Graham Ryder

Mass flux in the ancient Earth‐Moon system and benign implications for the origin of life on Earth

The origin of life on Earth is commonly considered to have been negatively affected by intense impacting in the Hadean, with the potential for the repeated evaporation and sterilization of any ocean. The impact flux is based on scaling from the lunar crater density record, but that record has no tie to any absolute age determination for any identified stratigraphic unit older than approx. 3.9 Ga (Nectaris basin). The flux can be described in terms of mass accretion, and various independent means can be used to estimate the mass flux in different intervals. The critical interval is that between the end of essential crustal formation (approx. 4.4 Ga) and the oldest mare times (approx. 3.8 Ga). The masses of the basin-forming projectiles during Nectarian and early Imbrian times, when the last 15 of the approx.45 identified impact basins formed, can be reasonably estimated as minima. These in sum provide a minimum of 2 x 10(exp 21)g for the mass flux to the Moon during those times. If the interval was 80 million years (Nectaris 3.90 Ga, Orientale 3.82 Ga), then the flux was approx. 2 x 10(exp 13) g/yr over this period. This is higher by more than an order of magnitude than a flux curve that declines continuously and uniformly from lunar accretion to the rate inferred for the older mare plains. This rate cannot be extrapolated back increasingly into pre-Nectarian times, because the Moon would have added masses far in excess of itself in post-crust-formation time. Thus this episode was a distinct and cataclysmic set of events. There are approx. 30 pre-Nectarian basins, and they were probably part of the same cataclysm (starting at approx. 4.0 Ga?) because the crust is fairly intact, the meteoritic contamination of the pre-Nectarian crust is very low, impact melt rocks older than 3.92 Ga are virtually unknown, and ancient volcanic and plutonic rocks have survived this interval. The accretionary flux from approx. 4.4 to approx. 4.0 Ga was comparatively benign. When scaled to Earth, even the late cataclysm does not produce oceane vaporating, globally sterilizing events. The rooted concept that such events took place is based on the extrapolation of a nonexistent lunar record to the Hadean. The Earth from approx. 4.4 to approx. 3.8 Ga was comparatively peaceful, and the impacting itself could have been thermally and hydrothermally beneficial. The origin of life could have taken place at any time between 4.4 and 3.85 Ga, given the current impact constraints, and there is no justification for the claim that life originated (or re-originated) as late as 3.85 Ga in response to the end of hostile impact conditions.

Lunar samples, lunar accretion and the early bombardment of the Moon

Recent advances in lunar sample science and planetary accretion theories demand a new look at the bombardment history recorded by the Moon. Among lunar samples there are no impact melts dated older than about 3.9 Ga, yet a heavy bombardment of the Moon from its birth until 3.9 Ga should have produced many melts to be sampled by Apollo, Luna, and the meteorites of lunar origin. The absence of older impact melts cannot be explained by continued isotopic resetting because ejecta blankets are mainly cold and ancient igneous rocks including mare basalts exist. The common 3.85-Ga melt ages cannot be ascribed to a single or even a few basin events (e.g., Imbrium) because the samples show wide differences in chemistry and real, if small, differences in ages; the ages also appear in the lunar meteorites and thus are Moon-wide.

Argon-40/Argon-39 Age Spectra of Apollo 17 Highlands Breccia Samples by Laser Step Heating and the Age of the Serenitatis Basin

We have obtained high-resolution (21–63 steps) 40Ar/39Ar age spectra using a continuous laser system on 19 submilligram samples of melt rocks and clasts from Apollo 17 samples collected from the pre-Imbrian highlands in the easternmost part of the Serenitatis basin. The samples include poikilitic melt rocks inferred to have been formed in the Serenitatis basin-forming impact, aphanitic melt rock whose compositions vary and whose provenance is uncertain, and granulite, gabbro, and melt clasts. Three of the poikilitic melts have similar age spectrum plateau ages (72395,96, 3893 ± 16 Ma {2σ} 72535,7, 3887 ± 16 Ma; 76315,150, 3900 ± 16 Ma) with a weighted mean age of 3893 ± 9 Ma, which we interpret as the best age for the Serenitatis basin-forming impact. Published 40Ar/39Ar age spectrum ages of Apollo 17 poikilitic melts are consistent with our new age but are much less precise. Two poikilitic melts did not give plateaus and the maxima in their age spectra indicate ages of ≥3869 Ma (72558,7) and ≥3743 Ma (77135,178). Plateau ages of two poikilitic melts and two gabbro clasts from 73155 range from 3854 ± 16 Ma to 3937 ± 16 Ma and have probably been affected by the ubiquitous (older?) clasts and by post-formation heating (impact) events. Plateau ages from two of the aphanitic melt “blobs” and two granulites in sample 72255 fall in the narrow range of 3850 ± 16 Ma to 3869 ± 16 Ma with a weighted mean of 3862 ± 8 Ma. Two of the aphanitic melt blobs from 72255 have ages of 3883 ± 16 Ma and ≥3894 Ma, whereas a poikilitic melt clast (of different composition from the “Serenitatis” melts) has an age of 3835 ± 16 Ma, which is the upper limit for the accretion of 72255. These data suggest that either the aphanitic melts vary in age, as is also suggested by their varying chemical compositions, or they formed in the 72255 accretionary event about 3.84–3.85 Ga and older relict material is responsible for the dispersion of ages. In any case the aphanitic melts do not appear to be Serenitatis products. Our age for the Serenitatis impact shows, on the basis of the isotopic age evidence alone, that Serenitatis is >20–25 Ma and probably >55–60 Ma older than Imbrium (≤3870 Ma and probably ≤3836 Ma [Dalrymple and Ryder, 1993]). Noritic granulite sample 78527 has a plateau age of 4146 ± 17 Ma, representing a minimum age for cooling of this sample in the early lunar crust. So far there is no convincing evidence in the lunar melt rock record for basin-forming impacts significantly older than 3.9 Ga.

40Ar/39Ar age spectra of Apollo 15 impact melt rocks by laser step-heating and their bearing on the history of lunar basin formation

We have obtained 26 high-resolution (16–51 steps) 40Ar/39Ar age spectra using a continuous laser system on submilligram fragments of recrystallized melt and single-crystal plagioclase clasts from 12 Apollo 15 impact melt rocks collected at the Apennine Front where the Imbrium and Serenitatis basins intersect. These melt rocks represent a wide range of compositions and at least half a dozen different impacts. Six of the melt rocks have reproducible, intermediate-temperature plateaus over 40% or more of the 39Ar released; the plateaus are interpreted as crystallization (impact) ages and much of the non-plateau behavior is attributable to recoil. Samples 15294,6,21,15304,7,69, 15314,26,156, 15357,15, and 15359,12 have mean 40Ar/39Ar plateau ages that are statistically indistinguishable and fall within the narrow range 3852 ± 14 (2σ) Ma to 3870 ±12 Ma with a weighted mean of 3865 ± 5 Ma. Sample 15356,9 has a mean plateau age of 3836 ± 11 Ma and may represent a distinctly younger impact. A seventh sample (15314,30,158) has a peculiar but reproducible double plateau; a low-T one at 3873 ± 9 Ma, which we think records the crystallization age, and a high-T one of 3831 ± 10 Ma, which we interpret as an experimental (39Ar recoil) artifact. Four of the remaining melt rocks (15308,9, 15414,2,37, 15436,2, 15445,253) have complex 40Ar/39Ar age spectra that indicate that they either formed in or were disturbed by impacts that occurred ≤3850 Ma but did not completely reset the K-Ar isotopic system. Sample 15414,3,36 is different. Its spectrum may represent release mainly from clasts that were not well degassed in a melt event at 3870 Ma; the melt phase contains little potassium. Because most of the Apennine Front material must be coeval with or predate formation of the Imbrium Basin, it seems likely that the Imbrium impact is no older than 3870 Ma and probably no older than 3836 Ma. So far there is no convincing evidence in the lunar record for melt-producing impacts, such as basin formation, older than about 3.9 Ga.

Lunar sample 15405 - Remnant of a KREEP basalt-granite differentiated pluton

Abstract Large, coarse-grained fragments of granite, containing plagioclase, a silica polymorph, potash feldspar, and exsolved pyroxene, with minor ilmenite, a phosphate, Fe-metal, and troilite, occur in sample 15405. A similar coarse-grained clast type (KREEP-rich quartz-monzodiorite) has a similar mineralogy but contains more ilmenite, large phosphates, less silica, and lacks troilite. One unusual KREEPy olivine vitrophyre fragment is also present. All the other fragments in 15405 are of Apollo 15-type KREEP basalt; ANT-suite and breccia fragments are conspicuously absent. The groundmass of 15405, of a KREEP basalt composition, is vesicular with a variolitic texture and is interpreted as an impact melt. Except for the olivine vitrophyre, the fragments are believed to be the remnants of a shallow-level KREEP basalt-granite differentiated pluton, in which granite was produced as the residual liquid without involvement of immiscibility effects. The large amount of melt required to produce the pluton, and the retention of the plutons integrity from crystallization until the formation of the source boulder of 15405 suggest that KREEP basalt magma is not ancient (∼4.3 b.y.), but was produced by the partial melting of the interior of the moon at around 3.90–3.95 b.y.; this conclusion is supported by the presence of KREEP basalt in soil breccia 15205, to the exclusion of other highland rock types. If this interpretation is correct, the source of Apollo 15-type KREEP basalt had a Rb/Sr ratio higher than anorthositic norite, commonly proposed as the source rock.

Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis

Abstract Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements (3.6–15.8 ppb Ir) with CI-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroid, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8–4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust.

Lunar ferroan anorthosites and mare basalt sources: The mixed connection

Global overturn of a hot, gravitationally unstable lunar mantle immediately following the solidification of a magma ocean [and essentially complete by 4.4 Ga] explains several characteristics of lunar petrology. Lunar mare basalt sources are inferred to be depleted in europium and alumina. These depletions are consensually attributed to complementary plagioclase floating from a magma ocean. However, the connection cannot be so simple and direct: in contrast to the mare basalt source parent magma, the ferroan anorthosite parent magma was more evolved by virtue of its lower Mg/Fe ratio and Ni abundances, although less evolved in its poverty of clinopyroxene constituents, flat rare earth pattern, and lower incompatible element abundances. The europium anomaly in mare sources is inferred to be present at 400 km depth, too deep to have been directly influenced by plagioclase crystallization. Massive overturning of the post-magma ocean mantle would have carried down clinopyroxene, ilmenite, and phases containing fractionated rare earths, europium anomalies, and some heat-producing radionuclides. These phases contributed to deep mare basalt sources. Upward-moving phases would have been magnesian mafic minerals; their immediate melting on pressure release contributed the magnesian suite of plutonic norites and troctolites that post-date the anorthosites in the highlands crust.

Lunar anorthosite 60025, the petrogenesis of lunar anorthosites, and the composition of the moon

Systematic variations of the mineral chemistry of ferroan anorthosite 60025, which is probably a mixture of closely related materials, suggest that lunar anorthosites formed by strong fractional crystallization and near-perfect adcumulate growth, without trapping liquid. The parent liquid for the most primitive samples was saturated with olivine, plagioclase, pigeonite, and chromite, and evolved to one saturated with plagioclase, pigeonite, high-Ca pyroxene, and ilmenite. The parent liquid also had a very low Na2O content, and combined with strong fractional crystallization this explains the steep trend of anorthosites on an Mg∗ (atomic 100 × Mg/(Mg + Fe)) v. An diagram. The mineral and chemical data for other anorthosites are consistent with such a model. Near-perfect adcumulation can occur if growth takes place at the crystal-liquid interface without the physical accumulation of crystals grown elsewhere, and is encouraged by the shifts in phase boundaries with pressure. Anorthosites are probably the remnants of a crust floating on, and crystallizing at the surface of, a magma ocean originally of bulk Moon composition. Mineralogical and trace element data suggest that the parental liquid for the most primitive anorthosites had previously crystallized no plagioclase and some but perhaps very little pyroxene. Hence the bulk Moon appears to be similar to that proposed by Ringwood (1976) but to have even lower alkalis, a subchondritic CaAl ratio, and REE abundances and patterns close to chondritic. The mare basalt sources are not directly complementary to the feldspathic crust, because experimental and trace element data indicate that they are too magnesian and contain too much high-Ca pyroxene. Other crustal rocks, such as the Mg-suite samples, are not closely related to anorthosites; in addition to their chemical differences they have a different crystallization sequence: ol → plag → px, in contrast with the ol → px → plag inferred for anorthosite parental liquid evolution.

The complex stratigraphy of the highland crust in the Serenitatis region of the Moon inferred from mineral fragment chemistry

Large impact basins are natural drill holes into the Moon, and their ejecta carries unique information about the rock types and stratigraphy of the lunar crust. We have conducted an electron microprobe study of mineral fragments in the poikilitic melt breccias collected from the Taurus Mountains at the Apollo 17 landing site. These breccias are virtually unanimously agreed to be impact melt produced in the Serenitatis impact event. They contain lithic fragments and much more abundant mineral fragments of crustal origin. We have made precise microprobe analyses of minor element abundances in fragments of olivine, pyroxene, and plagioclase to provide new information on the possible source rocks and the crustal stratigraphy in the Serenitatis region. These data were also intended to elucidate the nature of the cryptic geochemical component in breccias such as these with low-K Fra Mauro basalt compositions. We chose the finest-grained (i.e., most rapidly quenched) breccias for study, to avoid reacted and partly assimilated fragments as much as possible. Most of the mineral fragments appear to have been derived from rocks that would fall into the pristine igneous Mg-suite as represented by lithic fragments in the Apollo collection, or reasonable extensions of it. Gabbroic rocks were more abundant in the target stratigraphy than is apparent from the Apollo sample collection. Some pyroxene and plagioclase, but probably not much olivine, could be derived from feldspathic granulites, which are metamorphosed polymict breccias. Some mineral fragments are from previously unknown rocks. These include highly magnesian olivines (up to Fo94), possibly volcanic in origin, that exacerbate the difficulty in explaining highly magnesian rocks in the lunar crust. It appears that some part of the lunar interior has an mg∗[= 100 × Mg/(Mg/Fe) atomic] greater than the conventional bulk Moon value of 80–84. Other volcanic rocks, including mare basalts, and rapidly-cooled impact melt rocks do not contribute significantly to the fragment population. Nor do ferroan anorthosites contribute more than a tiny part of even the plagioclase fragment population. A few mineral fragments that are consistent with the cryptic low-K Fra Mauro chemical component were found, and these appear to be from gabbroic sources. The mineral fragment populations cannot be mixed in their observed proportions to produce the whole rock composition, because the fragments are more refractory and deficient in Ti, P, and alkalis. A preferential contribution to the melt from a rock similar to sodic ferrogabbro can partly resolve the discrepancy. The population of mineral fragments requires a very diverse population of igenous rocks that are not all related to each other, demonstrating the existence of a complex crust built of numerous separate igneous plutons. Many of these plutons may have crystallized at shallow depths. The chemical composition of the melt breccias, in combination with the mineral fragment data and an understanding of the cratering process, suggests that the deepest crust sampled by the Serenitatis impact (not necessarily the deepest crust) was basaltic in composition, including KREEP and gabbroic rocks like sodic ferrogabbro, and lacking abundant olivine-rich material. These were overlain by Mg-suite rocks of varied types, including norites and troctolites that supplied most of the olivine mineral fragments. Granulites, which are metamorphosed and more feldspathic breccias, were abundant near the surface. Remote sensing indicates that the entire Serenitatis region lacks ferroan anorthosite, consistent with the results of our study.

Chemical variation of the large Apollo 15 olivine‐normative mare basalt rock samples

Most chemical analyses of Apollo 15 olivine-normative mare basalts have been conducted on subsamples of 4 g) to obtain greater whole-rock representivity. These subsamples were individually ground and homogenized, and splits were taken for analysis. Furthermore, we used both X-ray fluorescence and neutron activation techniques to analyze for a comprehensive set of elements suitable for petrogenetic interpretation. The analyses show that the samples form a single coherent suite with almost all of the variation corresponding with olivine control (15% range). A few of the coarser rocks are not quite represented even at this sampling size. The analyses show that the rocks are individually distinct and that analyses are not merely of unrepresentative pieces of a single rock, undifferentiated rock unit, or rocks differing only by short-range unmixing of residual fluids. The petrographic features, including the low abundance of olivine and its small size, and the vesicularity of even some of the coarser samples, show that the olivine that controlled the chemical variation is not accumulated in any of the rocks. The Apollo 15 olivine-normative mare basalts were extruded as a series of magmas from a shallow but not locally surficial, olivine-accumulating magma system and formed a sequence of thin flows. A greater understanding of the relationships within and among other mare basalt sequences would be obtained by obtaining comprehensive chemical analyses on splits taken from subsamples of 5 g of all rocks large enough to obtain such subsamples.