Apollo 15 impact melts, the age of Imbrium, and the Earth-Moon impact cataclysm

The early impact history of the lunar surface is of critical importance in understanding the evolution of both the primitive Moon and the Earth, as well as the corresponding populations of planetesimals in Earth-crossing orbits. Two endmember hypotheses call for greatly dissimilar impact dynamics. One is a heavy continuous (declining) bombardment from about 4.5 Ga to 3.85 Ga. The other is that an intense but brief bombardment at about 3.85 +/- Ga was responsible for producing the visible lunar landforms and for the common 3.8-3.9 Ga ages of highland rocks. The Apennine Front, the main topographic ring of the Imbrium Basin, was sampled on the Apollo 15 mission. The Apollo 15 impact melts show a diversity of chemical compositions, indicating their origin in at least several different impact events. The few attempts at dating them have generally not produced convincing ages, despite their importance. Thus, we chose to investigate the ages of melt rock samples from the Apennine Front, because of their stratigraphic importance yet lack of previous age definition

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) Ar-40/Ar-39 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 (2sigma); 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 Ar-40/Ar-39 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 greater than or equal to 3869 Ma (72558,7) and greater than or equal to 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 q 16 Ma to 3869 q 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 greater than or equal to 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 greater than 20-25 Ma and probably greatr than 55-60 Ma older than Imbrium (less than or equal to 3870 Ma and probably less than or equal to 3836 Ma (Dalrymple and Ryder, 19931). 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

A Glass Spherule of Questionable Impact Origin from the Apollo 15 Landing Site: Unique Target Mare Basalt

A 6 mm-diameter dark spherule, 15434,28, from the regolith on the Apennine Front at the Apollo 15 landing site has a homogeneous glass interior with a 200 microns-thick rind of devitrified or crystallized melt. The rind contains abundant small fragments of Apollo 15 olivine-normative mare basalt and rare volcanic Apollo 15 green glass. The glass interior of the spherule has the chemical composition, including a high FeO content and high CaO/Al2O3, of a mare basalt. Whereas the major element and Sc, Ni, and Co abundances are similar to those of low-Ti mare basalts, the incompatible elements and Sr abundances are similar to those of high-Ti mare basaits. The relative abundance patterns of the incompatible trace elements are distinct from any other lunar mare basalts or KREEP; among these distinctions are a much steeper slope of the heavy rare earth elements. The 15434,28 glass has abundances of the volatile element Zn consistent with both impact glasses and crystalline mare basalts, but much lower than in glasses of mare volcanic origin. The glass contains siderophile elements such as Ir in abundances only slightly higher than accepted lunar indigenous levels, and some, such as Au, are just below such upper limits. The age of the glass, determined by the Ar-40/Ar-39 laser incremental heating technique, is 1647 +/- 11 Ma (2 sigma); it is expressed as an age spectrum of seventeen steps over 96% of the Ar-38 released, unusual for an impact glass. Trapped argon is negligible. The undamaged nature of the sphere demonstrates that it must have spent most of its life buried in regolith; Ar-38 cosmic ray exposure data suggest that it was buried at less than 2m but more than a few centimeters if a single depth is appropriate. That the spherule solidified to a glass is surprising; for such a mare composition, cooling at about 50 C/s is required to avoid crystallization, and barely attainable in such a large spherule. The low volatile abundances, slightly high siderophile abundances, and the young age are perhaps all most consistent with an impact origin, but nonetheless not absolutely definitive

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

The Hawaiian-Emporer volcanic chain, part 1, geologic evolution

Chapter 1 of Volcanism in HawaiiThe Hawaiian-Emperor volcanic chain stretches nearly 6,000 km across the North Pacific Ocean and consists of at least t 07 individual volcanoes with a total volume of about 1 million km3. The chain is age progressive with still-active volcanoes at the southeast end and 80-75-Ma volcanoes at the northwest end. The bend between the Hawaiian and .Emperor Chains reflects a major change in Pacific plate motion at 43.1 ± 1.4 Ma and probably was caused by collision of the Indian subcontinent into Eurasia and the resulting reorganization of oceanic spreading centers and initiation of subduction zones in the western Pacific. The volcanoes of the chain were erupted onto the floor of the Pacific Ocean without regard for the age or preexisting structure of the ocean crust. Hawaiian volcanoes erupt lava of distinct chemical compositions during four major stages in their evolution and growth. The earliest stage is a submarine alkalic preahield stage, which is followed by the tholeiitic shield stage. The shield stage probably accounts for >95 percent of the volume of each volcano. The shield stage is followed by an alkalic postshield stage during which a thin cap of alkalic basalt and associated differentiated lava covers the tholeiitic shield. After several million years of erosion, alkalic rejuvenated-stage lava erupts from isolated vents. An individual volcano may become extinct before the sequence is complete. The alkalic preshield stage is only known from recent study of Loihi Seamount. Lava from later eruptive stages has been identified from numerous submerged volcanoes located west of the principal Hawaiian Islands. Volcanic propagation rates along the chain are 9.2 ± 0.3 cm/yr for the Hawaiian Chain and 7.2 ± 1.1 cm/yr for the Emperor Chain. A best fit through all the age data for both chains gives 8.6±0.2 em/yr. Alkalic rejuvenated-stage lava erupts on an older shield during the formation of a new large shield volcano 190±30 km to the east. The duration of the quiescent period preceding eruption of rejuvenated-stage lava decreases systematically from 2.5 m.y. on Niihau to <0.4 m.y. at Haleakala, reflecting an increase in the rate of volcanic propagation during the last few million years. Rejuvenated-stage lava is generated during the rapid change from subsidence to uplift as the volcanoes override a flexural arch created by loading the new shield volcano on the ocean lithosphere. Paleomagnetic data indicate that the Hawaiian hot spot has remained fixed during the last 40 m.y., but prior to that time the hot spot was apparently located at a more northerly latitude. The most reliable data suggest about 70 of southward movement of the hot spot between 65 and 40 Ma. The numerous hypotheses to explain the mechanism of the hot spot fall into four types: propagating fracture hypotheses, thermal or chemical convection hypotheses, shear melting hypotheses, and heat injection hypotheses. A successful hypothesis must explain the propagation of volcanism along the chain, the near-fixity of the hot spot, the chemistry and timing of the eruptions from individual volcanoes, and the detailed geometry of volcanism. None of the geophysical hypotheses proposed to date are fully satisfactory. However, the existence of the Hawaiian ewell suggests that hot spots are indeed hot. In addition, both geophysical and geochemical hypotheses suggest that primitive undegassed mantle material ascends beneath Hawaii. Petrologic models suggest that this primitive material reacts with the ocean lithosphere to produce the compositional range of Hawaiian lava

Annotated record of the detailed examination of a ferromanganese crust retrieved the Emperor Seamount area in the Pacific Ocean by the R/V Kana Keoki during one of its KK76 expedition

40Ar/39Ar incremental heating experiments on three samples dredged from Jingu Seamount indicate that Jingu is 55.4 ± 0.9 m.y. old — older than the Hawaiian-Emperor bend and younger than the two dated Emperor Seamounts to the north. Major-oxide chemistry and petrography show that the samples are similar to hawaiites and mugearites from the Hawaiian Islands. By analogy with Hawaiian alkalic volcanic rocks, groundmass plagioclase compositions (An40-47) indicate that the three Jingu samples are probably mugearites. These results suggest that Jingu is a Hawaiian-type volcano and that the Emperor volcanoes become progressively older from south to north, as predicted by the hot-spot hypothesis

Potassium-argon dating : principles, techniques, and applications to geochronology /

Bibliography: p. [227]-240