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•Background: Self-harm is a major public health challenge, and repeated self-harm is common in those attending hospital following an episode. Evidence suggests psychological interventions could help people who self-harm, but few definitive studies have assessed their clinical and cost-effectiveness. Repeated self-harm is associated with poor quality of life, depression, suicide and increased health service costs which justify the development of psychotherapeutic interventions tailored for people with repeated self-harm. •Methods: FReSH START is a multicentre individually 1:1 randomised controlled trial evaluating the clinical and cost-effectiveness of standard care plus psychological therapy or standard care alone for adults (≥ 18 years) presenting at an emergency department (ED) with repeated self-harm. Recruiting 630 participants, it includes an internal pilot, economic evaluation and process evaluation. The intervention will be delivered by mental health staff working in acute settings, with experience of assessing and managing risk in people presenting to emergency services with self-harm. Staff will be trained and supervised to deliver one of three specially adapted therapies: psychodynamic interpersonal therapy, cognitive behavioural therapy or acceptance and commitment therapy. Participants allocated to the intervention will receive one of the adapted therapies according to therapist allocation for up to 6 months via 12 weekly, one to one, 45–50-min sessions. The primary outcome is quality of life measured by the Clinical Outcomes in Routine Evaluation Outcome Measure at 12 months post-randomisation. Secondary outcomes include suicidal intent, depression and cost-effectiveness. Data are collected using hospital attendance records and online/postal/telephone questionnaires at 6 and 12 months post-randomisation, with resource use additionally collected at 3 and 9 months. •Discussion: This protocol outlines a randomised controlled trial to investigate whether mo

Urbanisation and climate change have increased the need for equitable access and visibility of urban green and blue spaces (GBS), to promote the sustainability and resilience of cities and to improve the well-being of their inhabitants. In this paper, we test an implementation of the newly proposed guideline to achieve equitable greening, the 3-30-300 rule, in three European cities: Paris Region (France), Aarhus Municipality (Denmark), and Grad Velika Gorica (Croatia). In this analysis, every residential building should have at least three viewable trees, 30% neighbourhood GBS cover, and a GBS of at least 1 hectare within 300 m. Our results show that none of the cities currently meet any of these three components, and the three cities differed in which rules were most closely met. In our implementation, substantial changes were needed in all cities to meet the guidelines: 12.6% of Paris, 10% of Aarhus, and 18.4% of Velika Gorica’s urban footprint were converted to grass or tree cover, with implications for >100,000 buildings and >900,000 inhabitants. Our study discusses how existing conditions in each city impacted the viability of meeting the rule and proposes key considerations for future implementations of such guidelines, drawing on examples of innovative GBS already implemented globally.

The first samples collected by the Mars 2020 mission represent units exposed on the Jezero Crater floor, from the potentially oldest Séítah formation outcrops to the potentially youngest rocks of the heavily cratered Máaz formation. Surface investigations reveal landscape-to-microscopic textural, mineralogical, and geochemical evidence for igneous lithologies, some possibly emplaced as lava flows. The samples contain major rock-forming minerals such as pyroxene, olivine, and feldspar, accessory minerals including oxides and phosphates, and evidence for various degrees of aqueous activity in the form of water-soluble salt, carbonate, sulfate, iron oxide, and iron silicate minerals. Following sample return, the compositions and ages of these variably altered igneous rocks are expected to reveal the geophysical and geochemical nature of the planet’s interior at the time of emplacement, characterize martian magmatism, and place timing constraints on geologic processes, both in Jezero Crater and more widely on Mars. Petrographic observations and geochemical analyses, coupled with geochronology of secondary minerals, can also reveal the timing of aqueous activity as well as constrain the chemical and physical conditions of the environments in which these minerals precipitated, and the nature and composition of organic compounds preserved in association with these phases. Returned samples from these units will help constrain the crater chronology of Mars and the global evolution of the planet’s interior, for understanding the processes that formed Jezero Crater floor units, and for constraining the style and duration of aqueous activity in Jezero Crater, past habitability, and cycling of organic elements in Jezero Crater.

Prior to the Perseverance rover landing, the acoustic environment of Mars was unknown. Models predicted that: (i) atmospheric turbulence changes at centimeter scales or smaller at the point where molecular viscosity converts kinetic energy into heat1, (ii) the speed of sound varies at the surface with frequency, and (iii) high frequency waves are strongly attenuated with distance in CO₂. However, theoretical models were uncertain because of a lack of experimental data at low pressure, and the difficulty to characterize turbulence or attenuation in a closed environment. Here using Perseverance microphone recordings, we present the first characterization of Mars’ acoustic environment and pressure fluctuations in the audible range and beyond, from 20 Hz to 50 kHz. We find that atmospheric sounds extend measurements of pressure variations down to 1,000 times smaller scales than ever observed before, revealing a dissipative regime extending over 5 orders of magnitude in energy. Using point sources of sound (Ingenuity rotorcraft, laser-induced sparks), we highlight two distinct values for the speed of sound that are ~10 m/s apart below and above 240 Hz, a unique characteristic of low-pressure CO₂-dominated atmosphere. We also provide the acoustic attenuation with distance above 2 kHz, allowing us to elucidate the large contribution of the CO₂ vibrational relaxation in the audible range. These results establish a ground truth for modelling of acoustic processes, which is critical for studies in atmospheres like Mars and Venus ones.

The Perseverance rover landed in Jezero crater, Mars, to investigate ancient lake and river deposits. We report observations of the crater floor, below the crater’s sedimentary delta, finding the floor consists of igneous rocks altered by water. The lowest exposed unit, informally named Séítah, is a coarsely crystalline olivine-rich rock, which accumulated at the base of a magma body. Fe-Mg carbonates along grain boundaries indicate reactions with CO2-rich water, under water-poor conditions. Overlying Séítah is a unit informally named Máaz, which we interpret as lava flows or the chemical complement to Séítah in a layered igneous body. Voids in these rocks contain sulfates and perchlorates, likely introduced by later near-surface brine evaporation. Core samples of these rocks were stored aboard Perseverance for potential return to Earth.

The Perseverance rover landed in Jezero crater, Mars, to investigate ancient lake and river deposits. We report observations of the crater floor, below the crater's sedimentary delta, finding that the floor consists of igneous rocks altered by water. The lowest exposed unit, informally named Seitah, is a coarsely crystalline olivine-rich rock, which accumulated at the base of a magma body. Magnesium-iron carbonates along grain boundaries indicate reactions with carbon dioxide-rich water under water-poor conditions. Overlying Seitah is a unit informally named Maaz, which we interpret as lava flows or the chemical complement to Seitah in a layered igneous body. Voids in these rocks contain sulfates and perchlorates, likely introduced by later near-surface brine evaporation. Core samples of these rocks have been stored aboard Perseverance for potential return to Earth.

Before the Perseverance rover landing, the acoustic environment of Mars was unknown. Models predicted that: (1) atmospheric turbulence changes at centimetre scales or smaller at the point where molecular viscosity converts kinetic energy into heat1, (2) the speed of sound varies at the surface with frequency2,3 and (3) high-frequency waves are strongly attenuated with distance in CO2 (refs. 2–4). However, theoretical models were uncertain because of a lack of experimental data at low pressure and the difficulty to characterize turbulence or attenuation in a closed environment. Here, using Perseverance microphone recordings, we present the first characterization of the acoustic environment on Mars and pressure fluctuations in the audible range and beyond, from 20 Hz to 50 kHz. We find that atmospheric sounds extend measurements of pressure variations down to 1,000 times smaller scales than ever observed before, showing a dissipative regime extending over five orders of magnitude in energy. Using point sources of sound (Ingenuity rotorcraft, laser-induced sparks), we highlight two distinct values for the speed of sound that are about 10 m s−1 apart below and above 240 Hz, a unique characteristic of low-pressure CO2-dominated atmosphere. We also provide the acoustic attenuation with distance above 2 kHz, allowing us to explain the large contribution of the CO2 vibrational relaxation in the audible range. These results establish a ground truth for the modelling of acoustic processes, which is critical for studies in atmospheres such as those of Mars and Venus.

Before the Perseverance rover landing, the acoustic environment of Mars was unknown. Models predicted that: (1) atmospheric turbulence changes at centimetre scales or smaller at the point where molecular viscosity converts kinetic energy into heat1, (2) the speed of sound varies at the surface with frequency2,3 and (3) high-frequency waves are strongly attenuated with distance in CO2 (refs. 2–4). However, theoretical models were uncertain because of a lack of experimental data at low pressure and the difficulty to characterize turbulence or attenuation in a closed environment. Here, using Perseverance microphone recordings, we present the first characterization of the acoustic environment on Mars and pressure fluctuations in the audible range and beyond, from 20 Hz to 50 kHz. We find that atmospheric sounds extend measurements of pressure variations down to 1,000 times smaller scales than ever observed before, showing a dissipative regime extending over five orders of magnitude in energy. Using point sources of sound (Ingenuity rotorcraft, laser-induced sparks), we highlight two distinct values for the speed of sound that are about 10 m s−1 apart below and above 240 Hz, a unique characteristic of low-pressure CO2-dominated atmosphere. We also provide the acoustic attenuation with distance above 2 kHz, allowing us to explain the large contribution of the CO2 vibrational relaxation in the audible range. These results establish a ground truth for the modelling of acoustic processes, which is critical for studies in atmospheres such as those of Mars and Venus.

Observations from orbital spacecraft have shown that Jezero crater, Mars, contains a prominent fan-shaped body of sedimentary rock deposited at its western margin. The Perseverance rover landed in Jezero crater in February 2021. We analyze images taken by the rover in the three months after landing. The fan has outcrop faces that were invisible from orbit, which record the hydrological evolution of Jezero crater. We interpret the presence of inclined strata in these outcrops as evidence of deltas that advanced into a lake. In contrast, the uppermost fan strata are composed of boulder conglomerates, which imply deposition by episodic high-energy floods. This sedimentary succession indicates a transition, from a sustained hydrologic activity in a persistent lake environment, to highly energetic short-duration fluvial flows.

A central objective of the NASA Mars 2020 Perseverance rover mission is to collect and document a suite of scientifically compelling samples for possible return to Earth by a subsequent mission [1]. Strategic planning by the Mars 2020 Science Team has thus far identified a set of notional sample caches. These arose from integrating the testable hypotheses that could be addressed by Mars 2020 within the framework of the geology of Jezero crater and its surroundings [2], identifying specific locations of high scientific interest by analysis of remotely sensed data, and traversability considerations [1]. Here we describe the general characteristics of the identified notional caches and compare them to the types of samples previously prioritized by the wider Mars science community [3]. While strategic planning will guide and streamline the decision-making processes once the rover lands at Jezero crater, the actual samples collected will depend on the landing location, the traverse taken, and decisions made by the Mars 2020 Science Team.

The Mars 2020 team plans to explore and collect samples from diverse ancient environments preserved in the rocks in and around Jezero Crater. Here, we offer a tour of diverse, ancient habitable environments planned for exploration and sampling.

Observations from orbital spacecraft have shown that Jezero crater on Mars contains a prominent fan-shaped body of sedimentary rock deposited at its western margin. The Perseverance rover landed in Jezero crater in February 2021. We analyze images taken by the rover in the 3 months after landing. The fan has outcrop faces, which were invisible from orbit, that record the hydrological evolution of Jezero crater. We interpret the presence of inclined strata in these outcrops as evidence of deltas that advanced into a lake. In contrast, the uppermost fan strata are composed of boulder conglomerates, which imply deposition by episodic high-energy floods. This sedimentary succession indicates a transition from sustained hydrologic activity in a persistent lake environment to highly energetic short-duration fluvial flows.

Observations from orbital spacecraft have shown that Jezero crater on Mars contains a prominent fan-shaped body of sedimentary rock deposited at its western margin. The Perseverance rover landed in Jezero crater in February 2021. We analyze images taken by the rover in the 3 months after landing. The fan has outcrop faces, which were invisible from orbit, that record the hydrological evolution of Jezero crater. We interpret the presence of inclined strata in these outcrops as evidence of deltas that advanced into a lake. In contrast, the uppermost fan strata are composed of boulder conglomerates, which imply deposition by episodic high-energy floods. This sedimentary succession indicates a transition from sustained hydrologic activity in a persistent lake environment to highly energetic short-duration fluvial flows.

An expanded sedimentary section provides an opportunity to elucidate conditions in the nascent Chicxulub crater during the hours to millennia after the Cretaceous‐Paleogene (K‐Pg) boundary impact. The sediments were deposited by tsunami followed by seiche waves as energy in the crater declined, culminating in a thin hemipelagic marlstone unit that contains atmospheric fallout. Seiche deposits are predominantly composed of calcite formed by decarbonation of the target limestone during impact followed by carbonation in the water column. Temperatures recorded by clumped isotopes of these carbonates are in excess of 70°C, with heat likely derived from the central impact melt pool. Yet, despite the turbidity and heat, waters within the nascent crater basin soon became a viable habitat for a remarkably diverse cross section of the food chain. The earliest seiche layers deposited with days or weeks of the impact contain earliest Danian nannoplankton and dinocyst survivors. The hemipelagic marlstone representing the subsequent years to a few millennia contains a nearly monogeneric calcareous dinoflagellate resting cyst assemblage suggesting deteriorating environmental conditions, with one interpretation involving low light levels in the impact aftermath. At the same horizon, microbial fossils indicate a thriving bacterial community and unique phosphatic fossils including appendages of pelagic crustaceans, coprolites andbacteria‐tunneled fish bone, suggesting that this rapid recovery of the base of the food chain may have supported the survival of larger, higher trophic‐level organisms. The extraordinarily diverse fossil assemblage indicates that the crater was a unique habitat in the immediate impact aftermath, possibly as aresult of heat and nutrients supplied by hydrothermal activity.

An expanded sedimentary section provides an opportunity to elucidate conditions in the nascent Chicxulub crater during the hours to millennia after the Cretaceous‐Paleogene (K‐Pg) boundary impact. The sediments were deposited by tsunami followed by seiche waves as energy in the crater declined, culminating in a thin hemipelagic marlstone unit that contains atmospheric fallout. Seiche deposits are predominantly composed of calcite formed by decarbonation of the target limestone during impact followed by carbonation in the water column. Temperatures recorded by clumped isotopes of these carbonates are in excess of 70°C, with heat likely derived from the central impact melt pool. Yet, despite the turbidity and heat, waters within the nascent crater basin soon became a viable habitat for a remarkably diverse cross section of the food chain. The earliest seiche layers deposited with days or weeks of the impact contain earliest Danian nannoplankton and dinocyst survivors. The hemipelagic marlstone representing the subsequent years to a few millennia contains a nearly monogeneric calcareous dinoflagellate resting cyst assemblage suggesting deteriorating environmental conditions, with one interpretation involving low light levels in the impact aftermath. At the same horizon, microbial fossils indicate a thriving bacterial community and unique phosphatic fossils including appendages of pelagic crustaceans, coprolites andbacteria‐tunneled fish bone, suggesting that this rapid recovery of the base of the food chain may have supported the survival of larger, higher trophic‐level organisms. The extraordinarily diverse fossil assemblage indicates that the crater was a unique habitat in the immediate impact aftermath, possibly as aresult of heat and nutrients supplied by hydrothermal activity.

Cambrian siliciclastic sequences along the Dead Sea Transform (DST) margin in southern Israel and southern Jordan host both detrital fluorapatite [D‐apatite] and U‐rich authigenic carbonate‐fluorapatite (francolite) [A‐apatite]. D‐apatite and underlying Neoproterozoic basement apatite yield fission‐track (FT) data reflecting Palaeozoic–Mesozoic sedimentary cycles and epeirogenic events, and dispersed (U–Th–Sm)/He (AHe) ages. A‐apatite, which may partially or completely replace D‐apatite, yields an early Miocene FT age suggesting formation by fracturing, hydrothermal fluid ascent and intra‐strata recrystallisation, linked to early DST motion. The DST, separating the African and Arabian plates, records ~105 km of sinistral strike‐slip displacement, but became more transtensional post‐5 Ma. Helium diffusion measurements on A‐apatite are consistent with thermally activated volume diffusion, indicating Tc ~52 to 56 ± 10°C (cooling rate 10°C/Ma). A‐apatite AHe data record Pliocene cooling (~35 to 40°C) during the transtensional phase of movement. This suggests that timing of important milestones in DST motion can be discerned using A‐apatite low‐temperature thermochronology data alone.

Defying common expectations, some of the oldest landscapes on Earth are found in the humid tropics. 40Ar/39Ar dating shows that Fe-rich regolith that armors the ∼700–1000 m elevation banded iron-formation plateaus at Urucum, Mato Grosso do Sul, Brazil, results from chemical weathering that began at least 70 million years ago. Cosmogenic 3He concentrations in residual hematite clasts and the underlying saprolite on one plateau imply an extraordinarily slow surface lowering rate of 0.07–0.10 m Ma−1 maintained over the last ∼17–70 Ma. Cosmogenic 10Be and 26Al concentrations in a quartz vein in the same plateau suggest a similarly low surface erosion rate of ∼0.11–0.18 m Ma−1 over the past few million years. Cosmogenic 3He concentrations in goethite cementing a ∼200 m elevation indurated pediment ∼3 km northwest of the plateau escarpment yields a minimum exposure age of ∼2.6 Ma, revealing that some of the low-lying areas had already been incised and locally stabilized by goethite cementation by then. In contrast, 36Cl measurements on limestone outcrops at ∼170 m elevation and ∼6.5 km northwest of the Urucum plateau reveal short-term erosion rates of 7.1 ± 0.4 and 10.1 ± 0.5 m Ma−1, showing that areas not stabilized by goethite cementation are undergoing erosion at a relatively fast pace. The ∼700 m of relief at Urucum is consistent with differential denudation over ∼70 Ma controlled by lithological differences in chemical and physical rates of weathering and erosion. Effective chemical weathering and protracted low rates of erosion for the high-elevation surface result in armoring by a resilient residue of hematite blocks that are relatively insoluble and too heavy to transport. Armoring by the surface lag of large chemically resilient hematite blocks and bedded hematite saprolite focuses meteoric water flow into the subsurface and along faults. Preferential subsurface flow along the permeable pathways between insoluble hematite blocks, combined with segmentation of the

The Atacama Desert of northern Chile hosts many of the world's largest porphyry copper deposits (PCDs), many of which have been upgraded through supergene enrichment (Sillitoe and McKee 1996). Enrichment of exhumed PCDs occurs in the near-surface weathering environment, when copper is leached from primary sulphides and reprecipitated beneath the water table to form a concentrated blanket of secondary copper minerals (e.g. chalcocite). The Atacama is one of the driest regions on Earth, but supergene enrichment requires precipitation to drive the aqueous redistribution of metals. Previous alunite dating studies have suggested enrichment stopped in the middle Miocene, due to an increase in aridity. This conclusion is supported by the youngest supergene alunite ages from several PCDs clustering around 14 Ma (Sillitoe 2005). Hematite also forms in the leached caps of PCDs and hematite (U-Th)/He geochronology provides a tool to track the downward progression of weathering fronts (e.g. Cooper et al. 2016). This study will combine Ar-40/Ar-39 dating of supergene alunite (a proxy for the timing of copper enrichment) with hematite geochronology (as an indicator of the progression of weathering) to better understand the link between water table movement and supergene enrichment of Central Andean PCDs.

What allows a planet to be both within a potentially habitable zone and sustain habitability over long geologic time? With the advent of exoplanetary astronomy and the ongoing discovery of terrestrial-type planets around other stars, our own solar system becomes a key testing ground for ideas about what factors control planetary evolution. Mars provides the solar system's longest record of the interplay of the physical and chemical processes relevant to habitability on an accessible rocky planet with an atmosphere and hydrosphere. Here we review current understanding and update the timeline of key processes in early Mars history. We then draw on knowledge of exoplanets and the other solar system terrestrial planets to identify six broad questions of high importance to the development and sustaining of habitability (unprioritized): (1) Is small planetary size fatal? (2) How do magnetic fields influence atmospheric evolution? (3) To what extent does starting composition dictate subsequent evolution, including redox processes and the availability of water and organics? (4) Does early impact bombardment have a net deleterious or beneficial influence? (5) How do planetary climates respond to stellar evolution, e.g., sustaining early liquid water in spite of a faint young Sun? (6) How important are the timescales of climate forcing and their dynamical drivers? Finally, we suggest crucial types of Mars measurements (unprioritized) to address these questions: (1) in situ petrology at multiple units/sites; (2) continued quantification of volatile reservoirs and new isotopic measurements of H, C, N, O, S, Cl, and noble gases in rocks that sample multiple stratigraphic sections; (3) radiometric age dating of units in stratigraphic sections and from key volcanic and impact units; (4) higher-resolution measurements of heat flux, subsurface structure, and magnetic field anomalies coupled with absolute age dating. Understanding the evolution of early Mars will feed forward to unde

The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth1,2. It was caused by the impact of an asteroid3,4on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago5, forming the Chicxulub impact crater6,7. After the mass extinction, the recovery of the global marine ecosystem - measured as primary productivity - was geographically heterogeneous8; export production in the Gulf of Mexico and North Atlantic-western Tethys was slower than in most other regions8-11, taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning12, on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors - trophic interactions13, species incumbency and competitive exclusion by opportunists14- and 'chance'8,15,16. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass

The development of an in-situ geochronology capability for Mars and other planetary surfaces has the potential to fundamentally change our understanding of the evolution of terrestrial bodies in the Solar System. For Mars specifically, many of our most basic scientific questions about the geologic history of the planet require knowledge of the absolute time at which an event or process took place on its surface. For instance, what was the age and rate of early Martian climate change recorded in the mineralogy and morphology of surface lithologies (e.g., [1])? In-situ ages from a few select locations within the globally established stratigraphy of Mars would be transformative, enabling us to place direct chronologic constraints on the timing and rates of impact, volcanic, sedimentary, and aqueous processes on the Martian surface.

The development of an in-situ geochronology capability for Mars and other planetary surfaces has the potential to fundamentally change our understanding of the evolution of terrestrial bodies in the Solar System. For Mars specifically, many of our most basic scientific questions about the geologic history of the planet require accurate knowledge of the absolute time at which an event or process took place. For instance, what was the age and rate of early Martian climate change faithfully recorded in the mineralogy and morphology of surface lithologies (e.g., [1])? Currently, our only means of assessing the absolute age of a surface on a planetary body is through the use of crater counting statistics. This technique is fraught with uncertainty for planets with active geologic surfaces, on the order of billions of years in some cases (e.g., [2]). Accordingly, there is much room for improvement in our understanding of the absolute chronology of the surfaces of rocky planetary bodies.

Contains fulltext : 83905.pdf (publisher's version ) (Closed access)

Contains fulltext : 83905.pdf (publisher's version ) (Closed access)

Contains fulltext : 83905.pdf (publisher's version ) (Closed access)

Two long-lived peaks in extraterrestrial ^3He flux have been identified in the sedimentary record of the Cenozoic Era: at 8.2 Ma (late Miocene) and 35.8 Ma (late Eocene). These peaks document the occurrence of important events in the recent history of the solar system. The timing and temporal evolution of the dust event in the late Miocene as well as the absence of major terrestrial impact craters are consistent with an origin in the catastrophic collision that produced the Veritas asteroid family at this time. In contrast, there is no known asteroid collision corresponding to the late Eocene peak. Instead, the late Eocene event has elements consistent with a comet shower produced by a close stellar encounter. Both the rise time and the fall time of the enhanced dust flux and the occurrence of two major terrestrial impacts at the peak of the event are predicted by considerations of comet shower dynamics. However, debris from one of these impacts has a Cr isotope ratio that appears to exclude a carbonaceous chondrite impactor. If the presumption that comets have a carbonaceous chondrite–like composition is accurate, then an alternative mechanism for the late Eocene event may be required. The recent suggestion of an asteroid shower, where the ^3He-bearing dust resulted from lunar impacts, is one such possibility, but it too fails to account fully for existing observations. The cause of the late Eocene event thus remains uncertain.

The Tehachapi complex lies at the southern end of the Sierra Nevada batholith adjacent to the Neogene-Quaternary Garlock fault. The complex is composed principally of high-pressure (8–10 kbar) Cretaceous batholithic rocks, and it represents the deepest exposed levels of a continuous oblique crustal section through the southern Sierra Nevada batholith. Over the southern ~100 km of this section, structural/petrologic continuity and geochronological data indicate that ≥35 km of felsic to intermediate-composition crust was generated by copious arc magmatism primarily between 105 and 99 Ma. In the Tehachapi complex, these batholithic rocks intrude and are bounded to the west by similar-composition gneissic-textured high-pressure batholithic rocks emplaced at ca. 115–110 Ma. This lower crustal complex is bounded below by a regional thrust system, which in Late Cretaceous time tectonically eroded the underlying mantle lithosphere, and in series displaced and underplated the Rand Schist subduction assemblage by low-angle slip from the outboard Franciscan trench. Geophysical and mantle xenolith studies indicate that the remnants of this shallow subduction thrust descend northward through the crust and into the mantle, leaving the mantle lithosphere intact beneath the greater Sierra Nevada batholith. This north-dipping regional structure records an inflection in the Farallon plate, which was segmented into a shallow subduction trajectory to the south and a normal steeper trajectory to the north. We combine new and published data from a broad spectrum of thermochronometers that together form a coherent data array constraining the thermal evolution of the complex. Integration of these data with published thermobarometric and petrogenetic data also constrains the tectonically driven decompression and exhumation history of the complex. The timing of arc magmatic construction of the complex, as denoted above, is resolved by a large body of U/Pb zircon ages. High-confidence thermoc

Helium isotopes are a robust geochemical tracer of a primordial mantle component in hot spot volcanism. The high 3He/4He (up to 35 RA, where RA is the atmospheric 3He/4He ratio of 1.39 × 10−6) of some Hawaiian Island volcanism is perhaps the classic example. New results for picrites and basalts from the Hawaiian-Emperor seamount chain indicate that the hot spot has produced high 3He/4He lavas for at least the last 76 million years. Picrites erupted at 76 Ma have 3He/4He (10–14 RA), which is at the lower end of the range for the Hawaiian Islands but still above the range of modern mid-ocean ridge basalt (MORB; 6–10 RA). This was at a time when hot spot volcanism was occurring on thin lithosphere close to a spreading ridge and producing lava compositions otherwise nearly indistinguishable from MORB. After the hot spot and spreading center diverged during the Late Cretaceous, the hot spot produced lavas with significantly higher 3He/4He (up to 24 RA). Although 3He/4He ratios stabilized at relatively high values by 65 Ma, other chemical characteristics such as La/Yb and 87Sr/86Sr did not reach and stabilize at Hawaiian-Island-like values until ~45 Ma. Our limited 3He/4He record for the Hawaiian hot spot shows a poor correlation with plume flux estimates (calculated from bathymetry and residual gravity anomalies [Van Ark and Lin, 2004]). If 3He is a proxy for the quantity of primordial mantle material within the plume, then the lack of correlation between 3He/4He and calculated plume flux suggests that variation in primordial mantle flux is not the primary factor controlling total plume flux.

An in-depth Sr-Nd-Pb-He-Os isotope and trace element study of the EMII-defining Samoan hot spot lavas leads to a new working hypothesis for the origin of this high 87Sr/86Sr mantle end-member. Systematics of the Samoan fingerprint include (1) increasing 206Pb/204Pb with time - from 18.6 at the older, western volcanoes to 19.4 at the present-day hot spot center, Vailulu'u Seamount, (2) en-echelon arrays in 206Pb/204Pb – 208Pb/204Pb space which correspond to the two topographic lineaments of the 375 km long volcanic chain – this is much like the Kea and Loa Trends in Hawai'i, (3) the highest 87Sr/86Sr (0.7089) of all oceanic basalts, (4) an asymptotic decrease in 3He/4He from 24 RA [Farley et al., 1992] to the MORB value of 8 RA with increasing 87Sr/86Sr, and (5) mixing among four components which are best described as the “enriched mantle”, the depleted FOZO mantle, the (even more depleted) MORB Mantle, and a mild HIMU (high 238U/204Pb) mantle component. A theoretical, “pure” EMII lava composition has been calculated and indicates an extremely smooth trace element pattern of this end-member mantle reservoir. The standard recycling model (of ocean crust/sediment) fails as an explanation for producing Samoan EM2, due to these smooth spidergrams for EM2 lavas, low 187Os/188Os ratios and high 3He/4He (>8 RA). Instead, the origin of EM2 has been modeled with the ancient formation of metasomatised oceanic lithosphere, followed by storage in the deep mantle and return to the surface in the Samoan plume.

Helium isotopes are a robust geochemical tracer of a primordial mantle component in hot spot volcanism. The high 3He/4He (up to 35 RA, where RA is the atmospheric 3He/4He ratio of 1.39 × 10−6) of some Hawaiian Island volcanism is perhaps the classic example. New results for picrites and basalts from the Hawaiian-Emperor seamount chain indicate that the hot spot has produced high 3He/4He lavas for at least the last 76 million years. Picrites erupted at 76 Ma have 3He/4He (10–14 RA), which is at the lower end of the range for the Hawaiian Islands but still above the range of modern mid-ocean ridge basalt (MORB; 6–10 RA). This was at a time when hot spot volcanism was occurring on thin lithosphere close to a spreading ridge and producing lava compositions otherwise nearly indistinguishable from MORB. After the hot spot and spreading center diverged during the Late Cretaceous, the hot spot produced lavas with significantly higher 3He/4He (up to 24 RA). Although 3He/4He ratios stabilized at relatively high values by 65 Ma, other chemical characteristics such as La/Yb and 87Sr/86Sr did not reach and stabilize at Hawaiian-Island-like values until ~45 Ma. Our limited 3He/4He record for the Hawaiian hot spot shows a poor correlation with plume flux estimates (calculated from bathymetry and residual gravity anomalies [Van Ark and Lin, 2004]). If 3He is a proxy for the quantity of primordial mantle material within the plume, then the lack of correlation between 3He/4He and calculated plume flux suggests that variation in primordial mantle flux is not the primary factor controlling total plume flux.

An in-depth Sr-Nd-Pb-He-Os isotope and trace element study of the EMII-defining Samoan hot spot lavas leads to a new working hypothesis for the origin of this high 87Sr/86Sr mantle end-member. Systematics of the Samoan fingerprint include (1) increasing 206Pb/204Pb with time - from 18.6 at the older, western volcanoes to 19.4 at the present-day hot spot center, Vailulu'u Seamount, (2) en-echelon arrays in 206Pb/204Pb – 208Pb/204Pb space which correspond to the two topographic lineaments of the 375 km long volcanic chain – this is much like the Kea and Loa Trends in Hawai'i, (3) the highest 87Sr/86Sr (0.7089) of all oceanic basalts, (4) an asymptotic decrease in 3He/4He from 24 RA [Farley et al., 1992] to the MORB value of 8 RA with increasing 87Sr/86Sr, and (5) mixing among four components which are best described as the “enriched mantle”, the depleted FOZO mantle, the (even more depleted) MORB Mantle, and a mild HIMU (high 238U/204Pb) mantle component. A theoretical, “pure” EMII lava composition has been calculated and indicates an extremely smooth trace element pattern of this end-member mantle reservoir. The standard recycling model (of ocean crust/sediment) fails as an explanation for producing Samoan EM2, due to these smooth spidergrams for EM2 lavas, low 187Os/188Os ratios and high 3He/4He (>8 RA). Instead, the origin of EM2 has been modeled with the ancient formation of metasomatised oceanic lithosphere, followed by storage in the deep mantle and return to the surface in the Samoan plume.

We measured He, Sr, Nd, Pb, and Os isotope ratios and major and trace element concentrations in stratigraphically and paleomagnetically controlled shield-stage lavas from Kauai, Hawaii. The range of 3He/4He ratios (17–28 RA) from Kauai is similar to that reported from Loihi and thus challenges the prevailing notion that high 3He/4He ratios are restricted to the preshield stage of Hawaiian magmatism. 3He/4He ratios vary erratically with stratigraphic position, and chronostratigraphic control from paleomagnetic data indicates very rapid changes in the 3He/4He ratios (up to 8 RA in ~102 years). These variations in helium isotopic ratios are correlated with variations in radiogenic isotope ratios, suggesting rapid changes in melt composition supplying the magma reservoir. A three-component mixing model, previously proposed for Hawaiian shield lavas, does not adequately explain the isotopic data in Kauai shield lavas. The addition of a depleted-mantle (DM) component with the isotopic characteristics similar to posterosional basalts explains the isotopic variability in Kauai shield lavas. The DM component is most apparent in lavas from the Kauai shield and is present in varying proportion in other Hawaiian shield volcanoes. Shield lavas from Kauai sample a high 3He/4He end-member (Loihi component), but while lavas from western Kauai have a larger contribution from the Kea component (high 206Pb/204Pb, anomalously low 207Pb/204Pb relative to 206Pb/204Pb), lavas from eastern Kauai have a larger proportion of an enriched (Koolau) component. The systematic isotopic differences between eastern and western Kauai reflect a gradual migration of the locus of volcanism from west to east, or alternatively east and west Kauai are two distinct shield volcanoes. In the latter case, the two shield volcanoes have maintained distinct magma supply sources and plumbing systems. Our new geochemical data from Kauai are consistent with the existence of a single high 3He/4He reservoir in the Haw

We measured He, Sr, Nd, Pb, and Os isotope ratios and major and trace element concentrations in stratigraphically and paleomagnetically controlled shield-stage lavas from Kauai, Hawaii. The range of 3He/4He ratios (17–28 RA) from Kauai is similar to that reported from Loihi and thus challenges the prevailing notion that high 3He/4He ratios are restricted to the preshield stage of Hawaiian magmatism. 3He/4He ratios vary erratically with stratigraphic position, and chronostratigraphic control from paleomagnetic data indicates very rapid changes in the 3He/4He ratios (up to 8 RA in ~102 years). These variations in helium isotopic ratios are correlated with variations in radiogenic isotope ratios, suggesting rapid changes in melt composition supplying the magma reservoir. A three-component mixing model, previously proposed for Hawaiian shield lavas, does not adequately explain the isotopic data in Kauai shield lavas. The addition of a depleted-mantle (DM) component with the isotopic characteristics similar to posterosional basalts explains the isotopic variability in Kauai shield lavas. The DM component is most apparent in lavas from the Kauai shield and is present in varying proportion in other Hawaiian shield volcanoes. Shield lavas from Kauai sample a high 3He/4He end-member (Loihi component), but while lavas from western Kauai have a larger contribution from the Kea component (high 206Pb/204Pb, anomalously low 207Pb/204Pb relative to 206Pb/204Pb), lavas from eastern Kauai have a larger proportion of an enriched (Koolau) component. The systematic isotopic differences between eastern and western Kauai reflect a gradual migration of the locus of volcanism from west to east, or alternatively east and west Kauai are two distinct shield volcanoes. In the latter case, the two shield volcanoes have maintained distinct magma supply sources and plumbing systems. Our new geochemical data from Kauai are consistent with the existence of a single high 3He/4He reservoir in the Haw

The (U-Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions. However, in phases with crystallization ages less than ∼ 1 Ma, chemical fractionation within the238U decay series may modify the He ingrowth rate, causing He ages computed from the secular equilibrium age equation to be incorrect. The resulting systematic error depends on the [230Th/238U] activity ratio of the dated phase when it is erupted, and on the eruption age. Zircons, which exclude Th relative to U, will likely have secular equilibrium He 'ages' that underestimate the eruption age by up to a few tens of %, decreasing with increasing eruption age. Apatites tend to accommodate U and Th with little fractionation, so apatite secular equilibrium He ages will be nearly concordant with eruption age. If minerals are erupted immediately after crystallization, the disequilibrium effect can be reasonably accounted for based on Th/U systematics. However, crystals are likely to reside for unknown but potentially long periods in a magma chamber, such that the degree of secular disequilibrium will be reduced prior to the onset of He accumulation. (U-Th)/He analyses of co-genetic phases that fractionate the U/Th ratio differently, like apatite and zircon, can be used to better constrain eruption age, as well as to provide insights into magma chamber residence time. We illustrate this approach with (U-Th)/He analyses of zircons and apatites of the Pleistocene-age Rangitawa Tephra, New Zealand.

The efficiency of many noble gas mass spectrometers to ionize gas species is known to be a function of the pressure of gas in the spectrometer. This work shows how the half plate voltage for maximum He or Ar signal depends on the spectrometer pressure and shows that the half plate voltage for maximum 4He sensitivity does not coincide with the half plate voltage for maximum 3He sensitivity. In addition, half plate voltage has a greater control on sensitivity at higher spectrometer pressures. Variations in He and Ar sensitivity and isotopic discrimination as a function of pressure are due, at least in part, to these variations in the position of maximum sensitivity with respect to half plate voltage. The maximum sensitivity settings shift to lower half plate voltage at high spectrometer pressures, irrespective of if the pressure increase is due to the gas being investigated or a different species. Therefore noble gas mass spectrometers should always be tuned at the maximum possible pressure; measurements at higher pressures should be avoided. Significant errors in the spectrometer sensitivity and discrimination can result from improper tuning and calibration of noble gas mass spectrometers.

The efficiency of many noble gas mass spectrometers to ionize gas species is known to be a function of the pressure of gas in the spectrometer. This work shows how the half plate voltage for maximum He or Ar signal depends on the spectrometer pressure and shows that the half plate voltage for maximum 4He sensitivity does not coincide with the half plate voltage for maximum 3He sensitivity. In addition, half plate voltage has a greater control on sensitivity at higher spectrometer pressures. Variations in He and Ar sensitivity and isotopic discrimination as a function of pressure are due, at least in part, to these variations in the position of maximum sensitivity with respect to half plate voltage. The maximum sensitivity settings shift to lower half plate voltage at high spectrometer pressures, irrespective of if the pressure increase is due to the gas being investigated or a different species. Therefore noble gas mass spectrometers should always be tuned at the maximum possible pressure; measurements at higher pressures should be avoided. Significant errors in the spectrometer sensitivity and discrimination can result from improper tuning and calibration of noble gas mass spectrometers.

Noble gas isotopic ratios in mantle-derived samples require variability in the time-integrated ratio of volatile to lithophile elements in the Earth. Documentation of mantle ³He/⁴He variability is becoming increasingly complete, but for the heavier noble gases, the picture is still partly clouded by the effects of atmospheric contamination of mantle samples. Nevertheless, clear variations in mantle Ne, Ar, and Xe isotopic ratios exist, are apparently correlated with ³He/⁴He, and may be the product of varying degrees of mantle degassing. However, uncertainties in noble gas geochemical behavior and several conflicting observations leave open other possibilities. Recent Ne isotopic data are particularly important because they require that the atmosphere has not been closed to exchange with space. Derivation of much of the atmosphere from a source other than degassing of the mantle is a strong possibility that complicates efforts to model the geochemical evolution of the Earth.

Noble gas isotopic ratios in mantle-derived samples require variability in the time-integrated ratio of volatile to lithophile elements in the Earth. Documentation of mantle ³He/⁴He variability is becoming increasingly complete, but for the heavier noble gases, the picture is still partly clouded by the effects of atmospheric contamination of mantle samples. Nevertheless, clear variations in mantle Ne, Ar, and Xe isotopic ratios exist, are apparently correlated with ³He/⁴He, and may be the product of varying degrees of mantle degassing. However, uncertainties in noble gas geochemical behavior and several conflicting observations leave open other possibilities. Recent Ne isotopic data are particularly important because they require that the atmosphere has not been closed to exchange with space. Derivation of much of the atmosphere from a source other than degassing of the mantle is a strong possibility that complicates efforts to model the geochemical evolution of the Earth.

In order to investigate the behavior of slab-derived volatiles in the subduction environment, helium isotope ratios have been measured in geothermal gases from the Tabar-Lihir-TangaFeni (TLTF) chain in the Bismarck Archipelago of Papua New Guinea. As recorded by several geochemical tracers, these volcanos carry an exceptionally large slab-derived component, and therefore may provide new insights to the old question of volatiles in subduction zones. Geothermal gases from Lihir Island have homogeneous ^3He/^4He ratios of 7.18±0.07 times the atmospheric ratio (R_A), while those from Ambitle Island (Feni Group) have lower ratios of 6.61±0.13 R_A. These ^3He/^4He ratios are within the range defined by more-typical arc volcanos, but lie at the low end of the spectrum observed in arc volcanos erupted through purely oceanic crust. Although a small slab-derived signature (^3He/^4He ratio lower than depleted mantle) exists in the TLTF gases, these data demonstrate that even in volcanos with a comparatively large slab component, He is overwhelmingly derived from the depleted mantle wedge. This observation further confirms the relative insensitivity of He isotopes to the presence of slab fluids. He isotope ratios of 6.25 R_A were measured in geothermal gases from the Rabaul Caldera on New Britain Island. Coincidentally, these samples were taken six months prior to the major 1994 eruption at Rabaul. In conjunction with samples taken from the same locality 8 years earlier, these data allow us to test whether increasing He isotope ratios associated with fresh ascending magmas precede volcanic eruptions. Although some of the 1986 samples had much lower ^3He/^4He ratios (5 R_A than observed in 1994, one did not. We thus find no strong evidence for a systematic rise in the He isotope ratio of the Rabaul fluids between 1986 and 1994. If a ^3He/^4He increase did precede the Rabaul eruption, then it occurred either prior to 1986 or sometime between our 1994 sampling and the eruption.

Helium isotope analyses of ground waters from the Mirror Lake drainage basin in central New Hampshire (USA) show helium in excess of air-saturated water by up to 200x. The freon ages of these waters are younger than 50 years, consistent with the local hydrology. This excess helium has an isotope ratio of ^3He/^4He = 1.65 ± 0.10 x 10^(-6). It is shown that this component cannot be the result of cosmogenic production or mixing with young water containing ^3He from the decay of (bomb) tritium. Measurements of the helium isotope ratio of local rocks indicate that they cannot be the source of this excess component. This strongly suggests that the excess helium component is the result of the addition of some external source of mantle helium. The generally accepted view suggests that mantle helium in continental environments is the result of active volcanism and extensional tectonics. The latest episodes of volcanism in this region of New England are related to the New England hot spot track (95-190 Myr) and the closure of the Iapetus (> 300 Myr). Thus, either the timescale for helium transport through the crust is of the order of 100's of Myr or the signature of mantle helium can be preserved in (e.g.) fluid inclusions for significant periods of time.

New noble gas measurements have been made on Samoan ultramafic xenoliths in order to characterize the composition and nature of entrapment of a postulated mantle metasomatic agent. The new measurements were performed on gases extracted from severely tectonized harzburgites and dunites by both bulk crushing and laser microprobe. The tectonized specimens have the highest noble gas concentrations yet reported from mantle materials and attest to deformation in a highly gas-charged environment. The noble gas isotopic systematics are similar to those observed in undeformed specimens from the same locality, and are consistent with mixing between a mantle component (e.g. ^3He/^4He = 12 R_A, ^(40)Ar/^(36) Ar > 10,000) and an atmospheric contaminant. Within the xenoliths, the mantle component is spatially associated with features previously attributed to metasomatism (e.g. HAURI et al., 1993). Although this metasomatic component has many characteristics suggesting derivation from material returned to the mantle by subduction, its relatively high ^3He/^4He ratio is enigmatic. Whatever its source, this fluid appears to have existed within the Samoan mantle over fairy large temporal and spatial scales, and plays an important role in the geochemistry of Samoan basalts. Just as with the mantle component, the deformed xenoliths are also enriched in the atmospheric contaminant. This enrichment suggests pervasive penetration of air into the ubiquitous micro fractures and decrepitated fluid inclusions of the deformed specimens. In addition to source and contamination effects, the noble gases within these xenoliths record variable degrees of elemental fractionation. While the gas-rich (deformed) xenoliths have ^4He-^(21)Ne*-^(40)Ar* systematics close to long-term closed-system behavior, the comparatively gas-poor samples have lost up to 90% of their helium without concomitant loss of neon and argon. This likely represents diffusive loss of helium after fluid inclusion entrapment.

Following K-Ar dating of a mudstone and a sandstone, a third sample has been dated by the Curiosity rover exploring Gale Crater. The Mojave 2 mudstone, which contains relatively abundant jarosite, yielded a young K-Ar bulk age of 2.57 ± 0.39 Ga (1σ precision). A two-step heating experiment was implemented in an effort to resolve the K-Ar ages of primary and secondary mineralogical components within the sample. This technique involves measurement of 40Ar released in low-temperature (500°C) and high-temperature (930°C) steps, and a model of the potassium distribution within the mineralogical components of the sample. Using this method, the high-temperature step yields a K-Ar model age of 4.07 ± 0.63 Ga associated with detrital plagioclase, compatible with the age obtained on the Cumberland mudstone by Curiosity. The low-temperature step, associated with jarosite mixed with K-bearing evaporites and/or phyllosilicates, gave a youthful K-Ar model age of 2.12 ± 0.36 Ga. The interpretation of this result is complicated by the potential for argon loss after mineral formation. Comparison with the results on Cumberland and previously published constraints on argon retentivity of the individual phases likely to be present suggests that the formation age of the secondary materials, correcting for plausible extents of argon loss, is still less than 3 Ga, suggesting post-3 Ga aqueous processes occurred in the sediments in Gale Crater. Such a result is inconsistent with K-bearing mineral formation in Gale Lake and instead suggests postdepositional fluid flow at a time after surface fluvial activity on Mars is thought to have largely ceased.

By combining the Sample Analysis at Mars (SAM) instrument’s capabilities with those of the alpha particle X-ray spectrometer (APXS) on the Curiosity rover, radiometric K-Ar ages and 3He, 21Ne, and 36Ar exposure ages have been measured on two samples as of sol 1430. The Cumberland mudstone was found to have a K-Ar age of 4.21 ± 0.35 Ga (all uncertainties here are reported in 1σ), consistent with crater-counting estimates of the surrounding terrains. A second geochronology experiment was undertaken on the potassium-rich Windjana sandstone, which resulted in the unreasonably young and unrepeatable ages of 627 ± 50 Ma and 1710 ± 110 Ma on two different aliquots. These results were attributed to incomplete Ar extraction arising from the coarse grainsize and Ar-retentive characteristics of sanidine, the major K-bearing mineral in the sample. Recently, a drilled bedrock sample from the Murray mudstone formation (termed Mojave 2) was found by the CheMin instrument to contain approximately 3.1 wt% jarosite. Jarosite is precipitated from acidic, sulfate-rich waters and is a suitable mineral for KAr dating. On a large scale, jarosite has been thought to signal the aridification of Mars and a shift towards a cold, dry environment. The small amount of jarosite in this sample prohibits generalization to a larger Martian context. However, since jarosite forms strictly through interaction with water, the K-Ar age of the jarosite in Mojave 2 puts a maximum age constraint on the last time liquid water was present at the sample site. By extension, it could also indicate the time of the very final stages of evaporation from the lake in Gale Crater. A two-step heating experiment was designed to obtain the K-Ar age of the jarosite, which accounts for about 20% of the K2O in the sample. The remainder of the K2O exists in plagioclase, an amorphous component, and possibly a small amount in K-bear

Variability in the sulfur isotopic composition in sediments can reflect atmospheric, geologic and biological processes. Evidence for ancient fluvio-lacustrine environments at Gale crater on Mars and a lack of efficient crustal recycling mechanisms on the planet suggests a surface environment that was once warm enough to allow the presence of liquid water, at least for discrete periods of time, and implies a greenhouse effect that may have been influenced by sulfur-bearing volcanic gases. Here we report in situ analyses of the sulfur isotopic compositions of SO2 volatilized from ten sediment samples acquired by NASA’s Curiosity rover along a 13 km traverse of Gale crater. We find large variations in sulfur isotopic composition that exceed those measured for Martian meteorites and show both depletion and enrichment in 34S. Measured values of δ34S range from −47 ± 14‰ to 28 ± 7‰, similar to the range typical of terrestrial environments. Although limited geochronological constraints on the stratigraphy traversed by Curiosity are available, we propose that the observed sulfur isotopic signatures at Gale crater can be explained by equilibrium fractionation between sulfate and sulfide in an impact-driven hydrothermal system and atmospheric processing of sulfur-bearing gases during transient warm periods.

Copyright © 2019 The Authors, The breakup of the L-chondrite parent body in the asteroid belt 466 million years (Ma) ago still delivers almost a third of all meteorites falling on Earth. Our new extraterrestrial chromite and 3He data for Ordovician sediments show that the breakup took place just at the onset of a major, eustatic sea level fall previously attributed to an Ordovician ice age. Shortly after the breakup, the flux to Earth of the most fine-grained, extraterrestrial material increased by three to four orders of magnitude. In the present stratosphere, extraterrestrial dust represents 1% of all the dust and has no climatic significance. Extraordinary amounts of dust in the entire inner solar system during >2 Ma following the L-chondrite breakup cooled Earth and triggered Ordovician icehouse conditions, sea level fall, and major faunal turnovers related to the Great Ordovician Biodiversification Event.