed ranges from -6.6 to 2.3‰, with no relationship to the δ13C of carbonates. As the population of carbonates in any one CM chondrite can include multiple generations of grains that formed at different conditions, these values represent the mode of the temperature of carbonate formation for each meteorite. We observe that in the more altered meteorites carbonate Δ17O values are lower and formation temperatures are higher. These correlations are consistent with aqueous alteration of the CM chondrites being a prograde reaction whereby the hotter fluids had undergone greater isotope exchange with the anhydrous matrix. Our data are broadly consistent with the closed system model for water/rock interaction, but carbonate mineral formation in the latter stages of aqueous alteration may be linked to fluid movement via fractures.

Large igneous province volcanism of the Columbia River Basalt Group (CRBG) has been suggested to play a causal role in elevated global temperatures and atmospheric carbon dioxide levels of the Miocene Climate Optimum (MCO). However, assessing the connection between volcanism and warming is dependent upon an accurate and precise chronology for the timing and duration of CRBG emplacement. Building on our previous work (Kasbohm and Schoene, 2018), we present fifteen new high-precision ages, using CA-ID-TIMS U-Pb on zircon and multi-collector 40Ar/39Ar on basaltic groundmass, to provide a detailed dual-chronometer timeline for CRBG eruptions. We use both sets of new ages and precise stratigraphic positions of our samples in an integrated Markov Chain Monte Carlo model to calculate average long-term emplacement rates for main-phase CRBG volcanism of 0.2-0.9 km3/a, with a high likelihood of one prominent hiatus of 60-120 kyr duration occurring after main-phase emplacement. We analyzed trace elements and hafnium isotopes of each dated zircon from CRBG interbeds. The compositions are consistent with both Cascades subduction volcanism and evolved syn-CRBG volcanism proximal to the depositional area. Our age model also yields ages for all magnetic field reversals during the main phase of CRBG emplacement, which can be used to improve calibrations of Miocene paleoclimate records. We find that main-phase CRBG emplacement is coincident with the greatest sustained warmth of the MCO in astronomically-tuned records. Our work shows the power of using both U-Pb and 40Ar/39Ar geochronology in an integrated stratigraphic context to assess data reliability and develop the most robust age model possible for large igneous province emplacement.

The shergottites are the most abundant and diverse group of Martian meteorites and provide unique insights into the mafic volcanic and igneous history of Mars. Their ages, however, remain a source of debate. Different radioisotopic chronometers, including 40Ar/39Ar, have yielded discordant ages, leading to conflicting interpretations on whether the shergottites originate from young (mostly <700 Ma) or ancient (>4,000 Ma) Martian volcanoes. To address this issue, we have undertaken an 40Ar/39Ar investigation of seven shergottite meteorites utilizing an innovative approach to correcting data for cosmogenic isotope production and resolution of initial trapped components which, crucially, do not require assumptions concerning the sample's geologic context. Our data yield statistically robust 40Ar/39Ar isochron ages ranging from 161 ± 9 Ma to 540 ± 63 Ma (2σ), synchronous with the U-Pb, Rb-Sr, and Sm-Nd ages for the respective meteorites. These data indicate that, despite experiencing shock metamorphism, the shergottites were sourced from the youngest volcanoes on Mars.

The Ratagain Complex is an enigmatic Late Caledonian granitic intrusion and a member of the high Ba-Sr Northern Highlands granite (NHG) suite that has been related to slab failure. Slab failure magmatism explains varying contributions of mafic and felsic magmas in post-collision orogenic settings. It is therefore of major importance in understanding crustal accretion. However, the source and nature of any mantle derived contri-bution is poorly understood. This study reveals that Ratagain is not only transitional in nature between the high Ba-Sr calc-alkaline granites and syenite intrusions of the Northern Highlands Terrane, but overlaps with the entire compositional range of the NHG suite. New lithogeochemical data from Ratagain confirm remarkably high Sr (>1600 ppm) and Ba (>2200 ppm) contents, high LREEs, notable depletions in Nb, U, P and Ti, low HREEs and negligible Eu anomalies, associated with high initial 87Sr/86Sr (0.7055 to 0.7062) and low epsilon Nd (-11.8 to-13.3). Although mafic parts of the complex have strong elemental and isotopic similarities with broadly coeval lamprophyres, signalling derivation from enriched mantle sources, details of the isotope array with respect to local crustal reservoirs indicate a significant Lewisian component. Such geochemical characteristics, combined with tectonic and petrological evidence, may be attributable to long-lived, incremental emplacement of suc-cessive magma batches originating from the same enriched mantle but differing in age and extent of assimilation -fractionation crystallisation. We therefore propose that some of the age dates for the Late Caledonian intrusions, particularly those obtained from older geochronology studies, are in need of review as they may record early crystallisation in the deep crust and not be a valid proxy for granite emplacement.

The accuracy and traceability of geochronometers are of vital importance to questions asked by many Earth scientists. The widely applied 40 Ar/ 39 Ar geochronometer relies on the co‐irradiation of samples with neutron fluence monitors (reference materials) of known ages; the ages and uncertainties of these monitors are critical to our ability to apply this chronometer. Previously, first principles, astronomical and optimisation calibrations have been made. The first principles method for determining the age of monitor minerals is the K‐Ar method, which involves measurement of their 40 K and 40 Ar* abundances. The AQuA (Absolute Quantities of Argon) pipette system, which emits calibrated quantities of 40 Ar* via the ideal gas law, was used to calibrate the sensitivity of the system across a range of source pressures and estimate 40 Ar* abundances in neutron fluence monitors. These 40 Ar abundances were combined with existing 40 K abundance data for these monitors. Ages for HD‐B1 and MD2 (GA1550) biotite fluence monitors were calculated and combined with intercalibration data for HD‐B1 and Fish Canyon sanidine (FCs) to determine ages for FCs. Current results do not have the targeted accuracy when compared with previous calibrations; however, we show how the extensive methodology development presented here can be used towards making reliable future measurements.

Harrat Rahat is the largest volcanic field in Saudi Arabia and has been active from similar to 10 Ma to the present day. Due to its proximity to population centers, recent eruptions at Harrat Rahat- the Medinah lava flows (<1.7 Ma)- have been extensively studied to identify volcanic risk. However, evolution of Harrat Rahat's most extensive and oldest lava flows, known collectively as the Shawahit Basalt (>2.5 Ma), is poorly understood. In this study, we collected, dated and geochemically analyzed lavas from Harrat Rahat, primarily targeting the under-sampled Shawahit unit. We obtained dates of between 9.4 and 2.7 Ma using 40Ar/39Ar analyses of 23 Shawahit samples. Over the lifetime of Harrat Rahat, we observe a geochemical transition from predominantly subalkalic to alkalic eruptions coupled with a counter-intuitive decrease in incompatible element concentrations. We attribute these changes to a decrease in melt productivity and a reduction in contamination by enriched lithospheric melts, respectively. Thermobarometric analysis of basalts from Harrat Rahat indicates that they were generated by melting of asthenospheric mantle with a potential temperature of -1456(-32)(+50) degrees C beneath lithosphere that is 50-60 km thick. These results indicate that volcanic activity at Harrat Rahat was initiated by the arrival of a mantle plume beneath lithosphere thinned by a combination of rifting of the Red Sea and thermal erosion. Furthermore, we propose that this plume, either acting alone or in combination with a number of other plumes, is responsible for the formation of the Arabian swell, as well as much of the Neogene-recent intraplate volcanic activity observed across western Arabia. Our conclusions are consistent with a wide range of geochemical, seismologic, gravimetric, thermochronologic and geomorphic observations.

New occurrences of early artefacts ascribed to the Oldowan tradition come from localities at high level within the caldera of the extinct Kilombe volcano, located in the central rift valley of Kenya. The trachyte cone and caldera of Kilombe volcano formed at ca. 2.5 Ma, and the record of >130 m of sediment-fill indicates that the caldera subsequently held a lake for long periods during the Early Pleistocene. The Oldowan artefact localities, dated by 40Ar/39Ar and palaeomagnetism to similar to 1.78 Ma, lie east of the centre of the caldera, on the west side of an ancient small lake, which later drained away as a gorge formed on the east side of the mountain. The artefacts are dominantly made of Kilombe trachyte, and are associated with a fauna of large animals including Hippopotamus gorgops. These are the first Oldowan localities to be discovered in a new area of the Kenyan rift valley in the last thirty years, and their presence at high level in rugged landscape indicates that the associated hominins were exploiting a full range of environments. (C) 2021 Published by Elsevier Masson SAS.

The origins of felsic low-delta O-18 melts (< +5.5 parts per thousand) are usually attributed to assimilation of high-temperature hydrothermally altered (HTHA) rocks. Very few alkaline (silica-undersaturated and/or peralkaline) examples are known. Here, we classify the Miocene Fataga Group in Gran Canaria, a silica-undersaturated to mildly saturated alkaline volcanic sequence consisting of trachytic to phonolitic extra-caldera ignimbrites and lavas, as a new low-delta O-18 felsic locality. We provide new mineral, glass, and bulk geochemical data linked to a well-constrained stratigraphy to assess the processes involved in the magma reservoir that fed the Fataga eruptions. New high-precision single crystal feldspar 40Ar/39Ar ages of the study area span 13.931 +/- 0.034 Ma to 10.288 +/- 0.016 Ma. Fractional crystallization at shallow depths of sanidine/anorthoclase, biotite, augite/diopside, titanite, ilmenite, and titanomagnetite is the main driving process to produce phonolitic magmas from trachytic melts. Evidence of mafic hotter recharge is not found in the field, but some units exhibit trachytic compositions characterized by positive Eu/Eu* anomalies and high Ba contents, interpreted as melts of feldspar-dominated cumulates, the solid remnants of fractional crystallization. Hence, recharge magmas halted in the crystal mush and provided the heat needed to sustain cumulate melting and volcanic activity. This cumulate signature might be lost if fractional crystallization continues before the eruption. The interplay among meteoric water, the caldera-fault system, intra-caldera ignimbrites (Mogan Group), and the Fataga magma reservoir favoured assimilation of up to ca. 30% of HTHA rocks. Such assimilation is variable through time and recorded by delta O-18(melt) values down to +4.73 parts per thousand. We did not find any direct relation between assimilation and silica saturation of the Fataga volcanic deposits.

Efforts to date the oldest modern human fossils in eastern Africa, from Omo-Kibish 1 – 3 and Herto 4 , 5 in Ethiopia, have drawn on a variety of chronometric evidence, including 40 Ar/ 39 Ar ages of stratigraphically associated tuffs. The ages that are generally reported for these fossils are around 197 thousand years (kyr) for the Kibish Omo I 3 , 6 , 7 , and around 160–155 kyr for the Herto hominins 5 , 8 . However, the stratigraphic relationships and tephra correlations that underpin these estimates have been challenged 6 , 8 . Here we report geochemical analyses that link the Kamoya’s Hominid Site (KHS) Tuff 9 , which conclusively overlies the member of the Omo-Kibish Formation that contains Omo I, with a major explosive eruption of Shala volcano in the Main Ethiopian Rift. By dating the proximal deposits of this eruption, we obtain a new minimum age for the Omo fossils of 233 ± 22 kyr. Contrary to previous arguments 6 , 8 , we also show that the KHS Tuff does not correlate with another widespread tephra layer, the Waidedo Vitric Tuff, and therefore cannot anchor a minimum age for the Herto fossils. Shifting the age of the oldest known Homo sapiens fossils in eastern Africa to before around 200 thousand years ago is consistent with independent evidence for greater antiquity of the modern human lineage 10 .

Crystal zoning plays a fundamental role in modern volcanology as a key to unravel the geometry and the dynamics of plumbing systems. In this study, a detailed textural and compositional study of clinopyroxene crystals entrained in intrusive, hypabyssal and effusive products from Cima Pape (Dolomites) is coupled with thermobarometric-hygrometric models to reconstruct the geometry and evolution of the feeding system beneath Middle Triassic volcanic edifices. Whole-rock major, trace element distribution and Sr-Nd isotopic signature (Sr-87/Sr-86(i) = 0.7045-0.7050; Nd-143/Nd-144(i) = 0.51223-0.51228) show that the rocks from Cima Pape are SiO2- saturated and have shoshonitic affinity, and likely belong to the acme of the Mid-Triassic magmatism that shaped the Southern Alps between 239 and 237.6 Ma. Highly porphyritic trachybasaltic to basaltic trachyandesitic volcanic rocks contain a large number of concentric-zoned clinopyroxene crystals. Here, high-Mg# and -Cr2O3, REE depleted bands (Mg# 80-91; Cr2O3 up to 1.2 wt%) with variable thickness grew between relatively low-Mg# and-Cr2O3 (Mg# 70-77; Cr2O3 < 0.1 wt%) augitic cores and rims. In contrast, the gabbroic to monzodioritic 50-to 300-m-thick sill cropping out below the volcanic sequences, though to represent a relic of the shallowest portion of the plumbing system, is mostly made up of unzoned clinopyroxene crystals. Thermobarometric and hygrometric models allowed us to define that a small "mush-type" batch was located beneath the Cima Pape volcano at depths between 7 and 14 km. Here, augitic clinopyroxene formed in equilibrium with a slightly evolved (basaltic trachyandesitic), H2O-rich melt (Mg# = 43-45; T = 1035-1075 degrees C; H2O = 2.6-3.8 wt%). Periodic replenishments of the magma batch by primitive (Mg# = 65-70), hotter and relatively H2O-poor (T = 1130-1150 degrees C; H2O = 2.1-2.8 wt%) basaltic magmas led to the formation of diopsidic bands mantling the already formed augitic cores. Later on, re-equilibration of clinopyroxene with the mixed melt resulted in the formation of low-Mg#, LILE-and LREE-enriched rims. The most Mg-poor micro-phenocrystic clinopyroxene in the volcanic rocks and in the sill records the ultimate and shallowest conditions of crystallization, occurring at T of 975-1010 degrees C and P comprised between 50 and 150 MPa. Based on the presence of similar zoning in clinopyroxene phenocrysts, a comparison between the Mid-Triassic Cima Pape and active volcanoes was put forward to highlight the potential of studying ancient, entirely exposed volcanic systems for interpreting the feeding system processes acting beneath active volcanoes. At a regional scale, this approach represents a new, powerful tool for investigating the evolution of the Mid-Triassic magmatism in the Southern Alps and shedding light on the interactions between mantle-derived melts and differentiated batches ponding in the crust. (C) 2021 Elsevier B.V. All rights reserved.

The Sierra Nevada Volcanic Range (SNVR), which includes Popocatepetl, Iztaccihuatl and Tlaloc-Telapon volcanoes, has been the source of multiple large explosive eruptions that have dispersed tephra across central Mexico. Several eruptions since 40 ka have previously been described, particularly from Popocatepetl, the southernmost volcano of the range. However, the longer-term eruption history of the SNVR is poorly understood, due to challenges with correlating limited exposures of older pyroclastic sequences, and in discriminating between tephras derived from different sources. Here we describe two extensive exposures located between Popocatepetl and lztaccihuatl volcanoes, which provide a more complete and longer-term explosive eruption record of the SNVR: the Nepopualco and Xalitzintla tephra sequences. A detailed tephrostratigraphic survey, together with new 40Ar/39Ar geochronological analyses and glass geochemistry, has permitted the characterization of identified eruption units further leading to the determination of geochemical fields for each volcano and the subsequent discernment of volcanic sources. Our results show that, since the collapse of the Los Pies Cone, which destroyed the Pale edifice at 631 +/- 44 ka (2 sigma), Iztaccihuatl has produced at least 6 explosive rhyolitic eruptions. After coeval activity with Popocatepetl, between similar to 600 and similar to 500 ka, Iztaccihuatl's explosive activity ceased while Popocatepetl's continued until present day. Popocatepetl has produced at least 27 medium to large explosive eruptions (inferred VEI 4-6), commonly of andesitic to dad tic compositions. Some of these eruptions deposited pumice fallout of >1 m thick in both the Nepopualco and Xalitzintla sequences (e.g. the 339 +/- 16 ka [2 sigma]NT-23/ WRT-7 eruption), suggesting that Popocatepetl has produced several eruptions similar in magnitude to the wellknown similar to 14 ka Tutti Frutti Pumice (a VEI 6 eruption with a similar to 5 km(3) tephra volume). The Popocatepetl and Iztaccihuatl tephras are interbedded with deposits from more distal volcanoes, including some mafic to intermediate products of unknown sources (possibly from nearby monogenetic cones) and tephras related to the late Pleistocene eruptions of TIaloc-Telapon ( including the tephra layer produced by the San Valentin Ignimbrite, recently 40Ar/39Ar dated in this study at similar to 102 ka; 2 sigma). Our new chemical, stratigraphic and geochronologic investigations of these pyroclastic deposits, predominantly from Popocatepetl and Iztaccihuatl, provide information on the scale and frequency of medium to large magnitude explosive eruptions over a longer-time period than currently known and that have had potential to disperse tephra across central Mexico since the middle to late Pleistocene. This new data can be used to determine the source of further unknown tephras in the region as well as to better assess the volcanic hazard to the densely populated megalopolis of Mexico City. (C) 2021 Elsevier B.V. All rights reserved.