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Imbrian

The Imbrian is a major division of the lunar geologic timescale, spanning approximately 3.85 to 3.2 billion years ago and marking a transition from intense basin-forming impacts to widespread volcanic resurfacing of the Moon's surface.^[1]^[2] This period is defined by the excavation of prominent multi-ring basins, including the Imbrium and Orientale basins, which occurred during its early phase around 3.85 to 3.80 billion years ago, contributing to the decline in large-scale cratering rates following the Nectarian period.^[1]^[3] The Imbrian is subdivided into the Early Imbrian and Late Imbrian epochs, with the former dominated by ejecta deposits from basin impacts, such as the Fra Mauro Formation—a widespread blanket of breccias averaging 550 meters thick—while the latter saw the emplacement of basaltic lavas that formed the dark lunar maria covering about 16% of the Moon's surface.^[1]^[3] Key stratigraphic units from this era include the Cayley and Apennine Bench Formations, representing plains materials post-dating the Imbrium event but pre-dating mare flooding.^[3] Volcanic activity intensified during the Late Imbrian, with magma erupting through the thinned crust to fill impact basins, though non-mare highland volcanism, as seen in features like the Gruithuisen and Mairan domes, also occurred and produced morphologically distinct extrusive rocks.^[1]^[4] The Imbrian period's significance lies in its role during the tail end of the hypothesized Late Heavy Bombardment, a spike in impacts that reshaped the lunar highlands, while also initiating the long decline of endogenic activity that shaped the Moon's visible terrain today.^[5] Tectonic features from this time, such as thrust faulting in basin margins, were largely confined to local scales due to the Moon's cooling interior, with minimal global effects post-Imbrium.^[6] Overall, the Imbrian represents a pivotal epoch in lunar evolution, bridging cataclysmic bombardment with the more quiescent eras that followed.^[1]

Overview

Definition

The Imbrian is a geologic period in the Moon's stratigraphic timescale, representing a key phase in lunar history characterized by the decline of major impact events and the onset of extensive volcanic resurfacing.^[7] It spans approximately from 3.85 to 3.2 billion years ago (Ga), following the Nectarian period and preceding the Eratosthenian.^[7] The period's name derives from the prominent Mare Imbrium basin, whose ejecta deposits form a stratigraphic marker for its base, as originally proposed in early lunar mapping efforts.^[8] The Imbrian is subdivided into two epochs based on stratigraphic and radiometric boundaries tied to major geological events. The Early Imbrian epoch extends from about 3.85 to 3.80 Ga, encompassing the formation of the largest preserved impact basins.^[1] The Late Imbrian epoch follows, lasting from roughly 3.80 to 3.2 Ga, and is distinguished by a marked reduction in basin-scale impacts and the initiation of prolonged mare basalt volcanism.^[7] This period signifies a critical transition in lunar evolution, shifting from the intense heavy bombardment of earlier epochs—marked by frequent giant impacts—to a phase dominated by internal magmatic processes that flooded impact basins with basaltic lavas, fundamentally altering the Moon's surface morphology and composition.^[1] These changes established much of the Moon's modern highland-mare dichotomy, with Imbrian deposits providing essential context for understanding solar system bombardment histories.^[7]

Timeline and Chronology

The Imbrian period in lunar geology spans approximately 650 million years, from about 3.85 billion years ago (Ga) to 3.2 Ga, marking a transitional phase from intense basin-forming impacts to widespread mare volcanism. The lower boundary with the preceding Nectarian period is defined by the formation of the Imbrium basin at ~3.85 Ga, based on stratigraphic superposition of ejecta units and calibrated radiometric ages from impact melt rocks. The upper boundary with the Eratosthenian period is placed at ~3.2 Ga, defined by the age of the Eratosthenes crater marking the transition to smaller impact features, although mare volcanism continued into the Eratosthenian and later periods.^[1] This period is subdivided into the Early Imbrian (~3.85–3.80 Ga), dominated by impact events, and the Late Imbrian (~3.80–3.2 Ga), characterized by prolonged volcanic resurfacing. Dating of the Imbrian chronology relies primarily on radiometric techniques applied to lunar samples returned by Apollo and Luna missions, including argon-argon (⁴⁰Ar/³⁹Ar) and rubidium-strontium (⁸⁷Rb/⁸⁷Sr) methods, which provide precise crystallization ages for impact melts and basaltic rocks. For instance, ⁴⁰Ar/³⁹Ar dating of Fra Mauro Formation samples from Apollo 14 yields ages around 3.85–3.92 Ga for Imbrium-related materials, anchoring the lower boundary. Complementary crater counting methods, using size-frequency distributions from imagery acquired during Apollo and Luna missions, calibrate relative ages against these radiometric anchors to estimate surface exposure times across unsampled regions. Uncertainties in the Imbrian timeline arise from the overlapping signatures of impact ejecta and subsequent volcanic overprints, which can obscure stratigraphic relations and lead to age discrepancies of up to 100 million years for boundary definitions. These challenges have been addressed through post-Apollo missions, notably the Lunar Reconnaissance Orbiter (LRO), which provides high-resolution imagery enabling refined crater counts and more accurate modeling of resurfacing rates, thus tightening estimates for Late Imbrian mare units to within 50–100 million years. Recent samples from the Chang'e-5 mission (as of 2021) confirm mare basalts as young as ~2.0 Ga in later periods, further refining the overall volcanic timeline beyond the Imbrian.^[9]

Geological Context

Position in Lunar Timescale

The lunar geologic timescale divides the Moon's history into five primary periods based on stratigraphic superposition, crater morphology, and radiometric dating: the Pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican.^[10] These divisions reflect a progression from early, high-intensity impact cratering to later, lower-rate processes dominated by volcanism and erosion.^[10] The Imbrian period constitutes the third major stratigraphic division, succeeding the Pre-Nectarian and Nectarian eras of predominantly cataclysmic basin formation. It acts as a pivotal bridge in lunar history, marking the culmination of large-scale impact events—such as those forming the Imbrium and Orientale basins—with the emergent phase of extensive mare volcanism that resurfaced significant portions of the lunar highlands.^[10] This transition is evident in the declining cratering flux, from around 40-50 basin-scale impacts in preceding periods to just two major ones (Imbrium and Orientale) during the Early Imbrian, alongside the initial outpouring of basaltic lavas that would define the maria.^[10] Key stratigraphic units assigned to the Imbrian encompass impact-related deposits like the Fra Mauro Formation, comprising ejecta blankets from the Imbrium basin that blanket vast highland regions, and the Hevelius Formation, consisting of layered ejecta and secondary craters from the Orientale basin.^[10] Basin rim materials, including massifs and knobby terrains such as the Montes Rook Formation, further characterize this period, while early mare basalts represent the volcanic infilling of these structures, forming smooth plains with low crater densities.^[10] These units collectively overlie Nectarian materials and are overlain by Eratosthenian deposits, establishing the Imbrian's clear position in the stratigraphic column.^[10] For comparative planetary geology, the Imbrian period aligns temporally with Earth's early Archean eon (approximately 3.85 to 3.2 billion years ago), an interval of analogous heavy bombardment that influenced crustal evolution on both bodies. This overlap underscores the Imbrian's role in reconstructing shared solar system dynamics, including impactor fluxes and their effects on planetary differentiation.

Preceding Nectarian Period

The Nectarian period, spanning approximately 3.92 to 3.85 billion years ago (Ga), represents a phase of intense impact bombardment on the Moon, marked by the formation of several large multi-ring basins that reshaped the lunar crust. This era began with the Nectaris basin impact, which produced the Janssen Formation ejecta blanket and served as the stratigraphic marker for the onset of the Nectarian.^[11] Subsequent basins, including Humorum in the late Nectarian, contributed to widespread ejecta deposition and crustal disruption, with Humorum's formation estimated at around 3.9 Ga based on crater counting and radiometric constraints. These events excavated deep into the lunar highlands, exposing anorthositic materials and altering the pre-existing highland terrain through shock metamorphism and melting.^[12] The transition from the Nectarian to the Imbrian period culminated the Late Heavy Bombardment (LHB), a hypothesized episode of elevated impact flux across the inner Solar System from roughly 4.1 to 3.8 Ga, with the Imbrium basin impact at approximately 3.85 Ga defining the boundary.^[13] The Imbrium event, producing the Fra Mauro Formation, overlaid and partially obscured earlier Nectarian deposits, signaling a shift from basin-dominated cratering to a declining flux that allowed mare volcanism to emerge later.^[11] This boundary reflects not only stratigraphic superposition but also a potential dynamical change in the impactor population, possibly linked to giant planet migration.^[13] Geological remnants of the Nectarian period persist as ejecta layers beneath Imbrian basins, influencing the overall lunar stratigraphy through buried highland materials and impact breccias. For instance, the Janssen Formation from Nectaris underlies the Fra Mauro Formation of Imbrium, creating a composite sequence that records the progressive buildup of impact debris.^[11] These layers, often mixed with pre-Nectarian highland rocks, provide critical evidence for reconstructing crustal evolution and have been mapped via remote sensing and Apollo samples.^[14] Debates persist regarding the LHB's intensity and duration, with some models favoring a short-lived cataclysmic spike around 3.9 Ga and others proposing a more prolonged decline from 4.2 Ga onward, potentially without a distinct peak.^[15]^[13] Evidence from lunar meteorites, including impact melt ages clustering between 3.5 and 4.0 Ga, supports elevated activity extending into the late Nectarian, suggesting a possible flux maximum just prior to the Imbrian transition at around 3.85 Ga.^[13] Lunar samples indicate basin-scale impacts around 4.22 Ga, challenging a purely terminal cataclysm and implying a sawtooth pattern in bombardment rates.^[16]

Early Imbrian

Basin-Forming Impacts

The Early Imbrian epoch is defined by a series of cataclysmic impact events that sculpted the lunar surface through the formation of large multi-ring basins, with Mare Imbrium serving as the primary example. These basins, exemplified by the Imbrium and Orientale basins, were primarily formed between approximately 3.85 and 3.80 billion years ago (Ga), marking the onset of this period following the Nectarian.^[1] Radiometric dating of impact melt rocks and crater counting from lunar samples, such as those from Apollo missions, confirm this temporal clustering, with Imbrium's formation anchoring the epoch's base.^[17] These basin-forming impacts resulted from collisions with large asteroids, typically tens to hundreds of kilometers in diameter, generating transient craters that expanded into complex multi-ring structures due to the Moon's elastic-plastic response. The mechanics involved high-velocity excavation, where the impactor's energy vaporized and ejected material to depths exceeding 20-30 km, penetrating the anorthositic crust (estimated 30-50 km thick at the time) and exposing underlying mantle peridotite.^[18] For instance, the Imbrium impact excavated over 5 × 10^6 km³ of crustal material, redistributing it radially and creating topographic rings through rebound and viscous relaxation.^[17]^[19] Similar processes shaped the other listed basins, though their ring morphologies vary due to differences in impact angle, target properties, and pre-existing terrain.^[20] Ejecta from these events blanketed vast regions, with the Fra Mauro Formation representing the primary Imbrium-derived deposit, consisting of shocked breccias and melt fragments transported ballistically and as fallback material. This formation extends over hundreds of kilometers from the basin rim, covering approximately 10% of the lunar nearside surface and burying pre-existing highlands.^[21] Apollo 14 samples from Fra Mauro confirmed its composition as dominantly Imbrium ejecta, with isotopic ages aligning to ~3.85 Ga.^[22] Ejecta from companion basins like Orientale contributed overlapping layers, forming regionally continuous plains that obscure older Nectarian features.^[17] The immediate consequences of these impacts were profound, triggering global seismic waves that propagated through the lunar interior, causing widespread fracturing and mass wasting observable in modern topography.^[21] Such seismic activity likely induced localized melting in the mantle, potentially accelerating subsequent volcanic resurfacing by thinning the lithosphere and creating pathways for magma ascent, as evidenced by temporal correlations between Imbrium impact melts and early mare basalts.^[23] Overall, these events resurfaced up to 15% of the Moon's surface with ejecta blankets, erasing much of the prior cratered terrain and setting the stage for later geological evolution.^[17]

Relation to Late Heavy Bombardment

The Late Heavy Bombardment (LHB) hypothesis posits a spike in impact rates across the inner solar system approximately 4.1 to 3.8 billion years ago (Ga), characterized by an elevated flux of asteroids and comets following the initial accretion phase of the planets.^[13] This period is thought to have resulted from dynamical instabilities, such as the migration of the giant planets, which destabilized the asteroid belt and scattered planetesimals inward, as simulated in the Nice model framework.^[24] In this model, Jupiter's orbital resonance with outer planets ejected material from the trans-Neptunian disk, leading to a transient increase in collisions with terrestrial bodies.^[25] The Early Imbrian epoch, spanning roughly 3.9 to 3.8 Ga, temporally overlaps with the terminal phase of the LHB, during which the largest lunar basins formed as the bombardment waned.^[13] Key examples include the Imbrium basin, dated to approximately 3.91 Ga via radiometric analysis of impact ejecta, representing one of the final major LHB events that reshaped the lunar highlands.^[25] Evidence for this connection comes from high-impact melt rocks recovered by Apollo missions, such as breccias from the Fra Mauro formation, which yield U-Pb and ^{40}Ar-^{39}Ar ages clustering around 3.9 Ga, consistent with widespread melting and mixing during the bombardment's decline. These rocks exhibit shock features and geochemical signatures indicative of basin-scale impacts, linking them directly to Imbrian-age events.^[26] Supporting observations include dynamical simulations from the Nice model, which reproduce the observed lunar cratering record by predicting a 100- to 1000-fold increase in impact flux over 100-200 million years, aligning with the formation of 10-20 major basins.^[25] Additionally, isotopic studies of lunar zircons reveal age clusters and potential impact-induced resetting around 4.0 Ga, with some grains showing positive cerium anomalies suggestive of oxidation during post-impact magmatism tied to LHB heating.^[27] These data, combined with meteorite shock ages, bolster the case for a punctuated end to the bombardment during the Imbrian.^[28] Debates persist regarding the LHB's scope, with questions about whether it was a solar system-wide phenomenon or primarily lunar, potentially exaggerated by sampling biases in Apollo-era rocks.^[15] Recent post-2020 analyses, including those from China's Chang'e-6 mission samples returned in 2024, challenge the uniformity of the LHB by revealing a lack of expected age clusters in far-side impactites from the Apollo basin (within the South Pole-Aitken basin), suggesting a more gradual decline in impacts rather than a discrete spike.^[29] These findings, supported by reevaluations of argon diffusion in lunar minerals, imply that the apparent Imbrian clustering may reflect analytical artifacts rather than a true cataclysm.^[30]

Late Imbrian

Volcanic Flooding Events

The volcanic flooding events of the Late Imbrian epoch represent the primary phase of mare basalt emplacement on the Moon, occurring between approximately 3.8 and 3.2 billion years ago (Ga), with peak activity around 3.5 Ga.^[31]^[32] These events infilled large impact basins formed during the preceding Early Imbrian, such as those underlying Mare Imbrium and Mare Crisium, creating vast dark plains that cover about 16% of the lunar surface, predominantly on the nearside.^[33] The basaltic lavas produced during this period lowered the overall albedo of affected regions, making the maria prominent features visible from Earth even to the naked eye.^[33] Prominent examples of these flooding events include the extensive basalt flows in Mare Imbrium, Mare Crisium, Mare Serenitatis, and Mare Fecunditatis, where successive eruptions partially or completely filled basin floors and surrounding lowlands.^[34] In Mare Imbrium, for instance, lavas spread across an area exceeding 1,200 km in diameter, forming a relatively flat, radar-smooth surface punctuated by subtle ridges and sinuous rilles.^[34] Similar infilling occurred in Mare Crisium, a more isolated circular basin, where basalts created a distinct dark "sea" contrasting with the surrounding highlands.^[35] These maria exemplify the widespread nature of Late Imbrian volcanism, which favored nearside topographic lows due to thinner crust and higher heat flow in that hemisphere. The eruptions were predominantly effusive, characterized by low-viscosity lava flows originating from linear fissures and vents rather than central cones, enabling rapid flooding of basin interiors.^[32] Individual flow units typically ranged from 20 to 220 meters in thickness, though cumulative deposits in major basins like Imbrium reached averages of about 1 kilometer, with some localized accumulations exceeding this depth.^[36]^[37] These flows advanced over distances of hundreds of kilometers at inferred initial rates of several meters per second, slowing over distance, ponding in depressions and smoothing pre-existing cratered terrain while preserving subtle lobate margins and wrinkle ridges from post-emplacement contraction.^[38]^[32]^[39] Compositionally, the dominant rock type was tholeiitic basalt, featuring high iron and aluminum contents with low silica, which contributed to the lavas' fluidity and dark appearance. Titanium-rich variants, containing up to 10-15 weight percent TiO₂, were particularly prevalent along the margins of Oceanus Procellarum, such as in the western portions of Mare Imbrium, where ilmenite enrichment enhanced the spectral contrast of these flows.^[40] These compositional trends reflect derivation from mantle sources with varying degrees of partial melting, resulting in a spectrum from low-titanium to very high-titanium basalts across different maria.^[40]

Mantle Melting Mechanisms

The primary mechanisms driving mantle melting during the Late Imbrian period involved impact-induced decompression melting, where large basin-forming impacts thinned the lunar crust, reducing overlying pressure and allowing partial melts to form in the underlying mantle. This process was particularly effective in regions like the Procellarum KREEP Terrane, where crustal thinning facilitated the ascent of buoyant magmas from depths of approximately 100–200 km.^[41] Additionally, the deposition of thick ejecta blankets from these impacts provided thermal insulation, trapping radiogenic heat and delaying conductive cooling of the subsurface, which extended the timeframe for melt generation by up to 200 million years post-impact.^[42]^[43] Mantle evolution during this epoch featured partial melting of ilmenite-bearing cumulates, dense layers formed from the solidification of the lunar magma ocean, which underwent overturn and sank into the upper mantle. These cumulates, enriched in titanium and iron, contributed to the production of titanium-rich basaltic melts as they interacted with overlying peridotitic mantle through upwelling plumes driven by gravitational instability. Geochemical analyses of returned lunar samples, such as those from Apollo missions, reveal source regions at these depths characterized by high TiO₂ contents (up to 15 wt%) and trace element signatures indicative of low-degree partial melting (1–5%) under anhydrous conditions.^[44]^[45]^[46] The prolonged duration of Late Imbrian volcanism, spanning over 600 million years from approximately 3.8 to 3.2 Ga, was sustained by residual heat retention linked to core-mantle interactions, including friction from precession-induced tidal effects that maintained elevated temperatures in the upper mantle. This mechanism allowed for sustained partial melting even after the cessation of the lunar core dynamo around 1.5 billion years ago, preventing rapid thermal decay and enabling episodic plume activity.^[47]^[48] Geochemical models supported by isotopic and trace element data from mare basalts highlight the role of KREEP-rich materials—residual liquids from the lunar magma ocean enriched in incompatible elements like thorium and uranium—in enhancing melt production through concentrated radiogenic heating. These materials, distributed asymmetrically in the nearside mantle, lowered the solidus temperature of source regions by 50–100°C, promoting higher melt fractions (up to 10%) in KREEP-contaminated cumulates compared to KREEP-poor domains. This is evidenced by elevated Th abundances (5–10 ppm) in Late Imbrian samples, correlating with prolonged magmatic activity in KREEP-enriched provinces.^[49]^[50]

Scientific Significance

Key Lunar Samples

The Apollo 15 mission returned several key samples from the vicinity of Mare Imbrium, providing direct evidence of Imbrian-age volcanic activity. Notable among these are green glass spherules, such as those from sample 15426, which have been dated to approximately 3.3–3.4 Ga via ⁴⁰Ar/³⁹Ar and other radiometric methods, indicating origin from fire-fountain eruptions during the Late Imbrian period.^[51]^[52] These spherules, enriched in volatiles and magnesium, represent primitive mantle-derived melts and constitute a significant portion (up to 28% on average) of the fine-grained glass component in Apollo 15 soils.^[53] Samples from other missions further illuminate Imbrian volcanism beyond Mare Imbrium. The Luna 24 mission retrieved a core from Mare Crisium containing basalts dated to around 3.3–3.4 Ga, with ⁴⁰Ar/³⁹Ar ages averaging 3.22 ± 0.04 Ga, showcasing low-titanium compositions typical of Late Imbrian flooding events.^[54]^[55] Similarly, Apollo 17 samples include high-titanium mare basalts, such as those from the Taurus-Littrow valley, with Rb-Sr ages of approximately 3.76 ± 0.06 Ga, reflecting Early Imbrian mantle sources enriched in ilmenite-bearing cumulates.^[56]^[57] Imbrian-period lunar samples encompass diverse lithologies, including mare basalts, impact breccias, and anorthosites exhibiting overprints from Imbrian events. Mare basalts dominate the returned collection, with varieties ranging from low- to high-titanium types formed by partial melting of the lunar mantle. Impact breccias, such as those from Apollo 15 near the Apennine Front, incorporate clasts ejected during the Imbrium basin formation and subsequent impacts, often showing shock metamorphism and mixing with volcanic materials. Anorthosites, primarily from highland sites like Apollo 16, display Imbrian overprints through brecciation and melt sheets from secondary impacts, linking pre-Imbrian crust to later modifications.^[58]^[59] Radiometric dating of these samples, primarily using ⁴⁰Ar/³⁹Ar, Rb-Sr, and Sm-Nd techniques, confirms their placement within the Imbrian epoch (ca. 3.85–3.15 Ga), with the majority of returned lunar basalts—over 80% by volume—dated to the Late Imbrian. This concentration underscores the peak of mare volcanism during this subperiod, as evidenced by isochron analyses across Apollo and Luna collections.^[60]^[61]

Modern Research and Debates

Recent missions such as the Lunar Reconnaissance Orbiter (LRO) and Gravity Recovery and Interior Laboratory (GRAIL) have provided high-resolution data that refine models of Imbrian basin formation, particularly regarding basin depths and crustal thinning. GRAIL gravity measurements reveal Bouguer anomalies that constrain the diameters of transient craters and impactor sizes for basins like Orientale, estimated at 80 km for a cold thermal profile around 3.81 Ga, while LRO's Lunar Orbiter Laser Altimeter topography data improves bulk density corrections for these models.^[62] These post-2010 findings indicate that Imbrian basins exhibit significant crustal thinning, with subsidence depths up to several kilometers in mare-filled regions, influenced by pre-impact crustal thicknesses of 40-60 km and varying thermal states.^[63] Ongoing debates center on the timing of the Late Heavy Bombardment (LHB) decline and the onset of Imbrian volcanism, with evidence suggesting a more gradual impact flux rather than a sharp spike at ~3.9 Ga. Analysis of Apollo impact melt ages indicates that apparent clusters around 3.9 Ga may result from argon diffusion loss and episodic crust formation, supporting a monotonically declining bombardment rate through the early Imbrian rather than a cataclysmic event.^[15] For volcanism onset, mare basalts primarily erupted between 3.9 and 3.1 Ga, but younger activity (<3.0 Ga) challenges models reliant solely on radiogenic heating, as KREEP abundances decline post-Imbrian while eruptions persisted.^[64] The role of tidal heating in prolonging Imbrian volcanic activity remains a point of contention, with models proposing it as a supplementary heat source alongside radiogenic decay. Viscoelastic deformation from Earth's tides could have generated heating comparable to radiogenic sources, sustaining mare eruptions until ~2 Ga, particularly in the nearside, as evidenced by low-viscosity mantle scenarios in Andrade models.^[65] This challenges traditional views that radiogenic heating alone drove post-Imbrian activity, given its insufficient power after 3.2 Ga.^[64] Significant gaps persist in samples from far-side Imbrian basins, limiting validation of crustal and mantle models due to the Apollo program's nearside focus. While China's Chang'e-6 mission returned ~2 kg of basalts from the Apollo basin in 2024, revealing cooler interior conditions (~180°C lower than nearside), these represent only localized Eratosthenian material and do not fully address Imbrian stratigraphy.^[66] The Artemis program, particularly Artemis III targeting the South Pole-Aitken basin, is essential for collecting diverse far-side samples to test bombardment and volcanism models, including Imbrian-aged anorthosites and KREEP-rich terranes.^[67] Future research directions emphasize integrating Imbrian geology with exoplanet studies to model ancient bombardment phases across systems. Lunar impact records, refined by LRO and GRAIL, serve as analogs for cratering histories on exomoons and rocky exoplanets, informing dynamical models of planetesimal delivery and habitability thresholds during early system evolution.^[13]

References

[1]
The geologic history of the Moon - USGS.gov
The latest basin-forming impacts created the giant and still-conspicuous Imbrium and Orientale basins during the Early Imbrian Epoch, between 3.85 and 3.80 ...
[2]
Lunar mare volcanism: Stratigraphy, eruption conditions, and the ...
Stratigraphic analyses show that the flux was not constant, but peaked in early lunar history, during the Imbrian Period (which spans the period 3.85-3.2 Ga).
[3]
[PDF] Stratigraphy and Chronology of the Moon's Crust
Clearly pre-Imbrian stratigraphy will ultimately be divided into many more time-stratigraphic units as the moon's stratigraphy is studied in more detail. The ...
[4]
Imbrian-Age Highland Volcanism on the Moon: The Gruithuisen and ...
The Gruithuisen and Mairan domes on the moon represent morphologically and spectrally distinct nonmare extrusive volcanic features of Imbrian age.
[5]
The Imbrian Crater Record and the Lunar Cataclysm - NASA/ADS
Multiple impact basins on the Moon have been dated to about 3.8 Gyr ago. This unexpected outcome of the Apollo missions has since been referred to as the ...
[6]
Post-imbrian global lunar tectonism: Evidence for an initially totally ...
Thus such accretional models predict that no significant global thrust faulting has occurred during the post-Imbrian and that the crust should currently be ...
[7]
(PDF) Stratigraphy and Isotope Ages of Lunar Geologic Units
Aug 6, 2025 · PDF | The absolute ages of cratered surfaces in the inner solar system, including Mars, are derived by extrapolation from the impact flux ...
[8]
Stratigraphic Basis for a Lunar Time Scale - NASA ADS
IMBRIAN SYSTEM The Imbrian system is the oldest widely exposed stratigraphic unit in the Copernicus region. It is continuously exposed from the Carpathian ...
[9]
None
Below is a merged summary of the Lunar Geologic Timescale and Imbrian Period based on all provided segments. To retain all detailed information efficiently, I will use a combination of narrative text and tables in CSV format where appropriate (e.g., for stratigraphic units, timelines, and key data). The response consolidates overlapping information, eliminates redundancies, and ensures comprehensive coverage of all points mentioned across the segments.
[10]
The Nectarian System, a new lunar time-stratigraphic unit - USGS.gov
A new system, herein named the Nectarian System, extends from the base of the Janssen Formation up to, but not including, the Fra Mauro Formation.
[11]
Compositional studies of the Orientale, Humorum, Nectaris
Feb 25, 2000 · Using these data to probe the crust of the Moon permits new and detailed study of crustal structure and evolution. Large impact basins, such as ...
[12]
The Late Heavy Bombardment - Annual Reviews
The Late Heavy Bombardment refers to impact events that occurred after stabilization of the planetary lithospheres such that they could be preserved as craters ...
[13]
Chronology, Local Stratigraphy, and Foreign Ejecta Materials at the ...
Dec 11, 2024 · The surface age of the Chang'e-6 mare unit is ∼2.8 billion years from crater density measurements, which can facilitate the refinement of lunar ...
[14]
Illusory Late Heavy Bombardments - PNAS
Sep 12, 2016 · The Late Heavy Bombardment (LHB), a hypothesized impact spike at ∼3.9 Ga, is one of the major scientific concepts to emerge from Apollo-era lunar exploration.
[15]
Ancient Bombardment of the Inner Solar System: Reinvestigation of ...
Feb 6, 2018 · The lunar cratering record provides valuable information about the late accretion history of the inner solar system.
[16]
Lunar impact basins: Stratigraphy, sequence and ages from ...
Feb 2, 2012 · Impact basin formation is a fundamental process in the evolution of the Moon and records the history of impactors in the early solar system.
[17]
Lunar impact basins revealed by Gravity Recovery and Interior ...
Oct 30, 2015 · These observations demonstrate that basin-forming impacts remove crustal materials from within the peak ring and thicken the crust between the peak ring and ...
[18]
Controls on the Formation of Lunar Multiring Basins - AGU Journals
Nov 5, 2018 · We explore how thickness of the lunar crust, size of the impacting body, and interior temperature of the Moon affect the formation of basins and their rings.
[19]
New observational evidence of global seismic effects of basin ...
Jun 5, 2012 · [3] The possibility of global seismic effects of large-scale impact events forming multiring basins on the Moon and other planetary bodies has ...
[20]
[PDF] Geology of the Apollo 14 Landing Site in the Fra Mauro Highlands
Graph showing rate of regolith formation versus age of original surface at ... Nearly all of the lunar surface is covered by a layer of comminuted rock ...
[21]
Coincidence in Time of the Imbrium Basin Impact and Apollo 15 ...
On the Earth there is no firm evidence that impacts can induce volcanic activity. However, the Moon does provide a very likely example of volcanism induced ...
[22]
https://pubs.usgs.gov/pp/0880/report.pdf
[23]
[PDF] The Late Heavy Bombardment - Southwest Research Institute
Jul 19, 2017 · ... Archean-era impacts on Earth may have been comparable to Orientale-formation events on the Moon. Now that is a late heavy bombardment! At ...
[24]
Ages of lunar impact breccias: Limits for timing of the Imbrium impact
We propose an age of 3922 ± 12 Ma for this event. Samples that yield ages within these limits can be considered as possible products of the Imbrium impact.
[25]
Coordinated U–Pb geochronology, trace element, Ti-in-zircon ...
Previous studies have identified some lunar zircons that exhibit positive Ce anomalies ... Post-Hadean transitions in Jack Hills zircon provenance: a signal of ...
[26]
[PDF] Coordinated U–Pb geochronology, trace element, Ti-in-zircon ...
Pre- vious studies have identified some lunar zircons that exhibit positive Ce anomalies, which is characteristic of crystalliza- tion from oxidized magmas ( ...
[27]
Collapse of the Lunar Late Heavy Bombardment Hypothesis
Sep 28, 2025 · Collapse of the Lunar Late Heavy Bombardment Hypothesis: Evidence from Chang'e-6 Samples ... Chang'E-6 mission. Article. Sep 2024.
[28]
Implications for testing the Late Heavy Bombardment hypothesis
Nov 15, 2024 · The discordancy between impact ages inferred from lunar impactites using two different radiometric systems suggests caution in acceptance of the LHB hypothesis.
[29]
Chronology, composition, and mineralogy of mare basalts in the ...
In the late Imbrian (∼3.8–3.2 Ga), volcanic events dominated the geological evolution of the PIV region, and lava flows filled topographic depressions in the ...
[30]
Lunar Mare Basaltic Volcanism - GeoScienceWorld
Dec 1, 2023 · The most significant recent developments in lunar volcanism have been three-fold: 1) the nature of young volcanism, 2) the origin of sinuous rilles, and 3) ...
[31]
Ages and stratigraphy of lunar mare basalts in Mare Frigoris and ...
Mar 20, 2010 · In Mare Frigoris, we dated 37 basalt units, showing ages from 2.61 to 3.77 Gyr, with most units being formed in the late Imbrian period between 3.4 and 3.8 Gyr ...
[32]
Investigating the stratigraphy of Mare Imbrium flow emplacement ...
Jul 30, 2016 · The lunar maria are the product of extensive basaltic volcanism that flooded widespread portions of the Moon's surface.
[33]
[PDF] selected aspects of lunar mare geology from-apollo orbital ...
The examples chosen for this study include Letronne, Mare. Imbrium, Mare Crisium and Mare Serenitatis, which are covered, at least in part, by Apollo ...<|separator|>
[34]
Thicknesses of mare basalts on the Moon from gravity and topography
May 11, 2016 · Excluding a few outliers, the average thickness is close to 1 km for most of Oceanus Procellarum and Mare Imbrium. The one major exception is ...
[35]
Lunar mare basalt flow units: Thicknesses determined from crater ...
Apr 27, 2002 · We found that the range of flow unit thickness is ∼20–220 m and the average is ∼30–60 m. This technique expands considerably the ability to ...
[36]
Volcanism and Deep Structures of the Moon | Space
Volcanism is the most important endogenic geological process of the Moon, which is closely related to its internal structure and thermal history.
[37]
[PDF] LUNAR MARE BASALTS: MINERALOGICAL VARIATIONS WITH ...
Early Ti-rich basalts flooded large regions in the eastern lunar hemisphere (Ap11, Ap17) in the early Imbrian Period (3.3-3.8 b.y.) [5]. These basalts were ...
[38]
Spectral interpretation of late-stage mare basalt mineralogy unveiled ...
Oct 10, 2022 · Basin-forming impact can induce pressure release melting of the mantle, but the impact-related heat may not last for such long time. KREEP ...
[39]
Remelting mechanisms for shallow source regions of mare basalts
The thermal insulation model predicts a delay of about 200 m.y. between impact and volcanism. ... Volume of material ejecta from major lunar basins and ...
[40]
Effect of a Giant Impact on the Thermal Evolution of the Moon
(2) Covering of the surrounding areas with an ejecta blanket and thus thermal insulation of the highlands with respect to the basin. In this paper we ...
[41]
Titanium-rich basaltic melts on the Moon modulated by reactive flow ...
Jan 15, 2024 · We suggest that titanium-rich magmas erupted on the surface of the Moon can be derived through partial melting of ilmenite-bearing cumulates.
[42]
The participation of ilmenite-bearing cumulates in lunar mantle ...
Apr 1, 2019 · We study the fate of lunar ilmenite-bearing cumulates (IBC) after they solidify. Around 50–70% IBC sink to the mantle, when urKREEP is still partially molten.
[43]
[PDF] N9 4- I -2.4
As a conductively thickening thermal boundary layer becomes unstable, the resulting mantle plumes rise, decompress and partially melt to generate the mare ...
[44]
Long-lived lunar volcanism sustained by precession-driven core ...
We show that core-mantle friction can sustain global-scale partial melting in the upper lunar mantle until ∼3.1 Ga, thus accounting for the intense volcanic ...<|separator|>
[45]
A shallow mantle source for the Chang'e 5 lavas reveals how top ...
Jul 18, 2025 · When KREEP is concentrated at the base of the crust, it has a large and prolonged effect on lunar magmatism, whereas a more diffuse vertical ...
[46]
2.2-billion-year-old KREEP-rich volcanism on the Moon
Aug 20, 2025 · Due to its enrichment in radioactive heat-producing elements, KREEP has long been considered a critical driver of lunar magmatism [5]. Indeed, ...
[47]
Distribution of Radioactive Heat Sources and Thermal History of the ...
Nov 22, 2018 · Mantle-derived Th could be enhanced by assimilation of Th from KREEP-rich rocks within the crust. ... lunar asymmetry by the creep of dense KREEP?
[48]
Apollo 15 green glass - Chemistry and possible origin - NASA ADS
(1974) that indicate an age of 3.34 275 276 J. W. Delano G.y. (3.38 ± 0.06 G.y. and 3.29 ± 0.06 G.y., respectively). This age is equivalent to that of mare ...
[49]
https://www.sciencedirect.com/science/article/pii/S2095927325008631
[50]
Apollo 15: Evolution of the regolith and origin of glasses
Green glass fragments and spherules, a characteristic component of Apollo 15 soils, form 22—33% (av. 28%) of all glasses and are enriched in the 0.1—0.02 mm ...
[51]
Laser 40Ar-39Ar age determinations of Luna 24 mare basalts - ADS
The ages obtained span a narrow range between 3.18-3.28 Ga with an average of 3.22 ( 0.04 Ga. This is interpreted as being the age of the basalts at the Luna 24 ...
[52]
[PDF] Luna 24 - Lunar and Planetary Institute
The Sm/Nd age (3.3 ± 0.05 b.y.) is probably the most accurate, but it is based on only two points!
[53]
Rb-Sr ages and initial Sr-87/Sr-86 for Apollo 17 basalts and KREEP ...
The weighted average age of four Apollo 17 basalts dated in our laboratory is 3.76 ± 0.06 AE. The significance of the result for 15386 is discussed in light of ...
[54]
[PDF] Petrogenesis of mare basalts: A record of lunar volcanism*
The high-Ti basalts from the Apollo 11 and 17 land- ing sites are generally older (3.6-3.9 Ga) than the low-Ti basalts from the Apollo 12 and 15 sites and the ...
[55]
[PDF] Apollo 15 Lunar Sample Catalog Part I 15015-15299 - NASA
This catalog characterizes each of 267 individually numbered rock samples in the Apollo 15 collection, showing what each sample is.
[56]
[PDF] Lunar Highlands Breccias Generated by Major Impacts1
This is the class of breccias that formed as a result of the major impacts—the impacts that gener- ated the mare basins and the largest of the lunar craters. ...
[57]
Ages of mare basalts on the lunar nearside - AGU Journals - Wiley
Dec 1, 2000 · ... most basalts erupted during the late Imbrian Period at about 3.6–3.8 b.y., (3) significantly fewer basalts were emplaced during the ...
[58]
[PDF] Implications of U-Th-Pb and Rb-Sr Systematics
Abundant Rb-Sr internal isochron and MAr/3'Ar ages of lunar basalts indicate that a major period of mare basalt flooding occurred from ~ 3.8 b.y. to ~ 3.2 b.y. ...
[59]
Numerical Investigation of Lunar Basin Formation Constrained by ...
Oct 20, 2021 · The free parameters in our models are the impactor diameter, the preimpact crustal thickness, and the thermal profile representing different ...
[60]
Evidence for structural control of mare volcanism in lunar ... - Nature
May 20, 2023 · Our findings suggest that lunar syn-tectonic mare emplacement along reactivated inherited faults provides important records of basin-scale structure-involved ...Missing: resurfacing | Show results with:resurfacing
[61]
The source and thermal driver of young (<3.0 Ga) lunar volcanism
Aug 22, 2025 · Different from the deep peridotitic sources of the early Imbrian-aged Apollo mission-returned basalts (11–13), CE-5 basalts are thought to ...Missing: percentage | Show results with:percentage<|control11|><|separator|>
[62]
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021JE006908
[63]
Nature of the lunar far-side samples returned by the Chang'E-6 ...
The Chang'E-6 (CE-6) mission successfully achieved return of the first samples from the far side of the Moon. The sampling site of CE-6 is located in the South ...Missing: Gaps Artemis Imbrian
[64]
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Below is a merged summary of the segments on "Gaps in Lunar Samples from Far-Side and Need for Artemis to Study Imbrian Geology," combining all the information from the provided summaries into a concise yet comprehensive response. To maximize detail and clarity, I’ve organized the key points into tables in CSV format, which can be easily read or converted into a tabular structure. The response also includes narrative text for context and retains all useful URLs.